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STRAUSS et al 2003 CoComposting of Faecal Sludge and Municipal Organic Waste

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STRAUSS et al 2003 CoComposting of Faecal Sludge and Municipal Organic Waste
Co-composting of Faecal
Sludge and Municipal
Organic Waste
A Literature and State-of-Knowledge
Review
May 2003
Martin Strauss
Silke Drescher
Christian Zurbrügg
Agnès Montangero
[email protected]
[email protected]
[email protected]
[email protected]
Swiss Federal Institute of Environmental Science & Technology (EAWAG)
Dept. of Water & Sanitation in Developing
Countries (SANDEC)
P.O. Box 611, CH-Duebendorf, Switzerland
ii
Olufunke Cofie
Pay Drechsel
[email protected]
[email protected]
International Water Management
Institute (IWMI)
PMB CT 112
Cantoments Accra, Ghana
S A N D E C / E A W A G --- I W M I
Table of Contents
Table of Contents
iii
Foreword
v
Abbreviations and Glossary
vi
Abbreviations
vi
Glossary
vi
1
1
2
3
4
5
6
Reuse of excreta and municipal organic waste
1.1
General practices of excreta and solid waste use
1
1.2
The resource potential of human excreta and municipal solid waste
1
1.3
Health consideration in re-use of human waste and solid waste
3
Faecal sludge treatment
7
2.1
Relevant FS characteristics and quantities
7
2.2
Faecal sludge treatment options
8
2.3
Pre-Treating FS for Co-Composting Solids-liquid separation and dewatering
10
2.4
Hygienic quality of biosolids
13
Municipal Organic Solid Waste Management
15
3.1
Relevant municipal solid waste characteristics and quantities
15
3.2
Approaches for municipal organic solid waste treatment
15
Composting and Co-composting
17
4.1
Process definition
17
4.2
Why co-compost feacal sludge with municipal solid waste?
17
4.3
Composting systems
17
4.4
Key factors of the composting process
18
4.5
Quality of compost
19
4.6
Quality of compost produced from human waste
19
4.7
Benefit of using compost in agriculture
22
Literature and case-studies on FS co-composting
23
5.1
Case-studies of co-composting
23
5.2
Literature studies
27
Conclusions and open questions
29
6.1
Conclusions
29
6.2
Open questions / researchable issues
29
Bibliography
31
Annex 1 Mixtures of faecal sludge and other organic material in combined
composting as reported in the literature
Annex 2 The science of composting
S A N D E C / E A W A G --- I W M I
37
39
iii
iv
S A N D E C / E A W A G --- I W M I
Foreword
This review on the combined composting of (faecal) sludges and organic solid waste was produced as part of the project entitled „Cocomposting of Faecal Sludge and Organic Solid Waste in Kumasi,
Ghana.“ The initial phase of the project was conducted within the programme „Sustainable Solid Waste Management and Sanitation“ which
was financed by the French Ministry of Foreign Affairs and coordinated
by the pS-Eau/PDM.
The pilot project is co-ordinated by the International Water Management
Institute (IWMI) in collaboration with the University of Science and
Technology in Kumasi, the city’s Waste Management Department
(WMD, Kumasi Metropolitan Assembly) and SANDEC. Results of the
investigation will help WMD to develop its biosolids management strategy and enable the project team to develop guidelines for planners and
engineers on the option of co-composting.
S A N D E C / E A W A G --- I W M I
v
Abbreviations and Glossary
Abbreviations
BOD
Biochemical Oxygen Demand
COD
Chemical Oxygen Demand
FC
Faecal Coliforms
MSW
Municipal Solid Waste
FS
Faecal Sludge
NH4-N
Ammonium Nitrogen
NH3-N
Ammonia Nitrogen
SS
Suspended Solids
TKN
Total Kjeldahl Nitrogen
TOC
Total Organic Carbon
TS
Total Solids
TVS
Total Volatile Solids
WSP
Waste Stabilisation Ponds
WWTP
Wastewater Treatment Plant
Glossary
Faecal sludge
Sludges of variable consistency collected from socalled on-site sanitation systems; viz. latrines, nonsewered public toilets, septic tanks, and aqua privies
Septage
Contents of septic tanks (usually comprising settled and floating solids as well as the liquid portion)
Public toilet sludge Sludges collected from unsewered public toilets
(usually of higher consistency than septage and
biochemically less stabilised)
Percolate
vi
The liquid seeping through a sludge drying bed
and collected in the underdrain
S A N D E C / E A W A G --- I W M I
1 Reuse of excreta and municipal organic waste
1.1 General practices of excreta and solid waste use
All around the world, people both in rural and urban areas have been
using human excreta for centuries to fertilise fields and fishponds and
to maintain or replenish the soil organic fraction, i.e. the humus layer.
Until today, in both agriculture and aquaculture this continues to be
common in China and Southeast Asia as well as in various places in
Africa (Cross 1985; Timmer and Visker 1998; Visker 1998; Timmer
1999; Strauss et al. 2000). Use practices have led to a strong economic
linkage of urban dwellers (food consumers as well as waste producers),
and the urban farmers (waste recyclers and food producers). Chinese
peri-urban vegetable farmers have reported that customers prefer excreta-fertilised rather than chemically fertilised vegetables. Thus vegetables grown on excreta-conditioned soils yield higher sales prices.
Like excreta, the use of organic solid waste has a long history mainly
in rural areas. Traditional reuse practices of organic solid waste are
shown to be especially strong in countries where population densities
are high. With the growth of urban areas, the importance of managing
municipal solid wastes to avoid environmental degradation and public
health risks has gained significance. Although informal recycling activities of waste materials is wide spread in developing countries the treatment and use of the biodegradable organic fraction is still fairly limited.
Increasingly, national and municipal authorities are now looking at ways
to manage their organic solid waste. In India national legislation was
adopted with the “Municipal Solid Waste (Management & Handling)
Rules 2000” (Ministry of Environment and Forests 2000) whereby one
section of the rules requires Urban Local Bodies to promote and implement waste segregation at source and treat organic waste.
1.2 The resource potential of human excreta and municipal solid waste
1.2.1 Excreta
Excreta are a rich source of organic matter and of inorganic plant nutrients such as nitrogen, phosphorus and potassium. Each day, humans
excrete in the order of 30 g of carbon (90 g of organic matter), 10-12 g
of nitrogen, 2 g of phosphorus and 3 g of potassium. Most of the organic matter is contained in the faeces, while most of the nitrogen (7080 %) and potassium are contained in urine. Phosphorus is equally distributed between urine and faeces. Table 1 shows that the fertilising
equivalent of excreta is, in theory at least, nearly sufficient for a person
to grow its own food (Drangert 1998). In a recent material flow study
conducted in the City of Kumasi, Ghana, it was found that for urban and
peri-urban agricultural soils, nutrients (N and P, Organic matter, could
be fully replenished by using all the human waste and recycling all the
organic market waste and the wastes from breweries, timber and food
processing factories and from chicken farms (most of the wastes would
have to be treated prior to use, though) (Leitzinger 2000; Belevi et al.
2000).
Excreta are not only a fertiliser. Its organic matter content, which serves
S A N D E C / E A W A G --- I W M I
1
as a soil conditioner and humus replenisher – an asset not shared by
chemical fertilisers – is of equal or even greater importance
Table 1
The Fertilization Equivalent of Human Excreta (after Drangert
1998)
Nutrient in kg / cap year
Nutrient
In urine
In faeces
Total
Required for
(500 l/year)
(50 l/year)
Nitrogen (as N)
4.0
0.5
4.5
5.6
Phosphorus (as P)
0.4
0.2
0.6
0.7
Potassium (as K)
0.9
0.3
1.2
1.2
Carbon (as C) 2
2.9
8.8
11.7
250 kg of cereals 1
1
= the yearly food equivalent required for one person
2
= indicative of the potential for soil conditioning, normally not designated a nutrient
New approaches in human waste management postulate that sanitation
systems should, whenever feasible, be conceived and managed that
again enable the recycling of organic matter and nutrients contained in
human excreta (Winblad 1997; Esrey et al. 1998). A change in the sanitation management paradigm from flush-and-discharge to recycling of
urine and faeces is gaining ground in Europe (Larsen and Guyer 1996;
Otterpohl et al. 1997 and 1999; Otterpohl 2000). As a consequence,
treatment strategies and technological options for faecal sludges and
solid waste will have to be developed which allow the optimum recycling
of nutrients and organic matter to peri-urban agriculture, while being
adapted to the local situation and needs.
1.2.2 Municipal organic solid waste
The resource potential of mixed municipal solid waste is more variable
than for excreta as it depends on the waste composition, which varies
considerably from city to city and also among city districts depending on
income levels and consumer habits. Low-income countries generate
significantly less waste than high- income countries. Cointreau (1985)
estimates average municipal solid waste generation (mixed) between
0.4 - 0.6 kg per capita per day in low-income countries, compared to 0.7
– 1.8 kg/cap and day in high-income countries. Typically in low-income
countries the biodegradable fraction is significantly higher (40-85 %)
than in high-income countries (20-50 %) where municipal waste consists mainly of packaging materials (paper and plastics). Assuming a
daily per-capita solid waste generation of 0.5 kg with a 60 % biodegradable fraction, 300 g/cap.day wet organic waste is being generated.
Based on an assumption of 50 % water content of this organic fraction,
this is equivalent to 150 grams dry organic solids/cap and day. Based
on contents on a dry weight basis of 30-40 % carbon (C), 1-2 % nitrogen (N) and 0.4-0.8 % phosphorus (as P), and 1 % potassium (as K),
the per-capita nutrient and carbon contributions from the organic fraction of MSW is as indicated in Table 2. The table shows that municipal
2
S A N D E C / E A W A G --- I W M I
organic solid waste although low in nutrients is particularly rich in organic matter can be thus be valued on its soil conditioning potential.
Table 2
The fertilization equivalent of municipal solid waste (org. fraction) before waste treatment
Nutrient
Contribution in kg / cap year
Nitrogen (as N)
0.55 – 1.1
Phosphorus (as P)
0.2 – 0.4
Potassium (as K)
Carbon (as C) 1
1
0.55
16 – 22
= indicative of the potential for soil conditioning, normally not designated as a nutrient
1.3 Health consideration in re-use of human waste and solid waste
In developing countries, excreta-related diseases are very common,
and faecal sludges contain correspondingly high concentrations of excreted pathogens - the bacteria, viruses, protozoa, and the helminths
(worms) that cause gastro-intestinal infections (GI) in man. The actual
risks to public health that occur through waste use can be divided into
three broad categories - those affecting consumers of the crops grown
with the waste (consumer risk), those affecting the agricultural workers
who are exposed to the waste (workers’, farmers’ risk), and those affecting populations living near to a waste reuse scheme (nearby population risk)
1.3.1 Health risks related to excreted pathogens
The agricultural use of excreta or excreta-derived products such as
stored or dewatered faecal sludge or co-compost can only result in an
actual risk to public health if all of the following occur (WHO 1989):
(a)
That either an infective dose of an excreted pathogen reaches
the field or pond, or the pathogen (as in the case of schistosomiasis) multiplies in the field or pond to form an infective dose;
(b)
That this infective dose reaches a human host;
(c)
That this host becomes infected; and
(d)
That this infection causes disease or further transmission.
(a), (b) and (c) constitute the potential risk and (d) the actual risk to
public health. If (d) does not occur, the risks to public health remain potential only.
Die-off or survival of excreted pathogens is an important factor influencing transmission. In principle, all pathogens die off upon excretion.
Prominent exceptions are pathogens whose intermediate stages multiply in intermediate hosts as the miracidia of e.g. Clonorchis or Schisto-
S A N D E C / E A W A G --- I W M I
3
soma which multiply in aquatic snails and are later released into the water body. Some bacteria (Salmonellae, Shigellae and Campylobacter,
e.g., have the potential to multiply outside the host primarily on food and
at warm temperature. The pathogens have varying resistance against
die-off, and worm eggs are among the more resistant with Ascaris eggs
surviving longest in the extra-intestinal environment. The main factors
influencing die-off are temperature, dryness and UV-light. Table 3 lists
survival periods at ambient temperature in faecal sludges for temperate
and tropical climates. Another important factor is the infective dose of
a pathogen. It is the dose required to create disease in a human host.
For helminths, protozoa (e.g. amoeba) and viruses, the infective dose is
low (< 102). For bacteria, it is medium (< 104) to high (> 106).
Table 3
Pathogen Survival Periods in Faecal Sludge (after Feachem
et al. 1983, Strauss 1985 and Schwartzbrod J. and L. 1994)
Average Survival Time in Wet Faecal Sludge at
Ambient Temperature1
In temperate climate
(10-15 °C)
[days]
Organism
• Viruses
In tropical climate
(20-30 °C)
[days]
< 100
< 20
< 100
< 30
< 30
<
< 150
< 50
< 30
< 15
2-3 years
10-12 months
12 months
6 months
• Bacteria:
-Salmonellae
-Cholera
-Faecal coliforms
2
5
• Protozoa:
-Amoebic cysts
• Helminths:
-Ascaris eggs
-Tapeworm eggs
1
2
Conservative upper boundaries to achieve 100 % die-off; survival periods
are shorter if the faecal material is exposed to the drying sun, hence, to
desiccation
Faecal coliforms are commensal bacteria of the human intestines and
used as indicator organisms for excreted pathogens
Scott in China conducted investigations on microbial risks from human
waste use in relation to the use of human excreta in agriculture as early
as the 1930-ies (Scott 1952). Rudolfs et al. (1950 and 1951) conducted
later major assessments of microbial contamination of soils and plants
using wastewater and sewage sludge in the U.S.. Akin et al. (1978) reported about continued work in this field done in the United States. A
thorough, basic compendium on the relationships between health, excreted infections and measures in environmental sanitation has been
published by Feachem et al. (1983). Strauss (1985) published a review
on the survival of excreted pathogens on soils and crops –a factor of
great relevance for the risk or non-risk of human waste use –. WHO,
UNDP, the World Bank, in collaboration with other multi and bilateral
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S A N D E C / E A W A G --- I W M I
support agencies commissioned reviews of epidemiological literature
related to the health effects of excreta and wastewater use in agriculture and aquaculture in the early eighties. The results are documented
in Shuval et al. (1986) and in Blum and Feachem (1985). This, in combination with the systematised assessment of gastro-intestinal infections by Feachem, aimed at developing a rational basis for the formulation by WHO of updated health guidelines in wastewater reuse (see
WHO 1989).
Birley and Lock (1997) and Allison et al. (1998) have highlighted health
impacts and risks of solid and human waste use in urban agriculture.
While touching upon the water and excreta-related diseases, they also
focused on health risks to farmers and consumers from chemical contamination of soils, occupational risks from poisoning through herbicides
and pesticides and from physical injury mainly when solid wastes are
recycled to agriculture. The non-pathogen related risks are discussed
further below.
The epidemiological evidence on the agricultural use of excreta can be
stated as follows (Blum and Feachem 1985):
•
Crop fertilisation with untreated excreta causes significant excess
infection with intestinal nematodes in both consumers and field
workers
•
Excreta treatment, e.g. through thermophilic composting, extended storage and/or drying, significantly reduces or eliminates the risk of transmission of gastro-intestinal infections.
Pathogen die-off or inactivation during composting is dealt with in Chapter 4.6
Ascaris eggs, being the most persistent of all pathogens, can be used
as a hygienic indicators of treated excreta. For sludge or biosolids,
Xanthoulis and Strauss (1991) proposed a nematode egg standard of ≤
3-8 eggs/gram of dry solids. This value is based on the 1989 WHO
nematode guideline of ≤ 1 egg/litre of wastewater for unrestricted irrigation. In municipal solid waste, the health risk by pathogens is determined by the amount of faecal matter contained in the solid waste or by
pathogenic hospital and clinical waste, which may enter the municipal
solid waste stream unintentionally. Non-pathogen risks can be more
significant depending on the waste composition and the way the waste
is managed (or not managed).
1.3.2 Non-Pathogenic Health Risks
Besides excreted pathogens, chemical contamination constitutes an
important potential risk associated with human waste (wastewater and
sludge) use. Relevant groups of chemical contaminants are (Environmental Research Foundation 2000; Holm 2001; McArdell 2002; Suter
2002):
•
•
Heavy metals (HM)
Hormone active substances (HAS; also termed “endocrine disrupting chemicals”, EDC). These subsume natural and synthetic
estrogens, and an array of substances (and/or their degradation
products) with, among them, polychlorinated biphenyl (PCB) and
S A N D E C / E A W A G --- I W M I
5
•
chemicals used in industrial detergents, as plastic additives, in
pesticides and antifoulings, body care products.
Antibiotics
It may be rightfully assumed that the use and occurrence of some of
these substances are still rather limited in developing countries. Others,
such as heavy metals, antibiotics, and HAS (through the use of pesticides, cosmetic products and contraceptive medicine), however, may
likely be found rather widely and in critical loads in urban waste
streams. Antibiotics, e.g., are widely and indiscriminately used in urban
societies of developing countries.
The risks associated with these substances may, in the long run, turn
out to constitute a greater threat to health and be more difficult to deal
with than the risks from excreted pathogens. Excreted pathogens die off
once they have been shed into the environment (at varying rates,
though, and with potential for transient re-growth of some bacterial
pathogens under exceptional conditions). In contrast to this, heavy metals are conservative substances accumulating in the environment, particularly so in waste-amended soils. HAS and antibiotics have become
of considerable concern and are now a focus of environmental research
in industrialised countries, as they, too, may accumulate and persist in
the environment over extended periods with potentially serious health
impacts for humans and animals. They originate from human and animal excreta (naturally produced and from human and veterinary pharmaceuticals) as well as from domestic and industrial wastewater and
sludges. Sludges accumulating in individual sanitation systems or produced during wastewater treatment, are the result of concentration
processes taking place during storage and treatment. Hence, they constitute a sink for substances, including chemical contaminants, which
are in non-dissolved form. As a tendency, therefore, keeping a close
eye on the chemical quality of sludges is of particular importance,
where they are used in urban agriculture regularly and over longer periods of time.
Contamination of the organic fraction of solid municipal waste by chemical constituents, notably heavy metals, must be presumed in most
cases as organic solid waste is usually stored and collected together
with other waste fractions. When applying the contaminated compost
product, these constituents can accumulate in soils. The contamination
of soils by chemicals, the potential but as yet uncertain uptake by crops,
and the possible chronic and long-term toxic effects in humans are discussed by Chang et al. (1995) and by Birley and Lock (1997).
Further non-pathogen risks result from impurities of non-biodegradable
origin such as glass splinters or other sharp objects contained in the
compost product. Such impurities can result from insufficiently sorted
municipal solid waste before or after the composting process. Birley and
Lock (1999) have highlighted these risks also including indirect health
risks due to the attraction and proliferation of rodents and other disease-carrying vectors.
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2 Faecal sludge treatment
2.1 Relevant FS characteristics and quantities
Table 4 contains the daily per capita volumes and loads of organic
matter, solids and nutrients in faecal sludges collected from septic
tanks and pit latrines, as well as from low or zero-flush, unsewered public toilets. Values for fresh excreta are given for comparative purposes.
The figures are overall averages, actual quantities may, however, vary
from place to place.
Table 4
Parameter
Daily per capita volumes; BOD, TS, and TKN quantities of
different types of faecal sludges (Heinss et al. 1998)
Septage1
Public toilet sludge1
Pit latrine
sludge2
Fresh
excreta
• BOD
g/cap·day
1
16
8
45
• TS
g/cap·day
14
100
90
110
• TKN
g/cap·day
0.8
8
5
10
• Volume
l/cap·day
1
2
(includes water for toilet
cleansing)
0.15 - 0-20
1.5
1
2
(faeces and
urine)
Estimates are based on a faecal sludge collection survey conducted in Accra, Ghana.
Figures have been estimated on an assumed decomposition process occurring in pit latrines.
According to the frequently observed practice, only the top portions of pit latrines (~ 0.7 ... 1 m)
are presumed to be removed by the suction tankers since the lower portions have often solidified to an extent which does not allow vacuum emptying. Hence, both per capita volumes and
characteristics will range higher than in the material which has undergone more extensive decomposition.
2.1.1 FS characteristics
In contrast to sludges from WWTP and to municipal wastewater, characteristics of faecal sludge differ widely by locality (from household to
household; from city district to city district; from city to city) (Montangero
and Strauss 2002).
A basic distinction can usually be made between fresh, biochemically
unstable and “thick” vs. “thin” and biochemically fairly stable sludges
(Heinss et al. 1998). Unstable sludges contain a relative large share of
recently deposited excreta. Stable sludges are those, which have been
retained in on-plot pits or vaults for months or years and which have
undergone a biochemical degradation to a variable degree (e.g. septage, which is sludge from septic tanks).
Based on numerous FS monitoring studies in West Africa, Rosario (Argentina), Bangkok and Manila, the authors found that FS can often be
associated with one of these two distinct categories. In contrast to fairly
S A N D E C / E A W A G --- I W M I
7
stable sludges, fresh undigested and biochemically unstable sludges
exhibit poor solids-liquid separability.
Table 5 shows typical FS characteristics and typical characteristics of
municipal wastewater as may be encountered in tropical countries.
Storage duration, ambient temperature, intrusion of groundwater into
vaults or pits of on-site sanitation installations; installations sizing, and
tank emptying technology and pattern are important factors influencing
the sludge quality.
Table 5
Faecal sludges from on-site sanitation systems in tropical
countries: characteristics, classification and comparison with
tropical sewage (after Strauss et al. 1997* and Mara 1978**)
Type “A”
(high-strength) *
Type “B”
(low-strength) *
Sewage **
( for comparison
purposes)
Example
Public toilet or bucket latrine sludge
Septage
Tropical sewage
Characterisation
Highly concentrated,
mostly fresh FS; stored
for days or weeks only
FS of low concentration;
usually stored for several
years; more stabilised
than Type “A”
Item
COD mg/l
20, - 50,000
< 15,000
5 : 1 .... 10 : 1
COD/BOD
500 - 2,500
2:1
NH4-N mg/l
2, - 5,000
TS mg/l
≥ 3.5 %
< 3 %
< 0,1 ? %
SS mg/l
≥ 30,000
≅ 7,000
200 - 700
20, - 60,000
≅ 4,000
300 - 2,000
Helm. eggs no./l
<
1,000
30 - 70
2.2 Faecal sludge treatment options
2.2.1 Treatment goals
Faecal sludge should be treated to render the treatment products (biosolids and effluent liquids) apt for discharge into the environment (including landfilling), or to produce biosolids, which may be safely used in
agriculture.
In the majority of developing countries, no standards or guidelines have
been set for the quality of biosolids. Standards have usually been copied from industrialised countries without taking the specific conditions
prevailing in the particular developing country into account. In most if
not all cases, the standards were enacted having wastewater treatment
and discharge in mind. Quite commonly, in such cases, standards or
the performance of infrastructure works are neither controlled nor enforced. Faecal sludges and products from their treatment were not or
still not taken into special consideration nowadays, thus, applying the
standards set for wastewater treatment plant effluents. In most cases,
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S A N D E C / E A W A G --- I W M I
these standards are too strict to be attained even for wastewater treatment schemes under the local conditions. For FSTP, the enacted effluent standards would call for the use of sophisticated and highly capitalintensive treatment, which is unrealistic. A suitable strategy would consist in selecting a phased approach, under the paradigm that “something” (e.g. 75 % instead of 95-99 % helminth egg or COD removal) is
better than “nothing” (the lack of any treatment at all or the often totally
inadequate operation of existing treatment systems) (Von Sperling,
2001).
The EU has adopted a rational strategy for public health protection in
biosolids use. The general principle is to define and set up a series of
barriers or critical control points, which reduce or prevent the transmission of infections1. Sludge treatment options, which were found to effectively inactivate excreted pathogens to desirable levels (e.g. cocomposting), are typical “barrier points”, where the transmission of
pathogens might be stopped (Matthews 2000).
Table 6
Suggested effluent and biosolids quality guidelines for the
treatment of faecal sludges (Heinss et al., 1998)
BOD [mg/l]
total
A:
NH4-N
Helminth eggs
FC
filtered
[mg/l]
[no./L]
[no./100 mL]
≤ 104
≤ 105
Liquid effluent
1. Discharge into receiving waters:
•
Seasonal stream or estuary
100-200
30-60
10-30
≤ 2-5
•
Perennial river or sea
200-300
60-90
20-50
≤ 10
2. Reuse:
•
Restricted irrigation
n.c.
1)
≤1
•
Unrestricted irrigation
n.c.
1)
≤1
B:
•
Treated plant sludge
Use in agriculture
n.c.
n.c.
≤ 3-8/ g TS 2)
≤ 105
≤ 103
3)
1) ≤ Crop’s nitrogen requirement (100 - 200 kg N/ha.year)
2) Based on the nematode egg load per unit surface area derived from the WHO guideline for
wastewater irrigation (WHO, 1989) and on a manuring rate of 2-3 tons of dry matter /ha·year
(Xanthoulis and Strauss, 1991)
3) Safe level if egg standard is met
n.c. – not critical
In Table 6, a set of effluent and plant sludge quality guidelines for selected constituents is listed. The suggested values are based on the
based on the principle of defining and setting up barriers against disease transmission, which can be used as critical control points for securing safe biosolids quality. Xanthoulis and Strauss (1991) proposed a
guideline value for biosolids (as produced in faecal sludge or in wastewater treatment schemes) of 3-8 viable nem. eggs/ g TS. This recommendation is based on the WHO guideline of ≤1 nematode egg/litre of
treated wastewater used for vegetable irrigation (WHO, 1989), and on
an average manuring rate of 2-3 tons TS/ha·year. It was used to estimate the allowable yearly deposition of eggs, based on an assumed
1
The principle follows the “HACCP” principle, which stands for Hazard Analysis and Critical control Points. It
was first developed in the U.S.A. for food safety in manned space systems
S A N D E C / E A W A G --- I W M I
9
yearly rate of irrigation (500-1,000 mm).Examples for faecal sludge
treatment standards are known from China and Ghana.
2.2.2 Treatment options overview
Figure 1 provides an overview of options for faecal sludge treatment,
which can be implemented by using modest to low-cost technology.
They may therefore be considered as particularly sustainable for use in
developing countries. They comprise:
Figure 1 Overview of potential, modest-cost options for faecal sludge
treatment
Some of these options were and are currently being investigated upon
by EAWAG/SANDEC and its partners in Argentina, Ghana, Thailand,
and The Philippines. Information can be retrieved from SANDEC’s
homepage2.
The fact that faecal sludges exhibit widely varying characteristics calls
for a careful selection of appropriate treatment options, notably for primary treatment. The separating of the solids and liquid fraction of FS, is
the process-of-choice to condition FS for co-composting.
2.3 Pre-Treating FS for Co-Composting
Solids-liquid separation and dewatering
2.3.1 Principles
If FS is still rather fresh it has to be biochemically stabilised first for solids and liquids to become separable. Anaerobic ponds, designed to also
cater for separated solids accumulation, may serve the combined purpose of stabilisation and solids-liquid separation. Solids-liquid separation of FS, which has undergone considerable biochemical stabilisation
(septage), may be achieved through sedimentation and thickening in
ponds * or in tanks**, or through filtration and drying in sludge drying
2
*
**
10
http://www.sandec.ch/sos/references.html
hydraulic ret. time = days to weeks
hydraulich ret. time = hours
S A N D E C / E A W A G --- I W M I
beds. Resulting from this are a solids and a liquid fraction (Figure 1).
The solids fraction, which may be designated as “biosolids" may require
additional dewatering/drying to achieve spadability and to meet hygiene
requirements for reuse in agriculture as a soil-conditioner and fertilizer.
Table 3 lists pathogen die-off periods in faecal matter in tropical and
temperate climates. It may be referred to for estimating additional storage periods required to render biosolids apt for use. Additional dewatering/drying might be required also for landfilling.
Additional treatment might be also necessary for the liquid fraction, to
satisfy criteria for discharge into surface waters and/or to avoid longterm impacts on groundwater quality. Reference is made to the literature available on options such as waste stabilization ponds (WSP), upflow anaerobic sludge blanket clarifiers (UASB), or constructed wetlands (CW). Liquids emanating from separation processes can not be
used for irrigation, as their salt contents exceed the salt tolerance limits
of cultured plants (≤ 3 mS/cm = 3 dS/m; FAO 1985).
2.3.2 Settling/thickening of FS
Total solids (TS) and suspended solids (SS) contents in faecal sludges
are by factors of 7 – 50 higher than in wastewater. The separation of
the solids and the reduction in volume of the fresh FS might be desirable e.g. when treating FS in ponds, be it separately or in conjunction
with wastewater; as an option to produce biosolids conducive to agricultural use, and when intending the joint composting of FS solids and
solid organic wastes.
Sedimentation/
thickening tank
Figure 2 Scheme of the Achimota Faecal Sludge Treatment Plant
Results from FS settling tests carried out at the Water Research Institute (WRI) in Accra have shown that Accra’s septage, which has an average TS contents of 12,000 mg/l (thereof, 60 % volatile solids, TVS),
exhibits good solids-liquid separability (Larmie, S.A., 1994; Heinss et
al., 1998). Separation under quiescent conditions is complete within 60
minutes. This holds also for FS mixtures containing up to 25 % by volume of fresh, undigested sludge from unsewered public toilets.
S A N D E C / E A W A G --- I W M I
11
Settling tests were also conducted at AIT in Bangkok using septage of
the City of Bangkok exhibiting an average SS concentration of 12,000
mg/l. Cylinder settling tests showed that separation is complete in 30-60
minutes and that SS concentrations in the supernatant of 400 mg/l are
achieved (Koottatep, 2001; Kost and Marty, 2000).
Field studies, conducted at the Achimota Faecal Sludge treatment plant
in Accra/Ghana from 1993-97 reveal that the performance of the sedimentation tanks strongly depends on the plant’s state of maintenance
and operation. For the existing twin settling/thickening tanks, the loading and resting periods should not exceed 4 to 5 weeks each. In practice however, the tanks are emptied at most every 4 to 5 months. Process disturbance by improper design and operation for solids separation
systems has been repeatedly observed (Hasler, 1995; Mara et al.,
1992).In septage settling ponds of the Alcorta (Argentina) pond
scheme, TS in the settled solids amounts to about 18% after 6 months
of septage loading (Ingallinella et al., 2000). Septage collected in
Alcorta exhibits an SS content of approx. 8,000 mg/l (which might be
associated with an estimated TS content of 12,000-15,000 mg/l). The
specific volume of accumulated solids was only 0.02 m3/m3 of fresh
septage, hence, 5-7 times less than that found in the settling/thickening
tanks of the Achimota FSTP in Accra.
2.3.3 Sludge drying beds
Sludge drying beds serve to effectively separate solids from liquids and
to yield a solids concentrate. Gravity percolation and evaporation are
the two processes responsible for sludge dewatering and drying. In
planted beds, evapotranspiration provides an additional effect. Unplanted and planted sludge drying beds are schematically illustrated in
Figure 3.
In contrast to settling and thickening of FS, dewatering and drying of
thin layers of sludge on sludge drying beds calls for comparatively long
retention periods. However, organic and solids loads in the percolate of
drying beds are significantly lower than in the effluent of sedimentation/thickening tanks. Hence, less extensive further treatment of percolate is required.
Sl udge l ay er
Sand l ay er
Gr av el l ay er
A ir
D r a in a g e
Coar s e
gr av el
l ay er
1 :2 0
Figure 3 Planted and unplanted sludge drying beds (schematic)
From 50 - 80 % of the faecal sludge volume applied to unplanted drying
beds will emerge as drained liquid (percolate). In planted drying beds,
this ratio is usually in the order of 60 %. Pescod (1971) conducted ex12
S A N D E C / E A W A G --- I W M I
periments with unplanted sludge drying beds in Bangkok. According to
these experiments, maximum allowable solids loading rates can be
achieved with a sludge application depth of ≤ 20 cm. To attain a 25 %
solids content, drying periods of 5 to 15 days were are required depend2
ing on the solids loading rates applied (70 - 475 kg TS/m ·yr).
Results from FS dewatering experiments on pilot sludge drying beds
(Figure 4) obtained by the Ghana Water Research Institute (WRI) in
Accra indicate their suitability for septage/public toilet sludge mixtures
and primary pond sludge (TS = 1.6-7 %). Experiments were conducted
during the dry season with sludge application depths of ≤ 20 cm.
Figure 4 Pilot sludge drying beds in Accra, Ghana
Results from pilot sludge drying beds obtained by
the Ghana Water Research Institute show a good
applicability of sludge drying beds for septage/public toilet sludge mixtures (with p. toilet
sludge shares not exceeding 30 %) and for primary
pond sludge
Sludge, dewatered to ≤ 40 % TS in the Accra/Ghana experiments, still exhibited considerable
helminth egg concentrations. This is not surprising
as the drying periods amounted to 12 days at the
most. Further rain-protected storage of the dewatered solids of several months is required to attain
a hygienically safe product for use in agriculture
2.4 Hygienic quality of biosolids
The residual concentration of helminth eggs in the biosolids is dependent on the prevalence and intensity of infection in the population from
which FS or wastewater is collected and on various factors influencing
parasite survival. Where biosolids use in agriculture is a practice or being aimed at, treatment or storage must be designed at reducing
helminth egg counts and viability to acceptable levels. Table 3 may
serve to estimate pathogen (including helminth egg) die-off in faecal
sludge during storage in tropical and temperate climates. Figure 5 allows to estimate the time required for Ascaris egg die-off in properly
operated, thermophilic composting. Table 7 shows values for helminth
egg counts and viability in untreated human wastes and in biosolids as
reported in published and unpublished literature for a few selected
wastewater and FS treatment schemes.
S A N D E C / E A W A G --- I W M I
13
Table 7
Place and scheme
Extrabes, Campina
Grande (Brazil); experimental WSP scheme
Chiclayo (Peru); WSP
schemes
Helminth eggs in biosolids from selected wastewater and
faecal sludge treatment schemes 1
No. of helminth eggs
per litre of untreated
Helminth eggs in biosolids
Faecal
sludge
Wastewater
No. of eggs /g TS
Egg viability
----
1,000
(nematodes)
1,400 – 40,000
(in the settled solids
accumulated across a
primary facult. pond;
avg.= 10,000, approx.)
----
10 – 40
(mostly
nematodes)
60 – 260
(in sludge from a primary facult. pond)
2–8%
(period of biosolids storage
not reported
but probably
several years)
1–5%
(biosolids stored for 4-5
years)
Asian Institute of Techn.
(Bangkok); pilot constructed wetland plant
(planted sludge drying
beds) for septage dewatering+stabilisation
600 – 6,000
(septage;
nematodes)
170
(avg. nematode levels
in dewatered biosolids
accumulated over 3.5
years in planted sludge
drying beds)
Kumasi Ghana;
faecal sludge from unsewered public toilets
and from septic tanks
900 – 6,900
(7 samples)
(Ascaris +
Trichuris)
20 – 85 (3 samples)
(in biosolids dewatered
on sludge drying beds
during 1 – 3 weeks;
TS = 20 %)
14
----
S A N D E C / E A W A G --- I W M I
0.2 – 3.1 %
45 – 82 %
(8 samples)
(biosolids dewatered for 1-3
weeks and
fresh sludge)
Reference
Stott et al.
(1994)
Klingel (2001)
Koottatep and
Surinkul (2000);
J. Schwartzbrod
(2000)
Gallizzi (2003)
3 Municipal Organic Solid Waste Management
3.1 Relevant municipal solid waste characteristics and quantities
It is the easily biodegradable fraction which is of immediate interest in
composting. This includes food waste, vegetables and fruits, and garden wastes (sometimes referred to as yard wastes) such as grass,
leaves and small woody? materials. Although organic waste materials
such as paper and timber? may also be composted, they are more resistant to microbial degradation due to their high lignin content needed?
(Richard 1996). If these materials are included in the composting process, their particle sizes are often reduced beforehand through shredding to allow for faster decomposition. Based on composition of solid
waste of cities of low- and middle income countries as quoted in Obeng
and Wright (1987) (from Algiers, Accra, Alexandria, Cairo, Sao Paolo)
easily biodegradable fractions range between 44 and 87 %. in weight.
Similar average ranges (40-85 %) are also reported by Cointreau et al.
(1985) for low-income countries. Data from the Kumasi Waste Management Department (2000) shows figures of 79 % biodegradable
waste for the city of Kumasi.
3.2 Approaches for municipal organic solid waste treatment
Given these high amounts of biodegradable waste organic waste recycling, treatment and reuse can have considerable advantages for the
city's solid waste management system. Zurbrugg and Drescher (2002)
describe the potential benefits of organic waste management and as:
-
reducing the environmental impact of disposal sites as the
biodegradable waste fraction is largely to blame for the polluting leachate and methane generation.
-
extending the existing landfill capacity as organic waste is
kept out of the landfill thus providing additional volume.
-
replenishing the soil humus layer with organic matter and
nutrients by applying compost and thus contributing to sustainable resource management.
A further significant benefit of waste minimisation can be achieved if a
decentralised approach is envisaged. In this case the organic fraction is
removed from the waste stream and recycled ,as near to the source of
generation as possible, thus reducing collection, transportation and disposal costs and reducing health and environmental risks resulting from
inappropriate handling and management.
Current treatment and reuse practices for municipal organic solid waste
– other than composting – include:
- the use of waste as source of food for urban animal livestock
(Allison et al. 1998)
- direct untreated application onto soils
- production of fuel pellets as energy source
S A N D E C / E A W A G --- I W M I
15
-
mining of old naturally decomposed waste dumps for application on farmland (Lardinois, Van De Klundert 1993).
Although not considered a treatment option, the frequent use of municipal organic solid waste as animal feed must be mentioned here. Preferred organic waste used for urban animal livestock raising consists of
fresh organic solid waste from sources such as vegetable markets, restaurants and hotels, as well as food processing industries. Health risks
associated to feeding of animals with solid waste are possible disease
transmission to animals and humans when feeding animals with meat
waste from slaughterhouses (Lardinois, Van De Klundert 1993). Further
risks to animals and humans are highlighted by Allison et al. (1998) with
regard to unintentional feeding to waste with toxic content.
16
S A N D E C / E A W A G --- I W M I
4 Composting and Co-composting
4.1 Process definition
Composting refers to the process by which biodegradable waste is biologically decomposed under controlled conditions by microorganisms
(mainly bacteria and fungi) under aerobic and thermophilic conditions.
The resulting compost is a stabilised organic product produced by the
above mentioned biological decomposition process in such a manner
that the product may be handled, stored and applied to land according
to a set of directions for use. Important to note is that the process of
"composting" differs from the process of "natural decomposition" by the
human activity of "control". "Control" has the goal to enhance the efficiency of the microbiological activity, to restrict undesired environmental
and health impacts (smell, rodent control, water and soil pollution) and
assure the targeted product quality.
Co-composting means composting of two or more raw materials together – in this case, FS and SW. Other organic materials, which can
be used or subjected to co-composting, comprise animal manure, sawdust, wood chips, bark, slaughterhouse waste, sludges or solid residues
from food and beverage industries.
4.2 Why co-compost feacal sludge with municipal solid waste?
Co-composting FS and MSW is advantageous because the two materials complement each other. The human waste is relatively high in N
content and moisture and the MSW is relatively high in organic carbon
(OC) content and has good bulking quality. Furthermore, both these
waste materials can be converted into a useful product. High temperatures attained in the composting process are effective in inactivating
excreted pathogens contained in the FS and will convert both wastes
into a hygienically safe soil conditioner-cum-fertilizer.
4.3 Composting systems
The technologies chosen for aerobic composting (or co-composting) will
depend on the location of the facility the capital available and the
amount and type of waste delivered to the site. Two main types of systems are generally distinguished which are: 1) open systems such as
windrows and static piles and 2) closed "in-vessel" systems. In-vessel
or "reactor" systems can be static or movable closed structures where
aeration and moisture is controlled by mechanical means and often requires an external energy supply. Such systems are usually investment
intensive and also more expensive to operate and maintain.
"Open” systems are the ones most frequently used in developing countries. They comprise:
ƒ
Windrow, heap or pile composting
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17
The material is piled up in heaps or elongated heaps (called windrows). The size of the heaps ensure sufficient heat generation and
aeration is ensured by addition of bulky materials, passive or active
ventilation or regular turning. Systems with active aeration by
blowers are usually referred to as forced aeration systems and
when heaps are seldom turned they are referred to as static piles.
Leachate control is provided by a sloped and sealed or impervious
composting pads (the surface where the heaps are located) with a
surrounding drainage system.
ƒ
Bin composting
Compared to windrow systems, bin systems are contained by a
constructed structure on three or all four sides of the pile. The advantage of this containment is a more efficient use of space. Raw
material is filled into these wood, brick or mesh compartments and
aeration systems used, are similar to those of the above described
windrow systems.
ƒ
Trench and pit composting
Trench and pit systems are characterised by heaps which are
partly or fully contained under the soil surface. Structuring the heap
with bulky material or turning is usually the choice for best aeration,
although turning can be cumbersome when the heap is in a deep
pit. Leachate control is difficult in trench or pit composting.
4.4 Key factors of the composting process
The key factors affecting the biological decomposition processes
and/or the resulting compost quality are listed below. They comprise:
•
•
•
•
•
•
Carbon to nitrogen ratio
Moisture content
Oxygen supply, aeration
Particle size
pH
Temperature
• Turning frequency
• Microorganisms and invertebrates
• Control of pathogens
• Degree of decomposition
• Nitrogen conservation
Detailed description of the significance of the specific factors is explained more in detail in Annex 2.
The same process parameters valid for composting must be adhered to
and play a role in co-composting of human waste with organic solid
waste. Special attention has to be paid, though, to the ratio at which
human waste are co-mixed with other compostable material given their
moisture as well as C and N content. Numerous mixing ratios of excreta
and co-composted material are provided by Shuval et al. (1981), which
are compiled in Annex 1 together with mixing ratios collated from other
publications. Dewatered or spadable sludges may be admixed at a
volumetric ratio of approx. 1 (sludge) : 3 (solid organic material),
whereas more liquid sludges (TS
5 %) may be mixed at ratios between 1:5 to 1:10.
18
S A N D E C / E A W A G --- I W M I
4.5 Quality of compost
Gotaas (1956) lists ranges of the main constituents in final composts as
reported in reviewed publications (Table 8). The quality varies widely
and depends on the initial mixture of material to be composted.
Table 8
Ranges of constituents in finished compost (Gotaas, 1956)
Constituent
•
•
•
•
•
Organic matter
Carbon
Nitrogen (as N)
Phosphorus (as P2O5)
Potassium (as k2O)
Range
(% of dry weight)
25 – 50
8 – 50
0.4 – 3.5
0.3 – 3.5
0.5 – 1.8
Compost which is dry (35% moisture or below) can be dusty and irritating to work with, while compost that is wet can become heavy and
clumpy. The Composting Council (2000) recommends 40 % moisture
for ideal product handling.
Usually, mature compost is sieved prior to sale and use. Sieves made
of a wooden frame and wire mesh are suitable and can be easily made.
Mesh sizes vary according to the compost users requirements. Used as
plant fertiliser, a mesh size of 10-20 mm could be chosen, for use as
seedling production mesh sizes may be around 3 mm. The compostable sieving residues of larger particle size are usually recycled to windrows for further composting.
4.6 Quality of compost produced from human waste
4.6.1 Nutrient Content
Nutrient contents of composts, which have been produced from cocomposting human waste (faecal or sewage treatment plant sludge)
and organic solid waste are shown in Table 9. In theory, such compost
should exhibit higher nutrients than compost, which is produced from
such material as organic municipal refuse, woodchips, sawdust, i.e. material with N contents lower than in human waste. However, the data
show that nutrient, notably N, contents do not range particularly high
when compared with the ranges listed in Table 9, which were collated
from many references and for composts produced from many different
raw materials, including human waste.
The reason for composts produced from human waste not exhibiting
higher nutrient contents than other compost (as judged from the limited
data available) might be due to nitrogen (ammonia) losses during precomposting storage and treatment (e.g. by dewatering on sludge drying
beds) of the human waste.
S A N D E C / E A W A G --- I W M I
19
Table 9
Nutrient levels in compost for which human waste was one of
the raw materials
Constituent
% of dry weight
Reference
1.3 – 1.6
1.3
0.35 – 0.63
0.45
Shuval et al. (1981)
Obeng and Wright (1987)1
Kim, S.S. (1981) 2
Byrde (2001)3
0.6 – 0.7
0.9
Shuval et al. (1981)
Obeng and Wright (1987) 1
Kim, S.S. (1981) 2
--1.0
Shuval et al. (1981)
Obeng and Wright (1987) 1
•
Nitrogen (as N)
•
Phosphorus (as P2O5)
•
Potassium (K2O)
•
Organic matter (% TVS)
12 - 30
Kim, S.S. (1981)2
•
Carbon (C)
46 – 50
13
Shuval et al. (1981)
Byrde (2001)3
1
Chosen as “typical values” by the authors in their chapter on the economic feasibility of cocomposting
Raw material composed of varying ratios of FS (TS = 4 %), household waste and straw
Raw material composed of municipal solid waste and FS
2
3
4.6.2 Control of pathogens
A good operation of aerobic composting should be able to kill all pathogenic microbes, weeds and seeds especially if the temperature can be
maintained between 60 and 70 degrees for 24-hour period. The table
below illustrates the thermal kill of pathogens and parasites.
Scott (1952) investigated Ascaris egg die-off during thermophilic composting in stacks, in which the composting material was turned every 510 days. The result is illustrated in Figure 5
The graph shows that complete egg die-off was achieved within seven
weeks. Greater than 95 % egg die-off was achieved within little more
than three weeks already, though. These periods reflect the time required for Ascaris eggs to “disappear” from all sections of a windrow,
hence it is dependent on the composting operations. This can be
achieved by windrow turning or, alternatively, by mechanically aerating
a static, non-turnable, pile.
The duration for thermal inactivation of excreted pathogens at the upper
temperatures attained in thermophilic composting, are much shorter,
though.
Table 10 lists die-off periods at temperatures constituting thermal death
points for a few selected pathogens.
20
S A N D E C / E A W A G --- I W M I
Figure 5 Ascaris egg inactivation in thermophilic stack co-composting
of faeces (69 % of raw material), vegetable matter (20 %),
soil (10 %), and ash (1 %) (Scott 1952)
A general rule of thumb for pathogen suppression is to maintain the
composting process at 55oC to 65 oC for 3 consecutive days (Tchobanoglous et al., 1993).
Table 10 Thermal Inactivation of Selected Excreted Pathogens (after
Tchobanoglous et al. 1993)
Microorganism
Duration for Thermal Inactivation
Escherichia coli
Death within 1 hour at 55 oC and within 15-20
minutes at 60 oC
Salmonella sp.
Growth ends at 46 oC; death within 30 minutes
at 55-60 oC and within 20 minutes at 60 oC
Entamoeba histolytica cysts
Death within a few minutes at 45 oC and within a
few seconds at 55 oC
Taenia saginata
Death within few minutes at 55 oC
Ascaris lumbricoides eggs
Death in less than 1 hour at temperatures over
50 oC
S A N D E C / E A W A G --- I W M I
21
4.7 Benefit of using compost in agriculture
The Composting Council (2000) summarises the benefits of compost as
follows:
-
improves soil structure, porosity and density thus creating a
better plant root environment
increases infiltration and permeability of heavy soils, thus reducing erosion and runoff
improves water holding capacity thus reducing water loss and
leaching in sandy soils
supplies a variety of macro and micronutrients
may control or suppress certain soil borne plant pathogens
supplies significant quantities of organic matter
improves cation exchange capacities of soils and growing media thus improving their ability to hold nutrients for plant use
supplies beneficial microorganisms to soil and growing media
improves and stabilises soil pH
can bind and degrade specific pollutants
Addition of compost to tropical soils, which are often low in organic matter will make the soil easier to cultivate and improve its water holding
capacity, preventing cracking and erosion by wind and water (Winblad
and Kilama, 1978 and 1980). Obeng and Wright (1987) have summarised published information on the impact of using compost on clayey or
sandy soils as shown in Table 11.
Certain microorganisms found in compost suppress detrimental organisms like root-eating nematodes and specific plant diseases. Strengthened root systems reduce the need for pesticide use (King County Department of Natural Resources and Parks 2002).
Table 11 Impact on clayey and sandy soils through the use of compost
(Obeng and Wright 1987)
22
Impact on sandy soils
Impact on clayey soils
Water content is increased
Aeration of soil is increased
Water retention is increased
Soil permeability is increased
Aggregation of soil particles is enhanced
Potential crusting of soil surface is
reduced
Erosion is reduced
Compaction is reduced
S A N D E C / E A W A G --- I W M I
5 Literature and case-studies on FS co-composting
5.1 Case-studies of co-composting
5.1.1 Septage co-composting – Massachusetts, U.S.A
A septage co-composting pilot plant was commissioned in the state of
Massachusetts in 1977 to test the feasibility of co-composting for septage collected from three neighbouring towns (Lombardi, 1977). The
initiative followed prohibition by the authorities to continue the admixing
of septage to the wastewater treatment plant. Septage of approximately
4 % TS was mixed with sawdust, woodchips and cow or horse manure.
Mixing ratios are reported, yet conflicting figures render it difficult to
know what actually used ratios were. Both forced and naturally vented,
static windrows were used. Reported temperature development, however, indicates that aeration was secured and thermophilic conditions
were achieved, with temperature rising to 73 C at windrow centres
within 8 days of pile formation. They levelled of to about 50 C after 50
days. Capital cost for a full-scale septage co-composting plant serving
the three towns and treating 60 m3 of septage p. day were estimated at
$ 240,000 (1977 base). The procuring of sawdust as liquid absorber
was found to constitute a major O+M cost item. The authors do not avail
of information whether the system is still operational, or if a full-scale
system was built and has become operational as a result of the pilot
works.
5.1.2 Latrine sludge co-composting –Port-au-Prince, Haiti
A pit latrine sludge co-composting pilot scheme was initiated at Saint
Martin, a suburb of Port-au-Prince with a population density of > 2,000
persons/ha in 1981 (Dalmat et al. 1982). Both the traditional and newly
constructed individual pit latrines of which several ones are attached to
each other to form a toilet block are shared by several families. The latrines have traditionally been manually emptied, but tractor-drawn vacuum tanks were introduced through a donor-aided programme. A
BARC-type composting system was installed, using forced-aerated
windrows. Pit latrine sludge and partially composted refuse were mixed
at a ratio of 5:1 to form piles of 21 m3. No figure is given for the TS content of the pit latrine sludge, but it may be assumed to have ranged from
4-8 %. Air was drawn through the windrows at 12 minutes “on” and 8
minutes “off” cycles. Exhaust gases were pushed through a pile of finished compost to minimise odours. Windrows were covered with a layer
of compost for insulation and odour control. No monitoring data were
reported on the co-composting operations. Preliminary results from
greenhouse planting trials indicated that the use of co-compost yielded
“significantly greater plant growth and yield response” as compared to
the use of refuse compost. Haitian soils reportedly have a very low organic fraction. Hence, it was anticipated that the use of co-compost
would have considerable impact and be a good marketing argument.
For this project, too, the authors do not have any information at hand
whether the pilot project was scaled up and/or applied elsewhere in
Haiti or the country, and whether such operations continue until today.
S A N D E C / E A W A G --- I W M I
23
5.1.3 Bucket latrine sludge co-composting – Rini/Grahamstown, South Africa
An example of recent co-composting operations using bucket latrine
sludge and MSW is the demonstration scheme at Rini near Grahamstown, South Africa (La Trobe and Ross 1992). The plant was commissioned in late 1992, following a two-year trial phase on pilot scale. The
scheme became redundant, though, following the conversion of the
bucket latrines into sewered toilets in 1997. In spite of this, the authors
consider worthwhile to provide here a description of the plant and its
operation.
Figure 6 Sprinkling FS over refuse at the Rini/
Grahamstown (South-Africa) cocomposting plant
Figure 7 Sieving of matured compost in a rotary
sieve at the Rini/Grahamstown (South
Africa) co-composting works
The plant in which refuse and bucket latrine sludge collected from Rini
(pop. =100,000) were co-composted, consisted of forced-aerated, static
windrows. The faecal sludge was delivered to the station by a tractordrawn vehicle in 20-L barrels. Approximately 20 m3 were delivered
daily. It was then screened and collected in a pump sump from where it
was pumped by a macerating pump to two overhead, cone-shaped settling/thickening tanks. The tank supernatant was treated in waste stabilisation ponds, which were earlier receiving the bucket latrine sludge.
The thickened FS (TS = 5 %) was gravitated over the windrow as the
mixed refuse was being heaped up (Figure 8). Final windrow size
amounted to around 100 m3. The windrow was covered with finished
compost for insulation and bird control. The volumetric mixing ratio was
approximately 1:10 (FS:refuse). Measuring temperature at different
spots of the windrow controlled the process. Temperatures of 55 °C
were reached and the windrows left to react for 3 weeks. The compost
was let to mature for another 3 weeks. The matured compost was
sieved (Figure 9) and the rejects landfilled. The Grahamstown garden
department used the compost. The finished compost was reportedly
free of helminth eggs. Unfortunately, no scientific data were generated
or published about this valuable co-composting experience.
24
S A N D E C / E A W A G --- I W M I
Figure 8 Co-composting of FS and sorted MSW in Niono, Mali (Montangero and Strauss 1999)
Co-composting of latrine sludge with organic refuse in Niono, Mali
A small fraction of the pit latrine sludges generated in the town of Niono,
Mali (pop. =28,000) is co-composted with sorted refuse by a microentrepreneur. Faecal sludges are collected manually or by tractor-drawn
vacuum tanks. The compost is sold to rice and vegetable farmers (Montangero and Strauss 1999). Figure 10 illustrates the processing of the
FS with refuse and lime. Sieved refuse, liquid FS and lime are made up
in batches of approx. 2.8 m3, let to sun-dry and then processed in the
heated pelletizer (ret. period approx. 1 min.). The ratio of sieved refuse
to liquid FS amounts to 1:1.3. Hence, lime (CaCO3) is added to dewater
the liquid sludge.
The process allows inactivating excreted pathogens considerably, yet
drying periods are too short and heating temperatures too low to
achieve a reasonably safe “compost” all the time.
S A N D E C / E A W A G --- I W M I
25
Co-composting of biosolids from an FS pond treatment scheme –
Cotonou, Benin
A pilot co-composting scheme is currently (October 2002) being implemented in Cotonou, Benin, as part of an action research programme of
CREPA aiming at improvements in FS management (CREPA Benin,
2002). Biosolids generated in an FS pond treatment system will be cocomposted with municipal refuse. Comparative planting trials will be
conducted with co-compost and other plant/soil amendments.
The authors are aware of but a very few, more recently initiated
schemes – pilot or full-scale – in which faecal sludge was or is being
co-composted with municipal refuse or other organic bulking material.
There are, doubtlessly, numerous co-composting activities and
schemes in operation in developing countries, both formalised and informally operated ones, yet respective information has not been publicised. The following are schemes or practices, which are known to the
authors either through retrievable literature, through personal communications or from own field visits:
ƒ
Septage co-composting
A pilot project in Massachusetts, U.S.A., initiated in 1977
(Lombardo, 1977)
ƒ
Latrine sludge co-composting
A pilot project in Port-au-Prince, Haiti, initiated in 1981 (Dalmat et
al., 1982)
ƒ
Bucket latrine sludge co-composting
A full-scale demonstration project in Rini/Grahamstown, South Africa, initiated in 1990 (La Trobe and Ross, 1991 and 1992; personal observations).
ƒ
Co-composting of latrine sludge with organic refuse
Small-scale co-composting to produce compost for rice and vegetable farming in Niono, Mali (Montangero and Strauss 1999; personal observations)
ƒ
Co-composting of biosolids from an FS pond treatment
scheme
A pilot-scale scheme comprising planting trials with finished compost to be initiated in Cotonou, Benin, in 2002 (CREPA-Benin,
2002).
26
S A N D E C / E A W A G --- I W M I
5.2 Literature studies
Scott (1952) reports extensively about the combined composting of faecal matter with a variety of other organic materials as practiced in China
over centuries. Experiments with material available on farms, i.e. human excreta, animal manure and crop residues focused on nutrient (notably nitrogen) conservancy and pathogen (notably helminth egg) inactivation. Scott and his co-workers found the following:
•
•
•
•
Ascaris egg destruction was 95 % complete after 22 days and
100 % complete after 36 days in a stack whose contents were
turned every 5-14 days and reached 60 C after each turning.
Nitrogen losses from raw materials and from compost exhibiting
differing degrees of degradation during drying is significant. The
losses found were approx. equal to the ammonia contents of
the fresh material. The loss of nitrogen during co-composting
amounted to about 50 % of the initial nitrogen present. The
greatest loss occurred during the initial 5-10 days of composting.
Omission of ash was assumed to have contributed to a lowering of N losses.
Cooling the stacks with soil after the first few days of hot composting helped to considerably reduce nitrogen losses.
Shuval et al. (1981)3 reviewed literature and collated information on historical and actual practices of co-composting “nightsoil”3 and (sewage)
sludge. Cases of excreta co-composting are reported about from India,
China, Malaya, Africa (e.g. Kano, Nigeria) where fresh faecal sludge
collected from bucket latrines and frequently emptied latrine vaults were
co-composted. The bulking material comprised various forms of household refuse and plant residues. Most of these composting initiatives and
operations are reported as having been rather successful and producing compost at a regular rate. While many of the reported schemes may
not be operational anymore nowadays, since they were initiated and
operated under colonial administration, considerable informal cocomposting is doubtlessly being practiced in many countries around the
world.
Shuval et al. (1981) and Obeng and Wright (1987)4 reported on numerous schemes in the U.S.A. and Europe, mainly, and on windrow or
open systems, in which sewage treatment plant sludge (“biosolids”) are
or were composted together with other organic material, notably municipal refuse. All these installations make use of lower or higher degrees of mechanization. While the biochemical and pathogen inactivation processes are the same as in non-mechanised systems, mechanised co-composting schemes are largely inappropriate for developing
countries except possibly in situations where there is a high demand for
the product and it can be sold at high prices.
Shuval et al. (1981) provides detailed accounts of static pile or windrow
3
4
This comprises, in most reported cases, the fresh faecal material, with or without urine, collected daily from
households bucket latrines or at larger intervals from latrine pits or vaults
Both Shuval et al. (1981) and Obeng And Wright (1987) use the term “composting” as encompassing either
anaerobic, ambient-temperature degradation or “hot”, aerobic and thermophilic dagrdation of organic matter.
The authors of this report, however, prefer the term “composting” to exclusively designate the hot process
S A N D E C / E A W A G --- I W M I
27
co-composting works operated with forced aeration according to the
Beltsville Aerated Rapid Compost (“BARC”) system developed by the
U.S. Department of Agriculture research station at Beltsville, Maryland,
in the 1970-ies. Several hundreds of this type of co-composting systems are in operation in the U.S.A. nowadays (Goldstein and Riggle,
1989). The original BARC system co-composts dewatered sewage
sludge (TS = 20-25 %) and wood chips in ratios of around 1 (sludge) : 2
(wood chips). Windrows are covered with finished compost for insulation, moisture conservation and to prevent birds from feeding on fresh
waste. Shuval et al. (1981) also report on a BARC-type scheme cocomposting faecal sludge collected from latrine vaults in a national park
with wood chips, sawdust and finished compost. The sludge (TS = 5 %)
is mixed at a ratio of 1 (sludge) : 3.2 (other org. material). Finished
compost contained 1.3-1.6 % nitrogen on a dry solids basis. Compost
storage for one year did reportedly not lead to nitrogen losses.
Shuval et al. (1981) and Obeng and Wright (1987) also reported on
economic, agronomic and marketing aspects of co-composting and its
respective product. In Europe and North America, mainly digested and
dewatered sewage sludge is being processed in co-composting works.
Cited investigations focused on the hygienisation effect of the process,
mainly, and on the fate and concentrations of heavy metals in the finished product. Shuval et al. (1981), citing Julius (1977), remarks on the
importance of proper and sustained compost marketing strategies,
which are to comprise the demonstration of agricultural benefits of
compost on trial plots, training, extension and awareness raising.
28
S A N D E C / E A W A G --- I W M I
6 Conclusions and open questions
6.1 Conclusions
Based on the above "state-of-art" review the following conclusions on
co-composting of FS and organic solid wastes can be made:
•
•
•
•
•
Faecal sludges can be co-composted with any biodegradable,
organic material if the rules of the art in process control for
composting are adhered to.
Mixing ratios reported about in the literature vary widely, depending on the type of organic bulking material co-composted
together with faecal matter, the consistency of the FS itself, the
degree of dewatering prior to composting, and the cocomposting practice and care.
Reported mixing ratios of dewatered FS (TS = 20-30 %) and
other, more bulky organic material tend to range from 1:2 to
1:4. For fresh, non-dewatered FS, ratios used and reported
about tend to range from 1: 5 – 1:10.
Factors contributing to minimising nitrogen losses during thermophilic composting comprise:
Keeping the maximum temperatures below 65 °C
Keeping the periods of maximum temperatures as short as
possible
Limiting the frequency of turning
Keeping the water content of the composting material as
high as possible (50-70 %)
Only scanty information exists on existing experiences, especially on organisational, institutional, and financial aspects of
co-composting practices and schemes operated in developing
countries
6.2 Open questions / researchable issues
Using the co-composting process as a treatment option for a city's faecal sludge and organic solid waste, raises the issues not only of the
technological approach used, but also of the necessary organisational
set-up for operation and management of the composting site as well as
the delivery of feedstock (raw material) and distribution of the compost
productHoornweg and Thomas (1999) list explanations why composting is not widely or successfully practiced in cities of developing countries:
-
insufficient knowledge and care in carrying out composting operations leading to inadequate compost quality and resulting in nuisances potential, such as odours and rodent attraction.
lack of markets for the product and lack of compost marketing efforts and skills.
neglect of the economics of composting which relies on externalities, such as reduced soil erosion, reduced water pollution and
avoided disposal costs.
limited support by municipal authorities who tend to prioritise cenS A N D E C / E A W A G --- I W M I
29
tralised waste collection services rather than promote and support
recycling activities and decentralised composting schemes.
The following issues related to FS/MSW composting warrant applied
research:
Pre-treatment of FS by sludge drying beds:
•
•
•
•
FS handling and FS pre-treatment requirements
Sludge drying bed performance in dry and wet weather conditions
Maximum share of public toilet sludge (vs. septage) to allow for
adequate rates of dewatering
Appropriate options for treating the percolate of sludge drying
beds
Solid waste
•
Appropriate methods of segregation at source or sorting procedures, to allow delivery or utilisation of pure organic solid waste
for the co-composting process and to limit risks of compost contamination by impurities and chemical constituents
Co-composting
•
•
•
•
30
Maximum ratio of dewatered or thickened FS in the FS/MSW
mixture, which allows for proper thermophilic composting
Process specifications required to ensure production of a hygienically safe compost
Advantages and disadvantages of static pile vs. turnable windrow composting
Occurrence of heavy metals in FS-derived biosolids vs. in cocompost
S A N D E C / E A W A G --- I W M I
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36
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Annex 1 Mixtures of faecal sludge and other organic material in combined composting as reported in the literature
Faecal Sludge
Other material
•
NS:
1 (vol.)
Village refuse
•
SI:
1 (wt.) (w=78 %)
2 wood chips
(wt.)
0.7 wood chips
(wt.)
1.4 screened comp. (wt.)
•
NS:
1 (vol.)
•
NS:
•
•
Remarks
References
“Indore”
Shuval et al. 1981
“BARC”
(sludge = 78 % w)
(chips = 35 % w)
Shuval et al. 1981
2 rice straw
(vol?)
0.5 powdered bone (“)
China (1940)
Shuval et al. 1981
1 (wt.)
0.4-0.5 veg.matter + ash + soil (wt)
Scott, 1952
NS:
1 (wt.)
4
China (pre-war)
(expt. I)
China
(expt. II)
(NS = 90 – 95 % w?)
NS:
1 (vol.)
(w = 95 %)
1.6 wood chips
1.5 sawdust
(wt.)
(wt.)
1
(wt.)
6 (vol.)
(mix)
(base)
(cover)
veg.matter + ash + soil (wt)
compost
Scott, 1952
“BARC”
(NS: 95 % w)
Shuval et al. 1981
•
Sludge (dewatered):
1 (wt.)
1.13 finished compost (wt.)
“BARC”
Shuval et al. 1981
•
Sept. tank sludge
(septage?):
1 (8 m3)
(w = 90 %)
3 (24 m3) refuse (w = 61 %)
Windrow experiment, AIT
Pescod, Jan. 1970
NS – Nightsoil
LS – Latrine Sludge
S – Septage
SI – Sewage Sludge
w = water content
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37
Faecal Sludge
Other material
Remarks
•
NS:
1 (wt.)
0.6 city refuse
(wt.)
(wet)
Accra
•
NS:
1 (vol.)
2
(vol.)
(dry)
(refuse:
w = 30 %)
(n/s:
w = 74 %)
city refuse
References
GOPA, 1983
w
mixture = 57 %
•
NS:
1 (wt.)
+0.6 city refuse
0.5 compost
(wt.)
(wt.)
(wet) →
Accra
GOPA, 1983
•
S + N:
1 (wt.)
5
city refuse
(wt.) (wet)
(..1.7 compost)
Accra
1991 (Eiling, Neff; GOPA)
•
NS:
1 (vol.)
(w = 95 – 96 %)
10
city refuse
(vol.)
Rini township,
Cape Province
South Africa
•
Faecal sludge
1 (vol.)
(~ 70 % septage + 30 % BV
latrine sludge)
3.5 city refuse
(vol.)
Hanoi, Vietnam
NS – Nightsoil
LS – Latrine Sludge
38
S – Septage
SI – Sewage Sludge
w = water content
S A N D E C / E A W A G --- I W M I
Personal comm. (1994)
Annex 2 The science of composting
C:N Ratio and other nutrients
The primary nutrients required for microorganism growth are carbon,
nitrogen, phosphorus and potassium. Although bacteria also need trace
amounts of sulphur, sodium, calcium, magnesium, and iron, these elements are usually present in adequate quantities and do not limit bacterial activity (Hoornweg and Thomas 2000). Carbon and nitrogen are
both the most important and the most commonly limiting elements for
microbial growth (occasionally phosphorous can also be limiting). The
ideal ratio of C to N is between 20-30 :1. When there is too little nitrogen, the microbial population will not grow to its optimum size, and
composting will slow down as nitrogen becomes a limiting factor to the
growth of microorganisms. Microorganisms are forced to go through
additional cycles of carbon consumption, cell synthesis, decay, etc, in
order to burn off the excess carbon as CO2 (Kiely, G., 1998; GTZ 2000).
In contrast, too much nitrogen allows rapid microbial growth and accelerates decomposition, but this can create serious odour problems as
oxygen is quickly depleted and anaerobic conditions occur. In addition,
some of this excess nitrogen will also be given off as ammonia gas that
generates odours while allowing valuable nitrogen to escape (Richard
et al. 1996). The bioavailability of carbon also needs to be taken into
account when considering the C/N ratio. This is commonly an issue with
carbon materials, which are often derived from wood and other lignified
plant materials, as increased lignin content reduces biodegradability.
Thus a C/N ratio of 30 where carbon has high lignin content would be
too low for ideal composting as the carbon is not easily available for microbial activity.
Mixing various feedstocks of different C/N ratios allows a control of the
total C/N ratio. Some raw materials are high in carbon others high in
nitrogen. In practice, the ideal combination of different feedstock types
can be determined by experimentation and experience. Generally one
can classify "green" high nitrogen materials and "brown" high carbon
materials which in a simple recipe mixture can be mixed together in
equal volumes. Examples for "green" materials are fresh grass clippings, manure, garden plants, or kitchen scraps; "brown" materials are
dried leaves and plants, branches, and woody materials.
Moisture
Maintaining adequate moisture content in the composting pile is important, as hu-midity is required by microorganisms for optimal degradation. Moisture also dissipates heat and serves as a medium to transport
critical nutrients. Moisture content between 40 to 60 percent by weight
throughout the pile is ideal. Higher moisture levels slow the decomposition process and promote anaerobic degradation because air spaces in
the pile are filled with water and can not be supplied with oxygen. Moisture levels less than 40 percent cause the microorganisms to slow their
activities and become dormant or die. Moisture can be easily added
during turning by sprinkling water or a mixture of urine and water in a
S A N D E C / E A W A G --- I W M I
39
mixing ratio of 1:4 as urine enhances the growth of the microorganisms.
For best control of moisture, composting in piles covered by a roofed
structure is ideal. If in an open area, at times with excessive rains, the
waste pile can be made as steep as possible and be covered with a
tarpaulin, plastic sheeting or gunny-bags to reduce water infiltration. In
times of excessive heat and drought, the same coverings can serve to
reduce evaporation. The optimal moisture level is achieved when the
composting material feels damp to the touch; that is, when a few drops
of liquid are released while squeezing a handful of material strongly.
You can also test for moisture level content by putting a bundle of straw
in the heap. If after five minutes, it feels clammy, then the moisture level
is good; if still dry after five minutes, the moisture level is too low. Water
droplets on the straw indicate that the heap is too wet for successful
composting.
Moisture content and coarseness of material are closely interrelated in
terms of displacement of air in the pores by water, promotion of aggregation and lowering of the structural strength of the material.
Particle size
The surface area of the organic material exposed to microorganisms is
another factor in determining the rate of composting. Waste material
shredded, chipped, or otherwise reduced in size can be degraded more
rapidly. This is significant especially with slow degradable woody materials. However, care must be taken to avoid compacting the materials
by too small material sizes, as this reduces the porosity of the pile and
possible air circulation. The optimum particle size ranges between 25
and 75mm (1 and 3-inches). GTZ (2000) recommends chopping all materials to be composted to the length of about 5-10cm. Obeng and
Wright (1987) reported that typical particle sizes should be approximately 1cm for forced aeration composting and 5cm for passive aeration and windrow composting.
The physical state and the size of particles affect the moisture content
and the composting process. The coarser the material the higher the
moisture content should be. A consistent particle size ensures a homogenous composting process and facilitates the further treatment of
the compost.
Aeration
The air contained in the interstitial spaces of the composting mass at
the beginning of the microbial oxidative activity varies in composition.
The carbon dioxide content gradually increases and the oxygen level
decreases. When the oxygen level falls below 10%, anaerobic microorganisms begin to exceed the aerobic ones. Fermentation and anaerobic
processes take over. This implies that the aerobic microorganisms must
have constant supply of fresh air to maintain their metabolic activities
unaltered. The oxygen needed for composting is not only needed for
aerobic metabolism and respiration by the microorganisms but also for
oxidising various organic molecules present in the mass. Oxygen consumption during composting is directly proportional to microbial activity;
therefore there is a direct relationship between oxygen consumption,
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S A N D E C / E A W A G --- I W M I
temperature and aeration.
The greater the aeration rates the more rapid the rate of degradation.
Aeration provides the necessary aerobic conditions for rapid odourless
free decomposition and for destruction of pathogenic organisms by
heat. The most common way for aerating the compost heap cheaply in
the developing country is by turning (Winblad and Kilama, 1980). Active
aeration refers to methods which actively blow air through the compost
pile. Passive aeration takes advantage of the natural diffusion of air
through the pile enhanced by ventilation structures such as perforated
pipes in the pile, openings in the walls of composting bins and of course
the particle size and structure of the raw materials in the heap. If air
supply in the pile is limited, anaerobic conditions occur; thus producing
methane gas and malodorous compounds such as hydrogen sulfide
gas and ammonia.
The consumption of oxygen is greatest during the early stages and
gradually decreases as the com posting process continues to maturity.
Temperature
In windrows which have been prepared according to the “rules of the
art”, i.e. with adequate porosity, humidity, and C:N ratio, and exhibiting
a minimal size to provide sufficient “body” for insulation (1x1x1 meters),
thermophilic temperatures develop independently of ambient temperatures. Heat is generated in aerobic decomposition as a result of the microbial activity in the pile as the aerobic degradation of organic material
is an exothermic process. As the temperature of the pile increases, different groups of organisms become active. With adequate levels of
oxygen, moisture, carbon, and nitrogen, compost piles can heat up to
temperatures in excess of 65 degrees Celsius. Higher temperatures
begin to limit microbial activity. Temperatures above 70 °C are lethal to
most soil microorganisms. If windrows don’t turn hot, this is a sign of
process failure and that windrows were not set up according to the rules
of the art.
The thermophilic composting process goes through several temperature
variations The class of bacteria involved in the degradation process are
psychrophilic (5-20 °C), mesophilic (20-50 °C) and thermophilic (50-70
°C) (Kiely, 1998; Winblad and Kilama, 1980). This diversity is necessary
for the stepwise decomposition of the organic substances to stable
compost (humic substances and nutrients). Although composting will
occur also at lower temperatures, maintaining high temperatures is
necessary for rapid composting as it controls the thermo-sensitive human pathogens as well as destroys weed seeds, insect larvae, and potential plant pathogens that may be present in the waste material.
After piling the organic material, the temperature rises to 60 – 70 °C
within 1-3 days. After several days of active degradation, the process
slows down and the temperature remains around 50 – 55 °C. After approximately 30 days the compost process will slow down further and the
temperature will drop below 50 °C.. The composting process now enters into the maturing phase with low microbiological activity at temperatures around 40 °C. As the compost becomes mature the temperature
approaches the ambient temperature conditions.
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Turning frequency
Usually the greater the turning frequency, the better the chances for
uniform and better degradation. For quality control, it is important that
all the waste has been through the thermophilic phase. This can be best
controlled by regular turning. However, frequent turning may also lead
to increased ammonia losses, particularly so during the first few days of
thermophilic activities, when temperatures and pH is highest.
pH
Organic matter with wide range of pH (between 3 and 11), can be composted. However, good pH values for composting are between 5.5 and
8; and between 4 and 7 for the end product (Winblad and Kilama,
1980). Whereas bacteria prefer a nearly neutral pH, fungi develop better in a fairly acid environment. In the first moments of the composting
process, the pH may drop to around 5 as organic acids are formed,
however then microbial ammonification will causes the pH to rise into
the range of 8-8.5. Only during maturation, when the ammonium compounds are nitrified to nitrate will the pH sink once more below 8. Thus,
a high pH is generally the sign of immature compost.
Microorganisms and invertebrates
A properly constructed compost pile represents a interactive biological
and ecological system. It involves a diversity of species that emerge in
response to changes in the nutritional and environmental conditions of
the pile. Chemical decomposition of organic compounds results predominantly from microorganisms. such as bacteria, actinomycetes,
fungi, and some protozoans. At the first stage of composting when temperature rises through the mesophilic stage into the thermophilic range,
bacterial population which can multiply rapidly while utilising simple and
readily available substrates dominate. As temperature rises thermophilic bacterial populations take over. If excess heat is removed by ventilation or turning these populations will be maintained and overall rates
of bacterial activity will remain high. Fungi nor actinomycetes can withstand temperatures as high as the thermophilic bacteria. When thermophilic bacteria have used up the most easily available substrates, bacterial microbial activity can no longer liberate heat fast enough to maintain
high temperatures. As temperatures drop, actinomycete population increase and more complex substrates can be attacked by extracellular
enzymes (Palmisano and Barlaz 1996). As temperatures drop further
the remaining substrates which are even more resistant to decomposition, are degraded by fungal populations. The role of activities and appetites of various invertebrates such as mites, millipedes, beetles, earwigs, earthworms, slugs, and snails for physical and chemical decomposition is not be underestimated.
Gotaas (1956) discusses the issue of inoculation to enhance microbial
degradation. Modern developments in science and practice of composting have, apparently, been accompanied ever since by the promotion of
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and continuous debate about the need and usefulness of inocula comprising specific, laboratory-cultured strains of bacteria, enzymes, “catalysts”, “hormones”, etc.. A product designated “EM” (“effective microorganisms”), has been aggressively marketed in Asia in recent years. It is
used by households and applied to pits and vaults of on-site sanitation
installations, as well as solid waste composting heaps and dumps, reportedly to help enhancing biochemical degradation, preventing odours
and formation of large aggregates which may block appurtenances. EM
is sold and used also to reportedly enhance or speed up composting
processes and to prevent odour formation. The authors of this report
are not aware of any independent, rigorous study, which have been
done to investigate the effects and usefulness of EM in the composting
process.
Early composting studies dealing with this issue and reviewed by Gotaas, appear to strongly indicate that inocula are not necessary. Gotaas
argues – and in fact most composting specialists share this view – that
indigenous bacterial and other microbial populations are not a limiting
factor in composting. They can produce rapidly the enzymes, vitamins
and other growth factors required in sufficient quantities and at adequate rates.
Nitrogen conservation
Gotaas (1956), provides a comprehensive and in-depth description of
the composting process and composting operations. In particular, he
also discusses problems in relation to nitrogen losses and means of
conserving it. Like other nutrients (phosphorus, potassium, micronutrients), nitrogen may also be lost through leaching, yet, in contrast to
those nutrients, by far the greatest portion is lost through volatilization in
the form of ammonia (NH3) and other nitrogenous gases. These losses
have impact on the fertilising value of the compost product, thus influencing crop yield, farm economics and, hence, farmers’ livelihood. Ammonia losses are affected by the C/N ratio, pH, moisture, aeration, temperature, the chemical form of nitrogen in the feedstock, adsorptive capacity of the composting mixture, and windrow turning frequency.
Ammonia (NH3) and Ammonium (NH4+) are in a pH and temperature
dependant equilibrium..
Figure 9 Ammonia – ammonium equilibrium as a function of different
temperatures and pH (Schreiner 1997).
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43
Figure shows that higher pH and higher temperature move the equilibrium in favour of ammonia. Thus, higher levels of pH developing during
the composting process or high pH in the initial feedstock might enhance ammonia volatilisation for instance if the raw material may contain appreciable portions of ash (ash exhibits a pH of 10-11).
Excessive dryness will enhance NH3 volatilization whereas sufficient
moisture contents, like those for optimum composting, from 50-70 %,
allow to keep the highly soluble ammonia in dissolved state (Gotaas,
1956).
Excessive aeration and windrow turning enhances loss of ammonia,
which escapes more easily when the composting material is exposed to
the atmosphere. Hence, an optimum frequency of turning must be
found, which balances the need for all parts of a windrow to be subjected to high temperatures for pathogen inactivation with the need to
limit nitrogen loss.
A similar balance has to be strived for in temperature development.
High temperatures of around 60 - max. 65° C are desirable to attain
good pathogen inactivation, yet long periods of around 70° C must be
avoided as ammonia formation and hence, nitrogen losses increase
considerably at this temperature.
Degree of decomposition or compost maturity
Indicators for the degree of decomposition are: the colour and smell, the
drop in pile temperature, the degree of self-heating capacity, the nitrateN / ammonium-N ratio, the amount of decomposable and resistant organic matter in the decomposed material, redox potential, and oxygen
uptake.
Immature composts, still exhibit, the microbial activities when applied on
soil continues and there is a danger of microorganisms competing with
the plants for the availability of soil nitrogen (nitrogen block). Immature
compost also may contain high levels of organic acids and can damage
plant growth when used for agricultural applications.
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S A N D E C / E A W A G --- I W M I
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