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D.R. Gajurel, Z. Li and R. Otterpohl
Technical University Hamburg-Harburg, Institute of Municipal and Industrial Wastewater Management,
Eissendorfer Strasse 42, D-21073 Hamburg, Germany (E-mail: [email protected], www.tuhh.de/aww)
Abstract High levels of nutrients recovery can be achieved with source control sanitation – technologies are
already available. Separation toilets for example separate urine that can be used in agriculture with some
crop restrictions as a fertiliser after about 6 months of storage. The grey water has very low loads of nitrogen
and can be treated in different combinations of biological and physical treatment and reused. Faecal matter
with flush water from the separation toilet can be discharged into Rottebehaelter (an underground precomposting tank) that retains solid material and drains liquid to a certain extent. Investigation of
Rottebehaelter in the different sites and laboratory experiments showed that retained faecal material still
contained a high percentage of water. However, odour was not noticed in those Rottebehaelters that have
been examined. One of the major advantages of this system over other forms of pre-treatment as the septic
tanks is that it does not deprive agriculture of the valuable nutrients and soil conditioner from human excreta.
It has to be stated that maintenance is a crucial factor. As an intermediate result of the intensive research of
Rottebehaelter it seems that these systems are rather a way of solids retaining, de-watering and long-term
storage before the contents are further treated.
Keywords Brown water; grey water; nutrients; Rottebehaelter; source control sanitation; yellow water
Introduction
The linear flow of nutrients from farming land to water bodies due to flush sanitation results
in high demands of mineral fertiliser. Production of fertiliser is energy intensive and causes
environmental problems. Generating nitrogen from air requires a considerable amount of
energy; and mining and refining the raw materials for phosphate production generates huge
amounts of hazardous wastes. Reserves of phosphate (P), potassium (K) and also sulphur
(S) are definitely limited on a time scale of a couple of human generations especially with
regard to economic constraints. Therefore, nutrients present in wastewater should rather be
reused instead of discharging them into the water body in order to minimise the production
of mineral fertiliser.
It is well recognised that transfer of nutrients from terrestrial to aquatic ecosystem causes on one hand, eutrophication in water bodies and on the other hand nutrient deficiency in
agricultural land. These problems are increasing greatly particularly in the developing
world, where there is hardly any provision to interrupt the nutrients discharge into the water
body and, in return, loss of nutrients from agricultural land is compensated only with hardly affordable commercial fertiliser in order to feed a rapidly growing population. In the
developed world, nutrient transfer to water bodies through wastewater has been relatively
controlled by costly high technology for the larger treatment plants. However, even with
high investment, which is not affordable for most of the developing countries, more than
20% of nitrogen, more than 5% of phosphorus and more than 90% of potassium are still
emitted from wastewater treatment plants into the aquatic environment (Otterpohl et al.,
1997). Those nutrients, which are captured in sludge are often contaminated with heavy
metals such as cadmium (Cd) and organic compounds such as PCB (polychlorinated
Water Science and Technology Vol 48 No 1 pp 111–118 © IWA Publishing 2003
Investigation of the effectiveness of source control
sanitation concepts including pre-treatment with
Rottebehaelter
111
D.R. Gajurel et al.
biphenyl), which pose potential toxic risks to plants, animals and humans (Metcalf and
Eddy, 1991). Therefore, large amounts of sewage sludge are disposed of in landfills or
incinerated. A very small part is applied to the agricultural land.
In order to recover and reuse nutrients ecologically and economically, household wastewater should be treated separately. High levels of nutrients recovery are possible with the
concept of source control sanitation (Henze et al., 1997; Esrey et al., 1998; Jönsson et al.,
1999; Larsen and Udert, 1999; Otterpohl et al., 1999a; Otterpohl, 2001). This paper does
show the effectiveness and limitations of source control sanitation concepts with pre-treatment by pre-composting tanks (Rottebehaelter) to recover the particulate fraction of nutrients from household wastewater.
Source control sanitation
A vision of source control sanitation for household wastewater is based on the fact of very
different characteristics of grey water (washing water from kitchen, shower, washbasin and
laundry), yellow water (urine with or without flush water) and brown water (faeces, toilet
paper and flush water). The typical characteristics of the streams of household wastewater,
shown in Table 1, clearly reveal that urine contains most of the soluble nutrients, whereas
grey water, despite its very large volume compared to urine, contains only a small amount
of nutrients. Furthermore, faeces, which is about 10 times smaller in volume than urine,
contains nutrients, high organic load and the largest part of pathogens.
If urine is separated and reused in agriculture, not only nutrients will be reused fully, but
also a high level water protection will be reached. Unlike wastewater containing urine and
faeces, grey water can be treated with simple and low cost processes and reused. There are
many cost efficient biological treatments and membrane technology that can produce high
quality water. Even with the combination of ground infiltration grey water can be processed
to tap water. If faeces is separated and kept in a small volume with non or low-flush toilet,
this will be a favourable condition for high hygienisation and production of soil conditioner. Therefore, separated treatment of different flows according to their characteristics
Table 1 Typical characteristics of household wastewater components (Compiled from: Geigy,
Wissenschaftliche Tabellen, Basel 1981, Vol. 1, Larsen and Gujer, 1996 and Fitschen and Hahn, 1998)
112
can lead to full reuse of resources and a high hygienic standard. In order to realise the source
control, technologies have already been developed in Europe (Esrey et al., 1998, Otterpohl
et al., 1999b and 2001). A separation toilet which has been increasingly used in Sweden,
Denmark and Germany, for example, is a suitable technology to separate the urine and
faeces at source. The toilet has two bowls, the front one for urine and rear one for faeces.
Yellow water treatment
Urine is relatively sterile and can be reused without further treatment (Wolgast, 1993).
However, due to faecal contamination, pathogens have been found in yellow water; but in
low concentration, which will pose low hygienic risk of using yellow water as a fertiliser, if
it is stored at least for 6 months before being used in agriculture (Jönsson et al., 1999;
Hellström and Johansson, 1999). Since the practising of separated collection of yellow
water, farmers in Sweden have been collecting it in the underground storage tank for applying to their agricultural land (Esrey et al., 1998). This practice is good for a region where the
farmland is near to the housing area; otherwise, transportation of a large amount of urine
solution for longer distance has many negative environmental impacts (Hellström and
Johansson, 1999).
Recently, many methods for treatment and reduction of volume of collected yellow
water have been studied. One method is dewatering by evaporation with and without nitrification and freeze concentration (Gulyas, 2000). By freezing, it is possible to concentrate
80% of nitrogen and phosphorus in 25% of the original volume (Ban et al., 1999). By nitrification in combination with drying, it is possible to concentrate over 70% of the nitrogen in
10% of the original volume (Hellström and Johansson, 1999). These methods could be
beneficial, when a larger volume of urine solution has to be transported a long way to the
agriculture farm. These methods have been investigated only in small scale so far.
D.R. Gajurel et al.
Treatment of separated flows
Grey water treatment
Since grey water contains very low nitrogen concentrations (Table 1), nitrification and
de-nitrification processes are not necessary for grey water treatment. Therefore, unlike
total domestic wastewater, grey water is relatively easy to treat although concentrations are
not necessarily lower than in end-of-pipe systems due to a far lower dilution. Choice of
household chemicals that can be mineralised (degradability is no useful indicator) is helpful for achieving good quality. Grey water treatment with vertical constructed wetlands
with sizes of 2 m2 per inhabitant in the Flintenbreite settlement has shown good performance (Otterpohl, 2001). Constructed wetlands are cheap in construction and operation.
However, because of space scarcity, it is not always appropriate for the densely settled
urban areas. For these areas, among other methods, SBR can be suitable. Treatment of grey
water from the Flintenbreit settlement in small scale SBR was studied. Details are given in
Zifu Li et al., 2001. The results showed that, SBR can greatly reduce organic matter, nutrients and turbidity. For reuse purposes, tertiary treatment is needed. Treatment with a slow
sand filter can meet quality requirements for groundwater recharge. E. coli was greatly
eliminated to acceptable levels for recharge. Therefore, grey water treated with the combination of SBR and slow sand filter is suitable for ground water recharge. Treatment with
membrane-bioreactors will probably be the choice of the future especially if reuse is
intended.
Brown water treatment
A relatively new technology called Rottebehaelter or pre-composting tank, which is
usually an underground concrete tank having two filter beds at its bottom or two filter bags
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D.R. Gajurel et al.
that are hung side by side and used alternately in an interval of 6–12 month, has been
increasingly applied in Austria, Switzerland and Germany for domestic wastewater pretreatment (Figure 1). Those investigated in Germany have demonstrated to be beneficial
and can be combined with concept of source separation (Gajurel et al., 2001). One of the
major advantages of this system over other forms of pre-treatment systems is that it does
not deprive agriculture of the valuable nutrients and soil conditioner from human excreta.
The retained materials that have already been de-watered and pre-composted in the
Rottebehaelter for 8–12 months can be further composted with biological kitchen and/or
garden waste in a local composter for at least a year and used in agriculture. This avoids
expensive tanker-trucks which are extensively used in conventional systems to transport
sludge. Moreover, compared to septic tanks methane emission can be very low as the outer
parts of the retained material maintains the aerobic conditions. However, handling of the
bags or the material is not a simple task and should be improved for the future. There are
concepts in Austria in areas with strong gradients in the ground where the tanks are accessible with agricultural fork-lifters and can be removed and emptied in a simple way.
Investigation of existing Rottebehaelter in Lambertsmuehle pilot project
Background
The source control sanitation system has been installed in the pilot project in
Lambertsmuehle near Cologne city in Germany since the summer of 2000. Details can be
found in Otterpohl et al. (2001a). The yellow water, by the means of a separating toilet, is
collected separately in an underground tank, where it is kept till it is ready for use in agriculture. The brown water is discharged into the Rottebehaelter, which is madesup of concrete monolithically and constructed underground outside the building (Figure 2). It is
covered with a prefabricated concrete slab and provided with ventilation. A shutter of a
concrete slab for changing the filter bag, inspection and cleaning has been provided on the
covering of the Rottebehaelter. Inside the Rottebehaelter, two filter bags are hung side by
side in such a way that when one is full, the influent is manually diverted with the help of a
diversion pipe into the next empty bag. The filtrate, due to urine separation, is nutrient poor,
and is mixed with grey water and treated in a constructed wetland.
Performance of the Rottebehaelter
In September 2001, samples from both active and inactive filter bags were analysed. The
results are shown in Table 2. In both filter bags – active as well as inactive-moisture content
was higher than the optimal range (40–60% for composting). Moisture content above 70%
leads to anaerobic conditions (Bidlingmaier, 1983). Thus, anaerobic conditions must have
114
Figure 1 Rottebehaelter (pre-composting tank)
D.R. Gajurel et al.
Figure 2 Lambertsmuehle project: Rottebehaelter with filter bag and separation toilet
Table 2 Characteristics of retained materials (FM: Fresh matter, DM: Dry matter)
Parameter sample from
Active filter bag
Inactive filter bag
Water
Loss on
C
N
content
ignition
(% DM)
(% DM)
(% FM)
(% DM)
87.72
83.14
95.48 46.60
93.26 50.30
6.74
7.16
C/N
6.91
7.025
P
K
(% DM)
(% DM)
0.69
0.61
1.07
1.61
pH
Temp
°C
7.21
6.30
18
20
taken place in both bags. However, no odour was noticed during the sampling. Also people
living in the house have not complained about odour problem so far. Low temperature and
low reduction of volatile solids suggest that a slow decomposition process took place in
both filter bags. It might have caused slow and low emission of odour, which was not
detected by the human nose in the open air.
Low C:N ratio showed that structural bulking agent was not added sufficiently in order
to maintain a C:N ratio of composting material in the filter bags of between 20:1 and 30:1.
Struciaral material also helps to reduce water content and increase air circulation inside the
material. In the inactive filter bag, the pH was lower than the optimal range, 7–8. It was
most probably due to formation of volatile organic acids. Phosphorus and potassium concentrations in both retained materials were lower than concentration in faeces. This was
mostly due to loss through the filter bag in filtrate.
Investigation of the performance of Rottebehaelter in controlled conditions
Methods and materials
The aim of this investigation was to find out the effectiveness of a filter bag in separating
and pre-composting solid stuff from brown water. For this, two small scale experiments,
one with straw as structural additive and another without straw, were carried out in a small
container of volume 30 l with filter bag having a pore size of 1 mm (Figure 3).
For the experiments, faeces was collected directly in the plastic bag at the time of
excretion and weighted prior to filling into a small container where toilet paper (10 g) and
flush water (6 l) were also added. After that, the mixture, called brown water, was filled
manually into the container having a filter bag inside once a day. The filtrate that fell at the
bottom of the container was pumped out everyday. Temperatures in the filter bag and surrounding were measured. The experiment was carried out in two phases – the filling phase
for 30 days and the silence phase for 45 days. After the silence phase, the pre-composting
115
D.R. Gajurel et al.
Figure 3 Experimental set up
materials were mixed thoroughly and analysed. In experiment 1, only brown water was
filled; whereas in experiment 2, 2.5 g of straw as structural additive was added at every
filling.
Results and discussions
Effectiveness of Rottebehaelter in pre-composting
Table 3 shows the characteristics of retained materials. Water content of the material was
too high in both the experiments and the water content in the lower part was higher than that
in the upper part. Therefore, anaerobic decomposition must have occurred at the inner of
the lower part. Slight odour near the container during the experiment also supports this
idea.
pH value was relatively lower in upper part of the experiment 1 – without straw, whereas
it was higher in the upper part of the experiment 2 – with straw. Loss of organic substance
was more in experiment 2 – with straw than experiment 1 without straw. Therefore, addition of sufficient amount of dry structural bulking agent e.g. straw, bark etc. is required for
the composting. It does provide optimal C/N ratio, water content and air circulation of the
pre-composting material, which are prerequisite parameters for the composting. In both
experiments, these parameters were not optimal. In experiment 2, straw was added, but too
little. Therefore, further investigation with addition of sufficient amount of dry structural
bulking agent is needed.
Effectiveness of Rottebehaelter in retaining carbon and nutrients
Figure 4 shows the percentage of carbon and nutrients retained in the filter bag. Among
nutrients, nitrogen was retained above 60% of the total influent in both experiments.
Despite the loss due to anaerobic conditions, nitrogen retained in the filter bag was considTable 3 Characteristics of retained materials (FM: Fresh matter, DM: Dry matter)
Parameter sample from
116
Experiment 1: withoutstraw
• Upper Part
• Lower Part
• Mix of both parts
Experiment 2:with straw
• Upper Part
• Lower Part
• Mix of both parts
Water content
Loss on
(%FM)
ignition (% DM)
66.81
81.67
85.95
85.87
C (% DM)
C/N
pH
7.15
7.85
44.2
74.14
82.12
N (% DM)
7.15
6.12
83.29
85.57
8.61
8.51
38.5
6.3
6.11
Experiment 2: with straw
70%
60%
50%
40%
30%
20%
10%
0%
Carbon (C)
Nitrogen (N)
Phosphorus (P)
Potassium (K)
Figure 4 Carbon and nutrients retained in the filter bag
D.R. Gajurel et al.
% of Retained Carbon and Nutrients
Experiment 1: without straw
erably higher than phosphorus and potassium. However, in experiment 1, phosphorus
retained was above 40% of the influent. In general, the performance of the filter bag in
retaining carbon and nutrient was satisfactory.
Conclusions
Technologies for source separation and treatment have been developing rapidly. So far,
performance of the small scale experiments and pilot projects have given a positive impression. Investigation of Rottebehaelter has shown its potential for solids-liquid separation
and pre-treatment (de-watering to a certain extent) and collection/storage of solid stuff in
household wastewater. One of the major advantages of this system over other systems is
that it does not deprive agriculture of the valuable nutrients and soil conditioner from
human excreta and does not require expensive tanker-trucks. However, water content of the
retained material was very high, which must be lowered to an optimal level. Moreover,
measures such as prevention of heat loss from the Rottebehaelter and good ventilation are
important for effective performance. Last but not least, addition of sufficient amount of
structural bulking agents such as bark, straw etc. into the retained material are important
factors for maintaining optimal C:N ratio, moisture and oxygen required for composting.
Combination of the urine source separation concept with Rottebehaelter was demonstrated
to be effective to recover nutrients. It can be the most appropriate system for application in
the regions where there is a demand for local reuse of the end product. It has to be stated that
maintenance is a crucial factor, removal and handling of the pre-composted material has to
be improved. In addition, proper procedures of further composting and usage will have to
be established. Compared to septic tanks, there are a couple of advantages that make further
development worthwhile.
Acknowledgement
We are thankful to Heinrich Böll Foundation, Germany for providing scholarship to carry
out the research.
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