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Overlevelse af indikatororganismer i komposttoiletter og ved simuleret centraliseret efterkompostering af afføring fra mennesker
Summary and conclusions
Background
There is an increased interest in the society for local handling and use of human feces from compost toilet systems. However, it appears that the associated health risks are inadequately described despite the
fact that compost toilets have been used for decades in many countries. Therefore, The Danish Environmental Protection Agency ("Miljøstyrelsen") decided that it was necessary to initiate investigations to
describe and assess potential hygiene and health risks associated with these practices.
The main objectives for the investigations were
Through analyses of existing compost toilets:
- To assess the concentration of indicator organisms and occurrence of pathogens in fecal material from compost toilets.
- To analyze physical and chemical parameters including temperature development in selected compost toilet systems and correlate such information with the occurrence of indicator organisms.
Conduct laboratory experiments in model systems:
- To investigate the survival of indicator organisms under conditions simulating those found in large-scale centralized composting systems.
Investigation of the occurrence of indicator organisms and pathogens in compost toilet systems
A total of 15 compost toilet systems were included in the study. Six systems were located in "Det Økologiske Andelssamfund i Hjortshøj" north of Århus. Three toilet systems were used in houses in "Det
Økologiske landsbysamfund Dyssekilde" in North Sealand. The remaining systems were situated in different households in the southern and eastern parts of Sweden. The toilet systems included five batch
systems and eight continuous one-chamber systems. Four of the batch systems had two containers in alternating usage (system 1-4) and one batch system had four chambers also in alternating usage (system
5). The continuous systems consisted of three systems with a flat bottom (system 6-8) and five systems with a sloping bottom (system 9-13). The systems used for composting of fecal matter from the
household containers (system 14 and 15) consisted of a number of 1 m3 containers placed outdoor. The inhabitants in the study site Hjortshøj and Dyssekilde used such containers for the final placement of
fecal material from the individual compost toilet systems.
In agreement with the Environmental Protection Agency, it was decided to analyze for a number of microbiological parameters in fecal samples from compost toilets, including analyses for the indicators
thermotolerant coliforms, enterococci and spores of Clostridium perfringens, together with the pathogenic bacteria Salmonella, Listeria and Campylobacter. In addition, the fecal samples were analyzed
for helminth parasite eggs (a list with all microbiological parameters is shown in Table 3.2 in Chapter 3). Finally, analyses were done for a number of physical and chemical parameters, including temperature,
in the compost toilet systems.
Selected results from the microbiological analyses of fecal samples from the containers are shown in Table 1. Overall, thermotolerant coliforms were found with large variation in numbers. The numbers of
enterococci showed only limited correlation with the number of thermotolerant coliforms with large variation in enterococci numbers. Spores of Clostridium perfringens were found in the majority of
samples with numbers between <10-40,000 colony forming units (cfu) pr. gram feces. The numbers of fecal indicator bacteria showed large variations and there was no clear trend towards lower bacterial
counts as a result of longer storage time.
Table 1. Selected results from the microbiological analyses of fecal samples from compost toilets (all results are shown in table 3.5 in chapter 3). The numbers shown are colony forming units (cfu) pr. gram
feces.
| Compost toilet unit |
Thermotolerant coliforms |
Enterococci |
Cl. perfrin- gens spores |
Estimated storage times of fecal matter at time of sampling |
| System no. 4: 140 L plastic container |
110,000 |
10 mill. |
9,000 |
Few days |
| System no. 1: 220 L plastic container |
20,000 |
2,600 |
27,000 |
More than 2 months |
| System no. 3: 220 L plastic container |
70 |
1.4 mill. |
5,000 |
More than 7 months |
| System no. 2: 220 L plastic container |
160,000 |
280,000 |
10 |
More than 14 months |
| System no. 5: "Snurredass" with 4 units of 280 L plastic containers |
10 |
Less than 100 |
2,000 |
More than 21 months |
| System no. 10: "Linden"-toilet system" with 2.9 m3 container |
Less than 10 |
Less than 100 |
Less than 10 |
The oldest material originated from 2000 |
| System no. 14: Common compost container. Unit size approx. 1 m3 |
590 |
20,000 |
5,000 |
Between 2-8 years |
The most important finding from the analyses of the physical and chemical parameters was that only minor temperature elevations were registered in the containers used for feces collection. At no time during
the investigation the measured temperatures were more than 15 °C above the ambient temperature. This prompted the initiation of experiments to optimize the composting process through the addition of
organic material to the 220 l containers studied. Long lasting temperatures at 55 °C was achieved, but it was not possible to achieve complete sanitization of the material, i.e. exposure of the material to
70 °C for one hour as stipulated by the departmental order no. 49 on the use of waste materials for agricultural purposes (Miljøstyrelsen, 2000).
Model experiments simulating conditions for centralized composting of feces
As no large-scale centralized systems for composting of human feces existed in Denmark, these investigations were done in a model system, which simulated the conditions in a full-scale composting plant.
The model system consisted of six computer-controlled compost reactors each with a volume of 9 l. Following some initial pilot experiments, the survival of the indicator organisms were investigated at
temperatures from 50-65 °C. Curves of survival were then calculated for the microorganisms as a function of exposure time at the different temperatures investigated. Based on the curves of survival, the
times needed for a 4-log reduction in numbers were calculated (Table 2). Similar calculations have been used in previous investigations assessing the effect of different sanitation measures (Bendixen et al.,
1995).
Table 2. Time constants for a 4-log reduction in numbers of microorganisms at different exposure temperatures in model experiments
| | Time needed for a 4 log-reduction in numbers of microorganisms (hours) |
| |
Termotol. coliforms |
Salmonella |
Salmonella phage |
Enterococci |
| 50 °C |
4.88 |
3.76 |
63.17 |
141.90 |
| 55 °C |
7.09 |
3.28 |
37.86 |
85.81 |
| 60 °C |
0.78 |
0.77 |
16.92 |
62.17 |
| 65 °C |
0.32 |
0.38 |
14.13 |
46.09 |
Interviews of users of compost toilets
A questionnaire-based interview was conducted with all users of toilets from which samples were collected. In general, the users were satisfied with their compost toilets, all of which were technically simple
systems. A total of 12/13 users had made their own decisions on the establishment of compost toilets, the selection of type of toilets (models), and the placement in the house of the toilet system components.
The interviews revealed a number of recurrent problems related to usage and maintenance of the toilets, including bad smells, problems with flies and obstruction of the pipes for collection and transport of
urine.
Conclusions
The following main conclusions can be made as a result of the investigation on composting of human feces:
- Feces from the investigated compost containers should not be used for agricultural purposes, e.g. in the garden of the owner of the compost toilet without additional treatment as such usage will be
associated with sanitation and health risks.
This is due to:
- No thermophilic temperature development was registered in the composted fecal material in compost toilet containers in Hjortshøj, Dyssekilde, and Sweden.
- There were large variations in numbers of fecal indicator bacteria and no clear association between lower bacterial counts and prolonged storage time periods of collected fecal matter.
The most important specific conclusions from the investigation of the compost toilet systems were:
- The bacterial pathogens, Salmonella, Campylobacter and Listeria, were not detected in any fecal sample. This indicates that the pathogens have a low survival rate in collected and stored feces and/or
that the pathogens initially occurred in low numbers in the fecal material.
- The numbers of fecal indicator bacteria showed large variations and there was no clear trend of low bacterial numbers when feces were stored for long time periods. These findings raise the question; are
the traditional fecal indicators (enterococci and thermotolerant coliforms) appropriate hygiene indicators for collected and treated human feces? Rather than enterococci and thermotolerant coliforms, it
appears that E. coli could be a better indicator to assess the degree of sanitation of fecal material.
- In controlled composting experiments at KVL using 220 l plastic containers, one placed indoor and the other outdoor, the maximum temperature in the middle of the feces material was 43 °C and 49
°C, respectively.
- Despite optimization of the controlled compost experiments by addition of grass and a sugar solution, it was not possible to achieve a temperature high enough for sanitation of the material, i.e. 70 °C for
at least one hour, as stipulated in the departmental order no. 49 regarding the use of waste products for agriculture (Miljøstyrelsen, 2000).
- Survival of the eggs of the round worm parasite Ascaridia galli of poultry was used as an indicator for the human round worm Ascaris lumbricoides. Eggs of A. galli could not develop to the infective
larval stage after storage in the middle of a 220 l plastic container where temperatures above 55 °C were achieved for more than 2 days.
- Addition of Salmonella typimurium bacteriophage 28B and Salmonella senftenberg to fecal material contained in semi-permeable capsules proved to be a useful technique. In this way, microorganisms,
e.g. bacteria, virus and parasites can be added in known concentrations to materials to test their survival over time.
The most important specific conclusions from the model experiments were:
- The reduction of microorganisms was investigated at temperatures of 50 °C, 55 °C, 60 °C and 65 °C in model experiments simulating large-scale centralized composting systems. For
example, four days composting at 55 °C resulted in more than a 4 log-reduction of the indicatororganisms. The reduction in microorganisms was in general in agreement with the literature, except for
enterococci.
- The enterococci were reduced much slower than anticipated in the model experiments when compared with the other parameters, including the bacterial phage. These findings are highly surprising and not in
agreement with the literature. Unfortunately, it was not possible to explain these findings.
The most important specific conclusions from the interviews were:
- Users of compost toilets were in general satisfied with their systems. An important reason for this seemed to be that the users decided themselves on the type of systems to be installed.
- Problems with bad smells, flies and obstruction of urine pipes were common important recurrent problems of the investigated toilet systems.
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Version 1.0 Oktober 2005, © Miljøstyrelsen.
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