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Pee and poo for food
‘Pee and Poo for Food’ is a research project that was inspired by the idea of a sustainable future where human waste is used as fertilizer for food crops and dry toilets are the norm. To determine if there was any possibility for this future we investigated four dry toilet systems on allotments in the north and south of London. These were good case studies because the allotmenteers have regular access to a dry toilet system; they grow food and are familiar with the process of fertilising soil.
What types of dry toilets were tested?
There were three types of dry toilets in our sample of four allotments. Two allotments had the same proprietary system. The proprietary desiccating system used passive solar energy to induce air ow
over the wet waste taking away both odour and moisture. In one allotment, lime was added to the waste and the desiccated waste was stored in compost bins amongst a hedge on site (Proprietary- Limed); in another allotment the waste was removed, or burnt and buried (Proprietary-Removed). The other two toilets were self-built with different systems. One allotment had a twin vault dry toilet.
This is where one vault is in use, while the other vault is used to rest the waste for about 6 months. After this, it is emptied into a strong polypropylene woven builders bag behind the toilet shed and allowed to rest for another 6 months before application onto an orchard (Self-Twin). Another allotment had built a raised toilet shed where the waste is collected in a wheelie bin below the toilet. When it is full, the bin was sealed, moved, and the waste allowed to rest inde nitely. A new wheelie bin is then placed under the toilet (Self-Wheelie).
What were the allotment toilets like?
All toilet users felt that the dry toilet installation was a success and were happy to use the facility. The expectations of a toilet and toilet area which was clean and odour free were met. “I was amazingly surprised at how this one is wonderfully clean, bright and modern...The ventilation worked extremely well at keeping the bad smells away”.
Would people use compost from toilets?
More people would consider using the compost from the toilet on their allotment site than would not.
YES! Most – 39% of people would use compost from the toilet on their allotment site.
“Yes of course I would use it...it is just a part of the cycle of nature, I do not see it as being much different to me putting all my kitchen waste in the compost bin with the manure. I have been doing it for years, so in one respect I am eating the same food over and over again”.
“Yeah absolutely...from a sustainability point of view”. “I see no use in not using it...”
Maybe – 27% were uncertain if they would use compost from the toilet on their allotment site.
“There are valid reasons to say yes and reasons to say no...I think if it is well composted which one would think that it would be I don’t see a problem with it honestly; everybody chucks in and buys all kinds of fertiliser and why not keep things truly local I suppose”.
“I’d be very cautious about it. I would have to think...I’d have to be convinced that it was...safe...”
“I would put it around the roses. It wouldn’t bother me to have it around...”
NO! – 33% of people would not use compost from the toilet on their allotment site.
“I really don’t want to be having compost on my plot that’s been made by meat eaters...I don’t know for instance with the loo who’s using what meds they might be on...There’s a lot of ladies on our site who will be on HRT for instance...”
There was a difference between the allotments and the acceptability of using compost from the toilet. Self-Twin and Proprietary-Removed were more likely to use composted toilet waste, while Self-Wheelie and the Proprietary-Limed were more unlikely.
Were there dangerous pathogens in the compost?
Some – but not enough to worry about. We used a special kind of medium that selectively grows faecal bacteria, including coliforms such as E. coli. During the analysis of the samples, each of the cells of faecal bacteria in the samples grows into a colony which we can
then see and count on the agar plate. The most faecal bacteria found in the compost was nearly 385,000 bacterial colonies per gram of compost. A gram of soil will typically contain 100,000,000 to 1,000,000,000 bacterial colonies. One site contained zero coliform bacteria in the samples analysed.
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Self-Twin Self-Wheelie Proprietary-Limed Proprietary-Removed
Figure 1. This is the average number of coliform and other bacteria cultured from a gram of compost from each of the allotment sites. The bacteria were cultured on an agar which only grows faecal bacteria. The numbers are shown on a logarithmic scale so that all samples can be seen on the graph. The whiskers on each bar show the level of variation seen during the sampling period.
What nutrients were in the compost?
The two main nutrients that plants require are nitrate and phosphate. The samples indicate that there are relatively high levels of available phosphorus at all sites. However, nitrates were only detected in
the south London allotments. The maximum available nitrates and phosphates were found at Self-Wheelie. The highest nitrate was 1261mg/gm of dry compost. The highest phosphate was 1372mg/ gm of dry compost. However, this was also the site where the results were the most variable over time.
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Self-Twin Self-Wheelie Proprietary-Limed Proprietary-Removed
Figure 2. This the average amount of available nutrients in the form of nitrate and phosphate found in each dry gram of compost at each of the allotment sites. The whiskers on each bar show the level of variation seen during the sampling period.
Conclusion
Different types of toilet systems led to different ways of composting the waste, which in turn led to different levels of nutrients within the compost. All the treated toilet waste had some faecal bacteria within it, but much less than would be found in an average soil sample. The interviewed allotmenteers indicated that a majority of people would consider using this compost on their allotment site. The research demonstrates that there is hope for a sustainable future recycling human waste as fertiliser for food crops.
Tse-Hui Teh & Lena Ciric – t.teh@ucl.ac.uk, l.ciric@ucl.ac.uk
mg nutrients per dry gram of compost Colories per dry gram of compost