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Possibilities for sustainable nutrient recycling by faecal separation combined with urine diversion
Household wastewater can be divided into three fractions by origin; urine, faeces and greywater. The largest nutrient and smallest heavy metal contents are found in the urine, which is easily collected separately using a urine-diverting toilet. The second most nutrient-containing fraction is the faecal matter.
The faecal matter can either be collected dry or, after a short waterborne transport, be separated from the flushwater in a separator that uses a combination of whirlpool effect, gravity and surface tension. Using this type of separation, between 58% and 85% of the faecal nutrients were separated in the measurements performed here. By recycling the urine and the faecal nutrients, much energy can be saved as the load on the wastewater treatment plant decreases and as mineral fertilisers are replaced in agriculture.
To avoid transmission of diseases, the faecal matter has to be sanitised before recycling. If the faecal matter is collected dry, it is possible to perform the sanitation by thermal composting, preferably together with household biodegradable waste. A calculation method for determination of the safety margins for sanitation was developed. In a pilot-scale study, the safety margin for thermal composting of faeces and food waste, with old compost as an amendment, was approximately 37 times total inactivation of Enteroviruses, the most thermotolerant organism evaluated.
Another sanitation method investigated was chemical disinfection using urea or peracetic acid. At a dosage between 0.5% and 1.0%, the highly reactive peracetic acid inactivated all investigated organisms within 12 hours of treatment. The high dry matter content (10% DM) meant that high dosages were needed. Lower dry matter content would decrease the dosage required for proper sanitation.
A very promising treatment was the addition of urea. Addition of 30 g urea nitrogen per kg of wet weight faecal matter resulted in total inactivation of the monitored organisms, E. coli, Salmonella spp, Enterococcus spp, Salmonella typhimurium 28B phage and Ascaris suum eggs, within 50 days of treatment at 20°C. The spore-forming bacteria Clostridium spp in its dormant state was resistant to this treatment. As the urea has to be degraded to ammonia before it functions as a disinfectant, there is some delay in this treatment. Therefore, urea addition followed by 2 months storage is the preferred treatment for disinfection of separated faecal matter. As additional effects, urea increases the fertiliser value of
the treated material and there is no risk of microbial regrowth.
Changing to urine-diversion combined with faecal separation and disinfection by urea seems to be an interesting way to decrease the resource usage and possibly improve the hygienic standard of wastewater systems.
PhD Thesis, Swedish University of Agricultural Sciences (SLU), Department of Agricultural Engineering, Uppsala, Sweden
Faecal separation, urine-diversion, nutrient recycling, wastewater composition, wastewater reuse, disinfection safety margins, sanitation, disinfection, thermal composting, chemical disinfection, urea, peracetic acid, Vinnerås
Vinneras, B. (2002). Possibilities for sustainable nutrient recycling by faecal separation combined with urine diversion. PhD Thesis, Swedish University of Agricultural Sciences (SLU), Department of Agricultural Engineering, Uppsala, Sweden.