Growth potential of faecal bacteria in simulated psychrophilic/mesophilic zones during composting of organic waste.

AIM: This study investigated the growth potential of Salmonella serotype Typhimurium and faecal indicator organisms in compost materials and the correlation between bacterial growth potential and the physico-chemical composition of the compost substrate and temperature. METHODS AND RESULTS: Survival of Salm. Typhimurium, Enterococcus spp. and total coliforms at 14, 24 and 37 degrees C was determined in material of different degrees of maturity collected from composting plants for household waste and manure. All three micro-organisms showed the potential for growth in the material from active composts (Solvita index 4) but inactivation generally occurred over time in mature compost material (Solvita index 7-8). CONCLUSIONS: Salm. Typhimurium had the potential for growth in psychrophilic/mesophilic (P/M) zones of immature compost material and its growth potential correlated negatively with the maturity of the compost and the temperature within the simulated P/M zone. SIGNIFICANCE AND IMPACT OF THE STUDY: The risk of pathogen regrowth in P/M zones during organic waste composting further emphasizes the importance of good management practices and of avoiding P/M zones in combination with low compost maturity.

Sanitation of faeces from source-separating dry toilets using urea.

AIM: To develop a reliable and simple method to produce safe fertilizers from human excreta using urea for sanitation of faeces. METHODS AND RESULTS: Urea was added to faecal matter (17% dry matter) at concentrations of 0.5-2% (w/w) and inactivation of Salmonella enterica subspecies 1 serovar Typhimurium (Salm. Typhimurium), Enterococcus spp. and the Salm. Typhimurium bacteriophage 28B was monitored at 14, 24 and 34 degrees C. Urea additions enhanced inactivation and inactivation rates were positively related to increasing NH(3) (aq) concentration and temperature. Salm. Typhimurium was the most sensitive of the organisms studied, while Enterococcus spp. showed more persistence, especially at lower temperatures. The bacteriophage was the most resistant organism studied. CONCLUSIONS: Salmonella reduction levels that meet requirements for safe reuse of faeces as fertilizer (i.e. 6 log(10) reduction) can be achieved for 1% urea within 2 months at 14 degrees C or within 1 week at 24 degrees C and 34 degrees C. SIGNIFICANCE AND IMPACT OF THE STUDY: The relationships between organism inactivation rates and temperature, ammonia and pH were identified. Urea treatment proved to be a robust and efficient option for safe recycling of plant nutrients.

In situ ammonia production as a sanitation agent during anaerobic digestion at mesophilic temperature.

AIM: To measure the sanitizing effect of mesophilic (37 degrees C) anaerobic digestion in high ammonia concentrations produced in situ. METHODS AND RESULTS: Indicator organisms and salmonella were transferred to small-scale anaerobic batch cultures and D-values were calculated. Batch cultures were started with material from two biogas processes operating at high (46 mmol l(-1)) and low (1.6 mmol l(-1)) ammonia concentration. D-values were shortened from c. 3 days to <1 day for the bacteria. MS2 had the same D-value (1.3 days) independent of ammonia concentration whereas PhiX174 and 28B were faster inactivated in the control (1.1 and 7.9 days) than in the high ammonia (8.9 and 39 days) batch cultures. CONCLUSION: Running biogas processes at high levels of ammonia shortens the time to meet EU regulation concerning reduction of salmonella and enterococci (5 log). Unless a minimum retention time of 2 days, post-treatment digestion is needed to achieve sufficient sanitation in continuous biogas processes. SIGNIFICANCE AND IMPACT OF THE STUDY: Running mesophilic biogas processes at high ammonia level produces residue with a high fertilizer value. With some stipulations concerning management parameters, such processes provide a method of bacterial sanitation without preceding pasteurization of the incoming organic waste.