The feasibility of co-composting as an upscale treatment method for faecal sludge in urban Africa

Despite the improvements made to deliver improved sanitation around the world, Faecal Sludge (FS) management is still a challenge affecting hundreds of millions of people. Composting is one of the well known and most preferred low-cost methods for treatment of FS in urban Africa. However, its effectiveness has not been thoroughly explored. With an aim of improving the FS composting process in terms of nutrient recovery and pathogen inactivation; this study investigated the optimum sand filtering media thickness (150 mm, 250 mm, 350 mm) for FS drying beds, as well as the effects of different bulking agent types (sawdust, coffee husks and brewery waste), turning frequency (3, 7 and 14 days) and organic solid waste types (market waste and chicken feathers) on the FS composting process. Field dewatering and composting trials were conducted in Kampala, Uganda. The investigation revealed that the media thickness had a significant effect on the removal and recovery efficiency of percolate contaminant loads, but not on the dewatering time. The dewatering time improved to 3.65 - 4.02 days resulting into 65 - 67% reduction in the land area requirement per capita (0.016 - 0.017 m2/capita) for the treatment of FS to about 37%TS. The 350 mm filtering media thickness was the most efficient, in terms of contaminant loads removal from FS. However, 150 mm media thickness had the greatest potential of optimising nutrient recovery (NPK) from FS, particularly were the resulting solids are to be composted. The results indicated that the solid loading rate of 441 kg/m2/year can be used as the design criterion for FS drying beds in urban Africa. The results showed that the bulking agent types, turning frequency and organic waste types had a significant effect on most of the key physico-chemical and biological composting parameters. Overall, sawdust was the most suitable bulking agent for co-composting with FS as it enhanced nutrient recovery. It exhibited the lowest N-losses of only 2.2% compared to coffee husk and brewery waste with 48.2% and 72.5%, respectively. The results indicated that composting with high turning frequency (3 days and 7 days) enhances pathogen inactivation efficiency, organic matter degradation and composting rate, and thus reduced the composting periods by about 33%. However, it was associated with higher N-loss of about 50.1 - 48.6% compared to 7.6% of low turning frequency (14 days). Overall, the 7 days turning frequency was the most suitable and economically feasible for use during the FS composting. On the whole, chicken feather waste was the most suitable organic waste type for co-composting with FS as it enhanced pathogen inactivation efficiency and nutrient recovery. The composting of FS with both chicken feather and market waste reduced the composting periods by 32% and enhanced pathogen inactivation rate as well as nutrient recovery through nitrogen conservation, by a factor of 3. The composting of FS with chicken feather reduced the pathogen inactivation periods by 42%. This could thus increase the capacity of FS treatment plants or reduce their required capital investment, operational costs by 42%. The study revealed that a composting period of 8 weeks with temperatures of around 50.7 - 58.7°C sustained in the piles for more than 31 days, using 7 days turning frequency, is sufficient to ensure complete inactivation of pathogens in FS. Pathogen inactivation in composting piles was not solely dependent on the temperature-time factor, but also other mechanisms such as microbial antagonistic mechanisms or antibiotic action induced by indigenous microbial, moisture content, change in pH, nutrient depletion, and toxic by-products such as NH4-N. The following indices were established for mature FS compost; NH4-N 100%, WSC < 12.3 g/kg, WSNH4-N < 0.03 g/kg and WSNH4-N/WNO3-N < 0.18. The results indicated that a composting period of 14 weeks is required to obtain both stable/ mature and pathogen free FS compost, thus co-composting can be a viable way of treating FS in urban Africa