Measuring faecal sluge stabilization and its relation to dewatering performance.

Stabilization and dewatering are indispensable treatment mechanisms applied in the management of Faecal Sludge (FS) that accumulates in onsite containment facilities such as pit latrines and septic tanks. This is because FS is mainly comprised of 80 – 95 percent water and readily degradable organic matter. Based on field observations, Faecal Sludge is normally stabilized (offering limited scope for further stabilization at treatment) due to the longer storage times it spends in the onsite containment systems where it undergoes digestion in mostly anaerobic and facultative conditions. In addition, a potential linkage has been reported between the observed level of Faecal Sludge stabilization and its dewatering performance. It has been noted that more stabilized FS is easier to dewater than fresh sludge which is not stabilized. However, it is not clear how FS stabilization and its relation to dewatering can be measured with practitioners relying on qualitative information such as colour and odour to distinguish between the so-called stabilized and non-stabilized or fresh Faecal Sludge. The study evaluated rapid and low cost methods that can be used to measure FS stabilization, including criteria or index for characterizing a sample as stabilized or not stabilized. The study also assessed the relationship between FS stabilization and its dewatering performance. Methods that can be used for measuring FS stabilization were selected through a two-stage process i.e. screening using a decision matrix and laboratory evaluation to determine method performance and suitability. The relationship between FS stabilization and dewatering performance was determined through laboratory anaerobic digestion and dewatering experiments. A total of 27 faecal sludge samples including a fresh sample were collected and nine parameters related to stabilization and dewatering performance were analysed. The study found that FS stabilization can be measured using low cost methods such as the Volatile Solids to Total Solids ratio and the Specific Oxygen Uptake Rate and is associated with dewatering performance measured as capillary sanction time. Stabilization was correlated to the age/ type of faecal sludge, though the differences based on sludge age were not significant among samples from pit latrines and septic tanks. The observed differences in dewatering performance were associated with differences in level of stabilization. The majority of the FS samples become stabilized after 60 days of anaerobic digestion and a corresponding improvement in dewatering performance was also observed. Based on these results, FS from onsite containment facilities is not fully stabilized, despite the longer retention times at containment. Further, the associations between anaerobic digestion and improvement in dewatering performance as well as stabilization suggests that application of a biological stabilization step at treatment before FS dewatering can be beneficial.