Co-treatment of faecal sludge and sewage in a SBR (Nepal)

The main aim of this study is to evaluate the effects of different volumetric mixing ratios of settled faecal sludge and sewage on the performance of a sequencing batch reactor (SBR). Sludge was collected from the lined two-chambered septic tank (without outlets) present at the staff’s accommodation unit of Kathmandu University (Dhulikhel, Nepal) while sewage samples were collected from the laterals of another accommodation units of the same campus premises. Two SBRs, reactor one (R1) and reactor two (R2) operated in 24 hours cycle at four different volumetric ratios (5, 10, 20 and 40%) of FS and WW. The reactors were operated for 68 days, where 22 days were the start-up period. The removal of COD, ammonia and phosphate was high during the start-up phase and the volumetric mixing ratio of 5% (sludge:sewage). However, the pH dropped when the volumetric mixing ratio was increased beyond the 10% mixing ratio which could be due to the nitrification. pH was maintained at neutral (7.0) in the process by adding Sodium Hydroxide (NaOH). Overall, the average ammonia removal rate was 89.6% and 87.6%, respectively for R1 and R2, and the average COD removal rate was 86% and 85%, respectively for R1 and R2 during all the tested mixing ratios. The phosphate removal rate was found to be low which could be due to the absence of an anaerobic stage during the SBR cycle. Sludge wasting was not practiced during the operation which led to high biomass (6,500 mg MLSS/L) and low food to microorganism ratio. Furthermore, the operational cycle could be reduced 18 hrs and perhaps further. The study shows that co- treatment of faecal sludge and sewage in SBR may be an attractive solution.