Effect of scum on the dewaterability of Faecal sludge from septic tanks

Abstract

Understanding the properties of Faecal sludge (FS) is critical for over all faecal sludge management. This study investigated the impact of faecal sludge scum (FSS) on the dewaterability of faecal sludge (FS) from septic tanks. Samples of FS and FSS were collected from 35 septic tanks across Kampala. In-situ measurements and data on demographic, environmental, and technological variables were also recorded. Dewaterability rate (CST) was assessed using the capillary suction time method, while the extent of dewaterability (EXT) was determined through centrifugation. FSS depth ranged from none to 50 cm, with a median of 0.05 mm. Most scum was black (61%) and solid (77.4%), while 22.3% was foamy and watery. FSS showed high variability in characteristics with statistically significant differences between FSS and FS observed for Chemical Oxygen Demand (p=0.046), Total Solids (p<0.001), density (p=0.032), Oxygen Reduction Potential (p=0.008), Magnesium (p=0.010), Electrical Conductivity (p=0.025), Moisture Content (p<0.001), and Nitrogen (p=0.003). FSS had higher capillary suction time (CST) values with a mean of 163.5s compared to FS with a mean of 59.5 s, indicating lower dewaterability. Mean cake solids were 12.8% for FSS and 11.4% for FS. In FS samples, TS and OP positively correlated with CST, while MC showed a significant negative correlation (r = –0.532, p = 0.002). Findings showed that the percentage of FSS was positively associated with CST, while total FS volume had a significant negative correlation with CST (r = –0.3245, p = 0.0337). FSS percentage had a weak positive association with cake solids, whereas FS quantity was negatively correlated (r = –0.2142).). Scum dewaterability was found to be strongly influenced by FS characteristics. COD, ORP, fat, total solids (TS), and orthophosphates (OP) were positively associated with CST with increase in these parameters generally leading to longer dewatering times. In conclusion, the amount of FSS in FS has a greater impact on the time it takes for water to be drawn out from the sample i.e. dewaterability rate. Therefore, more scum in the sludge slows down the dewatering process but does not significantly affect the dryness of the resulting cake.