Sawdust as a filtering media in sludge drying beds

Abstract

Conventional wastewater treatment is a common method of domestic wastewater treatment in Sub Saharan Africa. Lubigi wastewater treatment plant (LSTP) in Kampala – Uganda is a unique wastewater treatment system combining treatment of on-site faecal sludge and domestic wastewater. High solids content of on-site faecal sludge mean large volumes of the same, thus limited sludge drying space. This means need to optimize sludge drying bed use (improving the sludge drying efficiency) by reducing their drying times. This study investigated use of wood sawdust as a filtering layer in the faecal sludge drying process. Comparison of performance between sand, fine and coarse sawdust as a filtering media was conducted at LSTP. Sludge shrinkage depth (cm) and moisture content (%) were key parameters used to analyze and determine the most ideal media for sludge dewatering. The study was conducted during both wet and dry seasons to determine the impact of seasonal changes. Dry season results showed a drastic decrease in the sludge depth (shrinkage) for all the three media types after a period of 8 days followed by a gradual decrease in sludge depth up to 28 days. This implies that effective dewatering happens for the first 8 days, which goes on for the rest of the remaining days. Overall, the best performing media was fine sawdust, coarse sawdust and lastly sand. Independent two sample t-tests assuming equal variances show that there is a significant difference between the mean sludge depth of sand and fine sawdust t(df) = 56, P<0.05). There was no significant difference in the mean sludge shrinkage depth of fine sawdust and coarse sawdust. Similar results were obtained during the wet season. Comparisons of similar media types during the dry and wet seasons shows that the mean sludge shrinkage for the dry season were significantly lower than that of the wet season (P<0.05). This implies that seasonal changes significantly affect the sludge dewatering. In terms of moisture content (MC), the results for dry season showed that fresh fine and fresh coarse wood sawdust achieved MC of 28% and 31% respectively after 28 days. Sand produced faecal sludge with a higher MC of 49% after 28 days. Similar results of the performance of the three types of media was observed during the wet season. In conclusion, fine sawdust performs better than coarse sawdust and sand media in faecal sludge dewatering. Sludge dewatering is affected by seasonal changes.