A thesis submitted m partial fulfillment of the requirements for the degree of Master of Engineering, School of Environment, Resources and Development

Thesis (M.Eng.) - Asian Institute of Technology, 1997

At present, disposal options for sewage sludge and nightsoil sludge generated in the Bangkok Metropolitan Administration area are under study. This study was conducted to evaluate the possibility of using sewage sludge and nightsoil sludge in agriculture. A laboratory scale study was undertaken to determine the dosages of quick lime (CaO) and slaked lime (Ca(OH)2) to be added to sludge to obtain a pathogen safe product. This was followed by a field study, where limed sludge was used as a source of fertilizer for sunflower cultivation. The loading rates to field were determined based on the nitrogen requirement (80 kg/ha) of the crop. The effect of using limed sludge in agriculture was determined by studying its impact on, soil pH, heavy metals in soil and plant, total nitrogen and exchangeable potassium and calcium concentrations in soil, and on plant yield. The effectiveness of the limed sludge as fertili.zer was studied by comparing with chemical fertilizer (control field). Experimental results revealed that to inactivate fecal coliforms in sewage sludge 0.22 g of Ca(OH)2 and 0.59 g of CaO were required per g of dry sludge solids. To achieve the same in nightsoil sludge 0.18 g of Ca(OH)2 and 0.48 g of CaO were required per g of dry sludge solids. These results indicate that Ca(OH)2 is the better of the two liming agents used. In the field study, all limed sludge treatments did not differ significantly from the control, for most parameters analyzed. Nightsoil sludge with Ca(OH)2 was the only treatment that displayed significantly higher concentrations of Cu in leaf and seed, close to the toxic limit. Plant yield in the limed sludge treatments was significantly less than that in the control. This is presumably due to the fact that sludge mineralization takes priority and delay availability of nutrients to plant. The yield in the limed sludge fields could be improved by providing a small quantity of chemical fertilizer at the start so that it will give time for sludge mineralization and release of nutrients. Considering the potential hazards (risk from pathogenic organisms and heavy metals) associated with application of sludge in agriculture, the treatments used in this study were relatively safe. It could be concluded that both nightsoil sludge and sewage sludge can be utilized in agriculture after the necessary lime treatment.