Municipal wastewater sludge dewaterability using bench-scale units

Abstract

The sludge produced by municipal wastewater treatment plants (MWWTPs) amounts to a small percentage by volume of the processed wastewater. However, it is handling accounts for approximately 50% of the total operating cost. Minimisation of bio-solids is critical in the wastewater treatment industry, but there are challenges in predicting dewatering performance. The dewatering performance can be overcome by optimising the sludge treatment process, especially conditioning. Sewage sludge is highly complex by nature and possesses unpredictable behaviour, due to the change in the composition of the raw inflow to the wastewater treatment works, hourly, daily, monthly, seasonally. Currently, there is a void in reliable bench-scale methodology and data to dewatering efficiency. This research study aimed to investigate sludge dewaterability at four MWWTPs using a gravity drainage test unit as well as a bench-scale press. The following observations were noted. Both experimental tests were able to predict sludge dewaterability, in terms of filtration using gravity drainage, cake height and cake moisture content. The volatile suspended solids (VSS) was found to be the most influential sludge characteristic. This was due to a relationship existing between cake dryness and the VSS. For a lower VSS of 3230 mg/l, a cake dryness of 18% was found, and for a higher VSS of 7162 mg/l, a cake dryness of 13 % was obtained. The experimental work also showed that polymer demand is dependent on feed solids % of the sludge. WWTP D with the highest feed solids, 3.14 %, required more polymer for flocculation to be established, at a minimum polymer demand of 30 ml. Whereas, WWTP A with the lowest feed solids of 1.87 % only required 12 ml of minimum polymer demand to establish flocculation. The alkalinity parameter for the treated effluents used was the only out of spec parameter that possibly interfered with the strength of polymer solution made-up. Treated effluent after filtration yielded a higher alkalinity value of 333 mg/l CaCO3. Therefore, during the gravity drainage experiment, the polymer solution made-up with treated effluent after filtration yielded the least amount of filtration in most of these experiments. The MBR treated effluent used as the solvent, gave to lowest polymer demand. The assessment of the dewaterability of sludge dewatering mathematically, the Specific Resistance to Filtration (SRF) on polymer dosage was obtained for the two polymers used, FLOPAM 4650 and FLOPAM 4800. It was found that SRF decreased with an increase in polymer dosage for both polymers used. For the bench press experiment, a direct correlation between sludge cake height and moisture content was observed. An increase in cake height increased the amount of solids in the cake. The Box Behnken Design model fitted the data and proved that a relationship existed between polymer dosage, cake solids concentration, and cake height during the bench-scale press tests.