Enhancement of sludge settling with chemical additives.

Solids in aeration tank effluents contribute to the total organic content and suspended solids concentration, and violates disposal and reuse standards. Therefore, improving the settling properties of sludge helps produce an effluent with a quality that conforms to effluent standards, assuming all other conditions for good settling in the sedimentation tank are favorable. In this paper, seven chemicals that are all cationic polyelectrolytes were tested individually as additives to improve the settling properties of aeration tank effluent from a full-scale wastewater treatment plant. Pilot settling columns designed and constructed at a local wastewater treatment plant were used in this project. Batch settling tests were performed on fresh activated sludge samples that were collected before the secondary sedimentation tank and mixed with different concentrations of the additives without cumulative additions. Each additive was accessed based on its particular sludge settling properties, that is, the shape of the settling curves, the values of the hindered zone settling rate or zone settling velocity, and the stirred sludge volume index. Each additive result was compared with the corresponding properties of a plain sludge sample collected on the same day and used as a control. Substantial and dissimilar improvements were obtained and are presented in the form of graphs and values using 92 settling runs. Shorter lag-times, quicker descending speed of sludge-water interface, higher descending distance, and lower stirred sludge volume indices values were attained with the use of these additives.