The effect of loess addition on the settling ability of activated sludge

Abstract

In this research, loess addition was investigated as a possible means of controlling the bulking sludge generated from a sequencing batch reactor (SBR) system treating a synthetic wastewater. The specific objective was to investigate whether loess changed the morphology of the sludge (i.e., influenced the relative abundance of filamentous species), as opposed to improving settling simply because the clay portion of the loess acted as a flocculating agent. To this end, two sets of batch tests were performed using 1 L reactors filled with bulking sludge from the SBR. The first set of batch tests investigated the effect of different loess concentration on the settling properties of the sludge; thus loess was added in concentrations of 0.0, 0.4, 2.0 and 5.0 g L⁻¹. The 5.0 g L⁻¹ loess concentration exhibited the most positive results on settling, bringing the modified sludge volume index (SVI) down into the target range of 150 mL g⁻¹. The second set of batch tests investigated filament length along with the modified SVI. It appeared that at the microbial level, 5.0 g L⁻¹of loess caused no reduction in filament length, suggesting no reduction in the amount of filamentous microorganisms. This means that adding loess to a system after it has bulked has the potential to mask the bulking problem by improving settling, while not fixing the problem microbiologically.

Keywords:

• Bulking sludge
• filamentous organisms
• loess
• settling

Acknowledgments

The authors would also like to thank Peter McGuigan and David MacPherson of the environmental engineering lab at the University of Canterbury for their excellent technical assistance. Also to be thanked is Dr. Elena Moltchanova of the Department of Mathematics for advice with the statistical analyses.