Efficient solids-liquid separation of the activated sludge wastewater treatment method depends on the formation of flocs in the biological step, and their rapid settling and proper compaction in the secondary clarifier. The effect of dissolved oxygen (DO) concentration on sludge floc morphology and settleability in three full-scale conventional activated sludge plants treating industrial and municipal wastewater was studied. Floc structure was considered in terms of size, shape, strength and filament index, while settleability was determined based on the sludge volume index (SVI). Generally, higher DO concentrations produced more round and compact flocs, while irregular and open flocs were associated with low DO concentrations. There was only a trend towards larger flocs at higher DO levels. SVI values did not correlate well with DO concentrations, with municipal plants operated at lower DO levels achieving better settling compared to industrial plants operated at higher DO levels. However, there was a positive relationship between SVI and levels of suspended solids in the effluent. Additionally, flocs with the same filament index but from different plants were morphologically different. These results indicate that activated sludge floc morphology and settleability are affected by other plant operating conditions, not DO exclusively.

activated sludge process, dissolved oxygen concentration, floc morphology and settleability