Effect of mixing on the rheological characteristics of conditioned sludge: Full-scale studies

Conditioning of wastewater sludges with polyelectrolytes prior to dewatering is necessary to increase the sludges' dewaterability. During this conditioning process adequate mixing of the conditioner and the sludge must be attained. This research reports on results from full-scale experiments conducted to examine the effect of mixing parameters upon the conditioned sludge and liquid stream rheological characteristics. Full-scale experiments were conducted at the 90 MGD Wilmington Pollution Control Facility. Varied mixing conditions were the rotational speed of the flocculation tank prior to dewatering and the filtrate recycling to the flocculation tank. The rheological changes in the conditioned sludge were detected using an on-line ZenoFloc(TM) system. Although previous investigations have shown effects of such variations with regard to CST, use of a rheometer as indication of the rheological changes also showed that G variations led to different rheological levels. This allows some insight into conditioning mechanisms as well as optimization: Results also showed that mixing must be optimized and controlled for better dewatering performance. (C) 1997 IAWQ. Published by Elsevier Science Ltd.