The present paper outlines the impact of hydrolysis occurring in DMAEA-Q type polymers in the dewatering facilities of municipal wastewater treatment plants. Laboratory studies of polymer hydrolysis kinetics were combined with full-scale dewatering experiments to investigate the influence of polymer charge loss on polymer consumption and cake dry matter yields. The results indicate that prolonged polymer storage leads to important charge loss effects, resulting in increased dosage requirements for satisfactory dewatering performance. Fortunately, charge loss effects due to hydrolysis upon polymer storage could be minimized by acidifying the polymer batches. Furthermore, cake dry matter contents obtained from dewatering tests with freshly prepared polymer batches suggested that acidification of the polymer batch also had a beneficial effect on the conditioning efficiency of the polymer solution. The results from this study thus stress the influence of pH on the shelf life and efficiency of polymer solutions in wastewater treatment plants. As a consequence, polymer preparation and storage procedures should be adapted to the polymer consumption pattern and pH of the medium used for polymer batch production.
Biological sludge dewatering is known to be a troublesome operation due to the nature of the raw material. Dry matter contents of more than 30% are seldom obtained, leaving a final product containing more than two thirds of water. Many approaches have been used to improve the dewatering step by influencing the conditioning or dewatering process. One of the physical conditioning methods used is freeze-thawing, in which the physical structure of the sludge is destroyed. Most studies so far have been focusing on freeze-thawing of sludge at low solids concentrations, making it an inefficient process with regard to energy and cost. This study focuses on the freeze-thawing of predewatered sludges for subsequent mechanical dewatering in order to reduce the final sludge volume and hence disposal costs. Predewatered sludge with a dry matter content of 25-30% was frozen for different times periods (1-96 h) and at different temperature gradients (slow to fast freezing) using a variety techniques (submersion in glycol, commercial freezer and liquid nitrogen). The results from this study indicated that thawing followed by a second mechanical dewatering resulted in dry matter contents of up to 50%. Slow freezing times and long storage times yielded the best dewaterability results. An economical assessment of the process showed that a depreciation time of 2 years already delivers a profitable process.
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