Effect of Digestion Processes on Dewatering and Bound Water Content of Sludge
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Solids handling can contribute to a significant portion of the operational costs of a wastewater treatment plant, contributing up to 50% of the total expenses in certain instances. Sludge dewatering and drying therefore become necessary not only from the operational perspective, but also from the economical viewpoint. The J-Vap process combines the above-mentioned processes, by pressure filtration of sludge followed by application of vacuum and heat. However, when cationic polymer conditioned sludge is dewatered in the J-Vap, the polymer is suspected to interact with the filter media at high temperatures, resulting in the formation of a skin layer that hinders efficient dewatering. The first part of the study has looked at various digestion processes and how they affect the skin formation phenomenon. The results showed that temperature played a significant role in determining the amount of polymer that adhered to the filter media. The second part of the study focused on different kinds of digestion processes and their effects on extracellular polymeric substances, bound water content and dewatering. Bound water tests were used to determine the maximum achievable solids concentration on dewatering. Bound water content of solids obtained from field centrifuges run at different torques and g values were evaluated and fitted on a standard graph obtained from lab pressed sludge with different solids concentration. The bound water was seen to decrease with increasing solids content till 20%, after which a nearly constant 1.0 g of bound water was present for every gram of dry solids seen. The results indicate that nearly 50% solids concentration could be achieved on mechanical dewatering. In reality, only 30 to 35% solids concentration was attained both in the lab and on the field. It was determined that dilatometry attributed the increase in cake solids to the decrease in bound water. However, the use of bound water as a predictive tool for determining cake solids was not practical since the bound water calculations use the solids content in the calculations.
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