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Deinking and kraft mill sludge dewatering using a laboratory sludge process Zhao, Hongmei

Abstract

A laboratory sludge press was used to investigate the dewatering of various wastewater treatment sludges generated in deinking mills and kraft pulp: mills. The effects of pressure applied to the sludge, pressing time and temperature were studied in terms of their effects on sludge cake solids consistency, filtrate flowrate and filtrate suspended solids (TSS). The effects of various combinations of primary and secondary sludge were tested. Secondary sludge alone could not be dewatered in the press. Addition of primary sludge made pressing possible and increasing the ratio of primary to secondary sludge improved performance. Sawdust and hog fuel were shown to be effective filter aids. The use of single polymer flocculants was compared to dual polymer coagulant/flocculant additions for various sludges. The use of a coagulant in a dual polymer system didn't improve dewatering compared to using a single polymer flocculant in term of cake solids consistency and filtrate flowrate although it did reduce the total amount of flocculant necessary. But a dual polymer system did reduce filtrate TSS. FeCl₃ and an inorganic polymer polyaluminum chloride (PAC) could be used to replace organic coagulants. FeCl₃ is cheaper than organic coagulants, but PAC is more expensive. Measurement of sludge particle charge was shown to be useful in detecting polymer overdosing. The laboratory sludge press was shown to be an effective tool for evaluating the use of polymeric conditioners in sludge dewatering. This press can distinguish the two mechanisms by which sludge dewatering occurs in practice, filtration and cake compression.

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