The paper describes two test procedures for gaining insight into the optimum polymer dose for conditioning an alum sludge prior to dewatering. The first method centres on a development of theory used to estimate the specific resistance to filtration (SRF) using the standard Buchner funnel test. The second is an ad hoc test (termed the ‘CML30 test’) which provides a simple means for treatment works operatives to gauge the optimum dose; it focuses on a particular type of alum sludge and there are no claims concerning its wider utility. Standard filtration theory has been developed to take account of the hydraulic resistance of the filter medium arising from clogging phenomena in overdose conditions. The modified theory yields an adjusted filtration resistance, namely SRFadj, which refers solely to the filtration behaviour of the cake. The CML30 test is based on 30 min settlement in a 100 ml measuring cylinder. With increasing dose, there exists a minimum interfacial height at a dose which coincides with the minimum in the SRFadj trend; this is regarded as an optimum and shows proportionality with the solids concentration. Simplified theory is presented to explain the settling behaviour.

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