Density and Strength of Deposits Formed During In-line Flocculation Filtration of Secondary Effluents

Abstract

The purpose of this work was to evaluate filtration efficiency by deposit characterization, applying various pretreatments during in-line flocculation filtration of secondary effluents. Shallow bed laboratory filtration columns were used. Attachment and detachment constants and deposit densities (on the basis of volume and mass of accumulated matter) of aggregates formed by effluents treated with alum, alum-anionic polymer or different cationic polymers as primary coagulants during in-line filtration, were calculated. The floc size-density relationship coefficients “a” and “b” for polydispersed primary particles were calculated and compared with some other already published coefficients which are mainly based on homogeneous particles. The calculated values for the different “a” coefficients were around 1.0. The values for the “b” coefficients were (1.15–1.45). Based on the calculated deposit densities and attachment constants, and results obtained for turbidity and TSS removal efficiencies, it was shown that filtration without any treatment caused weak deposits. 10–20 mg/l alum doses, at natural pH values of the effluents used (7.3–8.5), produced deposits of relatively low attachment strength. The deposit density for the 10 mg/l dose was higher than that for the 20 mg/l dose. High molecular weight low anionic polymers strengthened the alum-particle bond. Medium cationic, high molecular weight polymers at 0.5 mg/l and high cationic medium molecular weight polymers at 7 mg/l performed equally or better than alum as primary flocculants and formed strong deposits with low deposit densities. High cationic low molecular weight polymers were not effective at doses up to 7 mg/l probably because of the low contact time.