The Impact of Residual Coagulant on the Respiration Rate and Sludge Characteristics of an Activated Microbial Biomass

Abstract

Pre-precipitation is widely used in wastewater treatment to increase the removal of suspended solids (SS) and chemical oxygen demand (COD) in the primary sedimentation process. During pre-precipitation, a portion of the coagulant is removed from the treatment system in the primary settled sludge. However a residual amount will remain in the supernatant which is then treated by the secondary, biological treatment process. The effect of direct coagulant addition to the secondary biological treatment stage, known as simultaneous precipitation, is well documented but the effect of residual coagulant on secondary biological treatment as a result of pre-precipitation has received little attention. Jar tests were optimized with respect to SS, COD and phosphorus (P) removal at influent pH—between pH 7–8. Optimum dose was found to be 25 mgl −1 as metal ion for both alum and ferric sulphate coagulants. The residual metal concentration in the primary settled wastewater from the jar test experiments was identified as being 1.94 and 1.98 mg Fel −1 for ferric sulphate coagulants and 0.90 mg All −1 1 for alum. Primary effluent was fed to a settled activated microbial biomass to give total solids concentration of 2.0–2.5 g1 −1 . The impact of residual metal concentration on (i) respiration rate and COD removal and (ii) species diversity and floc structure were investigated. This paper presents the results of a laboratory-scale screening experiment undertaken to consider the impact of residual iron and aluminium coagulants on the secondary treatment process. Results identify a link between low level concentrations of coagulant, and reduction in microbial activity, changes in species diversity and floc morphology highlighting the importance of a bench/pilot-scale study.