Optimization of alum-coagulation/flocculation for COD and TSS removal from five municipal wastewater

In this study, the coagulation process is evaluated in treatment of municipal wastewater on the basis of organic material (e.g. chemical oxygen demand, COD) and suspended solids (TSS) removal efficiency. Alum-coagulation was optimized on the samples (24 sampling campaigns) taken from 4 wastewater treatment plants and a pilot plant at the University laboratory (Naples, Italy) to meet the Italian water quality discharge limits. A series of jar test experiments was run at 100 rpm for 1 min, 30 rpm for 20 min and 30 min for settling. 150 mg/l and 450 mg/l doses of alum were applied at pH ranging from 4 to 10 and room temperature. An anionic polyelectrolyte was used for flocculation. Raw and coagulated wastewater samples were analyzed for their COD, TSS and aluminium (RA) concentrations. The jar test experiments provided evidence that coagulation process could not provide sufficient COD removal efficiency in the Cuma and the University pilot plant wastewater even at an alum dose of 450 mg/l whereas the treatment with coagulation process using 150 mg/l alum in Nola and S. Giovanni plants was sufficient to meet COD (< 160 mg/l) and TSS (80 mg/l) limits. The highest COD removal (80%) was obtained at the range of 6.0-8.0 pH values in the Nola plant, whereas COD removal was lesser in the Marcianese wastewater although its initial COD value was in the same order with the Nola plant. COD removal of the university plant improved from 55 to 75-85% in parallel to TSS removal by pH increase (up to 8.0). The statistical analyses showed different correlation values/behaviour between COD and TSS removals in each plant due to wastewater origin, pH and applied alum dose. RA was found significantly related to pH of coagulation process. RA concentration increased at pH value < 5.0. These obtained results should contribute to, in particular, further RA studies dealing with the RA risk in the coagulated effluents. Moreover, this study evidenced once again that coagulation process can assure the limits of COD, TSS and RA for municipal wastewater treatment plants if the process is well optimized and operated.