Coagulant toxicity and effectiveness in a slaughterhouse wastewater treatment plant

Attempts were made in this study to examine the toxicity of polymer/alum addition to the aeration tank effluent prior to sludge flotation as practiced in a slaughterhouse wastewater treatment plant. Based on the Microtox toxicity assay, alum at concentrations 100–200 mg/L was found to slightly increase the toxicity level of slaughterhouse wastewater effluent. However, at higher concentrations (300–1000 mg/L), significant residual chronic toxicity remained in all slaughterhouse wastewater effluents, independent of the treatment process. Polymer, on the other hand, removed organics and solids, but polymer effluents are more toxic than alum at extremely low concentration. Results indicated that alum and polymer caused inhibitory effects to the system at soluble concentrations of approximately 400 and 60 mg/L and above, respectively. The data also indicated that the solids collected in both tests (polymer/alum) were much more toxic than those from the effluents. Sediment samples from the polymer tests were the most toxic. Furthermore, the effluent toxicity of the coagulants was dramatically more significant when used after the settlement of solids than when used in mixed liquor. In addition, strong correlations were observed between the observed toxicity for a series of supernatants and the coagulant concentrations of alum/polymer processes, and between supernatants and solids collected in both tests.