Chemical clarification of secondary effluents for organics

Department of Civil Engineering, College of Engineering, King Saud University, P. O. Box 800, Riyadh 11421, Saudi ArabiaJar test experiments of chemical clarification for organics and phosphorus removal were performed on 24-hour composite trickling filter effluent samples from Riyadh sewage treatment plant (South plant). Tests were conducted using the major coagulants; lime, ferric chloride, alum, and caustic soda. Improvement of clarification was also tested using three polymers; SuperfIoc A-lOO (anionic), Superfioc N-I 00 (cationic), and Nalcolyte-81O0 (polycationic). Analysis of the results has indicated that the average COD and total-P of secondary trickling filter effluents during the study period were 100 mg/I and 6.4 mg/l, respectively. The lowest COD level obtained injar tests using any of the chemical combinations was 40 mg/I. All coagulants, with the exception of lime, were able to lower total-P to below I mg/I without the need for polymers. Lime, however, required an additional 0.4mg/l of polymer A-lOO for proper total-P removal. The selected optimum dosage for lime, alum, and caustic soda was 250 mg/I, whereas, it was 70 mg/I for ferric chloride. Using lime and polymer resulted in the lowest estimated chemical cost of 121 Saudi Riyals (US$ 32) for 1000 m3 of wastewater