Chemical Clarification of Secondary Effluents for Organics and Phosphorus Removal

Jar 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; Superfloc A-100 (anionic), Superfloc N-100 (cationic), and Nalcolyte-8100 (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/1 and 6.4 mg/1, respectively. The lowest COD level obtained in jar tests using any of the chemical combinations was 40 mg/1. All coagulants, with the exception of lime, were able to lower total-P to below 1 mg/1 without the need for polymers. Lime, however, required an additional 0.4 mg/1 of polymer A-100 for proper total-P removal. The selected optimum dosage for lime, alum, and caustic soda was 250 mg/1, whereas, it was 70 mg/1 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