This paper describes the development of a cost-effective modified primary treatment method which can significantly increase the removal of both soluble and colloidal COD without resorting to the use of any chemical coagulant. It involves recycling a small portion of the primary sludge and then activating it to enhance its biosorptive and flocculent properties through 30-min aeration. The aerated sludge is then brought into contact with raw sewage to induce rapid biosorption of soluble COD and flocculation of colloidal organic before the mixture is settled in a regular primary clarifier. In this treatment approach, the raw sewage is not subjected to aeration; thus the sludge aeration tank volume and the aeration energy requirements can be minimized. In fact, with 30-min sludge aeration, theyamount to only 1.25% and 8.5%, respectively, of those required in the conventional activated sludge system. Through this modified approach with a 15% sludge recycle rate and 30-min sludge aeration, the COD removal efficiency is 43.9% higher than that of the regular primary treatment. This would result in a significant improvement of the primary effluent quality, or achieve a substantial reduction of organic loading to the secondary biological system.

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