Control of sludge settleability based on organic load and ammonia nitrogen load under low dissolved oxygen

Controlling dissolved oxygen (DO) at low level can save energy for wastewater treatment plants (WWTPs), but it is easy to induce filamentous sludge bulking. Through establishing the kinetic equation of sludge settleability, ammonia nitrogen (NH₄ ⁺-N) load and organic load (food-to-microbe ratio, F/M), the mechanism of the competitive relationship between filamentous and floccular bacteria under low DO was analyzed. The results showed when DO, NH₄ ⁺-N load and F/M were in the range of 0.15-0.35 mg/L, 0.035-0.15 d^-1 and 0.12-0.42 d^-1, respectively, the mass transfer limitation of organic matter was the main factor determining the dominant growth of filamentous bacteria. When DO, NH₄ ⁺-N load and F/M were in the range of 0.35-0.65 mg/L, 0.035-0.065 d^-1 and 0.12-0.22 d^-1, respectively, the mass transfer limitation of NH₄ ⁺-N was the main factor determining the dominant growth of filamentous bacteria. When DO was low, no matter how NH₄ ⁺-N load and F/M changed, the growth of filamentous bacteria was promoted. When DO and F/M were in the range of 0.35-0.65 mg/L and 0.22-0.42 d^-1, respectively, no matter how NH₄ ⁺-N load and F/M changed, the growth of filamentous bacteria was inhibited. Therefore, in actual operation, ensuring relatively low DO and high F/M was beneficial for the sludge settleability improvement.