Energy Use Considerations in the Activated Sludge Process in Los Angeles, U.S.A.

The City of Los Angeles has four secondary-tertiary treatment plants receiving wastewater flows of 1,590,000, 151,000, 114,000 and 75,000 m³/d. Operational difficulties with fine bubble air diffuser combined with low energy costs had resulted in the use of 4–5% efficiency diffusers as standard design. Increasing energy costs made revaluation of higher efficiency diffusers necessary. Long term treatment plant tests of ceramic domes, fused silica tubes and jet diffusers indicated that long term 8–10% air diffusion efficiency was possible. Energy savings of the order of 40% could be achieved by such diffuser retrofit. The type of diffuser to use would depend upon ambient air and waste characteristics, tank design and the sizing of existing equipment such as blowers.

Data from respirometer studies in operating plants indicated a further 10–30% decrease in energy use could arise from computer control leading to an equalized F/M ration throughout each day. Further tests of this possibility are planned.