Abstract
In 1981, the City of Houston's Southwest Wastewater Treatment Plant (144,000 m3/d) was experiencing occasional afternoon bulking with loss of solids to the effluent. The plant employs sludge reaeration with about even allocation of tank volume to mixed liquor and return sludge aeration. A dynamic model of solids transfer from influent to mixed liquor to settling tank to return sludge aeration and wasting was developed for use on a desk-top computer. The model predicted that a counter-intuitive reduction in the return sludge rate would redistribute the solids to less in the mixed liquor and more in the sludge reaeration tanks, but would also generate a sludge blanket during the transition period. However, if the change were made when there would be several low-flow days, then the blanket would stay within the settling tanks. The Thanksgiving Holidays of 1981 were chosen for the change in the return sludge rate. Low flow occurred as expected and there was no loss of solids. The effluent suspended solids improved as expected, but the SVI also improved from 172 ml/g to 72 ml/g. The reduction in the return sludge flow rate allowed extra hydraulic capacity for storm flows. The improved control of solids was demonstrated by simulating a storm flow of 4.0 m/h overflow rate by taking settling tanks out of service with resulting effluent TSS of less than 20 mg/L.
Published Version
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