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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.

use of the diluted sludge volume index (OS VI) in mUg, based on a diluted 30-minute settling test (DSV30) in percent (mUdL), in which the sample is diluted to give a settled volume of less than 30%. The settling velocity of an undiluted sample was correlated with the diluted 30-minute settled sludge volume in an unstirred I-L graduated cylinder. For return sludge, the OSV30 was determined as DSVI X C, where C is the solids concentration in percent.At the time of the report by Garrett et al. (1984), principal models used to relate the settling velocity to solids concentration were the power model used by Yoshioka et al. ( 1957) and Dick and Young (1972) and the exponential model used by Vesilind (1968). These models, respectively, are as follows:ABSTRACT: Aspects of operation and design for stonn flows at a 190-m 3 /h (1.2-mgd) dry weather design flow activated-sludge plant were tested. A significant plant enlargement allowed recycle of test clarifier effluent to the main plant versus discharge to a stream. Thus it was possible to test high overflow rates without risk of a permit violation.On-line monitoring showed that the return sludge concentration increase lagged approximately I hour behind the start of the storm flow. This required an adjustment of the power equation model used. Furthermore, when the overflow rate exceeded the settling velocity of the feed solids, the sludge blanket rose faster than the model prediction. This phenomenon required developing a new model to predict blanket behavior under various loading conditions. The new model allows for plant design to more accurately include a clarifier zone for storing solids during peak flows associated with rain stornlS. An exponential equation, fitted to settling data, was also used in the new model with test data for comparison. The exponential equation gave a low blanket prediction. Results are compared with those of other research for solids storage during peak flow. Waler Environ. Res., 71,432 (1999).KEYWORDS: activated-sludge clarifier, diluted sludge volume index, floc volume flux, sedimentation model, sludge blanket model, storm flow. and Where Vs Vs = a X Xi-" at X e-n'XXi (I) (2)

A historical overview is presented of instrumentation control and automation as applied in US wastewater treatment plants. A description is given how computer equipment has progressed through direct digital control, distributed control, programmable logic controllers, to the use of personal computers along with PLC's or I/O boards in minimum systems. Software developments have brought systems analysis, expert systems, fuzzy logic control, and artificial neural network control. An instrumentation test house has been organized in the US to improve the odds of purchasing reliable instruments. Technical Conferences on ICA are presented regularly by the American Water Works Association and the Water Environment Association. There is still need for designers to consider a complete system that includes a data base for automatic preparation of reports on the plant for use at other levels as was called for by delegates to the early workshops.

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