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Process Alternative Comparisons Assisted with Biowin Modeling

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Permitted at 6.4 mgd on an annual average day basis, the City of Avondale Wastewater Treatment Plant currently employs Nitrification/Denitrification process to produce effluent with maximum total nitrogen of 10 mg/L. The process consists of Aeration Basins followed by Secondary Clarifiers to treat the screened influent after the grit removal. On the solids handling side, the Wasted Activated Sludge is thickened with Dissolved Air Flotation, aerobically digested, and dewatered with centrifuges. As a result of the rapid growth and the City's desire to utilize reclaimed water as a renewable water supply, the City selected Damon S. Williams Associates, L.L.C. (DSWA) to prepare a master plan for the expansion of their wastewater treatment and reclaimed water facilities to 15 mgd on an average daily maximum month (ADMM) basis. The effluent will need to meet more strict turbidity requirements, for which tertiary filtration will be added. Meanwhile, a noticeable increase in influent strength compared with original ADMM design values had been observed in the years 2000 to 2004, namely 35%, 32%, and 44% increases to influent BOD, TSS, and TKN concentrations, respectively. This makes redefining the wastewater characteristics inherent to the master planning.

During the master planning, various liquid stream process alternatives. The components involved in the expansion include the addition of influent Equalization Basins and Primary Clarifiers, expansion of Aeration Basins and Secondary Clarifiers, and converting the existing conventional activated sludge process to membrane bioreactors. Anaerobic digestion was selected as the future solids stabilization process for being most economical for the plant's buildout capacity. Both the liquid and solids processes were modeled using BioWin version 1.2.1, a commonly used process simulator in the consulting industry. Steady state and dynamic modeling was conducted for each alternative. The modeling results were used to size the secondary treatment units, and thus provide a basis for cost comparisons and the selection of liquid stream process to be used in the expansion.

Plant historical records on the combined influent, which includes dewatering centrate, were evaluated to project the buildout loadings, concentrations, and peaking factors of the combined influent at various design conditions. Two weeks of intensive sampling were conducted to obtain supplemental influent characteristics as well as plant operating and performance data, averages of which were used to calibrate the Biowin model. The raw influent strength was determined by subtracting the loadings contributed by dewatering centrate, which was predicted by the calibrated Biowin models, from the projected combined influent loading.

Based on sizes required by achieve satisfactory model predictions of operating conditions and treatment performance, Alternative 4, which adds new influent Equalization Basins and Primary Clarifiers, would be the most cost effective expansion option. Influent equalization minimizes the size of the tertiary filters, chlorine contact tanks, and the effluent pump station and associated equipments, which are primarily sized for hydraulic peaks. Primary clarification reduces the organic loadings to Aeration Basins and thus the required expansion of aeration basins and aeration system. Meanwhile, feeding combined primary and secondary sludge to anaerobic digesters would improve the feasibility of the process and maximize the energy that could be recovered.
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Document Type: Research Article

Publication date: 2006-01-01

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