Process Alternative Comparisons Assisted with Biowin Modeling

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ABSTRACT
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.

INTRODUCTION
With a population of approximately 55,000, the City of Avondale is one of the fastest growing communities in the Phoenix metropolitan area. 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 their wastewater treatment and reclaimed water facilities. The treatment plant is currently allowed for an annual average daily (AAD) domestic wastewater flow of 6.4 mgd per the draft Arizona Pollutant Discharge Elimination System Permit (AZPDES), which was issued in February 2004. However, 35%, 32%, and 44% increases in average daily maximum month (ADMM) influent BOD, TSS, and TKN concentrations have been observed in years 2000 to 2004, compared to original design values. This reduces the rated capacity of the plant decreases to 4.5 mgd on ADMM basis. At the end of the planning period in year 2030, the population is projected to grow to 161,400, requiring plant’s capacity to increase to 15 mgd ADMM flow.

Currently the liquid stream process of the plant employs Nitrification/Denitrification, which consists of Aeration Basins (ABs) followed by Secondary Clarifiers (SCs) to treat the screened influent after the grit removal. On the solids handling side, the Wasted Activated Sludge (WAS) is thickened with Dissolved Air Flotation (DAF), aerobically digested, and dewatered with centrifuges. The treatment goals of the expansion is to produce effluent that meets Class A+ standards and has maximum reuse potentials, as defined in the Arizona Administrative Code R18-11-303 and applicable BADCT requirements. Major effluent quality requirements include the limits of 10 mg/L total nitrogen (TN) and an average daily turbidity of 2 NTU, which will be achieved by adding tertiary filters. The plant also decided to change from aerobic digestion, the existing sludge stabilization technique, to anaerobic digestion. The latter has shown to be more economical at the plant’s buildout size, using significantly less energy compared to the aeration demand of the aerobic digesters and producing methane gas which can be converted to energy.

During the master planning, various liquid stream process alternatives to meet the capacity expansion and effluent quality improvement goals were evaluated. The components involved in the expansion include the addition of influent Equalization Basins (EQ basins) and Primary Clarifiers (PCs), expansion of Aeration Basins (ABs) and Secondary Clarifiers (SCs), and converting the existing conventional Activated Sludge (AS) process to Membrane Bioreactors (MBRs). Both the liquid and solids processes were modeled using BioWin version 1.2.1, one of the AS process simulator commonly used in the consulting industry. The model is based on the industry standard for AS modeling, IWA (International Water Association) series of Activated Sludge Models (ASMs). The modeling results were used to size the secondary treatment for various alternatives, and thus provide bases for cost comparisons and the selection of liquid stream process to be used in the expansion.

This paper presents the model calibration and simulation results of various process alternatives.

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