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Value Engineering Results in the Novel Reuse of Existing Infrastructure for a 150 MGD Plant Expansion

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The City of Indianapolis is currently implementing its Long Term Control Plan (LTCP) to mitigate the overflow impacts from its combined sewer system. One of the control measures in the LTCP requires the elimination of the Belmont Advanced Wastewater Treatment (AWT) Plant primary effluent bypass. This bypass contributes significant amount of pollutants to the receiving stream and is the result of insufficient secondary treatment capacity at the facility. Currently, the plant has a peak primary treatment capacity of 13.1 m3/s (300 mgd) and a peak secondary treatment capacity of only 6.6 m3/s (150 mgd). The existing secondary treatment system consists of a biological roughing system (BRS) followed by an oxygen nitrifying system (ONS). The City's approved LTCP stipulated that expansion of the plant's wet weather secondary treatment capacity to 13.1 m3/s was to provide treatment of the 6.6 m3/s wet weather flow through a Trickling Filter/Solids Contact process (TF/SC).

The project was to be completed in four phases. When the Phase 1 (site preparation) construction was near completion and the Phase 2 design was at 90 percent completion stage, the estimated construction cost for the project was found to exceed 150,000,000. With the project budget set at 90,000,000, a value engineering (VE) workshop was held to develop and evaluate alternative process flow schemes that would make the best use of the existing infrastructure at the facility to bring the cost of the project back in line with the original budget. In response to the VE team's recommendation, further refinement of a newly proposed contact stabilization air activated sludge system was undertaken. This consisted of a detailed review of the basis of design, an evaluation of the final clarifier capacity, a detailed hydraulic analysis, and various layout configurations to identify the most cost-effective alternative.

Full-scale stress tests conducted by the plant demonstrated that existing final clarifiers could handle flows up to 15.3 m3/s with minor modifications to the sludge removal mechanisms and effluent collection system as long as the mixed liquor suspended solids concentration in the aeration tanks was below 1,700 mg/L. This confirmed that no additional final clarifiers would be required, and the contact stabilization process should be utilized during peak wet weather flow conditions. Computational Fluid Dynamic (CFD) modeling concluded that the inlet baffles of the final clarifiers should be reconfigured to improve clarifier performance.

Based on a comparison of the advantages and disadvantages of the alternatives in terms of cost, schedule, provision for future treatment needs, regulatory compliance, and additional environmental benefits, a new air nitrification system (ANS) placed in series with the existing ONS was recommended as the best treatment scheme. In this scheme, the ANS system will consist of two parallel ANS tanks, each having four anoxic zones alternating with three aerobic zones. During dry weather the primary effluent will be fed to the four anoxic zones of each ANS tank in equal amounts, and return activated sludge (RAS) will be fed to the first anoxic zone of each ANS tank, a typical step-feed process. During wet weather, when flow exceeds 6.6 m3/s, the operation of the treatment system will be changed to the contact stabilization mode to enable the plant to handle a peak flow of up to 13.1 m3/s. In this operation mode, the primary effluent will be fed to the ONS aeration tanks and the RAS will continue to be fed to the ANS aeration tanks.

A dynamic model utilizing a commercial whole plant simulation package was utilized to validate the dynamic inventory management concept of the proposed treatment scheme and assess activated sludge process performance during wet weather events and the recovery period following wet weather events.

The value engineering efforts during the planning phase lead to cost-effective reuse of the existing infrastructure for the 6.6 m3/s wastewater treatment plant expansion project. Additional value engineering conducted during the design phase simplified the facility layout and further reduced the construction cost. The project, which has an estimated construction cost of 90 million, is currently under construction and the new facility will be fully operational by the end of 2012.
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Keywords: Air Nitrification System; Contact Stabilization; Secondary Treatment; Wastewater; Wet Weather

Document Type: Research Article

Publication date: 2010-01-01

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