The City of Wichita is committed to investing in $300 million wastewater facility improvements in preparation for anticipated changes in effluent discharge permit requirements expected from the Kansas Department of Health and Environment (KDHE) by 2028. A significant component of the city’s investment is improvements to its largest water quality reclamation facility, the Lower Arkansas Water Quality Reclamation Facility, also known as “Plant 2.” The improvements include a new biological nutrient removal (BNR) treatment process that will reduce the nitrogen and phosphorus in the effluent to meet the expected National Pollutant Discharge Elimination System (NPDES) limits. The project also includes converting the plant to a single-sludgeactivated sludge process and the use of hydrocyclones, which is an innovative and new technology to improve sludge settleability and intensify treatment. The new BNR facility will treat up to 92 million gallons per day (mgd) of primary effluent (PE) and 45 mgd of return activated sludge (RAS) for a total capacity of 137 mgd between three process trains. The BNR treatment performance is dependent on equal flow distribution of both PE and RAS to the three BNR treatment trains. The Plant 2 improvements include three RAS pump stations that convey varying RAS quality that needs to be completely mixed before conveying a slipstream to the hydrocyclone facility, and then must be completely mixed with the PE (generating mixed liquor suspended solids, or MLSS) prior to distributing flow equally to the three BNR treatment trains. Loading individual treatment trains with excess flow and solids in one train and lower flows in a different train can result in poor treatment performance, potential permit violations, or increased chemical costs to make up for the imbalanced flow split. Computational fluid dynamics (CFD) modeling was used as a design tool to evaluate hydraulic performance with respect to mixing the three different RAS streams and mixing and flow distribution of PE and RAS. The initial design included a single structure where the PE and RAS combined and distributed flow over weirs to the BNR treatment trains. The CFD analysis revealed unacceptable mixing performance of the initial design (which relied on mixing and flow distribution in the same structure) and subsequently re-designed the layout to achieve RAS and PE/RAS mixing upstream of the flow distribution box to ensure equal distribution of PE and RAS to the three BNR process trains. Analysis of the long, straight RAS channel for mixing of RAS from the three aeration batteries also resulted in unacceptable results that dictated the addition of mixing baffles to optimize mixing performance before the RAS is conveyed to the hydrocyclone facility. The CFD results from this analysis yielded unexpected conclusions that contradicted conventional wisdom and engineering practice, demonstrating the value of CFD as a design tool. This paper will present the evolution of the design to optimize the RAS mixing, PE/RAS mixing and BNR flow distribution.
