Managing Risk for Cost-Effective Design – Alternative Process Design for Annual Loads

With capital budgets exerting ever-greater pressure on the rate structures, wastewater treatment utilities are looking for more cost-efficient designs and expect their engineers to re-evaluate traditional approaches to design. In traditional approaches, the design effluent quality may be set at or below the permitted concentration and the design is completed using conservative assumptions and typically incorporates the use of safety factors. This approach may minimize the risk of the plant not performing as intended, but the costs can be significant and many utilities are searching for more cost-effective approaches. The advent of annual loading permits for nitrogen and phosphorus has provided an opportunity for alternative design approaches, but also introduces various challenges. Conservative design approaches may be costly, but more aggressive design approaches result in greater uncertainty and risk. Designers need tools that will allow them to address these risks.This paper discusses some of the issues that confront designers and proposes an approach that directly evaluates risk and provides utility managers and engineers with the tools to make an informed decision about risk vs. cost.