SIDESTREAM TREATMENT VS. MAINSTREAM TREATMENT OF NUTRIENT RETURNS WHEN AIMING FOR LOW EFFLUENT NITROGEN AND PHOSPHORUS

Many wastewater treatment utilities are facing a common problem: the combination of decreasing discharge limits, increasing flows and loads, reduced budgets, and limited land for plant expansions. When planning an expansion of a facility with stringent permit limits (3 mg/L total nitrogen, TN; 0.3 mg/L total phosphorus, TP), it is important to create and make use of a dynamic mass balance that considers all nutrient loads to the plant, whether it be raw wastewater, hauled biosolids, or sidestreams generated during biosolids processing. Sidestream loads are often over-looked in plant design and operations, and when dewater occurs during the day shifts only, the impacts of sidestream loads are worsened because they are being recycled when the plant influent loads are the highest. Regionalization of biosolids processing is a current trend to reduce costs by processing biosolids from two or more facilities at one central facility, but this practice can generate high-strength sidestreams. As the facility imports biosolids, it also imports nutrients tied within the solids which are released during digestion and returned to the mainstream process in the dewatering sidestream. This can double or even triple the raw influent nutrient concentrations. In extreme cases, sidestream treatment may be warranted, but only after an evaluation has been conducted to show that constructing a sidestream treatment facility is more economical than handling the sidestream at the existing mainstream plant. Simulators such as BioWin (Envirosim) and GPS-X (Hydromantis) are powerful tools for generating plant-wide nutrient mass balances, and this paper will present examples of how they can be used to weigh two options: 1) managing sidestreams in the mainstream treatment processes, or 2) treating sidestreams before they are returned to the main plant.