DESIGN FLEXIBILITY KEY TO ADDRESSING BNR OPERATIONAL CHALLENGES AT A REGIONAL WATER RECLAMATION FACILITY

The Greenfield Water Reclamation Plant (GWRP) was designed to reliably produce the State of Arizona's highest classification of reclaimed water (Class A+) with variable design capacities ranging from 16 mgd [700 L/s] annual average day flow (AADF) to 48 mgd [2,100 L/s] peak hour conditions. The plant is also a regional solids handling facility designed to produce Class B biosolids, with 24-mgd [1,050 L/s] AADF equivalent of solids treatment capacity (based on 16 mgd [700 L/s] from the GWRP and 8 mgd [350 L/s] from another WRP).The lack of a good historical influent wastewater quality data, along with concerns relating to the high nutrient loadings expected in the solids recycle streams, posed several challenges during design. The paramount concern and focus was to design a facility capable of handling a variety of influent and recycle load scenarios, without merely over-designing the facility that would result in a high construction cost. The outcome was a highly flexible design that provided options for handling a wide variety of scenarios, without over-committing on any one scenario, and without a large sunk investment in construction cost. The capabilities of this design were severely tested during the facility startup beginning in the fall of 2006. Influent nitrogen loads that were higher than design assumptions stressed the secondary treatment process and its ability to meet the stringent 10 mg/L Total Nitrogen (TN) limit.This paper will present the details of how the design flexibility was utilized to overcome the different operational challenges posed at the startup of the facility, and how this flexibility will continue to be key throughout the life of the facility. While the case study focuses on the results from one specific facility, the lessons learned are applicable to any biological nutrient removal (BNR) plant. They provide a good example of how the process flexibility to address a wide range of scenarios can be incorporated into any BNR design at very little initial cost, but with significant operational value.