BALANCING INDUSTRIAL GROWTH AND WATER QUALITY IN THE SONORAN DESERT: LOCAL LIMITS EVALUATION IN CENTRAL ARIZONA

The Sub Regional Operating Group (SROG) has conducted these local limits evaluations for over fourteen years to control the wastewater discharges from over one hundred fifty significant industrial users (SIUs) to the 91st Avenue and 23rd Avenue Wastewater Treatment Plants (WWTPs). The 91st Avenue and 23rd Avenue WWTPs are secondary and tertiary treatment facilities, respectively, that employ the nitrification/denitrification (NdeN) activated sludge process and anaerobic digestion to treat municipal and industrial wastewater generated in the metropolitan Phoenix area by the SROG partners of Glendale, Mesa, Phoenix, Scottsdale and Tempe.The purpose of the local limits is to control industrial discharges of pollutants that pose risks to the WWTPs. The WWTPs can accept limited loadings of some pollutants with minimal risk of pass-through or interference. These loadings, which can be determined objectively, are known as maximum allowable headworks loadings (MAHLs). The MAHL for each pollutant is sitespecific, depending upon WWTP flow and processes, the ability of the WWTP to remove the pollutant, and the regulatory requirements (such as permit limits) that the WWTP must meet.Re-evaluation of the current local limits concluded with updating, maintaining, and/or removing some of the existing limits, or recommending the future evaluation of best management practices (BMPs). An important technical challenge was to develop the grounds for elimination of the limits. The development and application of computer models, methodologies, and decision matrices contributed significantly to the progress and success of this study, and provided the benefit of consistent, repeatable and defensible results.