Best management practice (BMP) programs for commercial and industrial dischargers of fluoride and molybdenum were considered and developed as an alternative to numeric industrial pretreatment limitations (local limits). This project, which encouraged the active participation of affected dischargers, was based on technical evaluations of the relative effectiveness of different control strategies. This paper describes case studies for fluoride and molybdenum and presents a step-by-step approach to developing and implementing non-numeric control strategies. The project was initiated in September 2004 and completed in July 2005.
The Sub Regional Operating Group (SROG) has been conducting local limits evaluations for over 14 years to control the wastewater discharges from more than 150 significant industrial users (SIUs) to the 91st Avenue and 23rd Avenue Wastewater Treatment Plants (WWTPs). The WWTPs treat municipal and industrial wastewater generated in the metropolitan Phoenix area by the SROG partners of Glendale, Mesa, Phoenix, Scottsdale, and Tempe, Arizona.
As part of a periodic local limits update in 2004, maximum allowable headworks loading (MAHL) calculations were performed to determine the maximum mass loadings that could be received at the WWTP headworks and not create pass-through or interference problems. The projected influent loadings were compared to the MAHLs. EPA recommends that limits be set for pollutants with average influent loadings exceeding 60% of their MAHLs and with maximum influent loadings exceeding 80% of their MAHLs. The project team determined that molybdenum exceeded United States Environmental Protection Agency (USEPA) criteria for acceptable influent loadings at the 23rd Avenue WWTP. Although fluoride did not exceed the criteria at either WWTP, the project team was also prompted to evaluate control options due to the potential for large increases in fluoride loadings from new or expanding semiconductor facilities. Neither pollutant was regulated by local pretreatment ordinances.
GOALS AND OBJECTIVES
The purpose of local limits is to control non-domestic discharges of pollutants that pose risks to the WWTPs. For a variety of reasons, numeric limitations cannot always meet pretreatment program objectives of technical validity, effectiveness, and enforceability. Molybdenum and fluoride posed a potential risk to the local WWTPs, but were not good candidates for numeric limitations. The objectives of this project were to identify and characterize the major sources of molybdenum and fluoride, develop non-numeric control strategies for the identified discharges, and define action plans to implement the BMPs.
BMPs include schedules of activities, prohibitions of practices, maintenance procedures, and other management practices to prevent or reduce pollution. BMPs can be more effective than numeric limits, when one or more of the following conditions exist:
- There is no current local limit for the pollutant.
- Non-industrial facilities or a small number of industrial facilities discharge significant loadings of the pollutant.
- The effectiveness of technologies to remove the pollutant is low.
Analytical and flow data from January 2003 to August 2004 were collected at domestic manholes, mixed commercial/domestic manholes, SIUs, and the WWTP influents. Statistical analyses were performed to determine representative concentrations at each sampling location. Population and per capita flow rate projections were used to estimate flow contributions from domestic and commercial sources and from discharges of water reclamation plant (WRP) residuals. Flow data collected at SIUs and the WWTPs were used to characterize industrial flow discharges and to confirm the total flows discharged to the WWTPs. Representative concentrations and flow estimates were used to calculate pollutant loadings and perform WWTP mass balances.