Fixed-bed biological test columns were used during the BIOBROx pilot testing at Magna Water District.
Click here to enlarge imageMagna Water Company, which serves potable water to approximately 28,000 customers in the northwest section of Salt Lake County, Utah, USA, is partially supplied by groundwater that’s impacted by perchlorate, arsenic, and total dissolved solids (TDS). A six-month pilot study conducted by Carollo Engineers in 1999-2000 showed that due to the presence of high silica and sulfate levels in the groundwater, electrodialysis reversal (EDR) would be the most feasible treatment alternative for Magna. EDR is a separation-based process in which an electrical potential drives charged species (e.g., perchlorate, arsenic, TDS) across ion-selective membranes, thereby removing them from the raw water.
Residuals Concerns
EDR generates a contaminant-concentrated residual stream that must be treated prior to disposal, as perchlorate cannot be discharged to the sewer. Historically, high-strength perchlorate-laden streams have been treated in dedicated biological reactors that convert perchlorate to innocuous chloride and oxygen.
Degradation kinetics of these systems, however, are generally slow (typical required contact times are ~24 hours), associated microbial communities are sensitive to changes in salinity, and high concentrations of a substrate (i.e., bacterial food) such as ethanol must be added to the system. In other words, these systems require large reactor volumes, lack robustness and exert high consumable costs.
Innovative Treatment
A novel approach to treating perchlorate-laden residuals streams, developed by Carollo Engineers, involves blending the residuals stream with municipal wastewater followed by treatment in a fixed-bed (FXB) bioreactor (Biodestruction of Blended Residual Oxidants or BIOBROx). The process eliminates perchlorate (or any other oxidant such as nitrate, bromate, or selenate) from the residuals stream prior to sewer discharge.
In effect, blending the residuals stream with municipal wastewater decreases the DO concentration and salinity of the untreated stream (i.e., increases degradation kinetics) and eliminates the need to add substrate or a bacterial seed to the bioreactor system. Resulting reactor volumes and consumable costs are minimized and process stability improves. Based on this treatment concept, Carollo submitted a patent application for the BIOBROx process, which is currently pending.
Bench-Scale Testing
As part of AwwaRF Project 2859 (Innovative Alternatives to Minimize Arsenic, Nitrate, and Perchlorate Residuals), a three-month bench-scale study was performed to investigate the feasibility of the above treatment concept. Perchlorate-spiked wastewater from the Magna Water District was used to biologically acclimate the activated carbon in a 2-inch diameter FXB bioreactor.
Seeded only with microorganisms indigenous to the raw wastewater, the bioreactor demonstrated efficient perchlorate removal while using only background organics as substrate. Perchlorate removal to below detection (8 mg/L) was achieved and sustained at empty bed contact times (EBCTs) as low as 30 minutes.
While these data were promising, additional research was required to evaluate process performance when a perchlorate-laden residual stream was blended with the wastewater prior to treatment in the fixed-bed bioreactor.
Pilot Testing
The U.S. Environmental Protection Agency funded a seven-month pilot study at Magna’s Barton Well Field in 2004-2005 to demonstrate the efficacy of the BIOBROx process for removing perchlorate from an EDR concentrate stream. An EDR pilot plant was operated to remove perchlorate, arsenic, and TDS from groundwater, and the resulting concentrate stream was blended with municipal wastewater and treated in a fixed-bed bioreactor.
