Pilot Testing of a Bioreactor for Perchlorate- Contaminated Groundwater Treatment

Groundwater at the Crafton-Redlands site in Redlands, California is contaminated with perchlorate in addition to chlorinated volatile organic compounds (VOCs). VOCs are removed from extracted groundwater using granular activated carbon, which is ineffective for perchlorate removal. Side-by-side pilot-scale bioreactors for perchlorate removal from extracted groundwater were tested at the site. Preliminary results showed that perchlorate was consistently removed from groundwater to concentrations less than 4 μg/L. Removal was dependent on consistent acetate feed, effective backwash strategy, and sufficient hydraulic residence time. INTRODUCTION Recent detection of perchlorate in several surface waters and groundwater wells used to supply drinking water has created an unforeseen water contamination crisis in the western states, and problems are likely to emerge at other sites where perchlorate has been used. It is estimated that as many as 12 million people could be affected by perchlorate contamination of drinking water. In March of 1997, the California Department of Health Services (CDHS) developed a method that reduced the detection limit of perchlorate from 400 μg/L to 4 μg/L. Based on EPA work, the CDHS established an action level of 18 μg/L for drinking water. Subsequent monitoring of 232 groundwater wells by the CDHS indicated perchlorate was present in 69 wells (30%) and at concentrations above the action level in 20 wells (9%) (AWWARF, 1997). In October of 2001, the Texas Natural Resource Conservation Commission (TNRCC) set an interim action level of 4 μg/L indicative of more stringent regulatory requirements for perchlorate treatment. New evidence of adverse health effects of perchlorate at low concentrations has prompted California to lower its action level to 4 g/L as well. Perchlorate contamination arises primarily from the manufacture and disposal of ammonium perchlorate, a highly reduced compound produced for use as the oxidizer in solid rocket propellant. It is highly soluble and not easily removed from water. Conventional water treatment technologies such as air stripping and advanced oxidation processes are ineffective for perchlorate removal or destruction. The costs associated with carbon adsorption and ion exchange processes are very high. While perchlorate may be difficult to chemically treat or physically remove from water, it is very biodegradable. It is known that some microorganisms are able to respire using chlorate (ClO3) and/or perchlorate (ClO4): that is, they can use either of these compounds as a terminal electron acceptor in the oxidation of many common substrates such as acetate, simple sugars, and amino acids (Logan, 1998; Herman and Frankenberger, 1998). However, oxygen and nitrate are preferred over perchlorate as terminal electron acceptors and will be microbially reduced before perchlorate is removed (Logan, 2001). Previous work in laboratory columns has shown that perchlorate can be removed in packed columns containing the perchlorate-respiring bacterium KJ (Kim and Logan, 2001). This report describes pilot testing of this technology at the Texas Street Well Facility that formerly supplied drinking water in Redlands, California. Groundwater at this site (the Redlands-Crafton plume) is contaminated with perchlorate and chlorinated VOCs. While the VOCs were successfully removed from groundwater using granular activated carbon, removal of perchlorate was not observed. Detections of perchlorate resulted in the shutdown of the Texas Street Well Facility. METHODS A pilot-scale reactor containing side-by-side plasticand sand-media modules was constructed at the Texas Street Facility as shown in Figure 1. Also shown in Figure 1 is a close-up of the plastic media in the reactor. The reactors were up-flow packed-bed reactors containing sand or plastic media. The plastic media floated in water and was held down with a perforated plate. The cross-sectional area for flow was 2 ft (0.19 m) and the reactor height was 7 ft (2.1 m). Groundwater was pumped to an equalization tank and then acetic acid and ammonium phosphate were added to the reactor feed at concentrations of about 50 mg/L and about 4 mg-N/L, respectively. The initial groundwater flow was 1 gpm to each reactor. The reactors were started by bioaugmentation of the columns on May 9, 2001, with perchlorate-respiring strain KJ grown in batch on a mineral salts medium containing acetic acid. The media and microorganisms were recirculated in each reactor for 11 days and then groundwater flow was initiated at 1 gal/min (3.8 L/min) to each reactor on May 20, 2001 (Day 0). Backwashing with an air scour was conducted to remove excess microbial growth and to minimize short-circuiting or flow channeling. This procedure resulted in movement of the sand media but not complete fluidization. The plastic media were held down by the perforated plate and were not fluidized. Perchlorate, acetic acid, nitrate, dissolved oxygen, sulfate, oxidation-reduction potential (ORP), pH, specific conductivity, turbidity, phosphate, ammonia, and backpressure were measured in the reactor influent and effluent. Only perchlorate results are given here as part of this preliminary report. RESULTS AND DISCUSSION Table 1 shows the groundwater composition that was treated by the reactors. The groundwater was saturated with dissolved oxygen and contained 4.3 mg-N/L of nitrate on average. Reduction of dissolved oxygen and nitrate is required prior to use of perchlorate as a terminal electron acceptor. Perchlorate concentrations were about 75 μg/L in the influent, and the treatment goal was 4 μg/L in the reactor effluent. FIGURE 1. Pilot-Scale Reactor and Plastic Media Detail. TABLE 1. Redlands Groundwater Composition. Parameter Value Units Dissolved oxygen 8.9 mg/L Nitrate as nitrogen 4.3 mg-N/L