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Effect of Air-Assisted Backwashing on the Performance of an Anaerobic Fixed-Bed Bioreactor that Simultaneously Removes Nitrate and Arsenic From Drinking Water Sources

Effect of Air-Assisted Backwashing on the Performance of an Anaerobic Fixed-Bed Bioreactor that Simultaneously Removes Nitrate and Arsenic From Drinking Water Sources

G. Upadhyaya, T.M. Clancy, K.V. Snyder, J. Brown, K.F. Hayes, and L. Raskin
Published In: 
Water Research,  
March 2012

Contaminant removal from drinking water sources under reducing conditions conducive for the growth of denitrifying, arsenate reducing, and sulfate reducing microbes using a fixed-bed bioreactor may require oxygen-free gas (e.g., N2 gas) during backwashing. However, the use of air-assisted backwashing has practical advantages, including simpler operation, improved safety, and lower cost. A study was conducted to evaluate whether replacing N2 gas with air during backwashing would impact performance in a nitrate and arsenic removing anaerobic bioreactor system that consisted of two biologically active carbon reactors in series. Gas-assisted backwashing, comprised of 2 min of gas injection to fluidize the bed and dislodge biomass and solid phase products, was performed in the first reactor (reactor A) every two days. The second reactor (reactor B) was subjected to N2 gas assisted backwashing every 3-4 months. Complete removal of 50 mg/L NO3- was achieved in reactor A before and after the switch from N2-assisted backwashing (NAB) to air-assisted backwashing (AAB). Substantial sulfate removal was achieved with both backwashing strategies. Prolonged practice of AAB (more than two months), however, diminished sulfate reduction in reactor B somewhat. Arsenic removal in reactor A was impacted slightly by long-term use of AAB, but arsenic removals achieved by the entire system during NAB and AAB periods were not significantly different ( p > 0.05) and arsenic concentrations were reduced from approximately 200 µg/L to below 20 µg/L. These results indicate that AAB can be implemented in anaerobic nitrate and arsenic removal systems.

Figure 1
Line diagram of the reactor system with photographs of reactors A and B. Inf, EA, and EB represent sampling locations for influent, effluent from reactor A, and effluent from reactor B, respectively. A5-A8 and B1-B4 represent the sampling ports along the depth of reactors A and B, respectively.

Upadhyaya, G., Clancy, T.M., Snyder, K.V., Brown, J., Hayes, K.F. and Raskin, L.”Effect of Air-Assisted Backwashing on the Performance of an Anaerobic Fixed-Bed Bioreactor that Simultaneously Removes Nitrate and Arsenic From Drinking Water Sources,” Water Research, 46(4)1309-1317, 2012.