Effects of Prolonged Shutdown on the Performance and Microbial Activity of Intermittently Operated Biofilters for Manganese Removal from Groundwater: Pilot Scale Study

Apart from aesthetic concerns, manganese (Mn) in drinking water has been associated with cognitive and neurobehavioral effects in children. Recently Health Canada has changed its drinking water guidelines for Mn from an aesthetic parameter to a regulated parameter with a maximum acceptable concentration (MAC) of 120 μg/L, an aesthetic objective (AO) of 20 μg/L, and a treatment objective limit of 15 μg/L. While biofiltration has proved to be an effective treatment option for Mn and iron (Fe), it is not widely used in North America. Also, in small-scale communities, treatment plants often adopt intermittent operation schemes, wherein, water treatment occurs for a few hours per day, based on demand. Among the various operational aspects that could potentially affect the performance of intermittently operated biofilters, prolonged filter shutdown remains a critical concern. However, limited insights exist regarding both the intermittent operation for Mn and Fe removal from groundwater and the impact of a prolonged shutdown. This study aimed to fill these knowledge gaps by comparing the performance of intermittently and continuously operated biofilters for Mn and Fe removal from groundwater, and by investigating the impact or a prolonged shutdown of these filters. To achieve these objectives, three pilot-scale biofilters were located at a drinking water treatment facility in Southern Ontario, Canada, out of which two biofilters were operated continuously and one intermittently (6 h per day). The biofilters were initially operated for a couple of months, then underwent a six-month shutdown period, and then resumed active operation for three months. Sand was used as the biofilter media providing an empty bed contact time (EBCT) of 15.6 minutes. Raw groundwater containing Mn (31 μg/L) and Fe (96 μg/L) was fed to the biofilters and the effluent flow rate was maintained at 5 m/h. Mn and Fe removal was assessed before and after the extended shutdown in order to gauge its impact on biofilter performance. In addition, turbidity, pH, DO, oxidation-reduction potential (ORP) and total organic carbon (TOC), were periodically monitored. Microbiological changes were analyzed using AxP (adenosine triphosphate (ATP), adenosine diphosphate (ADP), adenosine monophosphate (AMP)) and other microbiological analyses. Lastly, the storage strategy (SS) utilized during an extended shutdown period might play a crucial role, therefore four different storage strategies were investigated. To briefly summarize the results, during the start-up phase, after 56 days, all biofilters achieved Mn removal to below 15 μg/L, with the intermittently operated biofilter exhibiting lower removal rates and not reaching a steady state, while Fe removal reached a steady state within a week for all biofilters. Next, during the six-month extended shutdown, a notable reduction in ATP occurred, but all biofilters seemed to have remained viable for 4 months, potentially due to the SS utilized. Also, it was found that manganese oxidizing bacteria (MnOB) were able to persist and actively oxidize Mn even after the six-month-long shutdown. Lastly, post-extended shutdown all biofilters approached the previous Mn removal rates within 20 days of restart and then reached a steady state within a month of operation. Notably, intermittently operated biofilter performed equally well as continuously operated biofilters. Also, Fe removal was not found to be affected by the extended shutdown. This suggests that the biofilters were able to quickly regain their performance even after a six-month extended shutdown highlighting the resilient nature of these biofilter systems. Overall, this study enhances the understanding of Mn removal in intermittently operated biofilters, and suggests ways to mitigate the effects of prolonged shutdowns, thereby expanding the applicability of these biofilters, especially to smaller communities.

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