Impact Of Disinfectant Enhanced Backwash On Biodegradation And Bacteria In Biological Activated Carbon Filter

Biological activated carbon filter(BAC filter),which combines the functions of filtration,activated carbon adsorption and microbial degradation,was able to effectively remove various pollutants.As a result,it had been widely applied in the drinking water treatment plants.Disinfectant enhanced backwash was an important operation to maintain the performance of BAC filter since it could effectively eliminate the over-growth of microorganisms and micro-invertebrates.However,the combined influence of backwash and disinfectant on physiological activity of microorganisms and the ability of biodegrading organic substances was still not clear.Moreover,the influence of disinfectant enhanced backwash on antibiotic resistance genes in BAC fitlers remained unknown.Therefore,this study focused on the impact of disinfectant enhanced backwash on the degradation effects,the physiological activities of microorganisms and the safety issues of BAC filters,which could lead to a better understanding of backwash mechanisms and further lay a foundation for optimaizing backwash conditions.Due to lacking accuracy or sensitivity of commonly used indicator chemicals,the influence of disinfectant-enhanced backwash on BAC filter performance cannot be evaluated promptly.After pre-screening,three chloroacetic acids(CAAs)with variable degradation rates due to the competition of dehalogenase were selected to build up a detection system.Three CAAs was proved to have both high detection accuracy and desirable sensitivity,hence was applicable for evaluating the influence of disinfectantenhanced backwash on organic substances with different biodegradable characteristics.Results showed that the removal efficiencies of monochloroacetic acid and dichloroacetic acid that represented degradable organic substances,were less affected by the backwash.The negative effects only occurred on the very surface of BAC filter,and could be fully eliminated along the filter bed.However,the removal efficiencies of trichloroacetic acid that represented organic substances which degraded by secondary matrix mechanism,decreased significantly with the increase of disinfectant concentration.This proved that trichloroacetic acid should to be used as a key monitoring index to measure the degradation effect of organic substances.Thereafter,the physiological changes of microorganisms adhering to carbon particles during the backwash processes with varied concentrations of disinfectants were studied and quantitatively determined using the combined methods of flow cytometry and adenosine triphosphate assay.When the disinfectant concentration was low,there was nearly no apparent lethal structural change of the cell membrane of microorganisms,however the leakage of intracellular ATP was observed since the cell membrane permeability was changed,which further reduced the overall cell activity to a certain extent.When the concentration of free chlorine and chloramine reached 3.0 mg/L,the destruction rates of microbial cell membrane(mortality rates)reached 13.2% and 3.1%,respectively,indicating that a high concentration of disinfectant assisted backwash would have irreversible effects on microorganisms,while chloramine had relatively milder effects than free chlorine.Moreover,backwash process could confine the concertrantion of antibiotic resistance genes in BAC filter by peeling off more microbial cells.The absolute concentration of bla-TEM and cat genes decreased by 0.2-0.4 log after backwash.In addition,no ARGs accumulation was found in BAC fitler after 10 consecutive backwash experiments assisted with two disinfectants.As a result,disinfectant enhanced bacwash was effective and secure for mangament ARGs in BAC fitlers.