Disinfection of faecal indicator organisms by iron electrocoagulation for reuse of sewage water

Abstract

Due to the quick growth in human population and subsequently a high rate of urbanization, fresh water sources are under pressure. Estimated 1.8 billion people drink water from potentially sewage-contaminated sources and also the presence of antibiotic resistant bacteria in sewage has made the improper treatment of wastewater effluent an emerging problem. Iron electrocoagulation (Fe-EC) has shown to be successful in reducing pathogen concentration, although to different extent for types of pathogens. Besides, the water quality was found to be of influence on the effectiveness. This study therefore aimed to determine the removal mechanism for both bacteria and virus as well as assess the effect of wastewater components on the Fe-EC treatment process. EC experiments were conducted in an aerated beaker with a volume of 1L, continuously stirred. pH and current were kept stable at 7.5 and 200mA, respectively. The total charge dosage was 180 C/L. After electrolysis the samples were left to settle. Blank measurements were conducted in a 230 mg/L NaCl solution. Wastewater components were added in concentration of 4.0 mg/L for phosphate, 60 mg/L calcium, and 10 mg/L humic acids. E. coli WR1 and 휙X174 were used as indicator organisms. Contributing disinfection mechanisms for E. coli were inactivation and physical removal. During electrolysis a log removal of 3.8 log units was found, ascribed to inactivation. Further increase in removal to 6.0 log units was observed after settling. In the presence of TEMPOL only 0.3 log units removal were observed during electrolysis, but removal after settling still amounted 5.5 log units. For 휙X174 removal reactive species were not effective. Disinfection after settling equaled 4 log units, when settling was complete. Settling was found to be dependent on mixing conditions, and removal was correlated as well. Extended mixing promoted the formation of larger flocs and thereby enabled sweep flocculation for phage removal. Phosphate decreased attenuation of E. coli either for inactivation, 2.0 log units as for physical removal, 4.7 log units. Phosphate has high affinity for iron and competes with bacteria for iron surface. Calcium as well inhibited abatement of E. coli. Inactivation only amounted for 1.4 log units and removal after settling equaled 2.1 log units. Calcium is expected to complex bacteria surface, increasing repulsive forces between E. coli and iron. The effect of carbonate was only seen during electrolysis, 1.3 log units. The formation of the carbonate radical is suggested as explanation for the decrease in effectiveness. The effect of NOM on E. coli attenuation was unclear. Inactivation was inhibited to only 1.4 log units, but no effect was found on the total removal. Therefore, it can be concluded that NOM does not successfully compete with bacteria for iron surface. 휙X174 attenuation was not effected by addition of phosphate, calcium, or carbonate. On the other hand, the addition of NOM inhibited phage removal, only 0.6 log units in comparison to 4.0 log units for the blank. Adsorption of NOM onto iron surface is expected to negatively change the surface charge, inhibiting association between iron and virus.