Efficient removal mechanism of an electrical conductivity-enhanced constructed wetlands under particle accumulated conditions

Abstract

The performance of constructed wetland (CWs) systems should improve through the electrochemistry of microorganisms, with the help of conductive fillers. To explore the strengthening mechanism of the CWs, three comparative experiments, including Co CWs (electro-conductive materials coke as filler), Co-P CWs (coke as filler with pickerel weed (Pontederia cordata L)), and QS-P CWs (quartz sand as filler with pickerel weed (Pontederia cordata L)), were tested to investigate the influence of the clogging process on the purification capacity from the electrochemical characteristics and the community of microorganisms in domestic sewage. The results showed that coke, as an electro-conductive filler, had the best organic removal and high electric output, even with solid particle accumulation. Co-P CWs were the most stable and had the best removal efficiency of COD (chemical oxygen demand) in urban sewage, with an average removal rate of 74.9% at 7 weeks, which was 14 and 7.3% higher than those of the QS-P CWs and Co CWs, respectively. The power generation performance of the Co-P CWs was also the best. During operation, the voltage remained the highest (377.9–222.0 MV). The microbial community diversity, richness, and utilization rate of the six categories of carbon sources of Co-P CWs were the highest, with the dominant genera being organic matter aerobic degrading bacteria, electrochemically active bacteria, and ammonia-oxidizing bacteria. The microbial richness in coke CWs exhibited high microbial activity in the entire bed, not only at the inlet. This indicates that the system transfers electrons to the conductive material by relying on the metabolism of microorganisms; electrons generated at the bottom of the bed are directly transmitted to the bed surface through the conductive material, resulting in a uniform distribution of electrons throughout the bed and high microbial activity. Therefore, the conductivity-enhanced CWs effectively alleviated the reduction in wetland purification capacity.