Efficient removal mechanism and microbial characteristics of tidal flow constructed wetland based on in-situ biochar regeneration (BR-TFCW) for rural gray water

• The tidal operation of BR-TFCW maintained the biochar’s adsorption capacity. • When F/R = 3:1, the BR-TFCW’s COD removal rate was 99.84 ± 0.21%. • The COD’s removal path and respective contribution rates were analyzed. • In the rest phase, biochar depicted a regeneration rate of 75.03 ± 1.00%. • The functional bacteria achieved biological regeneration of biochar. In this work, a biochar regeneration tidal flow constructed wetland (BR-TFCW) was built and its pollutant removal performance in response to different flood/rest time ratios (F/R), microbial community characteristics, pollutant removal mechanisms, and biochar in-situ regeneration mechanisms was explored. The results showed that under the same hydraulic load (1.97 m 3 /m 3 filler), the BR-TFCW depicted the worst effluent chemical oxygen demand (COD) concentration was 45.20 ± 7.90 mg/L (F/R = 3:1), while COD concentration in control group Ⅰ (CG1), which had no rest phase, was 152.85 ± 10.88 mg/L. When F/R was 1:3, 1:1, and 3:1, the BR-TFCW COD removal rates were 90.92 ± 3.97%, 94.95 ± 1.17%, and 99.84 ± 0.21%, respectively. During the flood phase, COD was removed by aerobic degradation and adsorption in the early stage, and anaerobic degradation in the later stage. The BR-TFCW and CG1 microbial communities differed significantly. The BR-TFCW functional bacteria were associated with organic matter degradation and resistance to dehydration and nutrient deficiencies. The CG1 microbial community was comprised of anaerobic bacteria that were associated with fermentation and methanogenesis. During the flood phase, biochar adsorbed COD in the liquid, serving as the primary mechanism by which COD was removed. In the BR-TFCW’s rest phase, microorganisms degraded the organic matter adsorbed by the biochar, which enabled the biochar to undergo biological regeneration. The biochar’s regeneration rate before and after the rest phase was 75.03 ± 1.00%. The cycle of adsorption and biological regeneration maintained the biochar’s adsorption capacity.