Removal of organic matter, nitrogen and faecal indicators from diluted anaerobically digested slurry using tidal flow constructed wetlands

The rapid implementation of anaerobic digestion for renewable energy production has resulted in increased generation of anaerobically digested slurry, which contains a variety of pollutants and therefore has the potential to cause serious environmental problems. Tidal flow constructed wetlands, which could generate beneficial oxygen conditions, were investigated for their success in removing nitrogen, organic matter and pathogens in anaerobically digested slurry. The results indicated that tidal operation had a positive effect on promoting NH4+-N and organic matter (chemical oxygen demand (COD)) removal. With an average influent NH4+-N concentration of 288 mg/L and COD concentration of 839 mg/L, the average removal efficiency reached up to 93% (325 g/m2 day) for NH4+-N and 53% (603 g/m2 day) for COD, with total inorganic nitrogen (TIN) removal efficiency of 51% (226 g/m2 day). The nitrogen removal in the tidal-operated CWs is highly dependent on the flooded and drained (F/D) time ratio. Changing flooded time from 3 to 5 h enhanced denitrification (nitrite reductase-K (nirK) abundance) and further resulted in improved TIN removal efficiency of 62% (237 g/m2 day). The removal of faecal indicators was also examined, with reduction rate of approximately 0.9 log10 CFU/100 mL for both Escherichia coli and total coliforms, which was independent of the influent loadings and differing flooded/drained time ratio. Tidal flow CWs were demonstrated to have the high potential to treat diluted anaerobically digested slurry.