The negative effect of the high pH of waste concrete in constructed wetlands on COD and N removal

This study aimed to investigate the effect of the high pH of constructed wetlands (CWs) with that use waste concrete as a substrate on pollutant removal efficiency and to identify whether this high pH effect can be mitigated by adjusting the influent pH. Laboratory-scale vertical flow constructed wetland (VFCW) simulators in batch feeding mode were established with waste concrete, waste bricks and common gravel as the substrates. At a hydraulic retention time of 24 h and an influent of pH 7.98, an average pH of 11.28 was observed for the effluent of the waste concrete treatment, while CWs with waste brick treatment or common gravel treatment exhibited an effluent pH of 7.57 +/- 0.03 and 7.57 +/- 0.14, respectively. Compared with common gravel or waste brick, waste concrete treatment led to a significantly lower COD removal efficiency (72.09 +/- 0.21 %) and a significantly higher effluent NO2--N concentration (6.62 +/- 0.15 mg/L). With the effluent pH decreasing to 8.15 through influent pH adjustment, waste concrete treatment exhibited a better COD removal efficiency (81.60 +/- 0.03 %) and a lower effluent NO2--N concentration (4.08 +/- 0.11 mg/L) without a significant influence on the NO3--N, NH4+-N, and phosphorus removal efficiencies. The high pH of waste concrete had a positive effect on amoA and nirS/nirK expression and a negative effect on anammox and nxrA expression, which could be mitigated by decreasing the influent pH. Therefore, waste concrete is suitable for the treatment of acid wastewater, while for non-acid wastewater, additional measures such as mixing with other substrates should be taken to avoid the too high pH induced by waste concrete.

Automated summary

This study investigated the effect of high pH from constructed wetlands (CWs) using waste concrete as substrate on pollutant removal efficiency. It was found that waste concrete treatment had a significantly lower COD removal efficiency and higher effluent NO2--N concentration compared to common gravel or waste brick treatments. However, by adjusting the influent pH, the COD removal efficiency improved without significant impact on other removal efficiencies. The high pH of waste concrete had positive and negative effects on gene expression, which could be mitigated by lowering the influent pH. Therefore, waste concrete is suitable for treating acid wastewater, but additional measures should be taken for non-acid wastewater.