Study on Improving the Purification Function of Constructed Wetlands with Construction Waste Substrates by Acid–Base Substrate Configuration

Construction and demolition waste, when used as the substrates of constructed wetlands, provide notable environmental benefits: purification performances and substantial economic advantages compared with conventional substrates such as gravels. However, the high effluent pH induced by waste concrete severely restricts its practical application in such systems. The body of research focused on overcoming this limitation is rather limited. To address this limitation, this study proposed a strategy based on the configurations of acid alkaline substrates. A pilot-scale vertical flow constructed wetland experiment was carried out to evaluate the feasibility of this approach through three treatments: (1) waste concrete alone (Concrete), (2) waste concrete as the upper layer combined with perlite (an acidic substrate (Concrete + Perlite)), and (3) a uniform mixture of waste concrete and perlite (Mixed). The results demonstrate that the Concrete treatment exhibited a persistent high pH problem, where the effluent pH values remained above 9, even after five months of operation. In contrast, the Concrete + Perlite and Mixed treatments effectively mitigated the excessive effluent pH (<8.2). Relative to the Concrete treatment, both the Concrete + Perlite and Mixed treatments significantly enhanced the removal efficiencies of chemical oxygen demand (COD) (from 43.7% to above 68.5%), total nitrogen (TN) (from 31.8% to above 86.5%), and ammonium nitrogen (NH4+-N) (from 96.7% to 96.9%), whereas the removal efficiency of total phosphorous (TP) showed only a slight decrease. No significant differences in pollutant removal performance were observed between the Concrete + Perlite and Mixed treatments. Moreover, the Concrete + Perlite and Mixed treatments substantially increased the bacterial diversity within the substrate biofilm compared with the Concrete treatment, although differences in the bacterial community composition between the Concrete + Perlite and Mixed were relatively minor. Overall, configuring pH-balanced substrates through the combination of acidic and alkaline matrices provided effective and sustainable integrity for promoting the resource of construction and demolition waste in constructed wetlands.