Effects of effluent recycle on treatment performance in a vertical flow constructed wetland

Abstract

As part of the effort to improve design and operation schemes of decentralized wastewater systems, the Danish EPA funded a research initiative, to re-evaluate the Danish guidelines for VFCW with an aim to improve its design. Therefore, the aim of this study is to assess the effect of recycling water from the effluent of the VFCW to the pump chamber at several recycle ratios and to compare the effects of recycle to the sedimentation tank in lieu of the pump chamber. In addition, water quality changes within the depth of the bed were assessed in order to make recommendations on an appropriate depth for effective treatment. The results showed overall the system performed very well regardless of the recycle ratio with removal rates of TSS, TOC, BOD5, NH4-N and TP around 99% and thus easily exceeding the national discharge requirements. Despite that, the better results for TN effluent concentrations, below 45 mg/L, were achieved for recycling water from the effluent of VFCW to pump chamber at recirculation rates of 50% to 100%. The changes in concentrations of measured parameters with depth suggest that almost all transformations occurred with the top 20 cm of the 1.0 m thick effective bed depth, thus, the results show that the VFCW dimensions given in the Danish guidelines may be overdesigned from the perspective of meeting current discharge limits.