Constructed wetlands for advanced treatment and reuse

Abstract

Constructed wetland technology is gaining increasing attention as a low cost-efficient alternative to high-tech treatment systems for treating municipal and industrial wastewaters especially in small communities. However, its application for grey water reuse has been rarely investigated whilst performance for nutrients (N and P) still remains relatively poor. Pilot scale study was conducted in which three differently configured subsurface constructed wetlands: a horizontal flow reed bed (HFRB), vertical flow reed bed (VFRB) and a novel system - Green Roof Water Recycling System (GROW) were investigated for their suitability and robustness in treating grey water for reuse across a range of influent strengths to represent the limiting conditions observed in the literature. The HFRB and the GROW systems were found to be generally limited to comply with reuse standards especially at high strength. The release of iron from the HFRB media and particulates from the GROW system contributed to the poor turbidity of the final effluent from these systems. Overall, all wetland configurations were able to effectively treat low strength greywater but only the vertical flow system maintained its robustness when high strength greywater was treated. Analysis of the systems reveals this was due to the fact that aerobic metabolism is a more suitable treatment pathway for greywater. Ultimately, the performance of the vertical system was slightly lower but comparable to that of a membrane bioreactor making constructed wetlands a suitable technology for greywater recycling. Also, Bauxol, Red mud, Bayoxide, Ochre, Filtralite-P, Steel slag, concrete, Zeolite and various form of limestones were investigated for potential removal of soluble reactive phosphorous (SRP) and metals (Cu and Ni) in final sewage effluent for post Constructed Wetland System. P capacities exhibited by the different adsorbents correlated with type of metal (e.g. Fe, Al, Ca) and their cation exchange capacities. Ochre exhibited the best P removal ability with a P capacity of 26 g Kg-1 based on a Freundlich isotherm model. The equilibrium sorption capacity of BauxolTM and Ochre based on a Dubinin-Radushkevich model was found to be 4.1 and 4.9 mg g-1 for Cu and Ni unto BauxolTM respectively and 2.6 and 10.2 mg g-1 for Cu and Ni onto Ochre respectively. Kinetic and thermodynamic study revealed a spontaneous and efficient adsorption process via a pseudo-second order mechanism where intraparticle diffusion was shown to be the rate limiting step. An aerobic post constructed wetland system using Ochre as the bed media for large scale applications is suggested.