Effects of porous media and plants in the performance of sub surface flow treatment wetlands

Constructed treatment wetlands provide low cost, low maintenance, eco-friendly options for treating polluted water. Our research examined gravel wetlands and consisted of two major objectives: (a) to examine the effect of plants on treatment efficiency and (b) determine the effect of gravel properties---especially cation exchange capacity---on treatment efficiency. Our research found that typical domestic wastewater flowing through sub-surface wetlands containing 18mm sized aggregate resulted in similar effluent quality regardless of the presence of plants (cattails). Organic removal from human wastewater was 78.8% planted and 76.1% unplanted; ammonium removal was 94.5% planted and 90.2% unplanted. Organic removal from artificial wastewater was 88.8% planted and 90.1% unplanted; ammonium removal was 96.9% planted and 97.3% unplanted. These results confirm the view held by many engineers that the main benefit of plants is their aesthetic function. The potential of wetland aggregate properties to affect treatment efficiency has received less attention than the plant issue. The main function of the aggregate is to provide surface area for colonization by bacterial biofilms, while still allowing hydraulic flow through the wetland. Four aggregates of differing porosity and cation exchange capacity were examined. Organic removal rates were similar for all four aggregates: scoria was 95.4%; expanded shale was 95.7%; zeolite was 96.8%; basalt was 96.4%. Ammonium removal rates (primarily by nitrification) were similar for three aggregates: scoria (96.6%), expanded shale (97.4%), and basalt (94.5%). Zeolite, however, removed 98.2% of the ammonium (and 93.4% of the total nitrogen) prior to the wastewater reaching the nitrification chamber. The high cation exchange capacity of the zeolite initially adsorbed the ammonium, but the total removed was more than twice the adsorption capacity of the zeolite---indicating that biological stripping of the ammonium ions had occurred. We postulate that the zeolite aggregate had been colonized by anammox bacteria---combining ammonium (electron donor) and nitrite (electron acceptor) under anaerobic conditions---and releasing di-nitrogen gas as the end-product. Even without biological regeneration, ammonium adsorption by zeolite provides a cost-effective alternative to nitrification and denitrification for constructed wetlands.