Is removal of organics and suspended solids in horizontal sub-surface flow constructed wetlands sustainable for twenty and more years?

Constructed wetlands with horizontal subsurface flow (HF CWs) were the most common type of constructed wetlands used for municipal sewage during the 1980s and 1990s. Because of predominant anoxic and anaerobic conditions in the filtration beds, this CW type was primarily used for removal of organics and suspended solids. HF CWs were replaced with vertical and hybrid CWs, however many HF CWs are still in operation but very scarce information exists on a long-term treatment performance. The present study evaluated treatment performance of 114 Czech HF CWs with special attention to 17 systems that have been in operation for at least 20 years. The study clearly revealed that if the HF CWs are properly loaded, their treatment performance is very steady with outflow concentrations < 15 mg l(-1) of BOD5 and TSS and < 50 mg l(-1) COD. The results from systems that have been in operation revealed that treatment efficiency actually increased over the years of operation with outflow concentrations lower as compared to initial phase of operation. The removal efficiency of 17 systems that have been in operation for at least 20 years amounted to 91.7%, 82.9% and 88.3% for BOD5, COD and TSS, respectively during the last five years of operation. Removal of both organics and TSS does not depend on the season. The results of this study indicates that HF CWs can provide a sustainably excellent treatment performance for 20+ years despite being designed according to a simple formula used in the early 1990s.