Tertiary sewage treatment by a full-scale compact vertical flow constructed wetland

Abstract

ABSTRACTA 208 m2 compact vertical flow treatment wetland (cVFTW) was studied, treating the rotating biological contactor (RBC) effluent of a busy UK visitor attraction, with widely varying daily sewage loads up to 111 resident population equivalent. Results are presented from monthly spot-sampling and analysis for inlet and outlet concentrations of BOD, SS, NH4-N and TN, including load rates, for the first four-and-a-half years of operation. From commissioning to 13 weeks, outlet BOD, SS and NH4-N were below 12 mg l−1, until a sudden outlet NH4-N spike, to 31.9 mg l−1, the reasons for which are discussed. Thereafter, final effluent quality was <3:<3:<0.5 mg l−1 BOD/SS/NH4-N almost without exception, regardless of the determinand load rates imposed. Recirculation of controlled proportions of cVFTW effluent to the RBC inlet eliminated foul odour emanating from the RBC, and considerably increased overall nitrogen removal via denitrification. Phragmites australis development was poor, with the majority of plants experiencing chlorosis, putatively owing to insufficient bioavailable iron. By year three, despite ongoing weed removal to select for Phragmites, the wetland was increasingly colonised by other species, predominantly Epilobium hirsutum. Glyceria maxima, planted in year three, outcompeted even Epilobium, recommending itself as an alternative to Phragmites in high redox potential treatment wetlands.