Municipal wastewater treatment in an anaerobic digester-constructed wetland system

An experimental plant was constituted for an upflow anaerobic sludge blanket (UASB) reactor with an active volume of 25.5 m3 and two horizontal constructed wetlands of 75 m2 each. The first wetland was a superficial flow (SF) system and the second a subsurface flow (SSF) system. The UASB reactor was fed with 60-73 m3 d(-1) of raw domestic wastewater from the municipal treatment plant (MTP) of the city of Santiago de Compostela. Part of the effluent from the UASB reactor (15 m3 d(-1)) passed through the SF wetland and then went into the SSF. In the first ten months of operation, from July 2005 to April 2006, the hydraulic retention time for the UASB system was in the range of 7-14 hours and the organic loading rate (OLR) was between 0.3 and 1.1 g COD l(-1) d(-1). The organic load in the wetlands was in the range of 5-40 g BOD5 m(-2) d(-1). The global results achieved for total suspended solids (TSS), chemical oxygen demand (COD) and biological oxygen demand (BOD5) removal efficiencies were 85-96%, 65-90% and 69-93%, respectively. The average monthly concentrations in the final effluent were 8-73 mg BOD5 l(-1), 25-120 mg COD l(-1) and 6-20 mg TSS l(-1). The global balance of VSS in the UASB reactor shows that purge accounted for 19.6% and hydrolysis accounted for 47.7% of influent VSS. Data from suspended solids balance in the constructed wetlands, at the end of the operation period, show a TSS and VSS accumulation of 48% and 16%, respectively.