Solids hydrolysis and accumulation in a hybrid anaerobic digester-constructed wetlands system

The properties and behaviour of solids retained in a pilot plant constituted of an up-flow anaerobic sludge blanket (UASB) reactor and two constructed wetlands (CWs) were monitored over a 3-year period. The UASB (25.5 m3) was fed with raw municipal wastewater at a flow rate of 61–112 m3 d−1 and a volumetric loading rate (VLR) of 0.75–1.70 kg TCOD m−3 d−1. The CWs (75 m2 each) were operated in series and received a fraction (17–20 m3 d−1) of the UASB effluent. The applied surface loading rates (SLR) were in the range of 3800–8700 g TCOD m−2 d−1 (UASB) and 11–15 g BOD5 m−2 d−1 (CWs). The overall system removed 95% TSS, 85% TCOD and 87% BOD5 on average. For influent VSS, the UASB removed 72.1% and gave a hydrolysis of 63.5%, while the average surplus sludge generation was 8.7%. Over the 3-year period, TSS and VSS accumulated in the CWs at rates of 1.07 and 0.56 kg m−2 year−1, respectively. The aerobic biodegradability of the accumulated solids ranged from 23 to 92 mg O2 g VSS−1 d−1 and increased downstream in the CWs. About 59% of the VSS that entered the CWs was removed by hydrolysis, while 24% accumulated on granular media. These low solids accumulation rates were especially remarkable considering the high COD and BOD5 loading rates applied. The system lay-out appear to be promising in terms of preventing clogging.