Anaerobic digesters as a pretreatment for constructed wetlands

The most commonly used pretreatment technologies for constructed wetland (CW) treatment of domestic sewage are septic tanks (ST) and Imhoff tanks (IT). These technologies have frequently suffered from failures and even in normal operation they offer insufficient removal of solids. As a result, combined ST-CW or IT-CW can experience substrate clogging, especially when high organic loads are applied. In the last 7 years, the operation of combined systems using high-rate anaerobic digesters as a pretreatment and CW as a post-treatment has been reported. A review of the literature indicates that CW in these combined systems operates with a similar organic loading rate (on a chemical oxygen demand basis) but with a lower total suspended solid (TSS) loading rate. In these combined systems, the TSS loading rate is 30–50% less than that applied in CW combined with classical pretreatment technologies. A low TSS loading rate could prevent substrate clogging in CW. This work presents the results of different case studies on the treatment of municipal wastewater with high-rate anaerobic systems. Our interest is focused on the capacity of these systems for removing suspended solids, and therefore on their potential as an appropriate pretreatment to avoid clogging in constructed wetlands and to improve efficiency. Average and 95 percentile TSS concentrations of anaerobic treated wastewater were below 60 and 100 mg/l, respectively, for all configurations. Therefore, the use of high rate anaerobic systems as a pretreatment for constructed wetlands could delay gravel bed clogging. Furthermore, according to the level of organic matter removal, anaerobic pretreatment provided a 30–60% reduction in the required wetland area. Both treatment alternatives can be combined to develop low-cost, robust, and long-term systems for treating municipal wastewater.