Phosphorus and nitrogen removal from tertiary treated urban wastewaters by a vertical flow constructed wetland

Abstract

Lake L’Albufera is a hypertrophic lake exposed to anthropic pressures. To reduce nutrient loads, a set of horizontal and vertical sub-surface flow constructed wetlands (CWs) were built to treat wastewater from a tertiary wastewater treatment plant (WWTP) before its discharge into the lake.These CWs were designed to remove nutrients, primarily total phosphorus (TP). This paper is focused on a vertical flow constructed wetland (VFCW), the primary objective of which was to remove TP by adsorption and biological uptake. Prior to construction, laboratory experiments were conducted to determine which materials and in what proportions are best suited to achieve that goal. Two different sands (types 0 and 1), as filling material, two types of clays (types 1 and 2) and two types of iron oxides (types a and b), as sorption agents, were used. The primary parameters studied were the phosphate adsorption isotherm, the filter medium hydraulic conductivity and the depletion of adsorption capacity in experimental columns.Laboratory results showed that the best mixture was formed by sand type 0 (Qmax=2.94mg Pkg−1) and 10% of iron oxide type b (Qmax=1666.67mg Pkg−1). Operation was established in a daily cycle with a hydraulic loading of 0.068md−1.Following this procedure, a VFCW planted with reeds and with 157.9m3 water capacity per cycle was constructed. The inflow contained 0.635mg TPl−1, 1.906mg NH4+-Nl−1, 8.9mg TNl−1 and 20.9mg CODl−1. During the first year of operation, total phosphorus removal was 77.0%, ammonium 95.0%, total nitrogen 24.4% and organic matter (COD) 49.3%.