Treatment efficiency of a vertical-flow reed bed with recirculation

A technical reed bed treatment system (RBTS) with vertical-flow (VF), which was filled with medium-grain sand and planted with reed (Phragmites australis), was intermittently charged with domestic wastewater of 29 pe (population equivalents) at a surface flow rate of 23.3 L/m(2).d. At effluent temperatures of between 4.7 and 18.0 degrees C, a COD (BOD5) mass loading rate of 11.7 (6.4) g/m(2).d sewage was degraded to effluent concentrations of <15-28 (<2-7) mg/L. Average COD (BOD5) elimination accounted for 94 (98) %. At effluent temperatures >6 degrees C (April-Dec.), the applied mean nitrogen load of 1.9 g NH4+-N/m(2).d was nitrified to leave < 1.0 mg/L NH4+-N in the effluent (mean influent concentration: 79.6 mg/L NH4-N). Simultaneous denitrification in the VF removed 23% of the nitrogen input. Due to an unfavourable NH4+-N/BOD5 inlet ratio of 0.30 N-anorg., the removal rate only increased to 41% by recirculating effluent water into the inlet at a ratio of 1.1. Depending on the purification grade required, loading rates of 20-50 L/m(2).d and areas of 3-6 m(2)/pe are recommended for dimensioning VF.