Treatment efficiency of a vertical‐flow reed bed with recirculation

Abstract

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/m²‐d. At effluent temperatures of between 4.7 and 18.0°C, a COD (BOD₅) mass loading rate of 11.7 (6.4) g/m²‐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°C (April‐Dec), the applied mean nitrogen load of 1.9 g NH₄ ⁺‐N/m².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/m²‐d and areas of 3–6 m²/pe are recommended for dimensioning VF.

Key words:

• Domestic wastewater
• vertical‐flow reed bed
• nitrification
• pre‐denitrification