A decade of compact vertical flow constructed wetlands

Abstract

Compact vertical flow constructed wetlands (1–2 m2/pe, down-flow, single bed, >500 mm sand depth) were first installed in the UK in 1998, since when over 40 such beds have been implemented. After ten years of experience, this approach to wastewater treatment is reviewed with respect to operation and maintenance, and water quality improvement. Despite considerable accumulation of organic solids on the surface of some beds after several years' operation, effluent distribution over and permeability of the bed surface, as well as oxygen transfer rates within the bed matrix, have remained satisfactory. Six years of data relating to a 432 m2 bed treating combined settled sewage and abattoir effluent have demonstrated consistently high performance. Suspended solids, biochemical oxygen demand and ammonia removal averaged 80, 98 and 91%, respectively, even at a hydraulic loading rate of 340 mm/d. At 12 mm/d, removal was even more thorough. In contrast, accumulation of iron-rich organic solids on the bed's surface, which led to the bed becoming completely flooded, markedly decreased nitrification, although percentage total nitrogen removal was double that of the non-flooded bed. Flow rate analysis through the bed allowed calculation of percentage matrix saturation by effluent during operation, and operational hydraulic residence time (3.83 h), the implications of which are discussed.