Vertical flow constructed wetlands for treating unscreened sewage in the UK

The use of two stage vertical flow constructed wetlands (VFCWs) for sewage treatment post coarse screening is an established option in France. The need to reduce the energy and maintenance requirements associated with small sewage works remains a key objective to the UK water industry. Two stage VFCWs have been identified as a candidate technology to meet these aspirations. However, there is a paucity of information concerning operation and performance during the start-up period which could last up to two years as well as knowledge transfer relating to differences in hydraulic and organic loading patterns. Accordingly, the UK’s first two stage VFCWs for municipal sewage treatment has been recently built and operated to assess its suitability. Overall, the site performed similar to values reported in the literature regarding total suspended solids, biological demand and ammonium-N being respectively 6.2 ± 3.4 mg·L-1 , 5.6 ± 2.6 mg·L-1 and 5.8 ± 3.8 mg·L-1 compared to literature values of 10 ± 10 mg·L-1 , 6 ± 4 mg·L-1 and 5 ± 6 mg·L-1 , based on composite sampling. However, a key difference compared to operating systems in France was sustained operating periods beyond the design hydraulic load leading to long periods of surface ponding. This had two major impacts: a limiting ability to re- oxygenate the filter body affecting the nitrification performance and retardation of the sludge mineralisation rate reducing the operating infiltration rate and hydraulics of the filters. This highlights the hydraulic limitations of the young filter (5 months of operation) especially in winter conditions. Future work has been suggested in order to adapt the technology to UK conditions such as extending first stage, optimising feeding strategy, using a storm and first stage overflow constructed wetland, aeration of the second stage or design based on infiltration rate.