Tertiary sewage treatment by a full-scale compact vertical flow constructed wetland.

A 208 m2 compact vertical flow treatment wetland (cVFTW) was studied, treating the rotating biological contactor (RBC) effluent of a busy UK visitor attraction, with widely varying daily sewage loads up to 111 resident population equivalent. Results are presented from monthly spot-sampling and analysis for inlet and outlet concentrations of BOD, SS, NH4-N and TN, including load rates, for the first four-and-a-half years of operation. From commissioning to 13 weeks, outlet BOD, SS and NH4-N were below 12 mg l-1, until a sudden outlet NH4-N spike, to 31.9 mg l-1, the reasons for which are discussed. Thereafter, final effluent quality was <3:<3:<0.5 mg l-1 BOD/SS/NH4-N almost without exception, regardless of the determinand load rates imposed. Recirculation of controlled proportions of cVFTW effluent to the RBC inlet eliminated foul odour emanating from the RBC, and considerably increased overall nitrogen removal via denitrification. Phragmites australis development was poor, with the majority of plants experiencing chlorosis, putatively owing to insufficient bioavailable iron. By year three, despite ongoing weed removal to select for Phragmites, the wetland was increasingly colonised by other species, predominantly Epilobium hirsutum. Glyceria maxima, planted in year three, outcompeted even Epilobium, recommending itself as an alternative to Phragmites in high redox potential treatment wetlands.

Establishing a design for passive vertical flow constructed wetlands treating small sewage discharges to meet British Standard EN 12566.

Owing to legislation change (which made General Binding Rules effective from 1 January 2015) unless discharge is to specified environmentally sensitive sites, small sewage discharges (SSDs) in England - that is, <2 m3 d-1 to ground; <5 m3 d-1 to surface waters - no longer require an Environmental Permit (EP) and need not be registered for exemption, provided discharge to surface waters is preceded by treatment using equipment complying with BS EN 12566. This effectively excludes the use of treatment wetlands, unless covered by an EP, because the cost of certification to EN 12566 for bespoke designs is prohibitive. EPs take up to four months to obtain. Therefore, the new legislation has created a commercial disadvantage for constructed wetlands treating SSDs, compared with mass-produced sewage treatment plants. However, the UK statutory pollution regulators have maintained a dialogue with the Constructed Wetland Association (CWA), with a view to assessing whether treatment of SSD using constructed wetlands might be allowable, without requiring EPs. This paper presents treatment performance data obtained over 15 years, from a variety of full-scale operational treatment wetlands, as supporting evidence for design guidelines, proposed by the CWA to the UK regulators, for the implementation of constructed wetlands continuously passively treating SSD to 20:30:20 mg l-1 BOD/SS/NH4-N under a wide range of loading rates. Relevant experience of UK designers, installers and operators since the early 1990s is included, resulting in recommended physical design criteria and loading rates for compact vertical flow reed beds, presented here as key elements of the draft guidelines.