Constructed wetlands have been used for tertiary treatment for years but recent developments have increased their versatility in effluent treatment. Increasing water quality standards demanded by the Water Framework Directive (WFD) is stimulating the use of tertiary systems for the removal of residual phosphate, ammonia, BOD and suspended solids from effluents, and as a result, constructed wetlands are receiving greater attention.

Not all constructed wetlands are new, with many having been used for decades, but the industry’s understanding of the design principles is developing rapidly and their extrapolation into new applications is generating some interesting treatment systems. Grass plots are widely used as a polishing system, and both vertical and horizontal flow reedbeds have been in use for some 20 years. Traditional systems comprise principally grass plots, vertical and horizontal flow reedbeds. More recent applications include sludge drying reedbeds and both passive and aerated floating reedbeds. Effluent is fed to a header channel above the grass plots and allowed to flow down grassed slopes to a collection drain and discharge point.

The purpose of the plots is to reduce concentrations of suspended solids, BOD and ammonia residues. Sizing is determined by influent and effluent characteristics and treatment requirements. Physical processes and biochemical activity in the root zone of the surface vegetation achieve treatment. Effluent flows in a thin layer over the soil surface and through the root zone, so a combination of soil microbiological activity, hydraulics and physical chemistry creates the treatment process:

  • suspended solids are physically retained within the vegetation and soil matrix,
  • BOD is reduced by aerobic bacteria at microsites within the soil,
  • ammonia is removed by a combination of cation exchange and oxidation by bacteria.
  • In horizontal flow reedbeds the effluent is distributed across the full width of the bed and flows horizontally across the bed to the outlet. A level control system is installed at the outlet to maintain sub-surface flows other than in extreme weather conditions when the bed may be flooded to protect the reeds. Within the reedbed the effluent comes into contact with a network of aerobic, anoxic and anaerobic zones, providing robust treatment. The growth of the reed rhizomes opens up the bed to provide new hydraulic pathways, maintaining its treatment capability and preventing clogging and short-circuiting.

    Horizontal flow systems are good for removing BOD and TSS but are usually oxygen-limited and hence provide only limited nitrification. De-nitrification can occur, however, under anoxic/anaerobic conditions that prevail in parts of the bed. Vertical flow reedbeds are akin to a biological filter – effluent is distributed evenly over the surface of the bed and percolates down through the media made up of graded sand and gravel.

    Oxygen transfer

    As in horizontal flow systems, the reed rhizomes maintain hydraulic conductivity and support large populations of bacteria. Oxygen transfer is considerably enhanced as a result of intermittent dosing. As the liquid drains down, air refills the interstices of the media creating suitable conditions for nitrification. TSS removal is less efficient and vertical beds tend to need more management than horizontal flow systems.

    Reedbeds have low construction and operation costs and are ideal for rural developments providing both secondary and tertiary treatment. Pond and reedbed systems consist of a series of shallow ponds, fringed with various species of emergent plants, which are linked by areas of aggregate-filled constructed wetlands. Although requiring a larger area than reedbeds, they provide excellent treatment and can look very attractive.

    In a secondary treatment system, the first pond is facultative, where the effluent is stabilised by a combination of aerobic, anaerobic and facultative bacteria. The subsequent maturation ponds provide tertiary treatment by the endogenous respiration of the residual biological solids and ammonia is converted to nitrate by algae and/or using the oxygen supplied by surface re-aeration.

    These systems form part of the traditional wastewater treatment techniques in France, Germany and Canada but have been slow to catch on in the UK, partly because of the land area required and also because of our preference for engineered solutions. Sludge drying reedbeds are a relatively new innovation in the UK and are a modified version of vertical flow reedbeds. The sludge is distributed evenly over the surface of the bed where the solids are trapped and gradually aerobically biodegraded, while the supernatant liquors percolate down through the bed and receive some treatment before being returned to the inlet of the works. Area requirements are a function of the total weight of dry solids/m2/pa and, once established, these beds can accommodate up to 60kg/m2/pa.

    Sludge drying

    Removal of digested, mineralised and dewatered sludge is typically carried out every eight-ten years. Sludge drying reedbeds are becoming an accepted technology in continental Europe, particularly in Denmark, and there is a substantial opportunity for this low-cost system in the UK.

    Floating reedbeds are manufactured in lightweight modular sections, which are usually planted onsite with common reed (Phragmites australis) or reedmace (Typha latifolia) and can be quickly installed without the need for heavy plant or lifting gear. Constructed from HDPE pipe, heavy duty UV-stable mesh and coir fibre as a rooting medium, the modules are tied together to form a continuous raft that is buoyant and robust enough to be accessed by operators, but highly flexible and manoeuvrable.

    The plants rapidly form a dense sward of aerial shoots and a complex interwoven mass of rhizomes and fine feeder roots (the rhizosphere) extending to a depth of 1m or more, binding the entire raft tightly together. The rhizosphere offers a large surface area for biofilm development but with its superior oxygen profile it also becomes densely populated with ciliates and other protozoans found in aerobic secondary treatment systems, as well as larger zooplankton and detritivorous invertebrates, which do not normally feature in the rhizosphere of a horizontal reedbed.

    Floating reedbeds have been shown to be very effective at removing suspended biosolids, though the process is far more dependent on floc formation and zooplankton grazing than on physical filtration in the root zone. There is evidence that the very high population of stalked ciliates on the fine roots enhances floc formation and settlement by secreting mucous droplets into the effluent, allowing the zone of influence of the reedbed to extend well beyond the confines of the rhizosphere.

    There is also evidence of significant orthophosphate removal, which cannot be accounted for by uptake and storage in the aerial vegetation alone. The main advantages offered by floating reedbeds are the ability to withstand extremely variable organic and hydraulic loadings without damage, ease of installation and suitability for quick retrofits in existing tertiary tanks and lagoons. The land requirement is 20-40% of that required by horizontal reedbeds because the containment can be up to 2m deep and void space is much higher at around 90%.

    Floating reedbeds

    The characteristics of floating reedbeds make them highly suitable for run-off remediation schemes and CSOs, where the treatment priorities are usually suspended solids removal and the confinement of floating solids and hydrocarbons. If sludge removal is required, the raft can be manually towed out of the way, or if space is limiting, divided into modules and lifted out by standard site plant.

    Floating reedbeds can be combined with aeration systems to provide more intensified treatment with the capability of treating stronger effluents. Industrial effluents, sewage and treated effluents requiring further polishing can all be treated in this way. Intensified wetland systems have been installed around the world and as a result the design and process engineering of these more sophisticated systems is becoming better understood. These systems utilise the widely recognised benefits of fixed biofilms to accelerate the natural processes found in rivers, lakes and ponds. They use efficient airlift pumps and fine-bubble air diffusion systems to introduce and circulate oxygen to the floating reedbed’s rhizosphere and to artificial media, which can be used as a biofilm substrate to support rich microbial, algae and animal communities.

    Great volumes of beneficial micro-organisms are also produced, which flow into the surrounding effluent and feed on excess nutrients and organic pollutants. This provides opportunities for benthic communities to be established in the bottom areas that were oxygen deficient.

    Effluent quality

    Floating photovoltaic-powered lagoon and reservoir circulators draw up to 40m3/min from the lower part of the water column and spread it across the top of the lagoon for continuous surface renewal. The almost laminar-flow mixing action greatly accelerates the biological processes that improve effluent quality. Used in combination with floating reedbeds, these systems can provide solar-powered tertiary treatment for sewage, contaminated groundwater and CSO discharges.

    The tighter discharge consents being applied to many WwTWs, contaminated groundwater and CSO discharges, and the corresponding installation or upgrading of existing tertiary treatment systems will send a positive message to the public. The fact many of these systems are constructed wetlands will be seen as an environmental benefit with spin-off benefits for wildlife. For water companies, local authorities and private discharges, there is the knowledge that constructed wetland systems in particular provide a competitively priced treatment option, which can buffer shock loadings and provide secure tertiary treatment.

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