Reeding towards a sustainable future
Reed bed technology is set to play an integral part in helping water companies comply with the WFD regulations and help them meet AMP5 targets. And in Wales, Forced Bed Aeration is already making a difference.
Higher energy costs and the drive to reduce carbon footprints are all contributing to make reed beds a more viable option compared with the alternative chemical and mechanical treatment technologies. Recent advances in reed bed technology mean the treatment of high strength pollutants such as petrochemicals, agricultural wastewaters and capped landfill leachate with high ammonia concentrations are now proven.
A market leader in natural wastewater treatment is ARM, which has collaborated with its US partner, Naturally Wallace, which developed Forced Bed Aeration (FBA) technology for which ARM has sole UK distribution rights. It has a wide range of applications across many industries but the advantages are most considerable for the water industry.
In reed beds fitted with FBA, a network of aeration pipes is laid beneath the gravel media. Air is pumped through the pipes then bubbles up through the reed bed and greatly increases the oxygen availability, intensifying the loads that the reed bed is then capable of treating.
Micro-organisms attach themselves to media and plant roots. These organisms degrade the pollutants using the oxygen available in the reed bed. The more oxygen available to the organisms, the more pollutants they can remove.
Due to the increased oxygen availability in FBA systems, micro-organisms degrade pollutants aerobically and this can boost treatment capability by at least ten times that of a traditional passive reed bed.
The increased treatment capacity due to oxygen availability also has several other benefits - it allows for far deeper reed beds meaning FBA systems can have a smaller footprint; and means FBA reed beds can handle stronger pollutants such as wastewater containing high concentrations of ammonia than traditional, passive systems.
Complete nitrogen removal can be achieved by dividing reed beds into aerobic and anoxic zones to allow both nitrification and denitrification. FBA technology can be retrofitted to existing systems.
Severn Trent Water treats sewage and stormwater at 370 sites using reed beds, and is currently trialling the new FBA technology as part of a PhD study with Cranfield University on three reed beds retrofitted by ARM. This technology is applicable to many different industries but is set to have far-reaching implications for the water treatment industry.
In recent years stricter regulations governing the level of pollutants in water discharged into the environment have come into force. One such piece of legislation is the Water Framework Directive (WFD) designed to protect and enhance the quality of water bodies. Instead of imposing restrictions on clearing up pollution incidents after they happen, the WFD focuses on pollution prevention.
Under the WFD, by 2015, all water bodies should meet or exceed "good" ecological and chemical status. The future will see sewage treatment works having to adhere to much stricter consents regarding ammonia and other pollutants.
Under AMP5, many water companies are required to upgrade existing treatment systems. Several companies have retrofitted FBA technology to existing reed beds to drastically enhance their treatment capacities thereby either meeting and, often, surpassing targets.
Whilst the new FBA technology is in early days at Severn Trent, it was successfully implemented at the National Botanic Garden of Wales, in Llanarthne, Carmarthenshire, last year. The new system comprises a septic tank feeding into a 128m2 vertical flow reed bed system with integral FBA technology.
The garden's previous system was an 11-year old Living Machine with aeration tanks.
The energy consumption difference between the old Living Machine system and the reed beds with FBA technology is significant. The new ARM reed bed system has reduced maintenance and energy costs by £7,000 compared with those incurred by the Living Machine.
For each cubic metre of wastewater, a Living Machine would consume more than nine times the power than that of an aerated reed bed system and more than 15 times the power consumption for traditional surface or subsurface flow wetlands.
Whilst FBA systems require more electricity compared with conventional passive surface and subsurface flow reed beds, FBA reed beds more than compensate with their increased treatment capability facilitated by greater oxygen transfer rates (OTR). A typical horizontal subsurface flow wetland may have an OTR of 7g/m2/day and a vertical subsurface flow wetland, an OTR of 15g/m2/day.
However, using FBA, the average oxygen transfer rate is a minimum of 50g/m2/day, more than three times that of a vertical flow reed bed and more than seven times that of a horizontal flow model.
A common test to evaluate pollutant removal of treatment systems is a carbonaceous biochemical oxygen demand or CBOD test. This measures the depletion of dissolved oxygen by microbial organisms.
Naturally Wallace retrofitted FBA technology to an existing subsurface flow wetland used for treating wastewater generated during the cheese-making process at Eichten Cheese farm in Minnesota, US. A CBOD test was performed on the wetland to measure the pollutant removal.
Commissioned in April 1999, the FBA system was fully functional by July 1999 and the graph clearly shows pollutant levels in the water leaving the reed bed dropped significantly at this time,
despite great fluctuations in the concentration of pollutants in the wastewater as it enters the reed bed.
"We're making further investigations into floating reed beds, drinking water sludge treatment and the removal of pollutants such as phosphorus and soluble copper," says ARM's director, Tori Sellers. "We're always looking to find new trials to investigate treating different effluents and testing new wetland technologies.
"We're now expanding our research and trials using FBA for secondary treatment reed beds - treating effluents which have undergone primary settlement.
"Commonly, this secondary treatment is conducted using soak-away or passive horizontal or vertical flow reed beds. We're trialling FBA in secondary treatment reed beds and are highly encouraged with the results."
ARM suspects climate change concerns can only serve to drive up reed beds' popularity in the water treatment industry. "Energy consumption and corresponding carbon footprints of reed beds make them the first port of call when considering water treatment options," says Sellers.
"We deal with many water companies and they're all searching for more sustainable ways to treat sewage to tightening consent levels. They see FBA as the answer. In one project, by retrofitting FBA to their existing reed beds, they optimise their assets to help them meet AMP5 targets, perform to tighter consents under WFD and
step up their environmental standards."
Comparing a sludge treatment reed bed with an incinerator treating 2,000 tonnes of dry solids over a year to reveal a huge difference in emissions, Sellers says: "Whilst the reed bed would emit 37 tonnes of carbon dioxide and 235kg of nitrogen oxide, the incinerator would emit 335 tonnes of CO2 and a staggering 200,000kg of NOx. Similarly the reed bed would release a minimal quantity of sulphur dioxide, the incinerator would produce a massive 10,000kg.
"For the incinerator you also have to take into account transporting the waste, chemical risks and dangerous machinery, all of which is needed minimally or is unnecessary with the reed bed.
"On top of this, reed beds produce negligible odour and noise. Few would disagree that reed beds are far more appealing to the eye!"
Reed bed technology is set to play an integral part in helping water companies comply with both the WFD regulations and help them meet AMP5 targets.
"Look again," encourages Sellers. "You may think you know and understand reed bed technology but technology is developing at a pace and you need to ensure you're up to date. Reed beds have radically evolved over the past 30 years and, if the research and development we're undertaking is any indication, that evolution is set to continue. That will serve to widen reed bed applications - both their format and capabilities.
"Constructed wetland technology is here to stay because it provides a more sustainable alternative to energy intensive alternative water treatment solutions."