Project Focus: Aerated Reed Beds at Cowdenbeath WWTW
An £8.7M constructed wetland project at Cowdenbeath Wastewater Treatment Works has enabled Scottish Water to treat sewage overflows and enhance the environment
Eddie Burns, project manager, Scottish Water:
“This was a site that nobody had cared for for a long period of time, and we’ve returned it to a state where it looks great, reeds are growing and we’ve enhanced the environment as well. Most importantly, the output from the wetlands and the impact on the burn was much improved, and when samples were taken the regulator was very pleased with the results.”
Patrick Hawes, consultant, ARM Ltd:
“Forced Bed Aeration offers higher performance and more consistent performance through aeration than other systems. Reed bed systems can be event-driven and offer space advantages, and in Cowdenbeath this suited the site very well. We hope this will build confidence within Scottish Water in the use of this technology and naturally lead on to other applications across the region.”
A constructed wetland scheme at Cowdenbeath Wastewater Treatment Works has allowed Scottish Water to treat discharges from combined sewer overflows (CSOs) in a way which meets regulatory demands and enhances the local environment.
The £8.7 million project involved the construction of two units: a flow balancing shaft settlement system for sedimentation of solids and a 4,000 square metre, two metre deep saturated, vertical flow aerated wetland fitted with Forced Bed Aeration (FBA). It is the first time an aerated reed bed has been used on this scale in Scotland.
Located to the south of Cowdenbeath Golf Course, the WWTW receives wastewater from CSOs serving 11,000 people in Cowdenbeath, which was previously stored and pumped to larger works for treatment. Scottish Water decided to upgrade the facility in 2011 after the Scottish Environment Protection Agency (SEPA) identified that the nearby Lochgelly Burn was being polluted during heavy storm events.
“The initial driver for the project was that our regulator, SEPA, identified water quality and aesthetic failings relating to burns in that area that they believed were being caused by sewage discharges from Scottish Water assets,” says Eddie Burns, project manager at Scottish Water. “The asset in this case pumps flow to another works, and what was happening was that anything above a certain pump volume would discharge. There was some storage at this facility but when it went above a certain level - in heavy storm conditions - the rest was discharged to the local watercourses.”
Scottish Water engaged engineering consultancy Atkins to investigate possible solutions: one option was the installation of a large new pipeline to the closest works in Lochgelly. However, an on-site natural treatment system, consisting of a tank and a constructed wetland, was favoured.
“The wetland was preferred because it could be put in close to our existing assets and close to the watercourse, meaning a reduced impact on the environment and a better relative cost,” Burns continues.
“The chosen site was right next door to our existing works – we own the road that leads down to this work as well - so it allows us to run three assets in that location and operate them using the same teams.”
Scottish Water engaged Barhale Construction as their main contractors for the project, who brought in reed bed specialists ARM Ltd to design and construct the wetland.
One of the main challenges was the poor ground conditions: the site contained landfill material including colliery spoil, domestic waste and sewage sludge, and some of this waste was found to extend down to depths of around 12 metres. In addition, there were disused mine shafts below the surface which needed to be filled and grouted before the main excavation work could start.
In this context, the choice of a vertical, aerated wetland system was ideal, explains Patrick Hawes, Consultant at ARM Ltd.
“The landfill site chosen for the project was a very old one, and there were mine workings underneath it, so bringing the ground back into use was going to cost a lot,” says Hawes. “For this reason, the objective was that the reed bed had to be as small as possible, while being able to cope with the predicted load. So the use of aerated reed bed technology was ideal, because you maximise your treatment per metre squared and therefore reduce the total area of the reed bed.”
The FBA system enabled the bed to be designed much deeper – 2m – than a conventional reed bed, which typically goes down only 0.6m in depth. A deeper bed gives improved performance because the air bubbles can stay in the bed longer and provide a better oxygen transfer rate.
The two-unit system is designed to ensure the reed bed can cope with the variable flows of CSO discharges, Hawes explains.
“The nature of CSOs is that they are event-driven: you don’t get a continuous flow, you get an intermittent flow and therefore the system has to be able to handle high peaks and then nothing, and you don’t know when these flows are going to arrive as it depends on the weather.
“The answer in this case was to have an upfront balancing system, a 20,000 cubic metre balancing tank, which collects the CSO water, which is then pumped through at a constant rate [4.6 litres per second] into the reed bed so we know and can control what the bed receives. There are limits to what the reed bed can handle, around 4000 cubic metres a day. If there’s any excess beyond that, then it will be spilled into the final burn, although in that instance it’s likely to be so diluted so as not to need treatment.”
Burns says that early designs for the flow balancing system had situated the tank underground, but Scottish Water thought there might be access issues when it needed cleaning. The eventual solution involved a tank with segmented rings, and a shaft which is open to the air. Cleaning can now be done from the surface using hoses, with any excess solids pushed through the sump so that they will be pumped into the wetlands in the next storm event.
The reed bed provides sufficient treatment to allow storm waters to flow back into the watercourses without compromising water quality. The FBA aeration system is energy efficient because it only needs to be switched on once a day, or when required in a storm event - a process that happens automatically.
“When the pumps come on and the flow is delivered to the bed, aeration starts automatically, ensuring that any displaced effluent in the bed is well oxygenated and treated,” explains Hawes. “But if there’s no flow for a 24-hour period, then the blowers come on for a few hours every day anyway to keep the bed well aerated.”
This automation, and the fact that the system requires little maintenance beyond a visual inspection and an occasional filter change for the aerators, will make life easier for Scottish Water’s operational team.
“For the operator, the benefit is that you have an asset which is controlled and monitored using our telemetry system, and there’s very little that can possibly go wrong,” says Burns. “This is a site that probably doesn’t need too much intervention.”
The construction phase of the project took around 15 weeks. Now complete, the site can treat around 230,000 cubic metres of water per year, giving it one of the largest treatment capacities in the UK. SEPA inspectors did not take long to pronounce themselves happy with the readings from the nearby burn when the project was completed in March. The site is also much more pleasing to the eye and to the environment than previously, with the wetland forming a new habitat for birds and other wildlife.
Burns concludes: “This was a site that nobody had cared for for a long period of time, and we’ve returned it to a state where it looks great, reeds are growing and we’ve enhanced the environment as well. Most importantly, the output from the wetlands and the impact on the burn was much improved, and when samples were taken the regulator was very pleased with the results.”
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