The Water Framework Directive from the European Union requires the Environment Agency to set strict targets for phosphorus discharge levels, and the introduction of new reduced targets will mean that many water courses in the UK will not comply. There is therefore increasing pressure on water companies to do more. Wessex Water joined forces with the team at Bath’s Water Innovation and Research Centre (WIRC), led by Tom Arnot from the Department of Chemical Engineering and Rod Scott from the Department of Biology and Biochemistry, to tackle the problem. Wessex Water is a major investor in WIRC and the collaboration has involved a number of projects since 2013.
The researchers are trialling a ‘managed eutrophication’ process at Wessex Water’s sewage treatment works in Beckington in Somerset, where algae is deliberately grown in the treated wastewater to remove phosphorus, producing a ‘polished’ wastewater with much lower nutrient levels which can be discharged into watercourses. The nutrient-rich algae can be collected for a number of purposes, including fertiliser, animal feed or feedstock for biofuel. One of the purposes of the trial is to determine exactly what the algae will be most suitable for.
They have installed a high-rate algal pond (HRAP) system at the site, which has two open shallow ponds with an area of 60 m2 and a capacity of 18,000 l. The ponds are seeded with the desired species of algae and fed with conventionally-treated wastewater. Paddles circulate the water and algae continually. As the algae grows, a proportion is removed daily, with more wastewater added in a semi-continuous manner. At present, the ponds are treating around 3,000 l/d of water and removing 80–96% of the phosphorus.
The algae used for the HRAP system has been specially selected. Arnot told The Chemical Engineer that the researchers went “bioprospecting” in the discharge from wastewater treatment works to find algal species which were particularly abundant. They then screened those algae for their ability to remove nutrients, and which could settle through gravity to allow for easy removal. They settled on a strain of Scenedesmus, which was first tested in the laboratory and then in raceways in a greenhouse before being trialled in the HRAP system at Beckington.
Arnot, Scott and the team are using the trial period to determine the long-term performance, cost, reliability and practicality of the system in a real-world setting. While HRAP systems are in use in other parts of the world specifically for growing algae, it is one of the first examples where the system has been adapted for phosphorus removal for wastewater in the UK.
“HRAPs could offer an environmentally-friendly, chemical-free, sustainable and economical way of removing phosphorus from wastewater due to this approach utilising a naturally occurring process and removing the need to buy, transport and store costly chemicals. Depending on the use of the algal slurry, the technology also offers a means to recycle and re-use phosphorus as fertiliser,” said Jane Youdan, an environmental scientist at Wessex Water.
The project is one of several in the UK testing new methods of phosphorus removal, and is being funded by UK Water Industry Research (UKWIR). Arnot said that at the end of the 12-month trial period, the researchers will share their results to help determine the most promising methods to take further.
