Replacing chemical treatments with algae farming could make wastewater treatment cheaper and more sustainable, according to researchers at Bath University who are testing the method in partnership with Wessex water. The method is particularly suited to removing phosphorus from sewage, an increasing problem for water treatment.
Phosphorus is an ingredient in detergents, cleaning products and agricultural chemicals, and also gets into the wastewater streams via human waste. It is a particular problem because it is a nutrient for plant growth, and if it gets into the environment it encourages the overgrowth of aquatic plants in rivers, streams and ponds. This depletes dissolved oxygen in the water in a process called eutrophication. This then leads to loss of fish and insect life, damaging the health of watercourses.
Currently, phosphorus is removed at sewage treatment works by dosing wastewater with iron salts. However, tightening regulations on water quality are creating a need for new methods.
The Bath team, led by chemical engineer Tom Arnot and Prof Rod Scott of the Department of Biology and Biochemistry, is trying an approach that turns the problems posed by phosphorus into an advantage.
Phosphorus-containing wastewater streams are fed into shallow ponds that have been seeded with algae, and phosphorus acts as a fertiliser encouraging the aquatic plant to grow. This reduces the level of phosphorus in the water. Some of the water and algae mixture is then removed into a settling pond, and the "polished" water, whose quality is suitable for it to be released into rivers and ponds, is separated, leaving the algae behind. The treatment pond is refilled with more wastewater.
The method is currently under test with a small pond at Beckington sewage treatment works. Known as a high rate algal pond (HRAP), it is seeded with algae that can be used as a feedstock for bioplastics, biofuels and agricultural fertiliser.
The pond, with a surface area of 60m2, treats around 3000l of wastewater per day, and is removing 80 to 96 per cent of phosphorus content. The team is hoping to confirm that HRAPs such as this are practical in the inconsistent light and temperature conditions of the UK weather, and believe this method may be particularly suited to smaller sewage treatment works that serve communities of around 1000 people, of which Beckington is a good example.
“In theory, HRAPs could offer an environmentally friendly and sustainable way of removing phosphorus from wastewaters and consequently improving the health of our rivers and lakes without a massive increase in consumer’s water bills,” said Dimitris Kaloudis, a research associate and operator of the trial.“In this trial, one of the first of its kind, we are looking to establish how the technology performs in realistic scales and conditions as well as to understand and address any challenges that may arise during the course of the trial.”
The trial is one of a number being carried out by water and sewerage companies in England and Wales to investigate new phosphorus removal techniques.
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