Water & Wastewater Treatment Magazine
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30 | FEBRUARY 2020 | WWT | www.wwtonline.co.uk P hosphorus, in the form of various phosphates, is a key cause of eutrophica- tion – the over- enrichment of lakes, rivers and other surface waters with mineral and organic nutrients. It stimulates excessive growth of algae, which can turn the water green or form thick float- ing mats. As well as depriving other plants and animals of sunlight, algal blooms consume huge amounts of the water's oxygen with their respiration at night. Eventually, large-scale bacterial decomposition of dead algae strips even more oxygen from the water, leading to mass die- offs of fish, invertebrates and microscopic animal life. In ad- dition, toxins produced by some algal blooms may be harmful to livestock, dogs, wildlife and humans. The big challenge for water companies While environmentalists, legislators and the public are pressing water companies to combat eutrophication more effectively, there is also pressure to reduce the cost of improvements. Ofwat's latest price review, PR19, challenges them to deliver more cost- effective and efficient solutions than their current framework designs allow. It wants them to bring customers improved services and better value for money, without increasing prices. More stringent phosphorus removal levels are expected in the water industry's next asset management period, AMP7. One effect will be a demand for treatment on smaller sites where chemical dosing was not previously necessary. O†en these will have little or no historical data on which to base dosing specifications, so testing will be required. Another challenge will be how to install dosing systems within the confined space of small sites. Climate change is likely to intensify eutrophication problems, as summers become hotter and drier. Higher tem- peratures, together with con- centration of phosphorus in water during drought periods, will boost algal growth. Removal options Treatment may begin with simple processes like sand filtration and solid settlement, which remove larger parti- cles of material containing phosphate. Biological treat- ment using anaerobic and/ or aerobic digestion is o†en the next step, using bacteria to remove soluble phosphates from the water by feeding on them. These bacteria, along with the phosphorus they have consumed, are then sepa- rated into the resulting sludge. Phosphorus removal The Knowledge: eutrophication l The main sources of phos- phorus reaching our waters include: l Run-off of agricultural ferti- lisers (artificial and organic) and animal waste from fields l Human waste from sewerage systems l Detergents from household drains l Waste discharged into water- ways from various industries Many waterways and lakes are currently failing to meet strict standards set for nutri- ent levels under the UK's Water Framework Directive. For local communities, the related health risks, wildlife impacts and loss of recrea- tional and amenity value are big concerns. There is also a worry that extra purification efforts to deal with the issue will inevitably mean higher water bills. Kevin Wheeler of WES summarises the growing challenge of eutrophication, and the treatment options available to tackle it.