Water companies are legally obliged to control the level of phosphorus (P) in the wastewater they discharge, because of the eutrophication effect it causes in aquatic environments. Phosphate is a nutrient and if released unchecked into rivers or other bodies of water, it promotes the growth of algae and other plants, turning waterways green and causing hypoxia (lack of oxygen) in the water which kills fish and other aquatic animals.
European directives have led to ever tightening consents on phosphorus removal. The Urban Wastewater Treatment Directive in 1991 set a consent of 2mg/l for larger works or those discharging into sensitive waters; however, the Water Framework Directive in 2000, with its focus on continuous improvement and achieving ‘good’ status for all watercourses, has led to 0.5-1.0 mg/l limits being typically imposed by the Environment Agency in AMP6. However, in order to achieve the ultimate aim of ‘good’ WFD status across the board, it is thought that limits as low as 0.1 mg/l will be necessary. Most experts agree this is not economically realistic given current technology, which is why it remains a focus for innovation.
Conventional biological treatment processes remove only 50% or less of sewage phosphate, and so substantial improvement is needed to achieve the 90% or more removal to reach effluent concentrations of 0.5 to 1.0 mg phosphate per litre. The most common approach currently used by water companies involves dosing with chemicals such as ferric sulphate at the primary stage. However, this has some limitations, notably the cost of the chemicals (which is likely to increase with the closure of UK steel plants since ferric sulphate is a by-product of making steel), the high operational cost of running a dosing system, and unwelcome effects such as elevated iron levels which may require further treatment.
A programme of trials of phosphorus-removing technologies is taking place throughout the AMP6 period (2015-20) supervised by the Environment Agency and involving UK Water Industry Research (UKWIR) and several water companies and universities. Most of these are tertiary processes which operate in addition to an activated sludge process to give the final effluent a ‘polish’.
Filtration through reed beds is one low opex-cost option which requires no energy, and is particularly suitable for rural sites where there is sufficient space. The reed bed acts a filter system: reeds are planted with a layer of aggregate laid below. In 2010, Wessex Water initiated a research project in partnership with the Environment Agency and Cranfield University to construct and monitor site trials of reed beds, with Tarmac contracted as materials partner to providing aggregates for the scheme. Six aggregates were trialled initially over two years, with steel slag being found to be the most effective, removing around 70% of phosphorus. A second trial was then set up to focus on steel slag, investigating its vertical and horizontal capacity for phosphorus removal. Reed beds also create a circular economy as the phosphorus-enriched steel slag can then be sold on as a fertiliser.
Another option is sand filtration using ferric salts. Anglian Water is currently carrying out a 12-month trial of Hydro International’s DynaSand Oxy vertical sand filters at its Watton Water Recycling Centre in Norfolk; this offers simultaneous ammonia and phosphate removal, and is the first project of its kind to aim for ultra-low phosphate consents of 0.3 mg/l and lower. By introducing ferric salts to the effluent before it enters the vertical sand filters, the phosphate is combined with the ferric to form a filterable precipitate that can be easily removed by the sand filter. The washwater is then co-settled in the primary tank before being removed from site to an anaerobic digestion plant.
Two proprietary technologies that build on the established chemical dosing methods are Blue PRO, made by Blue Water Technologies, and CoMag, made by Evoqua Water Technologies. Blue PRO uses reactive filtration and adsorption to achieve P removal with a lower chemical load than simple dosing; meanwhile CoMag infuses magnetite as a weighing agent into conventional chemical floc, enhancing settling rates and increasing the performance of existing wastewater treatment facilities.
Finally, enhanced biological phosphorus removal can be deployed, in which specific bacteria are selectively enriched and accumulate large quantities of phosphorus within their cells (up to 20% of the mass). When the biomass enriched in these bacteria is separated from the treated water, these biosolids have a high fertiliser value. A well-buffered system maintains the pH where biological activity is optimised and allows for a more efficient use of oxygen, leading to reduced energy costs. Treating wastewaters using lime-based reagents (such as Lhoist’s Neutralac SLS45) increases the wastewater’s buffering capacity and helps the phosphorus removal step.