Phosphorus is critical to life and is needed by both plants and animals. It enables the normal functioning of every cell in our bodies, and to make things like bones, teeth and shells. It is also one of the three essential nutrients for plant growth. Humans and animals obtain phosphorus from consuming food, while plants obtain it from the soil through their roots.
By Eve Germain-Cripps
Wastewater Innovation Process Manager, Thames Water
Phosphorus cannot be manufactured and there is no substitute for it. Most of the phosphorus currently comes from phosphate rock, which is a finite resource created millions of years ago when phosphate from seawater, bones and waste products from sea creatures deposited as sediments at the bottom of the primeval seas. Most of the phosphate rock reserves being depleted are in China, Morocco and the USA and, given the finite nature of phosphorus, there is an ongoing debate on the sustainability of these reserves to meet current and future demands. Over time the quality and accessibility of the phosphate rock will deteriorate, which will lead to ever increasing mining costs and therefore higher prices for phosphorus products.
In Europe the main use of phosphorus is in the production of fertiliser for food crops. Some of it is also used for making feed grade additives for livestock. In the water industry, a very small amount is added to drinking water supplies to prevent pipes from leaching lead and other metals into the tap water.
We find phosphorus in sewage as, while some of the phosphorus contained in the food and drinks we consume is taken up by our body, most of it is excreted. Phosphorus can also be found in some toothpastes and detergents. Some industrial and trade effluents can also contain phosphorus adding to the amount of phosphorus found in sewage.
However, while phosphorus is essential for all living matter, too much of it (i.e. eutrophication – nutrient enrichment of a water body), especially in rivers and lakes, can damage the local ecosystems and impact on
biodiversity and water quality. As phosphorus is a key nutrient for vegetation, a surplus of phosphorus can lead to excessive growth of algae and other aquatic plants, which can choke the river, impacting flows and increasing flood risk. Furthermore, when these algae and plants die and decompose, they use up the oxygen available in the water, causing the fish and other aquatic animals to either leave the affected area or die. The effect of climate change and potential reduction in river flows might also exacerbate the phosphorus concentrations in the watercourses leading to more eutrophication.
Eutrophication is often a consequence of human activity with run-offs from agricultural land and effluent from sewage treatment works typically the largest two sources – but to a different extent depending on the location (rural or urban). Therefore, in order to protect the aquatic ecosystem, we need to ensure the phosphorus we discharge from our sewage treatment works is low enough to at least not deteriorate the water quality in the receiving watercourses or to improve its ecological condition.
This amount of phosphorus we can discharge to a specific watercourse is regulated, amongst other regulations, by the Water Framework Directive (WFD). This establishes a framework for the protection of European waters to achieve good ecological status for all the water bodies across the EU. Each water body and each sewage treatment works’ discharge are assessed on a case by case basis to calculate the total phosphorus permit needed for a specific watercourse to attain good ecological status. In the UK, the agreed Technically Achievable Limit (TAL) is currently 0.5 mg/l for AMP6. However, following extensive technology trials carried out by Thames Water and nine other water and sewerage companies to inform treatment requirements post 2020 and the planning for AMP7 the minimum total phosphorus permit was tightened to 0.25 mg/l for AMP7.
Under the Urban Waste Water Directive (UWWTD) any sewage treatment works discharging to a nutrient sensitive watercourse with a population equivalent (PE) between 10,000 and 100,000 had to meet a total phosphorus concentration limit of 2 mg/l, while any sites with a PE. over 100,000 had to meet a limit of 1 mg/l. Therefore, most of the medium to large sewage treatment works in the Thames Water region already have the required infrastructure and resources to remove phosphorus down to concentrations of 1 or 2 mg/l.
These types of permits are usually achieved using well known methods based on chemical or biological treatment. The chemical solution uses iron or aluminium salts as a coagulant, which results in the precipitation of metal-phosphorus complexes as solids in primary settlement tanks. The biological method relies on a specific configuration of activated sludge plant, which facilitates the so-called “luxury phosphorus uptake”. However, it often requires an additional carbon source. This can be achieved by either fermenting the primary sludge in the primary settlement tanks or by fermenting the return activated sludge (RAS) or mixed liquor in a dedicated fermentation zone to increase the amount of volatile fatty acids (VFAs) available. Another option is to dose an external carbon source such as methanol; however, in this case, it is probably easier and more cost effective to dose a metal salt.
With these types of processes, the phosphorus removed from the wastewater is not lost as it is retained in the sludge. After anaerobic digestion, the sludge is recycled to land as fertiliser or soil conditioner. As sewage sludge contains major plant nutrients (such as nitrogen and phosphorus) and organic matter, it is a sustainable fertiliser source which is inexhaustible and always available.
For the new low phosphorus permits of 0.5 mg/L, removal by chemical dosing alone or bio-P uptake is not sufficient and additional removal steps are needed. This is commonly achieved by adding a chemical dosing point after the secondary treatment process. A tertiary solids removal plant is also usually required as the solids present in the sewage effluent will contain a level of phosphorus high enough to compromise its quality.
Chemical dosing followed by tertiary solids removal is a well understood and easy to implement solution for medium to large works. It is more problematic for small unmanned rural sewage works, which usually use low tech, low energy processes. These sites do not tend to have the infrastructure required to implement such solutions, such as access roads for chemical deliveries, potable water for safety showers or sufficient power. To overcome this challenge at Thames Water we have developed a passive solution using constructed wetlands. Instead of using gravel to fill the wetlands, we use specific selected media with a high adsorption capacity for phosphorus. This way, the phosphorus is captured in the wetlands without the need to add chemicals. We were the first company to trial an Apatite based media at full scale in the UK. As our trials have demonstrated that it was the most suitable media for use in such systems, we are now looking at optimising the solution by developing different configurations of various footprints to make it suitable to most small sites. We are also looking at ways of recovering the phosphorus that has been captured on the media to be able to reuse the media and recycle the phosphorus.
For the ultra-low phosphorus consents of 0.25 mg/L, more complex technologies will most likely be needed. These usually involve a combination of metal salts and polymer addition to increase the phosphorus capture and combined with a ballast (e.g. sand) to speed up the settlement time (and therefore reduce the process footprint). These are well used in countries already implementing ultra-low phosphorus permits, such as the US, and are now starting to be installed in the UK.
The key issue with all tertiary treatment chemical based solutions is the carry-over of coagulant (iron or aluminium) in the sewage effluent. However, this can be managed by controlling the dose of chemical based on the actual phosphorus load to be removed using real time control systems. Phosphorus analysers and flow meters need to be installed as well as a control system with an algorithm to calculate the required chemical dose. This prevents the over dosing of metal salts and therefore protect the effluent quality.
Other technologies, which combine removal and recovery of phosphorus, have started to emerge such as biocatalysts using bio-P organisms or ion-exchange systems.
Currently phosphorus is reused through the application of sewage sludge to land, but other processes exist for the recovery of phosphorus from wastewater, sludge dewatering liquor, sludge or ash/char (for sludge incineration or pyrolysis). These are usually recovered as struvite (like the slow release fertiliser we are producing at our phosphorus recovery plant located at Slough Sewage Treatment Works), calcium phosphate or phosphoric acid.
Developing resilient solutions to remove and recover phosphorus to protect and enhance our environment is a key part of our Research and Development programme.