y Pete Vale
Innovation Technical Team, Severn Trent
The importance of phosphorus
Phosphorus (P) is essential to life, it is part of the structure and function of all living cells. Without it we can’t grow crops, fruit or vegetables and it’s an essential ingredient in the feed that farmers give to livestock. It is also a non-renewable, finite resource that is becoming rapidly depleted.
However, P is also the most common cause of water quality failures in England. When P enters our rivers and streams its fertilising effect (termed eutrophication) can cause major ecological problems through the excessive growth of algae and plants. That’s why at Severn Trent we’re investing more than £100m on new P removal technologies at our sewage treatment works to improve the quality of water that we discharge into the environment.
Our modern way of life is very inefficient in its use of nutrients. We apply P to land as fertiliser, some of this may leach directly into the aquatic environment, some will accumulate in soils and some will be converted to food. The P that is converted to food will be consumed and thereafter a substantial proportion will be excreted ending up in sewage. In addition to the P excreted, sewage also contains P from detergents (although due to a ban on the use of P in laundry and dishwasher detergents, this has substantially declined in recent years), from phosphoric acid added to drinking water to prevent plumbosolvency and from trade effluent inputs. Untreated sewage typically contains 6 – 10 mg/l of P, and therefore removing P from the discharge of sewage treatment works (STWs) is necessary to protect our aquatic environment.
Progress in tackling P
The water industry has made great progress in controlling the discharge of P. Since the 1990s, under the EU Urban Waste Water Treatment Directive the removal of P was introduced at STWs discharging to some of the worst affected waters. Over the last 20 years the number of STWs with P removal processes installed has increased dramatically, so that by 2015 P reduction was in place at some 650 large STWs, with phosphorus discharge limits typically set at 1 or 2 mg/l total P. It is estimated that the UK water industry will have invested £2 billion by 2020 to improve treatment specifically to remove P. As shown in Figure 2, this has led to a substantial reduction of 60% in the amount of P discharged to rivers from STWs.
Tighter limits now required
Despite major progress in reducing P inputs to water, over half of assessed river water bodies and 3/4 of lake water bodies currently exceed their P standard. P is the most common reason for English water bodies not achieving “Good Ecological Status” as required under the Water Framework Directive (WFD).
To respond to the challenge of improving water quality still further, Severn Trent is investing over £100 million in P removal technology in AMP6 (2015 – 2020). An investment that will see around 100 STWs upgraded by March 2020. The WFD requires us to meet much tighter P limits than we have in the past (0.5mg/l), and Severn Trent have invested in technologies to meet even lower limits (to as low as 0.2mg/l). To meet these new targets, our existing technology needs to be upgraded, and we’re trying to do this in the most cost efficient and sustainable way.
Packington STW Low P demonstration facility
To increase cost efficiency, we have invested £4 million in a state of the art test rig at our Packington STW in order to trial a variety of innovative new phosphorus removal technologies.
These trials have led to us roll out a number of these technologies to our sites, delivering AMP6 totex efficiencies of over £13 million, a rapid return on innovation investment. For AMP7 we envisage further totex savings of at least a similar scale.
The technologies trialled employed a range of different techniques using physical, chemical and biological processes and we have evaluated different solutions for our different sized sites. The technologies assessed included proprietary technologies that have been implemented this AMP and truly novel processes that offer the possibility of more sustainable solutions in the future. The six technologies evaluated are described below:
1 Magnetite ballasted coagulation process (CoMag): this process combines a coagulant, a magnetite ballast and a polymer to produce a weighted precipitate that settles very quickly and effectively. The trial proved that very low P levels can be achieved with this process. As a result we are currently installing the process at full scale at Finham STW in Coventry, one of our largest sewage works to achieve a very tight P limit of 0.22mg/l. The CoMag process is now included in our design manual matrix and is seen as particularly suitable for large works with very tight P limits.
2 Pile cloth media filters (Mecana): This technology uses iron dosing to convert soluble P to particulate form and a fine weaved cloth to filter the precipitate. Pile cloth filters use relativity little energy and have proven effective at removing solids. We are installing these filters at over twenty sites in AMP6 and anticipate that this will become an even more widely used process in AMP7. These filters are included in our design manual as a low totex solution for tight P limits on small to medium sized works.
3 Iron dosed tertiary membrane filtration: This technology uses iron-dosed ultra-filtration membranes to ensure virtually all solids are removed from the effluent after iron is used to convert soluble P to particulate P, delivering extremely low effluent P levels. The trial at Packington demonstrated that the process would be capable of meeting the very tightest P permits (of 0.1 mg/l and below) although the totex is likely to be high.
4 Reactive media reedbeds: This replaces the conventional media (gravel) used in reedbeds with a media that reacts with the P and then filters it out. This process would be ideal for small treatment works where delivering and storing chemicals can be problematic. The steel slag media evaluated at Packington is a by-product of the steel industry. Although the process was shown to be effective at removing P, we still need to find a solution to the elevated pH of the effluent. Development work in this area continues with alternative media being evaluated, for example apatite, a natural rock media with an affinity for P. We have plans to run further trials this AMP with a view to full scale roll-out in AMP7 if successful.
5 Immobilised Algal Bioreactor: This utilises algae to remove P (and ammonia) rather than using chemicals like iron. This offers a more sustainable approach to meeting the challenge of the WFD. This extremely novel (world first) approach encapsulates algae in a bead, significantly reducing the energy required to separate the algae from the treated effluent. Further development is required to increase the readiness of the technology to a point where we can consider its full scale implementation.
6 Nano-particle embedded ion exchange: The final technology we have been evaluating is an ion exchange process, which removes the P by adsorbing it onto a media bed, meaning there is no need to dose chemicals. The media bed can be regenerated, allowing the P to be recovered in the form of a useful mineral (calcium phosphate). Although the ion exchange/adsorption element of the technology is well developed, further work is required to optimise the regenerant clean-up and P recovery. Cost modelling indicates that this process can be extremely cost effective if these technical issues can be overcome.
Perhaps the greatest attraction of this last technology is the opportunity for P recovery. Severn Trent has set out its vision to changing the way it delivers wastewater services to make the most of resources which are becoming scarcer over time, and to provide more innovative services for customers in urban catchments. By embracing this circular economy approach, Severn Trent will deliver energy neutral, bio-refineries, creating valuable products from what has traditionally been viewed as waste.
Although the drive to meet tighter and tighter P limits in our treated wastewater discharges remains technically challenging, the successes achieved this AMP indicates that the future is bright both for the health of our rivers and in conserving the Earth’s precious resources.