Eutrophication can impact a water company’s bottom line as well as having a significant effect on the environment. In AMP 7 there will be heightened pressure to tackle this growing problem. Kevin Wheeler, managing director at chemical dosing specialist, WES Ltd, explains how to tackle P removal.
Phosphorus, combined within various mineral phosphates and organic materials, is a key cause of eutrophication in rivers, lakes and other surface waters. Eutrophication is the over-enrichment of water by mineral and organic nutrients, which stimulate the excessive growth, or blooming, of algae. Algal blooms are a problem as they deprive submerged plants and creatures of sunlight and, ultimately, suffocate them. Decomposition of the dead algal mass by bacteria, then strips even more oxygen from the water and leads to the mass killing of fish, invertebrates and planktonic animal life.
The key sources of phosphorus reaching our waters include the run-off of agricultural fertilisers and animal waste from fields, human waste from sewage systems, detergents from household drains and waste discharged into waterways from various industrial processes.
Environmentalists, legislators and the public are putting increasing pressure on water companies to tackle eutrophication. In the water industry’s Asset Management Period covering the years 2020 to 2025 (AMP 7), more stringent requirements for the removal of phosphorus are expected. This will not only increase demands relating to larger sites but will create a need for treatment on smaller sites where chemical dosing was not previously required. Many of the smaller sites have limited historical data on which to base dosing specifications, so testing will be required. A further challenge will be how to install dosing systems in the confined space of those small sites. At the same time, these improvements must be achieved without impacting too heavily on profits.
Options for phosphorus removal
Sand filtration and solid settlement are initial treatments which involve simple processes such as the removal of large particles of material containing phosphate. Often the next step is biological treatment using anaerobic and/or aerobic digestion. These processes provide conditions favourable to the growth of anaerobic and aerobic bacteria, respectively, which remove soluble phosphates from the water by feeding on them. The bacteria, along with the phosphorus they have consumed, are then separated into the resulting sludge. Digestion tends to be applied ahead of chemical treatment, which then reduces phosphorus to lower levels (below 1.0 mg/L).
Chemical treatments are an economical means of removing phosphates from wastewater. They typically involve the dosing of water with metallic salts, which react with dissolved phosphates to produce solid precipitates. These materials can then be removed using a solids separation process, such as clarification or filtration.
After these treatments, some companies may wish to reduce phosphorus levels even further by passing the water through a membrane filtration system.
Dosing test packages
If the chemical dosing needs of a site are unknown, as is the case for many smaller sites, there is a need for testing. Jar tests are of limited value as they only provide a snapshot of the conditions. To gain a full picture of the upper and lower dosing limits, economically, operators can hire a dosing rig on a short-term basis and still get value for money.
The smallest and simplest packaged systems available from WES for this purpose comprise a WES DosingCube™ and an IBC (intermediate bulk container) of chemical, sitting on a double IBC bund. Larger options include a self-contained system within a waterproof enclosure with a DosingCube™ and a 1,000-litre storage tank. There is the option of a containerised system with storage tank, duty and standby pumps and a local control panel for further capacity.
Using a package like this, tests can be run in real time over a typical hire period of between four and 12 weeks. Dosing flow requirements established by testing in these situations tend to be very low– often as little as 0.1 L/hr. This has important implications and potential pitfalls for system design, but with the benefit of extensive experience in this field WES has engineered any problems out of its temporary and permanent dosing packages.
Long-term or permanent packaged solutions
WES can supply a long-term packaged solution appropriate to the site, once dosing needs are known. Specifications typically start with a 1,500-litre storage tank, which gives enough capacity to refill from an IBC without having to interrupt the dosing. However, storage tanks of up to 5,000 or even 10,000 litres can be used for larger sites.
Eutrophication is a significant issue and one that is only going to get worse. Industry regulator Ofwat has challenged water companies to deliver more cost-effective and efficient solutions than their current framework designs allow. Chemical treatments offer an economical means of removing phosphates from wastewater and a positive result for the environment.
www.wes.ltd.uk
