Water companies need to adopt efficient ways of reducing phosphorus levels in water, in order to banish harmful algae from rivers and lakes
Water companies are under pressure from all corners – environmentalists, the public and legislators – to tackle eutrophication, the over-enrichment of waterways by mineral and organic nutrients from sources such as run-off of agricultural fertilisers. The main culprit is phosphorus, in the form of mineral phosphates, resulting in excess growth of algae and other plants. Freshwater algae are simple plants divided into many microscopic species; planktonic algae (or phytoplankton) can clog waterways, while filamentous algae may form thick, floating mats.
Algal blooms deprive submerged plants and creatures of sunlight, and ultimately suffocate them. In daylight, they generate oxygen through photosynthesis. However, they consume it during respiration at night, reducing it to levels where some animals cannot survive. Eventually the algae die and are decomposed by bacteria, further stripping the water of oxygen, and killing fish, invertebrates and plankton.
Some algal blooms also produce toxins that can be lethal to any animals that drink the water. Contact with these toxins can cause skin irritation and other symptoms in humans. Health may also be damaged by toxins that enter the food chain.
Communities are concerned by these dangers, by the impact on wildlife and the loss of recreational and amenity value in affected habitats. Many waterways and lakes do not meet strict standards set for nutrient levels under the UK’s Water Framework Directive. It seems inevitable that water bills will rise, as extra purification efforts are needed to solve the issue.
AMP7, the water industry’s Asset Management Period for the years 2020-2025, demands stringent phosphorus removal. Larger sites must improve, while many small sites, which previously had no need for chemical dosing, must introduce it. Many smaller sites have little or no historical data on which to base dosing specifications, so will require testing. Installing dosing systems in the confined space of small sites is also likely to be challenging.
Meanwhile, industry regulator Ofwat’s latest price review, PR19, challenges companies to deliver more efficient, cost-effective solutions than their current framework designs allow. It insists that customers receive better value for money, with better services but no increase in bills. For this reason, water companies are under pressure to make improvements, but without spending too much.
Phosphorus reaches our waterways from several sources. As already stated, these include run-off of agricultural fertilisers (artificial and organic), as well as animal waste from fields, human waste from sewage systems, and detergents from household drains. Waste discharged into waterways – from various industries – also contributes to higher phosphorus levels and increased eutrophication.
Without action, the potential for eutrophication will grow – and is likely to be compounded by climate change. Heavy rain and flooding are becoming more frequent, while summers are becoming hotter and drier. Higher phosphorus concentrations in water during drought periods, together with higher temperatures, will boost algal growth.
Chemical treatment involves dosing the water with metallic salts, such as iron sulphate or calcium carbonate, that react with dissolved phosphate to produce solid precipitates. The precipitate can be removed using a separation process such as clarification or filtration.
This method can be combined with biological treatment using anaerobic and aerobic digestion. This promotes the growth of anaerobic and aerobic bacteria, which feed on soluble phosphates and remove them from the water. The bacteria, and the phosphorus they have consumed, are then separated into the resulting sludge. When used in combination, digestion usually comes first. Chemical treatment then reduces phosphorus to a lower level (below 1.0 mg/L).
Before chemical treatment and digestion, larger phosphate-containing particles can be removed by processes such as sand filtration and solid settlement. Afterwards, phosphorus can be further reduced by passing the water through a membrane filtration system.
These phosphorus removal techniques should not be confused with sanitisation, where the aim is to kill undesirable organisms rather than remove the pollutant. Prior to human consumption, water tends to be chlorinated. This kills harmful bacteria and any algae present and continues to disinfect after it leaves the plant. Chlorinated water should not be released directly into natural waters. The chlorine and its by-products would also kill microscopic life forms that form part of a healthy ecosystem. Doing this would also feed phosphorus into the waters, if this method is used as an alternative to phosphorus removal.
UV irradiation and ozone treatment also kill microorganisms, but do not adversely affect life forms downstream. Again, they do not remove phosphorus.
When planning chemical treatment at a site whose dosing needs are unknown, such as smaller sites, testing is required. Jar tests are of limited value, as they give only a snapshot of the conditions. Instead, operators need to gain a full picture of the upper and lower dosing limits. This can be done economically by hiring a dosing rig.
The smallest, simplest packaged systems for this purpose consist of a dosing device and an intermediate bulk container (IBC) of chemical. Larger options include a self-contained system within a waterproof enclosure with a 1,000-litre storage tank and a dosing device. For extra size and functionality, there is the option of a containerised system with storage tank, duty and standby pumps and a local control panel. With this type of equipment, tests can be run in real-time over a typical hire period of four to 12 weeks. The levels established in these situations usually require very low dosing flows, often down to 0.1 L/hr.
Once dosing needs are known, a long-term solution appropriate to the site can be supplied. Specifications usually start with 1,500 litres of storage, giving enough capacity to refill from an IBC without having to interrupt dosing. The system would normally include a fill point panel, pipework, a set of duty and standby pumps, and a control system.
Components are installed within a suitable enclosure, chosen from a range of options. They vary from a rotationally moulded model to a fabricated bund with a GRP (glass-reinforced plastic) weatherproof kiosk and walk-in enclosure. WES’ larger packages fit the footprint of a 20-foot shipping container. Depending on the operator’s needs, designs are available to accommodate storage tanks of any size up to 5,000 litres. For the largest sites, a 10,000-litre option can be made available. Space for extra pumps can also be added.
Complete chemical dosing set-ups can typically be bought or hired, complete with storage tanks, bunds, filling and safety systems, pipework, connectors and control features. Components are pre-assembled and pre-tested to cut installation time. Hired systems can be ideal for dealing with urgent needs, short-term increases in demand and scheduled shutdowns. They also help to conserve capital expenditure. Eutrophication is a blight on our natural waterways. However, it can be managed by applying appropriate phosphorus removal techniques.
By Graham Ward, WES