Quality act

Mott MacDonald has completed one of the most intensive surveys of aquatic ecology ever conducted in the UK for water and sewerage company Southern Water

Farmers rather than water companies can contribute most to improving river water quality, one of the UK’s most thorough ecological studies has found.

(Article taken from our customer magazine, Momentum)

If a river or stream is being polluted the automatic response is to look for the source. Organic, phosphate and nitrate pollution is often assumed to come from sewage treatment plants, from which treated effluent is discharged to local watercourses.

But such an assumption is not necessarily correct, cautions Mott MacDonald hydrologist Jon Pavey.

Mott MacDonald has completed one of the most intensive surveys of aquatic ecology ever conducted in the UK for water and sewerage company Southern Water. “Under the European Union Habitats Directive we had an obligation to study the impact of effluent from our wastewater treatment plants on receiving water courses,” explains Southern Water project manager Mike Dearsley. Meanwhile, the UK’s Wildlife and Countryside Act required Southern Water to look at the impact of discharging into watercourses forming parts of Sites of Special Scientific Interest. In all, watercourses affected by 22 sewage treatment plants were studied over a period of two years.

“The ecological picture is like an Impressionist painting. Up close you see only dots. From the other side of the room you have a convincing picture.” John

“The fundamental point that came out of the research was that to achieve a marked improvement in water quality you need to tackle diffuse pollution,” Pavey emphasises. Soil, fertiliser and animal manure are washed from fields into watercourses when it rains, along with run-off from poorly managed farmyard slurry tanks or silage bins. The problem of diffuse pollution is well recognised, but extremely difficult to control.

Research data and conclusions have been presented to environmental regulator the Environment Agency and water industry economic regulator Ofwat. “The results of the investigations will be used by the Environment Agency in its review of discharge consents to determine whether improvements in effluent quality are needed,” Dearsley says. “To augment existing treatment with further processes to remove phosphates would incur significant costs, which would need to be considered against the potential environmental benefits.” Costs of providing extra treatment would have to be passed on to customers. All of the UK water companies are currently preparing their draft business plans for the next five year regulatory period, 2010-2015. Findings will be central to Southern Water’s submission.

Detective work

Investigating the impacts of Southern Water’s 22 treatment works involved grouping them into 12 areas. “We also set up control sites which were not on the flow paths,” says Mott MacDonald ecologist Laura Henderson. Typically 30 sampling points were established in each area and the ecology at each point was surveyed twice a year for two years. At its peak the project employed over 50 people.

“We looked at large aquatic plants and at algae on plant stems and stones, which tell us about eutrophication – the quantities of phosphates and nitrates – which causes increased plant growth. We also looked at invertebrates, which give an index for organic pollution,” Henderson explains.

At each of the 30 sites more than 30 plant and animal species were monitored. Species data was correlated with water quality and flow readings. Because “ecology is incredibly complex and interlinked, pinning down the significance of the study’s raw statistics was a painstaking process,” Pavey says.

“We asked: If you altered the discharge characteristics, would it change anything? The study involved a comprehensive literature review which revealed that previous results had been interpreted inappropriately. “The ecological picture, consisting of biological information, water quality and flow readings is rather like an Impressionist painting. Stand up close and you see only dots. View it from the other side of the room and you have a convincing picture.”