Phosphorus river pollution traced back to sewage

3 March 2010 by Sara Coelho

Phosphorus pollution in English rivers comes mostly from sewage, rather than agricultural fertilisers. However, cleaning up household effluent is not a silver bullet for the problem of pollution in rivers - the whole ecosystem needs to be fixed to improve water quality.

Algal bloom

Algal bloom

Eutrophication, or over-fertilisation of rivers, is a huge problem because it leads to excessive growth of algae that smothers and shades out other aquatic plants and animals.

Phosphorus pollution is one of the main causes of eutrophication and that is why "it's important to know where phosphorus comes from," explains Professor Colin Neal, an ecologist from the Centre for Ecology & Hydrology (CEH) in Wallingford.

The problem cannot be solved just by removing phosphorus - the whole ecosystem needs fixing.

- Professor Colin Neal, Centre for Ecology & Hydrology

There are two major sources of phosphorus in rivers: the runoff from agricultural fields fertilised with phosphorus compounds, and household effluent from sewage treatment works and septic tanks.

Scientists have debated for many years which of these sources supplies the phosphorus which causes river eutrophication, but Neal and colleagues devised a way to tell them apart. The clue is the chemical element boron.

"Boron is used as a whitener in washing powders," says Neal. It's been the breakthrough ingredient that has made laundry 'whiter than white' over the years, so the commercials have said, and ends up in rivers through the sewage system.

What Neal and colleagues found is that the concentration of phosphorus in English rivers is closely related to the amount of boron. When and where boron goes up, so does phosphorus. This means the two chemicals are very likely to come from the same source: sewage.

"The phosphorus used in fertilisers for agriculture is tied to sediments and soils and stays there until it's washed away by the rain," says Neal. The timing is important and so is the bioavailability.

We need cost effective solutions that do not damage the rural economy.

- Professor Colin Neal, CEH

Most of the phosphorus from agriculture gets into the rivers during winter, when the water level is high, whereas the phosphorus from sewage is steadily supplied to rivers throughout the year. The biggest effect is felt in summer, when the river level is low and the sewage entering the river is not diluted, so phosphorus concentration is at its highest. This is also the time of the year when algae and plants are most active. So the bad effects add up.

Getting rid of phosphorus does not solve the problem

So if the pollution problem is phosphorus from household waste, then the simple solution is to clean the sewage, right? Not really.

Neal's long-term studies, published in Science of the Total Environment, show that the problem is more complicated. For example, along the river Kennet (a tributary of the Thames) all large sewage treatment works have had a lot of the phosphorus removed from their effluents.

However, ecological damage due to excessive algal growth is still a major problem in the Kennet. If anything, the conditions have become worse with the erratic weather patterns of recent years changing the timings and durations of high and low flows in our rivers.

This is probably due to decreased river flow, stagnant waters and a combination of factors that led to profound changes in the ecosystem. To Neal, the Kennet example shows that "the problem cannot be solved just by removing phosphorus - the whole ecosystem needs fixing."

"We have to think more about how to make our rivers clean and how to restore the ecosystem back to what was before," says Neal, and this, he argues, demands an integrated approach.

Neal considers that "there are big national issues here, linked to the requirements of the Water Framework Directive, that demands both clean and biologically healthy rivers". Vast amounts of money are being spent each year on removing phosphorus from effluent - almost a billion pounds in total over the past 15 years in England and Wales - and costs will increase, because the population continues to grow.

At the same time, there are policy pressures to reduce phosphorus loss from agriculture, which could affect farming livelihoods by imposing restrictions that may not significantly help to achieve these environmental goals.

"We need cost effective solutions that do not damage the rural economy," says Neal. "We need to address far more than phosphorus concentrations in rivers, such as flow, habitat, and water resources which requires new science that looks at the complex relationships between hydrology, biology, chemistry and habitat, as well as our interactions and needs."

'The strategic significance of wastewater sources to pollutant phosphorus levels in English rivers and to environmental management for rural, agricultural and urban catchments' - Colin Neal, Helen P Jarvie, Paul JA Withers, Brian A Whitton, Margaret Neal. Science of the Total Environment 408 (2010) 1485-1500, Available online 25 January 2010. doi:10.1016/j.scitotenv.2009.12.020

'Declines in phosphorus concentration in the upper River Thames (UK): Links to sewage effluent cleanup and extended end-member mixing analysis' - Colin Neal, Helen P. Jarvie, Richard Williams, Alison Love, Margaret Neal, Heather Wickham, Sarah Harman, Linda Armstrong. Science of the Total Environment 408 (2010) 1315-1330

'Decreasing boron concentrations in UK rivers: Insights into reductions in detergent formulations since the 1990s and within-catchment storage issues' - Colin Neal, Richard J Williams, Michael J Bowes, Michael C Harrass, Margaret Neal, Philip Rowland, Heather Wickham, Sarah Thacker, Sarah Harman, Colin Vincent, Helen P Jarvie. Science of the Total Environment 408 (2010) 1374-1385