Unlocking industrial growth with catchment management of trade effluent

28 February 2025

Everything we consume, from food to furniture, produces trade effluent, so by managing it more effectively, you encourage much needed economic growth

Trade effluent is the forgotten cousin of the water industry. Less immediate than the need for clean, safe drinking water and less emotive than sewer overflows, it is effectively invisible to the public. And yet, everything we consume or buy, from food to cars to pharmaceuticals to textiles, produces trade effluent. As a result, it has bearing on all areas of the UK economy.

The Water Industry Act (1991) defines trade effluent as: “any liquid, either with or without particles of matter in suspension in the liquid, which is wholly or partly produced in the course of any trade or industry carried on at trade premises”. The law clearly states that domestic sewage is not trade effluent, meaning that, if your business discharges any sort of wastewater other than domestic sewage, you’re discharging trade effluent. This opens up a world of legislative compliance for traders (those who discharge the effluent) and regulation for the water companies (who monitor and manage it).

How trade effluent is managed

Publicly-available, though incomplete, data from Freedom of Information (FoI) requests suggests that there are at least over 12,000 trade effluent consents in the UK – the real figure is likely to be higher. These range from tiny discharges of perhaps 100 litres per day up to volumes of 7 million litres per day or more from a single site.

Wastewater treatment works (WwTW) have permits that allow the discharge of treated wastewater of a specified composition and volume. Domestic sewage is not regulated and water companies have little control over what is in it. Because WwTW are designed with headroom to cope with the variations in domestic sewage, one of the few routes that water companies have for regulating what their WwTWs receive is by controlling trade effluent.

Think of this process as protecting the four ‘P’s: people, pipes, processes and permits. Some effluent can contain toxic substances that could harm workers in the sewer network and at WwTWs. In addition, corrosive substances can damage sewers, pumps, valves and other equipment, and common contaminants in trade effluent can inhibit biological sewage treatment processes and reduce their effectiveness.

Which brings us to the fourth ‘P’: the water company’s permit to discharge to the environment. The flow and load of domestic sewage follows trends that are reasonably predictable and that can be incorporated into the design and operation of a WwTW. Trade effluent, however, can contain more exotic contaminants, including PFAS and microplastics, many of which, if not regulated, can affect discharges to the environment.

Managing trade effluent catchments

There are around 10,000 WwTWs in the UK. These range from tiny sites that treat only domestic sewage from a few properties, to works that treat sewage and trade effluent from a population of over two million. Each WwTW has a surrounding catchment, with all the wastewater in that catchment draining to that WwTW.

The WIA states that water companies must, “have regard…to its existing and likely future obligations to allow for the discharge of trade effluent into its public sewers”. A wide-ranging statement if ever there was one. One interpretation is that water companies need to know how much trade effluent enters a particular WwTW so that it, and the domestic sewage in the catchment, can be treated to the required standard. An important word is “future”. This should direct water companies to make sure that they account for changes in trade effluent volumes in catchments over time.

All water companies know how many treatment works they have and the population equivalent (PE) they serve. The PE is the domestic population plus the quantity of non-domestic wastewater, expressed as an equivalent number of people. For example, a WwTW might treat a PE of 10,000. Of that, 8,000 could be real people and the trade effluent load is equivalent to another 2,000. It is very important that water companies quantify the trade effluent load, its sources, volumes and composition.

This is where catchment management is important: having as complete a knowledge as possible of trade effluent and domestic discharges enables a water company to understand the headroom at any given WwTW. Ideally, there should be a numerical record that links each WwTW to each trader in its catchment. This enables the water company to do several things. It can calculate the headroom for accepting additional domestic sewage (from new housing, for example). It can also understand its capacity to serve industry in the catchment, estimate any risks to the ‘four Ps’, and monitor threats to environmental water quality. The outcome of this analysis is a decision about whether the WwTW needs to expand or operate differently.

This is what should happen but each water company does things differently. Here at Blackwell, we work on trade effluent projects throughout the UK and we know that sometimes water companies might know how many trade effluent consents there are in a catchment but could do more to quantify the effects of those discharges on WwTW capacity and performance. By doing so, they could potentially identify situations where traders’ consent limits could be varied, thereby effectively taking the brakes off economic growth in a particular catchment.

The problem with quantification for trade effluent discharge

The effect that a trade effluent discharge has on a WwTW can be quantified using an Environment Agency (EA) procedure that uses sewage treatment reduction factors (STRFs). These are the amount of any given contaminant which will be removed by a WwTW.

A water company should use the EA procedure to estimate the effect a trade effluent discharge could have on the composition of the water into which the treated wastewater is discharged. The problem is that some of the STRFs are too low; using them in the EA procedure might lead to the trader being told that the WwTW cannot remove enough of a contaminant. The trader might then feel they need to install pre-treatment when, in reality, none is needed. This can require significant expenditure, with costs ultimately passed on to customers.

In extreme cases, the water company might say that it cannot accept the discharge at all. This can have devastating impacts: one project we worked on almost saw a multi-million pound investment in a factory written off because the water company made a poor trade effluent decision. Our work proved that the WwTW could handle the trader’s discharge, and the trader ultimately secured the consent. We are also aware of traders who have felt they have no choice but to shut down permanently as a result of trade effluent consent decisions.

Incomplete catchment data only compounds the problem, with traders penalised unfairly based on inaccurate assumptions about their discharges.

We would like to see a national review of STRFs. Water companies could also do a lot more to sample the influent to their WwTWs, which would enable a better understanding of the catchment, as well as helping to determine when STRFs need adjusting for a specific site.

In many cases we’ve worked on we’ve found that a water company will claim a lack of capacity for trade effluent or a potentially significant effect on the environment resulting from accepting a TE discharge. This is a questionable argument because the WIA requires water companies to take catchment growth into consideration and provide infrastructure that is fit for purpose.

Solutions for the challenges of quantification for trade effluent

The UK water industry has more data from more sources than it has ever had before. It started from a very low bar, however, and more data could be collected. It could also be analysed more effectively to extract more value.

In addition to a national review of STRFs, we’d like to see STRFs of all known pollutants reviewed regularly – pooling water company data would be very useful. The STRF procedure itself also needs to be reviewed because, in its current form, it can give unrealistic decisions. We have developed a variant of this procedure that we have used on several projects and which gives a more realistic and rigorous assessment of the data.

We showed our procedure to the EA during two projects and on both occasions the agency staff acknowledged, informally, that the approach was comprehensive and produced useful data. We used the same procedure on work on highway run-off discharges for the campaign group Stormwater Shepherds. This was not reviewed by the EA but we believe it shows that different approaches to data analysis can provide better insight into catchment management.

An EA spokesperson said: “The STRFs provide a cautious but flexible framework designed to protect water quality and environmental standards, which water companies are encouraged to adapt based on specific circumstances.

“We welcome discussions on matters that impact the regulation of trade effluent discharges, and the EA remains committed to supporting evidence-based practices and fostering partnerships to achieve these shared goals.”

Fair and effective trade effluent management is essential to the health of the UK economy. Without it, large parts of our manufacturing base would struggle to operate. Water companies work hard to manage trade effluent but there are inconsistencies and gaps in practice across the UK, especially where quantifying what happens in a catchment is concerned.

There is a huge opportunity here to improve catchment management through better use of data. This benefits everyone: by maximising the efficiency of sewer and wastewater treatment assets, it enables operational cost savings for water companies; it also provides a more accurate picture of the capacity available for trade effluent, enabling growth in the UK manufacturing sector.

In this way, trade effluent impacts us all. More intelligent catchment management really can reduce the trade effluent burden on UK plc without increasing environmental risk.

Author: David Brydon MCIWEM is technical director at Blackwell Water Consultancy.