Five Star Customer Service
Stress Free Switching
Utility Management
Competitive PricesWhat trade effluent is
Trade effluent is, in the legal definition used by UK water wholesalers, any wastewater discharged from a business premises that is not domestic sewage. The boundary isn’t always obvious. A small office with kitchenettes and toilets produces only domestic-strength sewage. A restaurant produces trade effluent the moment it scrapes a plate. A car wash produces it from the day it opens.
Sectors that generate trade effluent as standard:
- Food production, restaurants, pubs, hotels with significant kitchens
- Manufacturing (paint, chemicals, textiles, paper, food production)
- Brewing, distilling, beverages
- Laundries, dry cleaners, commercial linen services
- Vehicle washes, garages, MOT centres, body shops
- Cooling water from refrigeration or HVAC plant
- Healthcare, dental, veterinary, laboratories
- Some agricultural operations (silage, dairy, livestock washdown)
Trade effluent is legally distinct from domestic sewage because of what it contains — food fats, chemical residues, oils, suspended solids, biological oxygen demand. The wholesaler has to treat it differently and charge accordingly.
Do I need a trade effluent consent?
Yes, if you discharge anything other than domestic-strength sewage into the public sewer. Discharging trade effluent without a consent is a criminal offence under the Water Industry Act 1991 and can carry an unlimited fine. The wholesaler can also disconnect the supply.
A consent is granted by your regional wholesaler. The application sets out:
- The type and origin of the effluent
- The maximum daily volume to be discharged
- The maximum rate of discharge
- Quality limits (biochemical oxygen demand, suspended solids, pH, temperature, prohibited substances)
- Any required pre-treatment
- Sampling and monitoring requirements
Consents are site-specific, not business-wide. If you have multiple premises, each needs its own. Once granted, consents are typically reviewed every 3 to 5 years, but the wholesaler can re-open them earlier if the operation changes materially or if monitoring shows breaches.
How trade effluent is billed
Trade effluent appears as a separate line on your water bill, distinct from the sewerage charge and the surface water drainage charge. The volume is either measured directly (if you have a trade effluent meter installed) or estimated as a percentage of incoming water — typically 95% by default unless you’ve argued for a lower figure.
The rate per cubic metre is calculated using the Mogden formula, which the next section explains.
The Mogden formula explained
The Mogden formula has been the UK standard for pricing trade effluent since the 1950s. It calculates a charge per cubic metre by adding four cost components together:
Charge per cubic metre = R + V + (B × Ot/Os) + (S × St/Ss)
Where:
- R = Reception cost. Fixed cost of receiving and pre-treating effluent.
- V = Volume cost. Cost of pumping and moving the effluent through the network.
- B = Biochemical treatment cost per unit of oxygen demand.
- Ot = Your effluent’s oxygen demand (mg/litre).
- Os = Standard sewage oxygen demand (used as a benchmark — typically 300 mg/l).
- S = Sludge disposal cost per unit of suspended solids.
- St = Your effluent’s suspended solids (mg/litre).
- Ss = Standard sewage suspended solids (typically 300 mg/l).
Each wholesaler publishes its R, V, B, and S coefficients annually as part of its charges schedule. They change every year — typically rising in line with treatment cost inflation.
Worked example. If your effluent has biochemical oxygen demand of 600 mg/l (double the standard) and suspended solids of 400 mg/l, your Mogden charge would scale accordingly. A site discharging at standard sewage strength might pay around 70 to 90 pence per cubic metre of trade effluent. A site discharging at higher strength might pay £1.50 to £3.00 per cubic metre or more.
Sewerage vs trade effluent
Two related but distinct charges sit on most business bills with trade effluent:
| Charge | What it covers | How it’s calculated |
|---|---|---|
| Sewerage | Domestic-strength wastewater from toilets and kitchenettes | Volumetric — usually 95% of incoming water as default assumption |
| Trade effluent | Process wastewater above domestic strength | Mogden formula on metered or measured volume |
| Surface water drainage | Rainwater running into the public sewer | Based on chargeable hard-standing area |
A site with trade effluent typically has all three on the bill. The sewerage portion bills the toilet flushes and sinks; the trade effluent portion bills the process discharge; surface water bills the roof and yard.
The three common overcharges
1. Estimated volume too high. If your bill assumes 95% of incoming water becomes trade effluent but your actual discharge is lower — because water is lost to product, evaporation, irrigation, or cooling tower bleed-off — you’re paying for effluent that never left the site. A volume study (measuring actual discharge over a representative period) can establish a lower percentage. Reductions from 95% to 70%, 60%, or lower are common in sectors with significant non-sewered uses.
2. Outdated Mogden coefficients on your consent. If your consent was written 10+ years ago, the assumed Ot and St values may reflect older, less efficient processes. Modern equipment, cleaner-in-place chemistry, or pre-treatment investments may have reduced your actual effluent strength. Sample testing and a consent re-negotiation can update the coefficients.
3. Wrong wholesaler. On some sites — particularly those with on-site treatment plants — trade effluent goes to a sewer owned by a non-incumbent operator. The bill may still be issued by the regional wholesaler when it should be issued by another party. Worth checking on industrial estates and large multi-tenant sites.
How to challenge a trade effluent bill
Five steps:
- Pull your consent. Read what it actually says about volume, strength, and limits. Many businesses have never read theirs.
- Pull your last 12 months of bills. Identify the trade effluent line, the assumed volume percentage, and the strength coefficients being applied.
- Compare to current reality. Has your operation changed? Equipment, processes, products, scale? If yes, your consent inputs are probably stale.
- Commission an effluent strength test if challenging the Mogden inputs. A standard 24-hour composite sample analysed for BOD and suspended solids costs £200 to £600.
- Apply for a consent review via your wholesaler if the test data supports lower coefficients. Most wholesalers have a formal process for this.
Backdated adjustments are typically limited to 12 months but can be argued further if the wholesaler accepts the data was wrong for longer.
Working with Clearsight on trade effluent
Every business water audit we run includes a trade effluent review where applicable — consent inspection, volume study, Mogden input review, and rebate or rebalancing claims where the data supports them. No upfront fees.
Get a no-obligation business water quote in 60 seconds.
Related guides: Business water pillar, How to read a business water bill, SPID number explained, Wholesale vs retail market.