The Mogden Formula forms the basis of trade effluent charges for businesses in the UK. Here we answer some of the most frequently asked questions about the complex calculation, including how businesses can reduce these charges with the right approach.
What is the Mogden Formula?
The Mogden Formula is the standard calculation used by UK water companies to determine the trade effluent charges that businesses have to pay to discharge wastewater to the public sewer system.
Why the Mogden Formula is used?
Trade effluent differs significantly based on the nature of the business and size of the site. For example, a diary processing facility will have a very different type of wastewater compared to a pharmaceutical manufacturing site.
Rather than a flat rate, the Mogden Formula is used to ensure that businesses pay a proportional fee based on the volume of wastewater and level / type of pollution, reflecting the difficulty of water treatment required. By linking costs to the volume and pollution level of wastewater, it encourages businesses to invest in water treatment and reuse to reduce long-term costs.
What is COD and why does it affect trade effluent charges?
Chemical oxygen demand (COD) is the amount of oxygen required to break down pollutants and organic matter in wastewater through chemical reactions. COD is one of the main metrics to evaluate the strength of trade effluent. Higher COD levels mean wastewater will require more intense and energy-intensive wastewater treatment. Therefore, the treatment cost per cubic metre (£/m³) will be higher, and this is reflected in the Mogden Formula.
What are suspended solids and why do they matter?
Suspended solids in wastewater are particles larger than two microns that are held in suspension by the water, rather than dissolved in it. The level of suspended solids is one of the main measures of water quality and a key part of the Mogden Formula. The more suspended solids that trade effluent contains, the higher the charge to the company will be.
The Mogden Formula equation
The Mogden Formula for calculating trade effluent charges is:
C = R + V + B(Os/Ot) + S(SS/ST)
What does each part of the Mogden Formula mean?
Each part of the Mogden Formula deals with a specific element of the profile of the wastewater, based on volume, pollutants and level of treatment required.
- R - Reception and conveyance charge (£/m³): the cost for the water company to transport and receive the trade effluent into the sewer
- V - Volumetric and primary treatment charge (£/m³)
- B - Biological oxidation treatment charge (£/m³)
- Bv - Additional charge if biological treatment is required (£/m³):
- M - Additional treatment and disposal charge where effluent is discharged into the sea (£/m³)
- Os - COD or settled COD of crude sewage effluent (mg/L)
- Ot - COD of settled crude sewage at the treatment works (mg/L), typically ~350 mg/L
- S - Sludge treatment and disposal charge (£/m³)
- Ss - Total suspended solids of crude sewage (mg/L)
- St - Suspended solids of crude sewage at the treatment works (mg/L), typically ~350 mg/L
A working example of how the Mogden charge is calculated
Below is a demonstration of how a Mogden charge is calculated for an existing UK food and beverage manufacturer. Essentially, the key message is that the percentage reduction in COD can have a similar proportional reduction in charges.
Halving the COD loading (by removing the total suspended solids) of the treated effluent can therefore potentially halve the Mogden charge. This is a good rule of thumb and can make the mathematics look a little easier to digest.
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Site profile: A dairy processing facility in Yorkshire that discharges to the public sewer under a trade effluent consent. Their effluent has been sampled and the following data is known.
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Parameter: value
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Discharge volume: 500 m³/day
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COD of trade effluent (Os): 2,500 mg/L
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Suspended solids of trade effluent (SS): 800 mg/L
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Operating days/year: 300 days
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Water company tariff rates (illustrative: Yorkshire Water 2024 basis)
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R (Reception): £0.30/m³
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V (Volumetric/Primary): £0.35/m³
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B (Biological treatment): £0.55/m³
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S (Sludge): £0.40/m³
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Ot (Reference COD of crude sewage): 350 mg/L
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ST (Reference SS of crude sewage): 350 mg/L
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Step-by-step calculation
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Step 1. Calculate the COD ratio (Os/Ot):
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2,500 ÷ 350 = 7.14
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Step 2. Calculate the SS ratio (SS/ST):
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800 ÷ 350 = 2.29
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Step 3. Apply the Mogden Formula:
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C = R + V + B(Os/Ot) + S(SS/ST) C = £0.30 + £0.35 + £0.55 × 7.14 + £0.40 × 2.29 C = £0.30 + £0.35 + £3.93 + £0.92 C = £5.50 per m³
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Step 4. Calculate the annual discharge cost:
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500 m³/day × 300 days × £5.50/m³ = £825,000/year
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The pre-treatment opportunity
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Now apply a Dissolved Air Flotation (DAF) system with coagulant/flocculant used to remove suspended solids, fats, oils, grease (FOG), and other contaminants from wastewater before it enters the main treatment process. This reduces:
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COD from 2,500 → 800 mg/L (68% reduction)
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SS from 800 → 150 mg/L (81% reduction)
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Recalculate:
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C = £0.30 + £0.35 + £0.55 × (800/350) + £0.40 × (150/350) C = £0.30 + £0.35 + £1.26 + £0.17 C = £2.08 per m³
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New annual cost:
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500 × 300 × £2.08 = £312,000/year
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Annual saving: £513,000/year
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How a business can reduce its Mogden charge?
Businesses can reduce wastewater charges by reducing the volume of water discharged to the public sewer and by improving the quality of the wastewater by lowering the level of pollutants and COD.
Can wastewater treatment reduce Mogden charges?
Wastewater treatment has a central role in reducing the Mogden charge. A treatment strategy tailored to the needs of the site and characteristics of the trade effluent can significantly improve the quality of the water and therefore reduce the charges applied through the Mogden Formula.
For example, if an organisation’s wastewater contains a high level of suspended solids, these can be removed using a range of technologies. These can include our Actiflo* clarification systems, Idraflot* Dissolved Air Flotation (DAF), or Hydrotech disc filter and drum filter mechanical filtration units. Treating water for reuse on site will also help reduce charges by lowering the volume discharged.
Is onsite wastewater treatment to reduce Mogden charges worth the investment?
Yes, most sites that incur a significant Mogden charge due to the nature of their wastewater will achieve a return on investment in onsite wastewater treatment technologies. Often the pay-back period will be just a few years. This is especially true if treatment for water reuse on site is implemented as this not only reduces the Mogden charge through lower discharge volumes but also minimises charges for mains water use.
Need support on working out the Mogden formula for your site? Get in touch with one of our experts now for a chat here.
*Trademark of Veolia; may be registered in one or more countries.
