What the formula does

The Mogden Formula works out a charge per cubic metre of trade effluent, based on the cost of carrying it away and treating it. It breaks the job into stages, reception and conveyance, treating the volume, breaking down the organic content, and dealing with the leftover sludge, and prices each one.

The parts of the formula

In plain terms, your charge reflects:

• Volume, how much effluent you discharge.
• Conveyance and treatment, the basic cost of carrying and treating that volume.
• Strength (organic load), measured as chemical oxygen demand and compared against an average-strength benchmark; stronger effluent costs more.
• Suspended solids, the amount of solid matter, compared against a benchmark; more solids cost more.

The dirtier and more concentrated your effluent, the higher each strength element climbs.

Why it matters

Because the formula is strength-based, your trade effluent bill isn’t fixed. It responds to what you put down the drain. Reducing volume, screening out solids, or pre-treating effluent before it leaves site all bring the charge down. So does making sure your effluent is sampled accurately, rather than billed against a cautious assumption. There’s more in our guide to trade effluent.

Which businesses pay Mogden charges

Mogden mainly affects businesses that put more than ordinary wastewater down the drain. Food and drink producers, breweries, laundrettes, commercial kitchens and a lot of manufacturing all sit in this group. If your waste is stronger or dirtier than normal sewage, the formula is how that gets priced.

It only applies where you hold a trade effluent consent. Our page on trade effluent explains when you need one and how it connects to the Mogden calculation.

How to bring your Mogden charges down

The charge rewards cleaner effluent, so anything that reduces the strength or the volume of what you discharge tends to lower the bill. Screening solids, balancing flows and basic pre-treatment all make a measurable difference over a year.

Accurate sampling matters too. If your effluent is being assumed rather than measured, the figures can run high. A business water audit is a practical way to check whether what you’re charged reflects what you actually discharge.

Mogden and your trade effluent consent

The consent sets the limits for what you’re allowed to discharge, and the Mogden Formula prices it within those limits. The two work together. Change what you discharge and both the consent and the charge can move with it.

The Mogden Formula in full

The formula is usually written as C = R + V + B(Ot/Os) + S(St/Ss). R is reception and conveyance, the cost of taking your effluent away. V is the basic volumetric cost of treating it. The B term covers biological oxidation and grows with the strength of the effluent, measured against a settled-sewage benchmark called Os. The S term covers sludge treatment and disposal, and grows with the suspended solids you discharge against a benchmark called Ss.

In plain terms, the dirtier and more solid-laden your effluent, the more the B and S terms pile on top of the basic cost. That’s the whole logic of the charge. You pay for volume first, then for how much work your particular effluent makes for the treatment works.

Reading the strength of your effluent

Two numbers drive most of the bill. Ot is the oxygen demand of your effluent, a measure of how much organic matter is in it. St is the suspended solids, the undissolved material carried along with it. Both are found by sampling, then compared against the settled-sewage benchmarks in the formula.

If your effluent is only ever assumed rather than sampled, the figures can sit higher than reality. That’s worth knowing, because a fair sample is sometimes all it takes to bring a Mogden charge back down to where it should be.

Mogden Formula FAQs