Home / Operator math / The Mogden formula (UK trade effluent charging)
Reference, not on any US operator exam · Reference only, not on any US operator exam · free · no signup
The Mogden formula (UK trade effluent charging)
What is the Mogden formula and does it appear on a US water operator exam?
The Mogden formula, C = R + V + (Bt/Bs) x B + (St/Ss) x S, is how UK water companies price trade effluent discharged to a public sewer under the Water Industry Act 1991: a flat reception-and-treatment charge (R, V) plus biological and solids charges scaled to how much stronger the effluent is than average sewage. It is a UK-specific trade-effluent charging calculation and is not part of any US state's ABC-aligned water or wastewater operator certification exam.
C = R + V + (Bt / Bs) x B + (St / Ss) x S. C is the charge per cubic metre of trade effluent discharged to the public sewer. R is the reception and conveyance cost per cubic metre (getting the effluent to the treatment works). V is the primary (volumetric) treatment cost per cubic metre. B is the biological-treatment cost per cubic metre for settled sewage of average strength, scaled by the ratio of the trade effluent's chemical oxygen demand (Bt) to the average settled sewage's chemical oxygen demand (Bs). S is the primary-sedimentation cost per cubic metre for average-strength sewage, scaled by the ratio of the trade effluent's suspended solids (St) to average crude sewage's suspended solids (Ss).
The formula, worked
Common slip Treating B and S as flat per-cubic-metre charges instead of scaling them by the trade effluent's actual strength ratio against average sewage. A stronger-than-average effluent (higher Bt/Bs or St/Ss) pays proportionally more for biological treatment and sedimentation; only R and V are flat per-volume charges. Every unit cost (R, V, B, S) is set locally by the individual water company and republished periodically, so a real bill uses that company's current published rates, never the example figures above.
Where this shows up
Nothing on this trainer's other domains uses the Mogden formula; it is included for wastewater-treatment readers researching UK trade-effluent charging specifically, not as exam prep. If you are studying for a US state water or wastewater operator certification, the tested calculations are the core and wastewater domains above, most directly the pounds formula (the US equivalent building block for chemical and loading calculations) and the activated-sludge math for BOD-related loading.
Drill the whole exam
Original problems and worked operator-math, free and ungated. No signup; your progress stays on your device.