The formula
The Mogden formula explained: how a trade effluent bill is built
Updated
Unit charge = R + V + (Ot/Os) B + (St/Ss) S. Two terms are fixed per cubic metre; two rise with how strong your effluent is.
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How this calculator works
- The formula. Unit charge = R + V + (Ot/Os) B + (St/Ss) S, where R is reception and conveyance, V primary treatment, B biological treatment and S sludge treatment and disposal, Ot and St your effluent's COD and suspended solids and Os and Ss the standard strengths. (Yorkshire Water wholesale charges scheme 2026/27)
- Severn Trent. 2026/27 standard tariff: R £0.4210, V £0.4207, B £0.2533 and S £0.2395 per m3, at standard strengths of 351 mg/l COD and 343 mg/l suspended solids. (Severn Trent non-household wholesale scheme of charges 2026/27)
- Hafren Dyfrdwy. 2026/27: R £0.5643, V £0.7503, B £0.4471 and S £0.3032 per m3, at 351 mg/l COD and 343 mg/l suspended solids. (Hafren Dyfrdwy NHH wholesale charges scheme 2026/27)
- Yorkshire Water. 2026/27, under 50,000 m3 a year: R 84.43p, V 83.49p, B 75.71p and S 50.02p per m3, at 840 mg/l COD and 335 mg/l suspended solids. (Yorkshire Water wholesale charges scheme 2026/27)
- Wessex Water. 2026/27, up to 80 Ml of total waste discharge: R £1.1777, V £0.4808, B £0.6909 and S £0.3593 per m3, at a deemed standard strength of 802 mg/l COD and 313 mg/l suspended solids. (Wessex Water wholesale charges scheme 2026/27)
- What is left out. Annual band, fixed, sampling and minimum charges, and any part of the treatment your effluent does not receive, are not included. Your retailer's bill is the figure you pay.
Each term
- R: reception and conveyance of the effluent, per cubic metre.
- V: preliminary or primary treatment, per cubic metre.
- B: biological treatment, scaled by your COD (Ot) against the standard (Os).
- S: sludge treatment and disposal, scaled by your suspended solids (St) against the standard (Ss).
Source: Yorkshire Water wholesale charges scheme 2026/27
A worked example
At Severn Trent, 1,000 m3 a year at 700 mg/l COD and 400 mg/l suspended solids: 0.4210 + 0.4207 + (700/351) x 0.2533 + (400/343) x 0.2395 = about £1.6262 per m3, or about £1,626 a year before fixed charges.
Source: Severn Trent non-household wholesale scheme of charges 2026/27
What lowers it
Less volume lowers every term; lower COD and solids lower the B and S terms. Where treatment such as biological oxidation is not provided for your effluent, Yorkshire Water says the whole or due proportion of that component is omitted.
Source: Yorkshire Water wholesale charges scheme 2026/27
This page is information, not legal or engineering advice.