A practical Q&A with Pete Cranney on understanding, managing and reducing industrial effluent costs.
Pete Cranney has been with Marlowe Environmental Services (formerly Atana) for 18 years. He is an expert in industrial trade effluent and has extensive experience with the Mogden Formula and its practical application.
Q: What is the Mogden Formula?
The Mogden Formula is the UK method for calculating trade effluent charges. A common unit-charge form is:
Charge per unit of effluent = R + [(V + Bv) or M] + B (Ot/Os) + S (St/Ss)
Where:
- R = reception and conveyance charge (£/m³)
- V = primary treatment charge (£/m³)
- Bv = additional volume charge for biological treatment (£/m³)
- M = treatment and disposal charge for sea outfall (£/m³)
- B = biological oxidation of settled sewage (£/kg)
- Ot = COD after 1-hour settlement at pH 7, Os = COD of crude sewage (mg/l)
- S = treatment and disposal of primary sewage sludge (£/kg)
- St = total suspended solids at pH 7, Ss = total suspended solids of crude sewage (mg/l)
This produces a unit charge applied to your discharge volume plus any fixed fees. For industrial sites, the practical factors are the volume discharged and the chargeable strength of that discharge, typically reflected in Ot (COD) and St (suspended solids). Even “within consent” bills can be high if these aren’t managed, usually due to process issues rather than billing errors. I work with clients to calculate charges and identify where costs can be reduced.
Q: Why do bills often feel high or unpredictable?
Many sites treat effluent as a fixed cost rather than a controllable process. If discharge strength is high, variable or poorly measured, the bill reflects it. The real drivers are volume and strength, not errors in billing. Treating effluent onsite is usually cheaper than leaving it all to the water company.
Q: What drives Mogden charges?
Two main drivers:
- Discharge volume – how much effluent leaves the site
- Strength – Chemical Oxygen Demand (COD) and Suspended Solids (SS) levels
Other overlooked contributors include production swings, heavy washdown peaks, poor stream segregation, weak equalisation, inconsistent pH, unstable chemical dosing and underperforming DAFs. Often the Mogden bill is the financial symptom of upstream operational issues.
Q: How can sites reduce costs practically?
There are three key stages:
Confirm billing basis – check tariffs, volume measurement, sampling points and how strength is averaged
Reduce avoidable load – improve stream segregation, meter accuracy and washdown practices
Stabilise treatment performance – screening, flow balancing, pH correction, coagulation/flocculation, DAF or settlement and sludge handling
Optimising each stage cumulatively lowers discharge strength and reduces bills.
Q: Why are our trade effluent charges so high if we are within consent?
A: Consent is the MOT while Mogden is the running costs. Passing the MOT does not mean the car is cheap to run. Compliance keeps you safe, but it is control and efficiency that keep your Mogden bills lower.
Trade effluent consents are usually pass or fail against limits. Mogden reflects how much you discharge and the average strength of that discharge over time. A site can stay within limits and still be expensive if it sends a lot of volume, a consistently strong effluent, or regular spikes that pull up the charging strength.
This is where sites get caught out. They focus on not breaching, but Mogden is still pricing the treatment effort. If your day-to-day effluent varies with production runs, washdowns, or minor spills, that variability can become your charging average. You might be operating legally, but you are still paying for load and instability.
Q: Do all sites need new plant?
Not always. Many savings come from better process control, dosing discipline and operational optimisation. Short on-site trials can prove improvements before committing to capital expenditure.
Q: What ensures long-term success?
Consistency is key. Make sure processes work reliably across shifts, product runs, cleaning cycles and seasonal changes. Track meaningful averages, tweak processes step by step and plan long-term changes thoughtfully rather than trying to fix everything at once. At MES we can help with that and with planning ahead.
Q: How can Marlowe Environmental Services help?
At MES we support sites with reviews and optimisation, sampling, chemical programmes, testing, trials and plant upgrades. We also offer free effluent testing to identify opportunities to cut costs and improve performance. Findings are presented clearly so you can make confident decisions and understand potential ROI.
What works in practice
From experience, sites that achieve reliable reductions don’t chase a single good result.
They build processes that perform consistently every day across shifts, products and operators. Many Mogden projects fall short when improvements are proven over a narrow window and the billing average doesn’t change because the process wasn’t stable or the evidence wasn’t representative.
The strongest outcomes come from a structured approach:
- Confirm the billing and sampling basis
- Establish a baseline
- Optimise processes in a controlled way
- Measure against representative averages
Only after this does it make sense to lock in long-term operational changes or consider plant upgrades.
Where to start
If your effluent bill is high and you’re unsure which costs are unavoidable, start with the basics: check billed volume, how strength is sampled and averaged and identify what’s causing spikes. Even a short review of bills and available flow or sampling data can make the main cost drivers obvious, and we can help with that.
The goal isn’t just a better treatment plant – it’s a more stable process and a more controllable operating cost.