Summary
Effluent treatment plant (ETP) operators use a small set of calculations every day: flow, hydraulic retention time (HRT), pollutant load, BOD and COD removal efficiency, the food-to-microorganism (F/M) ratio, the sludge volume index (SVI) and chemical dose. This post gives the formulas with worked examples and links the full reference PDF. Discharge limits come from your permit and local law, such as the EU Industrial Emissions Directive, Germany’s Abwasserverordnung, UK environmental permits, the US Clean Water Act and Canada’s Fisheries Act, and many buyers also apply ZDHC or IFC limits.
ETPs rely on quantitative calculations to keep treatment efficient and to meet discharge limits. Operators calculate BOD (biochemical oxygen demand), COD (chemical oxygen demand), flow rates, hydraulic retention time (HRT), sludge volume and chemical dosing rates. Limits are set by national or local authorities and often referenced against the IFC Environmental, Health and Safety Guidelines.
Which formulas do ETP operators use every day?
- Hydraulic retention time: HRT (hours) = tank volume (m³) ÷ flow rate (m³/h). Example: a 600 m³ aeration tank receiving 100 m³/h has an HRT of 6 hours.
- Pollutant load: load (kg/day) = concentration (mg/L) × flow (m³/day) ÷ 1,000. Example: 800 mg/L COD at 2,000 m³/day equals 1,600 kg COD/day.
- Removal efficiency: efficiency (%) = (inlet − outlet) ÷ inlet × 100. Example: BOD falling from 400 mg/L to 30 mg/L gives (400 − 30) ÷ 400 × 100 = 92.5%.
- Food-to-microorganism ratio: F/M = (flow × inlet BOD) ÷ (aeration volume × MLVSS), using consistent units. It tells you whether the biomass is under- or overloaded.
- Sludge volume index: SVI (mL/g) = settled sludge volume after 30 minutes (mL/L) × 1,000 ÷ MLSS (mg/L). A rising SVI points to poor settling.
- Chemical dose: dose (kg/day) = flow (m³/day) × dose rate (g/m³) ÷ 1,000. Example: 1,000 m³/day at 150 g/m³ coagulant needs 150 kg/day.
- Sludge production: estimate from the solids removed, then check against the dewatering plant’s capacity and your disposal route.
These formulas are standard teaching examples. Use your plant’s own design data, calibration records and laboratory methods, and follow your permit’s sampling rules.
Where can you download the full reference?
Download the reference document, which covers the calculations used in the daily operations of a typical effluent treatment plant:
Frequently Asked Questions
What is the difference between BOD and COD?
BOD measures the oxygen that microorganisms consume to break down organic matter, usually over five days. COD measures the oxygen needed to oxidise organic and some inorganic matter chemically, and it is faster to measure and usually higher than BOD.
Which discharge limits apply to my plant?
Those in your discharge permit. These follow national and local law, such as the EU Industrial Emissions Directive and Germany’s Abwasserverordnung, and many brands also require ZDHC wastewater limits from their suppliers.
Related reading: Effluent Treatment Plant Processes | Best Available Techniques for the Leather Industry | Chemical Safety | What is Compliance?
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