Calculators — water and wastewater
The 18 everyday calculations of water treatment: laboratory, biological process, membranes, dosing and disinfection. Each calculator shows its formula and offers practice problems.
Laboratory
Suspended solids (TSS / VSS)
Mixed liquor: 10 to 25 mL; effluent: 100 to 1000 mL. Typical VSS/TSS: 65 to 85 %.
Sludge volume index (SVI)
Typical: 80 to 150 mL/g. Above 150 the sludge settles poorly (possible filaments). If SV₃₀ is over 800 mL/L, dilute the sample and multiply by the dilution.
Total alkalinity (titration)
Titrate to pH 4.5. To nitrify, keep at least 50 to 70 mg/L CaCO₃ in the tank.
Diluted sample
The reading must fall inside the test range; if not, dilute further and repeat.
Biological process
Removal efficiency
Works for BOD, COD, TSS, ammonia or phosphorus: same parameter in and out.
F/M ratio (food to microorganisms)
Conventional activated sludge: 0.2 to 0.5; extended aeration and membrane bioreactors: 0.05 to 0.15.
Sludge age (SRT)
Conventional: 5 to 15 d; to nitrify, more than 8 to 10 d depending on temperature; membrane bioreactors: 15 to 30 d.
Wasting rate for a target sludge age
Change wasting in small steps (10 to 15 % per day) and watch the MLSS trend before adjusting again.
Hydraulic retention time (HRT)
Use the volume actually in service (real tank level) and the day's average flow.
Return sludge ratio (RAS)
If the current ratio is lower than the required one, sludge builds up in the clarifier.
Organic load
Compare the daily load with the plant's design load: that is what tells you whether the process is being pushed.
Observed yield (sludge production)
Typical: 0.4 to 0.8; lower when the sludge age is high.
Oxygen uptake rate (OUR / SOUR)
Typical SOUR: 8 to 20. Very low: inhibited or under-fed biomass; very high: overload.
Nitrification: oxygen and alkalinity
Denitrification gives back about half of the alkalinity consumed. If alkalinity runs short, nitrification stops and the pH drops.
Membranes
Flux and permeability (corrected to 20 °C)
Follow the K₂₀ trend rather than a single value: a 20 to 30 % drop signals fouling. Clean as the manufacturer's procedure says.
Chemical dosing and disinfection
Chemical feed: mass and pump rate
Dose and strength must be on the same basis (active product). A dosing pump's real output is confirmed with a calibration test.
Alum for phosphorus removal
Common starting point: Al:P ratio of 1.5 to 2.5. Then adjust with jar tests and effluent P results. Too much alum eats alkalinity.
Chlorination: dose, demand and CT
The CT required depends on the target organism, pH and temperature: compare with the regulatory tables that apply to your plant.
Learning and checking tools. The ranges shown are general guidance: always follow your plant's procedures, permits and set points. Nothing is sent anywhere: the calculations run in your browser.
The same maths, with a plant that reacts: the Beedy simulators.