ID Water/WW Operator cheat sheet

CT (contact time) = disinfectant residual concentration (mg/L) × contact time (min); required CT depends on disinfectant, temperature, pH, and log inactivation target.

Free chlorine: 1 mg/L as Cl2 equals 1.0 CT unit; chloramine CT credit is far lower, so chloramination is usually for residual maintenance, not primary inactivation.

Turbidity limits (conventional filtration): ≤0.3 NTU in 95% of monthly samples and never >1 NTU; combined filter effluent is the compliance point.

Log inactivation: log(N0/N) = log N0 − log N; 2-log = 99%, 3-log = 99.9%, 4-log = 99.99% reduction.

Detention time t = V/Q; convert units consistently (e.g., gallons ÷ gpm = minutes). Baffling factors reduce theoretical time to actual contact time.

Distribution residual: maintain a detectable disinfectant throughout; typical entry-point free chlorine 0.2–4.0 mg/L with a minimum 0.2 mg/L in the system (state rules vary).

Lead and copper: 90th-percentile action level 0.015 mg/L Pb and 1.3 mg/L Cu; corrosion control (pH, alkalinity, orthophosphate) is the primary lever.

Hardness (as CaCO3) = 2.497×Ca (mg/L) + 4.118×Mg (mg/L); alkalinity and pH govern whether scale or corrosive water forms.

Langelier Saturation Index (LSI) = pH − pHs; LSI near 0 is balanced, negative is corrosive, positive is scale-forming.

BOD5 measures biodegradable organics consumed over 5 days at 20 °C; carbonaceous BOD is roughly 60–70% exerted by day 5, so ultimate CBOD ≈ BOD5 ÷ 0.68.

Solids: total solids = total suspended solids + total dissolved solids; volatile suspended solids approximate the organic (biomass) fraction.

Sludge age (MCRT) = mass of solids in aeration ÷ mass of solids wasted per day; SRT controls nitrification and sludge quality.

F/M = (Q × BOD in) ÷ (mass of MLVSS in aeration); low F/M favors nitrification and lower sludge production but risks bulking at extremes.

Hydraulic loading rate = Q ÷ surface area (gpd/ft² or m³/m²·d); organic loading rate = BOD mass ÷ volume (lb BOD/1000 ft³·d).

Secondary clarifier solids loading = (Q + Qr) × MLSS ÷ surface area; exceeding design rate causes solids washout.

Nitrification consumes about 4.57 mg O2 per mg N oxidized and about 7.14 mg alkalinity as CaCO3 per mg N; denitrification returns roughly half that oxygen demand.

Collection system: 1% slope = 0.01 ft/ft = 1.0 ft fall per 100 ft; minimum self-cleansing velocity is typically 2 ft/s at design flow.

Chlorine disinfection of effluent: CT = residual × contact time; typical requirement is a coliform limit with a minimum residual and 15–30 min contact at peak flow.