OH Water/Wastewater Operator cheat sheet

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

Log removal credits: conventional filtration (coagulation, flocculation, sedimentation, filtration) provides 2.5-log Giardia and 2-log virus; disinfection must supply the remainder of required 3-log Giardia and 4-log virus.

Turbidity limits: filtered water must be ≤0.3 NTU in 95% of monthly samples and never exceed 1 NTU; individual filter effluent must be ≤0.3 NTU in 95% of samples.

Chlorine demand = applied dose − residual after contact time; demand must be satisfied before free residual is available for disinfection.

Jar test determines optimum coagulant dose by varying dose and measuring settled turbidity and pH.

Flocculation basin design: tapered velocity gradient (G) typically 10–70 s⁻¹, with higher G at the start and lower at the end; Gt (velocity gradient × time) typically 20,000–150,000.

Filter media: anthracite (effective size 0.9–1.1 mm, uniformity coefficient ≤1.7) over sand (effective size 0.45–0.55 mm, uniformity coefficient ≤1.65); support gravel layers graded from coarse to fine downward.

Backwash: minimum 20% bed expansion; backwash rate typically 15–20 gpm/ft² for anthracite/sand; air scour may precede water backwash.

Lead and copper rule: 90th percentile lead action level = 0.015 mg/L; copper action level = 1.3 mg/L; samples collected at high-risk taps after 6-hour stagnation.

Minimum residual disinfectant: free chlorine 0.2 mg/L or total chlorine 0.5 mg/L in distribution system; maximum contaminant level (MCL) for total chlorine residual is 4.0 mg/L (as Cl₂).

Total coliform rule: no more than 5% of samples per month may be total coliform-positive for systems collecting ≥40 samples; any positive requires repeat sampling within 24 hours.

Water age increases with low demand; target less than 5 days; flush dead ends and storage tanks to reduce age.

Distribution system pressure: minimum 20 psi under all conditions; normal operating pressure typically 50–80 psi; maximum 100–125 psi to prevent pipe/joint damage.

Storage tank turnover: maintain turnover every 1–3 days; tanks with poor turnover require mixing or operational cycling.

Hydrant flow test: measure static pressure, residual pressure, and pitot pressure to calculate flow; use flow coefficient and pitot pressure to determine discharge.

Pipe bedding and cover: minimum cover 4–6 ft to prevent freezing; bedding material must be free of rocks and debris to prevent point loading.

Cross-connection control: backflow prevention assembly (RPZ, DCVA) must be tested annually; air gap is the most reliable backflow prevention method.

Water main disinfection after repair: chlorinate to 25–50 mg/L for 24 hours or 100 mg/L for 2 hours, then flush and test for bacteriological safety before returning to service.

BOD₅ = (D₁ − D₂) × dilution factor, where D₁ = initial DO, D₂ = final DO, and dilution factor = (sample volume + dilution water volume) / sample volume.

F/M ratio = (BOD or COD loading, lb/day) / (MLVSS, lb); typical activated sludge F/M = 0.2–0.5 day⁻¹ for conventional systems.

Mean cell residence time (MCRT) = (MLSS in aeration tank, lb) / (solids leaving system per day, lb/day); typical 5–15 days for conventional activated sludge.

Sludge volume index (SVI) = (settled sludge volume, mL/L after 30 min) × 1000 / MLSS, mg/L; SVI 80–150 mL/g indicates good settling; >200 indicates filamentous bulking.

Hydraulic retention time (HRT) = aeration tank volume / influent flow; typical 4–8 hours for conventional activated sludge.

Dissolved oxygen: maintain 1.5–3.0 mg/L in aeration tank for nitrification; below 1.0 mg/L nitrification slows and filamentous bacteria may proliferate.

Nitrification stoichiometry: 4.57 lb O₂ consumed per lb NH₃-N oxidized; alkalinity destruction 7.14 lb as CaCO₃ per lb NH₃-N oxidized.

Chlorine disinfection: CT = residual chlorine (mg/L) × contact time (min); typical required CT for fecal coliform reduction depends on pH and temperature; maintain residual after contact.

Anaerobic digester: volatile solids reduction 40–50%; typical detention time 15–30 days; optimum temperature 35°C (mesophilic) or 55°C (thermophilic); monitor pH 6.8–7.4 and volatile acid/alkalinity ratio <0.3.

Minimum sewer slope: 0.005 ft/ft for 8-inch pipe; 0.0033 ft/ft for 10-inch; 0.0025 ft/ft for 12-inch; use Manning equation for design velocity.

Manning equation: Q = (1.486/n) A R^(2/3) S^(1/2) (English units); n = 0.013 for concrete pipe, 0.009 for PVC.

Minimum self-cleaning velocity: 2 ft/s at design flow; maximum velocity 10 ft/s to prevent scour.

Infiltration/inflow (I/I): infiltration is groundwater entering through defects; inflow is direct stormwater connection; both increase treatment costs.

Lift station wet well: cycle time = (wet well volume between pump-on and pump-off) / (inflow rate); pump capacity must exceed peak inflow.

Force main: minimum velocity 2 ft/s to prevent solids deposition; air release valves at high points; check valves at pump discharge.

Manhole spacing: typically 300–500 ft for straight runs; provide drop manhole when invert difference exceeds 2 ft.

Sewer testing: low-pressure air test (4 psi for 5 minutes) or water exfiltration test (head of 2 ft above pipe) to verify tightness.

Confined space entry: test atmosphere for oxygen (19.5–23.5%), flammable gas (<10% LEL), and hydrogen sulfide (<10 ppm) before entry; use forced ventilation and supplied-air respirator if needed.