Wastewater Treatment Plant Operator cheat sheet

Percent removal = (influent − effluent) ÷ influent × 100; applied to TSS, BOD5, and ammonia across every unit process.

Coarse bar screens have 1/4–1.5 in (6–38 mm) openings and fine screens 1/8 in (3 mm) or less; headloss rising above roughly 6 in signals the rack needs cleaning.

Grit has a specific gravity near 2.65 and a settling velocity of about 0.1 ft/s; grit chambers hold horizontal velocity near 1 ft/s so grit settles while organic solids stay suspended.

Aerated grit chambers typically provide 1–3 minutes of detention at peak flow and use a rolling roll to separate grit from organics.

Primary clarifier design: surface overflow rate 600–1,200 gpd/ft² at average flow, detention 1.5–2.5 h, weir loading 10,000–20,000 gpd/ft.

Expected primary removal is 50–65% TSS and 25–40% BOD5; rising blanket, carryover, or short-circuiting indicates hydraulic or solids-loading problems.

Solids loading rate = (influent TSS mg/L × flow MGD × 8.34) ÷ clarifier area in ft², reported as lb/ft²/day; typical design is 25–30 lb/ft²/day.

Settling regimes are Type I discrete, Type II flocculant, Type III zone (hindered), and Type IV compression; blanket depth is measured with a sludge judge or core sampler.

1% solids equals 10,000 mg/L; dry tons/day = mg/L × MGD × 8.34 ÷ 2,000.

F/M = lb BOD5 applied per day ÷ lb MLSS (or MLVSS) in aeration; conventional 0.2–0.4, extended aeration 0.05–0.15, high-rate 0.4–1.5.

MCRT (SRT) = lb solids in the system ÷ lb solids leaving per day (waste sludge plus effluent solids); conventional 5–15 days, nitrifying systems commonly 10–20 days or more.

SVI = settled sludge volume (mL/L after 30-min settle of a 1-L sample) × 1,000 ÷ MLSS mg/L; 80–150 mL/g is good settling, above 200 suggests filamentous bulking.

RAS rates are commonly 0.5–1.5 times influent flow; return sludge from a well-formed blanket is typically 4,000–10,000 mg/L.

Hold DO at 1.5–2.5 mg/L for carbonaceous oxidation; nitrification needs about 2 mg/L DO, pH 7.0–8.5, and 7.14 mg/L alkalinity as CaCO3 per mg/L NH3-N oxidized.

Oxygen demand is 4.57 mg O2 per mg NH3-N nitrified; denitrification recovers roughly 2.86 mg O2 per mg NO3-N reduced.

Trickling filter loading is lb BOD5 per 1,000 ft³ of media per day: low-rate 5–25, high-rate 25–100; recirculation dilutes strength and helps control sloughing.

Trickling filter problems include media ponding, Psychoda (filter flies), and seasonal sloughing; facultative lagoons rely on algae-bacteria symbiosis with 20–180 day detention.

Clarifier troubleshooting: rising blanket from denitrification or short MCRT, cloudy effluent from young sludge or toxic shock, pinpoint floc from low F/M.

Granular media filters are rated by hydraulic loading, typically 2–5 gpm/ft²; backwash runs 15–20 gpm/ft² with 20–30% bed expansion.

Membrane pore ranges: MF about 0.1–0.2 µm, UF about 0.01–0.1 µm, NF about 0.001 µm, and RO effectively non-porous, rejecting dissolved ions.

Flux is reported in gallons per square foot per day (gfd); recovery % = permeate ÷ feed × 100; a rising trans-membrane pressure at constant flux signals fouling.

Feedwater targets ahead of RO/NF are turbidity below 0.1 NTU and a silt density index (SDI15) below 5.

RO rejects 95–99% of dissolved salts; the concentrate stream must be managed by permit-approved disposal, deep-well injection, or evaporation.

Nutrient polishing options include anoxic denitrifying filters with external carbon, chemical phosphorus precipitation with alum or ferric salts, and effluent TP limits below 0.1 mg/L.

Chemical phosphorus removal typically requires roughly 1.5–2.5 mg of alum per mg of P removed; polymer is added to capture precipitated floc.

Membrane integrity is verified by pressure-decay (vacuum hold) tests and by permeate turbidity or particle counting.

CT = disinfectant residual (mg/L) × contact time (min); required CT increases as temperature falls and, for chlorine, as pH rises.

Free chlorine speciation: HOCl dominates and is far more biocidal below pH 7.5 (pKa ≈ 7.5 at 25°C); above pH 8 the less effective OCl⁻ dominates.

Chlorine demand = applied dose − residual after contact; breakpoint occurs near a Cl2:NH3-N weight ratio of about 7.6:1, beyond which free chlorine persists.

Dechlorination: about 1 mg/L SO2 (or roughly 1.5 mg/L sodium bisulfite) removes 1 mg/L free chlorine; sodium thiosulfate is used for small flows and sample quenching.

UV dose = intensity (mW/cm²) × exposure time (s), reported in mJ/cm²; wastewater disinfection typically uses 30–40 mJ/cm² at 254 nm and depends on UV transmittance and sleeve fouling.

Ozone is generated on site from dried air or oxygen at typical doses of 2–5 mg/L with 5–15 min contact; off-gas must be destroyed thermally or catalytically.

Contact chambers are designed for at least 30 minutes detention at peak flow with baffling to approach plug flow and limit short-circuiting.

Typical secondary effluent limits are a fecal coliform geometric mean of 200 cfu/100 mL with a 7-day maximum of 400 cfu/100 mL; dechlorination residual limits are commonly 0.01–0.05 mg/L.

Gravity thickening raises waste activated sludge from about 0.5–1% to 3–5% solids at solids loadings of 20–30 lb/ft²/day and overflow rates of 400–1,000 gpd/ft².

Mesophilic anaerobic digestion runs at 35–37°C (95–98°F) with 15–20 day SRT; thermophilic runs near 55°C; VS loading is typically 0.1–0.2 lb VS/ft³/day unmixed and up to 0.4 mixed.

Normal volatile solids destruction is 40–60%; gas yield is 5.5–6.5 ft³ per lb VS destroyed at 60–65% methane and roughly 600–650 BTU/ft³.

Digester upset indicators: pH below 6.8, rising volatile acids with falling alkalinity, VA/alkalinity ratio above 0.3, and increasing CO2 in the gas.

Belt filter presses and centrifuges commonly produce 15–30% cake; polymer conditioning doses are typically 5–20 lb dry polymer per ton of dry solids.

Solids capture = lb dry solids in cake ÷ lb dry solids in feed × 100; well-run centrifuges and belt presses achieve 90–95% or better.

40 CFR Part 503 governs biosolids: Class A (below 1,000 fecal coliform/g TS plus pathogen alternatives) versus Class B, vector attraction reduction, and nine pollutant limits.

Dry tons/day = mg/L × MGD × 8.34 ÷ 2,000; use the mass balance to check thickening, digestion, and dewatering performance.

Lime stabilization raises pH to 12 or above for 2 hours to reduce pathogens; composting and heat drying are alternatives that can produce Class A material.

Sanitary sewers should carry design flow at 2 ft/s or more to stay self-cleaning; Manning n is about 0.013 for concrete and 0.011 for PVC.

Manning's equation in English units: Q = (1.486/n) A R^(2/3) S^(1/2), with R = area ÷ wetted perimeter.

Infiltration is groundwater entering through defective pipes and joints; inflow is direct stormwater entry; together they form RDII and drive peak wet-weather flows.

Pump affinity laws: capacity varies with speed, head with speed squared, and power with speed cubed; impeller trimming follows similar relationships with diameter.

Wet-well cycling: inflow (gpm) = pump-out rate × run time ÷ total cycle time; minimum cycle time is set by the motor's allowable starts per hour.

Operating far left of the pump curve risks upthrust and overheating; far right risks cavitation and motor overload - compare NPSH available to NPSH required.

Air release and vacuum valves, surge relief, and thrust blocking protect force mains from water hammer during pump start and stop.

SSOs and CSOs must be reported to the state within the required time (often 24 hours) and logged with volume, duration, receiving water, and corrective action.

Smoke testing, dye testing, CCTV inspection, and flow monitoring are the standard methods for locating infiltration and inflow sources.

BOD5 uses a 300-mL bottle incubated 5 days at 20°C in the dark; depletion must be at least 2 mg/L with at least 1 mg/L residual DO, or the sample must be diluted or seeded.

Seeded BOD: BOD = [(D1 − D2) − (B1 − B2) × f] ÷ P, where f is the seed volume ratio and P is the sample volume fraction.

TSS is retained on a glass-fiber filter (1.5 µm nominal) dried at 103–105°C; VSS is the portion lost on ignition at 550°C.

Pounds formula: lb/day = mg/L × MGD × 8.34; conversely mg/L = lb/day ÷ (MGD × 8.34).

Holding times: pH and chlorine residual are run immediately; BOD 48 h refrigerated; TSS 7 days; preserved ammonia 28 days.

A grab sample represents one moment; a flow-proportioned 24-hour composite represents average mass loading and is what permits usually require.

Reference methods: Standard Methods 5210 B for BOD, 2540 D for TSS, and 4500-NH3 for ammonia.

COD by 2-hour dichromate reflux approximates total oxidizable matter and typically runs 1.5–2.5 times BOD5 in raw wastewater.

Calibrate DO meters by Winkler titration or saturated air, and pH meters with 4.0/7.0/10.0 buffers, recording temperature compensation.

Centrifugal pumps must be primed; running dry damages seals and wear rings, and cavitation (rattling, vibration, lost head) means NPSH available is below NPSH required.

Vibration analysis, lubricant analysis, and infrared thermography are predictive tools; a rising bearing vibration trend typically precedes failure.

Calibrate 4–20 mA loops with a signal generator: 4 mA = 0% and 20 mA = 100% of span; a live zero lets the system detect broken wiring.

pH probes need two- or three-point calibration and periodic cleaning; slow response usually indicates a fouled or coated bulb.

Rising blower discharge pressure with unchanged airflow points to diffuser or air-header plugging; check inlet filters as well.

Motor checks: voltage imbalance under 1%, balanced phase currents, insulation resistance, and running amps below nameplate full-load amps.

Lockout/tagout per 29 CFR 1910.147 precedes any repair on pumps, blowers, conveyors, or mixers.

Flow metering: magnetic meters need a full pipe and adequate conductivity, ultrasonic meters need a clean acoustic path, and open-channel flow uses a weir or Parshall flume with a stage reading.

Lubricate at manufacturer intervals; over-lubricating motor bearings causes overheating and is as damaging as under-lubrication.

Permit-required confined space entry (29 CFR 1910.146) requires pre-entry and continuous atmospheric testing, a signed permit, entrant, attendant, entry supervisor, and rescue provisions.

Entry atmosphere limits: oxygen 19.5–23.5%, flammable gas below 10% LEL, hydrogen sulfide below 10 ppm, and carbon monoxide below 35 ppm (or stricter site limits).

H2S is heavier than air, deadens the sense of smell at higher concentrations, and has an IDLH of 100 ppm; methane LEL is 5% and UEL 15%; chlorine IDLH is 10 ppm.

Never enter a space to rescue a downed worker without respiratory protection and backup - most confined-space fatalities are untrained would-be rescuers.

Chlorine gas is heavier than air and forms hydrochloric acid with moisture; respond with SCBA and a chlorine repair kit, keep cylinders away from heat, and never spray water on a leak.

Fall protection is required at 4 ft in general industry; hatches, guardrails, and grating over wet wells must be secured before opening.

Methane, hydrogen sulfide, and carbon monoxide are the classic wet well and digester hazards; use intrinsically safe multi-gas meters calibrated before each use.

Electrical work requires GFCI protection, lockout/tagout, and NFPA 70E arc-flash PPE; never work alone on energized equipment.

Chemical handling: add acid to water, follow the SDS, and keep an eyewash and safety shower within about 10 seconds' travel of feed areas.

The NPDES permit (CWA §402, 40 CFR Part 122) sets effluent limits, monitoring requirements, and reporting; the DMR is submitted monthly or as the permit specifies.

NPDES records must be kept at least 3 years (longer if compliance action is pending); biosolids records under Part 503 are kept 5 years.

Bypasses, upsets, and noncompliance require oral notification as soon as practicable (commonly within 24 hours) followed by written notice within 5 days.

SSOs must be reported to the state and often to the public; CSOs are addressed through the nine minimum controls and a long-term control plan.

40 CFR Part 503 sets biosolids pollutant limits, pathogen reduction classes, vector attraction reduction, and management and reporting practices.

Whole effluent toxicity (WET) tests use indicator species to detect unmeasured toxics; a failure triggers a toxicity identification evaluation (TIE).

The Clean Water Act (1972) established technology-based (BPT/BAT) and water-quality-based limits along with antidegradation policy.

Every exceedance must appear on the DMR even when caused by collection system or treatment problems; falsifying a report is a criminal violation.

Operator certification is a state requirement with grade levels tied to facility classification, and renewal requires continuing education.