PA Water/Wastewater Operator cheat sheet

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

Conventional train: coagulation → flocculation → sedimentation → filtration → disinfection → clear well/storage → distribution.

Rapid sand filtration rate 2–4 gpm/ft² (dual media up to 4–6 gpm/ft²); backwash at 15–20 gpm/ft² for roughly 20–25% bed expansion.

Combined filter effluent turbidity ≤0.3 NTU in at least 95% of monthly samples and never >1 NTU for conventional/direct filtration; individual filters ≤1 NTU.

Sedimentation surface overflow rate = flow ÷ surface area (gpm/ft²); typical conventional basin design 0.3–1.0 gpm/ft².

Free chlorine residual must be maintained at ≥0.2 mg/L (or detectable) in distribution; chlorine demand = dose applied − residual measured.

MCLs: nitrate 10 mg/L as N, nitrite 1 mg/L, fluoride 4.0 mg/L (SMCL 2.0), arsenic 0.010 mg/L, TTHM 0.080 mg/L, HAA5 0.060 mg/L; lead action level 0.015 mg/L, copper 1.3 mg/L.

Softening removes hardness with lime/soda ash; Langelier Saturation Index near 0 is balanced, positive indicates scaling tendency, negative indicates corrosion tendency.

Alum consumes about 0.45–0.5 mg/L of alkalinity as CaCO3 per mg/L of alum fed, so pH must be checked and adjusted after coagulant addition.

Maintain at least 20 psi at all points in the distribution system at all times; typical operating pressure is 50–80 psi to satisfy customers and fire flow.

Unidirectional flushing must achieve ≥2.5 ft/s in the main to scour sediment and biofilm; flush one direction at a time with valves sequenced clean-to-dirty.

AWWA C651 new main disinfection: 25–50 mg/L chlorine, at least 24 hours contact, flush to ≤2.0 mg/L, then collect bacteriological samples.

Backflow protection hierarchy: air gap (highest) > reduced pressure zone assembly > double check valve > pressure vacuum breaker > atmospheric vacuum breaker.

Hazen-Williams head loss: h = 10.44 L Q^1.852 / (C^1.852 d^4.87); C ≈ 130–150 for new cement-lined or PVC pipe, 100 or lower for old unlined cast iron.

High water age depletes chlorine residual, raises TTHM/HAA5 formation, and can trigger nitrification in chloraminated systems (monitor ammonia and nitrite).

Thrust restraint is required at bends, tees, reducers, valves, and dead ends, using concrete thrust blocks bearing on undisturbed soil or restrained joints.

Coliform monitoring frequency scales with population served; every routine positive requires repeat samples within 24 hours at the original site plus upstream and downstream.

Storage tanks should be operated for turnover, screened at vents and overflows, and inspected regularly for sediment, corrosion, and security.

Common secondary effluent limits: 30 mg/L BOD5 and 30 mg/L TSS as monthly averages; many permits are considerably tighter.

Activated sludge targets: MLSS 2,000–4,000 mg/L, DO 1.5–2.5 mg/L, F/M 0.2–0.5 lb BOD/lb MLVSS-day, MCRT 5–15 days, SVI 80–150 mL/g.

F/M = lb BOD5 applied per day ÷ lb MLVSS under aeration; SVI = (30-minute settled volume in mL/L × 1000) ÷ MLSS in mg/L.

Primary clarifiers: overflow rate 600–1,200 gpd/ft², detention 1.5–2.5 hours, removing roughly 50–65% TSS and 25–40% BOD.

Secondary clarifiers: overflow rate 400–800 gpd/ft² and solids loading 20–30 lb/day/ft²; a rising sludge blanket signals need for more wasting.

Nitrification consumes about 7.14 mg/L alkalinity as CaCO3 and 4.57 mg/L oxygen per mg/L NH3-N oxidized, and can depress pH enough to stall nitrifiers.

Mesophilic anaerobic digestion: 95–99 °F, 30–60 day SRT, 40–50% volatile solids destruction, gas about 60–65% methane; keep VA/alkalinity ratio low.

Effluent chlorination dose = chlorine demand + desired residual; fecal coliform limits are often 200 CFU/100 mL as a monthly geometric mean.

Solids handling sequence: thicken (gravity or DAF) → stabilize (digestion, composting, lime) → dewater (belt press, centrifuge); typical cake 15–30% solids.

Design gravity sewers for at least 2 ft/s at average daily flow (self-cleaning) and no more than about 10–15 ft/s to limit scour and H2S release.

Manning: Q = (1.486/n) A R^(2/3) S^(1/2); n ≈ 0.013 for PVC and concrete, 0.015 for vitrified clay and ductile iron.

Common minimum slopes: 4-inch lateral 1/4 inch per foot, 6-inch 1/8 inch per foot, 8-inch 1/16 inch per foot; confirm by computed velocity, not slope alone.

Infiltration is groundwater entering through defective pipes and joints; inflow is storm water entering through direct connections such as roof and area drains.

Acceptance testing: low-pressure air test per ASTM practice, deflection test with a mandrel sized to 95% of inside diameter (maximum 5% deflection), and CCTV inspection.

Force mains are designed for 2–5 ft/s; air/vacuum release valves go at high points, and surge is controlled with slow-closing valves or surge relief.

Excavation protective systems are required at 5 feet deep or more; spoil and equipment stay at least 2 feet from the trench edge and ladders within 25 feet of workers.

Manholes and wet wells are permit-required confined spaces: test for oxygen (19.5–23.5%), flammables under 10% LEL, and H2S under 10 ppm before and during entry.

H2S control includes ventilation, chemical addition (nitrate salts, iron salts, peroxide), and crown liners where sulfuric acid corrosion occurs above the waterline.