CO Water Operator cheat sheet

CT = disinfectant residual (mg/L) × contact time (min); the required CT rises as pH climbs above 7.5 and as water temperature falls.

1 mg/L of alum consumes about 0.5 mg/L of alkalinity as CaCO3; keep coagulation pH near 6.5–7.5 and finished alkalinity above roughly 20 mg/L.

A jar test compares coagulant doses across a pH range; select the lowest dose that meets the settled/filtered turbidity goal.

Combined filter effluent must be ≤0.3 NTU in at least 95% of monthly samples and never above 1 NTU; individual filters ≤1 NTU in 95% of samples.

Free chlorine residual entering the distribution system must be at least 0.2 mg/L; LRAA limits are 80 µg/L for TTHM and 60 µg/L for HAA5.

Rapid-gravity filters are loaded at roughly 2–6 gpm/ft² and backwashed at 15–20 gpm/ft² with about 50% bed expansion.

Langelier Saturation Index: negative values mean corrosive water, positive values mean scale-forming water; target roughly -0.5 to +0.5.

Fluoride: optimum dose about 0.7 mg/L, primary MCL 4.0 mg/L, secondary standard 2.0 mg/L.

At pH 7.5 free chlorine is about half HOCl and half OCl⁻; HOCl is the far stronger disinfectant, so lower pH improves inactivation.

psi = feet of head × 0.433; feet of head = psi × 2.31.

Maintain at least 20 psi at all points under normal demand and during fire flow; a negative pressure can draw contamination into the main.

Gallons per foot of main = 0.0408 × (inside diameter in inches)²; total volume = gallons per foot × length in feet.

Hazen-Williams: head loss rises with Q^1.852 and falls with C^1.852 and d^4.87; typical C is 130–140 for new lined pipe and 80–100 for tuberculated iron.

Separate water mains at least 10 ft horizontally and at least 18 in vertically from sanitary sewers; closer crossings require encasement or pressure-rated pipe.

Unidirectional flushing should achieve at least 2.5 ft/s in the main to scour and carry away sediment and biofilm.

A detectable free chlorine residual must be maintained throughout the system; 0.2 mg/L is the regulatory minimum and no sample may be non-detect.

An air gap is the only absolute backflow protection; an RPZ assembly is used for high-hazard connections.

Thrust blocks or restrained joints are needed at bends, tees, caps, and valves; thrust at a 90° bend equals about 1.41 × pressure × pipe cross-sectional area.

F/M = lb BOD5 applied per day ÷ lb MLVSS under aeration; conventional activated sludge runs about 0.2–0.5 per day.

MCRT (sludge age) = lb solids in the aeration basin ÷ lb solids leaving per day in waste plus effluent; nitrification needs roughly 10 or more days in cold water.

SVI = 30-minute settled sludge volume (mL/L) × 1000 ÷ MLSS (mg/L); 80–150 settles well, while values above 200 suggest filamentous bulking.

Oxygen demand is roughly 1.0 lb O2 per lb BOD5 removed, plus about 4.6 lb O2 per lb of ammonia nitrogen nitrified.

Nitrification consumes about 7.14 lb of alkalinity as CaCO3 per lb NH3-N oxidized; below roughly 50 mg/L alkalinity nitrification stalls and pH drops.

Secondary clarifier solids loading is typically designed at 0.8–1.2 lb/day/ft²; exceeding it raises the sludge blanket and carries solids over the weir.

Mesophilic anaerobic digestion at 95–100 °F destroys about 50% of volatile solids; the gas produced is roughly 60–65% methane and 35–40% carbon dioxide.

Typical permit limits include effluent pH 6.5–9.0 and a fecal coliform monthly geometric mean of 200 cfu/100 mL after disinfection.

Primary sludge thickens to about 3–6% solids while waste activated sludge is only 0.5–1.5%, which is why WAS is thickened before digestion.

Manning: Q = (1.486/n) × A × R^(2/3) × S^(1/2); n is about 0.013 for new PVC and capacity varies with the square root of slope.

Design for at least 2 ft/s at average daily flow to keep solids moving; velocities above roughly 10 ft/s scour the pipe.

Minimum slope for an 8-inch public sewer is about 0.004 ft/ft (0.4%); slope percent = (drop ÷ run) × 100.

Infiltration is groundwater entering through defective pipe and joints; inflow is direct stormwater from roof drains, sumps, and manhole covers.

Confined space entry limits: oxygen 19.5–23.5%, flammable gas below 10% LEL, hydrogen sulfide below 10 ppm, carbon monoxide below 35 ppm.

Lift station cycle time = wet well volume between pump-on and pump-off ÷ (pump capacity − inflow); short cycling indicates too little storage or oversized pumps.

Sulfide formed in anaerobic slime becomes sulfuric acid on the moist crown above the waterline, destroying concrete pipe and manholes.

OSHA requires cave-in protection at 5 ft depth unless solid stable rock, a ladder within 25 ft of every worker, and spoils kept at least 2 ft from the edge.

Manholes are typically spaced 300–500 ft on straight runs and placed at every change in grade, size, alignment, or pipe junction.