NC Water/Wastewater Operator cheat sheet

Pounds formula: lbs = mg/L × 8.34 × million gallons (MG).

Chlorine dose = chlorine demand + desired residual; residual is measured after the required contact time.

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

Combined filter effluent turbidity limit: ≤0.3 NTU in at least 95% of monthly samples and never >1 NTU under the surface water treatment rule.

Rapid sand filter loading rate = flow (gpm) ÷ filter area (ft²); typical design loading is 2–4 gpm/ft².

Filter backwash rates are typically 15–20 gpm/ft²; verify adequacy by bed expansion of roughly 20–50%.

Jar test: vary coagulant dose and pH to find the minimum dose that produces the lowest filtered turbidity.

Alum consumes about 0.45–0.5 mg/L of alkalinity as CaCO3 per mg/L applied; add lime when raw-water alkalinity is too low for coagulation.

Langelier Saturation Index: positive values indicate scale-forming water, negative values indicate corrosive water, near zero is balanced. Optimal fluoride is about 0.7 mg/L.

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

Q = A × V; 1 cubic foot per second = 448.8 gpm and 1 cubic foot = 7.48 gallons.

Pipe volume in gallons = 0.785 × diameter² (ft) × length (ft) × 7.48.

Hazen-Williams head loss increases with flow raised to about the 1.852 power and decreases as the C factor increases (smoother pipe).

North Carolina requires a minimum of 20 psi at all points in the distribution system; normal service pressure is typically 35–60 psi.

Flush water mains at 2.5 ft/s or faster to scour and remove sediment and stale water.

Water hammer is a pressure surge from a rapid velocity change; control it with slow valve closure, surge tanks, and air/vacuum valves.

Backflow protection ranked by hazard: air gap (highest) > reduced-pressure zone assembly > double check valve assembly.

Elevated storage: each foot of elevation gives 0.433 psi, so a tank 100 ft above a service point delivers about 43.3 psi.

1 mg/L = 8.34 lb per million gallons; use this to convert concentrations to mass loadings.

F/M ratio = lbs BOD5 applied per day ÷ lbs MLSS under aeration; conventional activated sludge operates around 0.2–0.5.

SVI = 30-minute settled sludge volume (mL/L) × 1000 ÷ MLSS (mg/L); 80–150 mL/g indicates good settling.

MCRT (SRT) = lbs of solids in the system ÷ lbs of solids leaving per day; wasting more sludge lowers MCRT and MLSS.

Clarifier surface overflow rate = flow (gpd) ÷ clarifier surface area (ft²).

Maintain about 2 mg/L dissolved oxygen in the aeration basin for carbon oxidation and nitrification.

Nitrification consumes about 7.14 mg/L alkalinity as CaCO3 and 4.57 mg/L oxygen per mg/L of ammonia nitrogen oxidized.

Mesophilic anaerobic digesters are held near 95°F (90–100°F) with a solids retention time of roughly 20–30 days.

Typical NPDES effluent pH range is 6.5–9.0 standard units.

Manning's equation (English units): V = (1.486/n) × R^(2/3) × S^(1/2).

Minimum self-cleansing velocity in gravity sewers is 2 ft/s; maximum velocity is about 10–15 ft/s to limit scour.

North Carolina minimum slope for 8-inch gravity sewer is 0.4% (0.004 ft/ft); larger diameters may use flatter slopes.

Infiltration is groundwater entering through defective pipes and joints; inflow is storm water entering manholes, roof drains, and direct connections.

Confined space entry limits: oxygen 19.5–23.5%, hydrogen sulfide below 10 ppm, and flammable gas below 10% of the lower explosive limit.

Hydrogen sulfide corrodes concrete above the waterline in sewers; control it with chemical dosing, ventilation, and odor-control covers.

Lift station wet wells are sized so pumps cycle within their recommended run and rest times; undersized wells cause short cycling.

Surcharging occurs when flow exceeds sewer capacity; check downstream restrictions and infiltration/inflow sources.

Grease, roots, and debris cause most blockages; high-pressure jetting and routine cleaning keep lines open.