NM Water/Wastewater Operator cheat sheet

Chlorine dose, lb/day = mg/L × MGD × 8.34.

Detention time, hr = basin volume (gal) ÷ (flow, gpm × 60).

Filter loading rate, gpm/ft² = flow (gpm) ÷ filter area (ft²); rapid sand filters normally run 2–4 gpm/ft².

Combined filter effluent turbidity must be ≤0.3 NTU in at least 95% of monthly samples and never exceed 1 NTU.

CT = disinfectant residual (mg/L) × contact time (min); required CT increases as pH rises and decreases as water temperature rises.

A jar test selects coagulant dose, coagulation pH, rapid-mix intensity, and flocculation time before full-scale changes are made.

Fluoride: primary MCL 4.0 mg/L, secondary standard 2.0 mg/L; concentrations above about 2 mg/L can cause dental fluorosis.

Langelier saturation index: negative = corrosive water, zero = balanced, positive = scale-forming water.

Concentration conversion: 1% = 10,000 mg/L; mg/L = (lb/day × 8.34) ÷ MGD.

Head and pressure: 1 ft of water column = 0.433 psi; 1 psi = 2.31 ft of head.

Flow: Q = A × V; Q (gpm) = 2.448 × inside diameter (in)² × velocity (ft/s).

Maintain a free chlorine residual of at least 0.2 mg/L throughout the distribution system; the maximum residual disinfectant level is 4.0 mg/L.

New main disinfection: 50 mg/L chlorine solution held 24 hours, then flushed and sampled for coliform bacteria before the main is placed in service.

Backflow protection hierarchy, best to least: air gap, reduced-pressure-zone assembly, double check valve, atmospheric vacuum breaker.

Distribution storage: equalizing volume commonly 15–25% of maximum day demand, plus fire flow and emergency reserve.

Water hammer is a pressure surge from rapid velocity change; slow-closing valves, surge tanks, and air chambers reduce it.

Cavitation occurs when available net positive suction head is less than required NPSH; symptoms are noise, vibration, and impeller pitting.

Unaccounted-for water = production metered − authorized metered sales − known system uses.

F/M ratio = lb BOD5 applied per day ÷ lb MLVSS in aeration; conventional activated sludge 0.2–0.4, extended aeration 0.05–0.15.

MCRT = lb solids in the aeration tank ÷ lb solids leaving daily (waste sludge plus effluent solids).

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

Nitrification consumes about 4.6 lb O2 and 7.14 lb alkalinity (as CaCO3) per lb ammonia-N oxidized; keep dissolved oxygen at 2 mg/L or more.

BOD5 is measured over 5 days at 20 °C using seeded dilution water and dissolved oxygen depletion.

Sludge yield in conventional activated sludge is roughly 0.5–0.6 lb VSS per lb BOD5 removed.

MLSS and MCRT are controlled by waste activated sludge rate; increasing WAS lowers both MCRT and MLSS.

Typical secondary effluent limits are 30 mg/L BOD5 and 30 mg/L TSS, with fecal coliform limits set in the discharge permit.

Increase return activated sludge rate when sludge blankets rise in the clarifier; decrease when blankets are thin and dilute.

Gravity sanitary sewer design velocity: minimum 2 ft/s (self-cleaning) and maximum about 10 ft/s to limit scour and hydrogen sulfide release.

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

Roughness n: about 0.013 for new concrete or vitrified clay pipe, 0.015 or higher for old or tuberculated pipe.

Pipe slope in ft/ft = fall ÷ length; percent slope = ft/ft × 100.

Infiltration is groundwater entering through defects; inflow is storm water entering through direct connections such as roof drains and catch basins.

Confined space entry requires oxygen 19.5–23.5%, hydrogen sulfide below 10 ppm, combustible gas below 10% of the lower explosive limit, a permit, and an attendant.

Use a drop manhole when the incoming sewer invert is more than about 24 in above the outgoing invert.

Smoke testing reveals illegal storm connections and broken pipe; dye testing confirms a specific identified connection.

Lift station wet well usable volume (gal) = pump capacity (gpm) ÷ (4 × allowable pump starts per hour).