NE Water/Wastewater Operator cheat sheet

Coagulant dose and optimum pH are determined by jar test; alum typically performs best near pH 6.0–7.0.

Sedimentation surface overflow rate = flow ÷ surface area; detention time = basin volume ÷ flow.

Rapid sand filtration rates are commonly 2–5 gpm/ft²; backwash rates are 15–20 gpm/ft² with 20–30% media expansion.

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

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

Chlorine demand = applied chlorine dose − residual remaining after the specified contact time.

Lime-soda softening raises pH; calcium carbonate precipitates near pH 9–10, and magnesium hydroxide precipitates above about pH 10.5.

Langelier Saturation Index: negative means corrosive water, zero means balanced, positive means scale-forming water.

Fluoride is fed to maintain about 0.7 mg/L; the fluoride MCL is 4.0 mg/L.

Minimum distribution pressure is 20 psi at all points under normal and emergency conditions; typical operating pressure is 40–80 psi.

Free chlorine residual must be detectable throughout the system; the maximum residual disinfectant level is 4.0 mg/L as a running annual average.

A positive total coliform result requires three repeat samples within 24 hours, collected at the original site and nearby upstream and downstream locations.

New water mains are disinfected with 50 mg/L chlorine for 24 hours, then flushed and left with at least 25 mg/L residual.

Main flushing should achieve at least 2.5–3 ft/s to scour and remove sediment.

In the Hazen-Williams formula, lower C-factor means greater head loss; new cement-lined pipe is about C = 130–140, while old tuberculated pipe may be C = 80–100.

An air gap provides the highest level of backflow protection; an RPZ assembly is used for high-hazard connections where backpressure is possible.

Water hammer is a pressure surge caused by rapid velocity change, such as fast valve closure or pump trip; slow-closing valves and surge tanks reduce it.

Water age increases in oversized mains and storage tanks, allowing chlorine residual to decay.

F/M ratio = BOD mass applied per day ÷ MLVSS mass in the aeration tank; conventional activated sludge operates near 0.2–0.5 day⁻¹.

MCRT (sludge age) = solids mass in the aeration system ÷ solids leaving per day; it controls sludge production and nitrification.

MLSS is typically 2,000–4,000 mg/L; SVI of 80–150 mL/g indicates good settling, while SVI above 200 mL/g indicates bulking.

Secondary clarifier surface overflow rate is commonly 400–800 gpd/ft²; solids loading rate is commonly 15–30 lb/day/ft².

Nitrification consumes about 7.14 mg alkalinity as CaCO3 per mg NH3-N oxidized and requires dissolved oxygen above 2 mg/L.

BOD5 is measured at 20°C over 5 days; a nitrification inhibitor is used when carbonaceous BOD alone is desired.

Mesophilic anaerobic digestion operates near 35°C (95°F), commonly with a 15–30 day solids retention time.

Facultative ponds rely on algae-bacteria symbiosis: algae supply oxygen, and bacteria release carbon dioxide and nutrients.

Manning equation: Q = (1.486/n) A R^(2/3) S^(1/2); n is about 0.013 for concrete or PVC sewer pipe.

Minimum self-cleaning velocity is about 2 ft/s; maximum velocity is about 10–15 ft/s to limit abrasion and erosion.

Typical minimum slope is 0.4% (0.004 ft/ft) for 8-inch sewer and 0.22% for 12-inch sewer.

Pipe drop = slope × length; for example, 0.005 ft/ft over 300 ft gives 1.5 ft of drop.

Infiltration is groundwater entering through defective pipes, joints, or manholes; inflow is direct storm or surface water from improper connections.

OSHA requires a protective system for excavations 5 feet deep or more unless the excavation is entirely in stable rock.

Spoil piles and equipment must be kept at least 2 feet back from the edge of an excavation.

Confined space entry requires atmospheric testing first; acceptable oxygen is 19.5–23.5%, and a permit is required for permit-required spaces.

A check valve on each lift-station pump discharge prevents backflow and keeps the wet well from refilling when the pump stops.