MN Water Operator cheat sheet

Rapid mix disperses coagulant in roughly 10–30 seconds at high intensity (velocity gradient G ≈ 300–1000 s⁻¹); flocculation follows for 15–45 minutes at gentle G ≈ 10–75 s⁻¹.

Sedimentation surface overflow rate = flow ÷ basin surface area; particles with a settling velocity equal to or greater than the overflow rate are removed.

Conventional filter loading is typically 2–6 gpm/ft²; backwash is typically 15–20 gpm/ft², producing about 20–30% bed expansion.

Dual-media beds use anthracite (≈0.9–1.1 mm effective size) over sand (≈0.45–0.55 mm), giving coarse-to-fine grading that stores solids through the bed depth.

A jar test identifies the coagulant dose and pH that meet the turbidity goal at the lowest chemical dose; it simulates rapid mix, flocculation, and settling.

Lime-soda softening precipitates calcium carbonate and magnesium hydroxide; excess lime raises pH above about 10.5, so recarbonation is used to stabilize the water.

Iron and manganese are removed by oxidation (aeration, chlorine, or permanganate) followed by filtration; manganese greensand and catalytic media are common.

Filtered surface water must be ≤0.3 NTU in at least 95% of monthly samples and must never exceed 1 NTU.

A minimum pressure of 20 psi should be maintained at all points in the distribution system, including during fire flow, to prevent backflow and contamination.

A detectable disinfectant residual must persist throughout the system; a free chlorine residual of about 0.2 mg/L or greater is a common distribution minimum.

Design velocities are generally 2–5 ft/s: below 2 ft/s settling and stagnation occur, while above 5 ft/s head loss and pipe erosion increase sharply.

Hazen-Williams head loss varies with flow raised to about 1.85 and with pipe diameter raised to about 4.87, so small diameter changes greatly affect friction loss.

Unidirectional flushing at 2.5 ft/s or more scours sediment and biofilm; valves are sequenced so flow travels in one direction.

An air gap is the most reliable backflow protection; reduced-pressure-zone assemblies are used for high-hazard connections where an air gap is impractical.

A main break that drops pressure below 20 psi triggers a precautionary boil water notice until repairs and bacteriological sampling clear the system.

Storage tanks require turnover and inspection; stagnant water loses residual, stratifies, and can develop biofilm and nitrification.

Head in feet of water = psi × 2.31; pressure in psi = feet of head × 0.433 × specific gravity.

Water horsepower = (gpm × total head in ft × specific gravity) ÷ 3960.

Brake horsepower = water horsepower ÷ pump efficiency; motor input horsepower = brake horsepower ÷ motor efficiency.

Affinity laws for a centrifugal pump: flow varies directly with speed, head with speed squared, and power with speed cubed.

Pumps in series pass the same flow and add heads; pumps in parallel operate at the same head and add flows.

Cavitation occurs when NPSH available is less than NPSH required; vapor pressure rises with water temperature, reducing available NPSH.

Three-phase electrical power in kW = (volts × amps × power factor × 1.732) ÷ 1000.

For a centrifugal pump, power draw generally increases as flow increases; operating far right or left of the best efficiency point reduces efficiency and stresses the pump.

Hypochlorous acid (HOCl) is a much stronger disinfectant than hypochlorite ion (OCl⁻); the two are roughly equal near pH 7.5, and lower pH favors HOCl.

Combined chlorine = total chlorine − free chlorine; combined chlorine consists mainly of chloramines, which are weaker disinfectants that cause taste and odor.

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

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

Breakpoint chlorination requires roughly 7.6 mg of chlorine per mg of ammonia nitrogen to oxidize ammonia; free chlorine persists beyond the breakpoint.

Bacteriological samples are collected in 100 mL sterile, thiosulfate-treated bottles, held near 4°C, and analyzed within 30 hours of collection.

Conventional filtration must produce turbidity ≤0.3 NTU in at least 95% of monthly samples and never exceed 1 NTU; membrane systems target ≤0.1 NTU.

Heterotrophic plate count is limited to 500 CFU/mL; any total coliform-positive routine sample requires repeat sampling and E. coli confirmation.

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

Lockout/tagout requires isolating the energy source, applying locks and tags, and then attempting to start the equipment to verify a zero-energy state.

A dilute ammonia solution held near a suspected leak forms a white cloud on contact with chlorine gas; responders wear SCBA and approach from upwind.

In Minnesota, the Minnesota Department of Health regulates public water systems and certifies water operators; systems submit routine compliance monitoring data to the state.

Primary MCLs include nitrate 10 mg/L as N, arsenic 10 µg/L, lead action level 0.015 mg/L, copper action level 1.3 mg/L, TTHM 80 µg/L, and HAA5 60 µg/L.

Tier 1 public notices are issued within 24 hours for immediate health risks; Tier 2 within 30 days; Tier 3 is included in the annual Consumer Confidence Report.

Community water systems must deliver the Consumer Confidence Report to customers by July 1 each year, covering source water, detected contaminants, and health effects.

Sanitary surveys of public water systems are conducted roughly every 3–5 years, depending on system type and compliance history.