ND Water/Wastewater Operator cheat sheet

CT (contact time) value = disinfectant residual concentration (mg/L) × contact time (minutes); CT requirements rise as temperature drops and pH rises for free chlorine.

Free chlorine is far more pH-sensitive than chloramines: at pH above ~8, hypochlorous acid (HOCl) converts to hypochlorite ion (OCl-), which is roughly 80–100 times less effective as a disinfectant.

Log removal credits: conventional filtration (coagulation, flocculation, sedimentation, filtration) can earn up to about 2.5 log Cryptosporidium and 3 log Giardia, but virus credit from physical treatment is limited to about 2 log; disinfection supplies the rest.

Turbidity limits under the Surface Water Treatment Rule: combined filter effluent must be ≤0.3 NTU in at least 95% of monthly samples and never exceed 1 NTU; individual filter effluent must never exceed 1 NTU.

Jar test procedure: rapid mix (about 1–2 minutes at high speed), slow mix/flocculation (about 15–20 minutes), then settle (about 20–30 minutes); select the coagulant dose giving lowest settled turbidity and appropriate floc size.

Optimum alum dose typically falls near pH 6–7; use alkalinity to predict pH depression - each mg/L of alum consumes roughly 0.45 mg/L of alkalinity as CaCO3.

Chlorine demand = applied dose − residual after the contact period; the breakpoint is where demand is satisfied and free residual begins to persist, commonly reached around a 7–9:1 Cl2-to-ammonia-nitrogen mass ratio.

Langelier Saturation Index (LSI) = pH − pHs; negative LSI is corrosive, positive is scale-forming, near zero is balanced. pHs depends on calcium hardness, total alkalinity, temperature, and total dissolved solids.

Minimum residual disinfection in distribution: a detectable residual must be maintained throughout the system; common state targets are around 0.2 mg/L free chlorine or 0.5–1.0 mg/L total chlorine minimum, with a 4.0 mg/L maximum at the entry point under Stage 2 DBPR.

Static pressure rule of thumb: 1 psi ≈ 2.31 feet of water head; a 100-foot elevation change equals about 43.3 psi.

Water hammer pressure surge ≈ a × Δv / g (a = pressure wave velocity, Δv = velocity change); closing a valve slowly reduces the surge.

Distribution storage provides equalization, fire flow, and emergency reserve; typical sizing guidance is that total storage should cover average-day demand plus fire flow plus emergency needs.

Pipe flow: Q = A × V. Doubling velocity doubles flow; doubling diameter quadruples cross-sectional area (A = πd²/4).

Flushing is complete when turbidity and chlorine residual stabilize at system baseline values; dead-end mains are the priority flush points.

Backflow prevention: an air gap provides the highest protection; reduced-pressure-zone assemblies protect against both backpressure and backsiphonage; a double check valve assembly protects against backsiphonage only.

Casing pipe and proper burial depth protect mains from frost and traffic loads; locate lines and valves before excavation, and maintain minimum separation from sewer lines per state rules.

Secondary clarifier solids balance: return activated sludge (RAS) and waste activated sludge (WAS) control the mixed liquor suspended solids (MLSS) and sludge age (SRT).

Sludge age (SRT, days) = MLSS mass in aeration basin ÷ (solids leaving in WAS + solids leaving in effluent) per day.

Food-to-microorganism ratio F/M = (BOD or COD load, lb/day) ÷ (MLSS mass in aeration, lb); low F/M means long sludge age and more stable nitrification.

Hydraulic retention time (HRT) = basin volume ÷ influent flow rate; solids retention time is always longer than HRT when RAS is returned.

Sludge volume index (SVI) = (settled sludge volume in 30 min, mL/L × 1000) ÷ MLSS, mg/L; SVI roughly 80–150 mL/g indicates good settling, above ~200 indicates filamentous bulking.

Nitrification consumes about 4.6 mg O2 per mg NH3-N oxidized and consumes alkalinity (about 7.1 mg/L as CaCO3 per mg/L ammonia nitrogen); denitrification recovers roughly half that oxygen demand.

Dissolved oxygen targets: typically 1.5–2.5 mg/L in the aerobic zone for nitrification; anoxic zones need near-zero DO and a carbon source to drive denitrification.

Effluent BOD5 and TSS permit limits are commonly 30/30 mg/L for secondary treatment under the federal minimum, with many states imposing stricter limits; disinfection often uses UV or chlorine with dechlorination.

Gravity sewer design velocity: self-cleansing minimum is commonly 2 ft/s at design flow, and maximum is typically 10 ft/s to limit scour and hydrogen sulfide release.

Manning's equation: V = (1.486/n) × R^(2/3) × S^(1/2) in US customary units; n for vitrified clay or concrete pipe is about 0.013.

Minimum sewer slopes are set by pipe diameter (a common rule is about 0.4% for 8-inch, 0.28% for 10-inch, 0.22% for 12-inch); flatter slopes require a velocity check.

Infiltration enters through defects such as cracked pipe, joints, and manholes; inflow enters through direct connections such as roof drains, sumps, and storm inlets.

Hydrogen sulfide corrosion: H2S forms under septic conditions, oxidizes to sulfuric acid above the waterline, and attacks concrete; control with ventilation, chemical addition, or liners.

Sewer cleaning: high-velocity jetting removes grease and sediment; roots are cut with mechanical augers; CCTV inspection documents defects and is required after rehabilitation.

Confined-space entry requires pre-entry atmospheric testing for oxygen (19.5–23.5%), flammables (below 10% LEL), and toxics such as H2S (often below 10 ppm), plus a permit and attendant.

Chlorine gas is heavier than air; evacuate upwind and uphill. NIOSH IDLH for chlorine is 10 ppm; the OSHA permissible exposure limit is 1 ppm as a ceiling.

Hydrogen sulfide: OSHA general-industry ceiling is 20 ppm; IDLH is 100 ppm. H2S is heavier than air and deadens the sense of smell at higher concentrations.

Confined space oxygen limits: 19.5% minimum and 23.5% maximum; below 19.5% is oxygen-deficient, above 23.5% is an explosion hazard.

Methane LEL is 5% by volume in air; entry is prohibited above 10% of the LEL without ventilation and controls.

Sodium hypochlorite and acid must never be mixed - mixing releases chlorine gas; store oxidizers separately from acids and fuels.

Lockout/tagout: isolate all energy sources, verify zero energy, and apply a personal lock; only the person who applied a lock may remove it.

Electrical safety: use ground-fault circuit protection in wet locations; water and electricity demand GFCI-protected circuits and dry hands.

Ladder and fall safety: maintain three points of contact, secure portable ladders, and use fall protection at heights of 4 feet or more in general industry.

Safe Drinking Water Act (1974, amended 1986 and 1996) authorizes EPA to set national primary drinking water regulations; states may be primacy agencies with stricter rules.

Total Coliform Rule: no more than 5% of samples may be total-coliform-positive per month for systems collecting at least 40 samples; any E. coli-positive sample triggers public notice.

Stage 2 Disinfectants and Disinfection Byproducts Rule limits: TTHM running annual average 80 µg/L and HAA5 60 µg/L; LRAA compliance is calculated at each monitoring location.

Lead and Copper Rule action level: 0.015 mg/L for lead and 1.3 mg/L for copper, based on the 90th-percentile of first-draw samples at high-risk sites.

Arsenic MCL is 0.010 mg/L (10 µg/L); nitrate MCL is 10 mg/L as nitrogen; fluoride MCL is 4.0 mg/L with a secondary limit of 2.0 mg/L.

Clean Water Act National Pollutant Discharge Elimination System (NPDES) permits set effluent limits and monitoring/reporting requirements; discharge without a permit is prohibited.

Operator certification: most states require a grade-specific exam plus continuing education units (often 2.0 CEUs or 20 contact hours per renewal cycle) to maintain certification.

Consumer Confidence Report (CCR) must be delivered to customers annually by July 1, summarizing source water, detected contaminants, and compliance status.

Recordkeeping: log daily flows, chemical feed rates, residuals, and lab results; permit compliance depends on accurate, contemporaneous records.

Chemical feed rate (lb/day) = flow (MGD) × dose (mg/L) × 8.34.

Percent strength of a solution: dose delivered = feed rate (gal/min) × 8.34 lb/gal × percent concentration; check units before calculating.

Pump horsepower: brake horsepower = (flow, gpm × head, ft × specific gravity) ÷ (3960 × pump efficiency).

Pump cavitation produces noise, vibration, and reduced flow; it is caused by low net positive suction head available (NPSHa) versus required (NPSHr).

Calibration frequency: verify analyzers and chemical feed pumps on a scheduled basis (often weekly to monthly), and after any repair or process change.

Chain of custody and sampling: use clean, appropriate containers, preserve samples per method (e.g., refrigeration, acidification), and record collection time and location.

Budgeting and asset management: track energy use, chemical consumption, and maintenance costs per unit of water or wastewater treated to spot inefficiencies.