MO Water/Wastewater Operator cheat sheet

CT (contact time) = disinfectant residual concentration (mg/L) × effective contact time (min); CT requirements rise as temperature falls and pH rises for chlorine.

Chlorine demand = dose applied − free chlorine residual remaining after the contact period; it reflects oxidizable matter in the water.

Turbidity limits under the Surface Water Treatment Rule: 0.3 NTU in 95% of monthly samples, never above 1 NTU; combined filter effluent monitored continuously.

Log inactivation credit for Giardia and viruses depends on disinfectant type, residual, contact time, pH, and temperature; Crypto is only creditable with UV or ozone.

Coagulation optimum pH is generally 5.5–7.5 for alum; jar tests determine the lowest dose producing settled and filtered clarity.

Filter loading rate = flow (gpm) ÷ filter area (ft²); rapid sand filters typically operate at 2–5 gpm/ft².

Backwash rate must expand the bed roughly 20–30%; typical rapid sand backwash is 12–20 gpm/ft².

Langelier Saturation Index near 0 means balanced water; negative is corrosive, positive is scale-forming.

Fluoride is dosed to about 0.7 mg/L; the MCL is 4.0 mg/L and the secondary standard is 2.0 mg/L.

Minimum residual: 0.2 mg/L free chlorine (or 0.5 mg/L chloramine) at points of entry and throughout the distribution system in Missouri.

Pressure must remain at least 20 psi at all points under normal conditions; a main break or pump failure that drops below 20 psi triggers a boil order.

Theoretical water horsepower = (gpm × head in ft) ÷ 3960; brake horsepower = water hp ÷ pump efficiency.

Detention time in a storage tank = tank volume ÷ flow rate; use consistent units (e.g., gallons ÷ gpm = minutes).

Hydrant flow test: static pressure, residual pressure, and pitot pressure determine available flow; a 20 psi residual is the standard reference point.

Coliform rule: total coliform positive triggers repeat sampling; E. coli-positive or repeat-positive results require public notice within 24 hours.

Water main flushing velocities of about 2.5–3 ft/s and above are needed to scour sediment; dead ends are flushed on a routine schedule.

Cross-connection protection: an air gap is the only true backflow prevention; RPZ, double check, and PVB devices protect specific hazard levels.

Pipe velocity = flow ÷ cross-sectional area; convert gpm to cfs by dividing by 448.8 before dividing by area.

BOD5 measures oxygen consumed over 5 days at 20°C; typical domestic wastewater is about 200–250 mg/L BOD and 200–250 mg/L TSS.

F/M ratio = (lbs BOD applied per day) ÷ (lbs MLVSS in aeration); typical conventional activated sludge runs 0.2–0.5.

MCRT (sludge age) = lbs MLSS in aeration ÷ lbs solids leaving per day; controls sludge blanket and effluent quality.

Solids loading rate on secondary clarifiers = lbs/day ÷ (surface area × 1,000); typical design is 12–30 lbs/day/1,000 ft².

Hydraulic detention time = tank volume ÷ flow; aeration basins typically hold 4–8 hours.

Return activated sludge rate typically runs 50–150% of influent flow; high SVI (>150) indicates poor settling (bulking).

Chlorine disinfection CT for wastewater uses fecal coliform limits (typically 200/100 mL monthly geometric mean in Missouri); ammonia interferes with chlorine demand.

Anaerobic digesters reduce volatile solids and produce methane; typical operating temperature is 95–98°F (mesophilic).

Effluent limits often include BOD 30 mg/L, TSS 30 mg/L, and ammonia; Missouri uses both monthly averages and weekly averages.

Minimum gravity sewer slope for 8-inch pipe is typically 0.40% (about 0.004 ft/ft); smaller pipe needs steeper slopes, larger pipe can be flatter.

Use Manning's equation: velocity = (1.486/n) × R^(2/3) × S^(1/2), where n ≈ 0.013 for new vitrified clay or PVC.

Self-cleansing velocity is about 2 ft/s; below that, solids settle and cause blockages.

Infiltration is groundwater entering through defects; inflow is direct stormwater entering through manholes, roof drains, or illegal connections.

Lift station wet well detention time should stay under 30 minutes to prevent septic conditions and odor.

Sewer cleaning: high-velocity jetting (1,000–2,000 psi) for grease and roots; bucket machines for heavy debris.

Confined space entry requires atmospheric testing for oxygen (19.5–23.5%), LEL (<10%), and H2S (<10 ppm) before entry.

Sewer smoke testing reveals inflow sources and illegal connections; dye testing confirms specific connections.

Manhole inspection frequency depends on pipe condition; CCTV inspection rates pipes on a condition scale of 1–5.