HI Water/WW Operator cheat sheet

CT (contact time) = disinfectant residual concentration (mg/L) × contact time (minutes); CT is expressed in mg·min/L and must meet or exceed the required CT for the measured temperature, pH, and disinfectant.

Rule of thumb: for chlorine disinfection, CT required decreases as temperature rises; colder water needs more CT for the same log inactivation.

Chlorine demand = chlorine dose − chlorine residual after the contact period; demand comes from reducing agents (Fe2+, Mn2+, H2S, organics, ammonia).

Breakpoint chlorination: adding chlorine to ammonia-containing water first forms chloramines (combined residual); past the breakpoint (roughly 7.6:1 Cl2:NH3-N by weight), free chlorine residual appears.

Jar test determines the coagulant dose: vary coagulant dose and pH, mix rapidly, flocculate slowly, settle, and measure turbidity; the lowest dose that gives acceptable settled turbidity is the working dose.

Detention time (hours) = volume (gal) ÷ flow (gpd) ÷ 24; use basin volume, not tank footprint.

Surface overflow rate (gpm/ft²) = flow (gpm) ÷ clarifier surface area (ft²); it governs settling performance independent of depth.

Filter backwash: backwash rate is set by temperature (water viscosity); cold water expands less, so backwash rates are lower in winter.

Turbidity is measured in NTU; the conventional treatment combined filter effluent limit is 0.3 NTU in 95% of monthly samples and never above 1 NTU.

Pressure conversion: 1 psi = 2.31 ft of water head; 1 ft of head = 0.433 psi.

Flow in a pipe: Q = A × V; doubling diameter quadruples area (A = πr² = 0.7854 d²).

Hazen-Williams head loss rises with flow^1.852 and falls with C^1.852; smaller C (rougher pipe) means more head loss.

Water hammer results from rapid velocity change; surge pressure ≈ 0.07 × velocity change (ft/s) × wave speed factor, mitigated by slow valve closure and surge tanks.

Cross-connection control: an air gap is the only non-mechanical backflow protection; RPZ (reduced pressure zone) assemblies protect against both backpressure and backsiphonage.

Distribution system minimum residual: maintain a detectable disinfectant residual throughout; 0.2 mg/L free chlorine is a common minimum at the farthest point.

Storage tank turnover: detention time (days) = tank volume (MG) ÷ average daily demand (MGD); long turnover ages water and depletes residual.

Pipe bedding and compaction: backfill is compacted in lifts around the pipe haunches first to prevent ovaling and joint deflection.

BOD5 measures biodegradable organic matter consumed by microbes over 5 days at 20°C; carbonaceous BOD excludes nitrogenous demand (NBOD).

F/M ratio = pounds of BOD applied per day ÷ pounds of MLSS under aeration; low F/M (long MCRT) favors nitrification and lower sludge production.

MCRT (sludge age, days) = pounds of MLSS in aeration ÷ pounds of solids wasted plus effluent solids per day.

SVI (mL/g) = settled sludge volume (mL/L after 30 min) × 1000 ÷ MLSS (mg/L); SVI above ~150 indicates poor settling (bulking).

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

Secondary clarifier solids loading rate (lb/day/ft²) = (Q + Qr) × MLSS × 8.34 ÷ clarifier area; overflow rate uses Q only.

Anaerobic digester: volatile solids destruction produces methane; typical gas is 60–65% methane, and digester heating plus mixing stabilize temperature around 95°F (mesophilic).

Disinfection: fecal coliform limits are typically a geometric mean of 200 CFU/100 mL (monthly) with a 400 CFU/100 mL maximum for secondary treatment.

Return activated sludge (RAS) rate controls MLSS; wasting rate controls MCRT - increasing wasting lowers MCRT and MLSS.

Manning's equation: V = (1.486/n) R^(2/3) S^(1/2); R = hydraulic radius = area ÷ wetted perimeter.

Self-cleansing velocity: design for at least 2 ft/s at full or half-full flow to keep solids moving; below ~2 ft/s grit and solids deposit.

Slope of a sewer: S (ft/ft) = rise ÷ run; a 1% slope drops 1 ft per 100 ft.

Infiltration enters through defects (groundwater); inflow enters through direct connections (downspouts, sump pumps, manhole covers).

Surcharge occurs when flow exceeds pipe capacity and the hydraulic grade line rises above the pipe crown; SSOs result if it reaches manholes or backups.

Lift station wet well: pump cycling time depends on wet well volume between lead and lag levels and inflow; short cycling damages motors.

Confined space entry requires atmospheric testing for oxygen (19.5–23.5%), LEL, H2S, and CO before entry and continuous monitoring during entry.

Grease and roots are the leading causes of blockages; jetting removes grease, and root cutters or chemical root control handle roots.

Inflow/infiltration (I/I) reduction: smoke testing locates inflow sources, and dyed water testing confirms illegal connections.