DC Water Operator cheat sheet

Disinfection CT = disinfectant residual concentration (mg/L) × contact time (min); required CT depends on pH, temperature, and disinfectant type.

Chlorine demand = applied dose − residual after contact period; free chlorine residual = total chlorine − combined chlorine.

Conventional treatment sequence: coagulation → flocculation → sedimentation → filtration → disinfection.

Filter loading rate = flow (gpm) ÷ filter area (ft²); rapid sand filters typically run 2–6 gpm/ft² at design rate.

Upflow through a filter bed loosens media, so backwash flow rates are set by percent bed expansion (typically 15–25%) and water temperature.

Jar test results set coagulant dose and pH; optimum pH and dose are chosen where settled turbidity is lowest.

Turbidity is reported in NTU; combined filter effluent limits are commonly 0.3 NTU in at least 95% of monthly samples and never above 1 NTU.

Log removal credit: 1-log = 90%, 2-log = 99%, 3-log = 99.9%, 4-log = 99.99% reduction of pathogens.

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.

Pressure = 0.433 psi per foot of water column; 1 psi = 2.31 ft of head.

Flow in a full pipe: Q = A × V; for a circular pipe, A = 0.7854 × D².

Hazen-Williams head loss hf = 10.44 × L × Q^1.85 ÷ (C^1.85 × d^4.87) in US customary units.

Minimum residual disinfectant in the distribution system is typically 0.2 mg/L for free chlorine; detectable residual is required at all points.

Minimum normal operating pressure is generally 20 psi; below that, a boil-water notice and investigation are triggered.

Water main disinfection after repair commonly uses 25–50 mg/L free chlorine for 24 hours or an equivalent CT.

Distribution storage must maintain turnover; long detention time raises water age, which depletes residual and increases DBPs.

Cross-connection control relies on backflow prevention by air gap (highest protection) or reduced-pressure-zone assembly.

Leak detection: acoustic listening and correlators locate leaks; minimum night flow indicates the size of system losses.

BOD5 measures biodegradable organic matter consumed by microbes over 5 days at 20°C; CBOD excludes nitrogenous demand.

Secondary clarifier solids loading rate = (Q + Qr) × MLSS ÷ clarifier area.

Sludge age (SRT) = mass of solids in aeration basin ÷ mass of solids wasted per day.

F/M ratio = pounds of BOD applied per day ÷ pounds of MLSS in the aeration tank.

Return activated sludge rate Qr = Q × MLSS ÷ (RAS concentration − MLSS).

Hydraulic detention time = tank volume ÷ flow rate; typical aeration basin detention is 4–8 hours.

Nitrification consumes about 4.57 mg O2 per mg N oxidized; denitrification recovers roughly 2.86 mg O2 per mg N.

Effluent limits are commonly 30 mg/L BOD and 30 mg/L TSS for secondary treatment, with 85% removal of both.

Sludge volume index SVI = settled sludge volume (mL/L after 30 min) × 1,000 ÷ MLSS (mg/L); SVI of 80–150 indicates good settling.

Manning's equation: V = (1.486/n) × R^(2/3) × S^(1/2) in US customary units.

Self-cleansing velocity in gravity sewers is about 2 ft/s at design flow; below that, solids deposit.

Maximum design velocity is typically 10 ft/s to limit scour and hydrogen sulfide release.

Minimum sewer slope for an 8-inch line is commonly 0.4% (about 0.0033 ft/ft); smaller pipes need steeper grades.

Infiltration enters through defects in pipe and joints; inflow enters through direct connections such as roof drains and sumps.

Surcharging occurs when flow exceeds pipe capacity; the hydraulic grade line rises above the crown.

Hydrogen sulfide in sewers causes odor and corrosion; it is controlled by ventilation, chemical addition, and preventing septic conditions.

Confined-space entry requires atmospheric testing for oxygen (19.5–23.5%), flammables, and H2S before and during entry.

Lift station wet-well level controls pump cycling; excessive cycles per hour shorten pump and motor life.

Disinfection CT = disinfectant residual concentration (mg/L) × contact time (min); required CT depends on pH, temperature, and disinfectant type.

Chlorine demand = applied dose − residual after contact period; free chlorine residual = total chlorine − combined chlorine.

Conventional treatment sequence: coagulation → flocculation → sedimentation → filtration → disinfection.

Filter loading rate = flow (gpm) ÷ filter area (ft²); rapid sand filters typically run 2–6 gpm/ft² at design rate.

Upflow through a filter bed loosens media, so backwash flow rates are set by percent bed expansion (typically 15–25%) and water temperature.

Jar test results set coagulant dose and pH; optimum pH and dose are chosen where settled turbidity is lowest.

Turbidity is reported in NTU; combined filter effluent limits are commonly 0.3 NTU in at least 95% of monthly samples and never above 1 NTU.

Log removal credit: 1-log = 90%, 2-log = 99%, 3-log = 99.9%, 4-log = 99.99% reduction of pathogens.

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.

Pressure = 0.433 psi per foot of water column; 1 psi = 2.31 ft of head.

Flow in a full pipe: Q = A × V; for a circular pipe, A = 0.7854 × D².

Hazen-Williams head loss hf = 10.44 × L × Q^1.85 ÷ (C^1.85 × d^4.87) in US customary units.

Minimum residual disinfectant in the distribution system is typically 0.2 mg/L for free chlorine; detectable residual is required at all points.

Minimum normal operating pressure is generally 20 psi; below that, a boil-water notice and investigation are triggered.

Water main disinfection after repair commonly uses 25–50 mg/L free chlorine for 24 hours or an equivalent CT.

Distribution storage must maintain turnover; long detention time raises water age, which depletes residual and increases DBPs.

Cross-connection control relies on backflow prevention by air gap (highest protection) or reduced-pressure-zone assembly.

Leak detection: acoustic listening and correlators locate leaks; minimum night flow indicates the size of system losses.

BOD5 measures biodegradable organic matter consumed by microbes over 5 days at 20°C; CBOD excludes nitrogenous demand.

Secondary clarifier solids loading rate = (Q + Qr) × MLSS ÷ clarifier area.

Sludge age (SRT) = mass of solids in aeration basin ÷ mass of solids wasted per day.

F/M ratio = pounds of BOD applied per day ÷ pounds of MLSS in the aeration tank.

Return activated sludge rate Qr = Q × MLSS ÷ (RAS concentration − MLSS).

Hydraulic detention time = tank volume ÷ flow rate; typical aeration basin detention is 4–8 hours.

Nitrification consumes about 4.57 mg O2 per mg N oxidized; denitrification recovers roughly 2.86 mg O2 per mg N.

Effluent limits are commonly 30 mg/L BOD and 30 mg/L TSS for secondary treatment, with 85% removal of both.

Sludge volume index SVI = settled sludge volume (mL/L after 30 min) × 1,000 ÷ MLSS (mg/L); SVI of 80–150 indicates good settling.

Manning's equation: V = (1.486/n) × R^(2/3) × S^(1/2) in US customary units.

Self-cleansing velocity in gravity sewers is about 2 ft/s at design flow; below that, solids deposit.

Maximum design velocity is typically 10 ft/s to limit scour and hydrogen sulfide release.

Minimum sewer slope for an 8-inch line is commonly 0.4% (about 0.0033 ft/ft); smaller pipes need steeper grades.

Infiltration enters through defects in pipe and joints; inflow enters through direct connections such as roof drains and sumps.

Surcharging occurs when flow exceeds pipe capacity; the hydraulic grade line rises above the crown.

Hydrogen sulfide in sewers causes odor and corrosion; it is controlled by ventilation, chemical addition, and preventing septic conditions.

Confined-space entry requires atmospheric testing for oxygen (19.5–23.5%), flammables, and H2S before and during entry.

Lift station wet-well level controls pump cycling; excessive cycles per hour shorten pump and motor life.

Disinfection CT = disinfectant residual concentration (mg/L) × contact time (min); required CT depends on pH, temperature, and disinfectant type.

Chlorine demand = applied dose − residual after contact period; free chlorine residual = total chlorine − combined chlorine.

Conventional treatment sequence: coagulation → flocculation → sedimentation → filtration → disinfection.

Filter loading rate = flow (gpm) ÷ filter area (ft²); rapid sand filters typically run 2–6 gpm/ft² at design rate.

Upflow through a filter bed loosens media, so backwash flow rates are set by percent bed expansion (typically 15–25%) and water temperature.

Jar test results set coagulant dose and pH; optimum pH and dose are chosen where settled turbidity is lowest.

Turbidity is reported in NTU; combined filter effluent limits are commonly 0.3 NTU in at least 95% of monthly samples and never above 1 NTU.

Log removal credit: 1-log = 90%, 2-log = 99%, 3-log = 99.9%, 4-log = 99.99% reduction of pathogens.

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.

Pressure = 0.433 psi per foot of water column; 1 psi = 2.31 ft of head.

Flow in a full pipe: Q = A × V; for a circular pipe, A = 0.7854 × D².

Hazen-Williams head loss hf = 10.44 × L × Q^1.85 ÷ (C^1.85 × d^4.87) in US customary units.

Minimum residual disinfectant in the distribution system is typically 0.2 mg/L for free chlorine; detectable residual is required at all points.

Minimum normal operating pressure is generally 20 psi; below that, a boil-water notice and investigation are triggered.

Water main disinfection after repair commonly uses 25–50 mg/L free chlorine for 24 hours or an equivalent CT.

Distribution storage must maintain turnover; long detention time raises water age, which depletes residual and increases DBPs.

Cross-connection control relies on backflow prevention by air gap (highest protection) or reduced-pressure-zone assembly.

Leak detection: acoustic listening and correlators locate leaks; minimum night flow indicates the size of system losses.

BOD5 measures biodegradable organic matter consumed by microbes over 5 days at 20°C; CBOD excludes nitrogenous demand.

Secondary clarifier solids loading rate = (Q + Qr) × MLSS ÷ clarifier area.

Sludge age (SRT) = mass of solids in aeration basin ÷ mass of solids wasted per day.

F/M ratio = pounds of BOD applied per day ÷ pounds of MLSS in the aeration tank.

Return activated sludge rate Qr = Q × MLSS ÷ (RAS concentration − MLSS).

Hydraulic detention time = tank volume ÷ flow rate; typical aeration basin detention is 4–8 hours.

Nitrification consumes about 4.57 mg O2 per mg N oxidized; denitrification recovers roughly 2.86 mg O2 per mg N.

Effluent limits are commonly 30 mg/L BOD and 30 mg/L TSS for secondary treatment, with 85% removal of both.

Sludge volume index SVI = settled sludge volume (mL/L after 30 min) × 1,000 ÷ MLSS (mg/L); SVI of 80–150 indicates good settling.

Manning's equation: V = (1.486/n) × R^(2/3) × S^(1/2) in US customary units.

Self-cleansing velocity in gravity sewers is about 2 ft/s at design flow; below that, solids deposit.

Maximum design velocity is typically 10 ft/s to limit scour and hydrogen sulfide release.

Minimum sewer slope for an 8-inch line is commonly 0.4% (about 0.0033 ft/ft); smaller pipes need steeper grades.

Infiltration enters through defects in pipe and joints; inflow enters through direct connections such as roof drains and sumps.

Surcharging occurs when flow exceeds pipe capacity; the hydraulic grade line rises above the crown.

Hydrogen sulfide in sewers causes odor and corrosion; it is controlled by ventilation, chemical addition, and preventing septic conditions.

Confined-space entry requires atmospheric testing for oxygen (19.5–23.5%), flammables, and H2S before and during entry.

Lift station wet-well level controls pump cycling; excessive cycles per hour shorten pump and motor life.