NC Phase W cheat sheet

Coagulation dose is set by jar test; optimum pH for alum is typically 5.5-6.5, and for ferric chloride roughly 4.5-6.0.

Alkalinity consumed by alum: about 0.45 mg/L as CaCO3 per mg/L of alum; add lime if alkalinity drops below ~50 mg/L to protect floc and prevent corrosion.

Flocculation design: tapered mixing with velocity gradient G of 10-75 s^-1 and detention time of 15-45 min; rapid mix G is 250-1000 s^-1 for under 30 s.

Sedimentation overflow rate: ideal settling velocity v0 = Q/A; particles with settling velocity greater than v0 are removed fully in a rectangular basin of length L, depth H, flow Q.

Filtration: slow sand at 0.1-0.4 m/h; rapid gravity at 5-15 m/h. Backwash with 15-25 m/h rise rate until turbidity clears, then filter-to-waste on restart.

CT disinfection credit: CT = residual disinfectant concentration (mg/L) x contact time (min); required CT depends on disinfectant, pH, temperature, and log inactivation target.

Chlorine demand = applied dose minus residual after contact time; free chlorine residual target at plant effluent is typically 0.5-1.0 mg/L.

Hardness removal by lime-soda softening: lime removes carbonate hardness, soda ash removes non-carbonate; practical limit is about 50-80 mg/L as CaCO3.

Turbidity limits: combined filter effluent typically 0.3 NTU or less; individual filter must not exceed 1 NTU in 95% of monthly readings.

Minimum residential pressure is typically 20 psi (140 kPa) at ground level under peak demand; normal service pressure is 40-80 psi, and over 100 psi requires pressure-reducing valves.

Hazen-Williams head loss: hf = 10.67 x L x Q^1.852 / (C^1.852 x d^4.87) in SI; C is 130-150 for new cement-lined ductile iron, 80-100 for old unlined cast iron.

Water hammer pressure surge: change in head = a x change in velocity / g, where a is wave speed (~1000-1400 m/s in metal pipe); close valves slowly to limit surge.

Fire flow capacity is checked by a hydrant flow test; available flow at 20 psi residual is estimated from static and residual readings with a pitot gauge and coefficient.

Storage sizing: equalization plus fire reserve plus emergency reserve; elevated tanks provide pressure by elevation at about 0.433 psi per foot of head (9.8 kPa per m).

Distribution main minimum size is typically 150 mm (6 in) with 200 mm (8 in) preferred for fire flow; dead-end mains should be avoided or flushed.

Coliform sampling: at least 5 samples per month for small systems, scaled to population; presence of total coliform triggers repeat samples and investigation for E. coli.

Disinfection residual must be maintained throughout distribution; chlorine residual is measured at representative points and must not be undetectable.

Design flow is peak wet-weather flow; peak factor commonly 2-4 times average dry-weather flow, and infiltration/inflow must be estimated separately.

Self-cleansing velocity in sanitary sewers is at least 0.6 m/s (2 ft/s) at design flow; maximum velocity is about 3 m/s (10 ft/s) to avoid scour.

Manning equation for gravity sewers: Q = (1/n) x A x R^(2/3) x S^(1/2); n is 0.013 for vitrified clay and concrete, 0.012-0.013 for PVC.

Minimum sewer slope is set by the self-cleansing velocity, not a fixed percent; typical minimums are 0.4-0.5% for 200 mm pipe with design flow checks.

Sewer depth of cover minimum is usually 0.9-1.2 m for frost protection; manholes at every change of grade, size, or alignment and at spacing of 90-150 m.

Surcharge protection: hydraulic grade line must stay below basement floor level; use backwater valves or relief structures where HGL exceeds the ground.

Hydrogen sulfide control: H2S forms in anaerobic slime above the waterline; ventilate manholes and dose nitrate or iron salts to control odors and corrosion.

Inflow and infiltration: I/I reduction by smoke testing, dye testing, and CCTV inspection; excessive I/I raises treatment cost and can cause sanitary sewer overflows.

BOD5 test: 5-day, 20 C incubation with nitrification inhibitor; seed correction is applied, and depletion should be between 2 mg/L and the residual limit for validity.

Activated sludge F/M ratio: 0.2-0.5 kg BOD/kg MLSS-day for conventional; MCRT/SRT controls sludge age, typically 5-15 days for conventional and 15-30 for nitrification.

Sludge volume index SVI = (settled sludge volume mL/L after 30 min) x 1000 / MLSS mg/L; SVI of 80-150 indicates good settling, above 200 indicates bulking.

Nitrification requires SRT long enough for slow-growing nitrifiers, dissolved oxygen above 2 mg/L, alkalinity about 7.1 mg as CaCO3 per mg NH3-N oxidized, and temperature above 10 C.

Denitrification consumes about 2.86 mg BOD per mg nitrate-N reduced; anoxic zone with carbon source and low DO, recycle ratio of 2-4 times influent.

Secondary clarifier overflow rate for activated sludge is 16-32 m3/m2-day at average flow; solids loading rate is 4-6 kg/m2-h; check both.

Chlorine disinfection of effluent: dose minus demand gives residual; CT approach applies, and dechlorination with sulfur dioxide protects receiving water.

Anaerobic digestion: mesophilic 30-38 C, thermophilic 50-57 C; volatile solids destruction is 40-60%, and gas is about 60-65% methane.

Biosolids land application is limited by pathogen reduction and vector attraction reduction; Class B requires site restrictions, Class A allows broader use.