Sanitary Engineer Licensure Exam cheat sheet

Design period rule of thumb: water treatment plants and transmission mains 10-20 years; distribution networks 20-30 years; pumping equipment 5-10 years; population projections use geometric or arithmetic growth, selecting the higher forecast for conservative sizing.

Peak factors: maximum day demand is typically 1.5-2.0 times average day; peak hour is 2.5-3.0 times average day in residential areas; fire flow is added to maximum day (not peak hour) for distribution main sizing.

Minimum residual pressure in distribution systems is commonly 10-15 m (approx. 100-150 kPa) at peak hour, with a minimum of about 7 m at fire flow conditions; velocities should stay between 0.6 m/s and 1.5-2.0 m/s to prevent sedimentation and excessive head loss.

Hazen-Williams head loss: hf = 10.67 L Q^1.852 / (C^1.852 D^4.87) in SI units (Q in m3/s, D in m); C values near 130-140 for new cement-lined ductile iron or PVC, 100-120 for older unlined steel or cast iron.

Manning equation for gravity sewers: V = (1/n) R^(2/3) S^(1/2); n = 0.013 for concrete and vitrified clay, 0.011-0.012 for PVC and HDPE; design at 0.6-0.75 full depth to maintain freeboard and ventilation.

Self-cleansing velocity in sanitary sewers is about 0.6 m/s at design flow (0.75 m/s where solids are heavy); minimum sewer diameter is 150 mm for laterals and 200 mm for public mains, with minimum slope of 1/100 to 1/200 for small pipes.

Biochemical oxygen demand is measured at 20 degrees C over 5 days (BOD5); first-order decay BODt = BODu (1 - e^(-kt)), with k about 0.17 per day at 20 degrees C for domestic sewage, adjusted by k_T = k_20 (1.047)^(T-20).

Standard-strength domestic wastewater: BOD5 about 200-220 mg/L, suspended solids 200-240 mg/L, total nitrogen 35-45 mg/L, total phosphorus 6-10 mg/L, and per-capita flow of 150-250 L/day (higher in areas with high water use).

Sewer appurtenances and rules: manholes at every change of direction, grade, or diameter and at intervals of 90-150 m; drop manholes where the invert drop exceeds about 0.6 m; sewer gradients follow ground slope where possible but must satisfy self-cleansing velocity.

Screening and grit removal: coarse bar screens with 25-50 mm clear spacing; grit chambers designed for a horizontal velocity near 0.3 m/s so grit (specific gravity about 2.65, 0.15-0.2 mm) settles while organics stay in suspension; detention time 45-90 s at peak flow.

Primary sedimentation: surface overflow rate 30-50 m3/m2·d at average flow and up to 80-120 m3/m2·d at peak flow; detention time 1.5-2.5 h; weir loading under about 250 m3/m·d; typical removal of 50-65% suspended solids and 25-40% BOD5.

Activated sludge is sized by solids retention time and food-to-microorganism ratio: conventional F/M 0.2-0.4 per day, extended aeration F/M 0.05-0.15 per day; aeration basin MLSS 2,000-4,000 mg/L; SRT 5-15 days conventional, 20-30 days extended aeration or nutrient removal.

Secondary clarifier design uses solids loading rate (about 100-150 kg/m2·d at peak for activated sludge) and surface overflow rate (16-32 m3/m2·d); hydraulic retention time 2-4 h; return sludge ratio 0.5-1.5 of influent flow.

Oxygen demand: about 1.0-1.4 kg O2 per kg BOD5 removed in conventional activated sludge, plus endogenous respiration; diffused aeration oxygen transfer efficiency 15-25% in clean water at 3-5 m submergence; aeration power roughly 1-2 kW per 1,000 m3 of tank volume.

Trickling filters: hydraulic loading 1-3 m3/m2·d for low-rate and 10-40 m3/m2·d for high-rate; organic loading 0.08-0.4 kg BOD/m3·d low-rate up to 0.4-1.0 kg BOD/m3·d high-rate; media depth 1.5-2.5 m low-rate; recirculation improves nitrification and wetting.

Sludge handling: waste activated sludge at 0.5-1.0% solids, primary sludge 4-6%; gravity thickener design solids loading 25-35 kg/m2·d achieving 4-6% solids; anaerobic digestion volatile solids loading 0.5-1.5 kg VS/m3·d, mesophilic 30-38 degrees C, SRT 15-30 days, 40-60% VS destruction.

Disinfection: chlorine CT concept (C in mg/L times t in minutes) with CT requirements increasing as temperature falls and pH rises; typical fecal coliform target 200-1,000 MPN/100 mL for discharge; UV dose 30-40 mJ/cm2 for secondary effluent; dechlorination where residual chlorine limits apply.

Nutrient removal: nitrification needs SRT above about 10 days at 20 degrees C and 20 days at 10 degrees C, dissolved oxygen above 2 mg/L, alkalinity about 7.1 mg as CaCO3 per mg ammonia-N oxidized; biological phosphorus removal requires anaerobic zone ahead of aerobic zone with VFA uptake.

Drinking-water quality limits commonly applied: turbidity below 1 NTU (ideally below 0.5 NTU), residual free chlorine 0.2-1.0 mg/L at the tap, pH 6.5-8.5, nitrate as N below 10 mg/L, fluoride 0.5-1.5 mg/L, and coliform absent in 100 mL.

Rapid sand filtration: filtration rate 4-12 m/h (about 100-300 L/m2·min), media depth 0.6-0.9 m of sand with effective size 0.45-0.55 mm and uniformity coefficient below 1.5; backwash rate 12-15 L/m2·s for 5-10 min with about 20-25% bed expansion.

Coagulation and flocculation: alum dose 10-50 mg/L with optimum pH 5.5-7.5; rapid mix velocity gradient G about 300-1,000 per second for 10-60 s; flocculation G 10-75 per second for 15-30 min, tapering G downward; Gt product 10^4-10^5.

Sedimentation in water treatment: surface overflow rate 20-40 m3/m2·d for alum flocs and 40-60 m3/m2·d for lime softening; detention 2-4 h; weir loading under 200-300 m3/m·d; the critical settling velocity equals the overflow rate for discrete particles.

Water demand and storage: average per-capita domestic demand 150-250 L/day; total demand includes commercial, industrial, public and non-revenue water (aim below 20%); service reservoir capacity 25-35% of maximum day demand, or 15-20% of maximum day plus fire storage.

Plumbing code essentials: minimum fixture flow 0.10-0.15 L/s for water closets and 0.20 L/s for lavatories; trap seal 50-100 mm; vent stack sizing based on fixture units; no trap may serve more than one fixture except approved combinations; backflow prevention by air gap or reduced-pressure zone assembly.

Cross-connection control and backflow: an air gap is the highest protection; reduced pressure zone backflow preventers protect against both backpressure and backsiphonage; double check valves protect only against backpressure; hose bibbs need vacuum breakers.

Septic tank and onsite systems: tank volume about 1,000-2,000 L for a household, retention 24-48 h, sludge and scum storage 1-3 years; soil absorption field loading 10-20 L/m2·d for sandy soils and 5-10 L/m2·d for clayey soils; setback from wells typically 15-30 m.

Solid waste and environmental health: per-capita municipal solid waste generation commonly 0.5-1.5 kg/day; sanitary landfill design includes a composite liner, leachate collection, gas venting, daily cover, and closure capping; composting requires carbon-to-nitrogen ratio about 25-30 and moisture 50-60%.