Drinking Water and Wastewater Treatment

Treatment Is Process Selection Plus Loading

The WRE blueprint gives drinking water 6-9 questions and wastewater 7-11 questions, so treatment is roughly one-fifth of the 80-question exam. NCEES lists Recommended Standards for Water Works and the companion wastewater standards — the Ten States Standards — among the supplied electronic references. You do not memorize their tables; you recognize whether a problem wants process logic, hydraulic loading, solids loading, chemical dose, or operational interpretation.

Drinking-Water Treatment Train

A conventional surface-water plant follows this sequence:

1. Rapid mix disperses coagulant in seconds.
2. Coagulation destabilizes fine particles and colloids (commonly alum or ferric salts; alum consumes alkalinity).
3. Flocculation gently grows settleable floc (tapered mixing, several minutes to ~30 min).
4. Sedimentation removes most floc by gravity; design by surface overflow rate, not just detention time.
5. Filtration polishes remaining particles (rapid sand loading ≈ 2-3 gpm/ft²).
6. Disinfection inactivates pathogens and provides a distribution residual.

Disinfection is governed by CT = C × T (residual mg/L × contact time min). Required CT rises for hardier organisms — Giardia and viruses need more CT than vegetative bacteria, and Cryptosporidium resists chlorine almost entirely, pushing plants toward UV or ozone. Demand terms set sizing: average day is typical use, maximum day stresses supply/storage, peak hour stresses pipes, pumps, and pressure. Storage balances demand and fire reserve — it is not a treatment substitute.

Wastewater Treatment Train

Collection is where the PE trap often sits. Infiltration is groundwater entering through pipe defects (slow, seasonal); inflow is direct stormwater through roof drains, cleanouts, and cross-connections (fast, storm-driven). Together (I&I) they overload lift stations and clarifiers.

Treatment then runs physical → biological → polishing. Preliminary removes rags and grit. Primary settles heavier solids and skims floatables (~30-40% BOD, 50-60% TSS removal). Secondary — activated sludge or fixed film — removes dissolved/colloidal biodegradable organics; well-run plants reach ~85%+ BOD/TSS removal, the classic secondary standard near 30 mg/L BOD₅ and 30 mg/L TSS. Nutrient removal is redox-driven: nitrification (NH₃→NO₃) needs aerobic conditions and oxygen, while denitrification (NO₃→N₂) needs anoxic zones with available carbon.

Activated Sludge Process Control

The activated-sludge questions reward a clear separation of water flow from solids flow from biology. The food-to-microorganism (F/M) ratio is the daily BOD load divided by the mass of mixed-liquor volatile suspended solids (MLVSS) in the aeration basin; a conventional plant runs F/M ≈ 0.2-0.5 lb BOD per lb MLVSS per day. The solids retention time (SRT), also called mean cell residence time, is the total solids inventory divided by the solids wasted plus lost in the effluent each day.

SRT is the master control variable: short SRT (2-5 days) favors carbonaceous BOD removal only, while a longer SRT (8-15+ days) is needed to retain the slow-growing nitrifiers that convert ammonia to nitrate.

The sludge volume index (SVI) flags settling problems: a settled volume measured in a 30-minute test, divided by MLSS concentration, gives mL/g. SVI above ~150 mL/g signals bulking sludge that will carry solids over the clarifier weir. Return activated sludge (RAS) recycles settled biomass to maintain MLSS; waste activated sludge (WAS) removes excess growth to hold SRT constant.

Calculations To Keep Ready

• Mass loading: lb/day = mg/L × MGD × 8.34.
• Detention time: t = volume / flow (convert units first).
• Surface overflow rate: SOR = flow / clarifier surface area (gpd/ft²).
• Filter loading: gpm / filter area (ft²).
• Food-to-microorganism (F/M) ratio: influent BOD load / MLVSS mass.
• Solids retention time (SRT): solids inventory / solids leaving per day.
• CT: disinfectant residual (mg/L) × contact time (min).

Worked dose: a plant treats 5.0 MGD and feeds chlorine at 3.2 mg/L, so 3.2 × 5.0 × 8.34 ≈ 133 lb/day — the 8.34 factor converts (mg/L)(MGD) to lb/day. Worked detention: a 0.5-MG clarifier passing 2.0 MGD has t = 0.5/2.0 = 0.25 day ≈ 6 hours, a typical primary value.

Exam Strategy

Read each stem twice — once for the treatment goal, once for the units. If the goal is pathogen control, do not answer with settling; if the goal is settleable solids, do not answer with activated carbon. Given mg/L and MGD, expect the 8.34 factor unless units are pre-converted. When a control term appears, classify it as water flow, solids flow, or biology — that single discipline prevents most wrong answers.

Common traps: confusing detention-time sizing with surface-overflow-rate sizing for clarifiers (settling depends on SOR, not depth or time alone); assuming chlorine handles Cryptosporidium (it does not — use UV or ozone); and running too short an SRT and then wondering why ammonia breaks through.