Drinking Water and Wastewater Treatment
Key Takeaways
- NCEES assigns 6-9 questions to drinking water distribution and treatment and 7-11 questions to wastewater collection and treatment.
- Conventional drinking-water treatment follows particle destabilization, floc growth, solids removal, filtration, and disinfection.
- Wastewater treatment separates physical removal, biological conversion, nutrient control, solids handling, and final disinfection.
- Common calculation forms include lb/day = mg/L x mgd x 8.34, detention time, surface loading, filter loading, F/M ratio, and solids retention time.
- The WRE exam supplies the listed Ten States Standards electronically, but many treatment items are solved by process purpose and unit consistency.
Treatment Is Process Selection Plus Loading
The WRE blueprint gives drinking water 6-9 questions and wastewater 7-11 questions, so treatment cannot be treated as a minor topic. NCEES also lists the Ten States Standards for Water Works and Wastewater Facilities as supplied electronic standards for the current WRE exam. You do not need to memorize every table in those standards, but you should know when a problem is asking for process logic, hydraulic loading, solids loading, chemical dose, or operational interpretation.
Drinking-Water Treatment Train
A conventional surface-water plant usually follows this sequence:
- Rapid mix applies coagulant and disperses it quickly.
- Coagulation destabilizes fine particles and colloids.
- Flocculation gently grows larger particles that can settle.
- Sedimentation removes much of the suspended floc by gravity.
- Filtration removes remaining particles and helps protect disinfection.
- Disinfection provides pathogen inactivation and a distribution-system residual when required.
Demand terms matter before treatment sizing. Average day describes typical use, maximum day stresses supply and storage, and peak hour stresses pipes, pumps, and distribution pressure. Storage is not a treatment substitute; it balances demand, supports fire or emergency reserve, and helps operations.
| Goal | Likely process | Exam clue |
|---|---|---|
| Reduce turbidity | Coagulation, flocculation, sedimentation, filtration | Colloids, particles, raw surface water |
| Inactivate pathogens | Chlorine, ozone, ultraviolet, or other disinfectant | Contact time, residual, CT, microbial target |
| Lower hardness | Lime softening, ion exchange, membranes, blending | Calcium, magnesium, scaling |
| Remove dissolved organics or taste compounds | Activated carbon, oxidation, membranes | Trace organics, odor, specific constituent |
| Manage DBP risk | Precursor removal or disinfectant strategy | Natural organic matter, THM, HAA concern |
Wastewater Treatment Train
Wastewater starts with collection, and collection is often where the PE trap sits. Infiltration is groundwater entering through defects; inflow is direct stormwater entry through roof drains, cleanouts, cross-connections, or openings. Both can overload lift stations and treatment units, but their timing differs.
Treatment then progresses from physical to biological to polishing steps. Preliminary treatment removes rags and grit. Primary treatment settles heavier solids and skims floatables. Secondary treatment removes dissolved and colloidal biodegradable organics through biological activity. Nutrient removal depends on redox conditions: nitrification converts ammonia to nitrate under aerobic conditions, while denitrification converts nitrate to nitrogen gas in anoxic conditions with available carbon.
Calculations To Keep Ready
The most common treatment calculations are simple but unforgiving:
- Mass loading: lb/day = concentration in mg/L x flow in mgd x 8.34.
- Detention time: tank volume divided by flow, with units converted before division.
- Surface overflow rate: flow divided by clarifier or basin surface area.
- Filter loading: gpm divided by filter area in square feet.
- F/M ratio: organic food loading divided by microorganism mass.
- Solids retention time: solids inventory divided by solids leaving per day.
Exam Strategy
Read every treatment stem twice: once for the treatment goal and 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. If a numeric item gives mg/L and mgd, expect the 8.34 factor unless the units are already converted. When a process-control term appears, identify whether it describes water flow, solids flow, or biology. That separation prevents most wrong answers.
A plant treating 5.0 mgd applies a disinfectant dose of 3.2 mg/L. Approximately how many pounds of disinfectant are applied per day?
A wastewater process has an aerobic zone followed by an anoxic zone with a carbon source. Which nitrogen pathway does that arrangement most directly support?