Water and Wastewater Treatment for the FE Exam
Water and wastewater treatment questions appear on the FE Civil and FE Environmental exams. These topics involve applying chemistry, biology, and hydraulics to protect public health and the environment. Understanding the treatment processes and their design parameters is key to scoring well.
Drinking Water Treatment
The typical drinking water treatment process follows these steps:
1. Coagulation and Flocculation
Raw water from rivers, lakes, or reservoirs contains suspended particles (turbidity) that must be removed. Coagulation adds chemicals (coagulants) that neutralize the negative charges on particles, allowing them to stick together:
- Common coagulants: Aluminum sulfate (alum), ferric chloride, ferric sulfate
- Rapid mixing distributes the coagulant throughout the water (1 to 2 minutes, high energy)
- Flocculation provides gentle mixing to promote particle aggregation into larger clumps called floc (20 to 30 minutes, low energy)
2. Sedimentation
Floc settles out in a sedimentation basin (clarifier). Design is based on the overflow rate (surface loading rate):
Overflow rate = Q / A (flow rate / surface area, units of m/day or gpd/ft^2)
Particles with settling velocities greater than the overflow rate will be removed. Typical overflow rates for water treatment: 20 to 40 m/day (500 to 1,000 gpd/ft^2).
Detention time: t = V / Q (basin volume / flow rate). Typical detention times: 2 to 4 hours.
3. Filtration
Remaining fine particles pass through granular media filters (sand, anthracite, or dual media). Filtration removes particles that did not settle in sedimentation and is the final barrier before disinfection.
Loading rate = Q / A (flow rate / filter area). Typical rates: 120 to 360 m/day (2 to 6 gpm/ft^2).
4. Disinfection
Disinfection kills or inactivates pathogenic microorganisms. Common methods:
- Chlorination: The most common method. Free chlorine (hypochlorous acid) is the primary disinfectant. The CT concept (concentration times contact time) determines disinfection effectiveness.
- Chloramination: Chlorine combined with ammonia forms chloramines, which provide a longer-lasting residual. Used for distribution system protection.
- Ozonation: Ozone is a powerful oxidant and disinfectant. Does not provide a residual in the distribution system.
- UV disinfection: Ultraviolet light damages the DNA of microorganisms. Effective against chlorine-resistant pathogens like Cryptosporidium.
CT = Disinfectant concentration (mg/L) * Contact time (minutes). Required CT values depend on the pathogen, temperature, pH, and disinfectant type.
Wastewater Treatment
Biochemical Oxygen Demand (BOD)
BOD is a measure of the amount of oxygen consumed by microorganisms to decompose organic matter in water. It is the primary measure of the strength of wastewater.
BOD kinetics follow first-order decay:
BOD_t = BOD_ultimate * (1 - e^(-k*t))
Where BOD_t is the BOD exerted at time t, BOD_ultimate is the total (ultimate) BOD, and k is the BOD rate constant.
Standard BOD test: BOD_5 (5-day BOD at 20 degrees C). Typical domestic wastewater BOD_5: 200 to 300 mg/L.
Primary Treatment
Primary treatment uses physical processes to remove settleable solids:
- Screening: Removes large debris (rags, sticks, plastics)
- Grit removal: Settles out sand and heavy inorganic particles
- Primary sedimentation: Removes settleable organic solids (primary sludge). Removes approximately 50 to 65% of suspended solids and 25 to 40% of BOD.
Secondary Treatment (Biological Treatment)
Secondary treatment uses microorganisms to decompose dissolved and colloidal organic matter. The most common processes:
Activated Sludge
The most widely used secondary treatment process:
- Aeration tank: Microorganisms (activated sludge) mix with wastewater. Air or oxygen is supplied to support aerobic decomposition.
- Secondary clarifier: Settled sludge is separated from treated effluent. A portion of the sludge is returned to the aeration tank (return activated sludge, RAS) to maintain the microbial population. Excess sludge is wasted (waste activated sludge, WAS).
Design parameters:
- Food-to-microorganism ratio (F/M): F/M = Q * S_0 / (V * X), where Q is flow rate, S_0 is influent BOD, V is aeration tank volume, and X is mixed liquor suspended solids (MLSS). Typical F/M: 0.2 to 0.5 day^(-1).
- Solids retention time (SRT or theta_c): The average time solids spend in the system. SRT = V*X / (Q_w * X_w), where Q_w is the waste sludge flow rate and X_w is the waste sludge concentration. Typical SRT: 5 to 15 days.
- Hydraulic retention time (HRT): HRT = V / Q. Typical HRT: 4 to 8 hours.
Trickling Filters
An attached-growth process where wastewater flows over a bed of media (rocks or plastic) coated with a biofilm of microorganisms. Simpler than activated sludge but generally produces lower-quality effluent.
Tertiary (Advanced) Treatment
Additional treatment beyond secondary to remove nutrients, remaining solids, or specific pollutants:
- Nitrogen removal: Nitrification (ammonia to nitrate) followed by denitrification (nitrate to nitrogen gas)
- Phosphorus removal: Chemical precipitation with aluminum or iron salts, or biological phosphorus removal
- Filtration: Removes remaining suspended solids
- Disinfection: Chlorination or UV to meet discharge requirements
Dissolved Oxygen and Stream Quality
When wastewater is discharged to a stream, it causes a dissolved oxygen (DO) sag. The Streeter-Phelps equation models the DO deficit downstream:
D = (k_d * L_0 / (k_r - k_d)) * (e^(-k_d*t) - e^(-k_r*t)) + D_0 * e^(-k_r*t)
Where D is the DO deficit, k_d is the deoxygenation rate constant, k_r is the reaeration rate constant, L_0 is the initial BOD, and D_0 is the initial DO deficit.
The critical point (maximum deficit) occurs at time:
t_c = [1/(k_r - k_d)] * ln[(k_r/k_d) * (1 - D_0*(k_r - k_d)/(k_d * L_0))]
Practice Tips
- Master BOD first-order kinetics and be able to calculate BOD at any time
- Know the design parameters for activated sludge (F/M, SRT, HRT)
- Understand the CT concept for disinfection
- Practice overflow rate and detention time calculations
- Know the Streeter-Phelps equation for DO sag analysis
Water and wastewater treatment questions follow predictable patterns. Know the treatment processes in order, understand the design parameters, and practice the key calculations.