WTO Water Quality & Treatment Principles 2 — Questions and Answers
Question 1: What is the maximum contaminant level (MCL) for total trihalomethanes (TTHMs) in drinking water under the EPA Stage 2 D/DBP Rule?
- 0.040 mg/L
- 0.080 mg/L (Correct answer)
- 0.100 mg/L
- 0.060 mg/L
Correct answer: 0.080 mg/L
The EPA Stage 2 Disinfectants/Disinfection Byproducts Rule sets the MCL for TTHMs at 0.080 mg/L (80 µg/L).
Question 2: Which physical property of water is used to measure the amount of suspended particles using light scattering?
- Color
- Turbidity (Correct answer)
- Taste
- Conductivity
Correct answer: Turbidity
Turbidity is the measure of water clarity based on how much light is scattered by suspended particles, expressed in nephelometric turbidity units (NTU).
Question 3: In water treatment, what process uses gravity to separate floc from water after coagulation?
- Flocculation
- Sedimentation (Correct answer)
- Filtration
- Aeration
Correct answer: Sedimentation
Sedimentation (also called clarification or settling) allows gravity to pull floc particles to the bottom of a basin after coagulation and flocculation.
Question 4: What unit is used to express chlorine residual in a distribution system?
- Nephelometric Turbidity Units (NTU)
- Colony-Forming Units (CFU)
- Milligrams per Liter (mg/L) (Correct answer)
- Moles per Liter (mol/L)
Correct answer: Milligrams per Liter (mg/L)
Chlorine residual is expressed in milligrams per liter (mg/L), which is equivalent to parts per million (ppm) in water treatment.
Question 5: Which of the following is a primary source of nitrate contamination in drinking water supplies?
- Industrial solvents
- Agricultural fertilizers and animal waste (Correct answer)
- Pipe corrosion byproducts
- Disinfection of surface water
Correct answer: Agricultural fertilizers and animal waste
Agricultural fertilizers and animal waste are the primary sources of nitrate contamination, as nitrogen compounds leach from fields into groundwater and surface water.
Question 6: What is the primary purpose of adding alum (aluminum sulfate) during water treatment?
- To raise the pH of acidic water
- To remove iron and manganese
- To destabilize and coagulate suspended particles (Correct answer)
- To disinfect the water
Correct answer: To destabilize and coagulate suspended particles
Alum is a coagulant that destabilizes the electrical charges on colloidal particles, allowing them to clump together into floc for removal.
Question 7: The term 'CT value' in disinfection refers to which combination of measurements?
- Color (units) × Temperature (°C)
- Chlorine dose (mg/L) × Contact time (minutes) (Correct answer)
- Concentration (mg/L) × Temperature (°F)
- Conductivity × Turbidity
Correct answer: Chlorine dose (mg/L) × Contact time (minutes)
CT value is the product of disinfectant residual concentration (C, in mg/L) multiplied by contact time (T, in minutes), used to measure disinfection effectiveness.
WTO Water Quality & Treatment Principles 2
What is the maximum contaminant level (MCL) for total trihalomethanes (TTHMs) in drinking water under the EPA Stage 2 D/DBP Rule?