Wastewater Operator Certification Disinfection Processes 1 — Questions and Answers
Question 1: What is the primary purpose of disinfection in wastewater treatment?
- Remove suspended solids from effluent
- Inactivate pathogenic microorganisms (Correct answer)
- Reduce biochemical oxygen demand
- Remove nitrogen and phosphorus
Correct answer: Inactivate pathogenic microorganisms
Disinfection targets the inactivation or destruction of pathogenic microorganisms to protect public health and receiving water bodies.
Question 2: Which form of chlorine is the most effective disinfectant in wastewater treatment?
- Combined chlorine (chloramines)
- Hypochlorous acid (HOCl) (Correct answer)
- Hypochlorite ion (OCl⁻)
- Chloride ions (Cl⁻)
Correct answer: Hypochlorous acid (HOCl)
Hypochlorous acid (HOCl) is the most germicidal form of chlorine because it is an uncharged molecule that penetrates cell membranes more easily than the hypochlorite ion.
Question 3: At what pH does hypochlorous acid (HOCl) predominate over the hypochlorite ion (OCl⁻)?
- Above pH 8.5
- Above pH 9.0
- Below pH 7.5 (Correct answer)
- Between pH 8.0 and 9.0
Correct answer: Below pH 7.5
Below pH 7.5, the equilibrium strongly favors HOCl, making lower pH conditions more favorable for chlorine disinfection effectiveness.
Question 4: The CT value used in disinfection calculations represents:
- Chemical temperature of the disinfectant solution
- Contact time alone in minutes
- Disinfectant concentration (mg/L) multiplied by contact time (minutes) (Correct answer)
- Chlorine total residual at the outlet
Correct answer: Disinfectant concentration (mg/L) multiplied by contact time (minutes)
CT is the product of residual disinfectant concentration (C) and the hydraulic contact time (T), used to predict pathogen inactivation efficacy.
Question 5: What occurs at the breakpoint in breakpoint chlorination?
- Chlorine begins to precipitate out of solution
- Enough chlorine has been added to destroy all chloramines, producing a free chlorine residual (Correct answer)
- The pH of the wastewater reaches neutral
- Chlorine demand equals zero
Correct answer: Enough chlorine has been added to destroy all chloramines, producing a free chlorine residual
At breakpoint chlorination, all ammonia-nitrogen has been oxidized and combined chlorine (chloramines) have been destroyed; any additional chlorine added appears as free chlorine residual.
Question 6: Chlorine demand is defined as:
- The total chlorine concentration added to the influent
- The free chlorine residual measured at the effluent
- The difference between the chlorine applied and the free chlorine residual remaining (Correct answer)
- The amount of chlorine consumed by ammonia only
Correct answer: The difference between the chlorine applied and the free chlorine residual remaining
Chlorine demand is the quantity of chlorine consumed by reactions with organics, ammonia, and other reducing substances, calculated as applied chlorine minus residual chlorine.
Question 7: Which disinfectant is most widely used in US municipal wastewater treatment plants?
- Ozone (O₃)
- Ultraviolet (UV) light
- Chlorine gas or sodium hypochlorite (Correct answer)
- Peracetic acid
Correct answer: Chlorine gas or sodium hypochlorite
Chlorine (as gas or sodium hypochlorite) is the most widely used disinfectant in US wastewater plants due to its effectiveness, low cost, and ability to provide a measurable residual.
Wastewater Operator Certification Disinfection Processes 1
What is the primary purpose of disinfection in wastewater treatment?