Wastewater Operator Certification Disinfection Processes 3

Question 1

Why is dechlorination performed on wastewater effluent before it is discharged to receiving waters?

Accepted Answer

To remove toxic chlorine residual that is harmful to aquatic organisms

Answer

To increase dissolved oxygen in the effluent

Answer

To lower the pH to acceptable levels

Answer

To reduce suspended solids prior to discharge

Question 2

Which chemical is most commonly used for effluent dechlorination at wastewater treatment plants?

Accepted Answer

Sulfur dioxide (SO₂) or sodium bisulfite (NaHSO₃)

Answer

Sodium hypochlorite (NaOCl)

Answer

Alum (aluminum sulfate)

Answer

Ferric chloride (FeCl₃)

Question 3

Trihalomethanes (THMs) are disinfection byproducts formed when:

Accepted Answer

Chlorine reacts with natural organic matter in the water

Answer

UV light contacts dissolved organic compounds

Answer

Ozone reacts with ammonia in the effluent

Answer

Sulfur dioxide reacts with residual chlorine

Question 4

A typical NPDES effluent permit for a wastewater treatment plant will set limits on:

Accepted Answer

BOD, TSS, pH, effluent chlorine residual, and other parameters specific to the receiving water

Answer

Only total suspended solids (TSS) and pH

Answer

Only nitrogen and phosphorus concentrations

Answer

Influent flow rate and influent BOD only

Question 5

What is the typical chlorine contact time required for adequate disinfection of secondary wastewater effluent in the United States?

Accepted Answer

15 to 30 minutes

Answer

1 to 5 minutes

Answer

60 to 90 minutes

Answer

2 to 4 hours

Question 6

Which statement correctly describes chloramines as used in wastewater disinfection?

Accepted Answer

Chloramines form when chlorine reacts with ammonia-nitrogen in the wastewater

Answer

Chloramines are stronger disinfectants than free chlorine on a mg/mg basis

Answer

Chloramines produce more trihalomethanes than free chlorination

Answer

Chloramines cannot be measured with a standard colorimetric DPD test

Question 7

When calculating the theoretical dose of sulfur dioxide required to dechlorinate an effluent, the approximate stoichiometric ratio of SO₂ to Cl₂ by weight is:

Accepted Answer

0.9 mg SO₂ per mg Cl₂

Answer

0.5 mg SO₂ per mg Cl₂

Answer

1.0 mg SO₂ per mg Cl₂

Answer

2.0 mg SO₂ per mg Cl₂