Disinfection Processes Flashcards
7 cards from real Wastewater Operator Certification practice questions. Tap to flip, then mark Knew It or Still Learning — missed cards come back until you master them.
Read the first 7 Disinfection Processes flashcards as text
Why is dechlorination performed on wastewater effluent before it is discharged to receiving waters?
Answer: To remove toxic chlorine residual that is harmful to aquatic organisms
Chlorine residual is acutely toxic to fish and aquatic invertebrates at very low concentrations, so NPDES permits typically require dechlorination to protect receiving water ecosystems.
Which chemical is most commonly used for effluent dechlorination at wastewater treatment plants?
Answer: Sulfur dioxide (SO₂) or sodium bisulfite (NaHSO₃)
Sulfur dioxide gas and sodium bisulfite solution are the most widely used dechlorination agents; they rapidly reduce free and combined chlorine residuals through a chemical reduction reaction.
Trihalomethanes (THMs) are disinfection byproducts formed when:
Answer: Chlorine reacts with natural organic matter in the water
THMs (such as chloroform) form when free chlorine reacts with naturally occurring organic precursors, primarily humic and fulvic acids, during disinfection.
A typical NPDES effluent permit for a wastewater treatment plant will set limits on:
Answer: BOD, TSS, pH, effluent chlorine residual, and other parameters specific to the receiving water
NPDES permits are tailored to protect the receiving water body and typically include limits for BOD5, TSS, pH, ammonia, fecal coliform or E. coli, chlorine residual, and may include nutrient limits.
What is the typical chlorine contact time required for adequate disinfection of secondary wastewater effluent in the United States?
Answer: 15 to 30 minutes
Most state regulations and design standards require a minimum chlorine contact time of 15 to 30 minutes for secondary effluent disinfection to achieve required pathogen reductions.
Which statement correctly describes chloramines as used in wastewater disinfection?
Answer: Chloramines form when chlorine reacts with ammonia-nitrogen in the wastewater
Monochloramine, dichloramine, and nitrogen trichloride form through reactions between free chlorine and ammonia-nitrogen, collectively referred to as combined available chlorine.
When calculating the theoretical dose of sulfur dioxide required to dechlorinate an effluent, the approximate stoichiometric ratio of SO₂ to Cl₂ by weight is:
Answer: 0.9 mg SO₂ per mg Cl₂
The stoichiometric reaction of SO₂ with Cl₂ requires approximately 0.9 mg of SO₂ per mg of Cl₂ (molecular weight ratio), though a slight excess is applied in practice to ensure complete dechlorination.