Water quality and treatment Knowledge-Based Practice

Questions

1. 1.

   In municipal water treatment, what is the primary chemical function of adding sodium carbonate ($\text{Na}_2\text{CO}_3$, or soda ash) during the lime-soda softening process?

   • A) To oxidize dissolved ferrous ions into insoluble ferric hydroxide precipitates
   • B) To provide carbonate ions that precipitate non-carbonate calcium hardness as insoluble calcium carbonate
   • C) To neutralize excess hydroxide ions and lower water pH prior to distribution
   • D) To destabilize negatively charged colloidal clay particles through charge neutralization
2. 2.

   In municipal drinking water treatment, granular activated carbon (GAC) filtration is primarily utilized to remove which category of contaminants?

   • A) Pathogenic bacteria and enteric viruses via physical size exclusion
   • B) Dissolved heavy metal cations through hydroxide precipitation
   • C) Multivalent inorganic ions that cause hardness, such as calcium and magnesium
   • D) Dissolved organic compounds responsible for objectionable tastes, odors, and synthetic micropollutants
3. 3.

   In municipal wastewater treatment, what is the primary chemical and biological objective of the secondary treatment stage?

   • A) To remove coarse suspended debris and dense grit through physical screening and gravity sedimentation.
   • B) To metabolize dissolved and colloidal biodegradable organic matter using microorganisms, thereby significantly lowering biochemical oxygen demand (BOD).
   • C) To eliminate pathogenic viruses and bacteria through terminal chemical oxidation using chlorine dioxide or ultraviolet radiation.
   • D) To precipitate dissolved heavy metal cations and orthophosphate ions through the addition of slaked lime and ferric chloride.
4. 4.

   Which statement best explains why chloramines are frequently utilized rather than free chlorine for secondary disinfection in municipal drinking water distribution networks?

   • A) They form a more persistent residual in distribution pipelines and generate fewer regulated trihalomethanes.
   • B) They exhibit substantially higher oxidation potential and inactivate viral pathogens much more rapidly.
   • C) They coagulate and settle dissolved natural organic matter before water enters the storage reservoirs.
   • D) They undergo precipitation reactions that simultaneously remove divalent heavy metal ions from the water.
5. 5.

   What is the primary chemical role of aluminum sulfate, $\text{Al}_2(\text{SO}_4)_3$, during the coagulation stage of drinking water treatment?

   • A) To provide trivalent cations that neutralize the negative surface charges of suspended colloidal particles, facilitating their aggregation into flocs
   • B) To drastically lower the pH of the water in order to sterilize bacterial pathogens prior to distribution
   • C) To react directly with dissolved calcium and magnesium ions to precipitate them as soluble sulfate complexes
   • D) To act as a strong oxidizing agent that converts non-biodegradable synthetic organic pollutants into carbon dioxide

Answers and step-by-step solutions appear after you finish the test.