CEE 4412: Environmental Engineering I - Water Treatment (Disinfection and Softening)

A complete set of standard Question and Answer flashcards covering water treatment operations, chlorination chemistry, lime softening, and ion exchange based on the CEE 4412 lecture transcript.

What is disinfection in the context of water treatment?

A unit operation for the inactivation of microorganisms.

What seven disinfection methods are outlined in the lecture?

Boiling, UV irradiation, Solar Disinfection (SODIS), Ozonation, Chlorine Dioxide, Reverse Osmosis, and Chlorination.

What are six key physical and chemical properties of chlorine gas?

It is a greenish-yellow gas, 2.5 times heavier than air, highly soluble in water (70 times that of air), a strong oxidant, corrosive, and toxic in gaseous form.

What are six advantages of using chlorine as a disinfectant?

Good disinfective properties, harmless to humans and higher animals in bacterial doses in water, provides relatively long-lasting residuals in water, easy to handle if precautions are observed, easy to detect and determine using comparators, and is the cheapest disinfectant available.

What four factors affect the effectiveness of chlorination?

Contact time, concentration, pH, and temperature.

What equation expresses the number of surviving microorganisms as a function of contact time during chlorination?

$$N_t = N_0 e^{-kt}$$

In the equation $N_t = N_0 e^{-kt}$, what do $N_t$, $N_0$, $k$, and $t$ represent?

$N_t$ is the number of survivors after time $t$, $N_0$ is the original number present, $k$ is the reaction constant, and $t$ is the contact time.

How does pH affect the effectiveness and required dosage of chlorination?

The lower the pH, the more effective chlorination is. Required dosage to produce the same treatment effect increases with increasing pH.

What chemical reaction occurs when chlorine gas dissolves in water?

$$Cl_2 + H_2O \rightarrow HOCl + H^+ + Cl^-$$

What chemical equilibrium equation defines the ionization of hypochlorous acid in water?

$$HOCl \rightleftharpoons H^+ + OCl^-$$

What constitutes Free Available Chlorine (FAC)?

Hypochlorous acid ($HOCl$) and hypochlorite ion ($OCl^-$).

How much less effective as a disinfectant is the hypochlorite ion ($OCl^-$) compared to hypochlorous acid ($HOCl$)?

$OCl^-$ is less effective by 80 to 100 times.

How does Free Available Chlorine (FAC) inactivate microorganism growth?

By disturbing cell metabolic reactions through selective inhibition of cell division.

What percentage of Free Available Chlorine is contained in Calcium Hypochlorite (HTH)?

70% Free available Chlorine.

What is the dissociation reaction for Calcium Hypochlorite in water?

$$Ca(OCl)_2 \rightarrow Ca^{2+} + 2OCl^-$$

What common consumer product name is given to Sodium Hypochlorite?

JIK

What harmful substances are formed when chlorine reacts with organic matter in water?

Halogenated hydrocarbons / Trihalomethanes (such as chloroform), which are carcinogens.

What is defined as chlorine demand in water?

The chlorine required to react with substances present in the water.

What is the fundamental mathematical equation relating chlorine dosage, demand, and residual?

$$Cl_{\text{dose}} = Cl_2\text{ demand} + Cl_2\text{ residual}$$

Why are satellite dosing stations required in large water distribution networks?

Because chlorine in water gets depleted over time.

What three chloramine compounds are formed during chloramination reactions?

Monochloramine ($NH_2Cl$), Dichloramine ($NHCl_2$), and Trichloramine or nitrogen trichloride ($NCl_3$).

What is Combined Available Chlorine (CAC)?

The chlorine present in chloramines.

How do the residual durations of Free Available Chlorine (FAC) and Combined Available Chlorine (CAC) compare?

FAC lasts about 15 minutes, whereas CAC lasts 1 to 2 hours.

What equation defines Total Available Chlorine (TAC)?

$$TAC = FAC + CAC$$

Under what two distribution conditions is Combined Available Chlorine (CAC) specifically applied?

Where chlorine demand is high, and where the distance to the consumer is excessive.

What chlorine residual range should typically be maintained in practical water supply systems?

$$0.2 - 0.5\text{ mg/l}$$

What contact time is required for effective disinfection during chlorination in practice?

An adequate contact time of 15 to 20 minutes.

What instrument provides the easiest way to check for residual chlorine in the field or laboratory?

A comparator.

What is precipitation in water treatment?

A unit operation for the removal of dissolved constituents by adding a chemical or compound to precipitate the unwanted compound.

What three specific constituents are commonly removed using precipitation in water treatment?

Excessive hardness (softening), excessive iron, and excessive manganese.

What chemical equations represent the removal of calcium and magnesium carbonate hardness during Lime Softening?

$Ca(HCO_3)_2 + Ca(OH)_2 \rightarrow 2CaCO_3 + H_2O$ and $Mg(HCO_3)_2 + 2Ca(OH)_2 \rightarrow Mg(OH)_2 + 2CaCO_3 + H_2O$

At what pH level does Magnesium Hydroxide ($Mg(OH)_2$) become insoluble and precipitate out during lime softening?

At pH $> 11$.

What chemical is added in Lime-Soda Softening to remove non-carbonate hardness?

Soda Ash (Sodium carbonate, $Na_2CO_3$).

What reaction describes the removal of non-carbonate calcium hardness using Soda Ash?

$$CaCl_2 + Na_2CO_3 \rightarrow CaCO_3 + 2NaCl$$

What are the required pH conditions for the removal of Iron ($Fe$) and Manganese ($Mn$)?

For Iron removal, pH should be around 7 or slightly less. For Manganese removal, pH must be $> 8.5$.

What is Ion Exchange in water treatment?

A unit operation for the removal of dissolved constituents (ions) where objectionable ions in solution are exchanged for less objectionable ions.

What chemical equation represents total hardness removal in a sodium-bed ion exchanger?

$$NaR + Ca^{2+} \rightarrow CaR + 2Na^+$$

How is a sodium-bed ion exchange resin regenerated upon exhaustion?

It is treated with a concentrated solution of Sodium Chloride ($NaCl$).

What is the typical capacity of an ion exchange resin?

Usually around $80\text{ g}$ of $CaCO_3$ per liter of resin.

What are four main disadvantages of the ion-exchange unit operation?

Resins and equipment are expensive, operations are expensive ($NaCl$ and acid), skilled operators are required, and disposal of waste (brine/acid) is problematic.