Disinfection

Question 1

Relationship between chlorine dose, chlorine demand, and chlorine residual.

Answer 1

Dose = demand + residual

Question 2

Basic purpose of wastewater disinfection.

Answer 2

Destruction of harmful or undesirable microorganisms

Question 3

Typical chlorination units used in the wastewater industry:
chlorine dose * chlorine demand * chlorine residual

Answer 3

• Dose: pounds/day * Determined by weighing the chlorine container daily
• Demand: mg/L * Determined by subtraction (dose - residual = demand)
• Residual: mg/L * Determined by measuring chlorine remaining after at least 30 minutes of contact time)

Question 4

Factors that enhance the effectiveness of chlorine disinfection of wastewater effluent.

Answer 4

• Warm wastewater temperature
• Low effluent BOD; and suspended solids
• Long contact basin detention time
• Neutral to slightly acidic effluent pH
• Adequate chlorine residual

Question 5

This chemical, when present in wastewater effluent, can cause serious interference with disinfection.

Answer 5

Nitrite, produced during nitrification, can cause high chlorine demand

Question 6

The type of chlorine residual found in most wastewater effluents

Answer 6

Combined residual chlorine, in the form of chloramines. Combined residual is typical because most effluents contain ammonia.

Question 7

Characteristics of chlorine gas:
Density
Color
Smell
Reactivity

Answer 7

• Approximately 2.5 times heavier than air
• Yellow green color, visible when gas is 100 ppm or higher
• Acrid, burning sensation detectable at 1 ppm
• Supports combustion and very corrosive in the presence of moisture

Question 8

Used to detect small chlorine gas leaks.

Answer 8

Waive an ammonia soaked rag near the suspected leak - white cloud indicates leak

Question 9

Typical containers used for gas chlorine.

Answer 9

• 150 lb cylinder
• 2000 lb container or “one ton” container

Question 10

“Rule of thumb” for removal of chlorine from cylinders.

Answer 10

Maximum removal rate: approximately 1/4 of full cylinder net weight/day
150 lb cylinder = 40 lb/day
2000 lb container = 500 lb/day

Question 11

The total weight of a full “one ton” chlorine cylinder.

Answer 11

A full “one ton” chlorine container weighs approximately 4000 pounds
Container wt. = 1800 lb
Chlorine wt. = 2000 lb

Question 12

Chlorine is available in these physical states

Answer 12

• Gas chlorine: Molecular chlorine. Cl
• Liquid, solution chlorine: Sodium hypochlorite, NaOCl
• Solid, granular chlorine: Calcium hypochlorite, Ca(OC1)2

Question 13

Microorganism group used to determine the effectiveness of disinfection of wastewater effluent.

Answer 13

• Fecal coliform bacteria (monthly ave. not to exceed 200/100 ml sample
• E. coli (monthly ave. not to exceed 126/1 00 ml sample
• enterococci (monthly ave. not to exceed 33/100 ml sample

Question 14

The fecal coliform limit for a secondary effluent.

Answer 14

Fecal coliform bacteria

Question 15

The disinfection standard for “reuse” of wastewater effluent.

Answer 15

Less than 2.2 total coliform bacteria/1 00 ml sample

Question 16

The most commonly used chemical for effluent de-chlorination.

Answer 16

Sulfur dioxide

Question 17

Method used to determine the 24-hour chlorine dose.

Answer 17

Weigh the chlorine container(s) at 24 hour intervals

Question 18

Observe this to determine the present chlorine dose rate.

Question 19

The difference between the amount of chlorine added and the amount of chlorine residual that remains after the contact time

Question 20

The chemical used to determine the chlorine residual in wastewater effluents.

Question 21

The color that develops when DPD reacts with residual chlorine

Question 22

These can cause chlorine demand in wastewater.

Question 23

Typical uses of chlorine in a wastewater treatment plant

Answer 24

• Chlorine (most common method in U.S.)
• Ultraviolet light (UV)
• Ozone (least common method in U.S.)

Answer 25

Approximately 158° F

Answer 26

* Dysentery * Polio * Typhoid fever * Cholera * Hepatitis A * Gastroenteritis

Answer 27

Pathogens

Answer 28

* Viruses * Bacteria * Protozoa

Answer 29

Coliform bacteria

Question 30

The effect of chlorine on the pH of wastewater.

Answer 30

Chlorine gas tends to lower the pH, due to the formation of hydrochloric acid and hypochlorous acid.

Question 31

High effluent water temperature will have this effect on the chlorine residual.

Answer 31

The chlorine residual will tend to be lower because the chlorine is reacting faster

Question 32

This remains after a specified contact time and the chlorine demand has been met.

Answer 32

Residual chlorine

Question 33

This is the most effective form of chlorine disinfectant.

Answer 33

Hypochlorous acid, also called "free" residual chlorine

Question 34

If the chlorine dose exceeds the ammonia concentrate by eight times, this will occur.

Answer 34

Breakpoint is reached. Combined forms of chlorine are destroyed and any additional chlorine dose will produce free residual.

Question 35

The term “available chlorine.”

Answer 35

The oxidizing power of a chlorine compound

Question 36

Baffles serve this purpose in a chlorine contact basin

Question 37

Bubbles observed across the surface of a chlorine contact basin indicate the need to do this.

Question 38

Chlorine reacts with ammonia nitrogen compounds in wastewater to form this.

Question 39

This may occur if chlorine is removed from a container too fast, or the container temperature is allowed to drop below 50° F.

Question 40

Characteristics of high test hypochlorite (HTH™ ).

Question 41

The wavelengths used for UV disinfection

Answer 42

Sulfur dioxide gas (SO2)

Answer 43

Sulfur dioxide reacts almost exactly 1:1 with chlorine

Answer 44

* Liquid chlorine drawn from lower valve on ton containers * Heat is applied in the evaporator to provide conversion to gas chlorine * High withdrawal of liquid chlorine does not cause container cooling and icing

Answer 45

* The complete destruction of all life * Processes designed to kill or inactivate most microorganisms in water and wastewater, including essentially all pathogens

Question 46

Characteristics of liquid hypochlorite chlorination: Delivered concentration Chemical formula Storage Dosing equipment Compatible piping, valves, storage tanks Reactivity Safety

Answer 46

Delivered concentration 12.5% Chemical formula NaOCl (sodium hypochlorite) Storage For 30 days can result in 15% drop in strength Dosing equipment Hypochlorinator which incorporates a variable output positive displacement pump (diaphragm or peristaltic) Compatible piping, valves, storage tanks PVC, tephlon™, polyethylene Reactivity Can react (sometimes with violence) with many substances: Metals Ammonia salts Organic compounds Acids (causes rapid release of Cl2 gas) Safety Pronounced irritant; can cause severe burns to skin and eyes. Inhalation hazard (Cl2