Lesson 14:
Bacteria and Disinfection

 

Disinfection

Water disinfection means the removal, deactivation or killing of pathogenic microorganisms. Microorganisms are destroyed resulting in termination of growth and reproduction. When microorganisms are not removed from drinking water, its usage will cause people to fall ill. Sterilization is a process related to disinfection. However, during the sterilization process all present microorganisms are killed, both harmful and harmless ones.

The disinfection of potable water and wastewater provides a degree of protection from contact with pathogenic organisms including those causing cholera, polio, typhoid, hepatitis and a number of other bacterial, viral and parasitic diseases. As an individual pathogenic organism can be difficult to detect in a large volume of water or wastewater, disinfection efficacy is most often measured using "indicator organisms" that coexist in high quantities where pathogens are present. The most common indicator organism used is Total Coliform (TC), unless there is a reason to focus on a specific pathogen. The most common indicator organism for wastewater evaluation is fecal coliform.

 

 

How does Disinfection Work?

There are a number of chemicals and processes that will disinfect wastewater, but none are universally applicable. Chlorination/dechlorination has been the most widely used disinfection technology in teh U.S., ozonation and UV light are emerging technologies. Chemical inactivation of microbiological contamination in natural or untreated water is usually one of the final steps to reduce pathogenic microorganisms in drinking water. Combinations of water purification steps (oxidation, coagulation, settling, disinfection, and filtration) cause drinking water to be safe after production.

Disinfection is usually the final stage in the water treatment process in order to limit the effects of organic material, suspended solids and other contaminants. Like the disinfection of wastewater, the primary methods used for the disinfection of water in small treatment systems are ozone, ultraviolet irradiation (UV) and chlorine. There are numerous alternative disinfection processes that have been less widely used in small treatment systems, including chlorine dioxide, potassium permanganate, chloramines and peroxone (ozone/hydrogen peroxide).

Disinfection commonly takes place because of cell wall corrosion in the cells of microorganisms, or changes in cell permeability, protoplasm or enzyme activity (because of a structural change in enzymes). These disturbances in cell activity cause microorganisms to no longer be able to multiply. This will cause the microorganisms to die out. Oxidizing disinfectants also demolish organic matter in the water, causing a lack of nutrients.

 

 

Chlorine Feed Rate

Chlorine is added to water to kill any disease-causing organisms which may be present in the water or may enter the water as it travels through the distribution system. The two expressions most often used to describe the amount of chlorine added or required are milligrams per liter (mg/L) and pounds per day (lbs/day). To convert from mg/L to lbs/day, or vice versa, the following equation is used:

(mg/L Cl2) (MGD flow) (8.34 lbs/gal) = lbs/day Cl2

 

Below are sample problems to determine the chlorine feed rate.

Determine the chlorinator setting (lbs/day) needed to treat a flow of 3 MGD with a chlorine dose of 4 mg/L.

First write the equation. Then fill in the information given:

(mg/L Cl2) (MGD flow) (8.34 lbs/gal) = lbs/day Cl2

(4 mg/L Cl2) (3 MGD flow) (8.34 lbs/gal) = 100 lbs/day Cl2

 

 

A flow of 875,000 gpd is to receive a chlorine dose of 2.7 mg/L. What should the chorinator setting be (in lbs/day)?

(mg/L Cl2) (MGD flow) (8.34 lbs/gal) = lbs/day Cl2

(2.7 mg/L) (0.875 MGD) (8.34 lbs/gal) = 19.7 lbs/day

 

 

A chlorinator setting is 35 lbs per 24 hours. If the flow being chlorinated is 1.15 MGD, what is the chlorine dosage expressed as mg/L?

(mg/L Cl2) (MGD flow) (8.34 lbs/gal) = lbs/day Cl2

(x mg/L)(1.15 MGD)(8.34 lbs/gal) = 35 lbs/day

Now solve for x:

 

 

 

Chlorine Dose: Demand and Residual

The chlorine dose required depends on two considerations: the chlorine demand and the desired chlorine residual.

Dose, mg/L = Demand, mg/L + Residual, mg/L

 

The chlorine demand is the amount of chlorine used in reacting with various components of the water such as harmful organisms and other organic and inorganic substances. When the chlorine demand has been satisfied, these reactions stop.

In some cases, such as perhaps during the initial phase of treatment, chlorinating just to meet the chlorine demand is sufficient. In other cases, however, such as at the end of the treatment process, it is desirable to have an additional amound of chlorine in the water available for disinfection as it travels through the distribution system. This additional chlorine is called the chlorine residual.

 

A water is tested and found to have a chlorine demand of 1.9 mg/L. If the desired chlorine residual is 0.8 mg/L, what is the desired chlorine dose in mg/L?

Chlorine Dose = Chlorine Demand + Chlorine Residual

Chlorine Dose = 1.9 mg/L + 0.8 mg/L

Chlorine Dose = 2.7 mg/L Cl2

 

 

What should the chlorinator setting be (lbs/day) to treat a flow of 2.46 MGD if the chlorine demand is 3.1 mg/L and a chlorine residual of 0.8 mg/L is desired?

First calculate the chlorine dose in mg/L:

Chlorine Dose = Chlorine Demand + Chlorine Residual

Chlorine Dose = 3.1 mg/L + 0.8 mg/L

Chlorine Dose = 3.9 mg/L

 

Then calculate the chlorine dosage (feed rate) in lbs/day:

(mg/L Cl2) (MGD flow) (8.34 lbs/gal) = lbs/day Cl2

(3.9 mg/L)(2.46 MGD)(8.34 lbs/gal) = 80 lbs/day Chlorine

 

 

 

Review

Bacteria are unicellular prokaryotes.  They are classified based on respiration, shape, and reaction to the gram stain.  In terms or respiration, bacteria may be either aerobic, anaerobic, or facultative.  In terms of shape, bacteria may be either spherical, rod-shaped, or spiral-shaped.  Finally, bacteria may be either gram positive or gram negative. 

When working with bacteria in the lab, operators use a specific set of equipment, techniques, and safety procedures.  Typical equipment used in a bacterial lab includes an autoclave, petri dishes, and an incubator.  Unique techniques used in bacterial labs include sterilization, preparing the media, inoculation, and incubation.  Safety procedures consist of wearing appropriate safety equipment, practicing general cleanliness, and following basic safety precautions. 

Water disinfection means the removal, deactivation or killing of pathogenic microorganisms. Microorganisms are destroyed resulting in termination of growth and reproduction. When microorganisms are not removed from drinking water, its usage will cause people to fall ill. Chlorination/dechlorination has been the most widely used disinfection technology in teh U.S., ozonation and UV light are emerging technologies. The chlorine demand is the amount of chlorine used in reacting with various components of the water such as harmful organisms and other organic and inorganic substances. At the end of the treatment process, the additional chlorine added is known as the chlorine residual.

 

 

New Formulas Used

Chlorine Feed Rate

(mg/L Cl2) (MGD flow) (8.34 lbs/gal) = lbs/day Cl2

 

 

Chlorine Dose

Dose, mg/L = Demand, mg/L + Residual, mg/L



Sources

Banks, Matt.  May 1, 2001.  "Zoogloea ramigera."  University of Missouri, Rolla. 

"Chapter 3: Sterilization."  June 5, 2001.  Guidelines on Standard Operating Procedures for Microbiology.  World Health Organization Regional Office for Southeast Asia. 

"The Gram Stain."  2002. University of Leicester, UK. 

What is Water Disinfection. Lenntech

Disinfection of Water and Wastewater. Norweco

 



Assignments

Complete the interactive exercises in Assignment 14.  

This assignment will give you practice with the math covered in Lesson 14 as well as information covered in the chlorine virtual lab.  You should print the assignment and write out your calculations on a separate sheet of paper so that you can go back and check for errors if you get a question wrong. You may do the Assignment online to get credit or print it out and send it to the instructor.


Lab

Read the Chlorine Residual and Chlorine Demand and Total Coliform Bacteria labs and do the assignment listed above, there are questions concerning the virtual lab included.




Quiz

Answer the questions in the Quiz 14When you have gotten all the answers correct, print the page and either mail or fax it to the instructor.