Disinfection

Introduction to Disinfection

Control of Waterborne Diseases

wastewater contains many types of human enteric organisms that are associated with various waterborne diseases. Typhoid, cholera, paratyphoid, and bacillary dysentery are caused by bacteria and amebic dysentery is caused by protozoa.?Disinfection?refers to selective destruction of disease-causing organisms in the water supply or in wastewater effluent.

Wastewater, after secondary treatment to remove BOD and solids, may still contain large numbers of microorganisms. Some of these organisms may be pathogenic and may cause epidemics if discharged to receiving waters. Wastewater treatment must reduce the possibility of this happening. As with other steps of treatment, there are many processes available to achieve disinfection. The most widely used process is chlorination. Other processes include ultra-violet (UV) light, ozonation, and bromine chloride additions.

Chlorination of the water supply has been practiced since about 1850. Presently, chlorination of both water supply and wastewater effluent is an extremely widespread practice for the control of waterborne diseases. However, chlorination may result in the formation of chlorinated hydrocarbons, some of which are known to be carcinogenic. Therefore, either dichlorination or alternate methods of disinfection are used.

?Chlorination

Chlorination, which follows all other steps of treatment, reduces the population of organisms in the wastewater to levels low enough to ensure that pathogenic organisms will not be present in sufficient quantities to cause disease when the wastewater is discharged.

In use, chlorine is added to the wastewater to satisfy all chemical demands. When these initial chemical demands have been satisfied, chlorine will react with substances such as ammonia to produce chloramines and other substances which, although not as effective as chlorine, have disinfecting capability. This produces a combined residual that can be measured using residual chlorine test methods. If additional chlorine is added, free residual chlorine can be produced. Due to the chemicals typically found in wastewater, chlorine residuals are normally combined rather than free residuals. Control of the disinfection process is usually based on maintaining total residual chlorine of at least 1.0 mg/L for a contact time of at least 30 minutes at design flow.

Certain factors affect the disinfection process. These include residual level, contact time, and/or effluent quality. Failure to maintain the desired residual levels for the required contact time will result in lower efficiency and increased probability that disease organisms will be discharged.

Chlorination Chemicals

Chorine used in the disinfection process normally is in the form of hypochlorite (similar to that used for home swimming pools) or free chlorine gas.

Chlorine Gas

Chlorine can be added to the water as a gas.?Once the chlorine enters the water, it produces hypochlorous acid .?This is the most concentrated form of chlorine, being 99.9% concentrated.

Chlorine gas is difficult to handle since it is toxic, heavy, corrosive, and an irritant.?

Hypochlorites and Bleaches

Chlorine can also come in a liquid form, known as?hypochlorite?or?bleach?.?These liquids work in the same general method as chlorine gas, by producing the disinfectant HOCl.?They are all much less concentrated than chlorine gas.

Hypochlorites have the disadvantage that they may decompose in strength over time while in storage. Temperature, light, and physical energy can all break down the hypochlorites before they are able to react with pathogens in water.?

Sodium hypochlorite?(NaOCl) is up to 12% chlorine. Like chlorine gas, it produces NaOH and HOCl when introduced to water.?

calcium hypochlorite?(Ca(OCl)2?).?This is the type of chlorine which is used in swimming pools.?Calcium hypochlorite is 65-70% concentrated.

The concentration of commercial bleach varies depending on the brand.?Chlorox bleach is 5% chlorine while some other brands are 3.5% concentrated.

Chloramines

When hypochlorites are mixed with ammonia, the result is a?chloramine?.?The ammonia and chlorine are mixed at a ratio of 4:5 to 1.

Chloramines are weaker than chlorine, but are more stable, so they are often used as the disinfectant in the distribution lines of water treatment systems.?Despite their stability, chloramines can be broken down by bacteria, heat, and light.?

Chloramines are effective at killing bacteria and will also kill some protozoans, but they are very ineffective at killing viruses.?

Chlorine Dioxide

Chlorine dioxide?, ClO2?, is a very effective form of chlorination since it will kill protozoans,?Cryptosporidium, Giardia, and viruses that other systems may not kill.?In addition, chlorine dioxide oxidizes all metals and organic matter, converting the organic matter to carbon dioxide and water.

Chlorine dioxide is generated on site, and it has the disadvantages that it is very costly and requires a great deal of technical expertise.?The operator must monitor the chlorite in a chlorine dioxide system.?

Chlorine dioxide also has other disadvantages.?It is dangerous around activated carbon, it can produce odors, and it is corrosive.???

Types of Disinfection

In the past, wastewater treatment practices have principally relied on the use of chlorine for disinfection.?The prevalent use of chlorine has come about because chlorine is an excellent disinfecting chemical and, until recently, has been available at a reasonable cost.

However, the rising cost of chlorine coupled with the chemical's toxicity to fish and other biota and the production of potentially harmful chlorinated hydrocarbons has made chlorination less favored as the disinfectant of choice in wastewater treatment.?

As a result, future wastewater treatment may see an increased use of ozone or ultraviolet (UV) light.?Both types of treatment are effective disinfecting agents and leave no toxic residual.?In addition, ozone will raise the dissolved oxygen level of water.?

Other Methods of Disinfection

Ozone

Oxygen in the air (O2) is composed of two oxygen molecules.?Under certain conditions, three oxygen molecules can be bound together instead, forming?ozone?(O3).

Ozone has many advantages as a disinfectant.?It kills all pathogenic organisms by a direct effect on their DNA.?Disinfection occurs 30,000 times faster than with chlorine, so a prolonged contact time is unnecessary.?There is no harmful residual left in the system.

The disadvantages of an ozone disinfection system are a corrosive nature, a high cost for the initial set-up, and a high electricity consumption.?

UV Light

Ultraviolet, or?UV, light is light outside the range usually detectable by the human eye.?It can be used to deactivate protozoans so that they can't reproduce and to significantly reduce the bacteria in water.?

The primary disadvantage of UV light is a high operating cost.?In addition, anything which blocks UV light from reaching the water will result in a lack of treatment.?

Process Control Calculations

There are several calculations that are useful in operating a chlorination system. These include chlorine feed rate, chlorine dose, and chlorine demand.

Chlorine Feed Rate

Here is an example using the equation above:

The chorine dose is 7.25 mg/L and the flow rate is 3.25 MGD. What is the feed rate for chlorine in pounds per day?

Chlorine Dose

Here is an example using the equation above:

The scale indicates that the plant has used 332 pounds of chlorine during the past twenty-four hours. The flow for the same period was 5.27 MGD. What is the dose of chlorine applied to the wastewater in milligrams/liter?

Chlorine Demand

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

Here is an exaple using the equation above:

If the chlorine residual for the previous example was 1.3 mg/L, what was the chlorine demand?

Demand, mg/L = 7.6 mg/L - 1.3 mg/L

Demand, mg/L = 6.3 mg/L