Technical Profile

Raw water that is treated for drinking water typically comes from either a groundwater or surface water source. Either source has the potential of being contaminated. The contamination may be either chemical or biological. Chemical contamination is typically the result of local or nearby industrial activities. Chemical contamination is abated by the organic removal process. Other, more specific chemical contamination, such as heavy metals, chlorinated organics or pesticides are treated with more focused approaches.

Microorganisms are common in nature - in soils, water, food and air. Many of these microorganisms contribute to some of the vital processes of the human body and most are harmless. However, there are some microorganisms capable of causing ill-health and disease.

Microorganisms fall into one of three types:

When selecting a disinfectant, the first criterion is typically focused on what compound can best destroy the microorganisms in the water being treated. An absolutely critical criterion, but disinfectants have other properties that must also be considered regarding how they may affect plant performance, safe handling, economics and non-lethality aspects.

The objective of the water disinfection process is to destroy or otherwise inactive pathogenic organisms, including bacteria, viruses and parasitic protozoa. This objective can be achieved with one of two approaches, each achieving the same outcome. The first approach is chemical based and the second is energy based. The former includes specific chemicals with high oxidation characteristics. The latter includes energy sources such as ultraviolet light and gamma radiation. Regardless of whether the approach is chemical-based or energy-based, the disinfection mechanism will follow one of the three modes of action:

The effectiveness of the disinfection process is a complex mix of many variables, primarily:

Considering these four variables together, as must be done, the selection of the proper disinfectant, dose and delivery becomes a complex process. One of the most common measures of the effectiveness of a disinfectant to oxidize organic material is the standard reduction potential. The standard reduction potential is an electrochemical attribute that varies depending on the oxidant. The higher the oxidation potential, the more effective that compound is at oxidizing organic matter. As such, the selection would seem simple; however, there are many other characteristics affecting the effectiveness of a disinfectant other than simple "oxidation power".

For example, the ability to diffuse into the cell, cell permeability and germicidal properties are dependent not on standard reduction potential, rather on molecular weight, chemical charge and other factors. In many instances, a highly effective disinfectant in one scenario of variables will be ineffective in a different scenario of variables. Consequently, to insure the proper disinfectant selection and dosage, it is prudent to collect and utilize empirical data resulting from pilot studies, bench studies and other similar methods to be able to target the proper agent and dose.

It is equally important to understand the characteristics of the microorganisms being targeted for destruction. When considering disinfection, the level of resistance of the target pathogenic microorganism is important. The relative level of decreasing resistance of various microorganisms is as follows:

The relative variance in resistance is attributed to differences in cytostructure. The resistance of the spore wall, cytochemical changes and the partially dehydrated state of the spore protoplasm may be reasons for the increased resistance of spores. At the other end of the resistance list, destruction of the metabolic systems occurs very rapidly with vegetative bacteria because respiration takes place on the surface of the cell and highly active systems are present very close to the cell wall.

Microorganisms in wastewater differ in their initial concentrations, die-off rates and susceptibility to various disinfectants. However, in most water supplies, bacteria are present at concentrations several magnitudes higher than viruses and other pathogenic microorganisms. As such, bacteria are typically included in water quality standards as the sole indicator of microbiological quality.

Contact time is a critical factor in determining the most effective disinfection system. The combination of dose and time contribute to the lethality of the disinfectant. Contact time is a very manageable factor to control. Further, empirical relationships have been developed that indicate a given percentage destruction of a certain organism at a specific dose and contact time.

There are other factors besides microbiological components that impact disinfection effectiveness. Some of these factors are as follows:

Turbidity and organics both have a negative influence on the disinfection process because they each, in different manners, protect the microbe from the disinfectant. Organics interfere by adhering to the cell surfaces and reacting with disinfectant to form different compounds, frequently of lesser effectiveness. The suspended solids simply surround the cell providing an effective shield from the disinfectant.

The pH can influence the effectiveness of a disinfectant of changing the chemical form of the disinfectant. For example, sodium hypochloride is a very effective disinfectant when used in a slightly acidic solution. If the pH is raised above 7.0 the effectiveness diminishes rapidly. The same holds true for temperature. All biological and chemical processes are impacted by temperature. As the temperature increases so does the rate of reaction. Thus, as the rate of reaction of a disinfectant increased so does the rate of disinfection, up to a limit.

Chlorine is the most commonly utilized chemical disinfectant used in water treatment. However, there are other available options, some of which may not be technically feasible or economically desirable. The most common chemicals used today for water disinfection are: