Water Treatment: Significant, Concepts, Objectives And Principles

WATER TREATMENT

PRE REQUISTE DISCUSSION

All surface water and some groundwaters require treatment prior to consumption to ensure that they do not represent a health risk to the user. Health risks to consumers from poor quality water can be due to microbiological, chemical, physical or radioactive contamination. However, microbiological contamination is generally the most important to human health as this leads to infectious diseases which affect all populations groups, many of which may cause epidemics and can be fatal. Chemical contamination, with the exception of a few substances such as cyanide and nitrate, tends to represent a more long-term health risk. An example of this is nitrate which can cause methaemoglobinaemenia in babies. Substances in water which affect the clarity, colour or taste of water may make water objectionable to consumers and hence ability to recover costs. As many microorganisms are found associated with particles in water, physical contamination may also represent a health risk as it extends microbial survival.

Most treatment systems are designed to remove microbiological contamination and those physical constituents which affect the acceptability or promote microorganism survival - largely related to the suspended solids in the water. A disinfectant is nearly always included in treatment plants of any size. This is done for two main reasons: firstly it is added to inactivate any remaining bacteria as the final unit of treatment; and, more importantly, to provide a residual disinfectant which will kill any bacteria introduced during storage and/or distribution.

CONCEPTS

Treatment processes usually function either through the physical removal of contaminants through filtration, settling (often aided by some form of chemical addition) or biological removal of microorganisms. It is usual for treatment to be in a number of stages, with initial pretreatment by settling or pre-filtration through coarse media, sand filtration (rapid or slow) followed by chlorination. This is called the multiple barrier principle.

 SIGNIFICANT

This is an important concept as it provides the basis of comprehensive treatment of water and provides a system to prevent complete treatment failure due to a breakdown of a single process. For instance, with a system which comprises addition of coagulation-flocculation-settling, followed by rapid sand filtration with terminal disinfection, failure of the rapid sand filter does not mean that untreated water will be supplied. The coagulation-flocculation-settling process will remove a great deal of the suspended particles, and therefore many of the microorganisms in the water, and the terminal disinfection will remove many of the remainder. Provided the rapid sand filter is repaired reasonable quickly, there should be little decrease in water quality.

A key element in the multiple barrier principle is to ensure that the source of water is protected and maintained at as high a quality as possible. This is sometimes easier for groundwater sources on a local scale, although there are obvious difficulties for both ground and surface water on a larger scale.

OBJECTIVES AND PRINCIPLES

Treatment processes usually function either through the physical removal of contaminants through filtration, settling (often aided by some form of chemical addition) or biological removal of microorganisms. It is usual for treatment to be in a number of stages, with initial pretreatment by settling or pre-filtration through coarse media, sand filtration (rapid or slow) followed by chlorination. This is called the multiple barrier principle.

There are many different treatment process available and whose suitability is a function of the source water quality, level of operator training and resources available for operation and maintenance. It is imperative that the selection of technology for treatment plants is done taking the above into consideration to ensure that they remain sustainable.