Most sources of water will require some treatment to remove undesirable constituents or simply as a precautionary measure to deal with accidental contamination. Similarly, most wastewaters need considerable purification before they can be safely discharged into the surrounding environment. With waters and wastewaters there is likely to be more than one undesirable constituent, so it is often necessary to utilize different treatment processes in series to achieve the desired product quality. This means that most treatment plants are a system of unit processes or operations linked together in the most effective way to produce the desired quality changes. In order to select the most appropriate combination of processes, a designer must be aware of the characteristics of the impurities which are to be removed and the principles of the available treatment processes.
Although the characteristics of waters and wastewaters vary widely, it is possible to classify the main types of impurities found into five groups.
- Floating or large suspended solids – leaves, twigs, fish, paper, rags, grit.
- Fine suspended and colloidal solids – clay, silt, microorganisms, proteins.
- Dissolved solids – alkalinity, hardness, salts, organics, metals.
- Dissolved gases – carbon dioxide, hydrogen sulphide.
- Immiscible substances – oils and greases.
In certain circumstances it is necessary to add materials as part of the treatment process; this could involve the use of coagulants in water treatment, the addition of chlorine for disinfection of water, or the aeration of sewage in biological oxidation. The divisions between different types of impurities are not, of course, sharply defined, and there may often be a degree of overlap in their properties. Nevertheless, the recognition of different types of impurities is useful as a means of characterizing the material which has to be handled and in giving some indication of the method of treatment which would be most appropriate. There are three main classes of treatment methods, each having several different forms:
- Physical processes which depend upon purely physical properties of the impurity such as size and density. Typical processes are straining, sedimentation and filtration.
- Chemical processes which depend upon the chemical properties of the impurity or an added reagent such as solubility and degree of ionization. Typical processes are coagulation, precipitation and ion exchange.
- Biological processes which use biochemical reactions to remove impurities which are utilized as sources of food for microorganisms housed in a suitable reactor. These processes, which may be aerobic or anaerobic, are mainly used for dealing with organic impurities in wastewater treatment. Typical processes are activated sludge, biological filtration, oxidation ponds and anaerobic digestion.
In my subsequent posts, I will cover physical, chemical and biological treatment processes in brief.