Effluent can be treated in a number of different ways depending on the level of treatment required. These levels are known as preliminary, primary, secondary and tertiary (or advanced). The mechanisms for treatment can be divided into three broad categories: physical, chemical and biological, which all include a number of different processes (Table 3). Many of these processes will be used together in a single treatment plant. Descriptions of the most commonly used processes are given in this chapter.
Chemical Unit Process
Chemical unit processes are always used with physical operations and may also be used with biological treatment processes, although it is possible to have a purely physio chemical plant with no biological treatment. Chemical processes use the addition of chemicals to the wastewater to bring about changes in its quality. They include pH control, coagulation, chemical precipitation and oxidation.
Waste from textile industries is rarely pH neutral. Certain processes such as reactive dyeing require large quantities of alkali but pretreatments and some washes can be acidic. It is therefore necessary to adjust the pH in the treatment process to make the wastewater pH neutral. This is particularly important if biological treatment is being used, as the microorganisms used in biological treatment require a pH in the range of 6-8 and will be killed by highly acidic or alkali wastewater. Various chemicals are used for pH control. For acidic wastes (low pH) sodium hydroxide, sodium carbonate, calcium carbonate or calcium hydroxide, may be added among other things. For alkali wastes (high pH) sulfuric acid or hydrochloric acid may be added. Acids can cause corrosion of equipment and care must be taken in choosing which acid to use. Hydrochloric acid is probably better from an environmental view point but can corrode stainless steel therefore plastic or appropriately coated pumps and pipes must be used.
Chemical Coagulation and Flocculation
Coagulation is a complex process but generally refers to collecting into a larger mass the minute solid particles dispersed in a liquid. Chemical coagulants such as aluminum sulfate (alum) or ferric sulfate may be added to wastewater to improve the attraction of fine particles so that they come together and form larger particles called flock. A chemical flocculent, usually a polyelectrolyte, enhances the flocculation process by bringing together particles to form larger flock, which settle out more quickly. Flocculation is aided by gentle mixing which causes the particles to collide.