Sedimentation: The suspended matter in textile effluent can be removed efficiently and economically by sedimentation. This process is particularly useful for treatment of wastes containing high percentage of settable solids or when the waste is subjected to combined treatment with sewage. The sedimentation tanks are designed to enable smaller and lighter particles to settle under gravity. The most common equipment used includes horizontal flow sedimentation tanks and centre-feed circular clarifiers. The settled sludge is removed from the sedimentation tanks by mechanical scrapping into hoppers and pumping it out subsequently.
Equalization: Effluent streams are collected into sump pit. Sometimes mixed effluents are stirred by rotating agitators or by blowing compressed air from below. The pit has a conical bottom for enhancing the settling of solid particles.
Neutralisation : Normally, pH values of cotton finishing effluents are on the alkaline side. Hence, pH value of equalized effluent should be adjusted. Use of dilute sulphuric acid and boiler flue gas rich in carbon dioxide are not uncommon. Since most of the secondary biological treatments are effective in the pH 5 to 9, neutralisation step is an important process to facilitate.
Chemical coagulation and Mechanical flocculation: Finely divided suspended solids and colloidal particles cannot be efficiently removed by simple sedimentation by gravity. In such cases, mechanical flocculation or chemical coagulation is employed.
In mechanical flocculation, the textile waste water is passed through a tank under gentle stirring; the finely divided suspended solids coalesce into larger particles and settle out. Specialized equipment such as clariflocculator is also available, wherein flocculation chamber is a part of a sedimentation tank.
In order to alter the physical state of colloidal and suspended particles and to facilitate their removal by sedimentation, chemical coagulants are used. It is a controlled process, which forms a floc (flocculent precipitate) and results in obtaining a clear effluent free from matter in suspension or in the colloidal state. The degree of clarification obtained also depends on the quantity of chemicals used. In this method, 80-90% of the total suspended matter, 40-70% of BOD, 5days, 30-60% of the COD and 80-90% of the bacteria can be removed. However, in plain sedimentation, only 50-70% of the total suspended matter and 30-40% of the organic matter settles out. Most commonly used chemicals for chemical coagulation are alum, ferric chloride, ferric sulphate, ferrous sulphate and lime.
The main purpose of secondary treatment is to provide BOD removal beyond what is achievable by simple sedimentation. It also removes appreciable amounts of oil and phenol. In secondary treatment, the dissolved and colloidal organic compounds and colour present in waste water is removed or reduced and to stabilize the organic matter. This is achieved biologically using bacteria and other microorganisms. Textile processing effluents are amenable for biological treatments . These processes may be aerobic or anaerobic. In aerobic processes, bacteria and other microorganisms consume organic matter as food. They bring about the following sequential changes:
(i) Coagulation and flocculation of colloidal matter
(ii) Oxidation of dissolved organic matter to carbon dioxide
(iii) Degradation of nitrogenous organic matter to ammonia, which is then
converted into nitrite and eventually to nitrate.
Anaerobic treatment is mainly employed for the digestion of sludge. The efficiency of this process depends upon pH, temperature, waste loading, absence of oxygen and toxic materials. Some of the commonly used biological treatment processes are described below:
Aerated lagoons: These are large holding tanks or ponds having a depth of 3-5 m and are lined with cement, polythene or rubber. The effluents from primary treatment processes are collected in these tanks and are aerated with mechanical devices, such as floating aerators, for about 2 to 6 days. During this time, a healthy flocculent sludge is formed which brings about oxidation of the dissolved organic matter. BOD removal to the extent of 99% could be achieved with efficient operation. The major disadvantages are the large space requirements and the bacterial contamination of the lagoon effluent, which necessitates further biological purification.