Role of Textile Effluent Treatment Plants (ETP) to Control Environmental Pollution

Secondary Treatment

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 color present in wastewater 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 [3]. 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:

1. Coagulation and flocculation of colloidal matter
2. Oxidation of dissolved organic matter to carbon dioxide
3. Degradation of nitrogenous organic matter to ammonia, which is then
4. 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, polyethylene 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.

Trickling filters:

The trickling filters usually consists of circular or rectangular beds, 1 m to 3 m deep, made of well-graded media (such as broken stone, PVC, Coal, Synthetic resins, Gravel or Clinkers) of size 40 mm to 150 mm, over which wastewater is sprinkled uniformly on the entire bed with the help of a slowly rotating distributor (such as rotary sprinkler) equipped with orifices or nozzles. Thus, the wastewater trickles through the media.

The filter is arranged in such a fashion that air can enter at the bottom; countercurrent to the effluent flow and a natural draft is produced. A gelatinous film, comprising of bacteria and aerobic micro-organisms known as “Zooglea”, is formed on the surface of the filter medium, which thrives on the nutrients supplied by the wastewater. The organic impurities in the wastewater are adsorbed on the gelatinous film during its passage and they are oxidized by the bacteria and the other micro-organisms present therein.

Activated sludge process:

This is the most versatile biological oxidation method employed for the treatment of wastewater containing dissolved solids, colloids and coarse solid organic matter. In this process, the wastewater is aerated in a reaction tank in which some microbial floc is suspended.

The aerobic bacterial flora brings about biological degradation of the waste into carbon dioxide and a water molecule, while consuming some organic matter for synthesizing bacteria. The bacteria flora grows and remains suspended in the form of a floc, which is called “Activated Sludge”. The effluent from the reaction tank is separated from the sludge by settling and discharged.

A part of the sludge is recycled to the same tank to provide an effective microbial population for a fresh treatment cycle. The surplus sludge is digested in a sludge digester, along with the primary sludge obtained from primary sedimentation. Efficient aeration for 5 to 24 hours is required for industrial wastes. BOD removal to the extent of 90-95% can be achieved in this process.

Oxidation ditch:

This can be considered as a modification of the conventional Activated Sludge process. Wastewater, after screening in allowed into the oxidation ditch. The mixed liquor containing the sludge solids is aerated in the channel with the help of a mechanical rotor.

The usual hydraulic retention time is 12 to 24 hrs and for solids, it is 20-30 days. Most of the sludge formed is recycled for the subsequent treatment cycle. The surplus sludge can be dried without odor on sand drying beds.

Oxidation pond:

An oxidation pond is a large shallow pond wherein stabilization of organic matter in the waste is brought about mostly by bacteria and to some extent by protozoa. The oxygen requirement for their metabolism is provided by algae present in the pond. The algae, in turn, utilize the CO₂ released by the bacteria for their photosynthesis. Oxidation ponds are also called waste stabilization ponds.

Anaerobic digestion:

Sludge is the watery residue from the primary sedimentation tank and humus tank (from secondary treatment). The constituents of the sludge undergo slow fermentation or digestion by anaerobic bacteria in a sludge digester, wherein the sludge is maintained at a temperature of 35^oC at pH 7-8 for about 30 days. CH_4, CO₂ and some NH₃ are liberated as the end products.