Tuesday, May 29, 2018

Textile Effluent Treatment Plant (ETP): Flow Chart, Description

Effluent Treatment Plant (ETP) in Textile Industry:
The demand for variety in textile materials and colors has propagated the utilization of a large number of dyes and chemicals. The majority of dyes are retained in the final textile material, but most of the chemicals and residual dyestuffs are discharged in the effluents. The toxic chemicals may be present as auxiliaries (surfactants) in the detergent or soap, dyeing auxiliaries (surfactants), dye, dyeing carriers and bacterial finishes used in textile dyeing industries.
Effluent treatment plant in textile
Fig: Effluent treatment plant in textile
A combination of more than one treatment process is usually necessary to achieve adequate removal of all contaminants. Because of the wide range of dyes in use, each plant will have to establish the most economical treatment for the effluent. Each waste seems to demand a tailored solution requiring a combination of methods.

Effluent Treatment Unit Operations in Textile Wet Processing:
Textile wastes are generally colored, high in BOD and total dissolved solids, highly alkaline and hot. Effluent treatment unit operations are usually combined in a site-specific scheme after evaluation of effluent characteristics. Unit operations are typically drawn from the following categories:

1. Primary treatment:
  • Physical-Equalisation, screening, setting
  • Chemical- Neutralisation, lime addition, alum addition, iron salt addition.
2. Secondary Treatment:
  • Biological- Activated sludge, extended aeration, lagoons
  • Physical or chemical- Powdered activated carbon addition to biological process.
3. Tertiary Treatment:
  • Physical- Secondary clarification, fixed media filtration, ultrafiltration, granular activated carbon, powdered activated carbon. 
  • Chemical- Ozonation, chlorination.

Effluent Treatment Plant Process Sequence in Textile Industry:
The various stages of a typical textile effluent treatment plant are shown in the figure.

The effluent is first passed through a bar screen to remove floating matters and oil. The screen raw effluent enters first at the equalization-cum-neutralisation tank, which is open, semi-underground rectangular RCC tank.
Effluent treatment plant process
Fig: Effluent treatment plant process
PVC perforated pipe-grids are fitted at the bottom through which air is blown at an angle of 45° towards the bottom of the tank to prevent settling of the suspended solids. The effluent is detained for 4-8hours in the tank, during which an automatic balanced dose of dilute hydrochloric acid is given from an overhead tank by gravity to maintain neutral pH of the effluent. Hydrochloric acid or sulphuric acid is generally used for neutralization, as the textile effluent is mostly alkaline.

The equalized and neutralized effluent is then pumped by means of open impeller to the flash mixer. In the flash mixer, alum and/or any other coagulant solution is mixed with the effluent and vigorously stirred by means of mechanical stirrers. Alum dosing produces flocculation due to chemical reaction.

The effluent is then sent to the clariflocculator, which is similar to an equalization tank, but circular in construction. Here, the effluent is retained for 8hours, during which period most of suspended solids (mostly inorganic) settle down at the bottom of the tank. Alum flocks settle slowly along with smaller suspended solids. The sludge from the bottom is swept by a mechanical rake arm and is sent to sludge beds. The scum layer containing oil, grease etc. floating on the surface is removed by a slow moving mechanical skimmer arm and is dumped into a scum chamber.

The chemically treated effluent is then sent to the aerator for biological treatment. In two successive circular, open, semi-underground RCC tanks fitted with surface aerator units, the effluent is vigorously churned and there by the BOD is reduced due to intimate contact with atmospheric oxygen. Optimum value of suspended solids is maintained by recycling some organic sludge from the succeeding clarifier using sludge pumps.

Clarifier, the final unit in the effluent treatment plant, is also an open, semi-ground, circular RCC tank. The effluent is held in the unit for about 8hours. A portion of the settled sludge is send back to the aeration tank and the excess sludge is sent to sludge-bed. Clear treated effluent flows by gravity into the drain, which is a small earthen channel.

Sludge-bed consists of graded metal layers with a thick layer of sand at the top. The excess of water passes through the layers into the drain. The solid sludge is allowed to dry and is removed from time to time. Before final disposal of the sludge containing hazardous materials like pesticides, chlorinated hydrocarbons etc. it may have to be incinerated a temperature of about 990-1, 480°C.