General Methods for Industrial Effluent Treatment Plant (ETP)
Introduction : Any Industrial Effluent Treatment Plant (ETP) must be studied for the possibility of the ” 3 R” principle, they are Reduce, Recycle and Reuse. After employing the basic and fundamental methods of Strength reduction, Volume reduction, Proportioning, Segregation and Combining of Effluents only the Wastewater must be subjected to treatment.
These are the Common methods of treatment of Industrial Wastes(effluent treatment plant |ETP):
A combination of any of the following methods is employed in industrial effluent treatment plant (ETP).
- Neutralization cum Equalization
- COD/BOD Ratio
- Chemical Coagulation and Precipitation
- Settling Treatment
- Monitoring of ET P
- Oxidation and Reduction
- Adsorption by Activated Carbon
- Thermal Processes
- Tertiary Treatment of Sewage
- Removal of Color
- Disposal of Solid Wastes
Neutralization cum Equalization
Acidic effluents of one process and alkaline effluents of another process can be combined for Neutralization. By neutralization the pH of the effluent will be brought in the range of 6 to 9. Neutralization may be achieved in the same Equalization tank. pH must be in the range of 6 to 8.5 for most of the biological treatment processes to function efficiently.
Acid + Base = Salt +Water.
Bases employed are: Slaked lime, Caustic Soda, NaOH; or Soda Ash Na2C03 & Acids like H2S04, HCI or CO2. The quality and quantity of effluent change from time to time of the day. However ETPs are designed for a particular flow and particular concentration of pollutants like, MLSS, BOD etc. The effluent is first taken to the Equalization Tank for Quality and Quantity Equalization and from this it is supplied to ETPs. Also, as all the Biological treatment processes are very sensitive to Quality and Quantity fluctuations, equalization is essential.
If COD/BOD Ratio is 2 or less i.e more than about 0.5, Biological treatment methods Will be economical. If COD/BOD ratio is very high, Biological treatment is suitable only if the Chemical Precipitation methods are first employed. Once the non-biodegradables (COD) are removed, the COD/BOD ratio decreases and if it is < 2, Biological treatment methods like Trickling Filters, Activated Sludge Process, Oxidation ponds etc. may be employed.
Chemical Coagulation and Precipitation
Chemical’ Precipitation employing i.e. FeSO4 7H20 or Alum i.e. Al2 (SO4)3 . 18H20 is recommended before Biological treatment methods.
The goal of settling process is the expulsion of settleable organic and inorganic solids by sedimentation, and the removal of materials that will float (scum or rubbish) by skimming. The sewage should first be subjected to settling process for removal of Screenings like wood, paper and plastic squanders and skimmings to oil and grease. Evacuation of toxic substances like Chromium, Cadmium, Cyanide, Arsenic etc., is essential before sending the wastewater for biological treatment. Almost all toxic substances like Arsenic, Selenium, Cadmium, Nickel etc., can be removed by Alkaline Chlorination (i.e. Chlorination at pH > 10) or Activated Carbon.
Streams with toxic substances and high concentrations of a particular pollutant like Cyanide and Chromium. BOD and pH should be treated separately. Separation of wastewater streams of different qualities is known as ‘Segregation’ or Separation’. Cooling Tower blow downs that are large in quantity and have no major pollutants in them should be treated separately, they are segregated from others. Similarly, small stream with high concentration of a pollutant should not be mixed with others.
Monitoring of ETP
The Effluent Treatment Plant needs a constant monitoring. Constant watch on the Operation of Biological treatment process is essential. Before starting any biological method, the wastewater must be made free from toxic substances like As, Hg, Chlorine etc. pH must be in the range of 6.5 to 8.5 by adding suitable neutralizing agents.
Oxidation and Reduction : Hazardous subsatnces are converted to harmless or less toxic / hazardous forms.
Boiling, KMnO4, UV Rays, Bromine and Iodine, Excess lime are a portion of the disinfectants utilized while Chlorination is the most utilized technique for sterillization. Disinfection regularly includes the injection of a chlorine solution at the head end of a chlorine contact basin. The chlorine dosage depends upon the of the wastewater and different factors, but dosages of 5 to 15 mg/l are common.
Adsorption by Activated Carbon
Adsorption is a surface phenomenon in which the pollutants to be removed or taken onto the surface of the adsorbent. Efficiency of adsorbent increases with availability of surface area per gram of adsorbent. Normal Carbon has a surface are of 0.001 m2/gram while Activated carbon has a surface area of more than 1000m2/gram. Molecular Sieves also are possible alternatives to treatment of effluents with toxic substances but the efficiency is more only if the effluent is free from primary pollutants like clay, MLSS and TDS.
Advance Demineralization Techniques
It is like Evaporation and Distillation, Ion Exchange, Reverse Osmosis and Freezing.
Gases dissolved in effluent can be removed by stripping or desorption. Effluent, if sprinkled or sprayed into atmosphere in the form of droplets through nozzles, gases are deabsorbed and temperature of effluent also decreases due to escape of steam.
Thermal processes i.e. by Incineration at temperatures of 8000C or above of C, H, O, S, N, Heavy Metals for thermal destruction of hazardous wastes- organic as well as inorganic.
Tertiary Treatment of Sewage:
After the Secondary Clarifier, an Oxidation Pond is always recommended as it gives a tertiary treatment to the wastewater and gives it a polishing nature. A Stabilization Pond using Algae is a very low cost treatment method and is very efficient also in the removal of BOD. Design Organic loadings as high as 300 kg BOD/ha/day and detention periods of 10 to 15 days, gives up to 90% of removal of BOD, if properly maintained. Anaerobic Ponds may be used if BOD is > 2000 mg/ l. Anaerobic Pond with 3 months detention time and 5m depth removes 60% of BOD/ COD. Also this has no maintenance costs at all.
Removal of Color
Iron imparts red or reddish brown color to Water while Manganese imparts black color to water. Zinc gives Opalescence, a whitish color to water while some dissolved organics etc., cause other colors and odors to water. Most of Iron’ Manganese and associated color and odor can be removed by Chlorination, in the final stages. Further removal of color and odor, as a tertiary treatment is done by using Activated Carbon, Activated Silica or Molecular Sieves. Activated Carbon at a pH Of 3 removes 90% of color, which is the main trouble shooting parameter in a Tannery . Textile or Paper and Pulp Industry. Consumer acceptance will be more and public uproar will be less once the color is removed effectively as color is a common man’s index of-pollution. Coagulation, Flocculation, Carbon adsorption and Chlorination are the common ruethods adopted for removal of color.
Disposal of Solid Wastes
Most of the toxic substances are present in the Precipitates or the solid wastes, which are considered hazardous and hence are taken away by the local Pollution Control Board once in a year. The solid wastes are to be stored in water tight storage tanks during this period and need not be further treated.
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