Pulp and Paper Mill Waste Treatments
Detailed information about Pulp and Paper Mill
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| Pulp and Paper Mill Waste Treatments |
⇒ This is one of the oldest and one of the largest industries in India.
⇒ This is one of the major industries which give a lot towards the pollution of air water environment.
⇒ Paper mills use the pulp as the raw material, which is again produced or utilizing different cellulose materials like wood, bamboo, other wood materials in the pulp mills.
⇒ The pulp and paper mill wastes commonly contain very high COD and color, the presence of the lignin in the waste, which is derived from the row cellulosic materials and it is not easily biodegradable, makes the COD and BOD ratio of the waste very high.
⇒ The pollution developing of paper mills are negligible compared to that of the paper mills.
⇒ So it is pulp making process which is responsible for the pollution problems.
Manufacturing Process and the sources of the wastes:
⇒ The process of manufacturing of paper may be divided into two phases that is Pulp making and then Making of the final product of paper.
⇒ Before an industrial waste is subjected to chemical or biological treatment it is required to separate the suspended matter by physical operations like Flotation and sedimentation.
⇒ Sedimentation tanks are to be provided only when the waste contains high percentage of settable solids and some other solids.
⇒ The Flotation is employed to separate fine particles with very low settling characteristics.
⇒ It consist of creation of fine air bubbles in the waste body by the introduction of air to the system. The rising air bubbles attach themselves to the suspended particles and there by increase the buoyancy of the particles.
⇒ The particles which are lifted to the liquid surface are removed by skimming.
⇒ In this pulp making part the chipped cellulosic raw material are digested with different chemicals in one tank under high temperature and pressure.
⇒ The process thus loosens the cellulose fibers and dissolves the lignin, resin and other non-cellulosic material in the raw material.
⇒ The Kraft process or the sulphate process of the pulp making uses sodium sulphate, sodium hydroxide, sodium sulphide, as the above mentioned digester chemicals.
⇒ And the another process of the pulp making is known as Sulphite process, which uses magnesium or calcium bisulphite and sulphourus acid as the digester chemicals, sodium hydroxide or lime is used in alkali process of pulp making.
⇒ The spent liquor produced by the above process of digestion which is called as the"Black Liquor".
⇒ The cellulosic fiber after being separated from the black liquor is wasted and then partially dewatered in a cylindrical screen called "Decker".
⇒ A concentrated wash water may be sent for chemical recovery, then the dilute wash water form the waste water.
- This volume of waste water is known as Brown stock wash or unbleached decker waste".
⇒ The wasted cellulosic fibers are then sent for the bleaching in three stages, where chlorine, caustic and hypochlorite are used in successive stages.
⇒ The waste water from the first and the last stages are light yellow in color, while that from the caustic extraction stage is highly colored.
⇒ The bleached pulp is then after sent for the paper mill.
⇒ The black liquor by the Kraft process will be concentrated by using the evaporation process, and then the incinerated with the addition of sodium sulphate.
- The organics which are like lignin, resin etc. are ignite out, and then the smelt will be dissolved in the water, then the resulting liquid is called as the "Green Liquor".
⇒ Lime is now added to this liquor, which resulting in the formation of the "White Liquor" and "Lime mud", containing calcium carbonate.
⇒ This White Liquor contains desired cooking chemicals and is sent for use in the digester.
Characteristics of Pulp and Paper Mill Wastes:
⇒ A well operated and well managed integrated pulp and paper mill, employing Kraft process for pulping, produces a waste volume in the range of 225 to 320 m3/ton, of paper manufactured.
⇒ Like the volume of the waste, the chemical composition of the wastes will also depend on the size of the plant, manufacturing process, and to great extent on the recovery of the chemicals and fibers.
| Small mill produces 20 ton of paper/day | Large mill produces 200 ton of paper/day | |
|---|---|---|
| Flow per day | 330 m3/ton | 222 m3/ton |
| PH | 8.2 - 8.5 | 8.5 - 9.5 |
| Total solids mg/l | - | 4410 |
| Suspended solids mg/l | 900 - 2000 | 3300 |
| COD mg/l | 3400 - 5780 | 716 |
| BOD mg/l | 680 - 1250 | 155 |
| COD/BOD | 3.9 - 5 | 4.6 |
Treatment of Pulp and Paper mill Wastes:
a) Recovery:
⇒ The recovery of the process, chemicals and the fibers reduced the pollution load to a great extent.
⇒ The lignin is recovered from the black liquor, by the precipitation by the acidulation with either carbon dioxide(C02) or Sulphuric acid (H2SO4).
⇒ The fibers in the white water from the paper mills are recovered wither by sedimentation or by the flotation using forced air in the sedimentation tank.
⇒ Lime is recovered from the lime and mud, by the process of calcination.
b) Chemical treatment of color removal:
⇒ The "Massive lime Treatment process, which was developed in United States Of America (USA) the main use of this is capable of removing 90 percent of color and 40 percent to 60 percent of Biochemical Oxygen Demand from the waste material.
⇒ In this process the entire quantity of lime , which is normally required for the recaustisation of green liquor into white liquor, is taken and allowed to recent first with the colored waste effluent.
⇒ The color is absorbed by the lime, and the sludge after settling is used in recaustising the green liquor.
c) Activated carbon for color removal:
⇒ Acid activated carbon can remove 94 percent color from the pulp mill water.
⇒ How ever, Ph should be reduced to 3.0 before this activated carbon treatment.
d) Physical treatment for clarification:
⇒ Mechanically cleared circular clarifier alone are capable of 70 to 80 percent removal of the suspended solids.
⇒ Here BOD reduction is 25 to 40 percent.
e) Biological treatment:
⇒ If sufficient area is available, the waste stabilization ponds, with 0.9 to 1.5m depth, and the detention period of 12 to 30 days is capable of removing of 85 percent BOD.
⇒ Aerated lagoons can be adopted to upgrade the main performance of already present stabilization ponds.
⇒ The BOD reduction of 50 to 95 percent can be achieved by aerated lagoons with a detention time of 3 to 20 days, at the loading range of 6970 to 1340 kg of BOD/hect/day.
⇒ Segregated the strong waste or may be the combined waste may be well treated in aerobic lagoons with nutrient supplementation.
⇒ A BOD loading of 0.048 kg/m3/d and a detention time of 20 days is adequate for 72.5 percent BOD removal.
⇒ This aerated lagoons may be employed after the anaerobic lagoons where a high effluent quality is utmost required.
f) Lagooning:
⇒ In the small mills where the black liquor is not treated separately for the chemical recovery the strong black liquor must be segregated from the other wastes and stored in a lagoon.
⇒ The content of the lagoon may be discharged into the stream under favorable conditions in the monsoon.
g) Land treatment method:
⇒ Some type of soil is capable of removing the color from, the waste.
⇒ The water is stored and allowed to be absorbed in such soil.
h) Disposal of the waste by Irrigation:
⇒ The main Pulp and Paper may be utilized for irrigation.
⇒ The yield is almost identical to that which conventional irrigation practices for wheat and the sugarcane.
Conclusion:
⇒ Here you can read detailed information about Pulp and Paper mill like Manufacturing Process and the sources of the waste and the Characteristics of Pulp and Paper Mill Wastes and Treatment of Pulp and Paper mill Wastes.
