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L- 28 Pulp and paper mill
Industrial Waste Treatment
Introduction
• The manufacture of pulp and paper is a resource-intensive industry that consumes large amounts of energy, water, and trees.
• It is one of the oldest and one of the • It is one of the oldest and one of the largest industry in our country.
• Waste water contains very high COD and colour.
• Presence of lignin in the waste is very high which is not easily biodegradable.
• Because of this COD/ BOD ratio is very highhigh
• Pulp mill causes more pollution than paper mills.
Manufacturing process1. Pulp Making• Chipped cellulose materials are digested
with different chemicals in one tank under high temp and pressure.
• The process thus loosens the cellulose • The process thus loosens the cellulose fibers and dissolves the lignin, resin in the raw material.
• Chemicals used are- sodium sulphate, sodium hydroxide and sodium sulphide
• Spent black liquor is produced from digestion process.
• This black liquor is very rich in chemicals therefore it is separated and treated for recovery of chemicals.
• Cellulosic fibers are washed and dewatered in ‘decker’ screen.
• Washed cellulosic fibers are sent for bleaching in three stages where chlorine, caustic and hypochlorite are used in successive stages.used in successive stages.
• The bleached pulp is sent for paper mill
2. Paper mill
• In paper mill pulp is mixed with Alum, talc and dyes in a tank known as ‘beater’
• After beating pulp is refined in a • After beating pulp is refined in a machine known as ‘jordan’.
• Refined paper is then diluted for consistency and passed through screen to remove lumps and knots
• The pulp is then passed through belt of fine screens to a series of rolls, where the product paper is produced.
• The drained water is called as white • The drained water is called as white water
Characteristics of combined effluentItem Small mill Large mill
capacity 20 tonnes of paper/day
2000 tonnes of paper/day
Flow per day 330 m3/tonne 222 m3/tonne
Colour 7800 units
pH 8.2-8.5 8.5-9.5
TS mg/l --- 4410
SS mg/lit 900-2000 3300
COD mg/lit 3400-5780 716
BOD mg/lit 680-1250 155
COD/BOD ratio 3.9 - 5 4.6
Treatment alternatives
Massive lime treatment
Objective Questions
Theory questions