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