a. Waste paper pulp is produced from two types of plants. The low quality of paper is produced by mechanically pulping the waste paper without chemical use. This pulp is used to produce brown paper and paper board. The good quality paper is produced by mechanical pulping followed by removal of inks/ pigments and bleaching. Most de- inking is done by ‘washing’ or floatation, or a combination of both the techniques. Washing is used to remove small particles of ink while flotation is used to remove ink particles which are too small to be removed by screens and cleaners but too big to be removed by washing. Both the processes involve the use of chemicals. The washing technique uses chemicals known as wetting agents and surfactants to detach ink particles from wastepaper. The particles are then removed through repeated washing. The floatation process is based on ink agglomeration chemistry. After the ink is detached from the wastepaper, the ink particles are made to stick together by using suitable collectors like fatty acid soap. The resulting slurry is then taken to a flotation cell where lime is added to make them hydrophobic (so that they do not dissolve in water). The ink particles then get attached to air bubbles passed through the slurry and are finally discharged as foam sludge. This process can handle both old newspapers as well as coated paper, which is used to print magazines. Starch and calcium carbonate are added to strengthen the pulp followed by blending with water to achieve proper pulp to water ratio. Alum, rosin, talc and acid are added to condition the paper before it is sent to paper machine, where steam is used for drying.
The major pollutants emitted from waste paper pulping are effluents and solid waste. Effluents are generated during pulping especially during de-inking, blending, conditioning and drying. Processing wastepaper generates sludge. In case the wastepaper is de-inked, the sludge contains heavy metals. Industrialized countries usually incinerate de-inking sludge. But in most of developing countries, most of it is disposed of in landfill or sold out.