Pulp and paper industry water treatment
This two kinds of industries are normally separated. The pulp (raw material for paper industry) and the paper industry, produce different kind of waste water. There is also a third related industry, the paper recycling industry, which has a similar kind of waste water. In the following text the three branches of industry are separated.
Production process, pulp industry
The composition of pollutants in waste water of paper industries, depend on the kind of raw material which is used. For paper production wood, bagasse (outer stalk sugar cane), hemp, straw, etc. are used. Five basic procedures are used to produce pulp in an industrial way. The conduction of case can be with or without a bleaching process.
If a bleaching process is also step of production, the Lignin will be destroyed. The effect is a whiter paper. The color depends also on the different kinds of wood or other raw materials which are used for the production. This step have the disadvantage to load the environment more, because of the use of chemicals like chlorine or chlorine dioxide (which are restricted, because of the process).
To produce pulp for fine and print paper, two chemical procedures are used.
The Kaft process is an alkaline process. The lignin is cracked by NaOH or Na2S, which is very effective at different kind of woods specially the wood contains pollutions.
Disadvantage is the odour problem, based on thiols and sulfides. The pulp has also to be bleached more, compared to the suphite process.
Process water of this kind of process contains SO2 and the pH is between 8 and 9. BOD and COD are depending on the kind of wood. Normally the BOD5 is between 1- 2 g/L, and the COD is 4- 7 g/L.
The sulphite process is a procedure based on acids. The effect is not the same compared to the alkaline process. The procedure is more sensitive, against pollution. Branches and bark disturb the chemical process and will not solute as well as the wood. Also resin disturbs the process.
Advantages of the “sulphite process” are more process variations and producing conditions without unwanted thiol and sulfide production.
The NSSC (Neutral Sulphite Semichemical) is most used. The yield of this process is approximately 75%,
The TMP (Thermo-Mechanical Process) is generally used for newsprint paper.
The new CTMP (Chemi-Thermo-Mechanical Process) is high efficient (approx. 95%) and the consumption of water is not as high than in other described processes. But it needs more energy to produce the pulp.
In modern pulp plants, this kind of waste water has to treated generally:
The amount of water which has to be treated is very high
Waste water is treated by anaerobic waste water treatment vaporizing, incineration and recovery.
Paper waste water
Paper is produced from “new” or recycled pulp. Depending on the quality of pulp, it’s used for different sorts of paper. During the process of paper producing, kaolin, CaCO3, talc and/or TiO2 are added to the pulp, to give the paper whiter color.
Also chemicals like organic fillers (starch, latex), colors, aluminum sulfate, etc. are used to make paper of different properties or making the process simpler.
Paper can be decolored, which can be done with two different processes. Washing the pulp with a high amount of water or washing with a low amount of water plus additions like sodium silicates, sodium carbonate, fatty acids or non ion detergents.
Pulp can be also made of recycled paper. But because of the color on written, old papers the pulp has to be bleached. The following procedures can be used.
Depending of the agents to bleach the pulp, waste water has to be treated. Bleaching with peroxides, oxygen and ozone is not as efficient as using chlorine or chlorine dioxide, but the water has generally a very low or no amount of to treatment chemicals. By using chloride or chloride dioxide, the water contains these agents which increase the AOX. On the other hand, chloride bleaching is the most efficient.
The waste water of paper recycling contains also particles which have to be filtrated. Rests of plastics, metal parts (paper clips, etc.) other waste have to be removed.
Find information about Water recycling in Paper industry