Sewage emerging from a pulp and paper industry contains various polluting constituents because in the processes involved in pulp and paper industry various chemicals are used. As such treatment of the sewage is necessary prior to its disposal.
Processes Involved in Pulp and Paper Industry:
(a) Raw Materials:
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The raw materials are-
i. Cellulosic and
ii. Non-cellulosic.
i. Cellulosic Raw Materials:
Bamboo is the principal cellulosic raw material. However, wood (hard or soft) is now being increasingly, used. Straw, mainly rice and wheat; grass; jute sticks; sunn hemp; old ropes; hessian; cotton linters and rags; bagasse; and waste paper are also used as raw material in small paper mills and also for specialty paper in bigger mills.
ii. Non-Cellulosic Raw Materials (Chemicals):
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These are – caustic soda, sodium sulphate, sodium sulphite, sulphur, bisulphites of calcium and magnesium, lime, limestone, chlorine, chlorine dioxide, hypochlorites of sodium and calcium, hydrogen peroxide, sodium peroxide, china clay, talc, rosin, starch, alum, glue, dyes and gums.
(b) Preparation of Raw Material:
The preparatory processes for the various types of cellulosic raw materials and pulping processes employed for them are mentioned below:
The pulping process also includes washing of pulp whereby the fibres are separated from other dissolved constituents either in diffusers, or vacuum or pressure filters. The separated fibres are screened to remove shieves, knots, etc., and bleached, if necessary, in several stages with chlorine, hypochlorite and other bleaching agents. One of the steps in chemical bleaching is the caustic extraction of the chlorinated pulp.
The dissolved constituents of raw materials, made up of the spent liquor and pulp washings, commonly known as black liquors, may be sent either for recovery of chemicals or byproducts, or for disposal, depending on the process adopted.
(c) Stock Preparation and Paper Making:
The making of paper from the pulp consists of two essential steps, namely – stock preparation and paper making.
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Stock Preparation:
In this process, the pulp is mechanically treated in beaters, refiners or other equipment to the required degree of fineness (freeness) depending on the quality of paper to be made. At this stage, sizing chemicals, loading materials, dyes, etc., are also added to impart the necessary characteristics to the paper.
Paper Making:
The refined pulp is then passed on in water suspension to the paper or board machines when the sheet is formed either on continuous running wire or on the moulds. The sheets are pressed and dried.
Sources, Quantity and Characteristics of Sewage:
Sources:
Sewage is contributed from different sections of the pulp and paper industry as indicated below:
(a) Raw Material Preparation Section (Chipper House):
The effluent from this section results from washing, cleaning, barking and chipping of the cellulosic raw materials.
(b) Pulp Mill:
The effluent from pulp mill consists of:
(i) The spent liquor known as Black Liquor (BL);
(ii) Effluents from brown stock washers, chlorination, caustic extraction and hypochlorite bleaching.
(iii) Chemical recovery process;
(iv) Spills and leakages; and
(v) Wash water from bleach liquor and chemical preparation plants.
(c) Paper Machines:
Stock preparation and paper machine effluents which include excess white drainage
(d) Caustic-Chlorine Plant:
The effluent from caustic-chlorine plant consists of-
(i) The sludges from brine purification and filtration and caustic filtration;
(ii) Condensates from vacuum dechlorinators and chlorine system;
(iii) Condensate from hydrogen cooling system; and
(iv) Other effluents from cell room and floor washes.
Quantity:
The quantity of sewage produced from a pulp and paper industry depends on the total production, the types of paper made, water supply and on the practices adopted for reuse of effluents within the mill operations. As such the quantity of sewage considerably differs from mill to mill, and it may vary from about 200 to 350 m3 per tonne of paper produced.
Characteristics:
The characteristics of effluents from different sections of a pulp and paper industry adopting conventional sulphate process of pulping are presented in Table 19.6. The sulphite process of pulping is, however, not being used now due to great pollution exerted by the spent liquor produced in this process.
Pollutional Effects of Sewage from Pulp and Paper Industry:
The main polluting constituents in the sewage from pulp and paper industry are suspended solids, colour, foam, inorganic materials such as sodium carbonate (when recovery system is not practised), toxic chemicals such as mercaptans, and inorganic sulphides. Mercury is present if mercury cells form a part of pulp and paper mill.
The sewage has high BOD and COD and when discharged untreated will damage the receiving water course due to high oxygen demanding organic and inorganic materials present in the sewage. Further the sewage imparts colour to the stream due to lignin and its derivatives present in the sewage. The colour persists for a long distance since lignin and its derivatives are not readily biologically degraded. The sewage may also impart odour to the receiving stream.
Methods of Treatment of Sewage from Pulp and Paper Industry:
The treatment of sewage from pulp and paper industry may consist of the following:
(a) Segregation of effluents from different sections.
(b) Primary Treatment, and
(c) Secondary Treatment
(a) Segregation of Effluents from Different Sections:
Segregation of effluents from different sections of pulp and paper industry according to their characteristics facilitates in devising treatment methods at source and reducing the pollution loads in the combined effluent requiring treatment.
It also makes possible recovery and recycling of valuable materials like fibres and water. Separate collection of lignin-bearing coloured effluents will provide flexibility for colour removal for a small-volume concentrated effluent rather than the dilute total effluent.
(b) Primary Treatment:
This involves removal of suspended solids and partial reduction in BOD and other constituents in the sewage. The primary treatment methods includes coagulation, sedimentation, save-all filtration and floatation. The primary sludge along with excess secondary biological sludge may be thickened and dewatered by any of the mechanical methods or lagooned or filtered on sludge drying beds. The dried sludge may be disposed of by incineration or land-fill.
(c) Secondary Treatment:
The secondary treatment methods that may be adopted includes:
(i) Lagooning;
(ii) Aerobic biological treatment including activated sludge process, trickling filters, aerated lagoons, oxidation ponds; and
(iii) Anaerobic lagoon followed by aerobic stabilization pond.
Colour bearing effluent, after suitable conditioning, may be used on land for irrigation wherever sufficient and suitable farm is available, since this method not only reduces the cost of treatment but also overcomes the problem of colour in the combined sewage discharged into river.
Two flow diagrams for the treatment of sewage from pulp and paper industry are shown in Figs. 19.7 and 19.8.
Fig. 19.7 Flow Diagram for Treatment of Effluents from Different Sections of Pulp and Paper Industry.
Fig. 19.8 Flow Diagram for Treatment and/or Utilization from Different Sections of Pulp and Paper Industry.