Pulp and Paper Mill Waste
The paper mills use the 'pulp' as the raw material , which is again produced utilizing different cellulosic materials like wood , bamboo,jute, straw mainly of rice and wheat, waste paper, bagasse etc in the pulp mills.
Manufacturing Process :
The Process of manufacturing of paper may be divided into two phases - Pulp making and then making of final product of paper.
The major portion of the pollution from papermaking originates in the pulping processes.
Raw materials are reduced to a fibrous pulp by either mechanical or chemical means.
The bark is mechanically or hydraulically removed from wood before it is reduced to chips for cooking.
Mechanically prepared (groundwood) pulp is made by grinding the wood on large emery(very hard mineral) or sand stone wheels and then carrying it by water through screens.
This type of pulp is low-grade , usually highly colored, and contains relatively short fibers, it is mainly used to manufacture non durable paper products such as newspaper. The screened bark effluent contains fine particles of bark and wood and some dissolved solids.
Chemically prepared pulps, as compared with mechanically prepared ones, are made by the soda, sulfate (Kraft) or sulfite process. In all these methods the wood is prepared, as in the making of groundwood, by reduction to chips and screening to remove dust. The chemical processes differ from one another only in the chemical used to digest the chips.
Pulping is the process in which wood or other cellulosic raw materials are digested with chemicals under high temperature and pressure so that cellulosic fibres of wood are relieved from its binders such as lignin, resin etc
To a digester holding about four cords(128 cft) of chips, a mixture of soda ash (Na2CO3) and Lime(Ca(OH)2 (alkali process) is added and the total contents are boiled under steam pressure for about ten hours. This digestion decomposes or separates the binding , non cellulosic materials , such as lignins and resins from the fiber.During digestion most of the lignins are hydrolysed to alcohol and acids.
The spent liquor produces by the above process of digestion is known as black liquor. Black liquor very rich in lignin content but also cintains a large amount of unutilised chemicals.The black liquor of the Kraft process is concentrated by evaporation, and then incinerated with the addition of sodium sulphate. The organics like lignin, resin etc are burnt out, and the spelt is dissolved in water. The resulting liquid is known as green liquor. Lime is added to this liquor, resulting in the formation of white liquor and lime mud containing chiefly calcium carbonate. White liquor contains desired cooking (digestion) chemicals and is sent for use in digester.
The sulfate process calls for a shorter digestion period of about five or six hours , with a mixture of sodium sulfide, hydroxide , sulfate and carbonate . The lignin and non cellulosic materials are dissolved , leaving a stronger fiber for paper formation.
After digestion, chemically prepared wood pulps are blown into a closed blow pit, where the black liquor is allowed to drain to the sewer or to the recovery processes.
The drained pulp is then washed . These wash waters may then be wasted, reused or sent through recovery operations while the washed pulp is passed through some type of refining machine to remove knots and other non disintegrated matter.
A cylindrical screen, called a Decker , revolving across the path of the pulp partially dewaters it, after which it is passed to bleach tanks, where it is mixed in a warm, dilute solution of calcium hypochlorite or hydrogen peroxide. The dried , bleached pulp is then ready for sale or delivery to the paper mill.
The washed cellulosic fibres are sent for the bleaching in 3 stages, where chlorine, caustic and hypochlorite are used in successive stages. Waste waters from first and last stages are light yellow in colour, while that from caustic highly coloured.
In the paper mill, the pulp mixture is disintegrated and mixed in a Beater to which are added various fillers like alum, talc etc and dyes, to improve the quality of the final paper product, and sizing to fill the pores of the paper.
The Beater is essentially an oblong tank equipped with a rotating cylinder, to which are attached dull knives to break up the knotted or bunched fibers and cause a through mixing of the entire contents of the tank.
Sometimes the pulps are washed in the 'breaker beater ' prior to the addition of chemicals. After beating, the pulp is usually refined in a Jordan, machine that consists of a stationary hollow cone with projecting knives on its interior surface, fitted over a rapidly rotating adjustable cone having similar knives on its outside surface. This machine cuts the fibers to the final size desired.
The pulp then passes to stuffing boxes, where it is stored, mixed and adjusted to the proper uniform consistency for papermaking.
Finally the pulp is screened to remove lumps or slime spots, which would lower the quality of the final paper.
The pulp is evenly distributed from a headbox over a travelling belt of fine wire screening , known as fourdrivein weir , and carried to rolls . A small portion of the water contained in the pulp passed through the screen while the longer fibers are laid down as a mat on the wire . A considerable portion of the fine fibers and some fillers also pass through the screen wire with the water. Because of its colour, this waste water is called white water.
The paper mat passes through a series of rolls as follows ; a screen roll to eliminate inequalities at the end of the wire, a suction roll to draw out more water, press and drying rolls to rid of the paper of most of the remaining water and finally finishing rolls(Calenders) which produce the final shape of the paper.
Characteristics of pulp and paper mill wastes :
The volume depends mainly on the manufacturing procedure, and the water economy adopted in the plant. It has been observed that a well operated and well managed integrated pulp and paper mill employing Kraft process for pulping , produces a waste volume in the range of 225 to 320 m3 per tone of paper manufactured. The mills manufacturing special quality of paper produce larger amount of water for washing and beleaching.
Like the volume of waste, the chemical composition of the waste will also depend on the size of the plant, manufacturing process. In most of the small paper mills in India, the chemical recovery is not practiced due to economical reasons.
The pulp and paper mill wastes are characterized by very strong colour, high BOD , high suspended solids and high COD/BOD ratio.
Small Mill - (<20 t of paper/day) Large Mill - (2000 t of paper/day) pH 8.2 - 8.5 8.5 - 9.5 Total solids, mg/l ---- 4410 Suspended solids , mg/l 900-2000 3300 COD , mg/l 3400-5780 716 BOD, mg/l 680-1250 155 Colour Dark Brown Dark Brown COD/BOD 3.9-5 4.6
The effect of wastes on receiving water courses or sewers:
Crude pulp and paper mill wastes, or insufficiently treated wastes cause very serious pollution problems, when discharged into the streams.
The fine fibers often clog the water intake screens in the down stream side. A toxic effect may also be induced upon the flora and fauna of the stream due to sulfites and phenols in the waste.
The bottom deposit of Lignin - Cellulosic materials near the point of the discharge of the waste in a stream undergo slow decomposition and may lead to the dissolved oxygen depletion followed by the creation of anaerobic condition and destruction of the aquatic life.
Treatment of the pulp and paper mill wastes :
The treatment of the waste may consist of all or a combination of some of the following processes :
Recovery : The recovery of the process chemicals and the fibres reduces the pollution load to a great extent. Where the economy permits the colour bearing black liquor is treated for the chemical recovery. In this process the lignin is destroyed. The same may also be recovered from the black liquor, by precipitation by acidulation with either carbon dioxide or sulphuric acid. The fibres in the white water from the paper mills are recovered either by sedimentation or by floatation using dissolved air in the tank.
Chemical treatment for colour removal : Massive lime treatment capable of removing 90% of colour and 40 to 60% of BOD from the waste. In this process, entire quantity of lime, normally required for the recaustisation of green liquor into white liquor, is taken and allowed to react first with the coloured waste effluent. The colour is absorbed by lime and the sludge after settling is used in recaustisting the green liquor.
Activated carbon for colour removal: In a study conducted by NEERI, it has been observed that acidic activated carbon can remove 94% colour from the pulp mill waste
Physical treatment for clarification: Mechanically cleaned circular clarifiers alone are found to be capable of 70-80% removal of the suspended solids from the combined mill effluent. About 95 to 99% removal of settable solids can be accomplished in the clarifiers.However the BOD reduction is comparatively small and of the order of 25-40% only.
Biological treatment of the waste: Considerable reduction of BOD from the waste can be accomplished in both conventional and low cost biological treatment processes. If sufficient area is available, the waste stabilization ponds offer the cheapest means for treatment. A minimum of 85% removal of BOD is fond to be achievable. Aerated lagoons are the improved forms of the stabilization ponds. It may be noted that the pulp and paper mill wastes does not contain necessary nutrients for the bacterial growth, and hence Nitrogen and Phosphorous are to added into the lagoons in the form of Urea or Ammonia and Phosphoric acid in BOD: N: P ratio of 100:5:1. Activated sludge process is the most satisfactory and sophisticated system for the effluent treatment.Trickling filter has got a limited use in the treatment of the pulp and paper mill effluent, due to the greater chances of clogging of the media with fibrous material. Also the trickling filter system is incapable to provide a high degree of treatment -even with the new plastic media with greater specific surface area the BOD removal is found to be only 40-50%.
Lagooning : In small mills , where the black liquor is not treated separately for the chemical recovery, the strong black liquor must be segregated from the other wastes and stored in a lagoon.