Treatment of Pulp and Paper Industries Effluents | Environmental Engineering

In this article we will discuss about the treatment and disposal of effluents of pulp, paper and board industries.

Processes Involved in Pulp and Paper Industry:

1. Raw Materials:

The raw material are cellulosic and non-cellulosic.

Cellulosic Raw Materials:

Bamboo continues to be 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 speciality paper in bigger mills.

Non-Cellulosic Raw Materials (chemicals):

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, resin, starch, alum, glue, dyes and gums.

2. Preparation of Raw Materials:

The preparatory process for the various types of cellulosic raw materials and pulping processes employed for them are mentioned below:

3. Pulping Processes:

The common pulping processes are listed below with their principal pulping chemicals:

The pulping process also included 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.

4. Stock Preparation and Paper Making:

The making of paper from the pulp consists of two essential steps, namely, stock preparation and paper making.

Stock Preparation:

In this process, the pulp is mechanically treated in beakers, 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 Effluents:

The sources of the effluents are:

(a) Raw Material:

Preparation Section (Chipper House) includes washing, cleaning, barking and chipping.

(b) Pulp Mill:

The effluents of pulp mill consists of:

1) The spent liquor known as black liquor (BL);

2) Effluents from brown stock washers, chlorination, caustic extraction and hypochlorite bleaching;

3) Chemical recovery process;

4) Spills and leakages; and

5) Wash water from bleach liquor and chemical preparation plants.

(c) Paper Machine:

Stock preparation and paper machine effluents which include excess white drainage.

(d) Caustic-Chlorine Plant:

The effluents from caustic-chlorine consist of:

1) The sludges from brine purification and filtration and caustic filtration;

2) Condensates from vacuum dechlorinators and chlorine system;

3) Condensates from hydrogen cooling system, and

4) Other effluents from cell room and floor washes.

1. Quantity of Effluents:

The quantity of effluents considerably differs from mill to mill and is dependent on the total production, the types of paper made, water supply and on the practices adopted for reuse of effluents within the mill operations. The volume of effluent discharged by different mills in India is highly variable as shown in Table 17.27.

2. Characteristics of Effluents:

Raw Material Preparation Section (Chipper House):

The effluent from the raw material preparation section (chipper section) is low in biochemical oxygen demand (BOD) and chemical oxygen demand (COD) but contains some amount of floating and readily settleable solids such as dust, grit and sand. The volume is also low in general but in some mills where water is utilized for washing the chips and removing the dust, the quantity of effluent becomes quite appreciable.

3. Pulp Mill Effluents:

Digester house effluent is low in volume and is dark in colour due to lignin. It has high BOD and COD. In integrated pulp and paper mills having chemical recovery system, black liquor goes to the recovery section for the recovery of chemicals and for utilization of fuel value of the spent liquor.

In general, effluent from this section will be from the drainage of the gland packing, floor washing etc. The main effluent, from pulping section in an integrated pulp and paper mill having a recovery system, is the effluent from decker or unbleached thickners only.

Pulp washing (brown stock washes) effluent is highly coloured with high pH value, suspended solids, BOD and COD. Sodium content is appreciable in this effluent.

Bleach plant normally discharges effluents from the operations of:

(a) Chlorination,

(b) Caustice extraction, and

(c) Hypochlorite treatment.

It is customary that chlorination and hypochlorite effluents are discharged together while caustic extraction effluent separately. The caustic extraction effluent is highly coloured. In mills where bleaching without caustic extraction is practiced, the problem of colour is eliminated to a great extent.

Bleach plant effluent is large in volume but low in BOD and COD compared to pulp washing effluent. It is also brown in colour due to caustic extraction effluent. The pH value of the waste water is near to 7.0 or slightly acidic. Suspended solids are low compared to other effluents.

Chemical recovery effluent consists of effluents from barometric condensers, surface condensers and floor washes. In the case of surface condensers, the quantity of effluent is much less and consists of condensers vapour only whereas in case of barometric condensers, the water is used to dissolve the vapours and thus the quantity is much more.

Normally, the effluent is colourless but occasionally shows brown colour due to black liquor spills and overflows that take place from evaporators. The effluents will have the characteristic odour of mercaptans and is toxic to aquatic life. BOD and COD in the effluent are not high. Recausticising gives rise to lime sludge which in some mills is disposed off as slurry. This effluent consists of calcium carbonate, small quantities of hydroxides of sodium and calcium, and sulphides.

Paper Machines Effluents:

It is large in volume but the quantity finally discharged to drains depends on the extent of its reuse. The effluent has low pH value (5.0) and contains high amounts of suspended solids.

The physico-chemical characteristics of effluents from different sections of mills adopting conventional sulphate process of pulping and the pollutional loads in the combined effluent are presented in Table 17.28 and 17.29 respectively. The characteristics and pollutional loads of effluents from sulphite mill and newsprint mill are given in Table 17.30 and 17.31.

Note:

The average values for suspended solids, BOD (5 days at 20°C) and COD in the combined effluent are 375,160 and 610 mg/l respectively.

* Salai wood and bamboo are the raw materials and salai is mechanically pulped. Ground wood pulp and chemical pulp (sulphate) in the ratio of 6:4 are used. Total quantity of newsprint made is 130 t/day

Pollutional Effects:

The main polluting constituents in pulp and paper mill effluents are suspended solids, colour, foam, inorganics such as sodium carbonate (when recovery system is not practiced), toxic chemicals such as mercaptans and inorganic sulphides. Mercury is present if mercury cells form a part of pulp and paper mill.

The effluents have high BOD and COD and when discharged untreated will damage the receiving water course due to the presence of high oxygen demanding organic and inorganic constituents present in the effluents.

Further the effluents impart colour to the stream and it persists for a long distance since lignin and its derivatives present in the effluent are not readily biologically degraded. The effluent may also impart odour to the receiving stream.

a. Pollutional Effects of Individual Effluents:

Raw Materials Preparation (Chipper House) Effluent from this section does not exert severe pollution problems since it has low biochemical oxygen demand. However, the heavy suspended matter, which settles rapidly, will have adverse effect by way of settling.

b. Sulphate Kraft and Soda Mill Effluent:

The combined effluent from sulphate mill will exert BOD in the range of 110 to 235 mg/l. In case of soda mills with smaller capacity for paper making, the spent liquors are not used for chemical recovery and hence the combined effluent from such mills will be very high in BOD (780 mg/l).

Suspended solids in big as well as small mills will be high. Traces of chemicals in the effluent, such as alkali, sulphides, merceptans will have toxic effect on aquatic life. Further the presence of lignin and its derivatives in the combined waste will persist for considerable time in the receiving waters.

c. Sulphite Process Effluent:

Spent sulphite liquor causes the greatest pollutional effect on receiving streams due to the high BOD in the liquor. The constituents present in it are calcium and magnesium salts of lignosulphonic acid, carbohydrates, salt of volatile fatty acids, sulphites, bisulphites, methanol, acetone etc. Between 40 to 50 percent of the total wood substance is present in the waste sulphite liquor. The pollutional effects of the combined effluent are mainly due to the discharge of spent liquor.

In general in calcium based sulphite pulp and paper mills there is no recovery system for recovering the cooking chemicals and the spent liquor obtained after pulping is discharged into the stream. Due to the great pollution exerted by this spent liquor, no more sulphite mills are being installed in the world and the existing ones are also being replaced by sulphate mills or soluble base sulphite mills.

With the introduction of soluble bases, namely, magnesium, sodium, ammonium etc., it is not possible to have a recovery system for the sulphite mills also. But, even for a sulphite mill having a recovery system, the BOD exerted will be much more as compared to sulphate mills due to the very high BOD being exerted by the condensate obtained during the concentration of sulphite spent liquor due to their low pH.

The sulphite effluents exert immediate oxygen demand due to the presence of reducing compounds.

Semichemical Pulping Effluent Oxygen demand, colour and high or low pH value are the main objectionable features of the effluents but these have not been reported to be toxic to aquatic life except through deoxygenation and change in pH value.

d. Pulp Washing and Bleaching Effluents:

The combined effluents from the pulp washing and bleaching operations contribute nearly 65 percent of the total BOD load of the entire mill’s effluents and hence the high oxygen demand of these results in the rapid depletion of oxygen in the receiving stream. The persistence of lignin results in discolouration of the receiving waters.

e. Paper Machine Effluent:

The pollutional effect of white water is not of a serious magnitude since the organic matter present in this effluent contributes only about 20 percent of the total BOD load exerted by the composite mill effluents. However, the white water contains fibre debris, small fibres, soluble matter and a high percentage of non-fibrous suspended matter such as pigment, filling materials etc. Stream pollution problems result mostly out of the fine suspended solids content which may settle at low stream velocities and cause deposits which might decompose very slowly.

f. Deinking Effluent:

Pollutional effects of these effluents are due to the high oxygen demand of the organic matter in solution and the suspended matter derived from the suspended debris. Deinking process is not so far practiced in India.

Methods of Treatment and Disposal:

Methods in Actual Practice within the Mills:

The methods higher to practised extensively towards abatement of pollution by the pulp, paper and strawboard mills’ effluents are described below:

a. Segregation:

Segregation of more concentrated and offensive effluents, such as blow- down liquors, spent liquor spills, wash liquors etc. from the relatively more voluminous but less offensive or contaminated effluents like condensates, cooling water and paper machine effluent is practiced. The concentrated effluents are either treated independently or sent back for chemical recovery wherever possible.

b. Chemical Recovery:

Efficient recovery of chemicals from the spent liquor is an integral part of modern sulphate, (kraft) and soda process. However, recovery of chemicals from spent sulphite liquor has not been successful but, in most mills, the spent sulphite liquor is subjected to a concentration process by evaporation and the concentrate thus obtained is used for a variety of industrial purposes such as fuel, road binder, adhesive, tanning agent, insecticide, ore-binder, fertilizer etc.

The concentrated sulphite liquor if not used for any of the purposes indicated above is burnt and the ash disposed off. Other useful byproducts obtained from spent sulphite liquor are sulphur, turpentine, alcohol, vanillin and yeast. In India, one mill concentrates part of its spent sulphite liquor and markets the products. In another mill, the liquor is concentrated and burnt to recover the heat and the ash is disposed off.

c. Reclamation and Recycling in Closed Systems:

Condensates from multiple effect evaporators in chemical recovery plants and cooling systems and paper machine waters, after clarification are mostly recycled into the system. It is reported that about 80 to 90 percent reduction in pollution load and 70 percent reduction in effluent volume in chipper house could be achieved through effluent reuse.

Further, reuse of water in pulping operations can reduce pollution loads appreciably and recovery of cooking liquors can result in reduction of 60 to 90 percent. With existing knowledge, it is reported to be possible to achieve 20 to 60 percent reduction in effluent from pulp screening process by adopting partial water reuse, but it is anticipated that complete elimination of this flow could be attained in future with additional in-plant recycle.

Similarly, recirculation could be attained in future with additional in-plant recycle. Similarly, recirculation in multi-stage bleaching operation reduces pollution loads by 30 to 80 percent. Effective fibre recovery from paper machine can reduce the pollution load by 20 to 60 percent and volume by 60 to 80 percent.

The Indian pulp and paper industry also employees recycling of effluent to a certain extent. It is reported that in one mill, 50 percent of pulp mill and recovery section effluent and 80 percent of paper machine water are being used for bamboo wetting and stock preparation respectively.

In another mill, the effluent alter fibre recovery is entirely used in pulp bleaching section and 50 percent of the discharge from the thickner is reused for pulp washing. Another example of reuse is that 60 to 80 percent of water requirements of the chipper house in a large mill are met from the paper machine water after filter recovery. In another situation it is reported that 50 percent of pulp mill effluent and 30 percent paper machine effluent are being reused.

Good housekeeping methods such as proper installation and operation of equipment will ensure substantial reductions in seepage losses, spills etc.

Removal of Suspended Matter:

By Floatation and Filtration:

Most of the mills use this technique as an in-built system to recover fibre from paper machine effluent.

The different types of equipment used include:

(a) Filtration save-all in which an inclined wire screen is used,

(b) A rotary screen or vacuum filter type save-all, and

(c) Floatation type save-all which operates under reduced pressure or at atmospheric pressure.

Alum in conjunction with glue or rosin is normally used to obtain optimum results. These devices are sensitive to variation in flow suspended matter air injected chemical dosage and fibre to filter ratio. Centre cleaner rejects results from pulp washing and paper section are small in volume but contain high concentration of fibres and grit or sand.

These rejects instead of discharging into the effluent drains can be segregated and passed over a side hill screen to separate out the suspended solids. It is reported that I to 20 tonnes of solids per day can be easily separated from the rejects of a200-tonne per day plant.

By chemical coagulation, flocculation and sedimentation-Paper machine water, combined mill effluent and de-inking effluents are amenable to treatment by this method.

The type of equipment used are:

(a) Multiple plain sedimentation basins or conical bottom tanks arranged to permit intermittent scooping out of solids, and

(b) Clarifers of thickener type with mechanical sludge collectors.

Clarifiers can remove suspended solids from 60 to 80 percent and BOD from 30 to 50 percent. Where high removals of suspended solids are required, coagulants such as alum, activated silica and polyelectrolytes may be used to improve performance to about 90 percent removal.

Clarifiers are commonly designed for over-flow rates to 25 to 40 m³/m²/day and a detention time of 2 to 4 hours. Substantial quantities of sludge withdrawn from a primary clarifier and biological treatment system with low solids concentration [0.75 to 1.5 percent (m/v)] requires thickening before disposal.

Techniques which have been employed include gravity thickening of mixed primary and secondary sludges solids concentration after thickening. 3.5 to 4.5 percent (m/v) air flotation, centrifugation [30 percent solids (m/v)] and vacuum filtration [20 percent solids (m/v)] and finally pressing to reduce the volume and increase the solids content [30 percent solids (m/v)].

Otherwise, sludge drying beds can be used for filtration and drying the gravity thickened sludge, in which case land requirements will be high. Ultimate disposal of sludge has been accomplished by land filling and lagooning with other solids wastes. Where land is not available, incineration of the sludge has been employed.

Methods Practiced for Treatment of Combined Effluent:

These methods are described below:

Disposal on Land by Lagooning and Irrigation.

a. Disposal on land by lagooning:

Lagoons of paper and strawboard mills’ effluents are shallow basins of almost any shape and size in which the effluents are held over a retention period ranging from a few days to several weeks depending on the local land conditions. During the period of storage, BOD, suspended solids and turbidity undergo reduction and the effluent is fit for discharge into the nearby land or water washed. Disposal of the sludge from the lagoons is a problem of great magnitude.

b. Irrigation:

Paper mill effluents which may lend themselves for disposal on land by spray irrigation are spent sulphite liquors, combined effluents from kraft and semi-chemical pulp mill, and white water from paper machine.

It is reported that the following guidelines for field application and land disposal of pulp mill effluents may be adopted:

(a) BOD load 225 kg/hectare/day,

(b) pH value -6.5 to 9.0,

(c) Sodium absorption ratio (SAB) – 8 on permeable soils,

(d) Colour – individual site investigations required, and

(e) Land requirements – 4.3 to 5.3 hectares per 1000 m³ effluent/day.

These guidelines are applicable to a system employing cover vegetation whose root system also provides substantial increase in percolation rate in dense soil where the lead system provides a means for additional water consumption by evapotranspiration. Substantial colour removal is also possible depending on the type of soil.

For example, clay loams are most effective in retaining the colour, followed by silt loam and sandy loam. However, the percolation rate will be in the reverse order, namely, clay loams are less porous permitting low hydraulic loading rates.

In most cases, land requirement will be high since the volume of effluent from pulp and paper mill is usually high. Some of the other factors to the considered in this method of disposal are:

(a) Land selected should have proper combination of soil characteristics;

(b) Land available should be adequate to permit easy distribution of the solids concentration in the effluent. It is essential to remove the settleable solids before the effluent is applied on land;

(c) Proximity of the mill premises to the disposal site;

(d) It should ensure absolute protection of ground waters from contamination;

(e) It should be located as far as possible along the direction of wind; and

(f) Availability of cheap power and suitable anti-corrosive equipment.

Pulp and paper mill sludge contain little or no nitrogen, phosphorous and potash and hence they have no fertilizer value. On the other hand, these sludged have been added to soils to increase their friability and water holding capacity. However, it is reported that the decay of cellulose increase, the acidity of the soil to detrimental levels. Due to these deficiencies, pulp and paper mill sludges have no application for agricultural uses.

Biological Methods of Treatment:

The BOD exerted by organic matter in pulp and paper mill effluents is derived mostly from carbohydrate material of the wood substance, salts of fatty acids, alcohols and other substance present in the dissolved state. The effluents are deficient in nitrogen and phosphorous which are essential for microbial metabolism.

The biological treatment systems most commonly applied to pulp and paper mill effluents are similar in principle to those used for many other types of industrial effluents, including municipal sewage. However, it is essential to develop and use appropriate design parameters specific to the effluent from the pulp and paper and pulp mill.

The methods considered suitable are the trickling filter, activated sludge process, aerated lagoons, the stabilization pond and anaerobic digestion or lagoon. Secondary biological processes indicated above, depending upon the type of design, can attain 30 to 95 percent BOD removal. The level of performance which may be expected in the different treatment systems treating pulp and paper mill effluents is shown in Table 6.

Trickling Filter High rate trickling filters for the treatment of pulp and paper mill effluents have been used along with further treatment if required. BOD loadings of 0.6 to 1.2 kg/m³/day would be practicable indicating that large land areas will be needed. Overall BOD reductions seldom exceed 60 percent.

Before treating the effluent on trickling filters, adequate removal of suspended solids and pH value correction to optimum limits conducive for biological growth are essential.

Modifications of the conventional biological filtration developed so far for treating pulp and paper mill effluents are:

(a) Two stage filtration using PVC filter media,

(b) Introduction of forced ventilation and recirculation with a high recirculation ratio, and

(c) Addition of supplemental nutrients to the effluent.

So far trickling filters are not used for the treatment of pulp and paper mill effluents in India.

With or without chemical addition and floccuiation.

As used to remove biological solids in secondary treatment.

Activated Sludge Process:

Technically, activated sludge process is the most successful method for treating effluents from kraft mill and semi-chemical mill. It is reported that mixtures of sewage and kraft mill effluents can be treated more readily than the raw effluents.

The following modifications of the conventional activated sludge process have been suggested while treating pulp and paper effluents:

(a) Longer periods of aeration,

(b) Higher concentration of seed sludge,

(c) Use in non-flocculent type of growths in the aeration tanks, and

(d) Aeration using a sparger or mechanical agitation or both.

For the activated sludge treatment of pulp and paper mill effluents, a loading of 0.2 to 0.3 kg/BOD (Max 0.4) kg per kg mixed liquor suspended solids (MLSS) was found to give a BOD reduction of 80 to 90 percent with a maximum of 95 percent. The Period of aeration was between 3 and 9 hours, MLSS concentration 3000 mg/l (2000 to 4000 mg/l) and the sludge recirculation 30 to 50 percent.

BOD.N.P of 100:5:1 was used. It is reported that in treating industrial effluents in aerobic systems, a minimum of 4.3 kg of nitrogen and 0.6 kg of phosphorous are needed for every 100 gm of BOD removed. It was also reported that in treating pulp and paper mill wastes by activated sludge process, if the nitrogen content is reduced below 4.4 Ag/100 kg BOD removed, the BOD removal efficiency decreased.

Removal of suspended solids from the effluent before sending it to aeration units and acclimatization of the seed sludge in the activated sludge plant are two prerequisites for the success of the process. The drawback in the process is foaming in the aeration tanks, colour intensification and coast of nutrient addition.

It was reported that if the effluents are given prior chemical treatment, the trouble due to foaming could be I these cases, the land requirements will be high and hence this method can be adopted in places where land is available in plenty and at cheap price, practically eliminated in the subsequent biological treatment.

Otherwise, foam in the aeration tanks has to be destroyed by fine water jets or by addition of antifoaming agents. There is only one installation in India where activated sludge process of treatment for part of the effluents from a speciality paper mill is being used. I makes use of pH correction with hydrochloric acid, primary clarifier with thickner and aeration tanks provided with surface aerators.

For nutrient supplementation, required quantities of nitrogen and phosphorous are added. The MAS from the aeration unit is clarified in a secondary clarifier. Excess biological sludge along with the primary sludge is disposed off by lagooning. The treated effluent mixed with the part of the primary settled effluent is used for land irrigation.

Aerated Lagoon:

Aerated lagoon is mostly an aerobic process and maintains relatively low MLSS (300 to 500 mg/l) as against 2000 to 3000 mg/l in conventional or extended aeration system. Aerated lagoons for the treatment of pulp and paper mill effluents have been adopted in several places. A detention time of 3 to 10 days is usually adopted and nutrient supplementation has to be provided.

It is reported that nutrient addition could be eliminated if the detention time in the aerated lagoon was kept about 15 days. Primary sedimentation for the removal of suspended solids is necessary before the effluent can be treated in aerated lagoon.

An overall BOD reduction of 40 to 85 percent can be obtained in this system. Aerated lagoons will be more suitable where land is moderate in cost or where natural depressions are available for constructing the lagoons. There will be problem of foam which requires control as mentioned earlier.

Stabilization or Oxidation Pond:

Where land is cheap stabilization basins or ponds can be used with advantage since long detention periods minimise process upsets and also keep down the operational cost to a minimum. This system requires little attention. Design of oxidation pond for treating pulp and paper mill effluents is similar to domestic sewage treatment practice.

The effluents will have to be given primary treatment in a clarifier for successful operation. Such ponds are reported to be in use in some places. Since shallow depth (0.9 to 1.5 m) is used in the construction of oxidation ponds, much area is required for treating the effluent. Another important consideration is that the brown colour due to lignin present in kraft pulp mill effluents is likely to prevent penetration of light which is very much essential for the process.

However, some of the less coloured effluents from pulp mill and paper machine can be treated by oxidation pond method. Since the pulp and paper mill effluents contain considerable amount of suspended solids as fibres etc. which resist easy degradation it is desirable to remove the settleable solids in clarifier before treating in stabilization ponds, otherwise it would reduce the depth vis-a-vis the detention time in the pond.

Anaerobic digestion or anaerobic lagooning:

Effluents which are amenable to treatment by anaerobic method are:

(a) Paper machine effluent from closed circuit recirculation system which results in heavy building up of dissolved organic solids,

(b) De-inkingg effluents,

(c) Rag, rope and jute pulping effluents,

(d) Semi-chemical effluents, and

(e) Brown stock wash water from kraft pulp mill.

Under controlled conditions of temperature, it is reported that 90 percent reduction in BOD is feasible with a detention time of 4 to 10 days. Addition of nitrogen and phosphorous in requisite quantities is essential. Presence of soluble sulphides up to 300 mg/l and sulphates upto 2000 mg/l have been observed to have no inhibitory effect on the anaerobic process.

Although anaerobic digestion in closed tanks can be carried out, it is not economically feasible. Instead, the stronger effluent (with high BOD) can be treated in anaerobic lagoons and such lagoons for the treatment of effluents from pulp, paper and strawboard mills have been reported in literature.

Pilot plant studies conducted in India using a combination of anaerobic lagoon and oxidation pond in series could reduce the BOD from 700 to 90 mg/l with a total detention time of 22 days. A full scale anaerobic lagoon to treat prehydrolysate waste (BOD 40000 mg/l) from a rayon pulp mill is in operation for the last 3 years giving 80 to 90 percent reduction in BOD.

Pilot plant experiments as well as full scale treatment studies, using brown stock wash water and caustic extraction effluent, in anaerobic lagoon revealed that 60 to 70 percent reduction in BOD is a achievable with a detention time of 20 days. Nitrogen and phosphorous are supplemented in the ratio of BOD: N: P as 100:2.0:0.5. In all these case, the land requirements will be high and hence this method can be adopted in places where land is available in plenty and at cheap price.

Methods under Experimentation and Development for Removal of Colour in pulp and paper mill effluents are due mainly to lignin and its derivatives. Tannins and other pigments present in the wood also contribute to colour in the effluent to a certain degree. A relationship exists between the lignin content of the pulp mill effluent and its COD value particularly in kraft mill waste.

Therefore, COD reduction in the effluent is dependent on colour reduction. The methods employed for colour removal from pulp and paper mill effluents include chemical treatment, massive lime treatment, activated carbon, resin adsorption and ozonation.

These treatment systems have not yet been employed to a significant extent in practice, except for experimental purposes and these are briefly prescribed as under:

a. Chemical Treatment:

Chemical coagulants such as alum, iron salts in combination with lime etc. have been used for colour removal from pulp and paper mill effluents. Among the coagulants, alum treatment is still being used with some promise. The coagulant dose depends on the initial colour in the effluent. The main drawback in the method in that chemical treatment produces large volume of sludge with poor settling properties.

b. Lime Treatment:

Massive time treatment for colour removal has been found to be technically and economically feasible. This method besides achieving over 90 percent colour removal also results in 20 to 40 percent reduction in BOD from the coloured pulp mill effluents.

A modified lime treatment for colour removal using only 1000 mg/l of lime for the total unclarified kraft pulp and paper mill effluent with time recovery system has been reported to be of great promise as this method also helps in the removal and disposal of the suspended solids in the combined effluent.

c. Activated Carbon Treatment:

Activated carbon treatment for colour removal from raw effluent is not economical. However, activated carbon treatment preceded by massive lime treatment will yield effluents fit for reuse in the mill and this combination will make activated carbon treatment an economic proposition.

Recently, a study on microlimecarbon sequence for removal of 90 percent colour and 40 percent total organic carbon (TOC) from clarified effluent of unbleached kraft mill has shown to yield reusable water at a relatively low cost compared to massive or minilime activated carbon treatment.

d. Resin Adsorption Treatment:

Synthetic resins have been reported to adsorb colour from coloured pulp mill effluents at pH 2.0 to 2.5. The resins can be desorbed by weak wash containing sodium hydroxide from the recausticization section. The coloured weak wash is recycled into the cooking system of the pulp mill. It is reported that by this process colour can be removed by 65 to 90 percent; BOD 25 to 40 percent; and COD 40 to 60 percent.

e. Ozone Treatment:

Ozone at a concentration of 0.3 to 0.35 kg per kg of lignin has been reported to remove over 90 percent colour from the effluent. These studies have been carried out only on laboratory scale and require further trials on pilot and plant scale.

Treatment Methods Recommended for Adoption in India:

From the information cited on the different methods that are available for treatment of effluents from paper and pulp mills, it is recommended that a complete treatment would consist of the following:

(a) Segregation of various types of effluents,

(b) Primary treatment of various effluents, and

(c) Secondary treatment of effluents.

a. Segregation:

Segregation of different effluents according to their characteristic facilities 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 dilute total effluent. Wherever possible, such segregation should be effected. This aspect should be kept in view by new mills at the stage of their planning and layout.

b. Primary Treatment:

This involves removal of suspended solids and partial reduction in BOD and other constituents in the effluents. The methods suggested, namely, coagulation sedimentation, save-all filtration and floatation are efficient and economical and shall be followed invariably as an essential treatment by each mill.

The primary sludge along with excess secondary biological sludge should be thickened and dewatered by any of the mechanical methods described or lagooned or filtered on sludge drying beds. The dried sludge should be disposed off by incineration or land-fill.

c. Secondary Treatment:

Any of the biological methods of treatment described can be successfully adopted to bring down the BOD to acceptable standards. The choice of the method depends on the location of the mill and the availability of land. Necessary design criteria are available for construction and operation of the full scale plants.

The secondary treatment methods that can be followed consist of:

(a) Lagooning.

(b) Aerobic biological treatment including activated sludge process, trickling filters, aerated lagoons, oxidation ponds, and

(c) Anaerobic lagoon followed by aerobic stabilization pond for board mill effluents.

Colour bearing effluent, after suitable conditioning, should 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 effluent.

Two flow sheets for the treatment of effluents from pulp and paper mill are shown in Figs. 17.7 and 17.8.