8 Waste water characteristics of pulp and paper industry

J.S. Laura

epgp books

 

 

 

Pulp and paper industry

 

9.1 Introduction

 

The pulp and paper industry is among India’s oldest and core industrial sector. The first paper mill in India was set up long back in 1812 at Seerampur (West Bengal). India is now 15th largest pulp and paper producing industry in the world, with top positions occupied by China and USA. Annually about 5 million people use about 400 million tonnes of paper and paperboard, globally. The key social objective of the Indian government that is, eradication of illiteracy through compulsory primary education have direct relation with the paper industry. So it is an important socio-economical industrial sector, but at the same time, is a highly capital, energy and water intensive industry. It is also highly polluting and requires considerable investments in pollution control equipment. Because of the serious nature of pollution, the pulp and paper industry in India has been included in the 17 categories of highly polluting industries. In India, the industry is classified into three categories based on its production capacity: Large scale (Greater than 33,000 tonnes per annum (TPA)), Medium scale (between 10,000 and 33,000 TPA) and Small scale (Less than 10,000 TPA).

 

Paper manufacturing process and chemical used

 

Paper production broadly involves two processes-first raw material is converted into pulp, second the pulp is converted into paper. Pulp may be produced from virgin fibre or from recycled paper. Wood is the main original raw material, but rags, flax, cotton linters, and bagasse are also used in some papers. The harvested wood is first processed so that the fibres are separated from the unusable fraction of the wood, the lignin. Pulp making can be done mechanically or chemically. The pulp is then bleached and further processed, depending on the type and grade of paper that is to be produced. In the paper factory, the pulp is dried and pressed to produce paper sheets.

 

Paper manufacturing involves the following steps-

 

  1. Raw material preparation: Wood is first cut into suitable lengths, debarked and chipped. The bark is used as fuel or sold outside. Wood chips are used as man product for paper making.
  1. Pulping: Wood is mainly composed of cellulose, hemicelluloses and lignin. Cellulose, which constitutes about 50 % of wood dry weight, is used in papermaking. Lignin is cementing material which keeps the wood fibres together. Pulping is done in order to separate the individual cellulose fibres from ligin in wood chips and to remove the other constituents of wood (like waxes and oleoresins) which can cause discoloration or disintegration of the paper. Hemicellulose plays an important role in fibre-to-fibre bonding in papermaking. Pulping is of two types-
    1. Chemical pulping: In this step raw material (wood chips) are cooked in aqueous chemical solutions at elevated temperature and pressure to extract pulp fibres. Kraft pulping is widely used chemical pulping process, which involves solution of sodium hydroxide and sodium sulphide as cooking liquor. Other method of chemical pulping is sulphite pulping in which acidic mixture of sulphurous acid and bisulphite ion is used. For preparing sulphite cooking liquors, cooled sulfur dioxide (SO2) gas is mixed with water containing any of magnesium, ammonia, sodium, or calcium chemical bases. Sulphite pulps are not stronger, but can be bleached more easily than kraft pulps because of having less color.

     

    1. Mechanical pulping: In this method fibres are separated by grinding and abrasion action alond with temperature. Mechanical pulps are weaker than chemical pulp, but it is cheaper and high yielding process.

     

    1. Semi-chemical pulping: In this wood chips are first softened with chemicals, steam

     

    and heat, and then pulping is completed mechanically. Chemicals used in this process include sodium carbonate, sodium hydroxide and sodium sulphate.

     

    In regions where wood fibres are less abundant (like in Asia & in Africa), non wood fibres such as agricultural fibres (straw), bamboo, bagasse etc are used. Non-wood plants usually contain higher amounts of silica than wood, which causes problems in chemical recovery. Silica reduces the conversion of lime mud (CaCO3) to calcium oxide, so higher lime mud is discharged by pulping plant. Recycled paper contributes to about 50% of raw material for paper industry. Pulping of recycled paper and paperboard is done mainly by mechanical means. Pulp yield of this process is 60-70%, so it contributes to more waste products.

     

    1. Pulp washing and screening: In this step pulp is washed to recover pulping liquor, to remove other unwanted contaminants and to obtain clean pulp product. Now uncooked wood and un-separated fibres are removed by screening.

     

    1. Bleaching: Unbleached pulp is used to make paperboard, grocery bags etc. Bleaching is done to increase the brightness and softness of pulp to produce high quality paper. Various bleaching agents include hypochlorite, chlorine (Cl2), chlorine dioxide (ClO2),
      1. Paper making: Now from bleached pulp paper is prepared through a series of processes. Various chemical additives required in this step include sizing agents, water repellents, acids, bases, strength and stiffness additives, fillers and agents for gloss, brightness and opacity (kaolin, TiO2) etc.Chemical recovery: This step involves the recovery of chemicals from spent liquor to make the process economic and environmental benign. The spent cooking liquor from pulp mills containing wood lignin, organic materials, oxidized inorganic compoundshydrogen peroxide and ozone (O3). Bleaching agents are added to the pulp in stages. Between each stage, spent bleaching chemicals are removed in the washers. Washer effluent is either re-used in other stages as wash water or sent to wastewater treatment.(sodium sulphate, sodium carbonate), is known as “weak black liquor”. The spent cooking liquor containing sodium sulphide and sodium hydroxide is known as white liquor. The chemical recovery process involves concentrating the spent liquor, combusting organic compounds, reducing inorganic compounds, and reconstituting the cooking liquor.

     

     

    Effluent generation

     

    Pulp and paper industry uses large amount of water and generate huge amount of solid, liquid and gaseous pollutants. The quantity of water consumed differs according to the type of pulping and quality of paper produced. In India, around 905.8 million m3 of water is consumed and around 695.7 million m3 of wastewater is discharged annually by this sector. Water utilization efficiency of Indian pulp and paper industry is very low as compared to global best. The global best specific water consumption is 28.66 m3 /tonne for large scale wood based pulp and paper mill while that of Indian mills average is 150 – 200 m3 /tonne of product. This is because of use of outdated technology/equipments and poor water management practices. The quality of wastewater generated from pulp and paper manufacturing vary significantly because of the process and chemical types used. Pollutant generating processes at different stages of paper making is given in the table (9.1).

 

 

 

From pulp and paper industry effluent is produced, on an average, at a rate of 20–250 m3/tonne of dried pulp which consists of wood debris and various pollutants.

 

Types of Pollutants

 

The typical pollutants found in wastewater of pulp & paper industry are discussed below:

 

  1. Suspended Solids (SS): Suspended solids come from wood fibres, bark particles, dirt, pigments and from losses and remains of fillers, process additives and coating substances used in paper making. On deposition on the bed of water body, anaerobic decomposition of organic SS releases nuisance gas H2S and BOD. Higher supplies of SS are dangerous for fish and other aquatic life.

 

  1. Biochemical Oxygen Demand (BOD): BOD in paper mill effluent is mainly contributed by wood debris which adds wood sugars and lignin to the water. Lignin makes the major part in wastewater from pulping plant, and it degrades very slowly, so it is responsible for long term BOD. Release of large quantities of effluents with higher BOD causes depletion of naturalFrom pulp and paper industry effluent is produced, on an average, at a rate of 20–250 m3/tonne of dried pulp which consists of wood debris and various pollutants.

 

Types of Pollutants

 

The typical pollutants found in wastewater of pulp & paper industry are discussed below:

 

  1. Suspended Solids (SS): Suspended solids come from wood fibres, bark particles, dirt, pigments and from losses and remains of fillers, process additives and coating substances used in paper making. On deposition on the bed of water body, anaerobic decomposition of organic SS releases nuisance gas H2S and BOD. Higher supplies of SS are dangerous for fish and other aquatic life.

 

  1. Biochemical Oxygen Demand (BOD): BOD in paper mill effluent is mainly contributed by wood debris which adds wood sugars and lignin to the water. Lignin makes the major part in wastewater from pulping plant, and it degrades very slowly, so it is responsible for long term BOD. Release of large quantities of effluents with higher BOD causes depletion of naturaldissolved oxygen in water. If the oxygen regeneration capacity of water body is overtaken, anaerobic conditions can lead to the emission of olfactory nuisances and the destruction of the aerobic fauna, including fish.
    1. Toxicity: Pulp & paper industry effluent is mixture of large number of chemicals. In aquatic environment these undergo chemical reactions and produces complex system of chemicals. Most of the toxic pollutants in waste water from pulp and paper industry come from its bleach plant. More than half of the bleached compounds of pulp mills are toxic chlorinated organic compounds such as chlorinated phenolics, chlorinated resin acids, furans and dioxins. Dioxins and furans are very toxic, non biodegradable and carcinogenic. Bleached kraft mill effluent affect the benthic invertebrates and responsible for high incidence of fish diseases and mutagenic effects on the aquatic fauna. Bleaching effluents have potential to impair the functions of liver, enzyme systems and metabolic cycles in the exposed fish. Low molecular weight fraction of bleach effluent can penetrate cell membranes and tends to bioaccumulate. The other chemicals found in paper and pulp effluent include acids, esters, aldehydes, pyrenes, aldehyde, ketones, hydrocarbons, VOCs (terpenes, alcohols, phenols, methanol, acetone), chloroform, sulphites, chlorolignin etc. The harmful effects of these pollutants are of ecological concern. In some papermaking processes Nonyl phenol ethoxylates (NPE) is used as non-ionic surfactants. It breaks down to nonylphenol, which is a suspected endocrine disrupter.

     

    1. Temperature: Pulp and paper plants involve some heating stages like pulping, deinking etc. Extra heat is discharged in wastewater. Increase in water body temperature by heated effluents can produce various effects on aquatic organisms. Increase in temperature involves the depletion of dissolved oxygen, which can affect the aquatic organisms. If the temperature increase is too great (e.g. in case of smaller water body), the affected fish can die.

     

    1. Colour, taste and odour: The effluent from bleach kraft mill is a complex mixture of chlorinated and non- chlorinated products of lignin and/or wood extractives that imparts it dark colour. Coloured effluent may result in the adverse effects upon the receiving water body. Lignin derived colour, is an indicator of the presence of potentially inhibiting compounds. Colour-imparting compounds can form complexes with metal ions (such as iron or copper) and form tar like residues, which have inhibitory effects on some of the lower organisms in the food chain. Colour bodies exert long term BOD that cannot be measured in terms of usual 5-day BOD. It increases the cost and difficulty in water treatment. Some compounds can give water an undesirable taste and odour.
  2. Characteristics of effluent from different stages of paper mill are given in table (9.2).

Typically, effluents from agro based paper mills have higher levels of SS, BOD and COD, as compared to that from wood based mills. Waste-paper based mills discharge lower levels of SS, BOD, COD and wastewater as compared to both of above two types of mills. Because of high level of pollutants in the effluents of pulp and paper industry and potential threat to the ecology of receiving water body, CPCB has laid down the standards for discharge of industrial effluents into the inland water bodies. These are given below in table (9.3).

Waste water treatment strategies

 

Pulp and paper industry utilize a wide range of treatment strategies for its wastewater treatment.

 

These are briefly being discussed here.

 

  1. Primary Treatment: The aim of primary treatment is to remove the suspended solid such as wood debris and fibres, bark particles, filler and coating materials etc. This is mainly achieved by screening, settling and sedimentation, which approximately removes 70-80% of suspended solids. Flotation and filtration are the other methods used to remove suspended solids from effluent of pulp and paper mills.
  1. Secondary Treatment: Secondary treatment also known as biological treatment, deals mainly with organic matter. Some common biological treatment applications for pulp and paper mills are as follows.
  1. Activated sludge systems: This system is conventionally used in treatment of wastewater in order to remove COD, BOD, SS, and AOX. In treatment of paper and pulp effluent it can be used to remove BOD, COD, resin, AOX, chlorinated phenols etc. But there are some operational problems with activated sludge system, for example limitation of nutrients (N and P) in pulp and paper effluents necessary for the growth of microorganisms.
  1. Aerated lagoons: Aerated lagoons or stabilization basins are the simple and economical biological systems. These systems have been used for removal of BOD, COD, low-molecular weight AOX and fatty acids at full-scale applications.

 

  1. Anaerobic treatment processes: For treatment of high strength wastewater such as from pulp and paper industry, anaerobic treatment process is more suitable. This is because anaerobic microorganisms are more efficient than aerobics to degrade the chlorinated organic compounds. A conventional pulp and paper mill produces 1.7-1.8 tons dry solid of black liquor per ton produced pulp and the potential energy of this liquor from anaerobic digestion is 250-500 MW. Main disadvantages for application of anaerobic systems presence of sulphur content in the wastewaters. Sulphate undergoes anaerobic biodegradation and produces hydrogen sulphide, which is another toxic, nuisance and corrosive gaseous compound.
  1. Fungal treatment: Some fungal species have also been used to remove the BOD, COD, lignin and colour from the pulp and paper effluents.
  1. Tertiary Treatment: Tertiary and advanced waste water treatment is used to remove specific waste water constituents that cannot be removed by secondary treatment. Different treatment processes are necessary to remove nitrogen, phosphorus, suspended solids, refractory organics and/or dissolved solids. Some of the tertiary treatment processes are given below.
  1. Coagulation/Precipitation: In this method metal salts are used to generate larger flocs from small particles for removing the pollutants. Some chemicals studied to remove AOX, COD, organic carbon and colour are chitosan, Al2(SO4)3, hexamethylene diamine epichlorohydrin polycondensate (HE), polyethyleneimine (PEI), alum salts and polyelectrolytes etc.
  1. Adsorption: This method uses the addition of an adsorbant such as activated carbon, activated coke, coal ash, fuller’s earth and activated charcoal to the wastewater to remove the pollutants like colour, AOX and COD.
  1. Chemical Oxidation: In this method destruction of pollutants is done by advanced oxidation methods such as photocatalysis, photo-oxidation, wet oxidation, ozonation etc. in pulp and paper mills.
  1. Membrane Filtration: This method have ability to remove colour, COD, AOX, salts, heavy metals and total dissolved solids from wastewater of pulp and paper mills. About 88% and 89% removal of BOD and COD respectively can be achieved by reverse osmosis (RO). After membrane filtration effluent can be used again in production process or discharge directly to the receiving water bodies
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References

 

  1. Stefen, Robertson and Kisten, 1990. Water and wastewater management in the paper and pulp industry. National Industrial water and waste-water Survey. ISBN-0947447830.
  2. Maminsky, R.T., 2017. Limits and perspectives of pulp and paper industry wastewater treatment- A review. Renewable and Sustainable Energy Reviews, 78:764-772.
  3. Kamali, M., Khodaparast, Z., 2015. Review on recent developments on pulp and paper mill wastewater treatment. Ecotoxicol Environ. Saf., 114:326-342.
  4. Tezel, U., Guven, E., Erguder, T.H., Demirer, G.N., 2001. Sequential (anaerobic/aerobic) biological treatment of Dalaman SEKA Pulp and Paper Industry effluent. Waste Management, 21:717-724.
  5. Astrafi, O., Yerushalmi, L., Haghighat, F., 2015. Wastewater treatment in the pulp-and-paper industry: A review of treatment processes and the associated greenhouse gas emission. Journal of Environmental Management, 158 :146-157.