Microbial remediation of waste from paper making and paper recycling industries
The waste from paper industry can be produced in solid, liquid and gaseous state. Liquid form is generally referred as waste water and its composition is highly dynamic depending upon the type of raw material/s and their physicochemical processing used in the making of paper. Waste water effluent from paper industries is one of the major environmental pollutants and therefore its remediation prior to the disposal in soil or aquatic ecosystems has now become a mandatory concern.
Composition of waste from paper making industry:
Preparation of raw material is the first step in the making of the paper. Agricultural byproducts like bagasse, straw, jute and wood are some of the raw materials used for production of pulp which is further processed to convert into the paper of desired quality. These raw materials are composed principally of cellulose and lignin fibres. Hence, cellulose, lignin, lignocelluloses, pectin, starch, phenolics and other complex carbohydrate derivatives contribute as major waste components. Fibres are separated from raw material by treatment with alkalis like sodium hydroxide or acids like sodium sulphite to produce the pulp. Bleaching or decoloration is done by chlorinated compounds and detergents to destroy lignin fibres and produce white bright print paper respectively. Bleached paper can also be colored using variety of chemical pigments/dyes. It is evident that every step of paper making from raw material processing to pulp formation is carried out by the application of different chemical agents. Treatment of raw material with alkali leads to offensive gaseous emissions containing hydrogen sulfide, mercaptans, sulfur dioxide, nitrogen oxides, thiols and particulate matter. Acid treated raw material processing generally emits sulfur oxides. Thermomechanical pulping units which are operated by using coal/fuel oils emit fly ash, sulfur and nitrogen oxides as chief pollutants. Waste water of alkaline pH, dark color and with varying degrees of COD and BOD is produced after alkali processing of the raw material. Waste water produced after bleaching usually contains silicates, carbonic acids, heavy metals, tannins, fatty acids and nonionic detergents/salts. In addition to this, waste water irrespective of composition of raw material always contains high concentration of phosphorus and nitrogen. Solid waste is generated as sludge, bark or straw fragments and major portion of ash produced during preparation of pulp.
Composition of waste from paper recycling industry:
Paper recycling industry is concerned with conversion of waste or old used papers to make cardboard sheets and other types of papers. Formation of pulp is also initial step of paper recycling process. Papers to be recycled are colored and written/printed with black ink; therefore chemical bleaching becomes mandatory step during the recycling process. Both chloride or peroxide/oxygen bleaching treatments are employed; but chlorination is the most effective treatment. Chloride bleaching generates waste water containing high amount of adsorbable organic halides which can be carcinogenic and mutagenic. Waste water also has high TSS, BOD and COD similarly like waste water from paper making industry. Solid waste consists of paper pins/clips, plastic wires and metal sheets/foils; such soild waste material can be filtrated which however increases total cost of processing during waste remediation. Pulp produced from used paper or paper to be recycled is also treated with whitening chemicals such as talc, calcium carbonate or kaolin. Organic filler compounds like latex, binders and various pigments/dyes for coloration are added to the finished product. These compounds being insoluble in nature contribute largely as xenobiotic recalcitrant constituents of waste water. In addition to this, waste water produced in paper making and paper recycling processes is an ideal environment for the growth of harmful microbes, especially the coliform group of bacteria.
Paper industry waste treatment by bioremediation:
Mechanical screening, sedimentation, floatation, incineration and filtration are some of the preliminary treatments which are given to the waste water from paper making and recycling processes prior to secondary treatments like bioremediation. Preliminary treatments help to remove solid waste while as gaseous emissions are removed by scrubbing or electrostatic filtration processes. Bioremediation methods that utilizes biological agents especially microorganisms has been one of the best ecofriendly techniques available today for treating pulp and paper industry wastes. Paper industry wastes are highly colored, odorous, enriched with N and P nutrients; contain heavy metals, chlorinated and acidic recalcitrant compounds; all of which are not easily remediated by activated sludge or oxidation pond treatments. However, some microbial species alone or in consortia have been found efficient in complete remediation of such waste. Many bacterial and cyanobacterial genera have been specifically developed and employed routinely for the biotreatment of paper industry effluents in controlled bioreactors or for direct on field applications. Bioremediation potential of these microorganisms is because of various degrading enzymes like ligninases, laccases, xylanases, pectinase, cellulases, peroxidases and peroxidases produced by them. Bacterial species of Alcaligenes, Azotobacter, Bacillus, Pseudomonas, Streptomyces, Klebsiella, Paenibacillus and cyanobacteria Phormidium, Pseudoanabaena and Chroococcus are important waste water degrading microbes. Parameters like denitrification, nitrogen fixation, phosphate removal, reduction in COD/BOD, color/odor and enhanced microbial growth are used to monitor progress of efficient process of bioremediation of waste water.
Not only bacteria but fungi, yeasts and algal species are also potential degraders of lignocellulose and other polymeric complex carbohydrates. Hence they have great potential to be used in bioremediation of paper industry waste. White rot fungi Phanerochaete, Gliocladium and Trametes have been exploited for their potential to deink and decolorize waste from paper mills. Fungal species of Aspergillus, Penicillium, Ganoderma, Rhizopus, Trichoderma, Fusarium, Pythium, Armillaria, Agaricus and yeasts Torulopsis and Rhodotorula are potential degraders of lignin, hemicellulose and cellulose compounds. Like white rot fungi, they are also very efficient decolorizers of waste water. In some case fungi and yeasts have been found more efficient and adventitious than bacteria. This efficiency is attributed to their ability to produce broad spectrum cellulolytic and ligninolytic enzymes, their ability to adsorb heavy metals, detoxify chlorinated compounds, ability to invade wood tissues and rapid growth. Like fungi and bacteria, microscopic diatoms and green algae such as Chlorella have been exploited for their waste water remediation potential. Algae have been found useful in decolorization and removal of halogenated (chlorinated) compounds. In this regard, their mixotrophic metabolism has been responsible the conversion of colored halogenated chemicals to decolored and nonhalogenated compounds. Bioremediated solid and water waste from paper industries have potential to be utilized as material for land filling and fertilizers respectively. Fertilizer value of bioremediated waste is in fact upgraded by nitrogen fixing activities of bacterial flora indigenous to waste water.
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