The pulp and paper industry is one of the core industrial sectors in world and play vital role in socio-economic development, while it is associated with significant environmental concern due to its large footprint on environmental resources. These paper mills generate large quantities of waste primary sludge every day in world to meet growing paper demands in India and abroad. Disposal of this solid waste has become a problem, as it causes abnormal increase in soil alkalinity and damage to vegetation in the vicinity of the dumping ground. Wastewater sludge from paper recycling mills contains high levels of heavy metals. Thus it cannot be used for land application as soil amendment and has to be disposed off to landfills, a prohibitively expensive and environmentally harmful end solution.
Waste paper industry sludge is believed to be one of the most promising feedstock for near-term commercial application of technology for converting cellulosic raw materials into commodity products. Therefore, utilizing this solid waste into novel value-added uses for this residue from the paper industry is alternative way to improve environmental and energy problem. Recycled paper sludge (RPS) is basically made up of secondary poor-quality non-recyclable paper fibres (fibres too short to be retained on fibre screens and paper machines). The high lignocellulosic content of this sludge material offers an opportunity as feedstock for bio-products. In fact, this substrate has some distinctive advantages among cellulosic feedstocks including negative cost at many locations and the potential availability of pre-existing facilities. The feasibility of biotechnological recovery of this potentially attractive substrate requires the conversion of all of its major components (cellulose and hemicellulose) to fermentable sugars, which could be further converted to fuels and chemicals, such as ethanol, organic acids or biodegradable plastics. Among these possible products, ethanol possesses a rapidly expanding market, either as an octane enhancer or primary fuel reducing the harmful effects of gasoline consumption.
Previous research has focused on using agricultural wastes like straw, and solid landfill wastes. These products are thought to be less "free" because they don't have much market value. However, there are costs associated with these "free" feedstocks. For example, there is the cost of transporting the material, and in some cases (such as straw, which is harvested once a year), stockpiling it. There is also a cost involved in separating the materials which contain cellulose from landfill waste. Some of this material would also have to be treated before it could be used. However, sludge from the pulp and paper industry could be considered a "free" feedstock because it is a waste product that would otherwise require disposal. It is generated on an ongoing basis, so there would be no need to stockpile it. The pulp and paper industry in India produces million tonnes of sludge each year. The year-round supply would also eliminate problems with spoilage, which could occur with agricultural products like straw. The wood fibres in pulp and paper sludge have also been pre-treated in the pulp-making process. Since the cellulose has already been partly broken down, the sludge could require less processing than other ethanol plant feedstocks. As according to literature pulp and paper sludge contains between 45 per cent to 70 per cent cellulose. That's the plant fibre which is converted to glucose for ethanol production. Straw is only about 30 per cent cellulose. In general, the researchers found that sludge is a cheaper option for ethanol production than material like straw. The process of making ethanol from pulp and paper sludge is similar to the process used in making ethanol from wheat.
Now at present scenario research work is focuses to evaluate the possibility of converting paper industry waste into valuable products. The approach is based on the enzymatic conversion of major sludge components (cellulose and xylan) and the simultaneous (simultaneous saccharification and fermentation--SSF) or sequential (separate hydrolysis and fermentation--SHF) fermentation of the resulting sugars to ethanol. Worldwide, a lot of research effort is spent on developing innovative biological conversion processes of biomass and improving existing biological conversion processes of biomass. The main focus of present research is to improve the technical performance and decrease the total costs of biological conversion processes, also including costs of pretreatment and post-treatment. A better overall performance may be possible by improvement of the pretreatment, focused on a better accessibility of the substrate component for microbiological conversion, application of new types of microorganisms, such as hyperthermophilic microorganisms or genetically modified microorganisms, improved reactor concepts, and better process control. Therefore, research in this direction is now hot topic for many graduate and doctoral students and become of interest for the biotechnological uses of large scale sewage sludge in the future.
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