Conversion of paper sludge to ethanol, II: process design and economic analysis

Simulation of ethanol recovery and economic analysis of pectin production on an industrial scale

Taking into account that the industrial processing of passion fruit generates significant amounts of waste (only the peels represent 51% of the total mass of the fruit), in the present study an economic analysis was conducted to evaluate industrial line viability for pectin extraction from passion fruit peels. Knowing that absolute ethanol (99.50% purity), used in the precipitation and washing steps, has a higher cost, a simulation of extractive distillation was performed using solvents ethylene glycol and glycerol, in the software Aspen Plus v.11, being possible to recover 99.63% of ethanol for both solvents. The results of the economic evaluation showed that the process using ethylene glycol has an advantage, mainly due to its higher profitability (1.13 times higher), lower production cost (94.86% of the price using glycerol), and a lower breakeven point (around 3% smaller). The financial indicators showed profitability and attractiveness for the implementation of this processing line.

Second Generation Bioethanol Production: On the Use of Pulp and Paper Industry Wastes as Feedstock

Due to the health and environment impacts of fossil fuels utilization, biofuels have been investigated as a potential alternative renewable source of energy. Bioethanol is currently the most produced biofuel, mainly of first generation, resulting in food-fuel competition. Second generation bioethanol is produced from lignocellulosic biomass, but a costly and difficult pretreatment is required. The pulp and paper industry has the biggest income of biomass for non-food-chain production, and, simultaneously generates a high amount of residues. According to the circular economy model, these residues, rich in monosaccharides, or even in polysaccharides besides lignin, can be utilized as a proper feedstock for second generation bioethanol production. Biorefineries can be integrated in the existing pulp and paper industrial plants by exploiting the high level of technology and also the infrastructures and logistics that are required to fractionate and handle woody biomass. This would contribute to the diversification of products and the increase of profitability of pulp and paper industry with additional environmental benefits. This work reviews the literature supporting the feasibility of producing ethanol from Kraft pulp, spent sulfite liquor, and pulp and paper sludge, presenting and discussing the practical attempt of biorefineries implementation in pulp and paper mills for bioethanol production.

Enzymatic treatment of deinking sludge - effect on fibre and drainage properties

This work focused on the enzymatic treatment of deinking sludge (DS), a waste stream arising from German deinked pulp mills. Three industrial DS samples were characterised with respect to chemical composition and fibre morphology. In this study, four cellulase enzyme preparations were tested under neutral pH conditions and one cellulase was identified as suitable for DS conditioning with no need of pH adjustment. Additionally, our approach shows that inorganic particles contained in the DS samples do not negatively affect cellulase activity, as illustrated by respectable hydrolysis yields of up to 61%. Enzymatic treatment of DS led to fibre shortening and fibre fines generation with increasing enzyme dose and incubation time. The dewaterability of enzyme-treated DS samples was tested and the results showed that high amounts of fibre fines enhanced filter cake consolidation during dewatering, resulting in higher cake solids. A 10-14 percentage point increase in cake solids was obtained depending upon the enzyme dose applied, the origin of the DS sample, and the exposure period. Through the enzymatic treatment the annually generated amount of DS in Germany could be reduced by up to 20%, which would mean considerable disposal cost savings.

Batch and fed-batch simultaneous saccharification and fermentation of primary sludge from pulp and paper mills

Primary sludge from a Portuguese pulp and paper mill, containing 60% of carbohydrates, and unbleached pulp (as reference material), with 93% of carbohydrates, were used to produce ethanol by simultaneous saccharification and fermentation (SSF). SSF was performed in batch or fed-batch conditions without the need of a pretreatment. Cellic^® CTec2 was the cellulolytic enzymatic complex used and Saccharomyces cerevisiae (baker's yeast or ATCC 26602 strain) or the thermotolerant yeast Kluyveromyces marxianus NCYC 1426 were employed. Primary sludge was successfully converted to ethanol and the best results in SSF efficiency were obtained with S. cerevisiae. An ethanol concentration of 22.7 g L^-1 was produced using a content of 50 g L^-1 of carbohydrates from primary sludge, in batch conditions, with a global conversion yield of 81% and a production rate of 0.94 g L^-1 h^-1. Fed-batch operation enabled higher solids content (total carbohydrate concentration of 200 g L^-1, equivalent to a consistency of 33%) and a reduction of three-quarters of cellulolytic enzyme load, leading to an ethanol concentration of 40.7 g L^-1, although with lower yield and productivity. Xylitol with a concentration up to 7 g L^-1 was also identified as by-product in the primary sludge bioconversion process.

Opportunities and prospects of biorefinery-based valorisation of pulp and paper sludge

The paper and pulp industry is one of the major industries that generate large amount of solid waste with high moisture content. Numerous opportunities exist for valorisation of waste paper sludge, although this review focuses on primary sludge with high cellulose content. The most mature options for paper sludge valorisation are fermentation, anaerobic digestion and pyrolysis. In this review, biochemical and thermal processes are considered individually and also as integrated biorefinery. The objective of integrated biorefinery is to reduce or avoid paper sludge disposal by landfilling, water reclamation and value addition. Assessment of selected processes for biorefinery varies from a detailed analysis of a single process to high level optimisation and integration of the processes, which allow the initial assessment and comparison of technologies. This data can be used to provide key stakeholders with a roadmap of technologies that can generate economic benefits, and reduce carbon wastage and pollution load.

Paper sludge (PS) to bioethanol: Evaluation of virgin and recycle mill sludge for low enzyme, high-solids fermentation

Paper sludge (PS) from the paper and pulp industry consists primarily of cellulose and ash and has significant potential for ethanol production. Thirty-seven PS samples from 11 South African paper and pulp mills exhibited large variation in chemical composition and resulting ethanol production. Simultaneous saccharification and fermentation (SSF) of PS in fed-batch culture was investigated at high solid loadings and low enzyme dosages. Water holding capacity and viscosity of the PS influenced ethanol production at elevated solid loadings of PS. High viscosity of PS from virgin pulp mills restricted the solid loading to 18% (w/w) at an enzyme dosage of 20 FPU/gram dry PS (gdPS), whereas an optimal solid loading of 27% (w/w) was achieved with corrugated recycle mill PS at 11 FPU/gdPS. Ethanol concentration and yield of virgin pulp and corrugated recycle PS were 34.2g/L at 66.9% and 45.5 g/L at 78.2%, respectively.

Bioconversion of kraft paper mill sludges to ethanol by SSF and SSCF

Paper mill sludge is a solid waste material composed of pulp residues and ash generated from pulping and paper making processes. The carbohydrate portion of the sludge has chemical and physical characteristics similar to pulp. Because of its high carbohydrate content and well-dispersed structure, the sludges can be biologically converted to value-added products without pretreatment. In this study, two different types of paper mill sludges, primary sludge and recycle sludge, were evaluated as a feedstock for bioconversion to ethanol. The sludges were first subjected to enzymatic conversion to sugars by commercial cellulase enzymes. The enzymatic conversion was inefficient because of interference by ash in the sludges with the enzymatic reaction. The main cause was that the pH level is dictated by CaCO3 in ash, which is two units higher than the pH optimum of cellulase. To alleviate this problem, simultaneous saccharification and cofermentation (SSCF) using cellulase (Spezyme CP) and recombinant Escherichia coli (ATCC-55124), and simultaneous saccharification and fermentation (SSF) using cellulase and Saccharomyces cerevisiae (ATCC-200062) were applied to the sludges without any pretreatment. Ethanol yields of 75-81% of the theoretical maximum were obtained from the SSCF on the basis of total carbohydrates. The yield from the SSF was also found to be in the range of 74-80% on the basis of glucan. The SSCF and SSF proceeded under stable condition with the pH staying near 5.0, close to the optimum for cellulase. Decrease of pH occurred due to carbonic acid and other organic acids formed during fermentation. The ash was partially neutralized by the acids produced from the SSCF and SSF and acted as a buffer to stabilize the pH during fermentation. When the SSF and SSCF were operated in fed-batch mode, the ethanol concentration in the broth increased from 25.5 and 32.6 g/L (single feed) to 45 and 42 g/L, respectively. The ethanol concentration was limited by the tolerance of the microorganism in the case of SSCF. The ethanol yield in fed-batch operation decreased to 68% for SSCF and 70% for SSF. The high-solids condition in the bioreactor appears to create adverse effects on the cellulase reaction.

Production of lactic acid from paper sludge using acid-tolerant, thermophilic Bacillus coagulan strains

Production of lactic acid from paper sludge was studied using thermophilic Bacillus coagulan strains 36D1 and P4-102B. More than 80% of lactic acid yield and more than 87% of cellulose conversion were achieved using both strains without any pH control due to the buffering effect of CaCO(3) in paper sludge. The addition of CaCO(3) as the buffering reagent in rich medium increased lactic acid yield but had little effect on cellulose conversion; when lean medium was utilized, the addition of CaCO(3) had little effect on either cellulose conversion or lactic acid yield. Lowering the fermentation temperature lowered lactic acid yield but increased cellulose conversion. Semi-continuous simultaneous saccharification and co-fermentation (SSCF) using medium containing 100 g/L cellulose equivalent paper sludge without pH control was carried out in serum bottles for up to 1000 h. When rich medium was utilized, the average lactic acid concentrations in steady state for strains 36D1 and P4-102B were 92 g/L and 91.7 g/L, respectively, and lactic acid yields were 77% and 78%. The average lactic acid concentrations produced using semi-continuous SSCF with lean medium were 77.5 g/L and 77.0 g/L for strains 36D1 and P4-102B, respectively, and lactic acid yields were 72% and 75%. The productivities at steady state were 0.96 g/L/h and 0.82 g/L/h for both strains in rich medium and lean medium, respectively. Our data support that B. coagulan strains 36D1 and P4-102B are promising for converting paper sludge to lactic acid via SSCF.

The potential of cellulases and cellulosomes for cellulosic waste management

Lignocellulose is the most abundant plant cell wall component of the biosphere and the most voluminous waste produced by our society. Fortunately, it is not toxic or directly harmful, but our major waste disposal facilities--the landfills--are rapidly filling up with few realistic alternatives. Because cellulose is pure glucose, its conversion to fine products or fuels has remained a romantic and popular notion; however, the heterogeneous and recalcitrant nature of cellulosic waste presents a major obstacle for conventional conversion processes. One paradigm for the conversion of biomass to products in nature relies on a multienzyme complex, the cellulosome. Microbes that produce cellulosomes convert lignocelluose to microbial cell mass and products (e.g. ethanol) simultaneously. The combination of designer cellulosomes with novel production concepts could in the future provide the breakthroughs necessary for economical conversion of cellulosic biomass to biofuels.