The main advantage of exploiting paper sludges as sources of
The main advantage of exploiting paper sludges as sources of cellulose-derived energy and chemicals in comparison to other lignocellulose substrates is their amenability, which is associated with an extensive pulping process that removes the majority of the lignin and exposes Lck Inhibitor sale fibers to enzymes. This results in substantial cost savings on energy for substrate pretreatment in comparison to other lignocellulose fuel production technologies (Gottumukkala et al., 2016). Studies on direct production of bioethanol from paper sludge have shown that tissue printed recycle sludge (a type of DIS) resulted in significantly lower ethanol yields when compared to corrugated recycle and virgin pulp mill sludges (Williams, 2017), so other possible ways for valorisation of this type of substrate need to be explored. One of the major limiting factors for bioconversion of cellulosic waste such as paper sludge to valuable products is the cost of the enzymes, as they are commercially produced using high cost feedstocks. Reducing the enzyme dosage per gram cellulose/feedstock has been a major research area for the past few years (Robus et al., 2016). Efficient transformation of cellulosic feedstock to value added products requires a mix of synergistically acting enzymes (CAZYmes) that are able to work at low dosages. White-rot fungi (WRF) are known to produce significant amounts of powerful extracellular oxidative and hydrolytic enzymes that degrade lignin and cellulose biopolymers (Manavalan et al., 2015). Major functional groups of glycoside hydrolases (cellulases and hemicellulases) produced by WRF involve endoglucanases (EG; EC 188.8.131.52), cellobiohydrolases (EC 184.108.40.206, EC 220.127.116.11), beta-glucosidases (18.104.22.168), endoxylanases (EX; EC 22.214.171.124), beta-xylosidases (EC 126.96.36.199) and alpha-glucuronidases (E.C. 188.8.131.52). On the other hand, lignin degradation enzyme systems are based on oxidative enzymes, such as lignin peroxidases (EC 184.108.40.206), manganese peroxidases (EC 220.127.116.11), versatile peroxidases (EC18.104.22.168), and laccases (EC 22.214.171.124) (Manavalan et al., 2015). Recently, oxidative enzymes, namely lytic polysaccharide monooxygenases, have also been shown to play an important role in the degradation of cellulose (Garajova et al., 2016). The composition of the enzyme mixtures produced on different substrates (ratios between different types of enzymes) reflects substrate composition (Elisashvili et al., 2008). The applications of fungal enzymes in paper industry involve biobleaching of pulp, pulp de-inking, degradation of dissolved and suspended organic compounds in concentrated effluents of mills and enhanced fibrillation. Enzymes usage is encouraged in paper industry in order to reduce the use of chemicals and energy consumption as well as to improve quality of the products (Bajpai, 2013). A desirable side-effect of processing paper mill waste by WRF enzymes is non-specific degradation of organic environmental pollutants, commonly present in these substrates (Kües, 2015). Most industrial enzymes currently produced by fungi are products of submerged fermentation (SF) which uses significant amounts of water and generates large quantities of liquid waste stream during the filtration process. Major cost contributor in commercial cellulase production is cost of the feedstock which is mostly glucose and accounts for 50% of the total process cost (Humbird et al., 2011). However, solid state fermentation (SSF) has emerged as an economically attractive alternative for the in situ production of lignocellulolytic enzymes due to lower energy consumption, direct use of low-cost lignocellulosic wastes as substrates, reduction in cost of dewatering in downstream processing, higher concentrations of enzymes and lower demand for the sterility of the equipments (Yoon et al., 2014). At present, the production of lignocellulolytic enzymes by SSF on several agricultural wastes has been reported, such as coffee pulp (Velázquez-Cedeño et al., 2002), spend brewery grains (Gregori et al., 2008), straw and fruit peels (Kurt and Buyukalaca, 2010), rice straw (Khalil et al., 2011) and tomato pomace (Iandolo et al., 2011). Conversely, there are few reports on enzyme production from paper sludge and no report specifically on fungal enzymes production from deinking paper sludge.