Sustainability 2023 , 15 , 6915
8of 21
paper waste [105]. Anaerobic digestion of paper is one of the major waste management options since it is not economically burdening and offers environmental benefits, producing renewable energy in the form of biomethane. Food waste and paper waste can be digested anaerobically individually (mono-anaerobic digestion) or combined (co-digestion). Co- digestion provides a better yield of methane compared to mono-digestion. Thermophilic (50–60 ◦ C) and mesophilic (30–40 ◦ C) are the two temperature ranges of anaerobic digestion, with thermophilic being better due to high methane gas content, low hydrogen sulfide content, and high rate of organic matter degradation [106]. When waste is anaerobically digested, methane is obtained in a high percentage (50–70%), followed by carbon dioxide and other gases such as hydrogen. Cellulosic wastes generally do not produce high yields of biogases due to their poor biodegradability. To overcome this, paper waste can be co- digested with other organic wastes to increase gas flammability [107]. A study conducted by Priadi et al. (2014) showed that the anaerobic digestion of paper sludge seeded with cow manure produced a significantly high yield of methane (269 mL/g) compared to the anaer- obic digestion of paper sludge alone (14.7 mL/g). The treatment of wastewater originating from the paper industry results in large quantities of paper sludge containing chlorinated organics, pathogens, and traces of heavy metals [108]. Wastewater from industrial and municipal sources can be treated by several different anaerobic reactors, such as an up-flow anaerobic filter, modified anaerobic baffled reactor (MABR), or up-flow anaerobic sludge blanket (UASB). UASB is the most commonly used anaerobic reactor for municipal and industrial wastewater treatment since it is stable and consumes energy efficiently. Using this reactor, wastewater containing organic matter released from papermaking or recycling industries can be used to produce biomethane [109].
Figure 2. Conversion of paper waste into biohydrogen or biomethane using dark fermentation and anaerobic digestion, respectively. Paper waste often undergoes pretreatments before anaerobic digestion to obtain a higher yield of biomethane [110]. Mechanical pretreatments such as shredding have shown no increase in biomethane production. Biological pretreatment, on the other hand, which consists of treating different types of paper waste with a thermophilic cellulose-degrading consortium, increases biomethane yield [111]. Moreover, biogas production is dependent on several different factors, such as the volatile solid content of the feedstock and the level of biological activity in the digester. Carbon and nitrogen balance in feeding material also affects biogas production, with the optimal carbon and nitrogen balance ratio being between 25:1 and 30:1. In addition, environmental factors, including temperature and pH, determine the obtained biogas yield [112]. Furthermore, variations in the pulp production procedures can alter methane production in anaerobic digesters. Primary pulp obtained from Kraft and sulfite pulping provides a high yield of methane when anaerobically digested due to the efficient removal of lignin from the pulp in papermaking processes. Another advantage
Made with FlippingBook Digital Publishing Software