Research paperOvercoming yard waste recalcitrance through four different liquid hot water pretreatment techniques – Structural evolution, biogas production and energy balance
Graphical abstract
Introduction
World energy consumption which was 575 quadrillion British thermal units (Btu) in the year 2015 is expected to rise to 736 quadrillion Btu in 2040, an overall increase of 28%. This increase in energy demand is mainly attributed to the rising world population. As per the report by U.S. even in the year 2040, 77% of world energy demand will be met by fossil fuel alone [1]. It is estimated that world produces 1.3 billion tonnes of solid waste per year which is expected to rise up to 2.2 billion tonnes by 2025 [2]. Municipal solid waste (MSW) is a valuable renewable energy resource [3] especially lignocellulosic fraction of municipal solid wastes, such as yard waste (YW) which provides a potential renewable biomass resource for biological conversion by anaerobic digestion (AD) technology for biogas production [[4], [5], [6]].
Cellulose, hemicellulose and lignin are the three main structural fractions in lignocellulosic biomass. Higher cellulose content of YW makes it a suitable substrate for AD. However, the presence of lignin and hemicellulose act as a physical barrier in the biodegradation of substrate. In order to reduce the recalcitrance of lignocellulosic biomass, an effective and cost-efficient pretreatment method is necessary. In lignocellulosic biomass, the cellulose fibrils are protected from hydrolysis by lignin and this eventually results in incomplete hydrolysis followed by lower methane yield. Several approaches towards pretreatment like physical, chemical, biological, and combined pretreatment methods have been extensively investigated in the past decades in order to make the substrate accessible to bacteria [[7], [8], [9], [10]].
All the pretreatment methods have their own advantages and disadvantages [[11], [12], [13]], but liquid hot water (LHW) pretreatment is a widely adopted pretreatment method which has been extensively studied in batch scale level and has been successfully applied in pilot plants [12,13]. Comparing with the other pretreatment methods the biogas yield from LHW pretreatment has several advantages such as (1) low environmental impact, (2) lowest total capital cost, (3) no chemical requirement, (4) formation of lower concentration of inhibitors for the subsequent biological conversion, (5) has little impact on equipment corrosion and (6) has potential to dissolve hemicelluloses [14,15]. Additionally, LHW processing not only allows the recovery of most of the pentosans but also achieves nearly theoretical cellulose enzymatic digestibility.
In LHW pretreatment method, the pretreatment efficiency depends on water i.e. pH of water. The pH of the water is the function of temperature. For example, the pH of deionized water at 25 °C is 7.0 compared with a pH of 5.6 at 200 °C. In thermal pretreatment, water, i.e. hot water, acts like an acid. In LHW pretreatment, acidic water penetrates into the biomass cell structure claves O-acetyl group, substitute uronic acid and liberates other acid moieties from hemicellulose [16]. The release of these acids helps to hydrolyze the hemicellulose into simple monomeric sugars. In addition, water at high temperature also has a potential to hydrate cellulose, slightly remove lignin, enlarge the accessible surface area and ultimately improve biomass degradability by microbes. Hot air oven (HAO), hot water bath (HWB), autoclave and microwave (MW) are the most commonly used LHW pretreatment techniques.The basic principle of hydrolysis is different in all the above mentioned LHW pretreatment techniques. In HAO and HWB the heat is circulated in the medium by convection and then the heat is transferred to the reactor by conduction through air and water medium, respectively. In an autoclave pretreatment, heat is transferred to the reactor by a hot steam fluid under a high pressure condition. In MW, heat is delivered directly into the biomass through molecular interaction with an electromagnetic field using MW energy. The pretreatment effect is due to both the thermal and athermal effect. In thermal effect, the disruption of lignin and cell walls is because of the increase in temperature and subsequent pressure. In athermal effect, biomass is subjected to a varying electromagnetic field in MW, the dipoles are unable to follow the rapid reversals in the field, as a result of this phase lag; power is dissipated in the material and results in breakage of hydrogen bonds present in cell biomass. This breakage of hydrogen bonds may result in destruction of cell walls and cracking of cellulose chains and breakdown of the crystalline configuration of cellulose molecules, leading to enhanced hydrolysis [17].
These pretreatment techniques reported in literature over many years for YW (Table 1). However, some of the results are contradicting each other. Such as in the study of Li et al. [18] a decrease in methane production rate and total methane production by 18% and 12% was found for a MW pretreatment where as in the study of Pecorini et al. [19] an increase in methane yield by 8.5% was found at the optimum conditions. In order to improve the understanding, and reduce commercialization risk, the efficiency of different type of LHW pretreatment techniques is necessary. Previous studies [[18], [19], [20]] tried to compare the effect of LHW pretreatment. However, the comparison study wasn't performed for all the four types of LHW pretreatment techniques, pretreatment techniques weren't performed under identical conditions, pretreatment conditions weren't optimized over a wider range, and the energy balance of the pretreatment process wasn't addressed.
Therefore this study compares all the four different type of LHW pretreatment techniques effectiveness on YW, over a wider range and the energy balance of the pretreatment process is also addressed. Although thermal pretreatment has lots of advantages in terms of disinfection and solubilisation of organic substrates but high temperature or prolonged thermal pretreatment can unexpectedly release of a high total ammonia nitrogen and it may stimulate Mallaird reaction by forming complex recalcitrant or inhibitory substrates and ultimately hamper the biogas production [19,21]. Therefore, the temperature and time required for the different LHW pretreatment were carefully determined. The effect of pretreatment on biogas production and its co-relation with organic matter solubilisation, compositional changes, structural features, biomass crystallinity and chemical changes were discussed. In addition, an energy balance analysis was also conducted to suggest opportunities for practical application.
Section snippets
Yard waste collection, preparation and characterization
The mixed YW was collected from Indian Institute of Technology (IIT) Kharagpur campus, West Bengal, India. Contaminants such as plastics, metals, and other debris present in YW was manually separated and discarded. The YW was composed of grass (33% w/w), dry leaves (65% w/w) and wood chips (2% w/w). The feedstock was dried at ambient temperature to remove the undesired moisture content. The air-dried biomass was milled (4000 rpm, 2 min) using a Havells Mixer Grinder (Havells Marathon, 2200 W,
Hot air oven pretreatment
HAO pretreatment of YW was investigated under different temperature (70, 80, 90,100, 110, 120, 140, 170 and 200 °C) over a treatment duration of 45 min. In order to avoid the formation of complex and difficultly biodegradable substrates, and occurrence of unwanted pyrolysis reactions, pretreatment temperature values greater than 200 °C were not selected [40]. A trend of decrease in sCOD concentration from 208.6 mg/L to 175.6 mg/L with the increase in treatment temperature from 70 °C to 120 °C
Conclusion
Four different types of LHW pretreatment were compared in this study. Solubilisation of organic matter was highest in MW followed by HAO, autoclave and HWB. MW pretreatment achieved relatively the highest solubilisation of COD followed by HAO, autoclave and MW. Solubilisation of organic matter i.e. 41.8% of initial COD, increased with increase in temperature; at 140 °C for 100 s. In addition, subsequent AD revealed that MW pretreatment can significantly improve the methane yield. The BMP
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2022, Bioresource TechnologyCitation Excerpt :In particularly, the high carbon and cellulose content of yard waste made the feedstock an attractive renewable source for biomethane production via AD. Moreover, the yard waste has the ability to produce 328 ± 15 mL/g volatile solids (VS)added in batch AD and 431 mL/gVSadded in co-digestion with food waste (Panigrahi et al., 2019). In most developing countries, the lignocellulosic waste generated from institute campuses, agricultural fields, and municipality areas is openly burned or thrown without any treatment facility (Panigrahi and Dubey, 2019a; Sarsaiya et al., 2019).