Elsevier

Waste Management

Volume 113, 15 July 2020, Pages 321-328
Waste Management

Circular economy in olive oil production – Olive mill solid waste to ethanol and heavy metal sorbent using microwave pretreatment

https://doi.org/10.1016/j.wasman.2020.06.017Get rights and content

Highlights

  • Microwave pretreatment better than conductive heating,

  • Acid and enzyme types are important for saccharification yield.

  • Microwave treatment with formic acid on OMSW showed highest saccharification rates.

  • The best ethanol yield was 9.1 g/ 100 g dry OMSW.

  • Hydrolysate solid remnants were effective for removal of Cu and Pb from water.

Abstract

Olive mill solid waste (OMSW) is an abundant agricultural waste without viable solution. The effects of OMSW different pretreatments (microwave or autoclave), different additives (water, formic, or sulfuric acid), and utilization of different saccharification enzymes (Cellic® CTec2 or Accellerase® 1500) were tested on saccharification process and sugar release, and on fermentation inhibitors formation and ethanol production. Microwave treatment with formic acid resulted in highest saccharification rates (90% of cellulose fraction) and fermentation yields (15.9 g/L ethanol), although loss of sugars and fermentation inhibitors production was notable. Microwave with water treatment resulted in less saccharification and ethanol (9.6 g/L). To facilitate economical process and to extract maximum value, solid remnants after saccharification were tested as heavy metal sorbent. Microwave with water resulted in the best sorbent, followed by microwave with formic acid. Addition of sulfuric acid, to either microwave or autoclave, resulted in very poor saccharification and absorbance capacity. Therefore, combination of ethanol and sorbent production from OMSW are suggested.

Introduction

Liquid fuel replacements such as ethanol are of global importance to help fulfill the energy demand due to the increasing population (Yilmaz and Atmanli, 2017). To-date, such biofuels are produced mainly from grown crops e.g. sugarcane, corn and grain starch (i.e. 1st generation ethanol), but production of ethanol from lignocellulose waste (2nd generation), such as agricultural waste is growing rapidly (Lin et al., 2012, Kumar et al., 2009). This pursue is of special importance for many Mediterranean countries where there is no enough land and water to grow specialized energy crops (Ayalon et al., 2008).

Olive oil production is an important agro-industrial sector in the Mediterranean region, with ~ 3.3 million olive growers (one-third of EU farmers) and average production of ~ 16 million metric tons of olives and more than 3 million tons of olive oil annually (International Olive Council, 2018). Olive oil extraction results in the production of a large amount of organic rich waste, where a one metric ton of olive oil using three-phase process resulting in average in ca 0.6 ton of olive mill solid waste (OMSW; i.e. 9.6 million metric tons annually) and 1.5 m3 of olive mill waste water (OMWW). Using the more modern two-phase process can reduce the amount of OMWW by 75%, but resulting in waste with much higher water content (Markou and Georgakakis, 2010).

Olive mill waste, especially the OMSW, is a notorious pollutant without a good solution (Morillo et al., 2009, Romero-García et al., 2014). Many valorization solutions were suggested, including composting and soil applications (Kavdir and Killi, 2008), cattle feed (Shabtay et al., 2009), methane production (Rincón et al., 2013), source of phenolic compounds (Aliakbarian et al., 2011), and biochar production (Abdelhadi et al., 2017), but to date none of them was found viable or economic (Azaizeh et al., 2020).

OMSW contain cellulose and hemicellulose (Abu Tayeh et al., 2016), suggesting it could be used as a feedstock for ethanol production. Furthermore, as it is pre-crushed as part of the oil extraction process, much less mechanical pretreatment is needed, making it a favorable waste. Indeed, such attempts were made (Abu Tayeh et al., 2014, Battista et al., 2016, Fernandes et al., 2016, Senkevich et al., 2012), but conversion efficiency was low due to the high lignin content, known to inhibit cellulase enzymes and making saccharification process inefficient (Berlin et al., 2006, Maobing et al., 2009, Ximenes et al., 2011).

Lignin in the lignocellulosic biomass such as OMSW can be removed by pretreatment. An effective pretreatment should minimize energy demands, be of low cost, limit formation of byproducts, increase cellulose accessibility to enzymes, and result in high concentration of fermentable sugars. Currently used pretreatment methods include physical (e.g. microwave, ultrasound), chemical (e.g. alkali, dilute acid, ozonolysis, organosolvents), physicochemical (e.g. steam explosion, hydrothermolysis and wet oxidation), or their combinations (Aguilar-Reynosa et al., 2017, Gabhane et al., 2014, Kumar and Sharma, 2017, Rosen et al., 2019 Peretz et al., 2020). When used on OMSW these pretreatment processes generated fermentation inhibitors. This was demonstrated by a dramatic improvement in fermentation process of OMSW after detoxification (Abu Tayeh et al., 2014, Alvira et al., 2010, Ballesteros et al., 2001, Cara et al., 2007, Felizón et al., 2000).

Microwave heating is considered energetically efficient process (Jones et al., 2002), with efficiency going up with volume and are easy to operate, even at large scale (Hoogenboom et al., 2009). Thus microwave pretreatment could improve energetic efficiency of biofuels production (Li et al., 2016). Indeed, microwave pretreatment of lignocellulosic materials have been shown to modify cellulose ultrastructure (Chen et al., 2011), degrade lignin and hemicellulose, and increase enzymatic susceptibility (Azuma et al., 1984, Ooshima et al., 1984), resulting in better sugar yields and ethanol production (Binod et al., 2012, Intanakul et al., 2003, Sasaki et al., 2011). Microwave based biomass pretreatment was demonstrated for different feedstock, including hardwoods and softwoods (Ooshima et al., 1984), sugarcane bagasse (Binod et al., 2012, Sasaki et al., 2011), rice straw (Zhu et al., 2005, Ma et al., 2009), wheat straw (Xu et al., 2011, Zhu et al., 2006), and switch-grass biomass (Hu and Wen, 2008).

Heavy metals (HMs) are a well-known contaminant of water worldwide, released to the environment from the industrial sector such as mining, textile, pulp and paper, fertilizers and plastic, and causing serious environmental pollution and source of concern (Abdolali et al. 2016). Different treatments to dilute or remove the HM pollutions in the wastewater, e.g. filtration, reverse osmosis, extraction, chemical precipitation, and others, were used, but these methods are expensive and are not effective in low concentrations. Many studies have been made to investigate the agro-industrial residue as biosorbents (Abdelhadi et al., 2017). The use of agricultural wastes as sorbents was previously suggested (Nguyen et al., 2013).

Ethanol production followed by HMs sorbent will support the circular economy model for olive oil industry sector. The gap in the OMSW management could be achieved through applying the circular economy model to this solid biomass.

The objectives of the current work were to test the use of microwave for the pretreatment of OMSW combined with using different additives and commercial enzymes, where microwave treatment time, production of fermentation inhibitors, saccharification efficiency, and ethanol yield were evaluated. In order to enhance economic viability and to obtain the maximum value of OMSW, we also tested the use of the hydrolysate solid remnants as HMs sorbent to remove HMs from water.

Section snippets

Raw material

Three-phase OMSW was collected from an olive mill (Iksal, Israel) during the 2015/2016 season. The OMSW was air dried in the shade, grinded, and sieved through an 18 mesh (i.e. 1 mm) to remove olive stone particles.

Composition of the feedstock

The composition of the OMSW was performed according to standard analytical procedure as described in NREL by Sluiter et al. (2008).

Pretreatment of OMSW

Schematic representation of the course of the pretreatments, experimental work and analysis were performed as shown in Fig. 1. Two alternative

Olive mill solid waste composition

The OMSW composition (%w/w) was: 20.0 ± 0.3 cellulose, 16.0 ± 0.2% hemicellulose, 37.5 ± 4.5% lignin (98% of acid insoluble), 19.9 ± 1.4% extractives, 2.1 ± 0.3% Acetyl, and 4.5 ± 1.6% ash dry basis.

Pretreatment

The different pretreatments and additives resulted in different sugars released and production of inhibitors and summarized in Fig. 2 and Table 1. Microwave treatment with SA resulted in the loss of 20% of the cellulose as glucose, while MW + FA resulted in lesser cellulose loss (~5%)(Fig. 2), in

Conclusion

OMSW is notorious agricultural waste without viable solution. Short MW + FA treatment resulted in efficient saccharification and ethanol production (90% of cellulose fraction hydrolysis and 91.5 mg ethanol per gram OMSW), followed by MW + DW. Solid remnants were effective for absorption of Cu and Pb from water without further treatment, especially after MW + DW pretreatment, followed by MW + FA. Sulfuric acid addition to either MW or Autoclave pretreatment resulted in the lowest sugar release,

Acknowledgements

This research was supported by grants from the Israeli Ministry of Science and Technology, grant number 3-13270 and the Israeli Ministry of Environmental Protection, project number 132-3-3. The authors also wish to thank Dr. Alon Silberbush, from the department of Biology and Environment University at Haifa-Oranim for his help in the statistical analysis, Mrs. Samya abdelhadi from the Galilee Society for the sorbent analysis, and Novozyme and Dupont for providing enzyme samples.

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