Biohythane production from food processing wastes – Challenges and perspectives

Journal Pre-proofs Review Biohythane production from food processing wastes - challenges and perspectives Ramakrishnan Anu Alias Meena, J. Rajesh Banu, R. Yukesh Kannah, K.N. Yogalakshmi, Gopalakrishnan Kumar PII: DOI: Reference:

S0960-8524(19)31679-7 https://doi.org/10.1016/j.biortech.2019.122449 BITE 122449

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Bioresource Technology

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30 September 2019 16 November 2019 18 November 2019

Please cite this article as: Anu Alias Meena, R., Rajesh Banu, J., Yukesh Kannah, R., Yogalakshmi, K.N., Kumar, G., Biohythane production from food processing wastes - challenges and perspectives, Bioresource Technology (2019), doi: https://doi.org/10.1016/j.biortech.2019.122449

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Biohythane production from food processing wastes - challenges and perspectives Ramakrishnan Anu Alias Meenaa, Rajesh Banu J b, Yukesh Kannah Rb, Yogalakshmi K Nc, Gopalakrishnan Kumar d* Department of Environmental Sciences, Bharathiar University, Coimbatore, India of Civil Engineering, Anna University Regional Campus Tirunelveli, India c Department of Environmental Science and Technology, School of Environment and Earth Sciences, Central University of Punjab, Bathinda 151001, Punjab, India d*Green Processing, Bioremediation and Alternative Energies Research Group, Faculty of Environment and Labour Safety, Ton Duc Thang University, Ho Chi Minh City, Viet Nam. a

b Department

Abstract The food industry generates enormous quantity of food waste (FW) either directly or indirectly including the processing sector, which turned into biofuels for waste remediation. Six types of food processing wastes (FPW) such as oil, fruit and vegetable, dairy, brewery, livestock and finally agriculture based materials that get treated via dark fermentation/anaerobic digestion has been discussed. Production of both hydrogen and methane is daunting for oil, fruit and vegetable processing wastes because of the presence of polyphenols and essential oils. Moreover, acidic pH and high protein are the reasons for increased concentration of ammonia and accumulation of volatile fatty acids in FPW, especially in dairy, brewery, and livestock waste streams. Moreover, the review brought to forefront the enhancing methods, (pretreatment and co-digestion) operational, and environmental parameters that can influence the production of biohythane. Finally, the nature of feedstock’s role in achieving successful circular bio economy is also highlighted. Keywords: biohythane; food processing wastes; anaerobic fermentation; methane; liquid fertilizer. d*Corresponding

author at: Dr. Gopalakrishnan Kumar: Green Processing, Bioremediation and Alternative Energies

Research Group, Faculty of Environment and Labour Safety, Ton Duc Thang University, Ho Chi Minh City, Viet Nam. Email Id: [email protected]

1

1.

Introduction Unlimited use of fossil fuels linked with environmental pollution has revealed the identity of

biomass derived renewable energy as future energy carrier (Alexandropoulou et al., 2018; Antonopoulou et al., 2008). Among the biofuels, biohydrogen (H2) is pinpointed as non-carbon and pollution free fuel which has high energy yield (122 kJ g-1) (Castelló et al., 2009). Subsequent to H2, biomethane (CH4) containing biogas generated from various waste biomass is a promising strategy for waste treatment and energy generation (Comino et al., 2012; Meena et al., 2019). Switchover of existing anaerobic digestion (AD) to a two stage process, which leads to the production of H2 in the first stage, followed by the production of CH4 in the second stage is receiving greater interest in recent years (Dahiya and Joseph, 2015). The resultant gases are applied separately or blended in such a way that it results in biohythane, a more advanced biofuel with the maximum percentage of CH4 (60%), followed by 30% carbon-di-oxide (CO2), and 10% of H2. Hydrogen occupies the least proportion of the biohythane gas (Cavinato et al., 2011). In the two-stage process, H2 is produced via dark fermentation (DF). The DF process is done in the absence of light and hence holds a prime biotechnology key (Kumar et al., 2016a). Multiple factors such as wide range of biomass, comparatively higher production rate, and its robustness in association with other technologies like photo-fermentation and methanogenesis, have made the DF process a viable approach (Ghimire et al., 2015a). Likewise the AD process which constitutes the second stage is really good at transforming organic wastes, especially food related wastes into biogas composed mainly of CH4, CO2, and liquid fertilizer (Tampio et al., 2016). DF and AD process plays a major role in treating biopolymer rich food processing waste (FPW) (Agyeman and Tao, 2014). The FPW is classified into expired solid baby foods, fruit and vegetable processing waste, oil processing waste, dairy processing waste brewery processing waste and livestock waste. The above listed FPW have different amount of protein and lipid content, but they show a similarity in their carbohydrate content, which paved the way for energy generation (Van Ginkel et al., 2005). Fig. 1 represents the composition of different FPW. In order to improve the energy yield and anaerobic biodegradability of substrate, many 2

researchers have suggest co-digestion and pretreatment techniques. Co-digestion of FPW satisfies the nutrient requirement for cost effective biohythane production (Dareioti and Kornaros, 2015; Sen et al., 2016). To overcome the complex nature of organic wastes, shift in pretreatment technique made H2 yield and rate easier by wiping H2 consuming bacteria(Costa et al., 2012). Longer retention time and low-acidic condition may favour methanogenic activity in DF and it leads to production of low hydrogen in DF. To overcome this pretreatment of substrate and inoculum is carried out. Among the pretreatment methods most promising are thermal and alkaline. Thermal pretreatment is easier and quicker compared to other techniques and is capable of deactivate the H2 consuming bacteria (Chavadej et al., 2019). Alkaline pretreatment of ensiled sorghum with NaOH and KOH prevents the methanogenic activity and thus fuel the rise of H2 level in digesters (Sambusiti et al., 2012). The biohythane production from food waste (FW) and FPW is governed by factors such as pH, temperature, hydraulic retention time (HRT), organic loading rate (OLR), reactor configuration, and microbial population. pH is having profound impacts on metabolic pathway of DF that extends to the hydrogenase activity of iron containing enzyme (Venetsaneas et al., 2009). Temperature during the reactions is tied to disintegration of feedstock, formation of metabolites, growth of the microbial population, and related enzyme activity which finally leads to the production of H2 and CH4. HRT which is otherwise defined as the duration which makes the amount of biomass widely available in the bioreactor over time fixes the economic cost of the process (Vo et al., 2019). OLR is often raised as an indicator to detect the conversion rate of biomass in digester system (Ferguson et al., 2016; Rico et al., 2015). Continuous exposure to specific micro flora (either pure or mixed cultures) in the digestion process not only raises the metabolites but also significantly increases the production of H2 and CH4 production (Karthikeyan et al., 2016). The bioreactor systems and its operation modes are selected to suit the nature of substrates and the microbial inoculum (Show et al., 2011). Biohythane production from FW and FPW are inhibited due to excess accumulation of volatile fatty acid (VFA) and ammonia in the bioreactor. VFAs such as acetic, butyric, propionic, caproic, formic, and valeric acids are nothing but intermediate formations which would be finally transformed into H2 and CH4. 3

Accumulation of VFAs in the left digesters has recorded low H2 levels and an imbalance between H2 producing and CH4 producing microbes (Atasoy et al., 2018; Khan et al., 2016). Although the microbial inoculum in the digestion process needed ammonia for their growth, the complex nature of FPW that is rich in protein results in ammonia generation within the system. Ammonia inhibits the total CH4 production and ends up in the accumulation of VFA (Yenigün and Demirel, 2013). Even though accumulation of VFA in the methanogenic reactor results in poor CH4 yield, it offer another value added by-product such as polyhydroxyalkanoates (PHA). PHA’s are labelled as eco-friendly materials (Amulya et al., 2014). PHA’s teeming with thermoplastic properties, draws attention in industrial uses including the medical field (Dahiya et al., 2018). Researchers revealed that PHA is a stored energy material and is produced by mixed cultures of bacteria under feast and famine conditions. The pathway of polyhydroxybutyrate (PHB) is mediated by three enzymes and their sequence of reaction mechanisms. Firstly, the enzyme, acetyl-CoA acetyl transferase catalyzes the condensation of two molecules of acetyl CoA (from Krebs cycle) into acetoacetyl – CoA. Secondly, the sysnthesized acetoacetyl –CoA undergoes reduction to form (R) – 3 – hydroxybutyryl – CoA. This reaction is catalyzed by the enzyme, NADPH dependent acetoacetyl – CoA reductase. At last monomers of (R) – 3 – hydroxybutyryl – CoA undergoes polymerization of form PHB. This reaction is mediated by the enzyme polyhydroxyalkanoate (PHA) synthase. Circular economy is a preferable strategy to promote the usage of resources as raw materials and products, boosting the maximum exploitation of resources and signaling minimum quantity of waste generation (Venkata Mohan et al., 2016). A series of bioprocesses, viz., acidogenesis and methanogenesis that may replace linear economy against circular bioeconomy hints at surprising biorefinery abilities in treating FPW (Dahiya et al., 2018). FPW has the ability to produce more biofuels and cut the exploitation of fossil fuels in response to GHG (greenhouse gases) emissions that contribute to biological sequestration and climate change (Amulya et al., 2015). Hence, the closedloop system of circular bioeconomy can be used as a tool to ease the reduction of FPW and deal with associated remediation in future (Venkata Mohan et al., 2016). Reviews reported so far demonstrated 4

both the DF and AD process (Bolzonella et al., 2018; Sen et al., 2016) and photo-fermentation (Bharathiraja et al., 2016) for the production of biohythane (H2 and CH4) from varied sources such as the FW, household food waste, and kitchen waste. Certain reviews also dealt with the production of either H2 or CH4 from FW. In addition to this review is also focuses on the production liquid fertilizer along with biohythane production. 2.

Category of FPW as feed stocks

2.1.

Oil type processing wastes Even though liquid olive mill wastes (OMW) is uniform (dark) in color, waste coming from

three phase centrifugation process is termed as olive mill wastewater unlike the two phase centrifugation process termed as olive pulp (Dareioti and Kornaros, 2014; Koutrouli et al., 2009). The OMW with levels of biochemical oxygen demand (BOD) up to100 g L-1 and chemical oxygen demand (COD) up to 200 g L-1 may increase the difficulty of high organic waste to be treated by direct biological process and disposal (Eroğlu et al., 2006; Ntaikou et al., 2009). The OMW wastes are reported to contain low pH and less macronutrients such as nitrogen and phosphorus (Seengenyoung et al., 2019). Besides, the presence of phenolic compounds with recalcitrant properties further increases its risk of degradation against treatment. Likewise, the palm oil mill effluent (POME) is brown colored and semi-solid in nature indicating low pH at higher temperature (O-Thong et al., 2008). Combination of the above two parameters in POME could fuel H2 production in fermentation process (Mamimin et al., 2017). POME characterized with high total solids (35 – 42 g/L), suspended solids (8.5 – 12 g/L), total nitrogen (0.83 – 0.92 g/L) and total phosphorus (0.097 – 0.125 g/L) is reported by researcher O-Thong et al. (2008). Notwithstanding, the presence of glycerine, crude oil, and dissolved oil in POME can turn the environment into nasty place if disposed untreated. Table 1 shows oil processing wastes and different reactor configuration used for biofuel production. 2.2.

Fruit and vegetable processing wastes 5

Globally, tomato and potato processing residues are the major vegetable processing wastes. Likewise, apple and grape processing contributes to the major fruit processing wastes (Achmon et al., 2019). The skins and seeds of extracted fruits, and vegetables are called as the pomace in industries. Even the stalks and leaves of the fruits and vegetables are considered as wastes (Achmon et al., 2019; Li et al., 2018). Besides, frozen FW generated from factories especially greens with noticeably high potassium are also considered as wastes (Carucci et al., 2005). The high organic content could give clues to the VS content of the fruit and vegetable waste which paves the way for biogas production (Ros et al., 2017). However, it is the protein that is equivalent to carbohydrate and not the lipid content. The lignin and cellulose content present in noticeable levels in fruit and vegetable wastes (21.3%. and 39.3% respectively) interferes with the DF and AD process and ultimately lowers the biogas production (Liang and McDonald, 2015). Moreover, these wastes also suffer from acidic or neutral pH conditions. Traditional waste management strategies like land disposal are not preferred for these highly degradable wastes due to their seasonal nature and varying physicochemical properties. Among the fruit wastes, fruits with lower nitrogen have higher treatment risk, especially mango and pineapple residues. The presence of essential oils in citrus wastes is believed to hinder the successful occurrence of AD process. A component of essential oil, limonene has left these wastes to record low CH4 level through interfering the waste treatment and digestion process (Zema et al., 2018). Table 2 shows fruit and vegetable processing wastes and different reactor configuration used for biofuel production. 2.3.

Dairy processing wastes Dairy processing, particularly cheese manufacturing industries buys cheese whey that is

produced in enormous amounts as a liquid by-product (Venkata Mohan et al., 2008). The liquid byproducts from dairy wastes are cheese whey and dairy waste permeate (lactose). Both of them are acidic in nature and consist of mineral salts, phosphorus, nitrogen, proteins, and oil and grease. The whole cheese processing generates both high and low strength wastewater from hard cheese production and cleaning operations, respectively (De Gioannis et al., 2014). Carbohydrate in the form 6

of lactose is one of the major component of cheese processing waste, and is thought to account for 70% of the liquid effluent in whey waste (Castelló et al., 2009; Kargi et al., 2012). Besides, proteins, lipids, trace minerals, salts such as NaCl and KCl, vitamins, and acids like citric and uric acids are also present in the cheese processing waste (Gadhe et al., 2013; Venetsaneas et al., 2009). High organic content in terms of high BOD (50 g L-1) and COD (up to 80 g L-1), acidic pH, and high biodegradable nature may turn these wastes into highly contaminated source and make it unfit for either direct land disposal or disposal into aquatic systems (Kavacik and Topaloglu, 2010; Rico et al., 2015). Thus, these abandoned, high lactose content wastes could be saved from landfill and turned into H2 and CH4 according to Azbar et al. (2009) and Banks et al. (2010). However, low alkalinity wastes pose rapid acidification which affect the digestion process to an extreme extent. The above phenomenon was exacerbated by low buffering capacity, a sharp decrease in pH, and subsequent accumulation of VFA which lead to the system failure (Castelló et al., 2009; Rico et al., 2015). Nonetheless, the acidic pH also lowers the CH4 levels and biogas production (Comino et al., 2012). Table 3 shows dairy processing wastes and different reactor configuration used for biofuel production. 2.4.

Brewery processing wastes Brewer’s spent grain, pot ale, and vinasse are the multiple forms of brewery processing

wastes from beer, whisky, and tequila processing and production industries, respectively (Chu et al., 2013). Pot ale is collected from malt whisky distilleries and vinasse is collected from brewing and sugarcane industry. The brownish black colour of brewery effluent has high organic content in terms of BOD (up to 5912 mg L-1) and COD (up to 32 g L-1) with acidic pH and are produced in enormous volumes (Estevam et al., 2018; Veeramalini et al., 2019). The presence of spent grains and yeast cells in effluent are responsible for its black color. Occurrence of amino acids and reducing sugars in the brewery effluent may induce browning reactions, which imparts brown colour to the effluent (Barrena et al., 2018; Buitrón et al., 2014). The cellulose and lignin content of these wastes are 925% and 7-28% respectively. These properties may boost contamination risk to the environment, if 7

discarded untreated (Malakhova et al., 2015). The amenability of these substrate for biofuel production can be increased by pretreatment methods such as physical, chemical, and biological (aerobic and anaerobic). In spite of switching, AD could reduce the negative impacts of the wastes by reducing the solid content, fermentable sugars, and macronutrients which in turn results in biogas formation (Estevam et al., 2018; Fu et al., 2017). However, the effluents rich in lignocellulosic materials such as cellulose, lignin and hemi cellulose, with few vitamins, and minerals may significantly lower the production of biogas (Veeramalini et al., 2019). The phenomenon is exacerbated by slow degradation of lignocellulosic feedstock and presence of polyphenols (Banu et al., 2019a). The existence of protein in pot ale fuels the rise of ammonia and cause an inhibition effect on methanogens (Barrena et al., 2018). Table 4 shows brewery processing wastes and different reactor configuration used for biofuel production. 2.5.

Livestock wastes The slaughterhouses leave behind various components such as dried blood, carcasses, hides,

bones, and feet as wastes (Gómez et al., 2006). To illustrate, pig-meat processing industries brought two types of wastes: brownish greaves, remains of pig tallow and whitish rinds, tough outer cover (Cavaleiro et al., 2013). Besides, the studies revealed the fact that not only the size but also the type of animals determine the composition of wastes coming from slaughterhouse (Moukazis et al., 2018). The slaughterhouse animal record a maximum of 50 % of the total body portion exists as wastes (Cuadros et al., 2011). These wastes account for high volume of proteins, lipids, COD, TS, VS, and lower level of carbohydrate along with acidic pH (Sittijunda et al., 2010). Both high COD and microbial flora turns these livestock effluents into more contaminated one and alters its direct disposal strategy by increasing the environmental concerns (Cuetos et al., 2008). Moreover, high protein content can trigger the ammonia production and can thus contribute to the inhibition effect of the digestion process (Alves et al., 2009; Ghimire et al., 2015b). The inhibition generally occurred during hydrolysis step which is predicted as the rate limiting step in treatment process (Masse et al., 2003). Notwithstanding, the anaerobic system is running out of digestion as the wastes are loaded 8

with high volume of solids along with microbial population other than H2 producing species. Table 5 shows livestock wastes and different reactor configuration used for biofuel production. 2.6.

Agriculture based (Agri-based) FPW The assured existence of lignocellulosic biomass in agri-based processing units influences the

biofuel generation (Kaparaju et al., 2009). The wastes that come under agri-based category for producing biogas include cassava, sugarcane, maize, cornstalk, rice grain, sorghum, sugar beet, wheat, peanut shell, and hazelnut shell. High solid content and low digestibility are equally bad for the bioreactor and biogas production (Chavadej et al., 2019). Besides, these wastes also have large volume of nitrogen and carbohydrates in addition to acidic pH (Ramos and Silva, 2018). Earlier, these types of wastes are applied to steam boilers. However, the initiation of the bio refinery approach began to replace the above traditional waste remediation at processing sites. In addition to vinasse, another solid waste, filter cake also comes from the sugarcane juice production during the clarification process. It is also reported to have profound effect on biogas production (Janke et al., 2016b). Contrary, the cane molasses generated from sugar processing were used with an aim of enhancing the H2 production (Park et al., 2010). Maize silage is also popularly used for biogas production especially in Europe countries. Homogenization and milling is reported to enhance the energy generation (Benito Martin et al., 2017). Regulating the valorization process for agricultural biomass like cornstalk, may reduce dumping and accumulation of enormous amount of waste after food processing. However, prolonged digestion may be killing the biogas productivity (Si et al., 2016). Sorghum crops teeming with fermentable sugars paved the way for high rate of degradation and biogas production. Moreover, sorghum is available at all seasons and methods like ensiling can save sorghum from destruction and ensures long term storage (Dareioti and Kornaros, 2015). H2 production of sugar pulp, a waste from sugar beet is larger than most agricultural biomass (Hussy et al., 2005). Although the pulp is not water soluble, the monomeric and dimeric form of sugars helped the pulp to produce the biogas. An organic based ingredient may be the source for H2 production from waste wheat powder (Karaosmanoglu Gorgeç and Karapinar, 2019). In addition, nitrogen and 9

phosphate composition are linked in nutrient supplementation for microbial growth. High cellulose and sCOD could give peanut shell wastes super powers in terms of microbial growth (Qi et al., 2018). Moreover, buffering capacity of peanut shell influences the production quality in the bioreactor. According to Roati et al. (2012), the organic content in the hazelnut shell is linked to level of H2 production within the system. Finally, a lignocellulosic biomass containing cellulose, hemicellulose and lignin (46.1 %, 5.6 % and 27.8 % respectively) has left bioreactors with records of low degradation levels, causing difficulty for the biogas production (Qi et al., 2018; Banu et al., 2019a). Table 6 shows agri-based processing and different reactor configuration used for biofuel production. 3.

Dual stage process for biohythane production

3.1.

Stage 1 -H2 production from DF DF process ends up in the production of H2 in the absence of light. This phenomenon is

exacerbated by the presence of carbohydrate-rich substrates, anaerobic degradation through H2 producing microbes, and activity of hydrogenase enzyme (Ghimire et al., 2015a; Banu et al., 2019b). H2 production was also produced through other modes of fermentation such as photo fermentation and dark photo fermentation. DF has gained attention due to its high production rate and ability to convert various types of wastes as feedstocks. Moreover, the potential of DF to break down different types of feed stock may help in cost cutting of the process (Ahmad et al., 2011). Besides, the lower retention time of the two-stage process, especially during the acidogenesis phase facilitates development of higher biomass and provides stability to the process. Overall, the process engineering and mixed microbial flora in two-stage system is believed to regulate the H2/CH4 ratio thereby paving the way to get biohythane products (Liu et al., 2013a). Alexandropoulou et al. (2018) demonstrated that suspended food industry waste with the addition of buffer solution can lead to a maximum H2production of 101.75±3.7 LH2 kg-1. This study highlighted how the non-carbohydrate sources play a role in enhancing H2 production despite the fact 10

that the carbohydrate sources are showing decreasing trend in removal rate. The presence of buffer solution has a profound impact on acidogenesis particularly at low HRTs. In contrast to the above study, cassava wastewater with added cassava residue showed a H2 production rate of 15 mL H2 g-1 CODremoved with lack of nutrients in a two-stage UASB system (Chavadej et al., 2019). The results of the study revealed the casualty of cassava residue: high starch content, starch has increased the VFA production, hit the H2 production and COD removal in acidogenesis. According to Estevam et al. (2018), an effective inoculum unlocks the H2 production mystery in brewery wastewater. A selective microbial species namely Klebsiella pneumoniae, an enterobacterium, isolated from aviary litter, promote H2 production (1.67 mol H2 mol-1 glucose) in brewery waste compared to natural fermentation. Lack of inoculum can wipe out the H2 production in the treatment process. Another factor that influences the hydrogen production is presence of nutrients in the waste mixture. Gadhe et al.(2013) demonstrated the importance of nutrients in dairy wastewater subjected to DF. To illustrate, a COD/N ratio of 100.5 and COD/P ratio of 120 is considered optimum for a H2 production rate of 29.91 mmol H2 g-1VSS.d. in dairy wastewater. However, excess N and P concentration prevents H2 production and at the same time, fuels the rise of organic solvents. A comparative account on the exposure of various feed stocks including slaughterhouse waste (dried blood) on H2 production has been revealed by Ghimire et al. (2015b). The dried blood with high protein content can lower the H2 production with a value of 87.6±4.1 ml H2 g-1 VS compared to potato and pumpkin waste with high carbohydrate content having a value of 171.7±7 ml H2 g-1 VS. Hence, the above findings confirm the fact that carbohydrate, not protein or lipid in FPW powers H2 production. Pretreatment methods inspired POME to pave way for efficient H2 production (Mohammadi et al., 2011). The pretreated inoculum harvested in a time period of not exceeding 48 h can increase the potential of the waste to produce more H2 (0.41 mmol H2 g-1 COD) with simultaneous removal of COD. The CSTR can speed up the H2 production with 25 g COD L-1 of sugarcane syrup obtained from the sugar processing waste (Nualsri et al., 2016). Varying HRT levels turn out to be threat to natural micro flora that exists in the sugarcane syrup. Moreover, as Clostidium butyricum was 11

inoculated into the syrup, butyrate fermentation pathway is believed to succeed the H2 production. Of the three type of coffee residues (green coffee powder, parchment, and defatted cake), both green coffee powder and defatted cake can boost the bioreactors with enhanced production of H2-rich biohythane, when co-digested with sugarcane vinasse (Pinto et al., 2018). Compared to green coffee powder, defatted cake possess two-fold higher COD and hence results in the production do CH4 in addition to H2. An optimal concentration of 67 g L-1 medium of chewing gum waste made H2 production possible on bioreactors (Seifert et al., 2018). It is interesting to note that concentration exceeding this level is more likely to decrease the H2 production which can be well corroborated to substrate inhibition. Seifert et al.(2018) concluded that minimizing the lag time would maximize the H2 production. In contrast, Torquato et al. (2017) concluded that a switch over to longer inoculum adaptation can end in increased production of H2 when treating effluents of citrus processing industry (wastewater and vinasse). The microbes overtake their normal cellular growth as they experience a longer adaptation period in the digester. The H2 producing bacteria can influence the citrus wastewater, respond to acidogenesis and at the end contribute to H2 production. Henceforth, DF can turn citrus wastewater into renewable energy resource by producing H2 compared to sugarcane vinasse. The phenomenon was exacerbated by the presence of fruit nutrients, longer lag time, and inoculum enrichment. 3.2.

Stage 2 –CH4 production from AD The waste treatment has become more bioenergy dependent after the application of AD.

Continuous exposure of FPW to AD not only reduces the organic load but also produces renewable energy like biogas while recycling the nutrients. Firstly, AD may achieve the waste treatment by two methods, viz., wet method and high solids method. To illustrate, wet AD method is suitable for TS, less than 15 % and the high solid method for solid content greater than 15 % but lesser than 40 %. Though wet AD becomes popular for treating wastes with high moisture content which direct towards the triggering action of microbes, high-solids AD could soon spell end of wet AD. Multiple factors like higher organic load, less installation and operating expenses, low water consumption 12

during digestion, and less maintenance of digesters have contributed to the popularity of the highsolids AD (Achmon et al., 2019). Hence, with respect to valorization, AD heads as a biological method that is technically feasible, reduces the differences in degradation rates in two stages and ecofriendly one. Achmon et al. (2019) demonstrated the high-solids AD with tomato or grape pomace as vegetable and fruit processing waste using dry bovine manure and green waste compost as cosubstrates reaching a TS content of 28 %. Tomato pomace makes high CH4 yield of 201.61 mL g-1 possible at 5 % feedstock loading level. Both bacteria and archaea communities get acclimatized to feedstock and operating temperature at a faster rate showing a metabolic shift resulting in high CH4 level. The CSTR type digester has the ability to reduce the COD by 97 % and contribute to CH4 production of 107 l CH4 kg-1, while treating sweet sorghum, an agri-based FPW (Antonopoulou et al., 2008). The extraction process also alters the CH4 production by efficient degradation of solid residues. Aslanzadeh et al. (2014) showed a comparative account of single-stage and two-stage AD on FPW. As anticipated, single-stage system is more likely to fail in handling the operating conditions such as HRT and OLR. Moreover, the two-stage process occupy less volume than singlestage that is good for high loading of feedstocks. A raw pot ale from whisky industry, has left the AD system to record low CH4 levels, compared to treated pot ale, creating disturbance in the total biogas production (Barrena et al., 2018). Surprisingly, high protein waste may turn the raw pot ale into CH4 source of 554±67 N L CH4 kg-1VS, whereas, the deproteinated pot ale record CH4 values lesser than the previous one (501±23 N L CH4 kg-1VS). Similar to the above study, meat processing wastes, such as greaves and rinds rich in protein and fats, with few carbohydrate content, may offer CH4 production potential, which is otherwise termed as good biodegradability, even in the non-pretreated state. However, CH4 production potential does not boost CH4 production rates (Cavaleiro et al., 2013). Thus, the authors concluded that pretreatment methods may help treat wastes with high CH4 production. Cheese whey co-digested with cattle slurry produced 621 L kg-1 VS biogas and COD removal of 82 % in the AD system (Comino et al., 2012). In this study, the start-up phase hints at 13

surprising production abilities of biogas (79 %) in digester. In addition, the lacking of chemicals made co-digestion of high volume of whey possible on AD. Pair of CSTR can treat FPW as effectively as single CSTR co-worked with other reactor including UASB. Dareioti and Kornaros, (2014) showed pair of CSTR can produce a CH4 level of 0.33 L CH4 LR-1d with OMW, a FPW from olive oil production. The presence of sodium and potassium in the influent inhibit the methanogens which ultimately results in low level of CH4. Park et al. (2010) reveals the production of CH4 at 1.48 (0.09) L-CH4 L-1 reactor d-1 while treating diluted molasses, a sugarcane processing waste. Production of ethanol may make less economic profit compared to the production of biohythane (H2 and CH4), particularly in the case of molasses. Moreover, the results confirmed that two-stage process favored the production of more renewable energy from the molasses when compared to single stage reactors. . Abandoned coffee residues could be saved from landfill and turned into biohythane, according to study (Pinto et al., 2018). Of the three coffee residues, (green coffee powder, parchment, and defatted cake), parchment residue yielded high CH4 rich biohythane through AD. The phenomenon is exacerbated by the presence of significant amounts of non-H2 components in the residue and low level of COD compared to other residues. A comparative account on the CH4 production of various FPW such as rice, hazel nut, and wine lees has been demonstrated in AD system (Roati et al., 2012). Wine lees is more likely to share high CH4 production potential (0.85 m3 CH4 kg-1 TS), whereas, hazelnut may offer potential biogas production ≥ 0.9 m3 biogas kg-1TS as compared to rice. Thus it is inferred that, switching to optimized operational conditions like C/N ratio and moisture could increase digestion efficiency and improve the economic feasibility. 3.3.

Mechanism of biohythane production Hydrolysis is the first stage of DF process for biohythane production. During this stage,

complex sugar polymers (cellulose, hemicellulose and lignin) are getting converted into monomers (glucose). This conversion occurs spontaneously occurs without external energy supply. At the end of this process, VFA rich effluent is obtained. Meanwhile high hydrogen partial pressure created inside the reactor environment result in inhibition of hydrogen producing microbes (HPM) activity. Only 14

methanogens has capability to survey under this environment. During two stage AD, methanogenic phase is the rate limiting step. Methane production takes place by two different pathway namely hydrogenotrophic and acetoclastic. During hydrogenotrophic pathway the hydrogenotrophs oxides hydrogen into methane. Similarly in the acetoclastic pathway, methanogens target acetate and convert it into CH4 and CO2. Among these two, acetoclastic is potential and capable of producing higher CH4 (approximately 60 to 70%) yield. At the end of second stage of AD a final product is obtained and it consist of H2, CH4, and CO2 in the ratio of 10, 60, and 30% respectively. Fig. 2 shows schematic of the metabolic pathway of DF. 4.

Influential parameters on H2 and CH4 production

4.1.

Hydraulic retention time (HRT) HRT in anaerobic reactors not only raises survival concerns for microbial flora but also

significantly affects the feedstock decomposition (Kumar et al., 2016b). To illustrate, while treating vinasse produced from tequila processing, Buitrón et al. (2014) obtained a maximum H2 production at HRT of 6 h and a maximum CH4 production at HRT of 24 h. Similarly, Dareioti and Kornaros, (2014) obtained H2 yield of 1.72 L-1 LR d, with a HRT of 0.75 d during acidogenesis and CH4 yield of 0.33 L-1 LR d with a HRT of 25 days during methanogenesis while treating OMW with cheese whey with. Likewise, agave bagasse from tequilana processing outcome, recorded a maximum H2 production of 105 mL H2 g-1 at 6h HRT and a maximum CH4 production of 225 mL CH4 g-1 at 14 h HRT (Montiel Corona and Razo-Flores, 2018). The above results demonstrated that assigning short HRTs to H2 production and longer HRTs to CH4 production increases the rate of production of biohythane (H2+CH4) (Antonopoulou et al., 2008; Anzola-Rojas et al., 2016). Concerning individual production of biogas, food processing industry waste produce high CH4 yield of 0.49 m3 kg-1 VS at a HRT of 7 days (Aslanzadeh et al., 2014). This is relatively higher compared to FW such as ripened fruits and waste pastry meant for H2 production (Han et al., 2016; Hwang et al., 2011) at a HRTs of 18h and 6h, respectively. This phenomenon is exacerbated by good digestion capacity of hydrogenogenic effluent, homoacetogenism, and increased biogas production (Vo et al., 2019). 15

4.2.

pH pH becomes better at the end of decomposition of feedstock and can raises the hope of

substrate and energy utilization (Chu et al., 2013). Irrespective types of FW, determination of hydrogen and methane production relies on pH of the substrate. For example, substrate pH around 5 promotes H2 production and substrate pH in the range of 7 to 8 promote CH4 production. Park et al. (2010) showed a maximum production of H2 (2.8 LH2 L-1reactor d-1) at pH 5.5 and maximum production of CH4 (1.48 LCH4 L-1reactor d-1) at pH 7.0, during the treatment of molasses, a by-product of sugar manufacturing industry. Likewise, H2 levels hit record high in ensiled sorghum, an agrobased food industry waste at a pH of 5.5. The pH levels for CH4 was around 8.0 (Dareioti and Kornaros, 2015). Similarly, pH values of 5.5 and 7.2 can help to treat the vinasse from liquor industry in China for maximum H2 (14.8 ml g-1VSsubstrate) and CH4 (274 ml g-1VSsubstrate) production, respectively (Fu et al., 2017). The above results demonstrated that assigning acidic pH for H2 and neutral to alkaline pH for CH4 increases the productivity of biohythane (H2+CH4) (Dhar et al., 2016; Nualsri et al., 2016). Concerning individual production of biogas, Peanut shells from grain processing mill produce maximum H2 yield of 39.9 m L-1gsubstrate at pH 6.5 (Qi et al., 2018). This is relatively in contrast to FW meant for H2 production that showed a value of 4.4±0.3 LH2 L-1d-1 at pH 11.0 (Jang et al., 2015). The neutral to alkaline pH is believed to be behind the activity of methanogens, whereas, acidic pH in digesters may favors acidogenesis (Li et al., 2018; Tenca et al., 2011). Notwithstanding, low pH values may be killing the syntrophic association that exists between acidogenesis and methanogenesis (Janke et al., 2016b). 4.3.

Temperature Like pH, temperature is also an environmental factor that may offer potential influence on

both acidogenesis and methanogenesis (Abreu et al., 2019; Algapani et al., 2016). Achmon et al. (2019) generated both H2 and CH4 from tomato pomace and grape pomace, major fruit processing wastes generated in California, USA by applying mesophilic condition (30 ºC) during acidogenesis 16

and thermophilic condition (55 ºC) during methanogenesis. The yields of H2 and CH4 are 73.17±32.76 mL-1gdry grape pomace and 201 mL-1gdry tomato pomace, respectively. Secondly, agave bagasse from tequilana processing produced maximum H2 and CH4 production of 105 mL H2 g-1 and 225 mL CH4 g-1 at a recorded temperature of 35 ºC and 23-25 ºC (below mesophilic), respectively (Montiel Corona and Razo-Flores, 2018). Contrast to the previous studies, thermophilic temperature may boost both H2 and CH4 production in FPW (Chavadej et al., 2019). The study explores the treatment of cassava wastewater with cassava residue, an agro-based FPW, that produced 15 mL H2 g-1CODremoved and 259 mL CH4 g-1 CODremoved at an applied temperature of 55 ºC. Likewise, mesophilic temperature (37 ºC) influenced both H2 and CH4 production in rice grain based distillery effluent with values of 150.7 mmol L-1 for H2 and 64.1 mmol L-1 for CH4 (Mishra et al., 2017). Concerning individual production of biogas, orange peels from citrus processing wastes produce maximum CH4 yield of 0.46 L g-1TVS at mesophilic condition and is four times higher compared to thermophilic condition (0.12 L g-1TVS). A high OLR and essential oil content of orange peel has resulted in low CH4 at thermophilic temperature creating disturbance in thermophilic strains (Zema et al., 2018). Hence, in general, mesophilic temperature in digesters could fuel H2 production in FPW such as brewery plant wastewater, sugarbeet, dairy wastewater and slaughterhouse waste (Estevam et al., 2018; Gadhe et al., 2013; Ghimire et al., 2015b). Therefore, the production of biohythane (sH2+CH4) depends on nature of feedstock and not on temperature. 4.4.

Microbial populations Identification of microbes that keeps the system vital can help to streamline the H2 and CH4

producing strains to enhance the biohythane production (Hung et al., 2011). The mixed cultures are more likely to share the decomposition of complex feedstock in FPW making the production of both H2 and CH4 possible within the system (Svensson et al., 2018). While treating fruit processing wastes, Achmon et al. (2019) found Syntrophomonas, Clostridium, Caldicoprobacter, Steroidobacter, and Thermacetogenium with potential for substrate degradation and thus 17

acidogenesis and H2 production, whereas, Methanoculleus and Methanosarcina, Methanomassiliicoccus, and Methanobrevibacter made methanogenesis process possible and started producing CH4. Similarly, Thermoanaerobacterium sp. and Methanosarcina sp. shall increase the H2 and CH4 production when subjected to POME (Seengenyoung et al., 2019). Likewise, Park et al. (2010) demonstrated that Clostridium butyricum in hydrogenogenic reactor caused enhanced H2 production and Methanobacterium beijingense and Methanothrix soehngenii that belonged to hydrotrophic and acetotrophic bacteria causes enhanced CH4 production. Some other researchers also decoded that the above said species involved in biohythane (H2+CH4) production (Algapani et al., 2018; Ferraz Júnior et al., 2014; Jariyaboon et al., 2015). Concerning individual production of biogas, brewer’s spent grain, produced CH4 yield with the help of Clostridium, Bacillus, and Bacteroides sp. during the first 35 days and later, Methanosaeta harundinacea, Methanocorpus culumlabreanum, and Methanobacterium sp. showed their abundance in reactors. Taking into account for H2 production, Veeramalini et al. (2019) choose Rhodobacter M 19 and Enterobacter aerogenes to achieve acidogenesis (1.96 mol H2 mol sugar-1) by taking effluent from brewery industry. Likewise, Alexandropoulou et al. (2018) identified Clostridiaceae/Ruminococcaceae and Enterobacteriaceae strains from expired solid baby foods, a food industry waste with a H2 production of 101.75±3.71 L H2 kg-1. However, other bacterial species such as Coprothermobacter, Anaeroturncus, Pectinatus, and Lactobacillus may also lead to H2 production in DF (Hung et al., 2011). 4.5.

Organic Loading Rate (OLR) The OLR at anaerobic digesters may predict the stability of the system (Cota-Navarro et al.,

2011). Hence, changes in OLR and its further optimization can be used as a tool to help ease negative effects during the feedstock shortage (Ferguson et al., 2016). Moreover, OLR is believed to be behind the activity of bacterial and archaeal strains (Supaphol et al., 2011). An OLR of 16 g COD L-1for vinasse obtained from tequila processing industry showed a maximum production of H2 57.4±4.0 mL 18

H2/L-h. Likewise, an OLR of 1636 mg COD L-1 was applied to produce a maximum CH4of 257.9±13.8 mL CH4 g-1 COD (Buitrón et al., 2014). Similarly, an OLR of 126.67kg COD m3 d and 3.37kg COD m3 d obtained by mixing OMW with cheese whey to produce more H2 and CH4 (Dareioti and Kornaros, 2014). Likewise, Wang et al. (2013) applied two different OLRs for treating sugary wastewater from sugar refining industry. Maximum rate of H2 and CH4 was produced at an OLR of 6 g L-1 and OLR of 2.1 g COD L-1 d-1, respectively. The above results demonstrated that assigning longer OLRs to H2 production and shorter OLRs to CH4 production increases the productivity of biohythane (H2+CH4) (Koutrouli et al., 2009; Montiel Corona and Razo-Flores, 2018). However, some authors reported contrast results to the above ones while treating organic wastes including FPW such as brewer’s spent grain (Cavinato et al., 2011). Concerning individual production of biogas, Ghimire et al. (2015b) treated slaughterhouse waste such as dried blood at on OLR of 3.8 g equivalent to 3.15 g VS and obtained a maximum H2 production of 87.6±4.1ml H2 g-1 VS. In the case of CH4, cheese whey from dairy milk processor arrived at a production of 1.37 m3CH4 m-3d-1 with an applied OLR of 5.9 kgVS m-3d-1 (Rico et al., 2015). 4.6.

Reactor configuration Though batch mode may offer easy operation and economically viability, continuous mode is

more advantageous and it is exacerbated by steady state, continuous production, and design of engineering systems (Braguglia et al., 2018; Ntaikou et al., 2009; Pinto et al., 2018). Looking for biohythane, two reactors either of same type or of different type heads for the production of H2 and CH4 (Buitrón et al., 2014; Dareioti and Kornaros, 2015). Fig. 3 Different bioreactor systems used for biohythane production. Nualsri et al. (2016) achieved acidogenesis through CSTR digester while working with a volume of 1 L and methanogenesis through UASB reactor with the working volume of 24 L while treating sugar cane syrup. Secondly, anaerobic SBR leads to higher production of H2 and UASB reactor leads to the production of CH4 during the treatment of POME (Seengenyoung et al., 2019). On the other hand, same type of reactors, i.e., both CSTR type are having profound impacts in the production of H2 and CH4 while treating dairy waste permeate from cheese 19

manufacture, which extends to the production of biohythane (Banks et al., 2010). Similarly, Chavadej et al. (2019) found that both UASB reactors are likely to increase biohythane production in terms of H2 and CH4 while treating cassava wastewater with cassava residue. In total, the first reactor is meant for fast growing acidogenic bacteria and the second reactor is meant for slow growing methanogenic bacteria which paves the way for biohythane production (Banks et al., 2010). Concerning individual production of biogas, especially in DF, compared to reactors such as anaerobic fluidized bed reactors, packed bed rectors, membrane bioreactors and UASB reactors, CSTR fuels the rise of H2 production in treating FPW (Alexandropoulou et al., 2018; Ntaikou et al., 2009). CSTR is believed to be behind the complete mixing and suspended form of acidogenic microbes in the reactor digestate (Wang et al., 2013), whereas, in methanogenesis, UASB reactors fuels the rise of CH4 production while treating FPW (Aslanzadeh et al., 2014; Zema et al., 2018). 5.

Biohythane production - biorefinery concept

5.1.

Biohythane production Biorefinery concept hints at surprising renewable abilities in FPW streams. This low-cost

solution reduces negative impacts of processing waste from landfills and other remediation options (Venkata Mohan et al., 2016). Moreover, circular biorefinery made FPW generate a wide array of value added chemicals, renewable energy and food and feed ingredients on DF/AD (Kumar et al., 2018). A proper upgrading of both H2 and CH4 may replace fossil fuel and can be used as gaseous transport biofuel (Xia et al., 2016). Biohythane, is a way to make greener economy due to its clean nature, high fuel value, enhanced heat efficiency, that develops engines more likely to work with less energy (Yeshanew et al., 2016). Moreover, ideal blending of H2 may help reverse GHG emissions (Liu et al., 2013a). Towards the production of biohythane, Fu et al. (2017) showed that vinasse, a processing waste from liquor production can degrade the wastes and generate both H2 and CH4 at 14.8 ml g-1VSsubstrate and 274 ml g-1VSsubstrate, respectively. Likewise, olive pulp from OMW may be used to produce 0.32 mole kg-1TS of H2 and 1.13 L L-1d-1 of CH4 (Koutrouli et al., 2009). Similarly, coffee residues including defatted cake has the ability to respond to sugar cane vinasse and can thus 20

contribute to the production of biohythane (30-40 % of H2 and 70 % of CH4) (Pinto et al., 2018). Regulating parameters such as OLR (Banks et al., 2010), microbes, temperature (Seengenyoung et al., 2019), VFA reduction (Mamimin et al., 2017), pretreatment (Kavitha et al., 2014) may significantly increase the production of biohythane. Finally, two-stage system could give clues to the enhanced production of biohythane and is exacerbated by higher COD removal, sustainable recovery, and existence of acidogens and acetogens (Si et al., 2016). 5.2.

Factors hindering the production of biohythane

5.2.1. Accumulation of VFA AD of FPW can create VFA, metabolic products in intermediate way during the occurrence of acidogenesis and acetogenesis phases. Switching to short reaction time after acidogenesis can prevent the activity of methanogens and enhance the accumulation of VFA (Algapani et al., 2018). Acetic and butyric acids are more likely to share the CH4 producer’s role (Atasoy et al., 2018). In addition, maximum amount of acetic acid could be turned into CH4 (Khan et al., 2016). The accumulation of VFA in digesters causes both environmental and operational problems. To illustrate, pH of 6 is fuelling the accumulation of VFA in digesters irrespective of the nature of feedstock (Jiang et al., 2013). Thermophilic temperature (55 ºC) may signal the butyrate and mesophilic temperature (35 ºC) may signal the acetate and propionate production. Moreover, mesophilic temperature is having profound effects in accumulation of VFA (Jiang et al., 2013). A higher HRT results in production of VFA, whereas, prolonged HRTs results in accumulation of VFA, irrespective of the nature and composition of the feedstock. HRT holds a key to yield and composition of VFA (Bolaji and Dionisi, 2017). OLR linked VFA accumulation is on the rise in the system up to a certain extent. Beyond the level, this could reverse and overall decline in VFA occurs. Jiang et al. (2013) denoted that an OLR of 11 g L-1 d favors the accumulation of VFA. Looking for biohythane, firstly, Montiel Corona and Razo-Flores, (2018) identified the accumulation of acetic and butyric acids while treating agave bagasse from tequilana processing. 21

Secondly, accumulation of mixture of VFAs tied to OMW and cheese whey has been identified (Dareioti and Kornaros, 2014). Likewise, in the case of sugarcane molasses from sugar manufacturing process, Park et al. (2010) found the accumulation of acetic, butyric, propionic, and lactic acids. Considering the individual production of biogas, slaughterhouse wastes especially pig meat processing wastes are more likely to accumulate VFAs including palmitic acids during methanogenesis (Cavaleiro et al., 2013). While at acidogenesis, Estevam et al. (2018) treating brewery wastewater, found the accumulation of acetic, butyric, propionic, lactic, and formic acids. As compared to biogas, VFA facilitates lower risk of storage and transportation. Moreover, the additional value of VFAs compared to CH4 paves the way for its recovery from AD of FPW. Therefore, the VFA, intermediate products become the low-cost raw materials for industrial applications such as biopolymers, biodiesel, biogas, and many more (Venkata Mohan et al., 2016). 5.2.2. Concentration of ammonia Protein-rich processing wastes, for example livestock effluents, release ammonia (as ammonium ion (NH4+) and free ammonia (NH3)) during hydrolysis and accumulate in the reactor to cause an inhibition effect and process instability leading to system failure (Giuliano et al., 2013; Polizzi et al., 2018). However, compared to acidogenesis, the level of ammonia could fuel inhibition in methanogenesis. High OLR also leads to ammonia inhibition. The neutral pH (6.0-7.0) is believed to be behind this phenomenon (Jiang et al., 2013). At thermophilic temperatures, the methanogens score higher in tolerating high ammonia compared to mesophilic temperatures. High ammonia level brought changes in the transcriptional profile of microorganisms such as bacteria and archaea (Yenigün and Demirel, 2013). Briefly, a highly unionized ammonia is toxic to the microbial population. It kills the population through its intrusion through the cell membrane. Such high ammonia concentration makes the survival of this bacteria to be difficult (Pan et al., 2013). Therefore, a hydrogenogenic effluent rich in ammonia, may significantly lower the production of biogas (Giuliano et al., 2013).

22

5.3.

Factors enhancing the production of biohythane

5.3.1. Co-digestion of FPW with other substrates Co-digestion of FPW with other wastes increased the possibility of dilution of toxic compounds (Sen et al., 2016). While treating fruit and vegetable processing residues, bovine dry manure and mature green waste compost as co-digesters, raises the hope of both H2 and CH4 production (Achmon et al., 2019). In the case of dairy waste, either individual treatment of cheese whey or mixture of cheese whey (with OMW or ensiled sorghum), cattle slurry otherwise known as liquid cow manure co-digestion, brings enhanced CH4 and biohythane, respectively (Comino et al., 2012; Dareioti and Kornaros, 2015, 2014). From the results of Comino et al. (2012), it was observed that without the use of chemicals, maintaining the pH is possible in co-digestion. Co-digestion with dairy manure and corn stover increases the CH4 production abilities (yield of 415.4 L-kg VS feed) in solid state digestion of tomato residues and the above mixture reduces negative impacts of inhibitors (Li et al., 2016). Therefore, frequently used co-digestion products such as animal manure, agrowaste, sewage sludge, and phytomass can replace the long term digestion of feedstock (Braguglia et al., 2018; Giuliano et al., 2013; Malakhova et al., 2015). 5.3.2. Pretreatment strategies Pretreatment ensures the easy availability and accessibility of bio convertible sugar compounds to mixed micro flora for the catalytic action of enzymes (Cheng et al., 2011; Karthikeyan et al., 2018). The applied pretreatments are listed as physical, mechanical, thermal, chemical (acid and alkali), physico-chemical, biological, catalyst saccharification, and combined treatment (Baêta et al., 2016; Barrena et al., 2018; Carucci et al., 2005; Cavaleiro et al., 2013; Chandolias et al., 2016; Dareioti and Kornaros, 2014; Ghimire et al., 2015b; Janke et al., 2016b; Kavitha et al., 2016). For biohythane production, combined thermal (95 ºC for 15 min) and mechanical (size reduction) pretreatments may help treat cassava wastewater and ensures accessibility to microbes for better production of both H2 and CH4 (Chavadej et al., 2019). Similarly, Montiel Corona and Razo-Flores, 23

(2018) demonstrated the blending of thermal (104 ºC for 24 h) and biological (enzymatic hydrolysis) pretreatments to cut off the toxic product formation and enhance the production of biohythane. In addition, individual pretreatment such as aerobic or thermal are also making the production of biohythane better, while treating mixture of OMW and cheese whey or mixture of maize silage, olive pomace, fruit/vegetable waste, and rice flour (Dareioti and Kornaros, 2014). 6.

Recovery of Liquid fertilizers Liquid fertilizers are recovered from digestate and it consist of considerable amount of

nitrogen (N), phosphorus (P) and potassium (K). Liquid fertilizer has potential of replacing the traditional chemical fertilizer and it significantly improves the soil nutrient content. Liquid fertilizers can be recovered from digestate by techniques such as pressurized membrane filtration, ammonia stripping, evaporation, forward osmosis, electrodialysis and phosphorus precipitation by struvite, (Tampio et al., 2016). Tampio et al. (2016) have used different techniques for concentrating the liquid fertilizer from FW digestate. They have used techniques such as ammonia stripping (AS), AS coupled with reverse osmosis (RO), evaporation coupled with RO, and AS coupled with both evaporation and RO to concentrate nutrients. As a result, a significant amount of N (70 %) is recovered by AS and AS coupled RO. About 63% and 67% of N recovery is achieved in evaporation coupled with RO and AS coupled with evaporation and RO respectively. Liquid fertilizer recovered from FW and FPW, can reduce the emission of greenhouse gases and promotes nutrient recycling. However, high amount of energy is required for the recovery of liquid fertilizer. 7.

Energy aspects of biohythane production The energy demand for biohythane production has to be assessed for its successful scaling up.

Energy assessment includes sum of energy spent and gained from various process associated with biohythane production. The main parameters considered for calculating energy spent include, energy required for heating, mixing, dewatering, pumping etc. Energy gain is calculated from the energy value of biohydrogen and biomethane produced. Xiao et al. (2018a) have employed 600 m3 of 24

thermophilic CSTR for hydrogen production and 2400 m3 mesophilic CSTR for methane production and obtained an energy yield of 13.68 kJ/g Fed VS, energy ratio of 13.43 and energy conversion efficiency of 16.67 kJ/g destructed VS was achieved. Similarly, in another study with 600 and 2400 m3 of thermophilic and mesophilic CSTR, Xiao et al. (2018b) have obtained an energy yield of 3410.93 kWh/Mg TS, energy ratio of 9.25 and energy conversion efficiency of 16.59 kJ/g removed VS. Yeshanew et al. (2016) have reported that energy recovery depends factors such as temperature, HRT, OLR and reactor size. Around 12.3 MJ/kgVSadded of total energy was recovered from continuously operated CSTR coupled anaerobic fixed bed reactor (AFBR). In first stage, hydrogen yield from CSTR was 115 L/kgVSadded and in second stage, methane yield from AFBR was 334 L/kgVSadded. Qin et al. (2019) have investigated the effect of food (FW) and paper waste (PW) co-digestion on energy yield. They varied total solid (TS) of food and paper waste from 0 to 50% and achieved hydrogen and methane yield of 157 NL-H2/kg-VSFW and 657 NL-CH4/kg-VSFW, respectively. A maximum energy yield of 28.1 MJ/kg VSFW was achieved for co-digestion ratio 1:1. Similarly, Liu et al. (2013b) have studied energy yield in food and waste activated sludge co-digestion and obtained a maximum hydrogen (106 mL H2/g-VS) and methane (353 mL CH4/g-VS) yield for 85% food to 15% waste activated sludge proportion. Voelklein et al. (2016) have calculated energy yield at varying organic loading rate in two stage anaerobic digestion. At an OLR of 2 g VS/ L, they achieved a maximum energy yield of 15.1 MJ / kg VS. Further increasing OLR beyond 2 g VS/ L d decreases energy yield. Luo et al. (2011) have examined energy recovery from mixed organic waste. It was evident from their study that energy yield from hydrogen and methane reactor was noted to be 0.4 ± 0.05 kJ/gVS and 12.3 ± 0.69 kJ/gVS, respectively. The overall energy recovery from two stage anaerobic digester was observed to be 12.7 ± 0.72 kJ/gVS. Chu et al. (2008) have digested 1000 kg of wet FW with 33.8% of TS in two stage anaerobic digester. Energy recovery from hydrogenic and methanogenic reactors were observed to be 139,333 kcal/t-wet and 1,722,925 kcal/t-wet, respectively. 25

8.

Techno-economic aspect of biohythane production So far, only limited literatures documented techno-economic aspect of biohythane production

from FW and FPW. It is considered to be an essential tool for scaling up of process. In technoecomonic analysis revenue gained and spent during the fired operational period of biohythane plant is accounted. The cost spent towards maintenance of treatment facility, equipment’s requirement, manpower expenditures, disposal, chemical requirement and transportation were to be considered for arriving net cost. Kavitha et al. (2017) and Kannah et al. (2019) have reported that revenue generated from 1 m3 of methane and hydrogen is about 1.26 € and 0.74 € , respectively. Micolucci et al. (2018) have arrived cost assessment for a pilot scale two-stage thermophilic anaerobic digester treating FW. From their study it was evident that pilot plant has generated an annual income of 540, 874 €/y. Ljunggren and Zacchi, (2010) have reported that two stage AD process requires a capital investment of 12687.7 € per 13.4 ton of waste biomass. 9.

Challenges and future perspectives for scale-up FPW is a waste of variable composition depending on the source of generation. Also, they are

generated in huge quantities in industrial sectors. Multiple factors like feedstock nature, selection of the production process, production site, and finally temporal variation in the production of the waste mirrored on its variability. Tonini et al.(2018) revealed that an average of 1 ton of food is expelled from processing sector to waste management. These wastes are ecotoxic, human toxic, and rich in particulate matter, precursors for cancer and photochemical ozone formation. They act as a main contributor for water depletion, fossil resource depletion, aquatic eutrophication (nitrogen and phosphorus), global warming, and terrestrial acidification. Hence, they concluded that the waste management of FPW has been partly mitigated with the replacement of incineration and AD. High moisture content of the waste turns out to be biggest threat to incineration, and hence, switching to biological process such as AD may offer potential consortium of microbes and pave the way for simple and cost-effective energy recovery. However, to overcome the limitations of AD such as 26

treating the wastes rich in COD and BOD content, suspended solids and emission of odours, Liu et al. (2018) demonstrated that two-stage anaerobic fermentation can replace one-stage AD. Assigning DF to H2 and AD to CH4 increases the productivity of biohythane. Even though the two-stage process evidenced for potential production, process parameters such as OLR, reactor design, microbes, and many more leads to the less production of biogas compared to the theoretical value. Hence, addressing these challenges could push the FPW a giant leap: from lab scale to industrial scale. As FPW became the point source of waste, successfully carry out the waste treatment at the source point is perhaps the best way to limit the cost of transportation of wastes. Though daily loading has its limitations, high rate reactors could be deigned to cope up with frequent feeding of load without any increase in volume, which leads to the higher production of biogas. Coupling of other techniques with the existing ones, for example, combining DF with microbial fuel cells is the best way to create a pipe line of new metabolic by-products. Similarly, CH4 production could also come up with hybrid techniques which would be a winning game plan for biohythane production. Degradation of feedstock are turning smarter with the new microbes. Thus, arriving at the identification of mixed consortium and decoding of unidentified microbes can act against feedstock and improves the way of biohythane production. 10.

Conclusion Unlike other wastes, FPW generation from industrial sectors is high in volume. Variability is

an even greater challenge in the face of waste management. However, this could increase the chance of higher production of biofuels. Combining DF with AD, is the best way to create H2, CH4 and thus biohythane. Both operational and environmental parameters are reviewed with special reference to FPW which might pave the way for enhanced production. Though this review reveals the potential of hindrance factors such as VFA accumulation and inhibition due to ammonia, it also highlights the significance of enhancing procedures like pretreatment and co-digestion. Acknowledgement 27

Mr. Yukesh Kannah R is grateful to CSIR, New Delhi, India for the award of Senior Research Fellowship (CSIR Direct SRF 09/468/0529). References 1.

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126. Venkata Mohan, S., LalitBabu, V., Sarma, P.N., 2008. Effect of various pretreatment methods on anaerobic mixed microflora to enhance biohydrogen production utilizing dairy wastewater as substrate. Bioresour. Technol. 99, 59–67. 127. Venkata Mohan, S., Nikhil, G.N., Chiranjeevi, P., Nagendranatha Reddy, C., Rohit, M. V, Kumar, A.N., Sarkar, O., 2016. Waste biorefinery models towards sustainable circular bioeconomy: Critical review and future perspectives. Bioresour. Technol. 215, 2–12. 128. Vo, T.-P., Lay, C.-H., Lin, C.-Y., 2019. Effects of hydraulic retention time on biohythane production via single-stage anaerobic fermentation in a two-compartment bioreactor. Bioresour. Technol. 292, 121869. 129. Voelklein, M.A., Jacob, A., O’Shea, R., Murphy, J.D., 2016. Assessment of increasing loading rate on two-stage digestion of food waste. Bioresour. Technol. 202, 172–180. 130. Wang, B., Li, Y., Wang, D., Liu, R., Wei, Z., Ren, N., 2013. Simultaneous coproduction of hydrogen and methane from sugary wastewater by an “ACSTRH–UASBMet” system. Int. J. Hydrogen Energy 38, 7774–7779. 131. Xia, A., Cheng, J., Murphy, J.D., 2016. Innovation in biological production and upgrading of methane and hydrogen for use as gaseous transport biofuel. Biotechnol. Adv. 34, 451–472. 132. Xiao, B., Qin, Y., Wu, J., Chen, H., Yu, P., Liu, J., Li, Y.-Y., 2018a. Comparison of single-stage and twostage thermophilic anaerobic digestion of food waste: Performance, energy balance and reaction process. Energy Convers. Manag. 156, 215–223. 133. Xiao, B., Qin, Y., Zhang, W., Wu, J., Qiang, H., Liu, J., Li, Y.-Y., 2018b. Temperature-phased anaerobic digestion of food waste: A comparison with single-stage digestions based on performance and energy balance. Bioresour. Technol. 249, 826–834. 134. Yenigün, O., Demirel, B., 2013. Ammonia inhibition in anaerobic digestion: A review. Process Biochem. 48, 901–911. 135. Yeshanew, M.M., Frunzo, L., Pirozzi, F., Lens, P.N.L., Esposito, G., 2016. Production of biohythane from food waste via an integrated system of continuously stirred tank and anaerobic fixed bed reactors. Bioresour. Technol. 220, 312–322. 136. Zema, D.A., Fòlino, A., Zappia, G., Calabrò, P.S., Tamburino, V., Zimbone, S.M., 2018. Anaerobic digestion of orange peel in a semi-continuous pilot plant: An environmentally sound way of citrus waste management in agro-ecosystems. Sci. Total Environ. 630, 401–408. 40

List of Figure Caption Fig. 1 Composition of different FPW Fig. 2 Schematic of the metabolic pathway of DF. Fig. 3 Different bioreactor systems used for biohythane production.

Highlights

1.

Carbohydrate rich food waste is a potential substrate for biohythane production.

2.

Accumulation of volatile fatty acid inhibit methane production.

3.

Acidic pH with low buffering capacity leads to accumulation of volatile fatty acid.

4.

Pretreatment and codigestion enhance biohythane production.

Table 1 Oil processing wastes and different reactor configuration used for biofuel production Substrate

Reactor configuration/ Mode

Biofuel Production

1.

POME

AnSBR 35 L (H2); UASB 175 L (CH4); fedbatch/ continuous

73 mL H2 g-1-COD 342 mL CH4 g-1-COD

pH. 5.5 (for H2); 7-8 (for CH4) Temp. 55 º C

2.

POME

Batch Reactor 200 mL

510 mL CH4 g-1 VS

pH 7.0 Temp. 60 º C

3.

OMW + Cheese whey (Liquid cow manure)

Two CSTR 0.5L (H2); 4L (CH4); Batch/continuous mode

H2= 1.72 L1 L d-1 R CH4=0.33 L CH4 LR d-1

pH 6.0 and 7.68 Temp. 37 º C

4.

POME

Batch Reactor 200 mL

0.41 mmol H2 g-1 COD

pH. 5.5 Temp. 35 º C

S. No

Environmental parameters

41

Operational parameters

Pretreatment

HRT 2 d (H2); 10 d (CH4) Mixing of OLR. substrate with 27.5gCODL-1.d methane (H2); effluent (1:1) 5.5gCODL-1.d (CH4) HRT 45 d OLR 11.8 gVS ------------L-1 HRT 0.75 d (H2); 25 d (CH4) Aerobic OLR 126.67 kg COD m3.d (H2); 3.37 kg COD m3.d (CH4) Chemical, HRT 72 h acid, heatOLR 0.19 and shock, 0.37 mmol g-1 freezing and COD thawing and

References

Seengenyoung et al. (2019)

Mamimin et al. (2017)

Dareioti and Kornaros, (2014)

Mohammadi et al. (2011)

base

Olive pulp (Urea (4.2 5. g L-1) + K2HPO4 (2 g L-1) CH4)

CSTR 0.5 L (H2); 3 L (CH4)

H2 = 0.32 (mole kg-1 TS olive pulp) CH4 =1.13± 0.08 (L L1d-1) 330.2 mL L diluted OMW-1

4.8 (for H2) 7.6(for CH4) Temp. 55 º C

HRT 28.7 (H2); 10 d (CH4) OLR 25.8 (g TS d-1) (H2); 7.9 (g TCOD L-1 d-1) (CH4)

Thermal

Koutrouli et al. (2009)

pH 4.7 Temp. 35 º C

HRT 24 h OLR 20.5 gO2 L1 (d-COD feed)

Thermal for 20 minutes for 2 days

Ntaikou et al. (2009)

6.

OMW K2HPO4 (1 g L-1)

CSTR 0.5 L; continuous

7.

POME

AnSBR of 150 mL

6.5 l H2 L-1 POME

pH. 5.5 Temp. 60 º C

------------

Thermal at 60 º C for 4 days

O-Thong et al. (2008)

8.

OMW

Batch reactor of 55 ml

H2 = 0.008 l l−1 h−1

pH 6.8 Temp. 35 º C

----------

With activated clay

Eroğlu et al. (2006)

OMW-Olive mill wastewater; POME-Palm oil mill effluent; H2-biohydrogen; CH4-biomethane; AnSBR-anaerobic sequencing batch reactor; CSTR- continuous stirred tank reactor; UASB-upward anaerobic sludge blanket reactor; FVWfruit and vegetable waste; OP-olive pomace

Table 2 Fruit and vegetable processing wastes and different reactor configuration used for biofuel production S. No

Substrate

1.

TP and GP (Bovine dry manure and mature green waste compost)

2.

TP (Dairy manure and cornstover)

3.

Ensiled OP (Dried poultry manure)

4.

TP and vegetable sludge (waste A); Pig slurry (waste B)

Reactor configuration/ Mode

Biofuel Production

Environmental parameters

Operational parameters

Pretreatment

Batch reactors; batch/fed-batch mode

73.17 mLg-1 dry GP (H2); 201ML g-1 dry TP; 132 ML g-1dry GP (CH4)

Temp 30 º C and 55 º C

OLR 5% (dry weight basis)

-----------

Achmon et al. (2019)

379.1 L kg-1 VSfeed (CH4)

pH 8.3 Temp 35±1 º C

Mechanical pre-treatment

Li et al. (2018)

0.46 L g-

pH 7.4-8.1 Temp 42 º C

-----------

Zema et al. (2018)

--------------

Ros et al. (2017)

Batch reactors with 80 % total volume is the working volume AD of 84 L; Semi continuous mode AD of 300 L total volume; Semicontinuous mode

1

TVS

6100 L (Biogas) (CH4)

pH 6.3 to 7.8 Temp 35±1 º C

42

OLR mixture of (36: 24: 40) % dairy manure, corn stover, and TP respectively (VS based) at 15% TS HRT 15-30 d OLR 1.38 gTVS L-1 d-1 OLR different loading rates including 70% (waste A ) / 30%: (waste B)

References

5.

Potato peel

6.

GP

Batch reactors of 800 ml with manual shaking

273 L kg-1 VSfed (CH4)

pH 7.0-8.0 Temp 35±1 º C

Bad = 0.72 m3biogas kg

OLR 6.4 % TS content with 5.2 % VS content

-------------

Liang and McDonald, (2015)

-----------

Chopping and grinding

Roati et al. (2012)

HRT 31 days OLR 3.5 g COD L1 d-1

Mechanical (size reduction)

Carucci et al. (2005)

-1

Ts

7.

Frozen food factory (fresh VW and precooked FW)

Batch reactors of 200 ml and stirred micropilot reactor of 1.2 L;

10 g COD

pH 7.0 to 8.0 Temp 35 º C

Food 0.17L-1g Thermal preVan Ginkel processing Batch reactors COD-1(H2); treatment of 8. pH 5.7 to 6.1 -------------et al. WW and of 250 ml 0.21Lg100 º C for 2 (2005) 1 -1 potato PW COD h TP-Tomato pomace; GP-Grape pomace; VW-vegetable waste; FW-food waste; WW-wastewater; OP-orange peel; PWprocessing waste, AD-anaerobic digester; UASB-upward anaerobic sludge blanket reactor; FV-Fruit and vegetable; Badbiogas adjustable

Table 3 Dairy processing wastes and different reactor configuration used for biofuel production S. No

Substrate

1.

CW (Dairy manure)

2.

CW (Mozzarella)

3.

Dairy WW

4.

CW (Cattle slurry)

Reactor configuration/ Mode

Biofuel Production

Environmental parameters

Operational parameters

Pretreatment

References

1.37m3 CH4 m-3 d-1

pH 7.1 Temp 35 º C

HRT 8.3 d OLR 5.9 kg VS m-3 d-1

-----------

Rico et al. (2015)

45.1 NL H2 Kg-1 VS

pH 6.0 Temp 39 º C

HRT 2-3 d

Thermal at 105º C for 30 min

De Gioannis et al. (2014)

Batch reactors of 125 ml

13.54 mmol g-1 COD (H2)

pH 5.5 Temp 37 º C

OLR 15.3g COD L-1

Thermal at 90º C for 20 min

Gadhe et al. (2013)

Anaerobic reactor of 102.8 L; batch mode

343.43 L-1 CH4 kg-1 VS.

pH near 7.7 Temp 35 º C

HRT 42 d OLR 2.65 gVS l1d

-------------

Comino et al. (2012)

5. CW powder

Batch reactors of 150 ml

1.03 mol H2 mol-1 glucose

pH 5.5 Temp 55 º C

Autoclaving of the substrate at 121º C for 15 min

Kargi et al. (2012)

6. CW powder

CSTR - 2 L ; UASB - 0.79 L; batch/continuous

28 L-1d-1 (H2); 5 L-1 d-1 (CH4)

pH 5.9 and 7.5 to 7.8 Temp 37 º C and 25-30 º C

Thermal at 100º C for 40 min

CotaNavarro et al. (2011)

CSTR of 21 L ; continuous mode Batch reactors of 0.8 L; (mechanical stirring)

7.

Dairy waste permeate (lactose)

CSTR of 4.5 L; batch mode

8.

CW (Dairy manure)

Anaerobic reactor of 20 L; Semi-continous mode

0.06 Lg-1 COD (H2); 0.33 L g-1 COD (CH4) 1.51 m-3 d-1 (daily biogas) (CH4)

pH 5.5-6.0 Temp 37 º C pH 6.5-7.5 Temp 34 º C

43

L-1

OLR 20 g (optimum)

HRT 6 h OLR 142 g lactose L-1 d-1 and 20 g COD L-1 d-1

HRT 12-14 d Sieving of OLR 15 g COD l-1 the digestate HRT 5 d OLR 28.75 kg COD m-3 d-1

-----------

Banks et al. (2010) Kavacik and Topaloglu, (2010)

7.9 l H2 L1d-1 Thermal preHRT 3.5 d 5417 ml CSTR of 2 L; pH 5.6 treatment of Azbar et al. 1 9. CW WW OLR 35g COD L batch/continuous (total Temp 55 º C 85 º C for 45 (2009) -1 d biogas) min (CH4) 122 mL LUnsterilised 1d (H ); UASB of 4.6 L; pH 5.0 HRT 12 h Castelló et 2 10. CW (0.2g -----------batch mode 1400ml d-1 Temp 30 º C OLR 20 g COD l-1 al. (2009) NaHCO3) (CH4) CW-cheese whey; CSTR-continuous stirred tank reactor; UASB-upward anaerobic sludge blanket reactor; WWwastewater

Table 4 Brewery processing wastes and different reactor configuration used for biofuel production S. No

1.

2.

3.

4.

Substrate

Whisky pot ale

BW

Biofuel Production

Environmental parameters

Batch mode

586 NL CH4 kg-1 VS

pH 7-8 Temp. 37 º C

AnSBR of 3.5 L

pH 5.5 Temp. 36±1 º C

Pre-treatment

----------

Centrifugation, decantation and deprotonation

Barrena et al. (2018)

--------

Estevam et al. (2018)

HRT 72 h OLR 86 g COD L1

References

Thermal preHRT 6 h and 14 h treatment of OLR 44g COD L-1 104 °C for 24 -1 d and 20 g COD h; enzymatic L-1 d-1 hydrolysis

pH 5.5-7.2 Temp. 37 º C

HRT 80 d

Thermal pretreatment of sludge at 100 º C for 30 min

Fu et al. (2017)

Batch reactor of 80 ml

150.7 mmol L-1 and 64.1 mmol L-1 (GDOC)

pH 7.0 Temp. 37 º C

HRT 24h (H2); 12 d (CH4)

-----------

Mishra et al. (2017)

UASB reactors of 1.3 L

239 mL g COD-1 (CH4)

pH 8.0-9.0 Temp. 40 º C

HRT 2.5 d OLR 9.6 g COD L-1 d-1

----------

Janke et al. (2016a)

9-11 L CH4 100 g-1

pH 7.0 Temp. 30 º C

---------

-----------

Malakhova et al. (2015)

57.4 mL H2 L-h ; 257.9 mL CH4 g-1 COD

pH 5.5 and 6.8 to 7.5 Temp. 35 º C

HRT 6 h and 24 h OLR 16 g COD L1 and 1636 mg COD L-1

Thermal treatment at 100º C for 24 h (SBR-1)

Buitrón et al. (2014)

CSTR of 1.0 L; UASB of 1.25 L

Vinasse

Batch reactor of 150 ml

Tequila vinasse (glucose adapted)

0.80-1.67 mol H2 mol-1 glucose 105 mL g-1 (H ); 225 2 mL g-1 (CH4) (bagasse) 14.8 ml g-1 VSsubstrate H2; 274 ml g-1 VSsubstrate CH4

Operational parameters

pH 5.5 and 7.5 Temp. 35 º C and 23-25 º C

Agave bagasse

Distillery effluent (GDOC, 5. MDOC, DDGS and AB) Sugarcane vinasse (Urea-2 g 6. L-1; NaHCO3, and TE) Brewer’s spent 7. grain (Jersalem artichoke) 8.

Reactor configuration/ Mode

SBR - 0.6 L (H2 ); UASB - 0.5 L (CH4) ; batch/continuous

44

Montiel Corona and RazoFlores, (2018)

Thermal treatment at Chu et al. 9. BW 14.6 H2 L-1 100 º C for 24 (2013) h Thermal preBatch reactor of treatment of -1 Sugar 2.8 H2 LR pH 5.5-7.0 2.5 L; 100 º C for 30 Park et al. 10. cane d-1; 1.48 Temp. 35 º C HRT 6 h continuous min; (2010) Molasses CH4 LR-1 d-1 mode enzymatic hydrolysis AnSBR-anaerobic sequencing batch reactor; UASB-upward anaerobic sludge blanket; H2-biohydrogen; CH4-biomethane; NaHCO3- sodium bicarbonate; TE-trace elements; GDOC-groundnut deoiled cake; MDOC-mustard deoiled cake; DDGSdried grain with solubles; AB-algal biomass; BW-brewery wastewater Batch reactor of 2L

pH 5.5 Temp. 35 º C

HRT 8 h OLR 40g total sugar L-1

Table 5 Livestock wastes and different reactor configuration used for biofuel production S. No

Substrate

1.

MIW (OP and OL)

2.

MIW (Dried blood)

3. MIW (pig meat)

4.

MIW

5.

MIW (Poultry)

6.

MIW (Poultry) (OFMSW)

MIW 7. (slaughterhouse)

8.

MIW (slaughterhouse)

Reactor configuration/ Mode Reactor of 200 ml; Semicontinuous mode Batch Reactor of 600 mL

Batch reactor of 80 ml

CSTR with total of 6 L

Batch Reactor of 70 mL; batch mode

Biofuel Production

Environmental parameters

727mL CH4 g-1 VSfed

pH 7.3-7.5 Temp 35 º C

87.6 ml g-1 VS (H2)

pH 6.8-7.4 Temp 35±1 º C

BMP 707 L kg-1 VS-1 and 941 L kg-1 VS-1 Vbiogas 10.8 L d-1 (70 % of CH4)

pH neutral Temp. 37 º C

pH around 6.9 Temp. 37 º C

Operational parameters

Pretreatment

HRT 379 days Thermal and OLR 0.8gVS mechanical L-1d-1 OLR 3.8 g dried blood (equivalent to 3.15 g VS) OLR 8 g (VS L-1) 14 g (VS -1 inoculum g CODtotal)

References

Moukazis et al. (2018)

Chemical, thermal and aeration

Ghimire et al. (2015b)

Alkali, thermal, enzyme and autoclaving

Cavaleiro et al. (2013)

HRT 17 d

---------

Cuadros et al. (2011)

Sittijunda et al. (2010)

136.9 mL H2 g-1 TS

pH 4.63 Temp 37 º C

------------

Dissolved air floatation and Aerobic thermophilic digestion at 55 º C

Mixed stirred digesters of 3 L reactor of 100 cm3; Semicontinuous mode

8.6 L d-1 (CH4)

pH 6.5 -7.5 Temp 34±1 º C

HRT 25 d OLR 1.70

---------

Cuetos et al. (2008)

71.3 N mL kg-1 VSrem (H2 yield)

pH 5.0-6.0 Temp 34 º C

HRT 3-5 d (for H2); 15 d (CH4)

----------

Gómez et al. (2006)

Batch reactors

------------

Temp 25 º C

-----------

Enzymatic hydrolysis

Masse et al. (2003)

BMP-biomethane potential; MIW-meat industry waste; AnSBR-anaerobic sequencing batch reactor; CSTR-continuous stirred tank reactor; OP-orange peels; OL- olive leaves; OFMSW-Organic fraction of municipal solid waste

45

Table 6 Agri-based processing wastes and different reactor configuration used for biofuel production S. No

1.

substrate

Reactor configuration/ Mode

Biofuel Production

Cassava WW (cassava residue)

UASB reactor of 4 and 24 L volume; continuous mode

15 mL H2 g-1 CODremoved 259 mL CH4 g-1 CODremoved

Reactor of 4.3 L volume; continuous mode

30-40 % of H2 in volume and around 70 % of CH4 in volume

pH 5.5-6.0 and 7.0-8.0 Temp 55 º C

Coffee residues 2. (Sugarcane vinasse)

Environmental parameters

pH 5.5 Temp. 55 º C

3.

Peanut shell

Batch reactors of 50 ml volume

39.9 mL-1g-1 substrate

pH 6.5 Temp 35 º C

4.

Wheat straw

CSTR 0.6 L

1.3 mol H2 mg-1 sugars added

pH 6.0 Temp. 55 º C

5.

Sugarcane syrup

CSTR 1 L (H2); UASB 24 L (CH4);

17.5 L-1d-1 (H2) 2.25 L-1d-1 (CH4)

pH 4.5-6.1 and 7.0-8.0 Temp 37±1 ºC

Mixture of ensiled sorghum 6. and CW (Cow manure)

CSTR 0.5 L (H2); 4 L (CH4); batch/continuous

2.14 L-1 LR d (H2) 0.90 L-1 LR d (CH4)

7.

Hazelnut, rice wine lees and GP

pH 5.5 and 8.0 Temp. 37 º C

Operational parameters

Pretreatment

OLR 10.3 kg m3d-1

Thermal treatment at 95 º C for 15 min; filtration, and mechanical

HRT 55d OLR 0.19 kgVS m3.d-1; -------------0.37 kgVS 3 -1 m .d and 0.16 kgVS m3.d-1 Pretreatment of substrate ------------undergone air-dry and grinding HRT 3 d OLR of 4.42 g COD L.d-1

Acid pretreatment

HRT 3 h and 3 d -----------OLR 25 g -1 COD L HRT 0.5 d and 16 d OLR 171.60 Mechanical kg COD m3 and alkali d-1 and 5.36 3 kg COD m d

References

Chavadej et al. (2019)

Pinto et al. (2018)

Qi et al. (2018)

Chandolias et al. (2016) Nualsri et al. (2016)

Dareioti and Kornaros, (2015)

1

Bad = 1.61 m3biogas kg Ts-1 (rice wine lees)

---------------

--------------

Chopping and grinding

Roati et al. (2012)

CSTR HRT 12 h 10.4 L H2 kg0.5 L (H2); pH 5.3 and 7.5 OLR 188.9 Extraction Antonopoulou 1 78 L CH 8. 4 process et al. (2008) 3 L (CH4); Temp. 35 º C mmol glucose kg-1 L-1 d-1 batch/continuous WW-wastewater; H2-biohydrogen; CH4-biomethane; CW-cheese whey; d-days; h-hours; TOC-total organic carbon; VSvolatile solids; FVW-fruit and vegetable waste; OP-olive pomace; GP-grape pomace Sweet sorghum

46