Critical challenges in biohydrogen production processes from the organic feedstocks

The ever-increasing world energy demand drives the need for new and sustainable renewable fuel to mitigate problems associated with greenhouse gas emissions such as climate change. This helps in the development toward decarbonisation. Thus, in recent years, hydrogen has been seen as a promising candidate in global renewable energy agendas, where the production of biohydrogen gains more attention compared with fossil-based hydrogen. In this review, biohydrogen production using organic waste materials through fermentation, biophotolysis, microbial electrolysis cell and gasification are discussed and analysed from a technological perspective. The main focus herein is to summarise and criticise through bibliometric analysis and put forward the guidelines for the potential future routes of biohydrogen production from biomass and especially organic waste materials. This research review claims that substantial efforts currently and, in the future, should focus on biohydrogen production from integrated technology of processes of (i) dark and photofermentation, (ii) microbial electrolysis cell (MEC) and (iii) gasification of combined different biowastes. Furthermore, bibliometric mapping shows that hydrogen production from biomethanol and the modelling process are growing areas in the biohydrogen research that lead to zero-carbon energy soon. Graphical Abstract Graphical Abstract


Introduction
The depletion of fossil-based fuel sources along with their increasing use day by day has created big concerns related to greenhouse gas (GHG) emissions and global warming. Increasing levels of CO 2 , which is a patent GHG emission and associated with burning fossil fuel sources, were found to exceed 409 ppm [1][2][3], which is aiding to the global temperature increase [4,5]. Moreover, growing industrial and economic development of the modern world is also demanding more sources of clean energy for the near future. The increasing gap between the growing energy demand and necessary energy supply due to at the rising human population has sparked a huge interest in new biofuel research as well as production in recent times [6]. Therefore, from the perspective of alternative energy sources, renewable energy sectors like solar, hydro, wind and biofuels like biodiesel, bioethanol and biohydrogen are finding its use in current development agendas across the world. Recently, hydrogen production by water electrolysis has gained global attention as one of the most promising and eco-friendly energy alternatives. H 2 is found to have a high energy content of around 122 kJ/g, about 2.75 times higher than other HC fuels [7,8]. It also possesses wide versatility in its production as well as its applications ranging from fuel-cells to biofertilisers and biofuels. H 2 produced from biological sources is known as bio-H 2 . Hydrogen produces no harmful greenhouse gases upon combustion but only water. Therefore, it is considered one of the energy sources to have the potential to replace part of the conventional fossil-based fuels shortly [9].
As far the production is concerned, fossil fuel is responsible for the majority of hydrogen production, out of which 60% is produced from dedicated primary hydrogen-producing facilities. It is also reported that around 71.27% of hydrogen is produced from natural gas (NG), 27.27% from coal, 0.7% from petroleum and the remaining 0.7% from water electrolysis [10][11][12]. Notably, the hydrogen production from fossil reformation is neither renewable nor carbon neutral as the production process involves high numbers of GHG footprints [4]. H 2 production is also achieved with water gas shift reaction (WGSR), thermal decomposition, catalytic oxidation, steam gasification, pyrolysis and autothermal reforming [13,14]. The recent popularity of waste-to-energy studies also creates an impact on research related to hydrogen production utilising waste materials effectively. Biohydrogen is produced from different organic wastes, thereby solving the issue of waste disposal and energy generation at the same time. Organic waste can be defined as the waste materials that are biodegradable and originates from plants or animals which can be broken into CO 2 , methane or simple organic molecules [15]. Organic wastes like industrial waste, municipal sewage sludge, solid waste, agricultural residues and poultry waste, manure, have the potential to be used for bioenergy production [16].
However, recent publications suggested further investigations are required on the production of H 2 using organic waste materials. The concept of using waste materials from different biological sources to produce environment-friendly biohydrogen can be potentially helpful to tackle the ongoing environmental challenges, while for all H 2 production processes (NG reforming, biomass and coal gasification, water electrolysis and others), there are requirements for better reliability and operating flexibility, a reduction in the capital costs and a significant enhancement in the plant efficiencies [17]. Herein, we assessed the routes of biohydrogen production derived from different organic waste materials and highlighted the key factors affecting the yield of biohydrogen. Furthermore, through bibliometric mapping, we suggest steps and future guidelines from the gaps in the literature for the optimisation of hydrogen production from organic waste streams. Overall, this critical review is aimed at helping the academics working in the biohydrogen production research area along with the industrial application and roll-out of a zero-carbon economy. It will also focus on themes that face the development and potential transformation of the biohydrogen market and its future.

Review methodology
Web of Science (WoS) was utilised herein to obtain the data within the core collection database and then the exported data files; some Boolean operator logic was implemented in the search to find suitable publications and identify evidence gaps in the knowledge and research concerning the biohydrogen topic. A broad timespan of biohydrogen research covering all available year option in the time frame of 1970-2020 is shown in Fig. 1. The bibliometric mapping generated from the WoS core collection is shown in Fig. 1. The overall number of data which was 1539 was exported to the VOSviewer software. Herein, we used the co-occurrence as the type of analysis and all keywords included and the fractional counting method employed. We have direct clusters in Fig. 1 linking specific keywords to general areas such as biohydrogen production. This approach enabled us to visualise the most distinguished keywords in publications in the last 50 years for biohydrogen production. For example, keywords like dark fermentation, water and ethanol production along with lignocellulosic biomass were the most frequently occurring keywords. Other common related keywords to the biohydrogen production are hydrolysis, pyrolysis, gasification, enzymatic hydrolysis, biodiesel production, sludge, microalgae, wastewater, anaerobic digestion, photo-fermentation, glucose, supercritical water and saccharification. Furthermore, the WoS search showed other keywords associated with the production conditions such as pre-treatment, pH, light and temperature. On the other hand, new keywords have been introduced to biohydrogen production recently such as methanol, modelling, storage, fuel-cells, energy recovery, organic waste, bioreactors, light intensity, methanogenesis along with the techno-economic and life cycle assessment (LCA) studies. This implies that areas such as hydrogen production from methanol need further investigations along with modelling, techno-economic analysis and other research areas.

Production of biohydrogen from organic waste through biological methods
Although most of the hydrogen production currently is based on fossil fuels, efforts to produce biohydrogen from different bioresiduals such as wastewater or organic wastes are seen to be increasing [18]. Currently, the most popular, widely discussed and developed processes of biohydrogen production using organic waste material are (a) biofermentation (dark fermentation and photofermentation), (b) biophotolysis (direct and indirect), (c) bioelectrochemical system such as microbial electrolysis cells (MEC) and (d) gasification [19,20]. Table 1 shows the recent status of the different biohydrogen production processes along with their energy conversion efficiencies. Energy conversion efficiency can be defined as the ratio between the useful output of an energy conversion machine/process and energy input [25]. In this section, these processes of biohydrogen production are discussed. Figure 2 shows the different biological processes involved in biohydrogen production. Biohydrogen production with dark fermentation, MEC and biomass gasification possesses high process efficiency, and biomass is the common feedstock in all of the aforementioned processes. Again, photofermentation and biophotolysis can be seen dependant on solar energy for the production of biohydrogen. Different process parameters are associated with all these processes, which have their own importance. Every process has its certain specification and operating conditions along with advantages and disadvantages which are discussed below.

Fermentation
Fermentation can be defined as the process of energy generation involving an endogenous electron acceptor from the oxidation of organic waste materials using a number of different microorganisms. The results of fermentation depend on the applied catalyst (isolated enzyme or microorganism producer) and used organic substrate (mostly carbohydrate or protein), along with the process parameters. The character of the fermentation process can be either aerobic or anaerobic [26]. Fermentation of organic waste materials using microorganisms under anaerobic conditions is a good way to produce H 2 along with other organic alcohols/acids as by-products. Depending on the necessity of light for the microorganisms, the biofermentation can be divided into two types: (a) dark fermentation and (b) photo fermentation. Dark fermentation is the process of fermentation carried out in dark anaerobic conditions, where breakdown of cellulosic organic feedstock results in the production of biological hydrogen along with organic acids and alcohols [27].
Unlike dark fermentation, photofermentation uses photosynthetic bacteria that use sunlight to produce CO 2 and H 2 from organic molecules under anaerobic conditions [28]. For improving the yield of biohydrogen, studies related to the integration of both the two fermentation processes can also be found. Figure 3 shows the two types of biofermentation processes used for H 2 production.

Dark fermentation
Dark fermentation has become one of the well-known technologies for biohydrogen production, which enables the microorganisms to produce H 2 in a dark anaerobic condition [29]. However, with the formation of many by-products, the low H 2 yield on substrates is a major disadvantage. Equations 1 and 2 show the main reactions that are involved in the dark fermentation process of hydrogen production. Equation 1 shows the reaction for H 2 production as a result of the proton reduction by generated electrons from C-source degradation.  Biomass gasification Biomass Commercial 88.1 [24] yield of 4 mol H 2 /mol glucose can be seen to be achievable in the dark fermentation process practically, though Eq. 2 shows a theoretical yield of 12 mol H 2 /mol glucose [31]. Higher yields can be achieved in thermophilic fermentations. This low yield in dark fermentation is mainly happening due to the production of other by-products such as acetic acid, propionic acid and butyric acid. Equation 3 shows the acetic acid pathway, where the reaction of glucose and two water molecules produce acetic acid (CH 3 COOH). Similarly, propionic acid can be found to be produced along with acetic acid from glucose, as shown in Eq. 4. Again, Eq. 5 shows the production of butyric acid from glucose reacting with six water molecules [32]. In all the three pathways, CO 2 and H 2 are seen to be produced in different quantities.
(Acetic acid pathway) (Propionic acid pathway) (Butyric acid pathway) Several types of waste with different chemical compositions are seen used as a substrate to produce H 2 in the dark fermentation process. Among those, the most widely used waste includes agricultural wastes (viz. rice/wheat/corn straw, animal manure), various wastewater types (viz. distillery wastewater, cheese whey effluent, palm oil mill effluent), food waste, municipal sewage waste and sewage sludge [33]. The sugar or carbohydrate-rich waste substrates tend to produce more H 2 compared with lipid or protein-rich substrates. A linear correlation between H 2 production and the proportion of carbohydrate-rich waste substrate was also found [34]. Waste like sewage sludge and palm oil mill effluent usually have a low H 2 yield compared with other waste due to the high presence of protein or lipid [33].
The pre-treatment is a crucial step in biohydrogen research. Table 2 shows various studies related to pre-treatment methods, like physical (high temperature, ultrasonication and microwave), mechanical (milling and grinding), enzymatic, radiation and hydrothermal pre-treatment for the improvement of H 2 yield [49][50][51]. Different types of substrates need different pre-treatment methods, which can enhance the production efficiency of hydrogen. Pre-treatment of dairy manure can be done mainly with three different methods: (a) acid (0.2% w/w HCI solution) treatment, (b) alkali (0.2% w/w NaOH solution) treatment and (c) 2 h infrared oven treatment [33,35]. In the case of sewage sludge, 15-min boiling at around 100°C completes the pre-treatment [36]. Pre-treatment of rice straw for hydrogen production was found with boiling at 80-100°C [37], and in another case, treatment with alkali solution (1% w/w) was found with cellulose hydrolysis after cutting and grinding (2 mm size) [41]. Distillery wastewater was also found to be pre-treated with pH neutralisation, centrifugation and sterilisation [39]. Food waste was found to be pre-treated in many ways. Sieving and 6 h boiling at around 100°C of food waste hydrolysate for hydrogen production were reported by Han et al. [42]. Kim et al.
[43] mentioned pre-treatment of food waste and sludge mixture with 30-min heating (at 120°C), alkalisation (3 M NaOH) and acidification (3 M HCl). Kitchen mill shredding was also applied as a pretreatment method to food waste combined with 5% glycerol [44].
The H 2 yield of 1130 mmol/g COD was reported for plain palm oil, while an improvement of 2760 and 1880 mmol/g COD was found for surfactant (Tween 80) and enzyme (Optimase BG) pre-treatment, respectively [52]. Efficient H 2 production with lignocellulosic materials like sugarcane bagasse rice/corn/wheat straw and corn stalk from agricultural waste needs pre-treatment as mentioned in several different studies [53,54]. An increase in 47.3% of biohydrogen production was seen for pre-treatment of rice husk with a commercial enzyme (Celluclast 1.5 L) compared with that of rice husk without pre-treatment (321 mL H 2 /g rice husk) [54]. Similarly, 35% high H 2 yield (155 mL H 2 /g VS) was seen in  Glucose -Thermotoga neapolitana 6.5 70.0 1.7***** [47] Wheat straw Overnight soaking in acetic acid, steam explosion at 190°C (for 10 min), enzymatic hydrolysis for 72 h Caldicellulosiruptor saccharolyticus 6.5 ± 0.1 70.0 134.0****** [48] *mL/mL wastewater; **mL/g-COD; ***L/L hydrolysate ****mL/L hydrolysate; *****mol H the case of cornstalk pre-treated with lime compared with that of the untreated stalk (115 mL H 2 /g VS) [55]. Song et al. [56] studied biohydrogen production from an aquatic weed, Alternanthera philoxeroides, pre-treated with 1% H 2 SO 4 at 135°C for 15 min, using Enterobacter aerogenes ZJU1. The optimum H 2 production was found to increase by 59.9% to reach production of 62.2 mL/g with pre-treatment compared with 38.9 mL/g VS for the raw material, without pre-treatment. That low hydrogen yield may be due to the utilisation of different feedstocks (115 mL H 2 /g VS). Shao et al. [57] used dilute acid (1% H 2 SO 4 )-pre-treated duckweed biomass for H 2 production using dark fermentation. They found a maximum H 2 yield of 169.30 mL/g dry weight under a temperature condition of 35°C and an initial pH value of 7.0. Acid pre-treatment (0.2% HCl) of dairy manure was also seen improving the H 2 yield by 36%; further 6.8 and 4.5% improvement in H 2 production from dairy manure was reported for base pre-treatment (0.2% NaOH) solution and infrared oven pre-treatment, respectively [35]. Thus, it can be seen that pre-treatment of substrates is highly recommended for good yield of biohydrogen in dark fermentation. Another important parameter for the dark fermentation biohydrogen yield is the pH environment value. pH level in the dark fermentation process is found to influence the metabolic pathway and microorganism activity of the microorganisms and thereby affect the substrate degradation and production efficiency. The pH levels at the start of operation and during the process were seen carefully maintained in many dark fermentation studies [58][59][60]. Using dark fermentation of cheese whey wastewater, the highest biohydrogen production was found at pH 5.5 and pH 6.5 for thermophilic and mesophilic conditions, respectively [59]. Xing et al.
[35] studied a wide variation in pH between 4.0 and 12.0 for fermentation of dairy manure. At pH 5.0, they found the highest biohydrogen yield of 31.5 mL/g VS. A pH below 4.0 and above 12.0 showed no biohydrogen production.
Hydrogenotrophic methanogens act as one of the major H 2 -consuming microorganisms which reduced the H 2 yield by consuming H 2 to produce methane. Therefore, inhibiting the production of hydrogenotrophic methanogens, which acts as an H 2 -consuming microorganism, is one of the major steps for dark fermentation. Pre-treatment of inoculum is considered for enriching H 2 -producing bacteria and suppressing H 2 -consuming methanogens. Since methanogens are strictly anaerobic microorganisms, aeration around the reactor can inhibit the methanogen production and thereby increase the H 2 yield [61]. The impact of pH in the growth of methanogen is another important aspect of biohydrogen yield. It has been reported that methanogens are capable of producing methane by consuming H 2 under an optimal pH range of 7-8 and optimal hydraulic retention time (HRT) of 15-20 days [62]. Kumar et al. [63] attained a hydrogen yield of 29.5 mL/g VS with pH 5.5 and methanogenic inhibitor from mixed microalgae biomass (Scendesmus and Chlorella).
Production of biohydrogen by dark fermentation is accomplished by various microorganisms that are capable of converting a wide range of organic waste substrates. Based on different living temperatures, these microorganisms are classified as thermophiles (45-65°C), mesophiles (25-45°C) and psychrophiles (0-25°C). The commonly used mesophilic cultures for H 2 production are Clostridium and Enterobacter (Clostridium beijerinckii, Clostridium butyricum, Enterobacter aerogenes and Enterobacter asburiae); while the most reported thermophilic one is T h e r m o a n a e r o b i u m ( T h e r m o a n a e r o b a c t e r i u m thermosaccharolyticum) [35]. Again, depending on their growth of metabolism in the presence of oxygen, they are divided as facultative (e.g. E. cloacae, Enterobacter aerogenes, Citrobacter intermedius and Escherichia coli) or obligate bacteria (e.g. C. paraputrificum, Ruminococcus albus and Clostridium beijerinckii) [13,64]. Facultative bacteria are the organisms that make ATP by aerobic respiration (in the presence of oxygen) and are also capable of anaerobic respiration or fermentation (in the absence of oxygen). On the contrary, obligate bacteria are unable to produce ATP (in the absence of oxygen) and cannot live in the presence of oxygen.
Enterobacter and Clostridium are two species of grampositive bacteria for large-scale production of hydrogen for their ability of fast-growing and forming endospores. Lactic bacteria like Klebsiella pneumoniae, Cellulomonas and some thermophilic archaea like Thermotoga neapolitana and Caldicellulosiruptor saccharolyticus were also found showing good results for H 2 production through dark fermentation [65].
HRT (hydraulic retention time) acts as one of the important parameters for proper fermentation of substrate and efficient H 2 production. The stability of the reactor and utilisation efficiency of the feedstock depends on HRT. Santiago et al. [66] found that HRT and solid retention time (SRT) have a great impact on the biohydrogen production and associated subproducts from organic solid waste (OSW) using a dark fermentation process. A 16 h of HRT and 55 h of SRT were found to be the optimum conditions to maximise the biohydrogen production. HRT was found as the main influencing parameter in the whole process. The substrate hydrolysis rate increased with decreasing HRT time. Moreover, substrate hydrolysis-ssolubilisation process time got reduced with an increase in SRT and a decrease in HRT. Fatty acid production was found maximum with long SRT and HRT of 60 h and 48 h, respectively. Lu et al. [67] studied the effects of HRT and concentration of substrate on the HPR (hydrogen production rate) from glucose in a pilot-scale bioreactor of 3 m 3 with three sequential chambers of 1 m 3 each. A HRT of 24 h and substrate concentration of 30 g/L with a maximum HPR of 100.2 mol/m 3 -d were found optimal for the reactor.
The production of biohydrogen using dark fermentation of two different cheese deproteinisation diary waste streams SCW (second cheese whey) and CCWP (concentrated cheese whey permeate) was studied by Colombo et al. [68]. With an increasing OLR (organic loading rate), H 2 production was seen increasing to 3.47 NL H 2 /d and 5.07 NL H 2 /d for SCW and CCWP, respectively. Similarly, organic acid yield was also found higher with increasing OLR (14.6 g/L/d and 12.6 g/L /d for SCW and CCWP, respectively). Table 2 describes different studies of biohydrogen production from wastes using a dark fermentation pathway. It can be seen that combined fermentation of different substrates leads to an increased biohydrogen yield. Moreover, pre-treatment processes such as acid treatment, base treatment, heat treatment and pH neutralisation have shown a significant impact on the yield of biohydrogen. Most of the studies were found to utilise a mixed culture process for good results.

Photofermentation
The production of H 2 with photofermentation involves decomposition of organic acids with the aid of light-dependant, sulphur and non-sulphur purple bacteria. A group of bacteria having the ability to do photosynthesis is known as purple sulphur bacteria. Again, purple non-sulphur bacteria (PNSB), commonly known as photobacteria, are a group of photoheterotrophic bacteria capable of degrading several carbon substrates like carbohydrate, organic matter, biowastes and organic acids for the production of H 2 [69] . Equations 6 and 7 show the reaction involved with the production of H 2 by photofermentive process from glucose and acetic acid, respectively. Oxidation of organic acids, like acetic acid, propionic acid, butyric acid, lactic acid and malic acid, by photofermentive bacteria, produces H 2 and CO 2 . Therefore, to obtain a higher H 2 yield, the two-stage dark fermentation process is often followed by a photofermentation process [70]. The energy needed for the growth of microorganisms is gathered from the production of adenosine triphosphate (ATP) using light through photophosphorylation [4]. Batch or continuous photofermentation process can be obtained using an artificial source of light or solar illumination as shown in Fig.  3b.
The photofermentation process offers the possibility of high H 2 production from a wide variety of substrates including wastewaters (such as olive mill wastewater, dairy wastewater, brewery wastewater) and wastes rich in organic acids (such as dark fermentation effluent, agricultural waste after hydrolysis) [33,71]. The best H 2 -producing microorganism for the photofermentation is PNS (purple non-sulphur bacteria), which include the Rhodobacter species (Rhodobacter capsulatus, Rhodobacter sphaeroides, Rhodovulum palustris and Rhodopseudomonas sulfidophilum) [72]. Some other bacteria used in H 2 production using nitrogenase and ATP production are Chlorobium vibrioforme, Allochromatium vinosum, Desulfuromonas acetoxidans, Thiocapsa roseopersicina and Chloroflexus aurantiacus [13]. Hydrogenase and nitrogenase are two different enzymes that help these bacteria to produce H 2 from organic acids using solar energy [73]. Nitrogenase are found to be the main enzymes responsible for H 2 production in limited-O 2 conditions. NH 3 is generally produced from N 2 by nitrogenase (in largescale production), but in absence of N 2 , ATP is used along with redundancy by nitrogenase to generate H 2 [13], as shown in Eq. 8.
Several studies can be found regarding photofermentive H 2 production in recent years. Mirza et al. [74] found a wide range of 148-513 mL H 2 /L photofermentive biohydrogen production using raw sugarcane bagasse with the help of PNSB (purple non-sulphur bacteria) isolated from the paddy rice field Rhodobacter capsulatus-PK. A maximum yield of 96 mol H 2 /mol sugar was achieved with initial pH 7.0 ± 0.2 and 10% (v/v) inoculum size, at a temperature of 30 ± 2.0°C along with a light intensity of 120-150 W/m 2 . The production of 671 mL/L of H 2 from glucose was also found with this process. For cost reduction of temperature control during summer, Rhodobacter capsulatus-PK was found as a good candidate for photofermentive bio-H 2 production. García-Sánchez et al. [75] used Rhodopseudomonas pseudopalustris to produce H 2 by tequila vinasses (VT) photofermentation. Compared with synthetic medium, they found a double H 2 yield with VT. With the replacement of H 2 by N 2 compared with unchanged headspace, three-time growth was seen in R. pseudopalustris up to 4.5 g/L, and the H 2 yield also increased to 860 mL H 2 /L. Laurinavichene et al.
[39] used PNS bacteria and anaerobic saccharolytic consortium to perform sequential dark photofermentation, which resulted in 17.6 L/L of distillery waste of maximum H 2 yield. Machado et al. [76] investigated the influence of milk whey permeate and glucose on the H 2 yield using PNS bacteria Rhodobacter capsulatus and Rhodopseudomonas palustris through co-culture. The maximum H 2 yield was found to be 287.39 ± 5.75 mmol of H 2 /L day. Keskin and Hallenbeck [77] used beet two major sugar mill waste-black strap and beet molasses for biohydrogen production using photofermentation. The H 2 yield found from pure beet sucrose, black strap and beet molasses are 14 H 2 /mol sucrose, 8 H 2 /mol sucrose and 10.5 mol H 2 /mol sucrose, respectively. A comparative study of different parameters involved in photofermetive biohydrogen production process is shown in Table 3. The  [88] temperature variation clearly shows that the optimum operating temperature range of photofermentation lies between 28 a n d 3 2°C . F u r t h e r , t h e h i g h e s t H 2 y i e l d w i t h photofermentation can be seen with a neutral pH value (around 7) in most of the cases [89]. Moreover, the light intensity and HRT play a very important role in the H 2 yield in photofermentation. Because of the slow metabolic activity of PNSB in photofermentation, usually longer HRT can be seen compared with dark fermentation [33]. Moreover, light source plays a very important role in the growth of microorganisms as well as the H 2 yield in photofermentation, which can be easily seen in Table 3.

Biophotolysis
Biophotolysis or water-splitting photosynthesis is the process in which by using oxygenic photosynthetic microorganisms like cyanobacteria and green microalgae, H 2 can be produced with only sunlight and water. For this process, FeFe-hydrogenase is needed for the green microalgae application and heterocystous cyanobacteria nitrogenase finds its use [13]. Biophotolysis H 2 production can be divided into two ways: (a) direct biophotolysis and (b) indirect biophotolysis.

Direct biophotolysis
In the direct biophotolysis, photosynthetic microorganisms like green algae and cyanobacteria absorb 400-700 nm solar radiation for their cell growth [90]. After accepting solar radiation, the microorganisms can evolve hydrogen through nitrogenase or hydrogenase. In direct biophotolysis, water splitting occurs with a light energy of 680 nm wavelength to produce protons, electrons and oxygen as shown in Eq. 9. The electrons derived from Eq. 9 are transferred through PS II and PS I to a potentially sufficient amount for ferredoxin (Fd) reduction. The reduced Fd then is used for the reduction of hydrogenase enzyme NADP+ to NADPH, which is responsible for the production of H 2 , as shown in Eq. 10 [13].

Indirect biophotolysis
Indirect biophotolysis involves a two-step photosynthetic conversion of light energy to carbohydrates as a form of chemical energy. As shown in Eq. 11, in the first step, using light energy O 2 and carbohydrate (starch and glycogen in green algae and cyanobacteria, respectively) are produced [91]. By limiting N 2 during Eq. 10, an increase in carbohydrate yield and reduction in O 2 amount can be achieved, which subsequently is advantageous for high H 2 yield. The second step involves the conversion of carbohydrate to CO 2 and H 2 with light energy under an anaerobic condition with less O 2 , as shown in Eq. 12 and Eq.13. [73].
Many recent research studies can be found producing biohydrogen from green algae and cyanobacteria as shown in Table 4

Bioelectrochemical system
Bioelectrochemical system of H 2 production from a wide variety of substrates using microbial electrolysis cells (MEC) is a new technology getting popularity in recent years. MEC technology is also known as biocatalysed electrolysis cells or electrofermentation [13]. As shown in Fig. 4, the MEC system has two electrodes, cathode and anode, which can either be placed in the same single chamber (single-chamber MEC) or be separately placed in two individual chambers (two-chamber MEC). In the two-chamber MEC, to separate the two chambers, commonly a proton exchange membrane is used.
Other recently developed membranes include a charge-mosaic membrane, cation/anion exchange membrane and bipolar membrane [102]. In the two-chamber MEC, the anode chamber is filled with the organic wastewater, while the cathode chamber can be filled with different solutions (like moderate acidified water, phosphate-buffered solution, bicarbonate buffers and salt solutions) [103,104]. The main working process in both the MEC types is the same. Electrons get generated by the oxidation of organic matter in the anode, which are transported to the anode. Then, they are transported to the cathode where upon combining with protons, H 2 gets generated [33]. The initial MEC systems comprised of two chambers avoiding interference of electrodes, which produced highpurity H 2 [105]. MEC acts as an anaerobic system sensitive to oxygen. Equations 14-16 show the production of H 2 using MEC for acetate. In addition to a potential generated by microorganisms (− 0.300 V), MEC needs a small external potential of more than 0.110 V for the production of H 2 [106]. The external power source use of the battery is generally considered, but the use of renewable power generated from solar, wind, MFCs and waste heat can be seen [19,107].
Overall : Many different substrates were found in use for MEC to produce H 2 . Some common pure chemical substrates used are butyrate, glucose, acetate and glycol. However, different waste streams like poultry farming wastewater [108,109], domestic wastewater [105,110,111], waste activated sludge [112][113][114] and industrial wastewater [115,116] are used in MEC. Tenca et al. [116] found a higher H 2 yield for methanolrich industrial wastewater compared with food processing wastewater, but the food processing wastewater was found to have high H 2 selectivity of around 86% compared with that of industrial chemical wastewater. Improvement in the H 2 yield can be seen in many studies with MEC coupled with anaerobic digestion and/or dark fermentation [114,115,117,118]. Huang et al. [117] studied the H 2 production from food waste from anaerobic digestion coupled with the singlechamber MEC. They found 511.02 mL H 2 /g VS of the H 2 yield from the continuous AD-MEC process which was much higher than the AD H 2 yield (49.39 mL H 2 /g VS). Dhar et al. [115] studied the H 2 yield from sugar beet juice using an integrated MEC dark fermentation process. Overall H 2 yield with the integrated process was found to be 25% of initial chemical oxygen demand (COD) (6 mol H 2 /mol hexose added ) which is much higher than that of dark fermentation alone (13% of initial COD). Li et al. [118] also found a maximum H 2 yield of 387.1 mL H 2 /g corn stalk with the integrated dark fermentation MEC process, which was around thrice that from dark fermentation alone with 20 g/L of corn stalk input and 7.0 initial pH value. Lu et al. [114] also found twice H 2 yield with waste activated sludge coupled with MEC (Table 5).
Raw materials, temperature, pH and operating voltage play an important role in determining the H 2 yield in MEC. However, with MEC, it has been noticed that operating temperatures from 0 can be used in producing biohydrogen from wastewater without having significant effects on the yield. An H 2 yield of 0.015 L/L/d was found with domestic wastewater within an operating temperature range of 13 to 21°C [105]. Heidrich et al. [120] found improvement in exoelectrogen activities with temperature while studying MEC with domestic wastewater within 1-22°C. Patil et al. [121] also demonstrated operating MEC with wastewater within 0 to 45°C. Better performance was seen within 10°C to 20°C, thereby showing the advantages of MEC over other fermentative biohydrogen production processes. Again, an increasing H 2 yield with increasing external applied voltage was reported in the literature [108,113].
In MEC, certain microorganisms which are capable of transferring electrons from the chamber to anode are used, known as electrogens. Shewanella spp. and Geobacter spp. are two popular electrogenic groups, out of which Shewanella oneidensis and Geobacter sulfurreducens are the most discussed species [122]. Acetobacterium woodii, Ochrobactrum anthropic, Sphingomonas strain DJ, Rhodopseudomonas palustris and Rhodoferax ferrireducens are some other exoelectrogenic species reported in recent studies [33, [122][123][124]. Rago et al. [125] found a high H 2 yield (2.6 L H 2 /L RE ACTOR /d) with alkaline MEC, using Alkalibacter sp. as exoelectrogen.

Biohydrogen production through gasification
Gasification of biowaste is another way of producing bio-H 2 . In gasification, syngas (a mixture of CO, CO 2 , H 2 and CH 4 ) and several by-products (tar, char, light HCs) are produced by partial oxidation of organic materials at high temperature and pressure [126]. Even though gasification is not a biological process, it is effective for organic waste conversion to hydrogen. The concentration of H 2 produced during gasification can be improved by optimisation of operating parameters. Equations 17-23 show the main reactions involved during gasification.
C þ CO 2 →2CO ð20Þ *mmol/g COD; **mL/g VSS; ***mL/g COD; ****mL/L/d Gasification of different types of waste materials like sewage sludge, municipal solid waste, agricultural and forest biomass, animal manure and food waste has been seen as a popular technology to produce hydrogen [33]. Prasertcharoensuk et al. [127] studied the effect of parameters on hydrogen production through lignocellulosic biomass waste gasification. H 2 content in the syngas was found increasing up to 67 mol% with pyrolysis temperature higher than 800°C and 0.5-1 cm 3 particle size. Su et al. [128] studied the effects of temperature (400-450°C), food additive (NaHCO 3 , NaCl and NaOH) and reaction time (20-60 min) on the supercritical water gasification of food waste. They found a maximum H 2 yield of 12.73 mol/kg with NaOH as a catalytic agent. Zhang et al. [129] found 28.9% H 2 content from food waste with an anaerobic digestion and gasification integrated process. Chang et al. [130] found a maximum of 29.72 g H 2 /kg substrate and 19.78 g H 2 /kg substrate H 2 yield with bagasse gasification and waste mushroom gasification, respectively. Shie et al. [131] studied plasma gasification of lignocellulosic municipal solid waste for H 2 production. The effect of different factors like biomass type, reaction temperature, feed size, catalyst type and SB (steam-to-biomass) ratio on the H 2 production in a steam gasification process is discussed by Parthasarathy and Narayanan [132]. Nanda et al. [133] studied supercritical water gasification of different agro-food residues and fruit wastes like a banana peel, Aloe vera rind, lemon peel, coconut shell, sugarcane bagasse, pineapple peel and orange peel. During the production of biodiesel, glycerol is produced in large quantities as a by-product. Recently, Osman et al. used glycerol along with the alumina foil waste using photocatalysis to produce a steady state of 4.2 millimole H 2 g/TiO 2 hr., which is a promising result of multifunctional cheap photocatalytic materials for the production of green biohydrogen [134].

Challenges with biohydrogen production through biological methods
Several studies have been made so far for enhancing the economic feasibility of the H 2 production process via biological methods. Although these processes have different advantages, there are many key challenges also which need to be addressed in future studies [4,13,33]. Table 6 describes the different advantages and challenges associated with these processes. As shown in Table 6, the biohydrogen production processes vary from process to process. The maximum yield of H 2 production was found to be 14.2 ± 0.2 mL/g VSS, and H 2 production rate was 0.13 mL/g VSS h [135]. It was found that for photofermentation, the maximum H 2 yield was 642 ± 22 mL, and the maximum H 2 production rate 77.78 mL/ L/h, with an initial pH of 7 [136]. In another case, the effect of adding corn stalk enzymatic hydrolysate H 2 yield was found to increase up to 1287.06 mL H 2 /g TOC, and the maximum H 2 production rate was found to be 10.23 mL/h [137]. Kossalbayev et al. [98] found a maximum H 2 yield of 0.348 μmol H 2 /mg Chl/h with Desertifilum sp. IPPAS B-1220. Moreover, energy conversion efficiency with biophotolysis was found to be around 2.4-4% [22]. Jayabalan et al. [138] found a maximum H 2 production rate of 4.38 ± 0.11 mmol/L/D from the sugar industry wastewater using MEC. A H 2 production rate of 3.48 L/L/d and an H 2 yield of 511.02 mL H 2 g −1 VS was reported from food waste anaerobic digestion coupled with MEC [117].
Water electrolysis is another way of producing hydrogen from water using electricity. To produce 1 kg H 2 , around 9 L of water is needed and 8 kg of O 2 occurs as a by-product in this process. The hydrogen produced with water electrolysis has a purity of 99.99 vol% (strongly depending on the type of electrolysis (AEL, PEM, etc.)) [139]. Yuzer et al. [140] found a maximum hydrogen production rate of 11.4 mmol/h with the use of a bipolar membrane. They found the highest energy efficiency of 82% and an exergy efficiency of 68% with the anion exchange membrane. Chakik et al. [141] found a maximum efficiency of 99.13% with a production rate of 2.34 mL/ min using a Zn 95 %Cr 5 % electrode in 20 g/L NaOH solution at 0.45 A, 5 V. Kovač et al. [142] studied H 2 production with a rate of 1.138 g/h from the electrolysis of alkaline water using solar energy.
Bio-H 2 production through the biological methods, for instance, dark fermentation, can produce H 2 without light along with in photofermentation, and photosynthetic bacteria can use a wide range of spectral energy. However, the energy conversion efficiency, in general, is low with 4.3 and 5.11% for dark and photofermentaion processes, respectively [21]. The major challenges herein are the low bio-H 2 production rate and yield and the high cost of the raw feedstocks; thus, using organic waste materials helps to address this issue.
Overall, hydrogen can be produced from various sources, with potential supply from renewable electricity, nuclear power and lignocellulosic biomass. However, it is currently dominated by using fossil-based fuels. From biomass sources, H 2 production comes mainly from anaerobic digestion, fermentation or gasification routes. While the former route is mature, it only processes specific feedstocks (food waste, sewage sludge and crops waste). While fermentation can utilise and process the non-edible cellulosic part of lignocellulosic biomass, gasification can process the whole portion of the biomass, but the technology is still not fully mature worldwide. H 2 production mostly comes from natural gas and coal, while during its production globally, a greenhouse gas in the form of CO 2 is released which is equivalent to the combined generated annual CO 2 emissions of the UK and Indonesia with an energy consumption of 275 million tonnes of oil equivalent (2% of total worldwide energy demand) [10]. Thus, carbon capture and storage (CCS) is crucial when producing H 2 from fossilbased fuels along with maximising our way of producing H 2 from clean electricity. Currently, the International Energy Agency (IEA) reported that the technical potential of producing hydrogen from renewable electricity is expensive. However, it is expected to decrease by 30% by 2030 due to the scaling up of H 2 production along with progress in renewables technology that comes with a reduction in the costing. Three major technologies could benefit from that: electrolysers (splitting water using electricity to produce H 2 ), fuel cells and refuelling equipment. With the progress in solar photovoltaic and wind renewable energy technologies along with batteries, renewable electricity could provide both low-carbon electricity and low-carbon H 2 , as well as using electrolysis, which accounts for only 2% of the global hydrogen production now. Economically, H 2 production from natural gas is the cheapest method in most of the countries around the world, such as in the Middle East which costs (1$/kg H 2 ). On the other hand, electrolysis cost is 10-40$/MWh along with full load hours of 3000-6000, so it can compete with natural gas coupled with CCSU (carbon capture storage and utilisation). Interestingly, countries that import natural gas and have available sources of renewables or nuclear power could easily find electrolysis as an attractive option. However, the production of H 2 -based fuel using hydrogen as a feedstock is not economically feasible at the moment.
Overall, electrolysis is a promising route where the efficiency of the electrolyser ranges from 60 to 80%, while for other green hydrogen routes such as dark fermentation, photofermentation, biophotolysis and microbial electrolysis cells, their energy conversion efficiencies are low which are 4.3, 5.11, 4.0 and 11.3%, respectively [21][22][23]. This is as a result of the complex structure of the biomass that requires complicated processing procedures during the production of green bio-H 2 . Also, finding the cheap feedstock of biomass is crucial herein. For instance, to meet the theoretical H 2 production demand in the USA, which is 60 MtH 2 , this would require nearly 100% of its biomass resources. However, by employing PV or wind power, only 1% or 6% will be required [143]. The factors that affect the costing of H 2 production from electrolysis are the cost for the electricity, capital expenditure requirements, conversion efficiency and annual operating hours.

Conclusion
For the future of the zero-carbon economy, biohydrogen is considered a promising candidate for fossil fuel replacement due to its zero-carbon emission. This review study provides a brief critical technological discussion and analysis of the processes that are used in biohydrogen production from organic biowastes along with the factors responsible for the efficient H 2 yield. Herein, raw materials, processing and production techniques and environmental influences of biohydrogen production have been reviewed. Wide varieties of biowaste materials, such as wastewaters, forest and agricultural residues, food wastes and municipal and sewage wastes, have been utilised in biohydrogen production. Regarding the high H 2 yield and feedstock availability, dark fermentation, photofermentation and gasification showed clear promising results. The combined fermentation processes also have shown promising results in different studies. Pre-treatment of the substrate, pH, temperature and hydraulic retention time (HRT) are crucial factors in regulating the optimum biohydrogen production route. The MEC method showed promising results with a good yield of biohydrogen using waste feedstock under low-temperature conditions. However, a large-scale production with these processes is still challenging. The need for future studies addressing more variants of microorganisms and waste varieties is highly observed. It is the authors' thought that the integration of more than one production process along with different biomass waste streams is required along with modelling to allow better processing for biohydrogen production. This would help alleviate issues concerned with fossil-based fuel, while also promoting environmental benefit as in the production of biohydrogen from sustainable waste materials and consequently working toward the zero-carbon economy.