Biogas, which contains abundant methane (approximately 60%) and carbon dioxide (approximately 40%), as well as small amounts of hydrogen sulfide, ammonia, hydrogen, and oxygen, is a common fuel. Therefore, increasing the use of biogas will alleviate the rising global energy and environmental crises ^[1][45].

Kitchen waste is partially converted into biogas through anaerobic digestion (AD) by anaerobes in the digestion pool. The production of biogas through AD is one of the best feasible options due to its low energy requirement and eco-friendly nature, in comparison with other methods ^[2][46]. The mechanism of the process is still under discussion. To date, mainly two-stage, three-stage and four-stage theories ^[3][47] have been proposed. The four-stage theory of hydrolysis→acidogenesis→acetogenesis→methanogenesis is now usually applied (Figure 1). Various anaerobic stages are induced by the corresponding anaerobic bacterial communities ^[4][48]. Clostridium, Micrococci, Bacteroides, Butyrivibrio, Fusobacterium, and Selenomonas are involved in rate-controlling steps ^[5][49], namely, they participate in the hydrolysis stage to convert insoluble organics into soluble organics.

Streptococcus, Lactobacillus, Bacillus, Escherichia, and Salmonella are involved in the conversion of hydrolytic products into low molecular weight products, such as propanoic acid, butanoic acid, ethanoic acid, aliphatic acid, alcohol, and carbon dioxide. All low molecular weight products in the previous step are transformed into acetic acid by acetic acidification. The products vary with the bacterial type and cultivation environment. Methane bacteria, such as Methanomicrobiales, Methanobacteriales, and Methanococcales, are used to generate methane via acetic acid, hydrogen and carbon dioxide, which are all formed in the second and third stages ^[6][50].

Although biogas production is a relatively mature technology, it has strict technical requirements because the composition of kitchen waste is complex and variable in time and space. The anaerobic or aerobic disposal of kitchen waste is challenging due to the presence of constituents that are difficult to decompose in kitchen waste ^[7][51]. However, only 40–60% of volatile solids are available in biomethane, which indicates that pretreatment is essential. The common pretreatment methods include chemical, thermal, pressure, biological, enzymatic, and ultrasonic methods, among others ^[8][9][10][52,53,54], as described in Table 1.

^][80].

Biohythane is produced in a dark fermentation stage and an acetyl-hydrogen stage, and each stage is controlled by microbes. Each stage has its own optimal conditions, such as pH value, fermentation temperature, partial pressure of methane, hydraulic residence time, organic load and nutrient composition ^[34][78].

Compared with the one-stage system, the two-stage biohythane system achieved a higher COD removal rate and energy recovery rate. The first stage of hydrogen fermentation benefited subsequent biomethane production ^[36][80]. Furthermore, two-stage anaerobic methods are more commercially valuable than one-stage methods; this is because dark fermentation results in both the poor yield and productivity of hydrogen, and is economically unsustainable ^[37][81].

However, because of its high required investment and operation complexity, the applicability of the two-stage method is severely limited. Recent research on the anaerobic one-stage production of biohythane is confined to the use of one digestion pool in order to simultaneously produce hydrogen and methane ^[38][39][82,83]. The reactor design, reactor operation strategy, and collection mechanism of biohydrogen in the separation pattern are unknown ^[40][84].

Chen’s research on the two-phase fermentation of kitchen waste in order to produce biohythane reveals that a low ammonia nitrogen concentration has a positive effect on hydrogen formation. However, a high concentration has a negative effect or even causes complete interruption. A 9.4 g VS /L.d organic load, 4 d hydraulic residence time, 1.0 reflux ratio for the hydrogen-producing phase, and 20 d hydraulic residence time for the alkane-producing phase are favorable conditions for the capture of biohythane, with a 14.2% H₂/CH₄ ratio ^[41][85].

The accumulation yield of biohythane varies with the organic load (OL, Table 2). At various OL values, the methane fraction increased with time, contributing to the presence of hydrogen and volatile aliphatic (fatty) acids (VFAs) at the top of the reactor. VFAs are good precursor substrates for methane because the residue of acid-producing bacteria is the substrate of methane-producing bacteria. Several steps were used to induce the formation of methane, with a maximum methane content of 35% at an OL of 100 g/L COD. Increasing the organic load produced a considerable increase in VFA production. The improved yield in the fatty acid can be attributed to the resulting suppression of methanogenic activity during the feeding stage ^[42][86].

The combustion of hydrogen in air produces water with a high LHV (lower heating value, 120 MJ/kg) that is 2.5, 2.3, 2.6 and 2.4 times the LHV of the combustion with the same amount of CNG (compressed natural gas), LNG (liquefied natural gas) and LPG (propane and methane), respectively (Table 3). Hydrogen is considered to be the most extractable clean and renewable energy source ^[43][87]. The fermentation of anaerobic organic microbes is a key pathway for biological hydrogen production. There is a considerable amount of organic garbage in agricultural residue, kitchen waste and industrial wastewater.

Anaerobic fermentation to yield biogas is affected by various factors, such as composition, salinity, alkalinity, residence time, solid–liquid ratio, temperature, pH, C/N ratio, organic loading, bacterial vaccination degree, toxicity type, hydraulic residence time, agitation/mixture, pretreatment process, and reactor ^[21][22][23][65,66,67]. The research showed that kitchen rice, vegetables, meat, and a mixture of kitchen waste produced 478.2, 433.3, 206.8 and 508.3 mL/g biogas, respectively, indicating that mixing waste is favorable for biogas production ^[21][22][23][65,66,67]. Salinity affects the normal growth of methane bacteria, resulting in an increase in salinity outside the bacterial cell and the loss of water from the cells, thereby reducing the activity of microorganisms and even resulting in the complete failure of the fermentation system ^[24][68]. Two methods can be performed as countermeasures to protect the bacterial cell from losing water in the high salinity system. The first method is the synthesis and accumulation of osmotic protectants. The second method is adding an influx of potassium and chloride into the cytoplasm ^[24][25][68,69]. Fermentation may produce free ammonia, volatile aliphatic acid and sulfide/sulfate, which are important nutrients for bacterial growth; however, their increased concentrations will inhibit biogas formation ^[26][70]. For example, in the fermentation process, ammonia nitrogen mainly exists in two forms, namely, free ammonia and ammonium ions, which directly or indirectly inhibit the anaerobic fermentation process, especially free ammonia, which has stronger toxicity. Free ammonia can penetrate the cell membranes of bacteria, disturb the proton balance, change the pH value in cells, inhibit the activity of specific enzymes, and eventually lead to the deterioration of the anaerobic digestion process ^[27][28][71,72]. A submersible microbial desalination cell was developed to lower the ammonia level by in situ ammonia recovery and electricity generation, with a significant increase in biogas production. By gradually increasing the ammonia nitrogen concentration, the bacteria could gradually adapt to the high ammonia concentration, which could also reduce the inhibition effect of ammonia on AD ^[27][28][71,72]. The research showed that mesophilic bacteria could be adapted to a total ammonia nitrogen (TAN) concentration up to 5 g/L by means of gradual TAN loading. The cumulative biogas production in reactors with gradual TAN loading (for maximum TAN of 10 g/L) was 1.9–3.1 times more than that with abrupt TAN addition ^[27][28][71,72].

In addition to the above factors, the addition of a third component, such as biocarbon or a trace element, can improve the biogas yield of kitchen waste. The gas production effect of the anaerobic digestion of food waste was the best, and the methane yield was 331.7 L/kg TS. When the inoculation amount of sludge was 20.98%, the initial pH was 7.05, and with the addition of 22.14 g/L biocarbon, the maximum kitchen waste anaerobic digestion was realized, namely, 331.7 L/kg TS ^[29][73]; however, the mechanisms of biocarbon synergism in anaerobic fermentation are still under discussion. For example, one mechanism is put forward that biocarbon can promote electron transfer between species, enhance cell colonization and increase enzyme activity to promote biogas production ^[20][21][64,65]; another proposed mechanism suggests that the addition of biocarbon can improve the availability of the trace elements and beneficial elements that are necessary for methane production, and can reduce the loss of nutrients during fermentation ^[30][74].

Proper supplementation with microelements can improve methane yield ^[31][75]. In semicontinuous acclimatization, the daily addition of Fe(0) species to kitchen waste in sludge for anaerobic fermentation resulted in a 17.74% higher average daily biogas production than the control group; i.e., Fe(0) was found to be more conducive to improving the stability and yield of biogas for the single-phase anaerobic digestion of kitchen waste ^[32][76]. The combination of FeCl₂ and EDTA was shown to promote protein polysaccharide hydrolysis and improve the degradation of volatile acids in the 13 d reaction. The yield of gas was 4.1 times that of when only FeCl₂ was added ^[33][77].

Biohythane is a hydrogen–methane blend with a hydrogen concentration of 10 to 30% v/v ^[34][78]. Biohythane is derived from organic degradable products in a secondary anaerobic digestion process. In two-stage anaerobic fermentation, hydrogen microbes play an important role in the short residence time in the first stage, even though they produce methane for a relatively long time. Compared with traditional fuels, biohythane is valuable on the market and is universally accepted as a commercial fuel for use in internal engines and automobiles ^[35][79]. Environmentally, biohythane is advantageous for decreasing greenhouse gases because the presence of hydrogen decreases the carbon content ^[36

The microbial fermentation of organic matter under anaerobic conditions is an important method for biohydrogen production. Agricultural residues, food waste, industrial wastewater, vegetable wastes and other wastes containing high organic matter are economic raw materials for biohydrogen production. The combination of hydrogen production and pollutant removal is a promising method for energy recovery from wastes ^[2][46].

Many studies have investigated the use of kitchen waste as a substrate in order to produce biohydrogen, but this method is limited by various obstacles, such as the composition of the waste, the type of microbes, and the processing environment (pH, atmospheric pressure, temperature, hydraulic residence time, feeding rate, operating conditions, etc.). Reasonable pretreatment in hydrogen preparation is necessary in order for complex organics to improve the production performance. Biohydrogen is produced by different approaches, namely, biophotonolysis, electrolysis by microbes and fermentation (dark fermentation, dry fermentation, and photofermentation).

Dark fermentation has the suitable characteristics of low energy demand, high yield and productivity, in order to be considered for large-scale hydrogen production. In dark fermentation, in which a single bacterium or community is available, the microbe variety is critical to hydrogen production. Commonly used bacteria include Clostridium, Enterobacteriaceae, Bacteroides and Thermoanaerobium, among others ^[47][91].

In photofermentation, which is dependent on rays, organic substrates are converted into hydrogen by photosynthetic bacteria ^[48][39]. Purple sulfur bacteria, purple non-sulfur bacteria and green sulfur bacteria are photosynthetic bacteria that use light and organic waste to produce hydrogen. The advantages of photofermentation include its good hydrogen yield and low COD level due to its utilization of dark fermentation products ^[49][92]. However, this technique has various shortcomings, such as poor photoefficiency, the requirement of transparent reactors and high energy consumption ^[19][63].

Dark fermentation produces hydrogen at a high rate, while photofermentation produces hydrogen at a good yield. Therefore, a suitable combination of the two could yield the maximum hydrogen output, which should be further investigated in the future ^[50][93]. For example, the researchers used the respective advantages of dark fermentation and light fermentation to implement the two-step coupling of dark–light fermentation, with kitchen waste as the substrate. By optimizing the H₂ production conditions of dark fermentation and light fermentation, the following results were obtained: (1) at 37 °C, a 50% feed ratio, and an initial pH = 6, dark fermentation produced 25.18 mL/g of VS hydrogen, and (2) at 30 °C, 100 W illumination, an initial pH = 8, and 10% vaccination, photofermentation produced 34.62 mL/g of VS hydrogen. The total for the two fermentation methods produced up to 59.80 mL/g of VS hydrogen, significantly increasing the hydrogen yield of kitchen waste ^[51][94].

Anaerobic fermentation to generate hydrogen is classified into wet fermentation (less than 15% solid content) and dry fermentation (greater than 15% solid content). Compared with wet anaerobic fermentation, dry anaerobic fermentation has many advantages, such as lower water consumption, higher gas production, reduced biogas slurry, and the lower moisture content of biogas residues ^[40][84]. Synergistic dry fermentation with different substrates could improve hydrogen production efficiency. For example, when the ratio of kitchen waste to garden waste was 60:40, the maximum cumulative hydrogen production reached 85.28 NmL/g VS by dry fermentation at 55 °C, which was 1.35 times higher than that obtained using single kitchen waste and 1.93 times that obtained using a single garden waste ^[40][84].

Despite these results, industrial-scale anaerobic fermentation to produce biohydrogen has not been adopted due to technical obstacles, its high cost and poor efficiency. Improving the hydrogen yield is a continuous concern in academia. New ideas, such as substrate addition, the continuous removal of products, and removal inhibition caused by substrate and product accumulation, have been proposed ^[52][95]. Genetic engineering to modify bacterial strains to reduce side products ^[53][96] and the use of nanoparticle additives to enhance enzymatic activity ^[54][97] have also been proposed.