3.1. Microstructural characterization of the additives
The SEM image of CaO2 represents a uniform particle size distribution of as shown in Fig. 3a. The structure and dispersion of particles show higher uniformity compared to the other additives which facilitate the hydrogen fermentation due to increased surface contact of the microorganism with the feedstock. On the other hand, a closely packed microstructure as well as a non-uniform distribution can be observed from the SEM image of CuO (Fig. 3b) due to the lower solubility which results in reduced microbial growth during the reaction. Similarly, the SEM image of ZnO (Fig. 3c) shows an unseparated and closed structure but it is slightly dispersed compared to CuO. The particle size distribution of both CuO and ZnO represents that the particles are highly agglomerated and incompletely separated promoting a less encouraging effect on the hydrogenase microbial action with the feedstock during the initial stages of the co-digestion. The crystallographic structure of CaCO3 (Fig. 3d) shows rhombohedral crystals with dense and grouped particles. The crystals are also undefined with agglomeration which deteriorates the performance of the system causing a reduced hydrogen fermentation during co-digestion. Moreover, the crystallographic characterization signifies that CaO2 leads to the enhanced hydrogen fermentation compared to the other additives. Similar results on the microstructural configuration of the additives have also been reported previously in the literature [26–29].
3.2. Effect of the additives on hydrogen concentration
The hydrogen concentration of the five reactors is shown in Fig. 4. It was observed that the hydrogen concentration in the control sample remains within the range of 17% whereas a higher concentration of hydrogen was found by supplementing the additives inside the reactors. The maximum hydrogen concentration after the addition of CaO2, ZnO, CuO, and CaCO3 was found to be 26.43%, 21.67%, 17.64%, and 20.84% respectively. The highest concentration of hydrogen was observed by adding CaO2 while the lowest concentration was observed by adding CuO as an additive with the feedstock. The hydrogen concentration gradually increases in all the reactors up to the fourth or fifth day when it reached the peak after that the concentration decreases significantly throughout the anaerobic fermentation. It was identified from the above results that additives could enhance the hydrogen concentration during the fermentation especially when the feedstock was supplied with CaO2. The addition of ZnO and CaCO3 has a moderate effect whereas CuO did not show any significant impact on the hydrogen concentration throughout the anaerobic fermentation process.
The fermentation process of hydrogen was significantly enhanced by adding CaO2 compared to the other additives. There are two main reasons involved in this observation. Firstly the dissociation of CaO2 results in the generation of OHˉ and ·O2ˉ that facilitate the hydrolysis of protein along with carbohydrate due to the increased breakdown of the microorganisms and the release of the amino acids . Secondly, the release of OHˉ and H2O2 as the intermediate products due to the reaction between CaO2 and water leads to an increased reaction rate during the initial stages of AD . These two factors are primarily responsible for the enhanced hydrogen generation during the fermentation process. The dissociation of ZnO and CuO results in the formation of Zn2+ and Cu2+ ions that lead to an increase in the toxicity of the medium and inhibit the microbial growth during the AD . Also, the toxic nature of the ions was responsible for the lower volatile fatty acid generation caused by the decreased hydrolysis of protein . The addition of CaCO3 causes less ion conductivity and lower pH which results in a lower concentration of hydrogen during the acidification . Also the reduced ion activity of Ca2+ and CO32− leads to the lower rate of reaction during the hydrolysis. Moreover, the combined effect of lower pH and higher toxicity results in lower hydrogen formation during the reaction. Baldi et al.  observed a maximum hydrogen percentage of 22.9% during the AD of activated sludge with food waste. A similar result was also reported by Yeshanew et al.  through the anaerobic fermentation of food dissipate.
3.3. Effect of the additives on cumulative hydrogen yield
Figure 5 shows the cumulative hydrogen yield of all the reactors. It was observed to be lowest in the control sample while the highest cumulative hydrogen yield was obtained by adding CaO2 with the feedstock. The cumulative hydrogen yield of all the reactors after 15 d of fermentation was found to be 101.57 mL g− 1 TS (control), 114.1 mL g− 1 TS (CaO2), 109.27 mL g− 1 TS (ZnO), 104.87 mL g− 1 TS (CuO), and 107.38 mL g− 1 TS (CaCO3) respectively. It was found to follow the order CaO2 > ZnO > CaCO3 > CuO > control. The hydrogen production was accelerated by the additives compared to the control samples during the reaction. Hydrogen generation was enhanced up to 11% by adding CaO2 with the feedstock compared to the control sample in which no additives were added during the fermentation process. The increase in hydrogen generation by adding ZnO, CuO, and CaCO3 was found to be 7%, 3%, and 5.4% respectively compared to the control one. It was clear from the observation that additives could increase hydrogen production significantly during the anaerobic fermentation of food waste.
The cumulative hydrogen yield was significantly enhanced by adding CaO2 due to the release of alkali, ·OH, and ·O2ˉ from the decomposition of CaO2 which facilitate the hydrolysis of the feedstock producing more amount of hydrogen during the early stages of AD . Also, CaO2 was more prone towards the inhibition of hydrogen consuming bacteria rather than the hydrogenase which is also responsible for the higher hydrogen generation during the reaction . The higher toxicity of ZnO and CuO restrict the growth of hydrogen fermentative bacteria during the earlier stages of AD which is the main reason for lower hydrogen generation when compared to CaO2 . However, the cumulative hydrogen yield by adding ZnO and CuO was found to be higher compared to the control sample due to the enhanced hydrolysis of the feedstock during the initial period of digestion. The lower pH of the feedstock along with the lower ion activity of Ca2+ and CO32− results in lower hydrogen generation by adding CaCO3 as an additive during the hydrogen fermentation . Nevertheless, an increase in hydrogen generation compared to the control sample was observed by adding CaCO3 due to the stimulated growth of hydrogenase caused by the release of ion during the acidification of the feedstock. The cumulative hydrogen production of 60.23 mL g− 1 VS was detected during the anaerobic fermentation of food waste by Li et al. . A comparable outcome was also reported by other authors during the AD of peanut shells .
3.4. Effect of the additives on carbon dioxide concentration
The effect of different additives on carbon dioxide concentration compared to the control sample is presented in Fig. 6. A significant increase in carbon dioxide proportion was observed through the initial period (7 to 8 d) of AD in all the cases. After the attainment of the peak hydrogen concentration it gradually decreases during the remaining days of digestion. The maximum carbon dioxide concentration in the control, CaO2, ZnO, CuO, and CaCO3 reactors was found to be 57.63%, 54.62%, 58.37%, 59.2%, and 58.4% respectively. The carbon dioxide concentration of CaO2 was lower than the control sample but ZnO, CuO, and CaCO3 result in higher carbon dioxide concentration compared to the control sample. The addition of CaO2 compared to the other additives significantly reduces the carbon dioxide formation which leads to improved hydrogen formation during the acidogenic stages of the reaction. However, the addition of ZnO, CuO, and CaCO3 reduced the hydrogen generation compared to CaO2 which was the consequence of higher carbon dioxide concentration during the AD.
Lower carbon dioxide concentration in the CaO2 reactor results in the higher hydrogen concentration compared to the other reactors. This may be due to the formation of alkali, ·OH, and ·O2ˉ which accelerate the hydrogen fermentation and suppress the generation of carbon dioxide during the reaction . The higher carbon dioxide concentration by adding ZnO, CuO, and CaCO3 was due to the increased toxicity of the medium caused by the ionic activity of Zn2+, Cu2+, and Ca2+ respectively . The acidification of the feedstock during the initial period of digestion was enhanced by the ions while the hydrogenase microbial growth was restricted due to the toxicity inside the reactor. A similar observation on carbon dioxide concentration was reported by the other researchers through the anaerobic fermentation of food residue [40, 41]. The results showed a significant variation in the carbon dioxide concentration by using the additives compared to the control sample. The addition of CaO2 exhibited a positive effect by reducing the carbon dioxide concentration whereas ZnO, CuO, and CaCO3 addition increase the concentration of carbon dioxide which is undesirable during the hydrogen fermentation.
3.5. Effect of the additives on maximum hydrogen and carbon dioxide concentration
The maximum hydrogen concentration was inversely proportional to the carbon dioxide concentration as shown in Fig. 7. The highest hydrogen concentration was detected in the CaO2 sample with the lowest carbon dioxide concentration in comparison with other reactors. As the hydrogen concentration increases with CaO2 compared to the control the carbon dioxide concentration was reduced simultaneously. Further decrease in the hydrogen concentration increases carbon dioxide concentration. Among the additives, the lowest hydrogen concentration was observed to be 17.64% in the CuO reactor that leads to the highest carbon dioxide concentration of 59.2%. The maximum hydrogen concentration of 26.34% was observed in the CaO2 reactor with the lowest carbon dioxide concentration of 54.62% among all the reactors. It was noted that the addition of CaO2 significantly promote the hydrogen generation by reducing the carbon dioxide concentration while the addition of ZnO, CuO, and CaCO3 has a less significant effect on hydrogen generation but still the additives have a positive effect on hydrogen and carbon dioxide concentration compared to the control reactor.
The disintegration of CaO2 leads to the generation of alkali, ·OH, and ·O2ˉ those are primarily responsible for the increased hydrogen production by increasing the microbial disintegration during the reaction . The enhancement of hydrogen generation simultaneously results in the reduction of carbon dioxide formed during the acidification. Also, the decomposition of CaO2 increased the alkali generation during hydrolysis that facilitates the higher bacterial activity inside the feedstock . As the hydrogen fermentation increases by adding CaO2 the microbial activity could not shift towards the carbon dioxide fermentation which results in the lower carbon dioxide concentration compared to the other reactors. Moreover, the formation of Ca2+ due to the decomposition of CaO2 did not show a significant effect on the reduced growth of hydrogenase compared to the combined effect of alkali, ·OH, and ·O2ˉ during the reaction . Lower alkali generation in ZnO, CuO, and CaCO3 reactors inhibited the hydrogenase bacterial growth during the acidification resulting in the lower hydrogen generation compared to the CaO2 reactor. The carbon dioxide fermentation was enhanced due to the inhibition of hydrogenase bacterial growth during the initial period of digestion. Also, the lower hydrogen concentration was the upshot of higher toxicity inside the reactors due to the formation of Zn2+, Cu2+, and Ca2+ during the decomposition of ZnO, CuO, and CaCO3 respectively . The increase in hydrogen generation with a simultaneous decrease in carbon dioxide concentration was reported by Algapani et al.  in which a maximum hydrogen concentration of 64% was observed with a reduced carbon dioxide concentration of 32% through the multi-stage digestate recirculated anaerobic fermentation of food residue.
3.6. Effect of the additives on hydrogen production
The cumulative hydrogen yield and the hydrogen concentration of all the reactors are presented in Fig. 8. The cumulative hydrogen yield was observed to be directly proportional to the corresponding hydrogen concentration of the reactor where the maximum hydrogen concentration and the cumulative hydrogen yield were observed to be 26.34% and 114.1 mL g− 1 TS respectively in the CaO2 reactor. The lowest value of the hydrogen concentration and the cumulative hydrogen yield among all the reactors were observed to be 17% and 101.57 mL g− 1 TS respectively in the control reactor. It is worth noting that the additives have more or less increased hydrogen generation in comparison with the control sample during the anaerobic fermentation. The lowest hydrogen production among the additives was observed with CuO while the highest production was found with the CaO2 sample. The maximum hydrogen concentration in the ZnO, CuO, and CaCO3 reactors was found to be 21.67%, 17.64%, and 20.84% respectively while the cumulative hydrogen yield was identified to be 109.27 mL g− 1 TS, 104.87 mL g− 1 TS, and 107.38 mL g− 1 TS respectively from the same reactors. The cumulative hydrogen production demonstrated a direct link among the hydrogen concentration when the additives are used with the feedstock through the anaerobic co-fermentation.
The increased hydrogen generation in the CaO2 reactor corresponds to the increased hydrogenase growth during the acidogenesis in comparison with the other samples. The higher pH in the control sample results in lower volatile fatty acid accumulation during the acidogenic stage of AD which was the main reason for lower hydrogen generation . Also, the microbial degradation of the feedstock was not significantly enhanced as the control reactor did not receive any additive that also results in lower hydrogen production. The lower hydrogen production in the ZnO, CuO, and CaCO3 reactors mainly resulted due to the increased toxicity of the medium as well as the lower alkali formation which restricts the hydrogenase microbial growth and volatile fatty acid consumption during the initial stages of AD [45, 48]. A similar trend of hydrogen generation was identified by Wainaina et al.  during the AD of food waste where the organic loading rate serves as the base for the comparison of hydrogen generation.