| Food waste is a biodegradable waste which has high organic contents. Bioenergy recovery through anaerobic digestion (AD) process is considered to be a sustainable food waste treatment practice, with hydrogen or methane rich biogas as the main product. Nitrogen control plays a vital role in the performance and stability of AD of food waste. Organic nitrogen in food waste is hydrolyzed to ammonia, and ammonia accumulation is potentially encountered during AD of food waste. The different characteristics and functions of microbial consortium during hydrogenogenesis and methanogenesis result in the hard synergetic control of microbial consortium. The ammonia characteristics of independent production of hydrogen or methane via one-phase AD, and co-production of hydrogen and methane via two-phase AD were investigated in this study. To optimize recovery of AD process following ammonia inhibition, ammonia control strategies were further explored. The main results are as follows:1) Effect of ammonia on hydrogen production from food waste via AD was studied. Successful hydrogen production from food waste was achieved by controlling feedstock to inoculum ratios (F/I), and ammonia concentrations at appropriate ranges. Hydrogen production could be enhanced by keeping ammonia at suitable levels. The proper range of total ammonia nitrogen (TAN) concentrations is0.5-6.0g/L at F/I3.9. The highest hydrogen yield (121.4mL/g VS) was achieved at the added TAN concentration of3.5g/L with an increase of57.3%compared to the control group without added ammonia. High concentration ammonia had negative effect on hydrogen production. The inhibition of ammonia on hydrogen production started to appear when the added TAN concentration was higher than6.0g/L. The addition of7.5and10.0g/L TAN resulted in14.3%and58.7%reduction of hydrogen yields, respectively.2) Effect of ammonia and nitrate on methane production from food waste via AD was investigated. At a VS loading of6.7g VS/L, lower added TAN concentrations (<1.0g/L) were beneficial to AD process, while higher TAN concentrations (>1.5g/L) caused an excessive inhibition of methanogenesis. Methane yield at the added TAN concentration of0.5g/L was increased by5.1%in comparison with the control group without added ammonia. TAN concentration of1.54g/L (with1.0g/L added TAN) corresponded to a threshold concentration for ammonia inhibition effect, above which ammonia would initiate inhibition of methanogenesis.Based on the study of ammonia effect on methane production, the suitability of nitrification process for ammonia removal from food waste digestate in the recirculated AD system was evaluated. In an attempt to simulate conditions of recycling digestate after nitrification treatment into the digestion system for ammonia dilution, the impact of nitrification products on AD performance was investigated by employing the nitrate as a variable compound with an added TAN concentration of1.0g/L. Results showed that added NO3-N concentrations in the range of100-750mg/L enhanced the methane production, while nitrate started to inhibit the methane production at added NO3-N concentrations higher than1.0g/L. Nitrification process can be potentially suitable for ammonia removal from food waste digestate with lower TAN concentrations (<2.29g/L). However, nitrification process would no longer be an appropriate technology for the digestate with higher TAN concentrations (>2.29g/L), since the ammonia was replaced by nitrate/nitrite that also had inhibitory effects on methanogenesis at certain concentrations.3) Comparison of high-solids to liquid anaerobic co-digestion of food waste and green waste was evaluated. Synergistic effects were found in co-digestion of food waste and green waste. Increasing the food waste percentage in the feedstock resulted in an increased methane yield, while shorter retention time was achieved by increasing the green waste percentage. Methane yields from high-solids AD (15-20%TS) were higher than the output of liquid AD (5-10%TS), while methanogenesis was inhibited by further increasing the TS content to25%. The high-solids AD system had much higher final TAN concentrations than that of the liquid AD system. Ammonia inhibition was more likely to be encountered in high-solids AD. A higher TAN concentration of4.2g/L at25%TS initiated inhibition of methanogenesis, leading to lower methane yields.During batch anaerobic co-digestion of food waste and green waste, no significant differences were observed in the final TAN concentrations at different mixing ratios. In the continuous AD trials, the TAN concentration in the digestion system with food waste as the single feedstock was much higher than that in the system with food waste and green waste as co-substrates. Higher TAN concentrations (>1912mg/L) caused an inhibition of methane production. Under high organic loading rates (OLR), higher process stability and ammonia tolerance capacity were achieved in co-digestion of food waste and green waste with respect to mono-digestion of food waste.4) Effect of ammonia on hydrogen and methane co-production from food waste by two-phase AD was investigated. A two-phase process with OLR9.4g VS/(Ld), hydraulic retention time (HRT)4d, recirculation ratio1.0for hydrogen reactor, and HRT20d for methane reactor, successfully achieved co-production of hydrogen and methane. The hydrogen yield in the hydrogen reactor was47.7mL/g VS, and the methane yield in the methane reactor was335.0mL/g VS. Lower TAN concentrations (<4044mg/L) improved hydrogen production in the hydrogen reactor, while higher TAN concentrations (>4044mg/L) caused an obvious inhibition of hydrogenogenesis. TAN concentrations of4256and4972mg/L caused51.8%and100%reduction in hydrogen production, respectively. Under higher TAN concentrations, both acid formers and hydrogen formers were subjected to severe inhibition of ammonia.Complete recovery was achieved in the methane reactor after acute inhibitory effects of lower TAN concentrations (<5800mg/L) on methanogenesis. Nevertheless, incomplete recovery to a level lower than the stable methane yield was followed after being subjected to higher TAN concentrations (>6200mg/L). The methane reactor long subjected to higher TAN concentrations (>6200mg/L) revealed chronic inhibition of methanogens. TAN concentration of9836mg/L caused53.2%drop in methane production. Effect of ammonia on the methane production was well simulated using the extended Monod equation (R2=0.959).An approach by adjusting HRT and recirculation ratio, and replacing reactor contents with the methane phase digestate retained at the steady stage was explored for controlling the ammonia inhibition. The approach was shown to effectively reduce the TAN concentrations in the two-phase system to levels comparable or lower than the ammonia inhibition threshold. Successful recovery was achieved in the hydrogen reactor after TAN reduction, while after chronic ammonia inhibition, the TAN reduction was still followed by a failed recovery in the methane reactor. |
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