Bio-hydrogen And Bio-methane Production From Food Waste In A Continuous Two-stage Anaerobic Digestion System

In China,approximately 6.0 × 107tons of food wastes are generated per annum,considered as the main category of municipal biomass waste.Food waste has higher biodegradable organic matter content,which can be anaerobically digested to recover clean energy as methane and minimize generated waste.However,food waste has high carbohydrate content resulting into high rate of biological hydrolysis and acidification,leading to rapid production of organic acid exceeding the rate of methanogenesis and causing anaerobic digestion imbalance.The accumulation of volatile fatty acid leads to inhibition of gas production in the traditional single-phase anaerobic digestion process.As a consequence,the development of anaerobic digestion process suitable for food waste has become an area of interest in the current studies on anaerobic digestion technology.In the current study,a long-term continuous experiment on anaerobic digestion of food waste was carried out to investigate two-phase anaerobic digestion system of thermophilic(bio-hydrogen production)and mesophilic(bio-methane production)conditions.Furthermore,kinetic models,microbial community structure analysis and material balance analysis was used to evaluate the process efficiency.The current long-term experiment comprised three steps,namely;(1)Food waste single phase bio-hydrogen production;(2)continuous temperature-phased two-stage system(thermophilic and mesophilic)for bio-hydrogen and bio-methane production(3)continuous temperature-phased two-stage system(thermophilic and mesophilic)for bio-hydrogen and bio-methane production with digestate recirculation.In the first step,an 80 days continuous experiment using a continuous stirred tank reactor was conducted at 55 ? and 70 ?.The thermophilic reactor could achieve stable bio-hydrogen production when the pH was controlled at 5.5,whereas there was no bio-hydrogen production in the hyper-thermophilic reactor.A stable bio-hydrogen yield(70.7 mL-H2/g VSin)at a concentration(58.6%)in the produced gas,was attained with hydraulic retention time(HRT)of 5 days and pH of 5.5 at temperature 55 ?.Batch analysis found that acidogenesis was the rate limiting step in anaerobic digestion of food waste.In the second step,a 270 days long experiment was undertaken to assess a temperature-phased system,bio-hydrogen production at thermophilic(55 0C)and bio-methane production at mesophilic(35 0C).It was perceived from the findings that the optimal yield were obtained at a hydraulic retention time(HRT)of 5 and 15 days with organic loading rate(OLR)of 16.2 and 3.2 kg-VS/m3.d respectively.Moreover,the total energy recovered was nearly 1,113.6 MJ/t-FW;the energy converted to bio-hydrogen and bio-methane was 8.2 and 82.5%respectively.Acetate was the main acid product in the bio-hydrogen reactor,the concentration of acetate achieved was 4.0 g/L.Furthermore,microbial community structure analysis showed Methanobacterium was the main methanogens,suggesting that part of the acetate from bio-hydrogen reactor was used to produce methane in methanogenesis reactor by syntrophic acetate-oxidizing bacteria and hydrogenotrophic methanogens(SAO-HM).Based on the two-phase bio-hydrogen and bio-methane production process,recirculation of the digestate from methanogenesis reactor to the bio-hydrogen reactor to reduce the chemical addition to the bio-hydrogen reactor.A continuous experiment for 120 days was conducted to investigate the influence of various recirculation ratios(RR)including,0.3,0.5,and 1 on the system energy efficiency and process stability.It was observed that the amount of NaOH require to maintain pH at 5.5,reduced by 54%,39%,and 50%at RR 0.3,0.5,1,respectively.The maximum bio-hydrogen production and yield were 3 LH2/L-d and 135 LH2/kgVSin,respectively at RR of 0.3.Likewise,at the same RR,the highest methane production and yield were 2.9 LCH4/L·d and 510 LCH4/kgVSin,respectively.The energy recovery from anaerobic digestion with recirculation system was found to increase by 8%for bio-hydrogen and decreased by 3%in bio-methane production,while the total energy recovered had no significant change.The highest removal of TS(67%)and VS(87%)was also observed at the same RR(0.3)in the two-phase recirculation system.Through long-term continuous fermentation experiments,the current study concluded that combination of thermophilic(55?)and mesophilic(350C)are a suitable process for bio-hydrogen and bio-methane production from food waste.The digestate recirculation from bio-methane reactor to bio-hydrogen reactor produced alkali and reduced the extra chemical which should be added to the reactor to maintain the pH,furthermore,could significantly reduce the cost of chemical addition.The two-phase fermentation system achieved higher organic removal efficiency and higher energy recovery and provides two kinds of clean fuel;bio-hydrogen and bio-methane in a stable anaerobic digestion system.