Study On Volatile Fatty Acid Production From Food Waste Anaerobic Fermentation

With the constantly improving of population quantity and living level, high-yield food waste(FW) is produced inevitably, which can severely harm environmental safety and human health if there is not suitable treatment and disposal. In addition, the excessive exploitation and utilization of fossil fuels caused more and more prominent problems on energy crisis and environment pollution. Human showed unprecedented enthusiasm and desire for exploiting renewable energies. FW was proven to be an ideal substrate of anaerobic fermentation, and volatile fatty acid(VFA) produced by anaerobic fermentation showed broad use of spaces. Recycling VFA from FW can simultaneously realize the dual goals of waste treatment and renewable energy production, which is beneficial for alleviate the environment pollution and energy crisis in 21 st century. However, the low efficiency of VFA production constrains the practical application of the technology. In this paper, the problem was solved by improving the hydrolysis efficiency of FW, inhibiting the activity of methanogens, and optimizing the influencing factors of anaerobic acidification process.Firstly, the hydrolysis efficiency was improved by choosing a suitable pretreatment method. The operation conditions of ultrasonic, acid and ultrasonic-acid pretreatments were optimized, and their effects on VFA productions were investigated. The results showed that, the optimized ultrasonic pretreatment conditions were energy density of 1 W/m L and the processing time of 20 min, the optimized acid pretreatment conditions were p H 2 and processing time of 20 h, and ultrasonic-acid pretreatment conditions obtained by response surface method(RSM) were energy density of 1.13 W/m L and p H of 1.43. The disgradation degrees obtained by the three pretreatments were 57.38%, 46.90% and 69.52%, respectively, and the VFA productions were 976.12、115.68 and 418.94 mg COD/g VS, respectively. Indicating that ultrasonic-acid and acid pretreatment were against VFA production. Therefore, ultrasonic was chosen as an appropriate pretreatment of FW, and the optimum operation conditions were energy density of 1 W/m L and the processing time of 20 min.Then, the decreased VFA production caused by the VFA consumption from methanogens was solved by adjusting the substrate to inoculum ratio(S/I). The buffer ability formed by fermentation system can keep p H in a range which is beneficial for hydrolysis and acidification, and inhibited for methanation. In the batch test, compared with non-pretreated FW and other S/I, higher VFA production(976.12 mg COD/g VS) and VFA/SCOD(72.27%) were obtained by ultrasonic pretreated FW and S/I of 6, which was attributed to ultrasonic pretreatment improved hydrolysis efficiency of substrates, at the same time, S/I of 6 indirectly adjusted the p H to a weak acidic range of 5.3-6.4. Under the circumstances, the hydrolysis efficiency was further improved, meanwhile only slight methane was detected. Therefore, ultrasonic pretreatment conbined with S/I of 6 can effectively promote the recover of VFA from FW anaerobic fermentation.Finally, to verify the feasibility of improving VFA production by ultrasonic pretreatment and S/I adjustment in semi-continuous production, a semi-continuous anaerobic fermentation was carried out. The optimum operation conditions were obtained by RSM, which were S/I of 5, temperature(T) of 40 oC, solid retention time(SRT) of 7 d and organic loading rate(OLR) of 9 g VS/L d. The high VFA production(867.42 mg COD/g VS) and VFA/SCOD(88.65%) were achieved under the optimum conditions. Both the variations of soluble organic matter concentration and EEM flurescence spectroscopy analysis showed that the hydrolysis efficiency of FW was obviously improved under the optimized conditions. And only low methane production(20.56 ml/g VS, only 5.34% of theoretical value) was dectected during fermentation process, indicating optimized operation conditions effectively inhibited the activity of methanogenes. The main reason was that, the release of ammonia provided a relatively high alkalinity, at the same time, the timed output of fermentation liquid and input of fresh substrates compensated for the decreased p H and kept the p H in a range of 5.2-6.4, which was proven to be beneficial for hydrolysis and acidification of organic matters and inhibited for methanation. Carbon mass balance analysis also showed that the substrates were efficiently and adequately utilized to produce VFA under the optimum conditions, and the main VFA compositions were acetic, propionic and n-butyric acids.