Feasibility Study On Treatment Of Food Waste By Using Two-phase Anaerobic Digestion System

The purpose of this study was to investigate the effects of short chain fatty acids(SCFAs),salinity and NH4+-N on the methanogenic activity of anaerobic sludge and to explore the feasibility of processing food waste in semi-continuous mode by using two-phase anaerobic digestion system.Firstly,the anaerobic biodegradability of selected SCFAs and their toxicity to anaerobic sludge 'were determined in a series of ASBR experiments and significant differences were observed between the tested SCFAs in either the optimal COD volumetric loading or their degradation efficiency in ASBR.The optimal organic loadings of lactic acid,propionic acid,acetic acid and butyric acid were 3 g/L-d,4 g/L·d,5 g/L-d and 7 g/L·d,respectively and the COD removal rates obtained at above organic loadings were 57%,60%,77%and 68%,respectively.All these four acids,which represent major components of food waste leachate,could be effectively degraded under appropriate loadings.However,three acids,namely lactic acid,propionic acid and acetic acid,exhibited significant inhibitory effects on the activity of anaerobic sludge when ASBRs were run at organic loadings higher than their optimal levels.Negative correlations could be established between.the methane conversion rate and the COD volumetric loading and expressed by linear equations in the range of toxic loadings.From corresponding linear equations,the organic loadings which would lead to 10%inhibition in the methanogenic activity of anaerobic sludge could be calculated to be 3.11 g/L·d,4.42 g/L·d and 5.48 g/L·d for lactic acid,propionic acid and acetic acid,respectively.The organic loadings which would lead to 50%inhibition in the methanogenic activity could also be calculated to be 4.53 g/L·d,5.94 g/L·d and 7.2 g/L-d for lactic acid,propionic acid and acetic acid,respectively.However,butyric acid didn't showed any inhibitory effect on the methanogenic activity of anaerobic sludge in the volumetric loadings ranging from 1 g/L-d to 7 g/L-d.Meanwhile the concentrations of isobutyric acid,valeric acid and isovaleric acid in food waste leachate were too low,which makes it impossible for them to become the limiting factor affecting subsequent methane fermentation.Secondly,toxic loadings of NaCl and NH4+-N were also studied in ASBRs feed with mixed SCFAs solution,which was prepared by simulating food waste leachate composition.Results showed that the anaerobic sludge could tolerate a certain amount of salinity or NH4+-N in the influent and its methanogenic activity remained unchanged when ?16 g/L of NaCl or ?1g/L of NH4+-N was added in the influent.However,introducing higher concentrations of NaCl or NH4+-N to the influent resulted in significant inhibition on the activity of anaerobic sludge.Concentrations,which lead to 10%or 50%decrease in the activity of anaerobic sludge were 22 g/L and 1.88 g/L for NaCl or 51 g/L and 6.69 g/L,for NH4+-N,respectively.In order to explore the feasibility of treating food waste in semi-continuous mode by using two-phase anaerobic digestion system,degradation efficiencies of food waste were determined in LB reactors at different feed loads.The results showed that the feed loading had a significant impact on food waste hydrolysis and acidification in leach-bed reactors.The SCOD concentration in the LB leachate increased with the increase in the feed loads when the feed loading was controlled in the range of 50-150 g/d.SCOD concentration remained quite stable and accumulation of feed materials was insignificant during the semi-continuous operation period,indicating a steady state was reached at this stage.When the feed loading was increased to 200 g/d,steady state could not be reached due to continuous accumulation of feed materials caused by lower degradation efficiency of organic matter.Once daily feeding was stopped the pH values of the LB leachate rose almost immediately until the pH value(8.29)of NaHCO3 solution was reached.Meanwhile SCOD concentration of the leachate dropped steadily till the low level of<2000 mg/L was reached.At the end of this experiment,the total solids removal rates for all LB reactors were ?97%.Finally,the leachate from LB reactors was introduced to ASBRs for methane production.During the methane fermentation period,high COD removal rates of 85.4%?97.6%were obtained and the highest organic loads were 2.63 g/L·d,2.36 g/L·d,2.87 g/L·d and 2.98 g/L·d for ASBR1?ASBR4,respectively.At the end of this experiment,accumulated methane yield was 62628 mL,89305 mL,123268 mL and 157925 mL for ASBR1?ASBR4,respectively.The methane conversion rate obtained were 206,214,232 and 243 mL/TS for LB1-LB4,respectively.Results from the present study demonstrate that it is feasible to convert food waste to methane gas through two-phase anaerobic digestion system(LB-ASBR).