Anaerobic Digestion Characteristics Of Lipid Waste And Co-digestion With Animal Manure

Biomass energy has attracted great attention in the world,since it is one of the most promising renewable energy.Organic wastes,as common biomass,can be converted into methane as clean energy by anaerobic digestion(AD).In China,a huge amount of lipid wastes are produced per year.They are very promising feedstock in AD,since methane production potential of lipid is much higher than that of carbohydrate and protein.However,the development and utilization of lipid waste is limited since the AD characteristics and degradation of lipid waste are unclear,and carbon to nitrogen ratio(C/N)of lipid waste is unsuitable for AD process.Moreover,lipid waste could lead to acidification and long-chain fatty acids(LCFAs)inhibition of AD reactor.In this study,AD process of typical lipids,LCFAs,and swine fatty waste(SFW)from slaughterhouse was investigated systematically,which was aiming at revealing the AD characteristics and degradation rule of lipid waste.In addition,considering the advantages of co-digestion such as improving C/N ratio and increasing buffer capacity of reactor,SFW was co-digested with pig manure in semi-continuous operation,which realized the stable and efficient methane production of AD system.The results are as follows.First,the AD characteristics of 8 common lipids and their main LCFAs were systematically investigated,and the relationship between LCFAs composition of lipids and the AD kinetics of lipids was studied.The results showed that methane potentials of the lipids were all higher than 800 mL gvs-1,however,great difference was observed in their degradation time.At the loading of 10-20 gvs L-1,the degradation time of chicken fat and plant oils which contain abundant unsaturated LCFAs was shorter than that of cattle,sheep,and pig fats which contain abundant saturated LCFAs.At the loading of 30-50 gvs L-1,the degradation time of chicken fat and plant oils was longer than that of cattle,sheep,and pig fats.The difference was caused by the content of unsaturated LCFAs.Lipids rich in higher content of unsaturated LCFAs had higher the maximum methane production rate,resulting in shorter degradation time at low loadings.However,high content of unsaturated LCFAs resulted in long lag phase at high loadings,resulting in long degradation time.The results of kinetic parameters in AD of LCFAs showed that unsaturated LCFAs had longer lag phase and higher the maximum methane production rate than that of saturated LCFAs.This was consistent with the difference of cattle,sheep,and pig fats with chicken fat and plant oils,which also indicated that the difference of lipids in AD process was caused by unsaturated LCFAs.Second,methane production performance,LCFAs profile,and microbial community structure in AD of SFW from slaughterhouse were studied.The results showed that SFW was very suitable feedstock for AD.SFW could be well digested at substrate to inoculum(S/I)ratio of 0.7 and 1,loadings of 20-40 gvs L-1,and methane potential were in the range of 920.9-999.2 mL gvs-1.Accumulation of LCFAs was observed in the initial digestion stage of SFW,indicating higher hydrolysis rate than LCFAs degradation rate.Nevertheless,with the proceeding of digestion,the concentration of LCFAs gradually decreased with the increase of methane production,indicating higher LCFAs degradation rate than hydrolysis rate.When the concentration of LCFAs decreased to a very low level,the daily methane production reached the peak,which indicated that the reaction rates of all steps were in equilibrium.The microbial community structure showed that predominant microorganisms were Clostridium,Syntrophomonas and Methanospirillum.Third,the AD performance and LCFAs profile was investigated in AD of pretreated SFW.The results showed that after pretreatment by 0.2%and 0.4%(w/v)lipase,lag phase in AD of SFW was shortened by 5 d,and T80 was shortened by 6 d.After pretreatment by thermal melting,lag phase in AD of SFW was shortened by 4.5 d,and T80 was shortened by 9 d.These two methods promoted the production and degradation of LCFAs in AD of SFW.However,after coupling pretreatment of thermal with lipase,severe accumulation of LCFAs occurred during digestion,and degradation process of SFW was largely prolonged.Consequently,lipase and thermal melting which promoted the degradation of SFW in AD were effective pretreatment methods.Fourth,SFW was co-digested with animal manure in semi-continues feeding mode.The methane production performance,stable parameters,and microbial community structure in the co-digestion were studied.Stable run of reactor could be achieved when the proportion of SFW was controlled within a certain range,and higher methane yield,methane content,and VS removal were obtained than mono-digestion of manure.Methane production performance in co-digestion of SFW with pig manure was higher than that in co-digestion of SFW with cattle manure and sheep manure.Pig manure had higher buffering capacity than cattle manure and sheep manure,due to its higher alkalinity and ammonia nitrogen.When SFW was co-digested with pig manure,stable run could still be achieved with proportion of SFW reaching 60%,while SFW was co-digested with cattle and sheep manure,stable run could not achieve with the same proportion of SFW.The dominant bacterias included Firmicute,Bacteroidetes,Proteobacteria,and Chloroflexi,and the dominant methanogenic archaeas were Methanothrix and Methanospirillum.Finally,AD performance in semi-continues co-digestion of SFW with pig manure at mixing ratio of 6:4 was investigated by increasing of organic loading rate(OLR).For co-digestion of SFW with pig manure,the highest VS methane yield was 534.3 mL gvs-1 d-1 obtained at OLR of 4 gvs L-1 d-1,and the highest volumetric methane yield was 2.52 L L-1d-1 obtained at OLR of 5 gvs L-1 d-1.For co-digestion of thermal pretreated SFW with pig manure,methane production performance greatly improved compared with untreated digester.The highest VS methane yield was 628.7 mL gvs-1 d-1 obtained at OLR of 3 gvs L-1 d-1,and the highest volumetric methane yield was 2.79 L L-1d-1 obtained at OLR of 5 gvs L-1 d-1.For co-digestion of lipase pretreated SFW with pig manure,no improvement of methane production performance was observed,and it accelerated the unstability of the digestion.