| The kitchen waste slurry,obtained after the pretreatment of kitchen waste for cooking at high temprature,oil extraction and solid separation,is a kind of high concentration organic wastewater containing carbohydrates,proteins,lipids,etc.It would seriously pollute the environment and harm human health,once it was discharged into the water body.Recently,the bioconversion into biogas via anaerobic digestion is a promising way to dispose this high-strength waste slurry and achieve energy recovery.However,the lipids contained in the substrate and the hydrolysis products,long chain fatty acids?LCFAs?,can adsorb on the biomass surface and present antibacterial effect.The bio-toxicity,together with the surface hydrophobic characteristics of the LCFAs,would result in sludge flotation and sludge washout,even deteriorate operation performance.To solve these problems,the anaerobic membrane bioreactors?AnMBR?supply a reliable method.With the aid of membrane,high loading rates,excellent effluent quality,robust stability and better anaerobic digestion performance are obtained.This makes the AnMBRs a very attractive alternative concept for the treatment of these high concentration waste waters/slurries.Therefore,the digestion performance and membrane filtration of the AnMBR treating the high lipids kitchen waste slurry were investigated in this study.Furthermore,some process control strategies was adopted to enhance the digestion efficiency and improve the sludge property and the membrane filtration.Moreover,the concentration of the characteristic product-LCFAs in the digestion process was determined to explore its metabolic process,cumulative effect and inhibition mechanism.Besides,the microbial community structure was analyzed via the 16S rRNA high throughput sequencing to illuminate the syntrophic metabolism between the proton-reducing acetogenic bacteria and H2-utilizing methanogens and analyze the stress reaction mechanism of the microbe.Finally,the fouling was analyzed by excitation-emission matrix spectroscopy?EEM?,confocal laser scanning microscopy?CLSM?,fourier transform infrared spectroscopy?FTIR?and scanning electron microscope-energy dispersive X-ray Spectroscopy?SEM-EDX?.Base on these results,a efficiency cleaning method was supplied.The conclusions provided theoretical guidance and technical support for the AnMBR engineering application,which were showed as follows:?1?The full scale continuous stirred-tank reactor?CSTR?was adopted to treat the kitchen waste slurry and its digestion performance and potential problems were investigated in this study.With the organic loading rate?OLR?of about 4 kg-COD/?m3·d?,the biogas production was 16000 m3/d and the chemical oxygen demand?COD?removal efficiency was 90%.Moreover,the mix liquid suspended solid?MLSS?in the effluent was up to 5 g/L,indicating of the serious sludge floating and washing out.To resolve this problem,a ultrafiltration membrane was adopt to filtrate the digestion sludge in the CSTR to obtain a optimum operating parameter in the long term operation.Results showed that the best operating pressure was 0.23 Mpa and cross flow velocity was 2.2 m/s.Under this operation condition,the flux was 32 L/?m2·h?.Furthermore,a excellent interception efficiency to the MLSS and macromolecular substances was achieved with the ultrafiltration membrane.?2?The pilot-scale AnMBR was built up according to the membrane operation parameters,and successfully started up when treating lipids-rich wastewater.The digestion performance,sludge characteristics and fouling of the AnMBR were investigated in this study.Results showed that with the aid of membrane,excellent treating efficiency and stability were obtained,with COD removal efficiency of 99%and Volatile Fatty Acid?VFA?of below 200 mg/L.When the OLR was 4.54.9 kg-COD/?m3·d?,the biogas prodction was 2.2 m3/d.However,the digestion efficiency gradually decreased in the late operation stage.Additionally,the hydrolysis products of the high lipids in the influent,the LCFAs,were gradually accumulated,which produced negative effects on the digestion efficiency and sludge characteristics as a result of its antibacterial effect and surface hydrophobicity.The MLSS sharply increased from 7.1 g/L to 39g/L and the sludge particle size decreased from 26.5?m to 6.5?m,while the soluble microbial products?SMP?increased from 47.7 mg/g-VSS to 98.0 mg/g-VSS and the relative hydrophobicity of sludge elevated from 28.2%to 68.1%.These variations further aggravated the fouling,resulting in an attenuation of flux from 32 L/?m2·h?to 10 L/?m2·h?.The pearson correlation test showed that the membrane filtration presented a significant positive correlation with the sludge particle size and a strong negative correlation with the MLSS,SMP and sludge relative hydrophobicity.?3?The process stability,microbial community structure,sludge characteristic and membrane filtration of the AnMBR were investigated under different solid retention times?SRTs?.When the SRTs decreased from 50 d,30 d and 20 d,the OLR increased from 5.9 kg-COD/?m3·d?to 7.5 kg-COD/?m3·d?and 9.3 kg-COD/?m3·d?.The digestion efficiency?i.e.conversion efficiency of COD to methane?increased from 78.1%to 81.5%and 84.4%,respectively,while the permeate COD all kept below 1500 mg/L.Additionally,the low VFA concentration?<300 mg/L?and stable pH value around 7.5 under the three SRTs confirmed the robust stability of the AnMBR system.Furthermore,with the decrease of SRT,the LCFAs accumulation decreased and the hydrolytic enzyme activity was enhanced.the amylase,protease and lipase activities increased by 1.9-fold,2.0-fold and 2.4-fold,respectively.Meanwhile,the decrease of SRT had no obvious effects on the bacterial community but remarkable influence on the archaeal community as the dominant archaea shifted from Methanobacterium to Methanosaeta.Moreover,when the SRT was shortened,the sludge characteristics were improved and membrane filtration was enhanced.Therefore,the optimized SRT was 20 d in this study.?4?As the temperature of the kitchen waste slurry was very high,to effectively recycle this energy,the thermophilic AnMBR?55?,t-AnMBR?started up based on the digestion sludge in the mesophilic AnMBR?39?,m-AnMBR?.The digestion performance,membrane filtration,LCFAs accumulation and discrepancy of microbe structure were compared between the t-AnMBR and m-AnMBR.Under the thermophilic condition,the OLR kept at 7.5 kg-COD/?m3·d?,while it reached 12.0 kg-COD/?m3·d?under the mesophilic condition.Compared with the mesophilic condition,the biotoxicity of the LCFAs was enhanced and the LCFAs accumulating rate was accelerated under the thermophilic condition,which presented negative effect on the performance and stability of the reactor.Furthermore,the temperature change showed obvious effects on the microbe structure.When the temperature rised from 39?to55?,the bacterial Bacteroidales and Anaerolineales that can not tolerate to the high temperature fade away,and a new species-MBA08 appeared under the thermophilic condition.In the m-AnMBR,the main pathway of methane production is aceticlastic methanogenesis and hydrogen methanogenesis,while the hydrogen methanogenesis played a dominant role in the t-AnMBR.?5?The performance and syntrophic metabolism of the AnMBR treating high-lipids kitchen waste slurry under different cross flow velocities were investigated in this study.When the cross flow velocity increased from 1.0±0.27 m/s to 2.5±0.17 m/s and 4.0±0.22 m/s,the tolerated OLR increased from 10.3 kg-COD/?m3·d?to 12.4 kg-COD/?m3·d?and 18.1 kg-COD/?m3·d?,respectively,while the corresponding biogas productions were 190 L/d,300 L/d and 520 L/d with methane percentages of 50.3%,55.2%and 58.4%.Moreover,with the increase of cross flow velocity,the COD removal efficiency improved from 86.2%to 90.4%and 96.3%based on the permeate quality,and the VFAs concentration decreased from 6860 mg/L to 4780mg/L and 1690 mg/L.Additionally,the higher cross flow velocity did not broke up the sludge flocs and affect the syntrophic interaction between microbes,and the flocs could still keep the same size of about 7.0?m under different cross flow velocities.On the contrary,it improved the mixing strength and enhanced hydrogen diffusion rate,promoting the syntrophic degradation of the toxic substances of long-chain fatty acids?LCFAs?.Meanwhile,the increased abundance of syntrophic microbe between Methanobacterium and Clostridiales favored the syntrophic metabolism.?6?The recovery of flux in the membrane cleaning process is very important for the AnMBR operation.However,it has a close relationship with the membrane fouling substance.Thus,the fouling component and characteristics were analyzed and a effective cleaning method was supplied in this study.Results showed that the SMP was the main organic fouling and easily accumulated in the reactor due to the interception of the membrane.The SMP could get into the membrane pore or deposited on the membrane surface,resulting in membrane fouling.Furthermore,the EEM presented that the tryptophan protein and F420 were the main substances in the SMP.Moreover,the high concentration calcium in the influent accumulated in the reactor during the AnMBR operation process would cause inorganic fouling.The FTIR and CLSM showed that the main fouling in the membrane surface were polysaccharides,proteins,and microorganisms.The SEM-EDX indicated that the calcium was the main inorganic element on the membrane surface.The combined chemical reagent of NaClO,HCl and ethylenediamine traacetic acid?EDTA?could supplied an effective cleaning method. |
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