| Excess sludge is an important by-product of the biological treatment of sewage sludge,containing a large number of toxic and hazardous substances（such as pathogens,heavy metals,organic pollutants,etc.）.Improper disposal will result in serious environmental pollution and human health hazards.Therefore,this study relied on the anaerobic membrane bioreactor（An MBR）as the main technology.In order to address the key technical challenges of slow sludge hydrolysis rate,low methane production potential,low organic solids degradation rate and membrane fouling control,free nitrous acid（FNA）coupled with electrochemical pretreatment,thermal hydrolysis（TH）pretreatment,bio-electrochemical system（BES）and nano zero-valent iron（n ZVI）addition strategies were applied to comprehensive enhance methane bioconversion and in-situ membrane fouling mitigation.The main research conclusions are as follows:1)An innovative coupling of FNA and electrochemical pretreatment was applied to comprehensively evaluate the synergistic effect of the combined pretreatment strategy on sludge fracturing and subsequent methane production.The results indicated that pretreatment with 10 V plus 14.17 mg N/L substantially enhanced sludge disintegration,with the highest soluble chemical oxygen demand（COD）concentration of 3296.7 mg/L,25.6-time higher than that without pretreatment（128.9 mg/L）.However,due to the potential toxicity of NO2-and NO3-to microorganisms and its bioprocesses,the methane productivity of sludge pretreated by FNA was significantly deteriorated.The maximum methane yield(152.0±9.6 m L/g-VSadded)was observed at10 V pretreatment alone,only 1.7%higher than that of the control(149.4±1.6 m L/g-VSadded).Digested sludge also achieved the best dewaterability under this condition.The FNA coupled electrochemical pretreatment strategy greatly improved the sludge hydrolysis,but failed to increase subsequent methane productivity.2)To enhance sludge hydrolysis and methane conversion,TH pretreatment was applied to sludge mesophilic fermentation in An MBR.The results showed that the best performance was obtained at a pretreatment temperature of 125°C,with SCOD concentration of 1116.2±62 mg/L,a 2.6-time increase over the raw sludge.During this stage,the methane yield reached to 73.2±7.5 m L/Lreactor/d,a 3.0-time increase compared to the control stage feeding raw sludge.In addition,the application of TH pretreatment effectively achieved membrane fouling mitigation,maintaining a stable membrane flux of 10.8±0.5 L/m2/h and low transmembrane pressure（TMP,≤4.7k Pa）.Cake layer deposited on the membrane surface accounted for 66.1±1.0%of total resistance.16s r RNA gene analysis demonstrated that utilization of TH could help to build a more robust microbial community and enhance the proliferation of organics-degrading bacteria（i.e.Bacteroidetes,Firmicutes,etc.）and methanogens（i.e.Methanosaeta,Methanobacterium,etc.）,which accelerated the biodegradation of organics while alleviating membrane fouling and upgrading methane production.The introduction of TH pretreatment into An MBR has great potential in promoting sludge hydrolysis,methane productivity and membrane fouling mitigation.3)BES was introduced into an anaerobic membrane bioreactor（An MBR）to treat TH sludge.A novel BES-An MBR system was constructed to investigate the feasibility of improving methane conversion and in-situ membrane fouling mitigation in treating TH sludge.The highest methane production was up to 77.5±5.9 m L/Lreactor/d at the applied voltage of 0.8 V and TH pretreatment of 85℃,66.7%higher than that of the control stage（46.5±4.3 L/Lreactor/d）.Due to the electrostatic repulsive force and electrochemical oxidation provided by the BES,BES-An MBR achieved excellent membrane fouling control,achieving stable membrane flux（8.6 LMH）and relatively low TMP（<4.0 kpa）.Furthermore,the anode and cathode graphite felt fibers provided a more suitable environment for the growth and reproduction of functional microorganisms（Proteobacteria,Smithella,etc.）,and promoted the growth of biofilm on the electrode and the"Smithella-pathway"metabolic pathway.BES also greatly enhanced the direct interspecies electron transfer by Geobacter on the anode and promoted the efficiency of methane production.In addition,COD mass balance and energy analysis suggested that the application of BES and TH pretreatment could further improve energy recovery in form of methane.4)nZVI was appied as an electron donor to enhance sludge methane bioconversion efficiency and functional microbial metabolism.To avoid the accumulation toxicity caused by membrane retention of n ZVI at An MBR,n ZVI was added to a continuous stirred tank reactor for long-term anaerobic fermentation.Results indicated that methane productivity strongly relied on n ZVI dosage（Rp=0.988,p0.05=0.001）and leached Fe2+（Rp=0.943,p0.05=0.005）.The methane production reached 59.2±4.9m L/Lreactor/d at 161.6 mg-n ZVI/g-TS,increasing by 87.3%than the phase without n ZVI supply.Further analysis demonstrated that the corrosion/dissociation of n ZVI could create a more thermodynamically favorable environment for the proliferation of anaerobic functional microbes such as organics-degrading bacteria（Firmicutes,Smithella,etc.）and hydrogenotrophic methanogen（Methanobacterium）,by improving system buffering capacity,decreasing oxidation reduction potential and serving as electron donors（H2/[H]）.More importantly,the continuous supply of n ZVI at 161.6mg/g-TS remarkably decreased the capillary suction time of the digested sludge from191.8 s to 88.8-97.6 s and its deterioration was alleviated.This study demonstrated that the continuous supply of n ZVI possessed a great potential for enhancing the methane bioconversion,improving the digested sludge dewaterability and reducing the ultimate disposal cost. |
