Enhancement Of Zero-valent Iron Addition On Anaerobic Permeable Biofilm Reactor:performance And Mechanism

Compared with anaerobic membrane bioreactors(AnMBRs),anaerobic dynamic membrane bioreactors(AnDMBRs),wherein coarse-pore filtration materials are employed instead of microfiltration/ultrafiltration membranes,have distinctive advantages in terms of low cost and energy consumption.Nevertheless,membrane fouling remains the main problem for AnDMBRs,hindering their more widespread engineering application.Suspended activated sludge in AnDMBRs is the main membrane foulants and its minimization thus presents an effective measure for membrane fouling control.Based on this concept,our research group previously proposed an novel anaerobic permeable biofilm reactor,wherein the organic pollutants removal were fully completed by the dynamic membrane(biofilm in essential)formed on the coarse-pore filtration materials with suspended activated sludge concentration nearly zero.The proposed novel anaerobic permeable biofilm reactor exhibit fairly good overall performance.To date,the R&D of the novel anaerobic permeable biofilm reactor was still in the infant stage.The reactor performance needs to be further enhanced,and the physicochemical and microbiological properties of the permeable biofilm and need to be further studied.According to the strong reducibility of zero-valent iron(ZVI)and its successful application in enhancing traditional anaerobic bioreactor performance,this research attempted,at the first time,to study the enhancement of ZVI addition on anaerobic permeable biofilm reactor performance and the corresponding underlying mechanisms.Two anaerobic permeable biofilm reactors(Control R1 and ZVI R2)were operated in parallel for the treatment of low-strength synthetic wastewaters.The biodegradation and filtration performance were investigated comparatively.Besides,the physicochemical and microbiological properties of the permeable biofilm(both inner and outer layer)were analyzed and their correlation with the rector performance were evaluated.Eventually,the underlying mechanisms based on which the ZVI addition enhanced reactor performance were revealed.The main conclusions were summarized as follows:(1)ZVI addition enhanced the biodegradation performance of anaerobic permeable biofilm reactor.The effluent COD concentration of R2 was 43.0±17.9 mg/L,31.0%lower than that of R1.The average COD removal efficiency increased to 90.1%.The biogas production increased to 340 mL/d.The effluent VFA concentration decreased to 9.5 mg/L.(2)ZVI addition enhanced the filtration performance of anaerobic permeable biofilm reactor.After long-term operation of 160 day,the TMP of R2 increased merely to 4 kPa,11.11%lower than that of R1.On the other hand,ZVI addition did not affect the effluent turbidity that was 3.63 NTU in average.Both reactors exhibit excellent solid-liquid separation performance.(3)ZVI addition changed the physicochemical properties of anaerobic perrmeable biofilm.The biofilm structure in R2 was relatively loose compared with that in R1.ZVI addition restraining the release of EPS,especially the amount of polysaccharides.These result in decreased filtration resistance and increased operation stability.As the depth of biofilm increased,the biofilm structure became more compact along with the increased average particle size,the decreased amount of protein,and the increased amount of polysaccharides.(4)ZVI addition optimized the bacteria structure,increased the efficiency of hydrolytic acidification and thus provided suitable substrates for the following methanogenesis.The relative abundance of dominant bacterial phyla(Proteobacteria and Bacteroidetes)in R2 playing important role in hydrolytic acidification and hydrogen/acetic acid production increased 12%compared with that in R1.The relative abundance of Syntrophobacterales in R2 increased 9.52%and 3.99%which were correlated with the biodegradation of VFA and the production of methane.(5)ZVI addition optimized the archaea structure,promoted the expression of the methanogenic archaea in the biofilm.Compared with that in R1,the relative abundance of Methanosaeta increased 7-10%in R2,indicating that ZVI addition further enhanced the dominance of Methanosaeta.The main microbiological mechanism responsible for the enhanced performance of R2 was attributed to be that the addition of zero-valent iron increased the relative abundance of the dominant bacteria and methanogens in biofilm.