Performance And Mechanism Of Bacteria-algae Symbiosis SBR Nitrogen Removal And Sludge Methanogenic Fermentation Based On Sewage Carbon Capture

To meet the needs of"carbon neutral"operation of sewage treatment,the new Adsorption/biodegradation（A/B）process with the technical concept of"carbon capture and redirection",that is,to capture and separate organic carbon in sewage to the maximum energy recovery by anaerobic digestion at stage A end and achieve efficient nitrogen removal at stage B,which is considered as the future direction of wastewater energy and resource recycling.However,the new A/B process is faced with such problems as the conventional sewage"carbon capture"technology is difficult to adapt to the sewage in China with low carbon,nitrogen,and phosphorus loading,the low C/N ratio effluent is difficult to be effectively denitrogenated by the conventional denitrogenation methods,and the methane production efficiency of sludge anaerobic digestion after carbon capture is not clear,so it is difficult to carry out a practical application in China.Based on the above problems,this study constructed a process of ferric chloride enhanced high load activated sludge（FC-HRAS）,and introduced ferric chloride coagulation on the basis of sludge flocculation and adsorption to improve the capture and enrichment efficiency of sewage organics and reduce the mineralization of organic matter;An algae-sequencing batch reactor（A-SBR）was constructed to remove nitrogen from sewage with a low C/N ratio based on the bacterial-algal symbiosis system,and the promoting effect of microalgae on nitrogen removal process in A-SBR was revealed;The methane production efficiency and the mechanism of anaerobic digestion of carbon-rich sludge were studied,which clarified the energy recovery efficiency of anaerobic digestion of carbon-rich sludge,and revealed the mechanism of methane production of carbon-rich sludge.The effects of sludge flocculation,ferric salt chemical coagulation,and ferric salt enhanced sludge flocculation on carbon capture and enrichment in sewage were investigated.The results showed that under the conditions of the sludge mixed liquid suspended solids concentration（MLSS）of 2000 mg/L and ferric chloride（FC）dosage of 24 mg Fe/L,FC enhanced sludge flocculation could achieve the maximum capture efficiency of sewage organics,and the removal rates of total chemical oxygen demand（TCOD）,ammonia（NH₄⁺-N）and total phosphorus（TP）reached 76.82±3.84%,33.59±1.68%,and 93.84±4.69%,respectively.On this basis,an FC-HRAS reactor with the continuous flow was constructed.It was found that when sludge retention time（SRT）=1 d,dissolved oxygen concentration（DO）=0.8 mg/L and hydraulic retention time（HRT）=40min,68.19±2.17%of TCOD,44.82±0.65%of dissolved chemical oxygen demand（SCOD）,91.20±4.03%of particulate chemical oxygen demand（PCOD）,33.52±1.95%of NH₄⁺-N and 82.75±4.87%of TP in sewage could be enriched effectively.The Zeta potential,molecular weight change,and organic matter composition of supernatant in FC-HRAS and HRAS were investigated.The results showed that compared with HRAS,FC-HRAS increased the Zeta potential of sewage to the ideal flocculation potential（-15.3±0.76 m V）,which destroyed the dispersion and stability of suspended colloidal particles in sewage and enhanced the efficiency of coagulation and sedimentation.Meanwhile,FC-HRAS further removed the tryptophan proteins and aromatic proteins from effluent,and reduced humic acids and other macromolecular components that were difficult to biodegrade,and increased the proportion of hydrophilic organic compounds（HPI）that were easy to biodegrade（54.61±2.70%）,thus enhancing the biodegradability of effluent.The optimum parameters of stable operation of A-SBR system under a low C/N ratio sewage condition were investigated.The results showed that when the optimal parameters were 200±12μmol/m²·s of light intensity,12 h light/12 h darkness of light cycle,and 0.5 L air/min of aeration rate,the A-SBR system has the best algae growth activity,sludge settling performance and pollutant removal efficiency.The changes in biomass,activity,and pollutant removal efficiency of microorganisms and algae in the A-SBR system during the 120 d of long-term operation were further investigated.It was found that under the same operating conditions,the A-SBR system has higher sludge specific oxygen uptake rate（SOUR increased by 41.25%）,microbial growth rate（increased by 31.2%）,and DO concentration（increased by 0.52 mg/L）.The A-SBR system could remove97.60%of COD,66.74%of total nitrogen（TN）,and 73.30%of TP,respectively.The effects of microalgae on biological nitrogen removal in A-SBR were investigated.The results showed that microalgae had a positive effect on the migration,transformation,and storage mode of nitride in EPS by improving the yield of EPS,changing the composition and electronegativity of EPS.The algal-bacterial symbiosis system enriched the bacteria and microalgae species related to nitrogen and phosphorus removal,improved the sludge microbial nitrification and denitrification activity,and enhanced the system nitrogen removal efficiency through the assimilation and uptake of microalgae.The methane production efficiency of carbon-rich sludge during anaerobic digestion was investigated.It was found that the methane production efficiency of anaerobic digestion of carbon-rich sludge from FC-HRAS was better than that from conventional waste activated sludge（WAS）,the results showed that the carbon-rich sludge with 12 mg Fe/g DS of FC residual had the largest cumulative methane production（89.04±2.70 m L/g VS）,which was 133.8%higher than that of WAS,of which the improved reduction of organics and the residue of FC in carbon-rich sludge contributed to 88.4%and 24.1%,respectively.Considering the potential effect of FC residues in carbon-rich sludge,the mechanism of FC on the key stages of anaerobic digestion was studied.The results showed that FC promoted sludge dissolution and hydrolysis through dissimilated iron reduction（DIR）,which increased 46%protein and 38%polysaccharide dissolution.Besides,FC could act as electron acceptors that consumed excess electrons in acetic acid production,which improved 38.1%and 69.0%acid production from amino acid and monosaccharides,respectively.However,the DIR process induced by FC could compete with methyl-Co M for electrons transferred from[CO]and HS-HTP,results showed that 29.2%and 28.4%of the methane produced from acetic acid and hydrogen was reduced in the methane production stage.