Study On Hydrogen Recovery And Phosphorus Release Mechanism In Electro-Fermentation System Enhanced By Free Nitrous Acid

Recovering resource and energy from waste activated sludge（WAS）is the urgent requirement for wastewater treatment in China.Considering the resource characteristic of WAS,converting WAS to valuable products,e.g.,volatile fatty acids（VFAs）,methane or hydrogen,have been widespread explored.Electro-fermentation（EF）,as an innovative technology to enhance the process of anaerobic fermentation by regulating microbial metabolism via electrodes,can accelerate high-purity hydrogen recovery from WAS.However,organics conversion and hydrogen yield in EF system was still limited when using raw WAS as the feedstock,due to most organics were enmeshed in semirigid cytoderm and extracellular polymeric substances（EPS）.In this study,a method for enhancing organics conversion and hydrogen recovery by coupling pretreatment and EF system was proposed.Strategies for improvement of hydrogen yield based on free nitrous acid（FNA）via cascading EF system were constructed.Transformation mechanism of phosphorus fractions in solid and liquid phases from WAS during the bioelectrolysis was also explored.This study may provide an innovative insight for maximal recovery of resource and energy from WAS.FNA pretreatment was conducted to effectively accelerate the WAS solubilization,providing abundant soluble organics for EF process.Organics release and utilization performance,hydrogen yield and microbial community structure were deeply investigated in both open and closed circuits of EF by using single chamber microbial electrolysis cell lacking a membrane（MEC）as main configuration.Moreover,aiming at the limited efficiency of extracellular electron transfer from anodic microbial consortia to solid electrode,signal molecule was used to achieve the interspecific regulation,treatment mechanism of EF system was analyzed.Results showed that 12 h was the optimal time under the pretreated condition of 2.13 FNA mg/L,the contents of soluble carbohydrates and proteins achieved the peak values,which were 6.7-and 18.4-fold of that obtained from raw sludge.Compared with the open circuit,organics release and utilization efficiencies were highest in the MEC bio-electrolysis system fed with FNA pretreated sludge（FMEC）.VFAs content peaked at 697.1 mg COD/L within 48h,which was 3.8 folds than that of MEC bio-electrolysis system fed with unpretreated sludge（RMEC）.The accumulative content of acetic acid（HAc）and propionic acid（HPr）accounted for 69.4%～75.3%.Meanwhile,hydrogen yield and current were 2.1 mg/g VSS and 1.9 m A in FMEC,respectively,which were1.5 and 3.2 folds of that obtained in RMEC.These two indicators were further enhanced by adding signal molecule and were 1.2 and 3.0 times of that of FMEC,respectively.Besides,the internal resistance especially the charge transfer resistance(R_ct)sharply decreased,and the electrochemical performance were enhanced accordingly.The predominating microbial consortia mainly consisted of anaerobic fermentation bacteria（AFB）,electrochemical active bacteria（EAB）and nitrate reducing bacteria（NRB）,among which the synergetic relationships collectively accomplished the processes of solubilization,hydrolysis,acidification,acetogenesis as well as electricity and hydrogen generation in EF system.Prefermentation-cascading bioelectrolysis system,also called cascading EF system,was constructed to further promote the performance of organics release and utilization.The role of FNA pretreatment on hydrogen production from WAS in cascading EF system was deeply investigated.The optimal technology was selected by comparing the performance of organics release and utilization,electrochemical characteristics,hydrolase activity and microbial community structure.Results showed that VFAs production was obviously promoted by FNA pretreatment during anaerobic fermentation,and peaked at 2201.6±18.9 mg COD/L within 4 d,which was 1.7 folds higher than that of unpretreated sludge.The accumulation of HAc and HPr accounted for 75.8%.In comparison with untreated sludge,various hydrolases activities increased by 14.4%～56.6%after FNA pretreatment during anaerobic fermentation.Soluble proteins and carbohydrates peaked at 1501.7±30.1 mg COD/L and 216.7±3.6 mg COD/L at16 h and 8 h in bioelectrolysis,and the utilization efficiencies reached 41.5%and62.1%at 72 h,which were 6.8%and 19.3%higher than that of unpretreated sludge.The utilization efficiency of VFAs was 74.8%,promoted 21.6%than that of the group of raw sludge.Compared with EF system,the cascading EF system manifested the best performance of organics release and utilization and lowest internal resistance.Current and hydrogen yield were 8.4 m A and 6.18±0.3 mg/g VSS,respectively.Moreover,functional microbes,including AFB（e.g.,Petrimonas and Proteiniclasticum）,EAB（e.g.,Pseudomonas and Geobacter）and NRB（e.g.,Ottowia and Thermomonas）accounted for 31.2%,8.7%and 3.8%,respectively,which were higher than that enriched in EF groups.In conclusion,cascading EF system was the optimal technology for hydrogen recovery from WAS.In view of the limitation function of EPS decomposition by FNA pretreatment,peracetic acid（PAA）combined FNA pretreatment was proposed to achieve the accumulative effect on EPS disintegration and cells inactivation.The condition of the co-pretreatment was optimized for the highest VFAs production during anaerobic fermentation.The performance of EPS exfoliation and utilization at different stages consisted of pretreatment and cascading utilization was explored.Results revealed that the optimal PAA dosage was 100 mg/g VSS,VFAs production achieved the peak of 7679±86 mg COD/L at 4 d,respectively,promoting 200.0%and 19.0%than that obtained from the sole FNA and PAA pretreated sludge.After the co-pretreatment,the release efficiencies of soluble proteins and carbohydrates attached to tightly bound EPS（TB-EPS）reached to47.6%and 68.0%,respectively,increased to 80.6%and 79.8%during prefermentation,and ultimately enhanced to 87.4%and 93.9%at the end of bioelectrolysis,which were both higher than that in FNA or PAA pretreatment group.Eventually,the utilization efficiency of organics in co-pretreatment group was 1.4～1.7 folds of that obtained from sole pretreated sludge,and the hydrogen yield and current were 10.8±0.3 mg/g VSS and 10.7 m A,respectively.Microbial community analysis showed that the proportion of AFB reached to 28.1%,e.g.,Macellibacteroides（6.3%）and Sedimentibacter（6.9%）,EAB accounted for57.0%,e.g.,Geobacter（39.0%）and Pseudomonas（13.6%）.Aiming at the release role of phosphorus during various treatment processes,the performance of phosphorus release from WAS in different system（anaerobic fermentation,EF and cascading EF）was explored.The migration and transformation mechanism for various phosphorus fractions existed in liquid and solid phases was revealed in cascading EF system.Results showed that organic phosphorus（OP）release was limited in prefermentation and EF system,but can be further improved via cascading EF system.OP content decreased from 6.61mg/g to 5.41 mg/g during prefermentation,and further weakened to the lowest value of 3.0mg/g at 16 h during cascading EF system.Ultimately,OP release efficiency peaked at 54.6%,and the corresponding release contribution rate was90.0%in cascading EF system assisted by FNA pretreatment,which was much higher than that in anaerobic fermentation and single EF system.