Impact And Regulatory Strategies Of Accumulated Aluminum Salts On Sludge Characteristics In MBR

As a coagulant,poly-aluminium chloride(PAC)is often used to enhance phosphorus removal and control membrane fouling in the membrane bioreactor(MBR).However,due to the excellcent filtration performance of the membrane module,a large amount of aluminum salt will be accumulated,and and its influences on the sludge characteristics has not been well assessed.In this paper,the effects of accumulation of aluminum salts on sludge activities,morphology,and microbial community structure in the A/O-MBR were discussed.Additionally,two strategies were proposed to reduce the the negative effects of aluminum salt on the MBR and their feasibility was also assessed.The concentration of aluminum salt on sludge surface continued to rise due to the chemical phosphorus removal by adding PAC(28.3 mg/L)to the A/O-MBR.When the percentage of atomic aluminium on sludge surface increased to 14.2%,the removal rate of ammonia nitrogen,phosphate and COD in the reactor were maintained above 80%.But the ammonia oxidation rate(AOB),nitrite oxidation rate(NOB),and specific oxygen uptake rate(SOUR)of sludge decreased by 82.1%,79.8%,and 46.4%,respectively,and the biological phosphorus removal of sludge almost disappeared.Additionally,the floc structure of the sludge was replaced by a closed and compact structure,the sludge particle size significantly decreased and the protein content in the extracellular polymeric substances(EPS)of sludge decreased substantially.The accumulation of aluminum salt had a significant effect on microbial community structure,and the continuoud increase of the relative abundance of genus Aridibacter might be the main reason for the stable operation of the reactor.The relationship between sludge SOUR and aluminum salt accumulation was studied,and the result indicated that the degree of sludge inhibition was positively correlated with the accumulation of aluminum salt.By comparing the operation of two A/O-MBR reactors under different SRT conditions(15d and 30d),the results showed that there was no significant difference in the removal rate of pollutants between the two reactors,but the short SRT was beneficial to control the accumulation rate of aluminum salts.On the 54 th day,the mass percentage of aluminum in the sludge reached 16.7%(SRT,30 d)and 13.2%(SRT,15 d),respectively.The AOB activity of the sludge decreased by 10.94% and 4.43%,respectively,and the NOB activity decreased by 22.99% and 18.25%,respectively.On the contrary,the denitrification activity and phosphorus uptake activity of the sludge were significantly enhanced.Therefore,the inhibition of the accumulation of aluminum salts on the nitrification of sludge could be slowed down by controlling shorter SRT,but the control of membrane fouling should also be consideredThe filler was added as a biofilm carrier in A/O-MBR.The effect of PAC addition on the performance of biofilm A/O-MBR process was studied.The results showed that the accumulation of aluminum salts could promote the increase of EPS,which increased the amount of biofilm by 17.5%.Additionally,the accumulation of aluminum salt promoted the enrichment of the genus Nitrospira on biofilm,which increased the AOB and NOB activities of the biofilm by 42.9% and 65.9%,respectively.Therefore,the biofilm-MBR system by adding carriers into the MBR could eliminate the negative effect of accumulated aluminum salt on the nitrification of sludge.