Effects Of Ni(?)on The Physicochemical Property And Phosphorus Removal Performance Of Activated Sludge In Enhanced Biological Phosphorus Removal System

In recent years,nickel(Ni)has been extensively used in many consumer products such as high quality iron-based alloys,catalysts,paints and batteries due to its unique physicochemical property.The extensive applications of Ni inevitably cause their releases into wastewater treatment plants(WWTPs),which can induce the potential effect on activated sludge.It is known that the main function of activated sludge is to remove organic pollutants,nitrogen(N),and phosphorus(P)from wastewater.Although a large number of endeavors have been dedicated to evaluating the effects of Ni(?)on the biological removal of organic matter and N,the potential effect of Ni(?)on biological phosphorus(P)removal has not been reported so far.In addition,the removal performances of activated sludge are relevant to or dependent upon their physicochemical properties,including the flocculability,settleability,and dewaterability.As a result,there is a need to explore the potential effects of Ni(?)on these physicochemical properties and enhanced biological phosphorus removal(EBPR)of activated sludge.Firstly,the effects of Ni(?)in the range of 0.1-10 mg/L on the physicochemical properties of activated sludge were investigated in this paper.In order to explore the details of how Ni(?)affects these properties,the compositions and structures of extracellular polymeric substances(EPS)by using excitation-emission matrix(EEM)fluorescence spectroscopy and Fourier-transform infrared(FTIR)spectroscopy,and the surface properties of activated sludge were therefore investigated.It was found that these properties were unaffected after short-term exposure to Ni(?)even at the level of 10 mg/L.After long-term exposure to 1 and 10 mg/L of Ni(?),however,the sludge flocculability has significantly deteriorated.Conversely,the settleability and dewaterability has gradually become better than the control.The mechanism studies revealed that long-term exposure to Ni(?)resulted in the decrease of protein content in EPS and the damage to EPS structures.Although Ni(?)did not bring any adverse effect on the integrity of cell membrane,the relative hydrophobicity of activated sludge was significantly decreased.Thus,the changes of EPS structures,and the decrease of the protein content and hydrophobicity were the major reason for the deteriorated flocculability.Then,further investigation about the effect of Ni(?)(0.1-10 mg/L)on biological P removal was carried out from the aspects of the transformations of metabolic intermediates,the activities of key enzymes,the production of reactive oxygen species(ROS)and the variations of microbial community.Compared with the control,the short-term exposure to 1 and 10 mg/L of Ni(?)resulted in the decrease of P removal efficiency from 99.7%to 38.3%and 0,respectively.The P removal was unaffected after short-term exposure to 0.1 mg/L of Ni(?),but it was completely collapsed after 30-day exposure.The mechanism studies revealed that the activity of exopolyphosphatase(PPX)was significantly suppressed by the short-term exposure to 10 mg/L Ni(?),likely due to the increase of ROS production induced by Ni(?),thereby resulting in the decrease of anaerobic P release.Moreover,the increase of intracellular Ni(?)concentration in activated sludge might be a reason for the increased ROS production.At the aerobic stage,the degradation of PHAs was significantly inhibited,which was the main reason for the severe inhibition on the aerobic P uptake.Unlike the short-term results,the long-term exposure to Ni(?)enhanced the transformations of poly-3-hydroxyvalerate and glycogen,suggesting that the microorganisms in these systems with Ni(?)exposure showed GAOs phenotypes rather than PAOs ones.Microbial community investigation with high-throughput sequencing analysis confirmed that alphaproteobacterial glycogen accumulating organisms instead of polyphosphate accumulating organisms became the predominant microorganisms in EBPR systems after long-term exposure to Ni(?).On the basis of above study,we attempt to find a valid strategy to mitigate the above-mentioned effect of Ni(?)in this paper.The results showed that the negative effects on the flocculability and P removal performance of activated sludge could be completely eliminated by adding the chelator such as EDTA or citrate.Moreover,EDTA might be a better complexing agent than citrate to reduce the toxicity of Ni(?).