Study On The Change Of Wastewater Treatment Efficiency And Response Mechanism Of Microalgal-bacterial Symbiosis System Under The Effect Of Heavy Metals

HMs are highly toxic,non-biodegradable,easy to bioaccumulate and bio-magnify,which poses a great threat to human health and ecosystem stability.Facing the environmental problems caused by HM wastewater,the MABS system has attracted widespread attention due to its better HM removal efficiency,lower cost investment and synergistic removal for nitrogen and phosphorus pollution.In this study,copper（Cu）and zinc（Zn）which are essential for microbial growth,were selected as the objects.The change of the growth characteristics in MABS system affected by HMs with different concentrations and types has been investigated.Besides,the role of phosphate in the response of MABS system to HMs was also investigated.The effect of HMs on the removal of pollutants by the MABS system was investigated.It was found that the removal efficiency of pollutants by MABS system decreased in the presence of Cu²⁺/Zn²⁺.Among them,the inhibition effect of 2 mg/L Cu²⁺was most obvious,resulting in the decreased removal efficiency of NH₄⁺-N and TP with values of 48.09%and 64.86%,respectively.Phosphate alleviated the deterioration of the pollutant removal effect of MABS system.The removal efficiency of COD and TN in the group of Cu²⁺co-dosing with phosphate were increased by 10.86%and 15.65%,compared with Cu²⁺alone.Similarly,the removal efficiency of TN in the group of Zn²⁺co-dosing with phosphate were higher than Zn²⁺alone,which increased by 37.24%.Analyzing the effects of HMs on the growth characteristics and structural characteristics of MABS system.It was indicated that 2 mg/L Cu²⁺had the greatest inhibitory effect on the growth of microalgae.The corresponding inhibition rate reached79.00%,which was higher than that of Zn²⁺（47.59%）.The maximum inhibition rates of Cu²⁺and Zn²⁺on the growth of total microbial biomass were 13.60%and 11.15%,respectively.Meanwhile,the addition of HMs significantly inhibited the microbial activity.The inhibition effect of Cu²⁺was still stronger than Zn²⁺.It was indicated that HMs affected the pollutant removal efficiency of MABS system by inhibiting the growth and activity of microorganisms.When adding phosphate to MABS system at initial stage,the inhibitory rates of Chl-a affected by 5 mg/L Cu²⁺and Zn²⁺were reduced by 9.32%and 23.26%,respectively.The inhibitory rates of total microbial volume decreased by10.59%and 20.31%,respectively.It was indicated that phosphate can relieve the inhibition of HMs on the growth and activity of microorganisms;then it was alleviated the HMs inhibition on pollutant removal of MABS system.However,replenishment of phosphate to the system in the later period play a less role in alleviating the coercion that the MABS system has suffered.Finally,the response characteristics and mechanisms of the MABS system to HMs were further analyzed.According to the performance of the MABS system in removing HMs,the removal efficiency of Cu²⁺were 89.90%and 80.61%in MABS system with and without phosphate,respectively;the removal efficiency of Zn²⁺were 98.31%and90.31%in MABS system with and without phosphate,respectively.At the same time,the maximum adsorptions of EPS for Cu²⁺and Zn²⁺were 23.13 mg/g EPS and 16.54 mg/g EPS,respectively.The microbial diversity of MABS system decreased after adding HMs,and meanwhile,the microbial community structure in the MABS system had been changed.Sphingomonas,Chryseobacterium,Desmodesmus,Scenedesmus and Parachlorella were dominant in the MABS system for the removal of Cu²⁺and Zn²⁺due to their high tolerance.In summary,the MABS system can effectively remove HM ions from wastewater through microbial and EPS adsorption,but the presence of HM ions can inhibit the activity of functional microalgae and bacteria in the MABS system;and it affects the pollutant removal performance of the MABS system.Moreover,phosphate not only provides nutrients for the growth of microalgae,but also reduces the concentration of HM ions through precipitation,thus alleviating the toxic effects of HMs to the MABS system.This study provides a theoretical basis and data support for future research on the treatment of wastewater mixed with HMs,nitrogen and phosphorus by MABS system;and it is beneficial to the migration and transformation of HMs in the water ecosystem.