| In recent years,with the improvement of people’s living standards,the demand for seafood is increasing,recirculating aquaculture system has developed rapidly in the world.Recirculating Aquaculture System uses biological method to remove harmful material,such as organic compounds and nitrogen salts.The biofilm method is widely used due to its highly efficiency micro-ecological system and strong impact resistance.Moving bed biofilm reactor(MBBR)is the core unit in the water treatment process of the recirculating aquaculture system.The quality of underground seawater is good and the water supply is stable.The underground seawater is one of the important water sources for marine aquaculture in China.Due to the strong reducibility and low p H of groundwater,the concentration of Fe and Mn in groundwater is much higher than that in seawater.The presence of iron and manganese ions in water may stimulate the microbial stress response,thereby changing the characteristics of extracellular polymer components,affecting the mass transfer and stability of the biofilm system,and thus affecting the operating efficiency of the biofilter.Some data show that the iron ion concentration of underground seawater in most areas reaches 8～10mg/L,and individual areas up to 30.05mg/L.In some areas,manganese ion concentration reached 7～8mg/L.Therefore,this paper intends to dynamically monitor the water quality treatment effect of artificially simulated seawater moving bed biofilter,and explore the effects of iron and manganese ions on the maturation process and operating efficiency of seawater moving bed biofilter.Using physicochemical analysis and high-throughput sequencing technology to study the effects of iron and manganese heavy metal ions on EPS composition and content of MBBR biofilm and microbial community structure characteristics,and then analyze the ecological mechanism of iron and manganese heavy metal ions affecting the biofilter’s maturation and operational efficiency,to providing theoretical reference for the development of high efficiency biofilter under complex water quality conditions.The experimental results of this paper show that:(1)To explore the maturation process and operating efficiency of MBBR in aquaculture wastewater with different concentrations of Fe and Mn by regularly monitoring the content changes of the ammonia nitrogen(NH4+-N),nitrite nitrogen(NO2--N)and nitrate nitrogen(NO3--N)in the start-up process and matured membrane of the reactor.The biofilm cultured on the PPC filler carrier successfully.In the Fe3+ experimental group,the treatment effect of three kinds of inorganic nitrogen salts in the aquaculture wastewater increased with the increase of Fe3+concentration,and the 8mg/L Fe3+ concentration group was the first to mature the membrane,and the removal rate of the first day of the seventh cycle reached 79.5%.Compared with the blank comparison group,it was 23% higher;in the Mn2+experimental group,the stable time of the five groups of systems was the 10 th,6th,10 th,9th and 11 th cycles respectively,and the experimental results of ammonia nitrogen cycle degradation indicated that 1mg/L Mn2+ contributes to the formation of biofilm and improves the treatment effect of wastewater,while 8mg/L Mn2+significantly inhibits the growth and activity of biofilm.(2)The results of continuous monitoring of water quality after 24 hours of maturity of the membranes showed that the microbial degradation activity of the8mg/L Fe3+ concentration group was the highest,and the NH4+-N could be completely removed at the 18 th hour,which was 35% higher than the blank group.The time of the highest value of NO2--N in the blank group,1mg/L Mn2+ concentration group and8mg/L Mn2+ concentration group was 18 h,13h and 21 h,respectively.In the 1mg/L Mn2+ concentration group NH4+-N could be completely removed at the 15 th hour.The removal rate was 31.64% and 42.76% higher than that of the blank group and the 8mg/L Mn2+ concentration group,respectively.(3)Fe3+ and Mn2+ play an important role in biofilm activity by affecting microbial physiology or indirectly affecting EPS components.The results of biofilm EPS analysis showed that the addition of Fe3+ could significantly increase the total EPS of biofilm,with the increase of Fe3+ concentration.The total EPS of the five groups were 155.7mg/g,167.45mg/g,184.44 mg,195.70mg/g and 212.06mg/g,respectively.Fe3+ is the most beneficial to the formation of EPS;In the Mn2+experimental group,the trend of low promotion and high inhibition was observed.Low concentration(1～2mg/L)Mn2+ significantly stimulated the secretion of adhesion-type EPS(P<0.05),which provided a buffer layer for microbes,which not only contributed to the formation of biofilm but also enabled the system to adapt to changes in the environment in a short time.However,the concentration of 8 mg/L Mn2+ produced significant toxic effects on microorganisms,and the total amount of EPS secretion was lower than that of the blank group by 36.89%.(4)The paper compared the differences of biofilm microbial communities in the blank control group and the 8 mg/L Fe3+ concentration group,the 1 mg/L Mn2+concentration group and the 8 mg/L Mn2+ concentration group.Analyze the ecological mechanism.It can be seen that there is a significant difference between the groups.The iron ions and manganese ions have a certain influence on the microbial community structure in MBBR.The number of OTUs in the 8 mg/LFe3+concentration group was 20.10% higher than that in the blank control group,and the Simpson index,Shannon index,Chao 1 and ACE index were higher than the blank control group,so the addition of Fe3+ made the biofilm have microbial community diversity.On the other hand,Proteobacteria and Planctomycetes get significant enrichiment,Make its system more resistant to external interference and adaptability to the environment,and obtained high level of nitrogen metabolism.The microbial diversity index of the 1 mg/L Mn2+ concentration group was higher than that of the blank control group,and the abundance of Pseudoalteromonas,Nitrosomonas,and Genus of Nitrospiraceaewhile is higher,and are even more suitable in this environment.The three genera play an important role in biofilm formation,nitrosation and nitrification.The 8 mg/L Mn2+ concentration group was the worst,and increases the abundance of pathogenic bacteria such as Vibrio and Bacteroides,and has an adverse effect on the formation of microbial membranes.Fe3+ and low concentration of Mn2+ increased the diversity of microbial communities and enhanced the efficient nitrogen removal performance of MBBR.The comprehensive experimental results show that at ρ(Fe3+)<8 mg/L,the biofilter was not inhibited but showed higher stability and efficiency.Fe3+ in seawatercan be used to improve the stability and efficiency of the biofilter,and reduce the cost of iron ion treatment.The effect of Mn2+ on biofilter showed a tendency of low promoting and high restraining.At ρ(Mn2+)=1 mg/L,the water treatment effect of the biological filter is the best.If the concentration exceeds this,toxic effects will be generated on microorganisms. |
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