Performance Of The UASB Reactor Under Microbial Actions With Different Dosage Of Exogenous Ferrous Iron

From 1960's,water eutrophication was focused by people all over the world,controlling and treating nitrogen and phosphorus pollution is the essential way to cure it.The existing wastewater biological nitrogen and phosphorus removal technology has the problems of lower nitrogen and phosphorus removal efficiency,poorer operational stability and more difficult nitrification in low temperature.Excessive nitrogen contained in municipal and industrial wastewater discharge is becoming a substantial environmental concern due to its potential for causing eutrophication in receiving water bodies.With the raising of environmental concerns,it is likely that there will be more stringent regulations on their disposal in municipal wastewater,which contains a mass of nitrogen in most countries.Up-flow anaerobic sludge blanket?UASB?reactors have emerged as one of the most commonly used methods for the treatment of municipal wastewater.Low capital investment and maintenance cost as well as less land and sludge generation have popularized the UASB process throughout the world.UASB reactor effluent must meet the discharge standards,especially in terms of pathogens and nutrients.However,nitrogen removal requires a lot of energy to create aerobic conditions for bacterial nitrification,and it also uses organic carbon to help remove nitrate by bacterial denitrification.Therefore,some fast and efficient ways for UASB reactors to remove nitrogen is necessary.Anaerobic digestion includes four key distinct stages:Hydrolysis,Acidogenesis,Acetogenesis and Methanogenesis,which are interdependent and proceed continuously.Complex organic wastes are decomposed through a series of processes based on interactions with microorganisms,and different microorganisms depend on and restrict each other,creating a favorable environment and symbiotic relationship.Thus,minimizing changes in microorganism dynamics is important to maintain stable performance under variable operational parameters,such as pH.The buffer capacity of the system can maintain the pH of the medium in the reactor in the optimal range?6.3-7.8?;however,the reactor stability would be affected significantly with a drastic pH change,and the chemical oxygen demand?COD?removal efficiency,one of the important indicators to evaluate the robustness and stability of UASB,decreases greatly.Previous studies showed that NaOH could be added to raise the pH value,but it is more expensive,and the final effluent will contain high sodium levels,which prevents the use of the effluent for irrigation purposes.In the current study,we investigated the functions of Fe²⁺on the stability and COD removal efficiency and explored the link among microorganisms,nitrogen and Fe²⁺in UASB reactors in different situations.The results are as follows:?1?The optimal concentration of Fe²⁺in the UASB reactorsCOD removal efficiency was detected in the UASB reactor with different concentration of Fe²⁺?0-7mM?.COD removal efficiency is about 79%in the UAAB without adding Fe²⁺.While COD removal efficiency increased after adding Fe²⁺and increased following the raising of the concentration of Fe²⁺,however,the COD removal efficiency decreased when the concentration of Fe²⁺is more than 5mM.Therefore,the best of concentration of Fe²⁺for the UASB reactor is 5mM.?2?Changes in pH,COD,NO₃^-,NO₂^-,NH₄⁺,Fe³⁺and Fe²⁺concentrations in different UASB reactorsTwo UASB reactors were performed in this study.After 19 days of running,5 mM of Fe?II?was added to one of the reactors?FER?,while the other reactor?CON?was run without any addition of Fe?II?.Between the 1st and 19th day,the COD effluent and pH remained relatively constant,and the COD removal efficiency remained between 80%and88%).The COD removal efficiency and pH were decreased after the 21st day in CON,and the COD removal efficiency dropped to 40%and 55%from day 49,and the pH was decreased to below 6 from day 43.However,the COD removal efficiency was gradually increased from 80%to 90%after the addition of Fe?II?for 20 days,and the pH of effluent of FER was kept constant.The COD removal efficiency and pH were decreased when the addition of Fe?II?was stopped and were kept low and constant from the 93rd day of operation.Importantly,the COD removal efficiency and pH in both FER and CON were gradually increased and were similar to the previous days between the 1st and 19th day.NO₃^-,NO₂^-,NH₄⁺,Fe³⁺and Fe²⁺concentrations were analyzed to learn more about the chemical reactions in FER.In the wastewater used in this study,NH₄⁺was the prominent nitrogen species?14±0.5 mM?.Within 19 days of the beginning of the operation,most of the ammonium was converted to nitrates and nitrites after the chemical reactions in the UASB.Both the NO₃^-and NO₂^-concentrations were reduced in the effluent with the addition of Fe?II?,suggesting that the presence of Fe?II?facilitated nitrogen removal,especially for nitrates and nitrites.The NO₃^-and NO₂^-concentrations were increased when the addition of Fe?II?was stopped.When the pH of the effluent was decreased to below 6 from day 89,both the NO₃^-and NO₂^-concentrations were increased to more than 5.1 mM and 2.4 mM,respectively,which were higher than were those concentrations within 19 days of the beginning of the operation.After the addition of Fe?II?in FER from day 105,the NO₃^-and NO₂^-concentrations in the effluent were gradually decreased to 3.4 mM and 1.4 mM,respectively.?3?Effects of heterologous iron on the composition and diversity of microbial communitiesThe different points in time for the collection of samples used for high throughput sequencing?Con,AdF,Aci and Rev?.At the genus level,the dominant genera in CON were Clostridium,Hydrogenophaga,Flavobacterium,Prevotella,Achromobacter,Paludibacter,Chryseobacterium,Treponema,Longilinea,Bacteroides,Cloacibacterium,Sulfuricurvum and Methylotenera.The relative abundances of Hydrogenophaga,Methylotenera,Zoogloea and Fluviicola evidently increased after the addition of Fe?II?,while for Clostridium,Flavobacterium,Prevotella,Achromobacter and Paludibacter,the relative abundances decreased dramatically.When the pH was below 6 without the addition of Fe?II?,the relative abundances of Paludibacter,Zoogloea and Methylomonas increased notably in Aci.When the pH of the reactor returned to the range from 7 to 8,the relative abundances of Hydrogenophaga,Methylotenera,Zoogloea and Fluviicola become the dominant bacteria in Rev,which is similar to those in AdF.The correlation coefficients between different chemicals and different relative abundances of genera were analyzed.The present results reconfirmed that some factors is significantly correlated with many bacterial communities.Fe?II?was confirmed to be significantly and positively correlated with the abundances of Hydrogenophaga,Methylotenera and Fluviicola.Importantly,the COD removal efficiency was confirmed to be significantly and positively correlated with the abundances of Hydrogenophaga,Zoogloea,Fluviicola and Syntrophus.?4?Identification and Screening of bacteria in the UASB reactorFour strains with strong denitrification ability and could grow in the artificial culture media,were identified from UASB reactor by bacterial isolation and purification method.The COD removal efficiency increased significantly by adding every bacteria in the UASB reactor.?5?Effect of exogenous iron on enzyme activity in UASB reactorActivity of different enzymes in UASB reactor were detected after the addition of exogenous iron.We found that protease,urease,phosphatase and amylase activity were increased 23%,36%,22%and 36%,respectively.