Exploring versatile applications of nitrite/nitrate-dependent anaerobic methane oxidation through modelling and experimental investigations

Abstract

Due to serious eutrophication in water bodies, nitrogen removal has become a critical stage ofnwastewater treatment plantsn(WWTPs) over the past decades. However, the conventional biological nitrogen removal process is suffering several major drawbacks, including substantial aeration consumption, high fugitive greenhouse gas emissions, requirement for external carbon sources, excessive sludge production and low energy recovering efficiency, thus unable to satisfy the escalating industrial needs. Recently, the discovery ofnanaerobic ammonium oxidation (anammox)nbacterianhasnpromoted the development of autotrophic nitrogen removal, which could partially solve the problems associated with conventional nitrogen processes. However, the applicationsnof anammox to treat wastewaternhave been hindered mainly by the unsatisfactory effluent quality withnnitrogen removal efficiencynbelow 80%.nThe discovery ofnnitrate/nitrite-dependent anaerobic methane oxidation (n-DAMO)nin the last decade provided newnopportunitiesnto removenthis barrier, by utilizing methane as an alternative carbon source. In addition to capacity of removing nitrogen, n-DAMO process also possesses significant potential for dissolved methane removal, which is a regular but unsolved problem for anaerobic process. Hence, this thesis aims to explore versatile applications (nitrogen and dissolved methane removal) of n-DAMO process from mathematical modelling to experimental investigations.nTo this end, the kinetic features of n-DAMO archaea were firstly investigated in an enriched suspended culture.nThe nitrate reduction to nitrite bynn-DAMO archaeanwas found to comply with Monod kinetics with a nitrate affinity constant of 2.1p0.4 mg N L-1, while displaying first order kinetics for methane oxidation in the tested range of methane concentration (up to 16 mg CH4nL-1), with a specific rate constant of 0.04~0.14 L h-1ng-1VSS.nFurthermore, these obtained kinetic characteristics were extended from suspended systems to biofilm systems, in order to investigate the feasibility of real application and to facilitate the process optimization. A 1-dimension biofilm model embedding the n-DAMO and anammox reactions was developed, which was calibrated and validated using comprehensive data sets from two independentnmembrane biofilm reactors (MBfRs), treating sidestream- and mainstream-strength wastewater, respectively. The system robustness towards dynamic influent flowrates and nitrite/ammonium ratios was assessed under different biofilm thicknesses. In addition, thicker biofilms were also favorable to achieve less residual methane emission.Based on the modelling results, two MBfRs operated under strictly anaerobic conditions were operated to study the nitrogen removal in high-strength (sidestream) and low-strength (mainstream) wastewater, respectively. The MBfR1 achieved high-level nitrogen removal performance from synthetic anaerobic digestion liquor by using the in-situ produced biogas from anaerobic digestors. Nearly complete nitrogen removal was achieved (g99%) by feeding the reactor with synthetic biogas (60%CH4n+ 40%CO2), while control strategies (alkali dosing and nitrogen gas flushing) were developed to successfully maintain the system at neutral pH. MBfR2 treating synthetic domestic wastewaternalso achieved a stable nitrogen removal rate (0.13 kg m-3nd-1), together with a high-level effluent quality (l5.0 mg N L-1). Interestingly, the nitrogen removal performance was robust at temperature as low as 10oC, which was explained as a result of overcapacity by the mathematical modelling developed above. Additionally, 16S rRNA gene sequencing revealed that anammox bacteria, n-DAMO bacteria and n-DAMO archaea jointly dominated the biofilm. Meanwhile, a series of batch tests were also conducted for mass balance to further support the existence of these functional microorganisms.Based on the results from MBfR1 and 2 (two-stage under strictly anaerobic condition), MBfR3 was established to couple aerobic microorganisms (ammonia-oxidizing bacteria, AOB) with anaerobic microorganisms (anammox bacteria, n-DAMO bacteria and n-DAMO archaea) in one single biofilm to achieve one-stage nitrogen removal.nWith feeding of 1030 mg NH4+-N/L at a hydraulic retention time of 16 h, the proposed one-stage MBfR3 achieved an average total nitrogen removal efficiency of 98% and a nitrogen removal rate of 1.5 kg N/m3/d (1.4- 1.8 g N/m2/d) by using methane as the sole carbon-based electron donor. The N2O emission was determined to be 0.34%p0.01%. Microbial analysis revealed that AOB, anammox bacteria, n-DAMO bacteria and n-DAMO archaea co-developed in the biofilm. Batch tests further validated this functional microbial community.Moreover, the last part of this thesis extended the objective from sole nitrogen removal to simultaneous nitrogen and dissolved methane removal. Whilenanaerobic technologies have been proposed as promising solutions to enhance bioenergy recovery, 20-60%nof methane produced remains dissolved in thenanaerobically treated effluent, which is a potent greenhouse gas and is easily stripped out in the aeration tank. By coupling anammox with n-DAMO microorganisms, 85% of dissolved methane and more than 99% of nitrogen were removed together in a biofilm system. Both mass balance andn16S rRNA gene sequencingnindicated that n-DAMO bacteria and n-DAMO archaea contributed jointly to the methane removal.nCollectively, this thesis investigated versatile applications of n-DAMO process in various practical scenarios in view from mathematical modelling to experimental studies. The resultsncan potentially be applied to advance the nitrogen removal technologies, reduce the carbon footprint, and save the organic carbon consumption in WWTPs.n