anMOgran · Anaerobic methane oxidation in granular sludge reactors for sustainable wastewater treatment

Anaerobic methane oxidation in granular sludge reactors for sustainable wastewater treatment

Research context Methane is the end product of anaerobic digestion and it can be used for energy generation. However, some of the methane remains dissolved in the effluent of anaerobic digesters and potentially escapes to the atmosphere during further downstream processing. Given the high global warming potential of methane, about 34 CO2 equivalents over a 100-year time scale, even small quantities of methane emissions can largely affect the carbon footprint of a wastewater treatment plant (wwtp) as stated by the International Panel for Climate Change. Recently, a bacterial species capable of nitrite-dependent anaerobic methane oxidation (ndamo), ‘Candidatus Methylomirabilis oxyfera’ was discovered (Raghoebarsing et al. 2006, Ettwig et al. 2010). These microorganisms oxidize methane to carbon dioxide coupled to the reduction of nitrite to dinitrogen gas. The ndamo process has so far not been applied in engineered systems, but microorganisms related to M. oxyfera were detected in nitrogen removal processes such as those treating anaerobic digester effluents, which contain high ammonium concentration and dissolved methane (Luesken et al. 2011a, Ho et al. 2013). Ammonium can be conveniently removed from such reject waters with a combined partial nitritation - anammox process, as demonstrated at full-scale (van der Star et al. 2007). The partial nitritation-anammox pathway forms the basis of innovative technologies for nitrogen removal from wastewater which imply significant benefitscompared to conventional nitrification/denitrification over nitrate, such as up to 60% reduction of aeration energy requirement, up to 100% savingsin organic carbon addition, lower sludge production and little CO2 emissions . During partial nitritation, typically half of the ammonium is converted to nitrite, while nitrate formation is prevented. This is followed by anaerobic oxidation of ammonium (anammox) to dinitrogen gas with nitrite as the electron acceptor. As reject water contains high levels of ammonium in proportion to methane, it is potentially interesting to combine anaerobic ammonium and methane oxidizing bacteria to remove nitrogen and methane simultaneously. Since Anammox and ndamo bacteria both have a doubling time of more than ten days, proper biomass retention is needed to handle large volumetric flows and loading capacities such as encountered in a wwtp (Strous et al. 1998, Ettwig et al. 2009). Granular sludge consists of biofilm aggregates in the form of dense, fast-settling granules, resulting in compact systems, which allows a high loading rate due to a large biofilm surface area in the reactor. Therefore, granules offer a good option for simultaneous growth of both ndamo and anammox bacteria. Research objectives The objective of this project was to investigate the feasibility of a combination of anaerobic ammonium and methane removal. Two possible configurations can be envisaged for this purpose; a one-stage and a two-stage process. In the two-stage process partial nitrification is performed in a separate tank to produce the nitrite needed for anaerobic ammonium and methane removal, which takes place in a second non-aerated reactor. Both processes are thus separated in space. However, in this process the methane might be stripped in the aerated partial nitritation reactor and therefore it is also of interest to investigate the possibility of simultaneous methane and ammonium removal in a one-stage system, operated with minimal aeration to avoid stripping effects. The objective of this study was therefore to evaluate the combination of Anammox and ndamo bacteria in a two-stage as well as one-stage reactor operation to test experimentally and by mathematical modelling if their co-existnce in a single granule is feasible to achieve stable and simultaneous methane and ammonium removal. Description of the work performed, main results and conclusions Simulation studies revealed that the coexistence of nitrite dependent anaerobic methane oxidizing bacteria (ndamo) and anaerobic ammonium oxidizing bacteria (anammox) in a single granule is possible. The nitrite and methane affinity constants of ndamo bacteria were measured experimentally. The biomass yield of ndamo bacteria was derived from experimental data and a thermodynamic state analysis. Through simulations, it was found that the possible survival of ndamo besides anammox bacteria was sensitive to the nitrite/ammonium influent ratio. If ammonium was supplied in excess, ndamo bacteria were outcompeted. At low biomass concentration, ndamo bacteria lost the competition against anammox bacteria. When the biomass loading closely matched the biomass concentration needed for full nutrient removal, strong substrate competition occurred resulting in oscillating removal rates. The simulation results further reveal that smaller granules enabled higher simultaneous ammonium and methane removal efficiencies. It was shown that simultenous methane and ammonium removal was possible, in two-stage as well as one-stage systems. However, in the one-stage system ndamo only appeared in a very narrow range showing that adequate control of the aeration intensity is crucial to obtain simultaneous methane and ammonium removal via mainly Anammox and ndamo bacteria. Our results show that a practical implementation of a one and two stage system is feasible. The implementation of simultaneous anaerobic methane and ammonium removal will decrease greenhouse gas emissions resulting from wastewater treatment. Potential impact and use and socio economic impact It is expected that, besides effluent quality, secondary objectives dealing with the sustainability of wastewater treatment will gain further importance in future, aiming for compact processes, reduced energy consumption, minimal addition of chemicals and reduced emissions of greenhouse gases. All these mentioned points are addressed in this project due to the compact structure of granular sludge reactor, which reduce energy, footprint and the costs related to wastewater treatment. In addition, this process can reduce methane emissions by using the newly discovered ndamo bacteria. Since methane is a greenhouse gas policy makers might set in future penalties for wwtp, which exceed a certain limit in methane emissions. Therefore, is the implementation of ndamo bacteria of interest to all public authorities, who might be obliged to meet low methane emissions. In addition, is the interactions of anammox and ndamo bacteria in man-made ecosystems similar to how these microorganisms interact in nature. Consequently, the results of the current proposal potentially have wider implications for our understanding of the nitrogen cycle in the environment and are hence of interest to broad field of researchers such as microbiologists, civil and environmental engineers. Provides the address of the project Website (if applicable) as well as relevant contact details. http://www.ugent.be/bw/biosysteemtechniek/en/research/biosystems-control/ http://www.ugent.be/bw/biosysteemtechniek/en/research/biosystems-control/research-winkler.htm contact: prof. dr. Eveline Volcke, Biosystems Control research unit, Ghent University, Eveline.Volcke@ugent.be References -Ettwig, et al. 2010. Nitrite-driven anaerobic methane oxidation by oxygenic bacteria. Nature 464:543-548. -Raghoebarsing, et al. 2006. A microbial consortium couples anaerobic methane oxidation to denitrification. Nature 440:918-921. -Strous, Met al. 1999. Key physiologies of anaerobic ammonium oxidation. Appl microbiol biotechnol 50:589-596. -van der Star, et al. 2007. Startup of reactors for anoxic ammonium oxidation: Experiences from the first full-scale anammox reactor in Rotterdam. Water Res 41:4149-4163

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