Abstract
The reject water of anaerobic digestors still contains high levels of methane and ammonium that need to be treated before these effluents can be discharged to surface waters. Simultaneous anaerobic methane and ammonium oxidation performed by nitrate/nitrite-dependent anaerobic methane-oxidizing(N-damo) microorganisms and anaerobic ammonium-oxidizing(anammox) bacteria is considered a potential solution to this challenge. Here, a stable coculture of N-damo archaea, N-damo bacteria, and anammox bacteria was obtained in a sequencing batch reactor fed with methane, ammonium, and nitrite. Nitrite and ammonium removal rates of up to 455 mg N-NO2− L−1 day−1 and 228 mg N-NH4+ L−1 were reached. All nitrate produced by anammox bacteria (57 mg N-NO3− L−1 day−1) was consumed, leading to a nitrogen removal efficiency of 97.5%. In the nitrite and ammonium limited state, N-damo and anammox bacteria each constituted about 30–40% of the culture and were separated as granules and flocs in later stages of the reactor operation. The N-damo archaea increased up to 20% and mainly resided in the granular biomass with their N-damo bacterial counterparts. About 70% of the nitrite in the reactor was removed via the anammox process, and batch assays confirmed that anammox activity in the reactor was close to its maximal potential activity. In contrast, activity of N-damo bacteria was much higher in batch, indicating that these bacteria were performing suboptimally in the sequencing batch reactor, and would probably be outcompeted by anammox bacteria if ammonium was supplied in excess. Together these results indicate that the combination of N-damo and anammox can be implemented for the removal of methane at the expense of nitrite and nitrate in future wastewater treatment systems.
Highlights
Aerobic wastewater treatment systems are energy-intensive and costly, as aeration for the removal of organic carbon and oxidation of ammonium requires large amounts of oxygen (Jetten et al 1997)
Nitrate/nitritedependent anaerobic methane oxidation (N-damo) by a consortium of bacteria belonging to the NC10 phylum and methanotrophic archaea related to the ANME-2d clade was first described in 2006 (Raghoebarsing et al 2006)
A 3-L laboratory-scale sequencing batch reactor (SBR) was started with a culture that contained both nitrate- and nitrite-dependent anaerobic methane-oxidizing(N-damo) microorganisms (Ettwig et al 2008)
Summary
Aerobic wastewater treatment systems are energy-intensive and costly, as aeration for the removal of organic carbon and oxidation of ammonium requires large amounts of oxygen (Jetten et al 1997). The N-damo bacteria, named “Candidatus Methylomirabilis oxyfera,” were shown to use nitrite as an electron acceptor for methane oxidation, while the N-damo archaea prefer nitrate (Ettwig et al 2008; Haroon et al 2013; Hu et al 2011). Studies have shown that anammox and Ca.Methylomirabilis-like bacteria and Ca.Methanoperedenslike archaea can be cultured together in laboratory-scale membrane reactors, together removing methane, ammonium, nitrite, and nitrate from the influent (Ding et al 2017; Lu et al 2017; Shi et al 2013; Xie et al 2017a, b). Other biomass retention systems might be more practical in full-scale use
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