Complete Nutrient Removal Coupled to Nitrous Oxide Production as a Bioenergy Source by Denitrifying Polyphosphate-Accumulating Organisms

Abstract

Coupled aerobic-anoxic nitrous decomposition operation (CANDO) is a promising emerging bioprocess for wastewater treatment that enables direct energy recovery from nitrogen (N) in three steps: (1) ammonium oxidation to nitrite; (2) denitrification of nitrite to nitrous oxide (NO); and (3) NO conversion to N with energy generation. However, CANDO does not currently target phosphorus (P) removal. Here, we demonstrate that denitrifying polyphosphate-accumulating organism (PAO) enrichment cultures are capable of catalyzing simultaneous biological N and P removal coupled to NO generation in a second generation CANDO process, CANDO+P. Over 7 months (>300 cycles) of operation of a prototype lab-scale CANDO+P sequencing batch reactor treating synthetic municipal wastewater, we observed stable and near-complete N removal accompanied by sustained high-rate, high-yield NO production with partial P removal. A substantial increase in abundance of the PAO Candidatus Accumulibacter phosphatis was observed, increasing from 5% of the total bacterial community in the inoculum to over 50% after 4 months. PAO enrichment was accompanied by a strong shift in the dominant Accumulibacter population from clade IIC to clade IA, based on qPCR monitoring of polyphosphate kinase 1 (ppk1) gene variants. Our work demonstrates the feasibility of combining high-rate, high-yield NO production for bioenergy production with combined N and P removal from wastewater, and it further suggests a putative denitrifying PAO niche for Accumulibacter clade IA.

Achieving energy neutrality in wastewater treatment plants through energy savings and enhancing renewable energy production

“…Ammonia and nitrous oxide are two N species found in wastewater that meet these criteria (WERF 2011). The CANDO process consists of three steps: first, ammonium oxidation to nitrite; second is reduction of nitrite to N 2 O; and eventually, N 2 O conversion to N 2 with energy generation (Scherson et al 2014;Gao et al 2017). The CANDO process converts reactive N to N 2 O, then captures the gas and recovers energy from it by using it as a co-oxidant in CH 4 combustion or decomposing N 2 O over a metal oxide catalyst.…”

Nitrous oxide emissions from biofilm processes for wastewater treatment

“…Behavior regarding N2O emissions may also vary based on the type of electron donor. For example, elemental-sulfur (S o ) oxidizing denitrifiers (Di Capua et al 2015;Liu et al 2017b), methane (CH4) oxidizing denitrifiers (He et al 2018), phosphate-accumulating (PAO) denitrifiers (Gao et al 2017;Wang et al 2011;Wang et al 2014;Zhou et al 2012), H2 oxidizing denitrifiers (Li et al 2017), and bacteria growing with an electrode as an electron donor (Jiang et al 2018) display different behavior with respect to N2O emissions. Methane-oxidizing denitrifiers appear to reduce NO2to N2 without forming N2O as an intermediate, and therefore are thought to minimize N2O emissions (He et al 2018).…”

Approaches to energy and resource recovery from municipal wastewater

“…It has been reported that about 60-80% of nitrite could be converted to nitrous oxide in the CANDO process and the specific nitrous oxide generation rate would be as high as 25.2 mg N/(g VSS • h) (Myung et al, 2015;Scherson et al, 2014). Moreover, the CANDO process is also capable of simultaneously removing nitrogen and phosphorus through alternating anaerobic/anoxic cycles (Gao et al, 2017).…”