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Nitrogen removal with energy recovery through N<sub>2</sub>O decomposition
Scherson, Yaniv D.,Wells, George F.,Woo, Sung-Geun,Lee, Jangho,Park, Joonhong,Cantwell, Brian J.,Criddle, Craig S. The Royal Society of Chemistry 2013 Energy & environmental science Vol.6 No.1
<P>A new process for the removal of nitrogen from wastewater is introduced. The process involves three steps: (1) partial nitrification of NH<SUB>4</SUB><SUP>+</SUP> to NO<SUB>2</SUB><SUP>−</SUP>; (2) partial anoxic reduction of NO<SUB>2</SUB><SUP>−</SUP> to N<SUB>2</SUB>O; and (3) N<SUB>2</SUB>O conversion to N<SUB>2</SUB> with energy recovery by either catalytic decomposition to N<SUB>2</SUB> and O<SUB>2</SUB> or use of N<SUB>2</SUB>O to oxidize biogas CH<SUB>4</SUB>. Steps 1 and 3 have been previously established at full-scale. Accordingly, bench-scale experiments focused on step 2. Two strategies were evaluated and found to be effective: in the first, Fe(<SMALL>II</SMALL>) was used to abiotically reduce NO<SUB>2</SUB><SUP>−</SUP> to N<SUB>2</SUB>O; in the second, COD stored as polyhydroxybutyrate (PHB) was used as the electron donor for partial heterotrophic reduction of NO<SUB>2</SUB><SUP>−</SUP> to N<SUB>2</SUB>O. For abiotic reduction with Fe(<SMALL>II</SMALL>), the efficiency of conversion of NO<SUB>2</SUB><SUP>−</SUP> to N<SUB>2</SUB>O was over 90% with 98% nitrogen removal from water. For partial heterotrophic denitrification, different selection conditions were imposed on acetate- and nitrite-fed communities initially derived from waste activated sludge. No N<SUB>2</SUB>O was detected when acetate and nitrite were supplied continuously, but N<SUB>2</SUB>O was produced when acetate and nitrite were added as pulses. N<SUB>2</SUB>O conversion efficiency was dependent upon the method of addition of acetate and nitrite. When acetate and nitrite were added together (coupled feeding), the N<SUB>2</SUB>O conversion efficiency was 9–12%, but when acetate and nitrite additions were decoupled, the N<SUB>2</SUB>O conversion efficiency was 60–65%. Decoupled substrate addition selected for a microbial community that accumulated polyhydroxybutyrate (PHB) during an anaerobic period after acetate addition then consumed PHB and reduced NO<SUB>2</SUB><SUP>−</SUP> during the subsequent anoxic period. The biological N removal efficiency from the water was 98% over more than 200 cycles. This indicates that decoupled operation can sustain significant long-term N<SUB>2</SUB>O production. Compared to conventional nitrogen removal, the three-step process, referred to here as Coupled Aerobic–anoxic Nitrous Decomposition Operation (CANDO), is expected to decrease oxygen requirements, decrease biomass production, increase organic matter available for recovery as biogas methane, and enable energy recovery from nitrogen, but pilot-scale studies are needed.</P> <P>Graphic Abstract</P><P>A new wastewater treatment process is introduced that removes and recovers energy from nitrogen waste through N<SUB>2</SUB>O decomposition. <IMG SRC='http://pubs.rsc.org/services/images/RSCpubs.ePlatform.Service.FreeContent.ImageService.svc/ImageService/image/GA?id=c2ee22487a'> </P>