• Energy Research

The carbon source used for denitrification is a key factor in the reduction of nitrous oxide (N2O) produced from wastewater treatment plants because it affects denitrification activity. In this study, two laboratory-scale Modified Ludzak Ettinger (MLE) processes were operated with methanol and sodium acetate as the sole carbon sources. Removal efficiency of soluble nitrogen was not affected by carbon source, but the N2O emission rate from the acetate-fed MLE process (1.6 ± 0.6 μg N–N2O/min) was lower than that from the methanol-fed process (3.0 ± 0.7 μg N–N2O/min). This is supported by the batch experiment data showing the acetate-fed biomass had a higher N2O reduction rate of 10.3 mg/gVSS/h than that of the methanol-fed biomass (3.3 mg/gVSS/h). In the methanol-fed process, 34.9 % of the total bacteria was the genus Methylotenera, which is reportedly incapable of N2O reduction. The acetate-fed process enriched the genera Dechloromonas and Rubrivivax, potential N2O reducers, accounting for 12.2 and 15.9 % of the total bacteria, respectively. The results indicated that acetate is a suitable replacement for methanol for wastewater treatment plants interested in mitigating N2O emission from the MLE process.

The goal of this study was to develop a startup strategy for a high-rate anaerobic ammonium oxidation (anammox) reactor to treat waste brine with high concentrations of ammonium from a natural gas plant. An upflow anaerobic sludge blanket (UASB) anammox reactor with an effective volume of 294 L was fed continuously with waste brine with a salinity of 3% and a NH4+ concentration of 180 mg-N/L, as well as a NaNO2 solution. By inoculating a methanogenic granular biomass as a biomass carrier, the reactor attained the maximum volumetric nitrogen removal rate (NRR) of 10.7 kg-N/m3/day on day 209, which was 1.7 times higher than the highest reported NRR for wastewater of comparable salinity. High-throughput sequencing of 16S rRNA gene amplicons revealed that Candidatus Scalindua wagneri was enriched successfully in granules in the UASB, and it replaced Methanosaeta and became dominant in the granule. The inhibitory effect of NO2- on the anammox reaction in the granules was assessed by a 15N tracer method, and the results showed that anammox activity was maintained at 60% after exposure to 300 mg-N/L of NO2- for 24 h. Compared with previous studies of the susceptibilities of Candidatus Brocadia and Candidatus Kuenenia to NO2-, the enriched marine anammox bacteria were proven to have comparable or even higher tolerances for high NO2- concentrations after a long exposure.

One-stage autotrophic nitrogen (N) removal, requiring the simultaneous activity of aerobic and anaerobic ammonium oxidizing bacteria (AOB and AnAOB), can be obtained in spatially redox-stratified biofilms. However, previous experience with Membrane-Aerated Biofilm Reactors (MABRs) has revealed a difficulty in reducing the abundance and activity of nitrite oxidizing bacteria (NOB), which drastically lowers process efficiency. Here we show how sequential aeration is an effective strategy to attain autotrophic N removal in MABRs: Two separate MABRs, which displayed limited or no N removal under continuous aeration, could remove more than 5.5 g N/m(2)/day (at loads up to 8 g N/m(2)/day) by controlled variation of sequential aeration regimes. Daily averaged ratios of the surficial loads of O(2) (oxygen) to NH(4)(+) (ammonium) (L(O(2))/L(NH(4))) were close to 1.73 at this optimum. Real-time quantitative PCR based on 16S rRNA gene confirmed that sequential aeration, even at elevated average O(2) loads, stimulated the abundance of AnAOB and AOB and prevented the increase in NOB. Nitrous oxide (N(2)O) emissions were 100-fold lower compared to other anaerobic ammonium oxidation (Anammox)-nitritation systems. Hence, by applying periodic aeration to MABRs, one-stage autotrophic N removal biofilm reactors can be easily obtained, displaying very competitive removal rates, and negligible N(2)O emissions.

Anaerobic ammonium oxidation (anammox) has been implemented as a cost-effective nitrogen removal process in wastewater treatment. To apply the process to saline wastewater treatment at temperatures below 30 °C, the effectiveness of marine anammox bacteria has been demonstrated in a pilot-scale reactor. Nevertheless, mesophilic conditions, often found in underground brine containing high NH4+ concentrations, have yet to employ an anammox process. The objective of this study was to enrich anammox bacteria capable of removing nitrogen from underground brine possessing a salinity of 3% and at a temperature over 30 °C. To select a promising inoculum, biomass from a brine settling tank in a natural gas plant was subjected to quantifying transcripts of anammox 16S rRNA and hydrazine oxidoreductase (hzo) genes by quantitative reverse transcription polymerase chain reaction. A fixed-bed column anammox reactor was fed with the selected inoculum and operated feeding underground brine mixed with NaNO2 solution at a temperature of 38 °C. As a result, a nitrogen removal rate (NRR) of 1.42 kg-N/m3/day was obtained on day 167. An average NRR of 1.21 kg-N/m3/day and nitrogen removal efficiency of 88% were maintained for 50 days. Amplicon sequencing based on the 16S rRNA revealed that anammox bacteria which are phylogenetically close to Candidatus Kuenenia were successfully enriched in the reactor. These results indicate that nonmarine anammox bacteria can be active and predominant under both high-salinity and mesophilic conditions, making it a likely candidate for effective nitrogen removal from underground and waste brine.

This study aimed to determine key factors for enriching methylotrophic populations for single-cell protein (SCP) production and to determine the predominant active methylotrophs. Microbial populations from rice paddy soils and roots, hotspots for CH4 oxidation, underwent serial incubation by feeding CH4 and O2. The protein content was primarily influenced by the incubation stage, with a lesser effect from the rice plant roots as an inoculum source, reaching a maximum of 20 % in biomass dominated by Methylomonadaceae and Methylophilaceae. While Methylomonadaceae as CH4 oxidizers exhibited lower abundance but high transcription activity, Methylophilaceae as methanol oxidizers were more abundant and strongly correlated with protein content (Pearson's coefficient >0.8, p < 0.05). Enhancing Methylophilaceae in the mixed cultures may improve SCP yields via interactions with Methylomondaceae, likely mediated by methanol. The cooperative relation plausibly offers a promising strategy for sustainable CH4-based SCP production.