• Energy Research

Quantifying oxygen demand and nitrifier yield are important in the design and operation of advanced wastewater treatment systems. However, the accurate stoichiometry of the autotrophic nitrification process has not been fully developed. In this research, stoichiometric links between nitrifier yield, ammonia and nitrite oxidization, ammonia assimilation, and oxygen uptake for each step of the nitrification process were determined. A pulse-flow respirometer was used to measure the oxygen uptake for complete nitrification and nitrite oxidation reactions. Results indicated that the specific oxygen uptake was 4.23 mg-O(2)/mg-N oxidized for complete nitrification, with 3.17 mg-O(2)/mg-N oxidized for ammonia oxidation (first step nitrification) and 1.06 mg-O(2)/mg-N oxidized for nitrite oxidation (second step nitrification). For the complete nitrification, fractions of ammonia used for electron donation, synthesis of ammonia oxidizers, and synthesis of nitrite oxidizers were 97.1%, 2.2%, and 0.7%, respectively. The fractions of electrons transferred into cell synthesis were approximately 7.5% for ammonia oxidation and 7.3% for nitrite oxidation. Biomass yield coefficients for ammonia oxidizers and nitrite oxidizers were 0.18 and 0.06 g-VSS/g-N oxidized, respectively. These parameters are critical when calculating oxygen needs and nitrifier biomass concentrations during the design of advanced wastewater treatment processes.

The effect of dissolved oxygen (DO) on the endogenous decay of active heterotrophic biomass and the hydrolysis of cell debris were studied. With the inclusion of a hydrolysis process for the cell debris, mathematical models that are capable of quantifying the effects of DO and sludge retention time (SRT) on concentrations of active biomass and cell debris in activated sludge are presented. By modeling the biomass cultivated with unlimited DO, the values of endogenous decay coefficient for heterotrophic biomass, the hydrolysis constant of cell debris, and the fraction of decayed biomass that became cell debris were determined to be 0.38 d(-1), 0.013 d(-1), and 0.28, respectively. Results from modeling the biomass cultivated under different DO conditions suggested that the experimental low DO (∼0.2 mg/L) did not inhibit the endogenous decay of heterotrophic biomass, but significantly inhibited the hydrolysis of cell debris with a half-velocity constant value of 2.1 mg/L. Therefore, the increase in sludge production with low DO was mainly contributed by cell debris rather than the active heterotrophic biomass. Maximizing sludge production during aerobic wastewater treatment could reduce aeration energy consumption and improve biogas energy recovery potential.

Our previous study indicated that a low dissolved oxygen (DO) could enrich and shift nitrifier community, making complete nitrification feasible under long-term low DO conditions. This research determined nitrifier kinetic constants, and quantified the chronic effect of low DO on the overall nitrification process. For ammonia oxidizing bacteria (AOB), the half-velocity constants of DO on the growth (KDO-g) and decay (KDO-d) were 0.29 and 0.48mgL(-1), respectively. For nitrite oxidizing bacteria (NOB), those values were 0.08 and 0.69mgL(-1), respectively. The low KDO-g values for both AOB and NOB suggest that a DO of greater than 1mgL(-1) does not provide further benefit to nitrification, and the lower KDO-g value for NOB suggests that nitrite oxidation is less impacted by a low DO. The KDO-d values of 0.48 and 0.69mgL(-1) for AOB and NOB, respectively, suggest that a low DO of less than 1mgL(-1) significantly inhibits the decay of both AOB and NOB, resulting in their enrichment. The relationship between the operational DO and required SRT for complete nitrification was developed to provide a theoretical foundation for operating an advanced wastewater treatment plant under low DO, to significantly improve aeration energy efficiency.

Nitrous oxide (N2O) is an important greenhouse gas and a dominant ozone-depleting substance. Nitrification in the activated sludge process (ASP) is an important N2O emission source. This study demonstrated that a short-term low dissolved oxygen (DO) increased the N2O emissions by six times, while long-term low DO operation decreased the N2O emissions by 54% (P < 0.01). Under long-term low DO, the ammonia oxidizer abundance in the ASP increased significantly, and thus, complete nitrification was recovered and no NH3 or nitrite accumulated. Moreover, long-term low DO decreased the abundance of ammonia-oxidizing bacteria (AOB) by 28%, while increased the abundance of ammonia-oxidizing archaea (AOA) by 507%, mainly due to their higher oxygen affinity. As a result, AOA outnumbered AOB with the AOA/AOB amoA gene ratio increasing to 19.5 under long-term low DO. The efficient nitrification and decreased AOB abundance might not increase N2O production via AOB under long-term low DO conditions. The enriched AOA could decrease the N2O emissions because they were reported to lack canonical nitric oxide (NO) reductase genes that convert NO to N2O. Probably because of AOA enrichment, the positive and significant (P = 0.02) correlation of N2O emission and nitrite concentration became insignificant (P = 0.332) after 80 days of low DO operation. Therefore, ASPs can be operated with low DO and extended sludge age to synchronously reduce N2O production and carbon dioxide emissions owing to lower aeration energy without compromising the nitrification efficiency.

Biofilm based systems and the hybrid between activated sludge and biofilms have been popularly applied for wastewater treatment. Unlike the suspended biomass, the biofilm concentration and kinetics on the media cannot be easily measured. In this study, a novel and easy-to-use approach has been developed based on pulse-flow respirometer to characterize the biofilm stoichiometry and kinetics in situ. With the new designed breathing reactor, the mutual interference between the magnetic stirring and biofilm media that happened in the conventional breathing reactor was solved. Moreover, Microsoft Excel based programs had been developed to fit the oxygen uptake rate curves with dynamic nonlinear regression. With this new approach, the yield coefficient, maximum oxidation capacity, and half-saturation constant of substrate for the heterotrophic biofilms in a fix bed reactor were determined to be 0.46 g-VSS/g-COD, 67.0 mg-COD/(h·L-media), and 4.4 mg-COD/L, respectively. Those parameters for biofilm ammonia oxidizers from a moving bed biofilm reactor were determined to be 0.17 g-VSS/g-N, 18.6 mg-N/(h·L-media), and 1.2 mg-N/L, respectively, and they were 0.11 g-VSS/g-N, 20.9 mg-N/(h·L-media), and 0.98 mg-N/L for nitrite oxidizers in the same biofilms. This study also found that the maximum specific substrate utilization rate for detached biofilms increased by 3.2 times, indicating that maintaining biofilm integrity was very important in the kinetic tests. Using this approach, the biofilm kinetics on the media can be regularly measured for treatment optimization.