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For in-situ observations or several steady-state reactor runs, the current approaches for calculating biomass yield require sophisticated sensors. The yield of particular groups of organisms in mixed cultures is still difficult to estimate quickly and simply. Based on the incorporation of 13C during activity measurements, the maximum biomass yield (Ymax) can be quickly estimated using the method described in this study. It was used on mixed cultures made up of thermophilic (50°C) and mesophilic (15°C –28°C) strains of nitrite-oxidizing bacteria (NOB) and ammonia-oxidizing bacteria (AOB). This approach makes it impossible to distinguish between AOB and AOA coexisting in a society. This approach estimates the carbon fixation activity of the autotrophic bacteria rather than the actual nitrifier biomass yield because 13 C redirection via SMP to heterotrophs may result in a modest overestimation of the nitrifier biomass. The difference in yields between thermophilic and mesophilic AOA (0.22 vs. 0.06-0.11 g VSS g-1 N) may be due to a more effective route for CO2 incorporation. Less biomass was produced by NOB thermophilic organisms, which could be explained by a greater maintenance demand that limits the amount of energy available for biomass synthesis. As a result of AOA dominating over AOB at higher temperatures, it is interesting that thermophilic nitrification yield was larger than its mesophilic cousin. Mesophilic AOB yield was affected by a sudden rise in temperature, supporting the effects of maintenance requirements on output. While sludge composition was affected by changing nitrifier yield, model simulations of two realistic nitrification/denitrification plants were robust in predicting effluent ammonium concentrations. In summary, a quick, accurate, and simple method for calculating the Ymax of ammonia and nitrite oxidizers in mixed communities was developed.
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