The effect of maintenance energy requirements on biomass production during wastewater treatment

The recognition that microorganisms satisfy their maintenance energy requirements in preference to producing additional biomass has revealed possible methods to reducing the generation of excess biomass during wastewater treatment. A relationship describing substrate utilization for maintenance and biomass production in substrate-limited continuous microbial cultures is presented. In continuously-fed, substrate-limited, microbial populations, it is proposed that the biomass production decreases proportionally to biomass concentration. To evaluate the significance of biomass concentration on biomass production, a laboratory chemostat containing Pseudomonas putida grown aerobically was fitted with a second stage in which the biomass in the reactor effluent was concentrated and a portion was recycled to enhance the biomass concentration. Overall substrate removal efficiency remained constant at each of the various biomass concentrations investigated. Increasing biomass concentration from 3 to 6 g l−1 reduced biomass production by 12% and analysis of a similar system observed that increasing biomass concentration from 1.7 g l−1 to 10.3 g l−1 reduced biomass production by 44%. Maintenance energy coefficients and true growth yields for operation both as a chemostat and with biomass recycle were found to be similar; qm=0.027±0.004 g substrate g biomass−1 h−1 and YG=0.359±0.003 g biomass g substrate−1 and qm=0.021±0.008 g substrate g biomass−1 h−1 and YG=0.385±0.022 g biomass g substrate−1, respectively. The relevance of these findings for biomass production in wastewater processes are discussed.