Because yeast treatment systems are generally open systems,the substrate also promotes the growth of bacterial ormold species present in the mixed liquor when the yeastspecies propagate continuously. The propagation of a specificyeast strain in the yeast treatment system is based on freecompetition among different microorganisms [22]. Therefore,the major challenge for yeast treatment processes is to properlydesign and control operational conditions to guarantee ayeast-dominated biomass, as well as good biomass settlingunder non-sterile conditions. Because sterilization for conventionalSCP process is expensive, the fact that yeast cangrow at low pH can be used to: (i) Sterilize the water atrelatively low temperatures, (ii) prohibit contaminating bacteriafrom growing, and (iii) provide sufficient environmentalselection pressure in a continuous system to direct a complexmicroflora towards yeast domination [35]. Therefore, an acidicpH (<6) is generally used for yeast treatment processes tomaximize yeast growth and limit bacterial growth under thenon-aseptic conditions [11,20,55,59]. However, based on fluorescentin situ hybridization-flow cytometry analyses, Zhenget al. [20] found that it was difficult to achieve yeastdominatedbiomass in continuous-flow systems at low CODloadings (e.g., 2 kg,m3,d1 COD), regardless of the acidic pHlevels, while acidic pH was sufficient to achieve yeastdominatedbiomass in a batch culture. In other words, theyeast treatment process should be conducted with both acidicpH and high COD loading (e.g., 8.7 and 21.0 kg,m3,d1 COD)levels as prerequisites [20]. If yeast cells are larger than bacterialcells, producing a lower surface-to-volume ratio, lowersubstrate adsorption/uptake efficiency at low substrate concentrationsor loading may result.
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