Effect of solids retention time on Microthrix parvicella growth
The objective of this study was to evaluate the effect of solids retention time (SRT) on M. parvicella growth and to calculate growth kinetic parameters of this filamentous species. Bench-scale continuous-flow experiments showed that M. parvicella growth can be significantly suppressed at an SRT of lower than 5.7 d for temperatures of between 14 and 18oC. According to the continuous-flow experiments the maximum sludge age for the avoidance of filamentous foaming problems caused by
M. parvicella is 6 d for temperatures lower than 18oC. At this sludge age M. parvicella loses its hydrophobicity and therefore its foaming potential. However, even lower SRTs are required in order to achieve a significant suppression of its growth. At SRT values of less than 5.7 d M. parvicella initially forms a shorter filament (< 150 μm) with clear spaces inside filaments and variable Gram stain reaction and eventually is eliminated from activated sludge biocoenosis. According to kinetic studies presented in this paper, M. parvicella is a slow growing bacterium with a low maximum specific growth rate of 0.67 1/d and 0.53 1/d under aerobic and anoxic conditions respectively. Maintenance energy requirements of M. parvicella were found to be significantly lower than the maintenance energy of floc forming micro-organisms as well as other filamentous species, thus providing the micro-organism with a significant advantage under starvation conditions prevailing at the majority of the extended aeration activated sludge systems.
Water SA Vol.32 (3) 2006: pp.315-321