Filamentous bulking is caused by the excessive growth of filaments over floc formers, and their competition was previously described using kinetic selection. However, recent studies reported that instead of kinetic selection, diffusion limitation inside the flocs may be the crucial factor in microbial selection. To clarify the roles of these factors in explaining filamentous bulking, a new conceptual qualitative framework was developed in this study. We hypothesize that the growth rates of filaments and floc formers are affected by the combination of kinetic selection and substrate diffusion limitation. Three different regions (bulking, transitional, and non-bulking region) based on substrate concentration are suggested. In the bulking region, kinetic selection controls the growth rate process and favors filaments. In the non-bulking region, kinetic selection also is controlling, and favors the growth of floc formers. However, in the transitional region, substrate diffusion limitation, determined by the floc size, plays an important role in causing bulking. To test this framework, sequencing batch reactors (SBRs) were operated with influent COD of 100, 300, 600 and 1000 mg/L, and the sludge settleability was measured at various floc size distributions developed using different mixing strengths. The experimental data in the bulking and transitional regions supported the proposed framework.
Filamentous bulking is caused by the excessive growth of filamentous organisms outside the activated sludge flocs (Jenkins et al. 2003). The difference in growth kinetics between filaments and floc formers was widely used for explaining their competition (Chudoba et al. 1973c; Chudoba et al. 1973b; Chudoba et al. 1973a). Floc formers are thought to have high values of μmax (maximum growth rate) and are favored at high substrate concentration, while filaments are thought to have low Ks and thus have high values of growth rate at low substrate concentration (Figure 1). Based on the substrate concentration, the bulking and non-bulking regions can be determined. In the non-bulking region, the growth rate of filaments is higher than that of formers.
However, a recent study (Martins et al. 2003a) suggests that instead of kinetic selection, substrate diffusion limitation inside the flocs may be the important factor in microbial selection. It was hypothesized that filaments and floc formers have the same kinetics. At low substrate concentration in bulk solution, because the diffusion of substrate inside the floc is very low, floc formers cannot access substrate for growth, while filaments, due to their morphology, can grow faster than floc formers and easily reach substrate outside the flocs. Extended filaments then cause bulking. At high substrate concentration, filaments are postulated to have no such diffusion limitation advantage to be predominant, and most grow inside floc, and bulking does not occur. If this mechanism is correct, there is no need to invoke differences in growth kinetics, μmax and Ks of the two types of organisms for explaining bulking. The implication of this hypothesis is that under the same substrate concentration, bulking tends to occur at large floc sizes due to higher substrate diffusion resistance.
To clarify the roles and to what extent diffusion and kinetics influence filamentous bulking, we used quantitative fluorescence in situ hybridization (FISH) to measure the filament length inside and outside the activated sludge flocs in bulking and non-bulking sludge. It was found that both types of sludge have high levels of total filament length. The difference is that in non-bulking sludge, most filaments grew inside the flocs, which was consistent with the diffusion limitation hypothesis (Liao, et al., 2004). However, our later study (Lou and de los Reyes III, 2005b) using substrate uptake tests and metabolic modeling showed that non-bulking sludge has a higher maximum growth rate than bulking sludge. This result agreed with the kinetic selection theory. Considering our previous experiments, we hypothesized in the current study that the growth rates of filaments and floc formers are affected by the combination of kinetic selection and substrate diffusion limitation, and the dominant effect depends on the substrate concentrations and the floc sizes in the activated sludge system.