Perhaps microbial aspect of MBR is the biggest interest of academic researchers perhaps due to its complexity. As a result there have been numerous research papers have been publish on this topic.
Microbial species found in the mixed liquor of MBR are basically same as those in conventional activated sludge (CAS) process, but they have some distinguished characteristics due to low F/M ratio, long SRT, and the perfect retention of non-floc forming microorganisms.
- Relative to activated sludge process, particles are generally smaller in MBR than in CAS. One convincing explanation is that MBR does not lose non-floc forming bacteria so that those can grow without being washed out. On the contrary, non-floc forming bacteira can be easily lost through effluent in CAS while floc forming bacteria are selected and proliferate in the system.
- Filamentous microorganisms tend to be more abundant in MBR. The cause is not completely clear, but it is popularly believed that filamentous microorganisms gain relative advantages over the other microorganisms at food-scarce environment due to their high surface to volume ratio, which allow better contact with foods. In one literature, filamentous population was one order of magnitude higher in MBR than in activated sludge (Merlo, 2004).
- Zoogloea population tends to be low in MBR perhaps due to low F/M ratio, where foods are scarce.
- Slowly-growing higher life forms such as rotifer, nematode, amoebas, ciliates, etc. are abundant in MBR due to the long SRT. The higher life forms consume fine particles that can be major membrane foulant. In one study, when sessile ciliates and free-swimming ciliates populations were high, population of particles smaller than 10 micron, especially the particles around 1 micron, were significantly reduced (Luxmy, 2000).
The high filamentous population does not necessarily mean high membrane fouling rates unless it comes with viscous bulking and severe foaming, where polysaccharides level rises possibly with a high zoogloea population. In fact, high SRT and low F/M ratio tend to increase filamentous population, but membrane fouling potential tends to decrease as discussed here. In contrast, in CAS excess filamentous population causes sludge bulking and interfere sludge settling.
Following are some microscopic images of MBR mixed liquor taken from municipal and industrial MBR.
Fig. 1. Abundant filamentous bacteria were observed at low F/M ratio, e.g <0.05 g COD/g MLSS/day. This mixed liquor had fairly low membrane fouling potential in a pilot study using synthetic feeds.
Fig. 2. The high salinity of the sludge (68 mS/cm vs. 48 mS/cm for seawater) prevents microorganisms from floc formation. Only a few species with distinguished appearances exist. This mixed liquor had extremely high membrane fouling potential and sustainable flux was below 5 LMH.
Fig. 3. Zoogloea population appeared connected with membrane fouling tendency in a municipal MBR plant. The lower the zoogloea population, the lower the membrane fouling tendency.
Fig. 4. Abundant filamentous and abundant higher life forms were observed from the mixed liquor from a municipal MBR.
© Seong Hoon Yoon