Reactor Configuration

Aerobic granules have been produced only in column SBR so far. It can be understood that reactor configuration has an impact on the flow patterns of liquid and microbial aggregates in the reactor. In a column SBR, air flow is subject to an upflow pattern. The air or liquid upflow pattern in a column reactor creates a relatively homogenous circular flow and localized vortex along the reactor height; and thus microbial aggregates are constantly subjected to circular hydraulic attrition (Liu and Tay, 2002).

The feasibility and efficiency of other types of bioreactors, such as completely mixed tank reactor (CMTR) in development of aerobic granular sludge have not been sufficiently demonstrated so far. In a hydrody-namic sense, column-type upflow reactor and CMTR have very different hydrodynamic behaviors in terms of interactive patterns between flow and microbial aggregates, as illustrated in Fig. 5.8a. According to the thermodynamics, the circular flow could force microbial aggregates to be shaped as regular granules that have a minimum surface free energy, provided those aggregates could be kept in the reactors under given dynamic conditions. Thermodynamically, such a phenomenon is very similar to the formation of benthic round-shape boulders in a natural flowing river system. It is obvious that in a column-type upflow reactor a higher ratio of reactor height to diameter can ensure a longer circular flowing trajectory, which in turn creates a more effective hydraulic attrition to microbial aggregates. However, in CMTR microbial aggregates stochastically move with dispersed flow in all directions. Thus, microbial aggregates are

(a) Circular movement of an aggregate

(b) Stochastic movement of an aggregate

(b) Stochastic movement of an aggregate

Fig. 5.8. Flow patterns in upflow column reactor (a) and completely mixed tank reactor (b) (Liu and Tay, 2002).

subject to varying localized hydrodynamic shear force, flowing trajectory and random collision (Fig. 5.8b). Under such circumstances, only flocs of irregular shape and size instead of regular granules occasionally form, and this is exactly like what happens in a conventional activated sludge aeration tank, which is a typical CMTR. The operation practice of conventional activated sludge process supports the above analysis because microbial granulation has hardly been reported in CMTR in the past one hundred years of operation practice.

It seems certain that not only the strength of hydrodynamic shear force, but also the interactive pattern between flow and microbial aggregates have effects on the formation of granular sludge. In this aspect, the column-type upflow reactor with high ratio of reactor height to diameter can provide an optimal interactive pattern between flow and microbial aggregates for granulation. It can ensure a circular flowing trajectory, which in turn creates a more effective hydraulic attrition for microbial aggregates. A high H/D ratio may also improve oxygen transfer and could result in a reactor with a small footprint (Beun et al., 2002). This may be a major reason why almost all of the granular sludge only forms in column-type upflow reactors. In an engineering sense, the desirable interactive pattern between flow and aggregates might be achieved by controlling reactor configurations and operation strategy. Consequently, a better understanding of the role of flow pattern in granulation process would lead to the development of novel types of granular sludge reactor. On the other hand, some researches also showed that aerobic granules could be developed in SBRs with different H/D ratios (Pan, 2003). It seems that aerobic granulation might not tightly associate with the H/D ratio.

Was this article helpful?

0 0
Solar Power Sensation V2

Solar Power Sensation V2

This is a product all about solar power. Within this product you will get 24 videos, 5 guides, reviews and much more. This product is great for affiliate marketers who is trying to market products all about alternative energy.

Get My Free Ebook

Post a comment