Oxygen diffusion and consumption in active aerobic granules of heterogeneous structure.

The interior structure of aerobic granules is highly heterogeneous, hence, affecting the transport and reaction processes in the granules. The granule structure and the dissolved oxygen profiles were probed at the same granule in the current work for possible estimation of transport and kinetic parameters in the granule. With the tested granules fed by phenol or acetate as carbon source, most inflow oxygen was consumed by an active layer thickness of less than 125 microm on the granule surface. The confocal laser scanning microscopy scans also revealed a surface layer thickness of approximately 100 microm consisting of cells. The diffusivities of oxygen transport and the kinetic constant of oxygen consumption in the active layers only were evaluated. The theoretical models adopted in literature that ignored the contributions of the layered structure of aerobic granule could have overlooked the possible limitations on oxygen transport.

Oxygen diffusion in active layer of aerobic granule with step change in surrounding oxygen levels.

High biomass density and large size limit the transfer of dissolved oxygen (DO) in aerobic granules. In the literature, the oxygen diffusivity is often employed as an input parameter for modeling transport processes in aerobic granules. The interior of an aerobic granule was observed to be highly heterogeneous. In this work, the distributions of extracellular polymeric substances (EPS) and cells in the interior of phenol-fed and acetate-fed granules were built up using a five-fold staining scheme, combined with the use of a confocal laser scanning microscope (CLSM). The steady-state and transient DO with step changes in surrounding DO levels at various depths were measured in the granules using microelectrodes. Cells were probed in a surface layer of thickness 125-375 microm. A marked fall in DO was also noted over this surface layer. No aerobic oxidation could occur beneath the active layer, indicating the oxygen transfer limit. Fitting the steady-state and transient DO data over the active surface layer yielded apparent diffusivities of oxygen were (9.5+/-3.5)x10(-10)m(2)s(-1) for the phenol-fed granule and (3.5+/-1.0)x10(-10)m(2)s(-1) for the phenol-fed granule. These values were lower than those adopted in models in the literature.

Diffusivity of oxygen in aerobic granules.

This work for the first time estimated apparent oxygen diffusivity (D(app)) of two types of aerobic granules, acetate-fed and phenol-fed, by probing the dissolved oxygen (DO) level at the granule center with a sudden change in the DO of the bulk liquid. With a high enough flow velocity across the granule to minimize the effects of external mass transfer resistance, the diffusivity coefficients of the two types of granules were estimated with reference to a one-dimensional diffusion model. The carbon source has a considerable effect on the granule diameter (d) and the oxygen diffusivity. The diffusivity coefficients were noted 1.24-2.28 x 10(-9) m2/s of 1.28-2.50 mm acetate-fed granules, and 2.50-7.65 x 10(-10) m2/s of 0.42-0.78 mm phenol-fed granules. Oxygen diffusivity declined with decreasing granule diameter, in particular, the diffusivity of acetate-fed granules is proportional to the size, whereas the diffusivity of phenol-fed granules is proportional to the square of granule diameter. The existence of large pores in granule, evidenced by FISH-CLSM imaging, was proposed to correspond to the noted size-dependent oxygen diffusivity. The phenol-fed granules exhibited a higher excellular polymer (ECP) content than the acetate-fed granules, hence yielding a lower oxygen diffusivity.