Abrasion of suspended biofilm pellets in airlift reactors: effect of particle size.

The detachment of biomass from suspended biofilm pellets in three-phase internal loop airlift reactors was investigated under non-growth conditions, and in the presence of bare carrier particles. In the experiments the size of biofilm pellets and bare carrier particles was varied. Results show that an increase in particle size drastically increases the abrasion rate caused by particle collisions. This increase is larger than predicted by conventional collision theory, which accounts for changes in collision frequency and collision impact. However, collision theory was formulated for neutrally buoyant particles which follow the liquid flow. This condition does not hold for biofilm pellets and carrier particles. The difference might therefore be caused by differences in particle responses to flow fluctuations. An empirical relationship, including this flow response, was formulated. The collision impact is also strongly affected by the roughness of a bare carrier particle: sharp and edgy particles cause much more damage than smoother ones.

Abrasion of suspended biofilm pellets in airlift reactors: importance of shape, structure, and particle concentrations.

The detachment of biomass from suspended biofilm pellets in three-phase internal loop airlift reactors was investigated under nongrowth conditions and in the presence of bare carrier particles. In different sets of experiments, the concentrations of biofilm pellets and bare carrier particles were varied independently. Gas hold-up, bubble size, and general flow pattern were strongly influenced by changes in volume fractions of biofilm pellets and bare carrier particles. In spite of this, the rate of biomass detachment was found to be linear with both the concentration of biofilm pellets and the bare carrier concentration up to a solids hold-up of 30%. This implies that the detachment rate was dominated by collisions between biofilm pellets and bare carrier particles. These collisions caused an on-going abrasion of the biofilm pellets, leading to a reduction in pellet volume. Breakage of the biofilm pellets was negligible. The biofilm pellets were essentially ellipsoidal, which made three-dimensional size determination necessary. Calculating particle volumes from two-dimensional image analysis measurements and assuming a spherical shape led to serious errors. The abrasion rate was not equal on all sides of the biofilm pellets, resulting in an increasing flattening of the pellets. This flattening was oriented with the basalt carrier inside the biofilm and independent of the absolute abrasion rate. These observations suggest that the collisions causing abrasion are somehow oriented. The internal structure of the biofilms showed two layers, a cell-dense outer layer and an interior with a low biomass density. Taking this density gradient into account, the washout of detached biomass matched observed changes in volume of the biofilm pellets. No gradient in biofilm strength with biofilm depth was indicated. (c) 1997 John Wiley & Sons, Inc.

Adhesion and biofilm development on suspended carriers in airlift reactors: hydrodynamic conditions versus surface characteristics.

Adhesion and biofilm formation by Pseudomonas putida was studied using suspended carriers in laboratory airlift reactors. Standard, roughened, hydrophobic, and positively charged glass beads, sand, and basalt grains were used as carriers. The results clearly show that in airlift reactors hydrodynamic conditions and particle collisions control biofilm formation. In the reactors, on surfaces subjected to different shear levels, biofilm formation differed considerably. This could be described by a simple growth and detachment model. Increased surface roughness promoted biofilm accumulation on suspended carriers. The physicochemical surface characteristics of the carrier surface proved to be less important due to the turbulent conditions in the airlift reactors. Adhesion of P. putida to glass beads was poor, and results of an adhesion test under quiescent conditions were not predictive for adhesion and subsequent biofilm formation under reactor conditions.

Detachment of biomass from suspended nongrowing spherical biofilms in airlift reactors.

In three-phase internal loop airlift reactors, the detachment of biomass from suspended biofilm pellets in the presence of bare carrier particles was investigated under nongrowth conditions. The detachment rate was dominated by collisions between bare carrier particles and biofilm pellets. The concentration of bare carrier particles and the carrier roughness strongly influenced the detachment rate. A change in flow regime from bubbling to slug flow considerably increased the detachment rate. Otherwise, the superficial gas velocity did not directly affect the detachment rate. The influence of particle size was not clear. The bottom clearance did not affect the detachment rate within the tested range. Other aspects of reactor geometry might be important. The main detachment processes were abrasion and breakage of biofilm pellets. During the detachment process, two phases could be distinguished. In the first phase the detachment was relatively high, and both breakage and abrasion of biofilm pellets occurred. During the second phase, breakage dominated and the detachment rate was lower. The two-phase behavior is explained by differences in strength between the inner and outer biofilm layers, possibly caused by variations in local growth rates during biofilm formation. Differences in growth history might also explain the various detachment rates observed with different biofilm batches. (c) 1995 John Wiley & Sons, Inc.

Heterogeneity of biofilms in rotating annular reactors: Occurrence, structure, and consequences.

A rotating annular reactor (Roto Torque) was used for qualitative and quantitative studied on biofilm heterogeneity. In contrast to the classic image of biofilms as smooth, homogeneous layers of biomass on a substratum, studies using various pure and mixed cultures consistently revealed more-dimensional structures that resembled dunes and ridges, among others. These heterogeneities were categorized and their underlying causes analyzed. Contrary to expectations, motility of the microorganisms not a decisive factor in determining biofilm homogeneity. Small Variations in substratum geometry homogeneity. Small variations in substratum geometry and flow patterns were clearly reflected in the biofilm pattern. Nonhomogeneous flow and shear patterns in the reactor, together with inadequate mixing resulted in significant, position-dependent differences in surface growth. It was therefore not possible to take representative samples of the attached biomass. Like many other types of reactors, the Roto Torque reactor is valuable for qualitative and morphological biofilm experiments but less suitable for quantitative physiological and kinetics studies using attached microorganisms. (c) 1994 John Wiley & Sons, Inc.