Terminal settling velocity and bed-expansion characteristics of biofilm-coated particles

Fluid dynamic behavior of biofilm-coated particles in ambient water has been investigated. New experimental results are presented and compared with published data. From experimental measurements of the single particle terminal settling velocity the corresponding drag coefficient was found to be larger (by a factor of 1.6) than that for a smooth, rigid sphere at the same Reynolds number. A new simple correlation describing this finding is suggested. For multiparticle systems the Richardson-Zaki equation, derived empirically for rigid particles, provided a satisfactory description of biological beds. Of the two numerical parameters characterizing the expansion law, i. e. the slope n and the extrapolation to voidage equal one ui, the first was found to be similar to that suggested by Richardson and Zaki (1954), whereas ui gave results smaller than the single-particle terminal settling velocity, in contrast with the mentioned work but in agreement with more recently published behavior.