flow; mass transport; and biofilm clusters?

Stephen Livesey stephen at livesey123.freeserve.co.uk
Wed Oct 11 14:12:43 EST 2000


I have been studying the effects of recirculation flow velocity (within a 
rectangular channel, 2cm * 10cm) on the rate of autotrophic 
denitrification. I would like to present some observations and ask for comment.

I have observed the following:

[1] at low flow rates Re ~ 300 a compact flat biofilm morphology may be 

[2] when the flow is progressively increased (Re~300-2400) the presence of 
anchored clusters of biomass becomes evident. The clusters oscillate/waft 
in the flow.

[3] further increasing the flow rate to Re~3000 leads to a significant drop 
in reactor performance (probably due to sloughing).

Analysis of the mass transport of nitrate (NO3-) in the system by relating 
mass flux to the hydraulic conditions (using Sherwood no.eqns 1 and 2), 
suggested that the system is not under mass transfer control.

(calculated mass flux of N03- was an order of magnitude greater than the 
experimentally observed flux).

[1] Sh = 1.8488*((Re*Sc*de)/L)^1/3 -0.4
Prasad et al., 1995 flow in rectangular channel.

[2] N (mol/m2s) = ((Sh*D)/de)*Cb

where Re =Reynolds no.; Sc = Schmidt no.; de= equivalent hydraulic diameter 
(m); L = length of channel(m); D = diffusion coeff.(m2/s); and Cb= bulk 
substrate conc(mol/m3).

Nevertheless the rate of denitrification was observed to increase almost 
linearly with increasing flow rate. This suggested that another mechanism 
was enhancing the rate of denitrification.

My question is: has anyone made similar observations; read of mechanisms by 
which oscillation of anchored clusters of biomass have enhanced reaction 
rates; or have suggestions for the observed results?

I was thinking that the movement of the clusters is in someway reducing 
diffusion limitations within the biofilm. Anyway as you can imagine I would 
be greatful for any comments.


Luke O'Brien
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