Hydraulic film diffusion
Example 5.4
Fig 5.7
156
By interpretation of tests it is necessary to take into consideration the possibility that
both hydraulic film diffusion and diffusional limitation in the biofilm have an effect. Fig
5.7 shows a number of measuring points for the removal rate per surface area of nitrate which is removed by denitrifying bacteria in a thick biofilm. Apparently the following
relationship is obtained:
rA = 0,09 · S 0 · 64
hence a deviation from the expected % order.
By substitution
h
0.15 mlh
k%A
0.13 g%m-%h- 1
the full line shown in Fig 5.7 is obtained. For S equal to approx. 50 g N03-N/m 3 , thereaction is half order. Hence the exponent found
(n = 0.64) can be attributed to the fact that the data are located in the transfer zone between a half order (n = 0.5) and a first order (n = 1.00); but the data do not have a sufficient span to distinctly show the first order reaction. Therefore it is sometimes tempting
to make a linear regression on something which is, in fact, a curved graph.
2
r ... g/(m ·h)
1.0
1!2 ·1!2 ·1
k 1128 = 0.13g m
h
S g/m 3
0.1
1.0
10
100
Surface area specific reaction rates of nitrate in a rotating disc as a function of the
nitrate concentration in the water phase.