2.3. Effect of SIS particle size
Since no significant effect of SIS on the flow rate was observed at
14 mJ cm2 UV dose, the same effect was evaluated at a reduced
dose of 7 mJ cm2 while still guaranteeing a turbulent flow regime.
A clear model solution (Table 1) and apple solids with two different
average particle sizes were used to assess the effects of particle size
(over a range of SIS concentrations) on the flow rate of the solution
through the UV unit. Apple solids (average diameter of 895 mm)
were obtained using a turbo extractor, following the procedure
indicated in Section 2.2. A second batch of solids, with a smaller
average diameter (199 mm), was obtained by the same method but
using a 0.4 mm screen. Solids were heat treated with steam at 99 C
for 5 min to soften the slurry and facilitate the second homogenization
step, which was carried out for 2 min with a high shear
mixer (model HSM-100 LSK, Charles Ross & Son Company,
Hauppauge, USA) equipped with a high shear rotor/stator attachment.
These solids were finally treated with the high shear mixer
for additional 2 min using the fine screen stator head at the
maximum speed of 10,000 rpm.
The effects of particle size over a range of turbidity values were
evaluated by adding different concentrations of apple solids (as
slurries) with two different average particle sizes (895 mm and
199 mm) to two independent model solutions, both resulting in
turbidities from 4 to 500 NTU. The flow rate of the solution through
the reactor was determined. After treated, samples were bottled
and stored at 2 C for up to 24 h. Model solutions were then
inoculated with E. coli ATCC 25922 and UV treated at a fixed flow
rate of 214.5 ml s1
. All treatments were executed in triplicate.
Inoculation and analysis were performed as indicated in the Section
2.6.