246 Organic waste recycling: technology and management
Problems encountered with microstrainers having large mesh openings
include inadequate solids removal and inability to handle solids fluctuations.
These problems may be partially over-come by varying the speed of drum
rotation. In general, drum rotation should be at the slowest rate as possible, that
is, consistent with the throughput, and should provide an acceptable head
differential across the fabric. The controlled variability of drum rotational speed
is an important feature of the process, and the speed may be automatically
increased or decreased according to the differential head best suited to the
circumstances involved (Middlebrooks et al. 1974).
5.3.2 Paper precoated belt filtration
Because conventional microstrainer fabric (mesh sizes greater than 23 ȝm) is
too coarse for the unicellular algae grown in HRAP and extremely fine fabric
media capable of effective separation of algal cells cannot be adequately
cleaned, a new harvesting process developed in Australia, utilizing a paper
precoated belt filter, seems to be able to overcome these difficulties (Dodd and
Anderson 1977). A schematic diagram of the paper precoated belt filtration
process is shown in Figure 5.11. The belt filter incorporates a coarse fabric belt
on which a precoat of paper fibres is deposited, as in a paper machine, forming a
continuously renewed filter medium. The paper precoat is continuously
reformed to provide a fresh filter medium having high trapping efficiency and
good throughput characteristics.
According to Figure 5.11, the algal water or influent is filtered by the
filtration drum. The filtered algal cells attached on the main belt are sandwiched
between the main and secondary fabric belts during application of vacuum and
water to separate the algae from the precoat. The paper precoat and the algal
cells are then removed from the belts by water showers (no. 14 and 15 in Figure
5.11) which are called first-stage and second-stage algal concentrates. These
algal concentrates are further washed to remove the algae, and the washed paper
fibers are recycled to form new paper precoat.
From their experiments, Dodd and Anderson (1977) found the average SS of
the first stage algal concentrate (no. 14 in Figure 5.11) to be 1.18 percent,
ranging from 0.81 to 1.49 percent. The average SS of the second stage algal
concentrate (no. 15 in Figure 5.11) was 0.43 percent. The second stage
concentrate appeared to have flocculent characteristics, which should be
beneficial in the thickening of the algal concentrate to reduce the cost of further
dewatering by centrifugation or filtration.