Cross-Flow Microfiltration
A term project by Mersey Marsh and Cindy Giannaris, Spring, 1997. Revamped
page
done previously by Kareemah Abdullah and Samantha Park, Fall, 1994. Our
intentions were to clarify and enlighten the subject matter covered.
Overview
In cross-flow microfiltration (CFMF), the suspension is pumped tangentially over the filtration
medium. Clear liquid permeates the filtration medium and is recovered as the permeate, while the solids
accumulate at the filtration barrier to form a fouling layer, or cake. The cake, constituting an increase in
hydraulic resistance, decreases the permeate flux. However, the tangential suspension flow tends to limit
the growth of the cake. Thus, after an initial rapid increase in cake thickness, cake growth ceases, and the
cake thickness becomes limited to some steady-state value. Correspondingly, after an initial rapid
decrease, the permeate flux levels off and either attains a steady-state, or exhibits a slow, long-term
decline with time.
Microfiltration is suited to separate larger sizes, such as suspended solids, particulates, and
microorganisms. This is accomplished because microfiltration membranes are thought to act like a
physical seive. The membranes are highly porous and have discernible pores even when the surface
skins are asymetric. Therefore, the separation is based mainly on size. Membrane material is usually
made up of ceramics, teflon, polypropylene, or other plastics.
At the Durban Corporation's Northern Waste Water Treatment Works (NWWTW) many studies have been
conducted which show that anaerobic digestors perform much better when coupled with a cross-flow
microfiltration (CFMF) unit. The CFMF unit enables the retention of biomass in the reactor and thus
the digestor may be operated at a higher biomass concentration. This increases the biomass loading
per unit volume of the digestor capacity and also the kinetics of the digestor process and
the rate of the gas production. By coupling a CFMF unit to the digestor, the liquid resistance time
(LRT) is decoupled from the solids resistance time (SRT). This enables the volumetric throughput
of the digestor to be increased while still maintaining the necessary solids residence time.
Advantages
The coupled process has the direct advantage of existing digestors which may be operated at
throughputs in excess of their current maximum values. This would enable existing works to cope with
increases in inflows and delay the necessity to construct new digestors. Effectively, the exploitation
of existing resources will be increased.
In addition there would be significant indirect disadvantages to upstream and downstream unit process.
The volumetric loading to downstream sludge dewatering equipment would be reduced. The supernatent
liquor that is returned to the head of the works, i.e. the permeate from the CFMF unit, will have a
negligible suspended solids content.
Other disadvantages can be read about from research on the
fouling of membranes.
In the development of the coupled CFMF/digester processes, the next stage involves
investigating various other aspects concerned with the long term viability, reliability,
and operability.
Economics
The economic feasibility of the coupled CFMF/digestor process was assessed, by comparing the cost
of the coupled CFMF/digestor system to the cost of conventional treatment equipment that would yield
a similar volatile solids destruction and final effluent solids concentration. As the basis of the
economic evaluation, the conventional process stream at NWWTW was considered. A total solids inflow
was 1.5 times that of the current maximum capacity of the NWWTW plat. The cost of the conventional
process and the coupled CFMF/digestor process were determined from data supplied by NWWTW personnel.
Over a project life of 20 years, the total cost of the coupled procedure including labour, electricity,
capital redemption and maintenance, is 11/240% less than that of the conventional process stream. Thus,
the economic evaluation indicates that the coupled CFMF/digestor system is economically feasible.
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