Table 1: Current Market Position of MF for Water Treatment Supplier
USF Memcor
Aquasource
Zenon
Pall
Koch
XwFlow
Pore Size
0.2 µm Polypropylene
0.01 µm Cellulose
0.03 to 0.1 µm Organic polymer Submerged
0.015 µm Polyacrylonitrlle
0.005 to 0.5 µm Polysulphone
Direct Filtration
Crossflow
0.1 µm Polyethersulphone/ polyvf nyl pyrro Iidone Dlrect;cross flow hybrid
>80 ML/d
>20 ML/d
>50 ML/d
Membrane
acetate
Material
Configuration
Direct and submerged
Cross flow
Total Installed or
contracted capacity > 500 ML/d
> 400 ML/d
>50 ML/d
Source:- Oosterom et a!. & information obtained from suppliers and their Hterature.
increases, until eventually the capacity of the plant is reduced. Note that this is significantly different to the mode of failure for a sand filter, which would typically foil by a reduction in water quality rather than quantity. Fouling can be avoided by selecting a membrane which does not foul with the particular foulant, using appropriate pretreatment or some form of chemical dosing. Unfortunately, most of these options are best dealt with at the design stage, when the fouling problem may not be evident. Therefore the best strategy is to run reasonable scale pilot trials to determine the best membrane and design on the water in guestion. In most cases fouling can be removed by chemical cleaning of the membranes. It may take the application of trial and error to determine the best chemical(s). This reinforces the benefit of pilot trials, as the cleaning chemicals to deal with particular fouling problems can be determined.
System Integrity The benefits of microfiltration are related to the fact that no particles of greater than 0.1 to 0.2 ~Lm will pass the membrane. However, this is not true if the membranes have holes significantly greater than the pore size. If enough fibres have failed, the system is not acting as a microfilter, and these benefits are lost. Fibres are essentially a small diameter plastic tube. They are subject to physical forces from movement (say during a backwash), and damage from abrasion, and other forces from the particles in the water. The fibres may suffer from chemical attack and may have manufacturing defects which only become evident over time. It is often impossible to determine why a membrane has failed. There are two main methods used to detect fibre failure. The first is to perform some sort of sensitive measurement on the guality of the filtrate, with a particle counter the obvious choice for an instrument. A particle counter will show a deterioration in the water quality if a significant amount of the flow is
bypassing the membranes through a fibre failure or some other leak path such as a faulty O-ring seal. Note that on a large plant, there may need to be several instruments, each on a separate 'bank', to avoid having any one failure averaged out. A problem with this test is that there must be a measurable, (and therefore significant) degree of failure before it can be detected, although particle counters are reputed to be able to detect two breaks in a bundle of 256000 fibres. The second method is to use an air pressure hold test. The laws of air bubble formation in water show that at a pressure of 50 to 100 kPa, the size of air bubbles will be greater than 0.2 µm. Therefore air at these pressures will not pass a wetted membrane. One investigation by a user demonstrated a straight-line correlation between the rate of leakage and the number of fibres deliberately broken. One supplier claims that failures of a single fibre can be detected using this method. A problem with this test is that it requires the plant to be offiine while it is performed and therefore it cannot be carried out continuously. Once a fibre failure has been detected, there are a number of different technigues used to detect which individual module has the problem. These techniques generally involve the use of air bubbles and then either sonic or visual identification. The repair method varies from manufacturer to manufacturer. Generally, the standard cure is to plug both ends of the offending fibre and then it cannot leak, or supply water. A large number of fibres need to fail before this is impractical, or before the overall capacity of the system is reduced. Service contracts which offer assistance with this, together with some form of commercial arrangement on membrane replacement cost arc becoming common. Waste Strea1ns: MF produces a number of waste streams: • Backwash, and blowdown of the tank water for immersed membranes. Thus MF produces a waste stream which is
around 5 to 10% of the total flow (at least double that produced by a sand filter.) If the MF plant is operating without prefiltration this stream may be high in solids, and may not settle easily. In many cases, chemical dosing of the backwash stream is advisable to ensure settling, and possible recycle of the supernatant. • Cleaning Waste: MF plants need chemical cleans, and eventually a concentrated and dirty chemical stream will need disposal. Possible routes for this include sewer, neutralisation and then to sewer or to a washwater system, or trucking offiite.
Available Systems Table 1 summarises some of the major suppliers, and some aspects of their technology. Microfiltration appears to be competitive on a capital cost basis with more traditional separation/sand filtration plants up to around 10 ML/ d. This is a significant reduction in price over the last 5 years. There is also greater acceptance of MF technology, as indicated by the number of projects specifying MF outright. Recent Australian examples of this are the 6 ML/d plant at 13amaga in North Qld, and two other projects; one at 20 ML/d and the other at 45 ML/d, where MF has been identified as the preferred technology. Where the specifications for treated water guality are more stringent, for example if high levels of guaranteed Cryptosporidi11111 and Giardia removal are required, then microfiltration appears competitive up to 100 ML/d. This is borne out by the number of 100 ML/d plus plants worldwide. Pertinent local examples using the submerged membrane system include the Bendigo, Victoria 'Agua 2000' project, where US Filter is constructing a 120 ML/d MF plant, and the Waikato project in NZ, which will incorporate Zenon membranes to an ultimate capacity of 150 ML/d. In terms of capital cost, it appears that the submerged process is quite competitive with conventional treatment WATER JANUARY /FEBRUARY 2000
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