Getting the best out of poor resources
Colin Reith of Memcor Ltd, outlines the key role which continuous microfiltration plays in keeping costs down and efficiency high in the treatment of potable water, industrial water pre-treatment and wastewater recycling.
Membrane separation processes are now an essential part of every water treatment
engineer’s repertoire but, whilst much is trumpeted about reverse osmosis and
ultrafiltration, it is continuous microfiltration (CMF) which, over the last
fifteen years, has quietly become indispensable in many areas of water and wastewater
Gas backwash system
Continuous microfiltration was pioneered in the UK by Memcor, now part of Vivendi
Water, in the mid 1980’s using hollow fibre membranes manufactured from polypropylene
or PVDF. The membranes are about 0.5mm diameter with pore size 0.2µm and
operate at a transmembrane differential pressure of 0.5 – 1.5 bar, which means
fine filtration at relatively low operating costs.
The CMF membranes are washed at intervals varying between a few minutes to
several hours, depending on the application, using Memcor’s unique gas backwash
system. A quantity of compressed air is applied to the filtrate side of the
membrane and released through the membrane wall, lifting accumulated solids
from the membrane surface and allowing them to be flushed out using raw feed
water. This ensures the wastewater volume is kept to a minimum.
In the production of potable water, the membrane pores are ideally sized for
the removal of the colloidal material that causes turbidity in river waters
as well as algae, bacteria (typically larger than 0.2µm) and protozoan
cysts including those of Giardia and Cryptosporidium (typically larger than
2µm). This means that a single stage of CMF can effectively replace the
‘conventional’ train of flocculation, clarification and sand filtration, giving
a more compact footprint with treated water virtually sterile and free from
Cryptosporidium. But is it competitive in capital cost?
In 1998 Montgomery Watson was in the process of designing a new conventional
plant at Kenosha in Wisconsin, USA, to treat 80Ml/d of lake water to potable
standards. Halfway through the design, the company reviewed the costs and changed
to a Memcor CMF plant. The plant, consisting of sixteen streams of ninety CMF
modules, each containing 20,000 hollow fibres, cost $29.5M – less than 50% of
the original budget for the conventional plant.
The worldwide increase in demand for drinking water, together with diminishing
resources, means that poorer quality sources are being exploited. CMF provides
a simple treatment process, without the need for chemicals, for remote sites
and has been used to treat raw waters with peak turbidities of up to 700NTU,
giving treated water turbidity consistently less than 0.1NTU.
The Memcor membranes have approval from the UK’s Drinking Water Inspectorate
for use in the removal of Cryptosporidium to comply with the current Drinking
Water Regulations and recent installations in the UK at Homesford WTW (65Ml/d)
and Ennerdale WTW (59Ml/d) reflect the importance of this legislation. The CMF
process also meets the needs of the food industry where, increasingly, major
customers such as supermarket chains, require their suppliers to provide on-site
filtration of all water to 1µm or better. Memcor’s fully automatic built
in integrity test procedure, which can be initiated and monitored remotely via
telemetry links, makes validation of CMF membranes particularly simple.
Industry is also turning to the use of membrane processes to provide process
water where mains water supplies are either too expensive or simply not available
in sufficient volume. Reverse osmosis has been used successfully for the treatment
of river water to produce power station boiler feed water for many years but,
when the massive 2940MW Eraring Power Station in New South Wales, Australia,
lost its potable water supply in 1995, the only reliable alternative source
was the final effluent from the local sewage treatment works. Reverse osmosis
was the obvious treatment choice to provide the necessary 3.8Ml/d of water,
but pre-treatment to prevent membrane fouling was critical to long term performance.
Once again it was a Memcor CMF plant which provided that security.
CMF’s bacterial removal capability has also been exploited in the treatment
of sewage works final effluent. A 1.1Ml/d plant in the arid Canary Islands recovers
sewage works effluent for use in agricultural irrigation and a 3.8Ml/d plant
at Aberporth sewage treatment works in Wales meets the challenging microbiological
standards of the UK legislation on discharges to bathing waters.
It was Memcor which, around 1990, was using membranes to replace final clarifiers
at an Australian sewage treatment works, effectively anticipating the role of
membrane bioreactors before the name had been coined. That original plant used
a standard CMF membrane format, that is with feed being pumped into the membrane
module as a separate unit operation external to the bioreactor itself. The latest
generation of Memcor membranes are packaged into modules which can be submerged
in the aeration tank with effluent being drawn through them under vacuum. This
arrangement further reduces the footprint and the air bubbles in the aeration
tank help to keep the membrane surface clean, reducing the backwash frequency.
But membrane bioreactors achieve rather more than simply replacing conventional
clarifiers to produce a final effluent of high clarity and good microbiological
quality in a small footprint. Because the Memcor MBR membranes can handle highly
concentrated suspensions, the mixed liquor suspended solidscan be maintained
at a much higher level – more than twice the concentration which is achieved
in a conventional activated sludge system.
This means more efficient biological oxidation and the ability to handle high
strength industrial wastewaters. And the effluent is suitable for direct feed
to a reverse osmosis plant for recycling – particularly important as the latest
UK legislation on Integrated Pollution Prevention and control (IPPC) comes into
As industry is forced to recycle more water, and as continuing demand for potable
water forces engineers to exploit poorer sources to produce a higher quality
product, so continuous microfiltration will become an essential component of
many water and wastewater treatment processes. It is already playing a major
role in the drive towards the water management engineer’s ‘Holy Grail’ – zero
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