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 treatment.
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 force.
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