Getting the best out of poor resources

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.

Legislative demands

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.

Secure pre-treatment

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.

Efficient oxidation

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

discharge.