Treatment at a stretch

Wastewater from the wool scouring industry is very polluting, and contains

substantial amounts of detergent, oil and grease. It can have a chemical

oxygen demand (COD) concentration of over 250,000mg/l, and a temperature in

excess of 60ºC. Historically, the preferred disposal option for this

wastewater was to discharge it to sewer after appropriate pre-treatment,

but

pressure for water re-use through implementation of environmental

management

systems and cost reduction measures now makes water recycling increasingly

attractive. Membrane treatment systems are helping to make this possible.

Sometimes, scouring also takes place after the yarn has been spun. This is

done to remove oily lubricants, which are added to the wool to improve

processing during spinning. Both types of scouring use essentially the same

kind of scouring machine.

Necessary penalties

A scouring machine typically consists of three or four double bowls, the

inner bowls having perforated sides to allow the scoured dirt to pass

through. The bowls also feature a series of forks or rakes which move the

wool through the length of the bowl to a pair of rollers which squeeze the

water and dirt, or the lubricating oil, out of the fibres.

The wool, or yarn, is fed into the first bowl, which contains a solution of

hot water and neutral detergent. It then passes to the second, and, if

relevant, third bowls, which contain a cooler and weaker scouring solution,

before passing to the final bowl for rinsing.

The alternative on-site technologies for the treatment of wastewater from

scouring have relied on evaporation and biological treatments. Both of

these

technologies bring with them necessary penalties in terms of space and

cost.

However, membrane technology, often in collaboration with evaporation, is

now providing a cost-effective and efficient treatment option.

Membranes are permeable or semi-permeable polymeric solids ­ often modified

(cellulose-based) natural products such as cellulose acetate, or synthetic

polymers such as polyamide, polysulphone, polyethersulphone or

polyvinylidene fluoride. The particulate fraction of the waste is separated

from the liquid fraction; retained behind the differentially permeable

membrane because of its inability to pass through the pores. Exactly what

is separated, and how efficiently the process works, is governed by the

specific composition of the membrane, the pore size, the particle size

distribution in the wastewater and its flow direction and velocity.

There are four main categories of filtration process applicable to the

treatment of wastewaters. These are, in order of decreasing membrane pore

size, microfiltration, ultrafiltration, nanofiltration and reverse osmosis.

At one extreme, microfiltration will separate colloidal solids with a

diameter in the range 0.1-1.0 µm, whilst reverse osmosis will separate out

most ions and retain ³particles² with diameters down to 0.0001µm.

Ultrafiltration is the mechanism most suited to the requirements of

wastewater treatment in the wool scouring industry. Operating at a pressure

of between 2-7bar, ultrafiltration membranes have a pore size with a

molecular cut-off of around 100,000, making them permeable to most soluble

salts.

Youghal Carpet Yarns is based near Cork in Ireland, and produces large

amounts of spun and dyed wool yarn for the carpet industry. The first

stage

of the production process consists of blending the supplied scoured wool to

achieve uniformity of quality and colour. Lubricants are then added to the

wool to ensure good processing and prevent static electricity build-up, and

the wool is spun into yarn. The lubricating oil is then removed in the

³yarn

scouring² process and the yarn is dyed before being despatched to carpet

manufacturers.

All wastewater produced by Youghal is discharged, after pH balancing,

directly into the sea under an Environmental Protection Agency licence. The

wastewater comes from two main sources: from the yarn scouring process

(incorporating oil from spinning); and from the dye baths. Most of the

effluent produced comes from the dye baths and requires only minimum

balancing to satisfy discharge requirements. However, without some form of

on-site treatment, the total effluent would exceed regulatory limits, which

restrict COD to a maximum concentration of 1,150mg/l and BOD to 300mg/l. In

order to achieve this quality, the wastewater from the yarn scouring

process

is treated, using an ultrafiltration plant from Koch Membrane Systems

comprising 136, 1in tubular membranes, providing 27m2 of active membrane

area, at an operating pressure of 65psi. The system is capable of expansion

to 35m2 of active membrane area, simply by adding more tubes, although this

extra capacity has never been required in eight years of continuous

operation, and the plant is still running on the original set of membrane

modules.

COD concentration

In operation, 150m3 of wastewater is generated by the three scouring bowls

each week, at a COD concentration of between 70,000-200,000mg/l. It is

first

pre-filtered to remove any fibres shed from the yarn during the scouring

operation ­ minimal because of the efficiency of the 1in tubular membranes.

It is then passed, in batch volumes of 60m3, through the membrane treatment

plant, which produces, on average, around 1,500l/h of treated permeate. The

initial permeate flow rate is around 3,500l/h but, as the batch becomes

more

concentrated, the permeate rate decreases to around 500l/h. Once the

retained volume has reduced to around 7m3, the residual concentrate is then

passed through to an evaporator and the remaining semi-solid residue is

burnt in the factory¹s own boilers to raise steam.

The treated permeate passing through the membranes has a residual COD

concentration of 1,000-2,000mg/l, and by the time this has been diluted by

the much larger volume of dye bath effluent, the final total mixture has a

COD of 300-500mg/l and is well within regulatory limits. It only needs

balancing before discharge to the sea.

Although the cost of installing water-recycling facilities on these sites

would currently be too high to justify expenditure, the option is there for

the future. Changing economic circumstances might be driven either by

potable water cost/availability or by a need to implement a more

sustainable

water use regime within a company environmental management system.

Nevertheless, both of these case studies indicate that membrane treatment

systems are able to provide a practical treatment choice for this kind of

process effluent.