During the last 50-75 years, there has
been ever-increasing efforts to somehow arrange manufacturing processes in
such a way that they cause
minimal damage to the environment. Efforts
focused on the technology of waste treatment development at the same time,
and an uneasy adversarial relationship developed between regulators and
industry. Waste minimisation is the application of a systematic approach to
reducing the generation of waste at source. In other words, waste
minimisation prevents the waste from occurring in the first place, rather
than treating it once it has been produced by end-of-pipe treatment
Waste minimisation applies to hazardous
materials, non-hazardous materials, water, energy, raw materials, all waste
emissions, and other resources. It is NOT a one-off activity, but an
ON-GOING programme. That is, it is a technique that can be applied to all
inputs to and outputs from, a process. Waste minimisation is important
because it reduces operating costs, risk of liability and end-of-pipe
treatment, improves process efficiency, enhances public image, protects
health and environment, and improves employee moral. Waste is not only
materials that are excess to requirements, but represents a loss of company
This article makes an attempt to review
the various waste minimisation techniques and possibilities that are
available for the textile industry today.
Waste minimisation is achieved through
source reduction by making process and product changes. Product changes
include increasing product life, and designing for less environmental
impact. Process changes include improved operating practices, improved
housekeeping, change in raw materials, change in technology, in-house reuse
or recycling. End-of-pipe treatment is only considered after source
Four main types of wastes are
particularly amenable to source reduction:
(1) Hard to treat,
(3) Offensive and
(4) High volume.
For each, there are major advantages in source reduction. Hard-to-treat
wastes are persistent or resistant to normal treatment and include colour,
metals, phosphates, and certain organic materials especially certain types
of surfactants which resist biodegradation. These are often extremely
expensive and difficult to remove or destroy via conventional wastewater
treatment, and therefore source reduction is an economical and attractive
alternative. Hard-to-treat wastes can be reduced at the source by chemical
substitution, control and conservation, waste segregation, capture, reuse
and recycle. In doing this, the goal is to make changes which either reduce
the amount of hard-to-treat waste or make the wastes more treatable and less
The second category, dispersible wastes,
usually exists in concentrated form when they are created and at that point
they can be captured or reduced by process modification at the source, or
recycled. But, once discharged, they tend to become widely dispersed and
hard to recapture and collect for treatment. Machinery design, chemical
substitution, procedural remedies or other primary control measures can
often accomplish excellent results at low cost for these wastes.
In addition, reclaimed wastes in
concentrated form have its highest potential for resale or recycle. Textile
examples of this type of waste include lint (eg, from air and water
filters), print paste (eg, foam drum washers), waste foam coating operations
(eg, foam back coatings), air emissions (eg, breathing losses from tanks),
waste solvents from machine cleaning (eg, shop), still bottoms from solvent
recovery (eg, dry cleaning), and batch dumps of unused mixes (eg, finishes).
Even if not reused, collection, handling and disposal is usually much easier
if the material is captured when least diluted or contaminated. Offensive or
hazardous textile wastes include toxic air or water emissions, eg, metals,
various organic materials and surfactants.
In many instances, chemical
substitutions can effectively reduce production of undesirable process
by-products or reduce their offensive nature. For these wastes, treatment
often leads to undesirable waste treatment solids, eg, metal bearing
sludges. Hazardous or toxic wastes are particularly offensive and they
deserve special attention to reduce either volume or offensiveness. For
textiles, hazardous or toxic wastes include metals, chlorinated solvents,
non-degradable surfactants, and other non-degradable or volatile organic
materials. These often arise from non-process sources, eg, machine cleaning,
biocides. Appropriate reduction strategies include conservation, --
substitution, process or equipment modification maintenance and housekeeping
The fourth group, high volume wastes,
can sometimes be reduced by process modification or chemical substitution,
or reused either on- or off-site. The most common examples of these in
textiles are hydraulic loading (high volume of wastewater), wash water from
preparation and continuous dyeing, sizes and alkaline waste from
preparation, air from hot air dryers, and dye waste containing large amounts
of salt, acid or alkali. These can often be reduced by segregation and
subsequent recycle, as well as by process and equipment modifications.
Especially important are wasted chemicals (batch dumps and cleanup
activities) of starch size, knitting oils and degradable surfactants.
Each of the above four types of waste
may originate from a variety of textile operations and can be addressed by
reduction conservation strategies, process modifications, chemical
substitutions and other techniques which reduce the sources of these wastes
in textile wet processing, including preparation, dyeing, printing,
finishing and other sources. Since every operation is different, it is not
possible to specify exact universally applicable method for source reduction
activities. The main ingredient in a successful source reduction program is
to encourage individuals to look for waste reduction opportunities and take
Source reduction techniques can
generally be viewed in terms of several classifications as shown in Figure
1. These include common sense, good work practices, known technologies
already in commercial use, known technologies not in commercial use, new
technologies based on known science and, finally, new science. The worst
mills do not even use good work practices and common sense. Average mills
use good work practices as well as some known technologies. A "state of
the art" mill will use all economically proven technologies. "Best
available technology" would involve the application of all know
technologies. Development of new technology or new science can be very
A common mistake of textile mills is to
jump into the middle of the waste reduction curve, thus encountering high
cost and poor results. The best strategy for a source reduction program is
to take things one step at a time, keeping the pollution source reduction
program simple and efficient at first and avoiding any attempt to substitute
high tech approaches for common sense and good work practices. Only after
laying a proper foundation of common sense, good attitudes and work
practices can a mill expect to successfully move on to the "high tech
source reduction approaches.
Steps onvolved in
The first and most important step is to
audit the wastes from an operation, including both process and non-process
items. In many cases, non-process sources account for over half the
pollution, especially the most offensive types. Most often found is
non-process cooling water, solvents from machine cleaners and shop use,
metals, herbicides, fungicides, boiler chemicals, insecticides and the like.
Once all wastes have been listed, each should be viewed as a potential
recycle product, and markets should be sought.
The next step is to identify specific
reduction strategies for each identified waste. Some broadly applicable
techniques include raw material control, conservation and optimisation of
chemicals, chemical substitution, process modification, equipment
modification, maintenance procedures, housekeeping, waste recovery (for
reuse and recycle), and segregation. Using a combination of the above
techniques has produced documented annual savings in the millions of
dollars. These savings will persist and even increase year after year as
time passes, in contrast to waste treatment losses and costs which produce
an ever increasing burden of cost and liability as time passes.
In any manufacturing operation, there
are two fundamental sources of product variation: Raw material variation and
processing variations. In order to achieve process optimisation and product
uniformity, consistent raw materials must be fed into the manufacturing
process. To try to optimise processes and equipment without first
controlling raw materials leads to constant adjusting of the process and
equipment with little if any permanent improvements. Thus the concepts of
source reduction, process efficiency, cost reduction and quality control go
Raw materials coming into a textile
process generally include commodities such as water, substrate (yarn, fibre,
and fabric), salt and size, as well as speciality process chemicals and
dyes. Raw material control can be exercised both by prescreening materials
before use and by testing shipments as they are received. These precautions
provide many important benefits, not the least of which are reduction of
off-quality goods, lower reworks, and improved product uniformity. Each of
these increases manufacturers’ profits while reducing waste associated
with inefficient processes, reworks and remakes. These techniques also allow
the mill to screen out raw materials which will ultimately produce offensive
In this process, be sure to include shop
and maintenance chemicals. It is not unusual to find dyeing and other
procedures in textile wet processing which use excessive amounts of
chemicals or unnecessary chemicals. Often chemicals are unnecessarily added
to procedures to counteract the side effects of other chemicals. For
example, defoamer is frequently added to reduce foaming caused by other
chemical specialties. In many cases, it is more judicious to adjust,
substitute, or to remove offending chemicals from a process than to add more
chemicals to offset undesired side effects. Even better, it is often
possible to accomplish the desired result by machine set up, time
temperature and other processing parameter adjustments, without using
chemicals at all. Such conservative use of chemicals can significantly
reduce waste loads, cut processing costs and increase quality.
Recycle/reuse is not a source reduction
activity, it does deserve comment here. It is good economic sense to avoid
dumping unused portions of chemical mixes, but this still happens in many
types of continuous textile operations such as slashing, preparation,
continuous dyeing, printing, coating and finishing. Batch dumps are
expensive and account for a major portion of a processor's waste load. To
salvage unused mixes, segregation of wastes is the first step in
recycle/recovery, since mixed wastes are generally not reusable. Even if
wastes are not recycled, there are several reasons that segregation of
wastes may be desirable. Specific treatments, such as neutralisation of
acid/alkali and oxidation/reduction wastes are more effective prior to
mixing. Recovery/reuse systems generally are more effective when the waste
stream is most concentrated. Mixing hazardous and non-hazardous (or hard to
treat and easily treatable) wastes can create unnecessarily large volumes of
hazardous (or hard to treat) wastes.
Water consumption in a textile factory
can be reduced by implementing various changes ranging from simple
procedures such as fixing leaks, to more complex options such as optimising
water use and reducing the number of process steps. They include:
# Repairing leaks, faulty valves,
etc: A simple method of determining if leaks exist is to take incoming
water metre readings before and after a shut-down period when no water is
being used. A difference in the readings could indicate a leak.
# Turn off running taps and hoses: Encourage workers to turn off taps
and hoses when water is not required. The fixing of hand triggers to hoses
also reduces water consumption.
# Turn off water when machines are not running: Encourage workers to
turn off machines and water during breaks and at the end of the day. Avoid
circulating cooling water when machines are not in use.
# Reduce the Number of Process Steps: This involves a study of all
the processes and determining where changes can be made. For example, fewer
rinsing steps may be required if a dye with high exhaustion is used.
# Optimise process water use: Examples include using batch or
stepwise rinsing rather than overflow rinsing, introducing counter-current
washing in continuous ranges, and installing automatic shut-off
# Recycle cooling water: Cooling water is relatively uncontaminated
and can be reused as make-up or rinse water. This will also save energy as
this water will not require as much heating.
# Re-use process water: This requires a study of the various
processes and determining where water of lower quality can be used. For
example, final rinse water from one process can be used for the first rinse
of another process.
# Using water efficient processes and equipment: Although replacing
outdated equipment with modern machines which operate at lower liquor ratios
and are more water efficient requires capital investment, the savings that
can be made ensure a relatively short pay-back period.
# Sweeping floors: Instead of washing the floors of the dye house and
kitchens, rather sweep up any spillages and wash down only when essential.
Not only will this reduce water use, but also the concentration of
contaminants to drain as the waste is disposed of as solids.
# Reusing water from auxiliary processes: The water used in the rinsing of
ion-exchange columns and sand filters can be reused elsewhere in the
The majority of chemicals applied to the
fabric are washed off and sent to drain. Therefore, reducing chemical
consumption can lead to a reduction in effluent strength and therefore lower
treatment costs, as well as overall savings in chemical costs. Various
options for reducing chemical use are listed below:
# Recipe optimisation: Recipes
are generally fail-safe designed which results in the over-use of chemicals.
Optimising the quantity of chemicals required will lead to more efficient
chemical use and lower costs. Continual updating of recipes should be
carried out when new dyestuffs enter the market as, in general, less of
these chemicals are required.
# Dosing control: Overdosing and spillages can be reduced by mixing
chemicals centrally and pumping them to the machines. Check that manual
measuring and mixing is carried out efficiently and automatic dispensers are
# Pre-screen chemicals and raw materials: Avoid dyestuffs containing
heavy metals, solvent-based products and carriers containing chlorinated
aromatics. Safety data sheets should be obtained from the chemical
manufactures to obtain information such as toxicity, BOD and COD. Check that
raw materials do not contain toxic substances. Check that companies will
accept expired raw materials for disposal.
# Chemical substitution: Review chemicals used in the factory and
replace those hazardous to the environment with those that have less of an
impact. Use dyes that have high exhaustion rates and require less salt.
Specifically replace metal-containing dyes, use bi-reactive dyes in place of
mono-reactive, avoid the use of APEO detergents and replace with more
biodegradable alternatives, replace stilbene optical brighteners with
alternatives, or eliminate altogether, dye wool with dyes that do not
# Correct storage and handling: More effective control of the storage
and handling of chemicals will results in less spillage reaching the
# Chemical recovery and reuse: Chemical use may be reduced through
recovery and reuse. For example, sodium hydroxide from mercerising can be
recovered through evaporation. Dye baths may be reused and size can be
recovered for reuse.
# Process changes: Investigate the feasibility of changing to
cold-pad batch dyeing. This results in less chemicals being used (and in
particular, salt) and reduces water consumption significantly.
# Improve scheduling: Review the scheduling of continuous processes such
as sizing, desizing, padding etc. to ensure that the same chemical bath is
used as many times as possible, thus reducing the number of dumps to drain
As with water conservation, reductions
in energy use can result in substantial savings and lower emissions from
boilers or generating plants. They include optimising compressed air
generation, installing compressor control systems, and general
Reduce cooling loads, decrease
condensing temperature (as a guideline, reducing condensing temperature by
1oC will yield savings of between 2% and 4% of annual refrigeration cost);
Increase evaporating temperature (as a guideline, increasing evaporator
temperature by 1oC will yield savings of between 2% and 4% of annual
refrigeration cost); Compressor control, incorrect control of compressors
can increase costs by 20%, or more; Boiler blowdown, economisers,
insulation, flash steam recovery, good housekeeping, installing heat
exchangers, optimising plant environmental conditions, shutting off of
lighting, air-conditioning, etc.
Reducing solid waste
In terms of volume, solid waste is the
second largest waste stream in the textile industry next to liquid effluent.
There are a number of waste minimisation options available to reduce solid
waste, and these include:
i) Reducing the amount of packaging material by improved purchasing
practices such as ordering raw materials in bulk or returnable intermediate
bulk containers (IBCs). This reduces spillages, handling costs, exposure of
workers to chemicals and the amount of storage space required. ii)Purchasing
chemicals in returnable drums. Enquire if vendors will accept unwashed drums
as this will reduce the waste water generated in the factory. If possible,
ordering chemicals in IBCs rather than bags as these are easily broken,
iii) Purchasing yarn on reusable plastic cones rather than cardboard
iv) Reducing seam waste through effective training programmes.
v) Selling waste fibres, sweeps, rags, yarn and cloth scraps.
Waste minimisation in
specific textile processes
The following sections will describe
various waste minimisation techniques that can be implemented in specific
# Size selection in sizing: Replace starch-based sizes with synthetic
sizes. The advantages of this are, a reduced pollution load as synthetic
sizes have lower BOD levels and they can be recycled for reuse.
# Sizing raw materials: Test incoming raw materials for toxic
compounds. Purchase size in bulk in drums rather than bags etc. as this
produces less solid waste and reduces the chances of spills due to
# Sizing recipe optimisation: Ensure that only the minimum required
size is added onto the yarn. This reduces chemical consumption as well as
the pollution load to drain during desizing.
# Preparation department: Preparation includes desizing, scouring,
bleaching and mercerising. Desizing accounts for > 50% of the pollution
load of preparation while scouring contributes between 10 and 25%. Good
preparation is essential for subsequent processing as any impurities
remaining on the fabric will interfere with the dyeing and finishing
Some waste minimisation options for the preparation department are listed
The effluent from desizing will contain the sizes that were added onto the
yarn before weaving/knitting. Using and recycling synthetic sizes in place
of starch sizes will reduce the pollution load from desizing.
Incoming raw material should be screened for toxic chemicals as these
will be removed during the scouring process. Detergents should be easily
biodegradable. Avoid the following detergents: linear
alkylbenzenesulphonate; nonylphenoletoxylate; dialkyldimthyl ammonium
chloride; distearyl dimethyl ammonium chloride; di dimethyl ammonium
chloride; sulphosuccinates; alkylphenolethoxylates; complexing agents with
poor biodegradability (eg, EDTA; phosphonic acid; NTA; phosphonates). Reuse
scours washwater for desizing. Recycle continuous scour washwater to batch
Replace the use of chlorites and hypochlorites with hydrogen peroxide.
Ensure that bleaching is carried out efficiently. Recycle bleach washwater
for scouring. Use vacuum slots to remove excess solution which can then be
Little or no pollution arises from singeing. Ensure that air scrubbers
are installed to trap particles that are burnt off the fabric. Cooling water
can be reused elsewhere in the factory. Remove lint from the pad solution to
reduce the frequency of dumping.
Recycling of sodium hydroxide through evaporation for reuse in
mercerising or scouring will decrease the pollution load and chemical
* Batch processing:
There are a number of waste minimisation options for batch processing.
These include cascading multiple rinsing operations. Reusing softening baths
with reconstitution. Reusing preparation baths (scouring and bleaching) with
reconstitution after filtration to remove impurities. Segregating coloured
effluent streams from clean streams (preparation and rinsing) to ensure that
only concentrated effluent is treated. This clean effluent may be used
elsewhere in the factory. Installing automatic shut-down of water in
overflow cooling when the required temperature has been reached. Replacing
outdated machines with high liquor ratios with more modern equipment.
Carrying out softening on a pad mangle. Replacing batch-wise rinsing with
continuous rinsing with counter current flow.
* Batch dyeing:
Careful selection of dyes is important. Dyes should have high
fixation/exhaustion, low toxicity, absence of metals, and be appropriate for
the end use. Correct and efficient application procedures must be used and
right-first-time production should be achieved. The main areas for waste
minimisation include : Using low liquor ratios; Using automated dye and
chemical dosing systems; Reusing dyebaths, rinse water and softening baths;
Ensuring a good cloth preparation; Optimising pH and salt for each recipe;
Avoiding the use of auxiliaries that reduce or retard exhaustion; Using
bireactive dyes; Using the newer low-salt reactive dyes; Optimising dyeing
temperatures; Avoiding the addition of more chemicals to offset the effects
of other chemicals -- use other non-chemical methods such as procedural or
mechanical alterations or change the dye selection; Replacing the use of
acetic acid in neutralising after dyeing with formic acid or dilute
hydrochloric acid (acetic acid adds to the COD of effluent).
* Continuous dyeing:
The main waste minimisation strategies in continuous dyeing are to: Maximize
dye fixation; Minimize wash-off; Avoid discards and machine cleaning wastes
during start-up, shut-down and changes of colour and style; Minimize the
number of times a dyebath has to be dropped and cleaned due to a colour
change by careful scheduling; Use automated colour kitchens to minimise the
working losses and discards; Improve washing efficiency through the
installation of flow restrictors to control water volumes; Use counter
current washing procedures; Optimise dosing of chemicals through monitoring
of relevant parameters such as pH, absorbance, turbidity, etc.
* General waste minimisation options for dyeing:
Operate at lowest possible bath ratio --
this leads to a reduction in operating costs, water consumption, chemical
use, energy use and less effluent discharge; Minimise stripping and/or
redyeing procedures; Avoid shading additions; Avoid the use of detergents to
wash fabric after reactive dyeing; high temperatures are just as effective;
Minimise auxiliary use.
The greatest costs in reprocessing are
associated with the cost of dyes and chemicals -- typically, the costs can
increase by as much as 30%. In dyeing polyester, avoid the use of carriers
by upgrading dye machinery or replace with less harmful alternatives. Good
fabric preparation increases the chance of right-first-time dyeing as
fixation is improved. Dye fixation onto cotton can be improved by
mercerising the yarn or fabric prior to dyeing.
Pollutants associated with printing
include suspended solids, solvents, foam, colour and metals, and in general,
large volumes of water are consumed during the washing-off stages. The main
areas of waste minimisation in printing include raw material conservation,
product substitution, process and equipment modifications, material
handling, scheduling and waste recovery.
Other options include:
Waste minimisation in the design stages can eliminate the need for dyes
Careful selection of surfactants.
Reducing air emissions by replacing solvents with water-based
Routine and careful maintenance of printing equipment.
Training employees in the practices of good housekeeping.
water from washing the print blanket.
Turning off wash water when
machine is not running.
Installing automated colour kitchens.
left over print paste.
Removing excess paste from drums, screens and
pipes by dry techniques (wiping with a squeegee, etc) before washing
with water. This reduces the colour load discharged to drain.
scheduling to prevent expiration of print pastes before use.
Investigating alternatives to urea as this increases the nitrogen in
There are a number of finishing processes that
are carried out on the fabric after dyeing and/or printing. These can be
achieved by chemical or mechanical methods.
Some waste minimisation
options are listed below :
Design fabrics such that the need for
chemical finishes are minimised.
Use mechanical alternatives to
Use low add-on methods.
Avoid mix discards through careful preparation.
Install automated chemical dispensing systems.
Train employees in good
Use formaldehyde-free cross-linking agents.
Reduce solid waste by reducing the need for selvedge trimming through
better width control, training workers and collecting selvedge trim for
Investigate the use of spray application of finishes as these
have a low add-on and require no residual dumping at the end of a run.
End-of-pipe treatment methods
Once waste minimisation has been carried
out in the factory, effluent will still be produced that will require
some form of treatment prior to disposal to sewage, river or sea.
Prior to the installation of any end-of-pipe
treatment method, it is essential to carry out segregation of the
effluent streams to separate the contaminated streams from the
relatively clean streams for treatment. This result in a more effective
treatment system as a smaller volume of waste water is treated
(resulting in lower capital and operating costs) and it allows for the
use of specific treatment methods rather than trying to find one method
to treat a mixture of waste with different characteristics. The
segregated clean streams can then be reused with little, or no,
treatment elsewhere in the factory.
two possible locations for treating the effluents, namely, at the
textile factory or at the sewage works. The advantage of treatment at
the factory is that it could allow for partial or full re-use of water.
The following technologies have all been used: Coagulation and/or
flocculation, membranes (microfiltration, nanofiltration, and reverse
osmosis), adsorbents (granular activated carbon, silica, clays, fly ash,
synthetic ion-exchange media, natural bioadsorbants, and synthetic
bioadsorbants), oxidation (Fenton's reagent, photocatalyis, advanced
oxidation processes, ozone) and biological treatment (aerobic and
Since the effluent from the textile industry is complex and
variable, it is unlikely that a single treatment technology will be
suitable for total effluent treatment and water recycling.
Chemicals are added that form a precipitate which,
either during its formation or as it settles, collects other
contaminants. This precipitate is then removed either through settling
or by floating it to the surface and removing the sludge. This is a
well-known method of purifying water. Both inorganic (alum, lime,
magnesium and iron salts) and organic (polymers) coagulants have been
used to treat dye effluent to remove colour, both individually and in
combination with one another. With the changes in dyes and stricter
discharge limits on colour, inorganic coagulants no longer give
satisfactory results. They have the added disadvantage of producing
large quantities of sludge.
Organic polymers show improved colour
removal and produce less sludge, but then may have detrimental effects
on the operation of the sewage works. Cationic polymers have also been
shown to be toxic to fresh water fish. Alum is effective in removing
colour from textile effluent containing disperse, vat and sulphur dyes,
but is ineffective against reactive, azoic, acid and basic dyes.
However, it does have the advantage of reducing phosphorous levels,
thereby improving the operation of sewage works.
methods that are available for effluent treatment are microfiltration,
ultrafiltration, nanofiltration and reverse osmosis. In general,
nanofiltration or reverse osmosis are the most effective processes for
removing colour and recovering water. The drawbacks of these processes
are the high capital costs, the fact that the concentrated effluent
still has to be treated, and membrane fouling.
The most frequently
tested method is reverse osmosis. The effluent is forced under moderate
pressure (1.5 to 4 MPa) across a semi-permeable membrane to produce a
purified permeate and a concentrate. This process can remove up to 99%
of salts and the complete removal of most organic compounds. The
concentrate will require further treatment prior to disposal as the
level of impurities are up to six times that of the original effluent
stream. In nanofiltration, the membrane acts as a molecular filter,
retaining polyvalent ions and compounds with a molecular mass greater
than 200. The concentrate contains almost all of the organic impurities
and a large proportion of the polyvalent inorganic salts and requires
further treatment prior to disposal. The permeate contains the
monovalent ions (eg, sodium and chloride ions).
This method of effluent
treatment has been found to be effective in the treatment of dyebaths
from reactive dyeing where sodium chloride is used as the electrolyte,
as the permeate produced contains the salt and is virtually colourless,
and therefore, suitable for reuse in the reactive dyeing process, saving
both water and the cost of the salt. Ultrafiltration and microfiltration
as stand-alone treatment methods are only suitable for reducing COD and
suspended solids from solution. They are effective in combination with
other treatment methods such as coagulation/flocculation. They are also
useful for the partial removal of colour and organics prior to discharge
to sewer. Microfiltration removes colloidal material such as disperse
and vat dyes.
In order for an adsorbent to work effectively,
the concentration of the impurities in the effluent stream must remain
fairly constant to prevent the release of the adsorbed material back
into the effluent if the concentration falls. Activated carbon is the
most commonly used adsorbent and it is effective in removing organic
components from the effluent (but not inorganic compounds). Once
saturated, it must be regenerated or disposed of.
costly, and in most cases it is trucked off site and disposed of in
landfill. Care must be taken with the disposal method as the organics
may leach out over time and cause pollution problems at a later date.
Other adsorbents include inorganic compounds such as silica, cinder ash
and various clays. Trade name adsorbents such as Macro-sorbs and COLFLOC
have been shown to be effective at removing colour from reactive dyebath
effluent, although disposal of the sludge may be problematic.
Bioadsorbants are naturally occurring polymers that are biodegradable
and have structures that allow the adsorption of species within them, or
which act as ion-exchangers. Synthetic cellulose bioadsorbants have also
been developed and preliminary investigations into their use for
removing colour due to reactive dyes show promising results
Oxidants decolourise dyes by breaking down the dye molecule. Commonly
used processes are ozone and Fenton's Reagent. Ozone has been
investigated in a number of studies. It has been found that dye
wastewaters react differently depending on the composition. Effluent
containing sulphur and disperse dyes are difficult to decolourise,
whereas colour due to reactive, basic, acid and direct dyes is removed
fairly easily. The main drawback with installing an ozonation plant is
the high capital and operating costs.
However, improvements in generator
and contacting equipment design, together with increasingly strict
environmental legislation will probably lead to a more widespread
application. Fenton's Reagent consists of ferrous salt (usually sulphate)
and hydrogen peroxide. The reaction is carried out at a pH of 3 and
involves the oxidation of ferrous ion to ferric ion with the
simultaneous production of the hydroxyl radical. This radical is a
powerful oxidizing agent and will attack organic compounds and cleave
the bonds. In the case of dye molecules, this would lead to
A disadvantage (in terms of costs for the discharger)
is the production of ferric hydroxide sludge, but it is thought that
this sludge is advantageous to the biological treatment system. Other
oxidation methods include the use of ultraviolet light in conjunction
with a photocatalyst (titanium dioxide), or other chemical agents such
as hypochlorite (the use of which is not encouraged as chlorinated
organic species may be formed which are themselves toxic to the
The main drawback of these above methods is that it is not
known what degradation products are formed from the oxidation process
and it may be the case that these end products, although colourless, may
be more toxic than the original dye molecules.
The majority of sewage works are based on the
principle of aerobic treatment, where the incoming effluent is exposed
to bacteria which convert the components into carbon dioxide and sludge,
which is then sent to an anaerobic digester for further treatment. It
has been found by a number of researchers that aerobic treatment methods
are not sufficiently able to treat the colour from the textile industry,
and any colour removal that does take place is due to adsorption onto
the sludge, rather than degradation of the dye molecule.
Anaerobic digestion is the biodegradation of complex organic
substances in the absence of oxygen to yield carbon dioxide, methane and
water. It is an effective process for treating high COD wastes (eg,
size, desize washing and scouring) and the methane that is produced can
be utilised as energy for heating, etc. The reducing conditions in an
anaerobic digester have been found to cause decolourisation of azo dyes
through cleavage of the azo bond and subsequent destruction of the dye
chromospheres. Complete mineralisation of these degradation products
does not take place and aromatic amines may be present in the effluent
from the digester.
The textile industry emits a wide
variety of pollutants from all stages in the processing of fibres and
fabrics. These include liquid effluent, solid waste, hazardous waste,
emissions to air and noise pollution. The consumption of energy must also be
taken into account as the fuel used to provide this energy contributes to
the pollution load. In general, effluents that are high in COD are most
effectively treated by biological methods, either aerobic or anaerobic.
There are a number of methods for
removing colour from effluents, depending on the class of dye used, but the
most effective over the range of dyes is oxidation methods (such as Fenton's
Reagent) or membrane treatment using reverse osmosis. Effluents that are
high in BOD and SS are best removed through coagulation and flocculation
methods followed either by settling or dissolved air flotation. Those
effluent streams containing alkaline (mercerising and bleaching) can be
treated by membranes (ultrafiltration) or evaporation and reused in the same
The same is true for synthetic sizes
where they can be recycled after filtration. As mentioned previously, there
is no one single treatment technology that can effectively treat the final
effluent from the textile industry and a combination of the available
methods is necessary in order to achieve the required discharge standards.
It is important to investigate all aspects of reducing wastes and emissions
from the textile industry, since this will result in not only improved
environmental performance, but also substantial savings for the individual
1. Susan Barclay and Chris Buckley:
Waste Minimisation Guide for the Textile Industry -- A Step Towards Cleaner
Production, Volume I.
2. Turner MT: Waste Water Treatment and Re-use within the Textile Industry,
Water Services Annual Technical Survey, 1978.
3. Anon: Economics of Reclaimed Water in Dyeworks, International Dyer and
4.Brent Smith: Waste Minimisation, NCSU College of Textiles, Raleigh, N C.
5. Montgomery V: Reclaiming Water, Modern Textiles, 1976, Vol 57, No. 8.
6 .Clarke E A and Steinle D (1995): Health and Environmental Safety Aspects
of Organic Colourants. Review in Progress Colouration 25, 1-5.
7. Laing I G (1991): The Impact of Effluent Regulations on the Dyeing
Industry. Review in Progress Colouration 12: 56 - 70.
8. Pollution Research Group (1983), A Guide for the Planning, Design and
Implementation of Wastewater Treatment Plants in the Textile Industry. Part
One: Closed Loop Treatment / Recycle System for Textile Sizing / desizing
Effluents. Water Research Commission, Pretoria, South Africa.
9. Textile Federation (1999), Textile Statistics and Economic Review
11. National Productivity Council (1994), From Waste to Profits: Guidelines
for Waste Minimisation, New Delhi, India.
Note: For detailed version of this
article please refer the print version of The Indian Textile Journal
September 2008 issue.
Department of Textile Technology,
Bannari Amman Institute of Technology,
Sathyamangalam, Tamil Nadu.
J Joshuva Alexander
Department of Textile Technology,
Bannari Amman Institute of Technology,
Sathyamangalam, Tamil Nadu.