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Pollution control in the textile industry - the chemical auxiliary manufacturer's role
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INTRODUCTION
With new legislation and the establishment of the National
Rivers Authority (NRA), pressure is being applied to all
effluent dischargers to reduce watcFborne pollution emission.
This even applies to the water service companies
themselves. The result has been a general tightening up of
consent limits and increased charges to cover the full
treatment costs incurred by the water companies.
The challenge facing manufacturing industry is to find
effective and inexpensive ways of treating its effluent prior
to discharge in order to meet new consents and reduce the
overall cost for disposal. However, some form of pretreatment
such as screening. equalisation or pH adjustment is
usually necessary, whether the discharge is to the municipal
treatment works or direct to a water course (Figure 1). After
pretreatment the effluent. still containing the full polluting
load, may be discharged to the local sewage works or may
undergo on-site treatment, either to obtain partial load
reduction for discharge to the sewer or to achieve more
complete reduction for discharge to a water course.
I JSDC Volume 107 May/June 1991 215
1
Industrial
treatment
Industrial
treatment
Receiving waters
Figure I - Wastewater management alternatives
The advantage for industry of relying completely on the
local sewage works for effluent treatment is that capital
investment and operational responsibilities are kept to a
minimum. Disadvantages are that problems may be encountered
in staying within consent limits and charges incurred
may be very high in order to meet the costs of higher
standards imposed on the water company by the NRA.
TREATMENT METHODS
Typically, textile waste waters have high biological oxygen
demand/chemical oxygen demand (BODKOD), a substantial
proportion ol which is represented by substances present
in a highly emulsified and/or soluble form. The organic
polluting load can be many times greater than that in
ordinary domestic sewage and can also be highly coloured.
A number of pretreatment processes such as equalking/
balancing, gravity adsorption or neutralisation are available,
and actual treatment can be achieved by chemical oxidation,
ultrafiltration, adsorption, and biological or physico-chemical
techniques. Selection of the appropriate method of
treatment is influenced by a large number of factors related
to each effluent characteristic, such as relative costs, levels
of treatment required or site restrictions, etc. For example,
biological and physico-chemicalt reatments areo ften used in
tandem to obtain maximum removal of organics in textile
waste water. The dual use of methods combats certain
organics that are not biodegradable, as well as othero rganic
constituents that may not be amendable to chemical precipitation.
PHYSICO-CHEMICAL TREATMENT
Precipitation/coagulation
Effluent will contain impurities in dissolved, colloidal and
suspended forms. The first stage of treatment involves the
precipitation and coagulation of these impurities to produce
microflocs, either by pH adjustment (such as acid cracking),
or by inorganic coagulants (Gultivalent metals) or by organic
coagulants.
Organic coagulantsare low molecular mass, highly charged
polyelectrolytes that are usually cationic, and can be used
either as an alternative to, or in conjunction with, inorganic
216 JSDC Volume 107 May/June 199 1
coagulants. Their mechanism can be explained in terms of
the charge patch model (Figure 2).
Inter-particle
Electrostatic
attraction brings
particles together
Figure 2 - The 'charge patch' coagulation model
Flocculation
When the impurities in the waste water are in the form-of
microflocs and other suspended solids, the second stage of
flocculation aggregates them into larger agglomerates. This
is usually achieved by adding lowly to moderately charged
polyelectrolytes with a very high molecular mass; the charge
may be anionic or cationic. Flocculation involves adsorption
of the polyelectrolyte onto particle surfaces. These form
loops and tails which act as physical bridges across particles,
thus binding them together into p oa lymer-particle matrix or
floc, i.e. a bridging mechanism (Figure 3).
Solidfliquid separation
This is achieved by various means. including gravity sedimentation,
filtration and centrifugation. Another method
gaining in popularity is dissolved air flotation, where solids
are induced to float by introduction of microscopic air
bubbles which attach to the flocs and accelerate their rise to
the surface. The flocs form a float which is skimmed by
mechanical scrapers in the form of sludge.
\ 0 A - 7 Adsorption
”and bri \ - r
I Adsorption continues,
bridges shorten and
multiply
Adsorption and
flocculation
complete
Figure 3 - The ‘bridging’ flocculation model
EXAMPLES OF PHYSICO-CHEMICAL TREATMENT
OF TEXTILE WASTE WATERS
Within the textile industry commercially successful processes
have been developed and used for treating wool scouring
effluent, permethrin removal and colour removal.
Wool scour effluent
Liquorproduced from scouring wool fleeces is rich in wool
grease. organic and inorganic excretions (suint), dirt, vegetable
matter and detergents. It is highly polluting, with a
BOD ranging from 20 000 to 40 000 mg/l and a COD as
high as 100 000 mg/l. Gravity settlement alone is effective
only in removing the ‘heavy solids’ but does little for the
reduction of BOD/COD. However, a substantial proportion
of the colloidal dirt (stabilised by detergent) can be removed
by flocculation with polyelectrolyte. The choice of polyelectrolyte
tends to be a high molecular mass bridging flocculant,
with slight anionic or cationic charge depending on the
particular characteristics of the liquor to be treated.
In the flocculation process (Figure 4) colloidal particles
and dense solids are bound together to form large, rapidly
settIing flocs. Typicarel ductions in suspended solids (and the
accompanying reduction in COD) are given in Table 1.
Scour bowl
-c
To drain
Figure 4 -Flow diagram of a typical wool-scour treatment
plant
‘6 .
This simple process provides partial treatment, sufficient
for meeting consent limits and for reduction of costs for
discharge. Recycling of the liquor to the scouring bowls is
also possible, thus enabling further economies to be made.
TABLE 1
Typical data from treatment of wool scour effluent
COD ss
Treatment (mg/l) Reduction (%) (mg/l) Reduction (%)
Untreated 100 490 30 950
Centrifugation 91 500 9 26 200 15
60 mg/l Zetag 92
Centrifugation 60 460 40 19 370 37
100 mg/l Zetag 92
Centrifugation 50 650 50 13 400 57
Permethrin removal
Permethrin is one of the most widely used mothproofing
agents in the textile industry. Currently the use of mothproofing
agents is under threat as a result of Department of
Environment legislation, due to come into effect in 1993,
which will severely limit their discharge into the environment.
This follows EEC directives that have included mothproofing
agents on the ‘dangerous substances’ list.
Allied Colloids was asked to devise a treatment process
for removal of permethrin from spent tapes cour liquor. The
liquor was from the end bowl of a continuous scouring
process, and contained permethrin, formic acid, detergents,
salts and extracted impurities. The high formic acid content
presented certain difficulties in establishing an effective
chemical pretreatment.
Despite this, an effective system was developed (Figures
5 and 6) involving adsorption of permethrin onto an inorganic
absorbent material (Organosorb), flocculation with
non-ionic polyelectrolyte (Magnafloc 351) and gravity settlement.
Typically, the final permethrin content was 0.04 mg/
1 from an initial value of 50 mg/l, a reduction of 99.92%.