|
Introduction
Bangladesh is heavily involved in textile production and export.
A lot of textile mills were established in the country mostly in
and around Dhaka city in last two decades. A large number of
these mills generate and discharge waste waters. It was reported
by the various organization and monitoring authority that the
pollution of the rivers and canals in and around Dhaka is well
above their acceptable level. Various agencies are trying to
safeguard the clean environment. Bangladesh Government has
already issued various forms of warning and red alerts to the
concerned industries. A lot of textile wet processing industries
have already installed effluent treatment plants (ETP) and many
of them are at various stages of installing ETP's.
There are various types of textile wet processing plants e.g. (i)
the woven dyeing plants and (ii) knit dyeing plants. The
technique for dyeing of textile materials mainly depends on the
type of fiber. Some widely used fiber that are produced and
exported from Bangladesh includes Cotton, wool, silk, linen,
polyester, nylon, viscose, acrylics etc. These fibers can also
be classed into two main classes e.g. (i) Natural fiber-those
produced naturally e.g. cotton, jute, wool silk etc. (ii)
Synthetic fiber- man made fiber that are produced from petroleum
product. Dyeing of the above mentioned fibers are somewhat
different and each of them requires a different class of dye.
Different dyestuffs require different types of chemicals and
auxiliaries to apply them into the fabric or textile product. As
a result the characteristics of the textile waste water vary
significantly. Due to variation of characteristics, textile
waste water requires various types of techniques to treat them.
Description of
Textile Indu-stries Polluting Environment
Textile wet processing plants of Bangladesh may be classed as (i)
Knit dyeing units, (ii) Woven dyeing units, (iii) Denim plants,
(iv) Printing units, and (v) Garments washing units.
Knit Dyeing Units
 Knit dyeing industries are one of the most highly environment
polluting textile industries in Bangladesh. These are mainly
engaged in processing export oriented knit fabrics. Different
types of textile products require different methods of treatment
procedure. The treatment procedure and machines of woven fabric
dyeing and knit fabric dyeing are quite different. This is
because knit fabrics are soft and require gentle handling while
woven fabrics are relatively stiffer and may be subjected to
tougher treatment options. For this reason knit fabrics are dyed
in winch types of dyeing machine where treatment is carried out
in a very high M:L ratio like 1:150-200. It was found in various
studies that nearly 150 to 200 liters of water is required to
dye one kg of knitted goods. Considering all the factors it was
found that a knitting factory of 10 ton production capacity
generates nearly 100 to 150 M3 of waters per hour. However all
the water mentioned above are not equally harmful. Some are very
severely polluted while some are mildly polluted. On average, it
was found that 50% of waters are polluted and needs to be
treated and the rest of the water can discharged directly or
subjected to very mild treatment. Thus a general guide line for
knit dyeing is that for a factory of 10 ton dyeing capacity may
require effluent treatment plant of 40 60 M3/hr treatment
capacity.
Woven Dyeing Units
Woven dyed fabric is dyed in a different way than knitted
fabrics. The volume of waste water generated from a woven dyeing
factory is very low in comparison to knit dyeing process. Apart
from this the characteristics of woven dyeing plants are
different from those of knit dyeing plants. Before weaving,
sizing is carried out to increase the strength of warp yarns.
The main component of sizing is Starch. Wet process starts with
desizing to remove the starch and other sizing agents from the
fabric otherwise subsequent process and dyeing will not be
perfect. The discharge from a desizing unit is highly polluted.
Some of the pollutants of woven dyeing units are as follows:
Starch and other sizing materials, unfixed dyes, poor
washing-off of dyes, machine cleaning wastes during start-up,
shut-down and changes of color and style, salts and alkalies
etc. Generally woven dyeing is carrie dout at a very low m:l
ratio which could be as low as 1:5 (for continuous dyeing) hence
the quantity of waste water is very low but the level of
toxicity of the effluent is very high.
Denim Plants
Denim plants are particularly weaving plants and produce denim
fabrics. In denim plant dyeing and sizing is carried out on the
warp yarns and after weaving various types of finishing
operations are carried out. The effluent discharged from sizing
and dyeing units are highly polluted as compared to that of all
other wet processing plants and cause substantial impact on the
environment. Off course the volume of effluent is relatively
lower than that of knit dyeing. The characteristics of dyed
effluent of the denim is somewhat different from that of knit
dyeing plants as because vat dyes are used for denim plant while
reactive and dispersed dyes are used for dyeing knit goods,
however the quantity of generated effluent is extremely low e.g.
1:1.5.
Printing Units
Pollutants associated with printing include suspended solids,
solvents, foam, color and metals, and in general, large volumes
of water are consumed during the washing-off stages. The main
areas of environmental pollution of textile printing includes
dyes containing metals, objectionable surfactants, air
emissions, water from washing the print blanket, leftover print
paste, excess paste from drums, screens and pipes, use of urea
as this increases the nitrogen in the effluent. Like denim the
quantity of effluent of textile printing industry is very low
but highly polluted.
Garments Washing Units
At present there are a lot of garment washing plants which are
operating in Bangladesh. There are basically two types washing
e.g. (i) normal washing and (ii) denim washing. The denim
washing involves a lot of operations like desizing, enzyme
washing, finishing etc.
These can be achieved by chemical or mechanical methods. Process
involves use of volatile chemical. The pollution loads from
garments washing plants are not much but quantity is large and
requires some sort treatment.
Important
Pollutants
Various types of toxicities are found in the textile
wastewaters. However in order to monitor them Department of
Environment (DOE) has identified the Biochemical Oxygen Demand (BOD),
Chemical Oxygen Demand(COD), Total Suspended Solids (TSS), Total
Dissolved Solids (TDS), pH, Oils and grease, color and
temperature etc. are most harmful for environment. The polluting
parameters of other industries will vary to some extent.
Pollutant concentration in effluents for various textile
industries varies significantly.
Effluent
Treatment Methods
Effluent treatment is an important subject from undergraduate to
Ph.D. level students. Various philosophies, thoughts and
approaches were suggested to deal with textile as well as other
effluents. Some of the important effluent treatment steps are
described below.
Effluent Segregation
 The effluent generated from a textile wet processing plant is of
varying type having different level of toxicity and chemical
characteristics. For example the effluent characteristics of
Scouring, Bleaching and Dyeing and printing will be quite
different from each other. One of the important task of any
effluent treatment approach would be to separate them according
to their level of toxicity. The idea is that at first vigorous
treatment is carried out with the most polluted waters and at
certain stage the less polluted waters are added to the streams.
Off course the cost of chemicals is almost same whether effluent
treatment is carried out together or separately, but it the cost
of power and capacity and cost of motors and pumps makes a big
difference. It can be summarize that from economic point of
view, it is better to treat small amount of highly toxic
effluent rather than a treating a large amount less toxic
effluent, because in the latter case the capacity of motor and
pump would be higher and more electricity will be required to
run those motors and pumps. Therefore one of the important task
of effluent treatment approach is to separate or segregate the
effluents according to their level of toxicity. 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 finding 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.
Treatment
Technologies
Effluent can be treated in a number of different ways depending
on the level of treatment required. These levels are known as
preliminary, primary, secondary and tertiary (or advanced).Three
mechanisms for treatment can be divided into three broad
categories: physical, chemical and biological.
ETP manufacturers use these mechanisms and processes together
either fully or partially in a effluent treatment plant.
Preliminary or Pretreatment
Removal of large solids such as rags sticks, grit and grease
that may damage equipment or result in operational problems.
Preliminary treatment consists of mainly (i) Screening which is
adopted in a effluent treatment plant to remove relatively large
solid wastes. and (ii) Equalization & Skimming - to remove
grease & oil and homogenize and to remove some BOD & COD.
Primary Treatment
Primary treatment is intended for removal of floating and
settleable materials i.e. suspended solids and chemical organic
matter. Primary treatment consists of the following treatments;
A. Coagulation - to coagulate the suspended solid to coagulate
B. Flocculation - to flocculate by coagulants
C. Neutralization - to adjust the pH between 6.5 to 8.5
D. Sedimentation to precipitate small suspended solids
All the above processes contribute to removal of substantial
amount of all the polluting parameters.
Secondary Treatment
Secondary treatment is carried out to removel biodegradable
organic matters which include removal of BOD and COD &
decomposition of organic matter. Biological treatment can be
aerobic and anaerobic.
Tertiary/Advanced Treatment
 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. Tertiary treatment includes removal of residual
suspended solids/ dissolved solids. Most of the effluent
treatment plants use the Adsorbents (granular activated carbon,
silica, clays, fly ash, synthetic ion-exchange media, natural
bio adsorbents, synthetic bio adsorbents). In Bangladesh the
following two types of filters are used:
A. Granular Media Filtration-to removes TSS
and any other pollutants in the form of particle. This filter is
also useful to protect the Activated Carbon Filter (ACF) from
overloading by pollutants such as particulates, organics.
B. Activated Carbon Filter- Carbon adsorption is a
proven process in tertiary treatment for the processing
biologically treated wastewaters, and is one of the many
processes used in the advanced treatment of waste-waters. The
ACF is used to remove relatively small quantities of refractory
orga-nics, as well as inorganic such as sulfides and heavy
metals remaining in an otherwise well-treated wastewater.
Sludge Management
& Disposal
Sludge disposal to separate the sludge from the thick slurry and
then dispose of the sludge as dried cake
Description of Important Treatment Process
Screening, Equalization and Skimming
 Screens are very simple materials having iron bars in the form
of square grids. Effluent is allowed to pass through the grid
when large and coarse solid materials are arrested by it
allowing smaller particles and effluent to pass through. In some
several grids are use with diminishing grid sizes.
Equalization tank is a large chamber which is designed for
retention time of 12 hours. This means if the rate effluent is
30 cubic metre then the capacity of the equalization tank has to
be 30 X 12 = 360 cubic meters. The equalization tank is
specially built where air is blown by two blowers alternately
round the clock on continuous basis. The purposes of
equalization are (i) to supply oxygen so that DO level increases
and (ii) to mix various types effluents and (iii) to reduce the
temperature of the water.
On the top surface of the equalization tank there is a scrapper
used to skim the oily substances.
Coagulation and Flocculation
From the equalization tank the effluent is taken to a small tank
which is also called flash mixing tank of small retention time.
In the flash-mixing tank coagulants like lime (Calcium
Hydroxide) and flocculants like ferrous sulfate (FeSO4) are
dosed. This is done for coagulation and removal of the total dye
particles. The basic idea of adding coagulant is to bring
together all the suspended and dye particles so that they can be
precipitated out in the flash mixing tank and flocculation tank
by coagulation and flocculation mechanism. The chemical reaction
that occurs in the coagulation and flocculation process is shown
below;
CaO + H2O Ca (OH) 2
The above reaction take place in flash mixing tank when lime
reacts with water and we get calcium hydroxide solution. This
solution reacts with the ferrous sulfate solution, which as
follows
Ca (OH) 2 + FeSO4 CaSO4 + Fe (OH) 3
+ FeSO4 (Unreacted) + Fe (OH) 2
Adequate quantity of polyele-ctrolyte polymer solution is dosed
in the flocculation tank followed by flash mixing tank to
enhance the process of color removal by the flocculation
process. A substantial amount BOD and COD etc. are removed in
the coagulation and flocculation process.
Precipitation and Sedimentation in Tube Settler -1. (To
remove the flocs materials)
From the flocculation tank the effluent is taken to the tube
settler- 1 tank where the dyes and suspended particles are
precipitated. The flocs formed are removed in the downstream
tube settler 1 by the help of tube settler media. The effluent
will further flow by overflow system to the pH correction tank
where requisite quantity of acid will be dosed and pH will be
adjusted as per the requirement.
pH Correction (To adjust the inlet pH)
The effluent from tube settler- 1 tank is then taken to the pH
correction channel for neutralization, where 33% HCl acid is
dosed for neutralizing the pH value around 7 to 8. It may be
mentioned that different dyed effluent may generate different pH
level. For acrylic and basic dyeing the pH is on the acidic side
as dyeing medium is acidic, in that pH correction should be made
by adding additional alkali. The pH correction tank is designed
for hydraulic retention time of around 1 -2 minutes and is
provided with slow speed agitator for thoroughly mixing of waste
with acid/alkali to maintained pH value.
Biological Treatment
The objective of biological treatment of industrial wastewater
is to remove, or reduce the concentration of organic and
inorganic compounds. Biological treatment process can take many
forms but all are based around microorganisms, mainly bacteria.
These microorganisms use components of the effluent as their
food and in doing so break them down to fewer complexes and less
hazardous compounds. In the process the microorganisms increase
in number. There are two main types of processes, these involve
suspended microbial growth (e.g. activated sludge) and attached
microbial growth (e.g. fixed film). With both approaches large
populations of microorganisms are brought into contact with
effluent in the presence of excess oxygen. In both systems the
microbial population has to be retained in the reactor. With
suspended growth systems microbes grow in small aggregates or
flocs (this is known as activated sludge).Activated sludge (AS)
leaves the reactor with the treated effluent but is settled out
in a clarifier and returned to the aeration unit. If the amount
of AS is excessive some may be disposed of rather than being
recycled.
In fixed film systems the microbial population grows as a thin
layer (a ìbio-filmî) on the surface of an inert support medium.
The classical fixed film system is known as a percolating or
biological filter and uses small stones as a medium to support
microbial growth. In the more modern system microbes grow on
plastic supports. In the traditional percolating filters
effluent is sprayed over the medium and trickles through a
packed bed with oxygen entering from the air. In more recent
reactor designs, the medium (usually plastic) is submerged in
effluent and air is blown into the base of the reactor.
Traditional percolating filters require large areas of land and
are unlikely to be of use in Bangladesh due to land costs.
Submerged fixed film reactors using plastic media require much
less land and are potentially of value in treating textile
wastes. These plastic media are now widely used and known as
Moving Bed Biological Reactor (MBBR).
MBBR systems require
a final clarifier to remove particles of biofilm that become
detached from the medium. However, this material is not recycled
to the reactor.
The chemical
reactions that took place in the MBBR reactor can be defined
according to the following three processes;
A. Oxidation process
COHNS + O2 + Bacteria + DAP and UREA CO2 + NH3 + Energy + Other
end Products
DAP and Urea are used as food for the microorganism.
B. Synthesis Process
COHNS+O2+ Bacteria C5H7NO2 (New bacteria)
C. Endogenous Respiration
C5H7NO2 + 5O2 CO2 + NH3 + 2H2O
Apart from the above basics reaction there are some other
reactions that take place in the MBBR reaction tanks. During
aeration the oxygen reacts with C, S and N which is shown below.
C + O2 CO2
S + O2 SO2
N + O2 NO2
Some untreated ferrous sulfate and ferrous hydroxide reacts with
oxygen and the reactions are as follows,
FeSO4 + O2 Fe2 (SO4) 3
Fe (OH) 2 + O2 Fe (OH) 3
While most of the activated sludge is recycled some may be
surplus to requirements and needs to be disposed of, as does
detached biofilm from film reactor. This material must be
disposed appropriately so that the pollutants now present in
this sludge do not enter the water cycle. The treated liquid is
discharged to the environment or taken for further treatment
depending on the desired standard of effluent quality or the
required use of the wastewater. Biological treatment plants must
be carefully managed as they use live microorganisms to digest
the pollutants. For example some of the compounds in the
wastewater may be toxic to the bacteria used, and pre-treatment
with physical operations or chemical processes may be necessary.
It is also important to monitor and control pH as adverse pH may
result in death of the microorganisms. The ETP must be properly
aerated and must be operated 24 hours a day, 365 days a year to
ensure that the bacteria are provided with sufficient food (i.e.
wastewater) and oxygen to keep them alive. Like humans
microorganisms need a balanced diet with sources of carbon,
nitrogen, phosphorus and sulfur. While textile wastes have
enough carbon and sulfur (sulfate) they are generally lacking in
nitrogen and phosphorous containing compounds. If the
microorganism are to grow and work effectively they are likely
to need addition of nutrients. Normally materials such as urea
and ammonium phosphate are added. It is possible to replace
these nutrients by substituting the liquid portion of effluent
from toilets, which is rich in nitrogen and phosphorus
containing chemicals (the solid portion may cause problems).
Both activated sludge and fixed film systems can produce high
quality effluent but both in have advantages and disadvantages.
In the AS process the settling and recycling of AS to the
aerobic reactor is vital, and the settling process can be
difficult to accomplish. Fixed film/MBBR systems do not require
recycling of biomass and so do not present this problem.
Anaerobic digestion
 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 (e.g. 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 decolorisation of azo dyes through cleavage of
the azo bond and subsequent destruction of the dye chromophore.
Complete mineralisation of these degradation products does not
take place and aromatic amines may be present in the effluent
from the digester (Carliell et al., 1995).
Effluent treatment plants in Bangladesh
 According to the Environmental Conservation Rules 1997,
industrial units and projects have been classified into four
categories (Green, Orange A, Orange B, and Red) based on
environmental impact and location. Fabric dyeing and chemical
treatment industries fall under the Red category and when
applying for site clearance must submit to the Department of
Environment an ETP plan, including the layout and location. When
the design has been approved by the Department of Environment
and the ETP has been constructed, then red category industries
can apply for an environmental clearance certificate.
From very rough estimation there are nearly 600-700 textile wet
processing industries in Bangladesh. So far nearly 300 to 400
factories have already set up or at various stages of setting up
effluent treatment plant (ETP)
Factors to Consider
Any factory needing to install an ETP has to consider several
factors. For example, information about the factory's wastewater
is required, including quantity and quality. To get this
information the factory will have to take samples and have them
analyzed at a reputable laboratory. Some of the factors to be
considered are presented in the schematic on the next page.
In Bangladesh mainly two types of effluent treatment plants are
operating e.g (i) Physico Chemical Plants(PCP), (ii) Biological
Plants (BP). Both of them uses the above mentioned treatment
processes fully or partially.
Some key information about two types of plants
1. Initial investment- Cost establishment of PCP is very low in
comparison to BP, nearly 1:15-20
2. Land area required by PCP is very small in comparison to BP
3. Running Cost of PCP is nearly 20 Tk/M3 of effluent while this
cost of BP is around Tk 5-6. Thus PCP adds nearly 3-5/kg of
knitted goods while BP adds only Tk 1-1.25.
Environmental Pollution by other means
Emissions to Air
Air pollution results from various textile processes and from
energy production and has been identified as the second greatest
pollution problem for the textile industry. Those from process
emissions include volatile organic substances and particulate
matter from the printing, dyeing, and curing of fabric, and the
handling of chemicals. The emission of volatile organic
substances may lead to the production of photochemical oxidants
and cause unpleasant odours. The textile industry requires a
great deal of heating and the type of fuel determines the nature
of the pollutants. Boilers are one of the major point sources
for air emissions, producing nitrogen and sulphur oxides.
Fugitive sources of air pollution include volatile emissions
from processed fabric stored in warehouses and chemical spills.
Emissions to air can be minimized by designing products that do
not require the use of volatile chemicals, optimizing boiler
operations and reducing the use of solvents.
Noise Pollution
There are various ways noise is created in textile industries.
The main source of noise is due to running of the equipments. So
far two types machines were diagnosed to be vulnerable for human
being and these are various types of looms and rotor spinning
machine. Apart from this noise is also arises from fan systems,
transport to and from the industry.
Hazardous Waste
Hazardous wastes are defined as wastes, or combination of
wastes, that pose a threat to human health or living organisms
because they are lethal, non-degradable, and persistent in
nature and can cause detrimental cumulative effects. These
wastes can be solids, liquids, gas or sludges Most textile
operations produce little or no hazardous waste, but some
(10-20%) may be generators of hazardous waste. The main sources
from the textile industry are heavy metals and solvents. Other
components include acids, alkalis, bleaches, adhesives,
polymers, cross-linking agents, detergents, dye carriers,
chemical finishes, biocides, weed killers, paint strippers and
solvents. The remainder (75% (m/m)) is considered to be
non-hazardous. Carriers containing chlorinated aromatics are
considered to be extremely hazardous and their use today is
limited (Swedish Environmental Protection Agency, 1989). Organic
phosphorous compounds (e.g. flame-proofing and completing
agents) may contain organic bromine compounds, such as
decabromine diphenyl oxide and antimony trioxide, which are
bioavailable and bioaccumulative.
The most effective means of reducing hazardous waste from the
textile industry is one of avoidance by pre-screening all
chemicals. This is most easily achieved by studying the material
safety data sheet (MSDS). If hazardous chemicals are required,
proper handling and storage is important and workers should be
trained in these areas.
Conclusion/ Suggestions
a) Proper management of wastewater will reduce the overall
volume of effluent. Not all effluent in a textile wet processing
plant are equally harmful for the environment, dyeing effluents
are more harmful than rinsing waters, the rinsing water should
be separated from dyed water and can be discharged without or
with minimum treatment.
b) If all the factories do not install appropriate ETP and do
not run them properly then there will be an unequal price
competition among fabric producers where the honest ETP owners
will always lose the battle.
c) Use of synthetic sizes rather than starch based sizes will
reduce the high toxicity of the effluent generated from denim
plants. The starch-based sizes contribute to highest amount of
BOD.
d) Government should reduce or exempt the tariffs and taxes on
the synthetic sizes, so that synthetic sizes can replace cheap
starch based sizes that contribute to very high BOD.
e) Attempt should be made to explore the idea of developing
woven sector. The material to liquor ratio of woven dyeing is
very low in comparison to knit dyeing. Less effluent means less
raw water will be required, this in turn will reduce the amount
of gas/electrical energy required to pump and heat water. The
profit margin of woven garments is also very high.
f) Attempt should be made to explore the idea of developing
synthetic production. The characteristics of effluent from
synthetic dyeing is different from that of cotton products. The
pollution load is less for synthetic. Less effluent means less
raw water will be required, this in turn will reduce the amount
of gas/electrical energy required to pump and heat water. The
profit margin of woven garments is also very high.
g) Reconstitution of processed effluent should be examined for
every stage of processing like scouring, bleaching,
mercerization, dyeing and finishing baths. This will reduce the
quantity of ultimate effluent.
h) Decisive and efficient use of dyes, chemical and auxiliaries
will reduce the quantity and toxicity of the effluent to a great
extent. Selection dyes, chemicals and auxiliaries that generate
lower pollutants will reduce ultimate pollution load.
i) Like many developed countries, Government can offer free
water and wastewater test facilities. Though some people may
take advantage of this facility but for greater interest of the
country this type of initiative will help in understanding,
administering and researching the over all-environmental
pollution scenario.
j) Environment is violated not only by the textile processing
plants but also by many other factories and establishments like
slaughtering houses, sweet manufacturers, bakeries, poultry
farms etc. Their pollution load should be examined and should
also be brought under strict scrutiny.
k) Universities may research on managing and handling
effluent..Research is essential to study the effect and toxicity
of various dyes, chemicals and auxiliaries used in our country.
Research is also necessary to study and upgrade the existing
treat-ment procedures in order to reduce operating cost and also
to combat future violation of the environment.
l) Entrepre-neurs can be encouraged to set up industries far
away from the crowded greater Dhaka. This will at least reduce
the pollution burden on the Dhaka dwellers to some extent.
Setting up of industries near big rivers or coastal areas could
be a much better option because effluents can be directly
released into the .¨
|