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(Late)
S.G.Rangan
Member - Energy and Power
Sub-Committee
of CII-Southern Region
Past President of IPPTA
(Indian Pulp & Paper Technical
Association)
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A Tribute to Mr S G Rangan
Mr.S.G Rangan who
passed away on 20th May
2002, was a doyen of the
Paper Industry. He was
the Past President
of IPPTA (Indian Pulp
& paper Technical
Association) and highly
respected by the Paper
Industry fraternity.
He was an active member
of Energy and Power
Sub-Committee of CII-Southern
Region. He contributed
immensely to the manual
on Energy Efficiency
at design stage"
prepared by CII- Energy
Management Cell. |
| Known
for his wit and sense
of humour as session chairman,
his absence would be felt
in the CII Conferences
and seminars. |
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Water as a natural resource, is a
vital element and necessity in the
manufacture of pulp, paper and paperboard
and for the generation of power in
the industry’s steam plants.
Mills using up to 350 cubic meter
per ton of paper are not uncommon
in our country. A large percentage
of the water requirements for the
mills comes from surface supplies
– rivers and lakes – and
the remainder comes from bore wells
of a few feet to over 6000 feet deep.
Good water in large quantities is
as essential to the manufacture of
pulp and paper as is cellulose. In
fact, water is one of the most critical
of all the raw materials used by the
pulp and paper industry. It is used
directly in the processing of pulp;
it dissolves or is mixed with the
various loading, sizing, and coloring
ingredients; and , in addition, it
is the medium which carries the fibers
through the screens and refiners to
the paper-making machine where it
plays a most important role in the
making of a sheet of paper.
The original pulp and paper mills
were generally located on the banks
of larger rivers, down which the logs
were driven; these rivers supplied
the mills with ample water for all
purposes including process demands,
generation of power, and disposal
of all the mill waste products. Improved
methods for generating and transmitting
electricity and the use of other transportation
means (particularly highway transport)
for bringing the wood and, other raw
materials to the mills and delivering
the product to the market have changed
this picture.
Recently-built mills are located at
the strategic points with regard to
distribution of products, and that
may be a long distance from a river
or lake, Sometimes, they may have
to depend on subsurface sources for
their water, and this situation affects
not only their water supply use but
also their water disposal procedures.
In any event, if sub-surface sources
are to be used as a potential primary
mill water supply, it is necessary
first to make a very through hydrological
survey of the area before making final
construction commitments. Otherwise,
the new mill may find either that
it has insufficient water for its
own growth or that its continuous
heavy demands have resulted in a reduction
of the quantity and quality of the
general water supply available in
the area. Lowering of the ground water
table in the region may result in
the failure of other wells, also.
The wide variations in water usages
may be accounted for in part by the
different requirements of the various
products produced, but this is only
one of many factors involved.
Conservation in water use is encouraged
when the cost of water is high. Another
economic factor is the relationship
between the cost of treating and recirculating
the white water and waste liquors,
(after all good fiber has been removed)
and the cost of fresh water. Local
regulations governing stream pollution
and water supply rights have marked
effects in controlling the quality
of water used.
REUSE
OF PROCESS WATER- A NECESSITY
The pulp & paper mills use large
quantities of water. Until fairly
recently in the history of paper-making,
the ingredient has been regarded as
a cheap, if not a free raw material.
This fact caused little attention
to be paid to the way in which water
was utilized in the mills, even if
it was clear to many people that this
utilization had a crucial influence
on the fiber losses to drain. Some
how, it was agreed that to make a
good sheet of paper a purge of the
system was needed the bigger the purge,
the system was needed the bigger the
purge the better paper in the end.
In today’s highly competitive
market with continuing emphasis on
more stringent effluent limits it
becomes inreasingly important that
water usage be closely monitored.
Every drop of water that is mixed
into a papermaking system incurs cost
in one or several ways. First it has
to be pumpe to the mill and often
from a considerable distance or depth.
If contaminated it is to be purified
in a plant built for the purpose and
operated at some expense. With the
exception of few cubic meters of water,
which is evaporated in the dryer section,
remaining has to be treated in wastewater
treatment plant at considerable capital
and running cost for its reuse or
disposal as effluent. Each drop of
water leaving the process, contains
some dissolved and dispersed solids
which are lost from the precess thus
causing increased production cost
and adverse effect on the environment.
The environment
and the economic considerations have
made the reuse of process water a
necessity in the operation of a pulp
and paper mill.
Literature survey and data received
against the questionnaire sent to
the Indian pulp nd paper mills (1989)
indicated that there was a wide variation
in the water requirement per ton of
paper in case of different mills which
differ in installed capacity, types
of fibrous raw material, and product
range etc. The fresh water requirement
ranges from about 100 to 350cm /tonne
of paper. By reuse of process of water
it should be possible for an integrated
mill to use not more than 100cm/tonne
of paper.
| S.No |
Section |
Water
cm/tonne |
| 1. |
Pulp mill |
30 |
| 2. |
Paper
machines and stock prep |
20 |
| 3. |
Boilers
including water |
30 |
| 4. |
Treatment
and cooling towerChemical
recovery area |
10 |
| 5. |
Miscellaneous |
10 |
| |
Total |
100 |
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WHITE
WATER
At this point, it must be remembered
that, in the washing, screening, bleaching,
and diluting of pulp and paper stock,
great saving in the water consumption
of the mill may be effected by the
reuse of white water. Whether or not
such reuse is advisable for particular
plant requires careful study. In a
mill making several grades from radically
different furnishes, there is danger
of contaminating one stock by the
non-fibrous materials which come from
another. Also, from a purely economic
viewpoint, the cost of re-circulating
white water is often far higher than
the cost of fresh water, even when
full account is the salvage values
of any valuable materials which it
contains and which would be lost when
the white water is dumped.
The relative merits of the use of
white water and fresh water in the
paper mill proper has been a subject
under discussion at every meeting
of every paper technical society for
the last thirty-odd year, and the
author does not wish to get involved
in any controversy on the subject
here.
The reuse of water is common in the
pulp and paper industry, and it is
increasing in extent. The general
criteria so far have been “How
much does it cost to treat, deslime,
and recirculate white water, and what
shall we gain over the use of fresh
water?” Most mills have answered
this question by closing the system
until they felt that they were losing
a bare minimum of valuable fibers
or chemicals and that any further
treatment and recirculation of white
water would cost many times more than
the value of the materials recovered.
The composition and volume of the
mill effluent must be considered together
in gauging the efficiency of a mill’s
efforts in water conservation. Regardless
of the degree of “closed circulation,”
the volume of the effluent will remain
that of the gross intake less the
losses due to evaporation and the
moisture content in the finished products
and the by-products, if any.
The most widely utilized of reusable
water are (a) paper machine white
water (b) decker and screen white
water, and (c) condenser cooling water.
These are employed with, or without,
screening and clarification for (a)
stock dilution to the paper machine
head box (b) shower on the paper machines
(wires, felt, etc.), thinning the
stock from deckers and to the screens,
screen showers, washing and slushing
the stock.
The reuse of paper machine white water
on the machines is common, and many
mills report that over two-thirds
of their needs are met in this manner.
EVERY DROP
of water that is mixed into a papermaking
system incurs costs in one of several
ways. First it has to be piped to
the mill and often pumped to a considerable
distance. Then it has to be purified
in a plant built for the purpose and
operated at some expense.
Attempts to close up an existing white-water
system often meet with serious difficulties
relating o runnability and, sometimes
product quality. Why is it then that
there exists today a number of mills
very successfully operating tightly,
or even completely, closed systems?
The grades produced range from fine
papers and newsprint to board and
tissue. By and large, the answer is
to be found in the layout of the systems.
Systematic design of the white-water
and broke systems goes a long way
towards eliminating potential problems.
THE WHITE
–WATER SYSTEM
Figure I shows a normal, relatively
open system. The pulp pumped in from
the pulp mill carries with it about
30 cm of water per ton of product
. Sealing water and shower water are
added in the stock prep area and on
the paper machine. In addition, sealing
and cooling waters are supplied to
the vacuum pumps, and cooling water
to the steam condensers, motors, sweat
dryers, etc. Water from all sources,
50 to 150c m /t, is collected in a
single sewer, so that all the water
becomes polluted with fiber. The white
water that is discharged is that with
the lowest concentration. The concentration.
The concentration of solids in the
effluent may be 250 mg/L
(ppm), which makes for a solids loss
of 20 to 50 kg per ton (kg/t) of the
product, i.e., 2 to 5 % of the production.
Figure 2 shows a conventional, well-closed,
but not tightly closed, system. The
net quantity of water entering the
system with the pulp is reduced by
a press or filter.
The white water from the paper machine
is divided so that there is a slight
deficiency in the white-water silo,
which receives the most fiber-rich
white water at the beginning of the
wire section and which is used in
the short circulation to the headbox.
The leaner white water from the seal
pit is divided. The more fibre-rich
water is used either to cover the
deficiency in the wire pit or the
returned via a white-water tank to
the stock preparation department for
use in diluting incoming stock. Only
the leanest seal pit water leaks out
of the system and is taken to a fiber
recovery unit. The water from the
press section has approximately the
same concentration as lean white water
and is collected in the seal pit.
It is important to collect properly
process the effluent from the press
section, because, in a well-closed
machine system, this effluent may
carry a major proportion of the total
solids load.
The wire shower water has a very low
fibre content and is therefore pumped
directly to the fibre recovery unit
Cleaned white water is used for the
wire showers and some other showers.
The seal water for the vacuum pumps
is clean because water separators
effectively disconnect the seal water
from the white-water system.
The water consumption in a closed
system of the type is typically about
20 m /t of product.(5000 gal/short
ton) . Since the amount of filtarable
meterial in the effluent can be kept
at a level of about 50mg/L, the total
permanent discharge is of the order
of 1 kg/t or 0.1 % of the production.
This does not include temporary discharges
caused by process disturbances, which
should be separately collected and
fed back into the system.
FIBRE RECOVERY,
BROKE SYSTEMS
The couch pit is probably the primary
reason why many would like to consider
broke handling and the white-water
system as one The couch pit is a convenient
place to dump all excess white water
from the wet end. The couch pit contents
are so dilute during normal running,
and during stock-off-the-wire conditions,
that it is not acceptable to the broke
system except after thickening. As
a result couch pit operation has always
been considered a problem.
Literature survey and data received
against the questionnaire sent to
the Indian pulp and paper mills (1989)
indicated that there was a wide variation
in the water requirement per ton of
paper in case of different mills which
differ in installed capacity, types
of fibrous raw material, and product
range etc. The fresh water requirement
range from about 100 to 350 m /ton
of paper (Table I,II).
RECYCLED
WATER
Water which is recycled at different
points is Pulp mill back water,Machine
back water,Wire pit ,Silo water,Seal
pit water,Sealing and cooling gland
water of pumps.
FOR
REDUCING FRESH WATER CONSUMPTION THE
RECOMMENDED STEPS
- Low pressure showers on the
wire return section should be
fed by clarified save-all water.
- Clarified krofta water should
be used for deckers & vacuum
washers.
- Rejects from 3rd centricleaner
stage should be sent over to sandtrap
(riffler) for removal of nonfibrous
solids & accepted water with
fibres should be returned to hill
screens for fiber reclamation.
- Steps were already initiated
for changing washing of CMP to
pressure washing (diffusion/extraction)
in place of deckers. Dilution
should use back water from paper
machine.
- Sealing water in vacuum pump
should be recirculated.
- Water temperature of back water
should be maintained as high as
possible for improving drainage
on wire and for reducing volume
of washing water (heat conservation).
- The pulps in mixing chest should
be neutralised with dilute Sulphuric
acid to pH 7-6 Alum addition should
be reduced step by step down to
approximately 1-1 5% preventing
precipitation of salts and subsequent
deposition due to enrichment by
closure of circulation.
FEED
BACK
Few of the above measures at the
first instance resulted in reduction
of fresh water consumption to the
extent of 25-30% i.e. 129 cM to
around 90c M /ton of paper. When
the systems were working efficiently
(double wire washer) this figure
further went down to around 60 cM
/ton of paper.
BENEFITS
- Reduced water consumption entailed.
- Reduction in Energy Consumption
due to
- Running lesser number of tube
wells
- Higher back water temperature
resulting in better drainage due
to which pulp washing efficiency
improved substantially which reduced
running period/number of washers.
- Reduction in Effluent Volume
- Reduced volume of total effluent
prevented overloading of the effluent
treatment plant.
FRESH WATER
REDUCTION IN THE PAPER MACHINE AREA:
In the paper machine area almost
the entire fresh water consumption
is in the showers of the machine
wire and the press felts.
Not much experience has yet been
gained regarding the use of white
water in felt showers. The only
method of reducing the water consumption
at the present time is to reduce
the amount used in various positions.
One factor which facilitates this,
is the introduction of synthetic
materials in the press felts, and
this has led to a noticeable reduction
of the quantity of shower water
required.
In the paper machine wire part greatest
reduction in fresh water consumption
and the best continuity has been
achieved by recirculation of the
white water and a number of systems
have been developed. The most common
is the use of cleaned white water
from the interal fiber recovery
unit (eg. Krofta save all). Thus
treated white water can be used
in the various shower of paper machine
wire by employing proper shower
design, for example self cleaning
showers. In a typical type of self
cleaning shower reduction in the
line pressure retracts a piston
to purge fibers and other suspended
solids from a clogged nozzle, which
makes them particularly well
suited for use white water.
Typical application of these showers
as claimed are for-
- Wire cleaning (return rolls)
- Knock of showers
- Pre wetting showers (Breast
roll)
- The amount of reduction in
fresh water in paper machine section
by using self cleaning showers
in the low pressure showers only
has been estimated to be around55%
to 60%. Preliminary laboratory
scale studies with a typical imported
self cleaning shower nozzle has
given encouraging results.
- There is good scope to reduce
fresh water consumption in the
pulp and paper mills, in particular
by improving the degree of recycling.
- Pulp washing & paper machine
area where substantial proportion
of freshwater is used need first
consideration.
- Studies carried out in a typical
mill, 25 to 30% reduction in fresh
water
requirement was achieved by carefully
auditing the water usage at different
points followed by simple &
inexpensive measures.
- For initiating water saving
measures the mill management should
precisely & accurately know
the total quantity of water in
use. A material balance study
therefore, is very essential in
the very beginning to know the
section and stage-wise water consumption
and the existing mode of recycling
and drainage.
- In the beginning only simpler
and cost effective methods to
reduce freshwater consumption
should be adopted.
- Reduced fresh water consumption
means saving of energy in pumping
& handling of relatively smaller
volumes, lesser material losses,
lesser effluent treatment cost
(capital & running) due to
reduction in volume. It will eventually
prove to be one of the most effective
means of preserving the environment
as paper industry in developed
countries has already started
talking of zero effluent discharge.
Many industries in India do not
give much importance to conservation
of water. The focus is mainly on
production, Capital mobilisation,
Profit making and other aspects
which have an immediate impact on
the bottom line of the company.
Environmental issue like water conservation
take a backseat. “Concern”
for the environment rather than
just “Awareness”, should
be there among these industries
for minimising the impact of pollution
on the environment.
The Ministry of environment and
Forests, Government of India vide
notification No.422(E) dated 19th
May 1993, has envisaged that the
effluent disscharge from large paper
mills of capacity above 24,000 MT
per annum must not be more than
175 Cu M per ton of paper produced.
In order to meet the targeted effluent
discharge level as declared by Government
of India. complete upgradation of
processes as well as strengthening
of in -plant control measure like
recycling of water has to be adopted
by Mill. Such measures should aim
to bring down the fresh water consumption
to around 200 Cu M per ton of paper
produced.
Generally, the reduction in water
consumption for paper industry can
be achived by adopting following
methods:
1. Maximum recycling of back water
at various stages.
2. Improvement in washing and screening.
3. Maximum recycle of condensate.
Strict monitoring is needed for
effective
recovery.
4. Prevention of accidental losses,
leakages from pump glands, valves,
pipe
line and other sources to be attended
immediately.
5. Collection of spillage and recycling
of concentrated spill to the system
and weak spill to spill lagoon to
avoid contamination of whole effluent
steam. Collection and recycling
from black liquor, fresh liquor
and chemic al dosing pump to the
system and immediate repair of units,
have been found to be very effective
in reducing colour and toxicity
and improving recovery of chemicals.
6. Maintaining the steam pressure
above digester cooking pressure
and
routing check-up of non-return valves
and timely replacement to avoid
return of cooking liquor to condensate
line.
7. Metering and monitoring of water
consumption of every unit
8. Washing of wood and bamboo to
avoid carryover of dust and other
foreign materials to subsequent
stages.
9. Separation of centricleaner rejects
from main drain and collection of
fiber
to avoid contamination of whole
system.
10. Recovery of all alkali from
dregs of green liquor by centrifuge.
11. Use of back water strictly in
hose pipe for floor washing and
other uses.
12. Use of clarified and treated
back water for causticising at various
stages in pulp mill and paper machines,
especially for kraft paper,
13. Use of back water in waster
paper pulping plant,
14. Use of evaporator condensate
in causticising section, pulp washing,
dreg washing, dilution in screening
and centricleaning.
15. Use of contaminated back water
from pulp mill and recovery section
drain in coal moistening, ash quenching
surface condenser and bamboo
washing, and
16. Good house keeping, general
conciousness regarding water conservation
and monitoring by a separate cell.
Some of the mills have separate
cells for monitoring water consumption
which have been found to be very
effective.
CONCLUSION
Need For Water
Conservation
Water is required in the process
for paper making. However, most
of the water, say about 90% (considering
10% evaporation losses) comes out
as effluent. By the time process
is completed fresh water is contaminated
with chemical and colors. This water
cannot be used again in the same
form and it should be disposed off
by pumping it. Due to social obligation
and stringent statutory stipulations
by the Government the effluent water
is to be treated properly and then
only can be disposed off. With the
enforcement of various acts by the
Government like Water (prevention
and control pollution) Act-1974,
Water (prevention and control of
pollution) cess Act-1977, Environmental
Protection Act-1986 etc. and with
a series of unending amendments
to these acts, it is difficult task
industry to meet the stringent standards
stipulated by the Government with
regard to the rate of water consumption
and effluent discharged per tonne
of paper produced. The latest standard
stipulated being 175 cm effluent
discharge per tonne of paper produced
in case of large paper mills as
per Environmental Protection Act
has come in force with effect from
January, 1994.
Keeping this in view, there is an
urgent need for the paper industry
to reduce the water consumption/effluent
discharged by way of reducing fresh
water consumption. Water conservation
schemes adopted to reduce the fresh
water consumption result in multifold
savings improving the profitability
of the company.
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