By Len Dewhurst
Manufacturing tissue has come a long way in the
last 30 years or so in the types and qualities
available, production machines and a range of furnishes
from virgin pulp to recycled fibre, everything
in between and back again. Converting has also
developed at a pace with all variations possible.
What has not changed is the dependency on water
in the manufacturing process or the methods of
treatment compared to the progress in tissue manufacture
or converting and generally the level of investment
in its conservation, re-use and treatment.
Legislation
and the need to improve discharge quality are,
however, making many mills look to improving water
treatment both inside and outside the mill. Not
only to meet discharge standards but especially
to reduce water consumption. In doing so, to save
much of the very high cost of raw materials, energy,
chemicals.
Microfilters as a water treatment concept have
been around for many years but are still considered
a new concept by many in the industry.
The method
of operation is very simple: no vacuum, low speed,
soft filter in action and a wide range of filtering
mediums from 10 up to 1,000 micron.
The flexibility of operation makes
microfiltration suitable for a wide range of treatment
applications from raw water to fibre recovery and
polishing after existing savealls to produce a
large flow of super clear for safe re-use, through
to primary effluent and treatment at final discharge,
after biosedimentation.
The key of the microfilter performance is its ability
to provide a filtrate quality that is virtually
fibre free, essential when used to replace fresh
water on showers.
A Reson For Return on Capital
Water: The number of mills that have to pay for
both incoming and discharge water is growing, as
are the costs. By replacing incoming water with
treated water, flows and consequently costs at
both ends are saved.
It is important, however,
to have the right quality of treated water, not
only low in solids but of the particle size of
any solids still present.
Recently two tissue mills
in Indonesia installed microfilters. They both
found re-using the filtrate in place of fresh water
trouble free. Here the TSS was less than 30 mg/l
and particle size of any solids in the filtrate <30
micron.
Energy: Replacing cold fresh water with warm treated
water, for example on wire showers, will increase
the machine operating temperature overall, improving
performance in the forming section as well as in
the dryers, reducing steam/energy cost there.
A
mill in Norway, having replaced DAF units on all
three machines with microfilters, had
savings of energy as follows: electricity 5%; gas
on the yankee hood 20%; and steam to the drying
cylinders 14%. As a result, the temperature of
the water discharged to the effluent plant has
increased from the previous 13ºC up to 23ºC.
Water consumption: Water consumption, which is
flow of water into the mill, has been reduced by
47% . Fibre savings: The amount of solids in the
waste water passing to the effluent system has
been reduced by 67%. Chemicals: Less water to be
treated in the effluent plant and a reduction in
the level of solids means less chemicals both in
the treatment and thickening area.
Other savings: Water on pump seals. Some mills
have replaced sealing
water on pumps, both in packing and mechanical
seals, with filtrate from microfilters. This was
the case in the mentioned reference mill in Norway.
Less ludge in the effluent plant means less to
transport and disposal and in some cases, less
disposal tax.
Effluent Treatment
A tissue mill in Germany, presented with ever increasing
discharge costs from the municipality water treatment
division, investigated methods of treating its
effluent other than in its existing sedimentation
basin.
After evaluation trials with microfilters, it designed
a plant that would reduce flows to the sewer, improve
the quality of any water that was discharged, and
solve a problem of disposing of the effluent sludge.
As the mill was using a furnish of recycled fibre,
much being coated magazine, the ash levels in the
effluent could be as high as 70%.
The first stage of the plant was two microfilters
installed in parallel, treating effluent from a storage
tank which was the original old settling basin.
The effluent consisted of general mill excess water,
sludge from DIP and stock waste systems, boiler blow
down, back wash from carbon filters making a consistency
mix of 1-2%. With polymer added, the mix was fed
into the microfilters, that produced clear filtrate
at 50 mg/l some of which was re-used, while the rest
was passed to the sewer, with the discharged solids
at around 6-8% dropping onto a gravity dewatering
table.
This stage increased the consistency to 15-20%. The
effluent was then passed into screw presses which
further increased the solids to a final 55- 60% dry.
The dry cake was then transported to a local cement
works for mixing with concrete to make building blocks,
which also saved the mill considerable dumping costs.
Separated Dip Sludge Treatment
DIP sludge is generally at too low a consistency
to be passed directly to presses, so normally goes
directly to primary effluent. The nature of DIP sludge
causes high contamination of the general effluent
flow and so mills are turning to treating this difficult
material in isolation.
A reference installation treated DIP sludge at about
1-1.6% inlet, thickening up to 6-8% and passing it
directly to belt presses. The filtrate at below 100
mg/l can be either returned to the DIP plant as dilution
water or passed directly to the sewer.
In this case substantia savings were made in chemical
use in the primary plant, an easier effluent to treat
with better efficiency in the system which in turn
reduced considerably the discharge cost by reducing
flow, TSS and COD.
Final Discharge Standard
A mill in Germany making packaging grades had for
years been treating the final outflow from the bio
sedimentation unit with sand filters. It soon had
to live with the high maintenance costs and handling
the back flush water from the sand filter which was
more than was originally expected.
In order to move forward, it installed a microfilter,
a concept that was being used successfully in other
group mills, to replace the old filter rather than
install a more modern type of sand filter.
A recently developed computer programme can calculate
from the input of information from the customer mill,
the savings that can be achieved. Details of water
flow, solids content and stock preparation power
in kWh/ton, water and energy costs etc can be collated
and presented.
Fig 7 showing a summary sheet based on a nominal
2,000 l/min flow with 1,500 mg/l solids. Based on
a raw material cost of $300/ton and adding associated
savings of water, power, energy, chemicals and sludge
disposal costs, saved by better water management,
total savings of $1.7 million/year can be achieved.
The savings from all these areas can be obtained
by sensible investment, providing a return on capital
that often is achieved within months rather than
years, far outstripping the relative payback achieved,
for example, from capital invested to increase production.
Water can no longer be taken for granted. Pollution
prevention pays. TW
Len W. Dewhurst is Director of Sales with ALGAS Fluid
Technology Systems AS based in Moss, Norway
