A long-neglected area is the target of new technology aimed at helping mills eliminate pouring more than just water down the drain
June 2008
By Len W. Dewhurst, sales manager Algas, Norway
Although water is the lifeblood of this industry, the attention given to its treatment and re-use has not seemed to correspond with its importance in the process. The limited budget often given for water treatment reflects this lack of attention.
More emphasis has been given to fiber recovery and effluent treatment where authorities have demanded improvements in discharge standards, either through legislation, charging for flows, solids and COD/BOD or, in a growing number of cases, limiting emission amounts.
Recently, however, there appears to be a growing awareness that saving water can be coupled with savings in the “investments” put into the water flow through the mill. In addition to saving fiber there are also potential savings in power, energy, chemicals, through to effluent treatment costs by reducing solids and hydraulic loading to that area.
Savings in these areas can be very considerable given the present cost of raw materials, energy, chemicals and the effect of increasing legislation and the growing water shortages affecting more and more areas around the world due to environmental awareness and global warming.
There are already many types of equipment that are used for water treatment. Perhaps the most common being disc filters or flotation cells used as savealls for fibre recovery and sedimentation basins and sometimes flotation cells for primary effluent treatment.
The former concepts, whilst being satisfactory in recovering fiber generally do not produce a quality of water that is suitable for safe re-use to replace fresh water.
In the effluent area, the traditional sedimentation method takes a lot of space and is not always easy to control, being especially susceptible to floating sludge problems from the settled solids rotting, producing methane gas in the process and causing solids to rise to the surface. The solids carryover can affect the efficiency of the subsequent biological treatment, increasing the amount of biological sludge that needs to be treated both chemically and mechanically.
Due to these “new” challenges and opportunities the industry is beginning to look for different (and better) methods of water treatment. To meet demand, Algas has developed its Microfilter concept.
Although being a drum filter, the Algas Microfilter has two main design differences from a conventional unit. The first is a convoluted surface that increases the surface area by up to 50% compared with a flat drum. The second is a filtering process that work from the inside out, using neither vacuum nor any high pressure to induce a flow through the filtering medium.
Depending on the application, the filter medium can vary from a very fine 10-micron for special polishing applications up to a coarser quality of 1,000 microns used, for example, for thickening applications.
Because of these features, Algas Microfilters can be used with equally high efficiency in most water treatment applications: treating raw water, filtrate from screw presses, vacuum sealing water, treating white water for fiber and water recovery, “polishing” the water from existing savealls to a “super clear” quality, through to primary effluent treatment and after bio-sedimentation, either for safe discharge or sometimes re-use.
There are more than 500 Microfilters installed throughout the world, as can be seen from the following examples.
In 2005, a specialty mill in Norway replaced flotation units with Microfilters on all three of its paper machines to treat white water. After commissioning, the mill set about rerouting the flow of clear filtrate from the Microfilters to replace fresh water. Some application areas were:
- Wire and high pressure showers
- Sealing water on mechanical seals and pump stuffing boxes.
- Sealing water on vacuum pumps
- Replacing fresh wash up water with with recycled water.
The mill reduced its electricity consumption by 5%. Gas usage in the Yankee hood and steam to the drying cylinders were reduced by 20% and 14%, respectively. This is illustrated by the fact that the temperature of the water discharged to the effluent plant increased from 13ºC to 23ºC.
Water consumption, flow of water into the mill, was reduced by 47%.
The amount of solids in the waste water passing to the effluent system was reduced by 67%.
Treating Filtrate from Roller Presses
A mill in France installed two Algas Microfilters to treat the filtrate from roller presses. The objective was to recover fiber and pass the treated water with minimum solids directly to the bio plant.
Considerable fibre savings were achieved and previous problems in the effluent plant were eliminated.
A high-quality tissue mill in North America experienced problems with fiber in the clear filtrate blocking nozzles in a newly installed dilution headbox. An Algas Microfilter fitted with a special fine cloth was installed to “polish” the clear filtrate from the disc filter, not only producing a low TSS quality but also one where any particles present were no greater than 25 micron.
A mill in the UK was experiencing problems in its effluent plant from DIP sludge, discharged into the general effluent flow, overloading the treatment plant.
After running trials with a Microfilter test unit, it ordered and installed a Microfilter to separately treat the DIP sludge, returning the clear filtrate from the unit at 100 mg/L back to the plant for dilution and the sludge at 6% directly to the effluent sludge presses.
Doing the “Impossible”?
A North American tissue manufacturer needed to not only improve effluent treatment in one of its German mills to meet new discharge standards but also overcome the problems of disposing of effluent sludge from the mill.
This particular mill was making products from mostly recycled fiber that needed DIP treatment. Coupled with high ash values, this produced a difficult-to-treat effluent.
After investigation the mill installed two Microfilters to treat the mill effluent at inlet levels of 1-2% consistency and up to 60% ash levels.
In one step through the units, a clear filtrate of maximum 100 mg/L was achieved. After running for more than six years the average is generally 50 mg/L.
The resulting sludge (around 6% consistency) passes through a gravity system of the drainage table and onto screw presses which further increases the consistency to about 50%. The sludge is then transported to a brick works to be used in the manufacture of internal wall building bricks.
Holding Space Reduced by 95%
Suffering from floating sludge in the primary clarifier, a UK producer of corrugated medium turned to Algas to solve the problem.
Installing two units as the first stage before the clarifier, the mill achieved its four objectives.
- With the cloudy flow the fines and filler go to the existing clarifier. And because there is no fiber and the flow is now lower, the floating sludge problem is cured and the efficiency of the following bio plant improved
- The clear filtrate is returned back to the mill for re-use.
- The recovered fiber, “washed” clean of the fines and fillers, and also fresh because the dwell time in the Microfilter is minutes rather than hours, is also returned back to the process for re-use.
- The improved quality of the final discharge after the bio plant met the new standards set by the local river board.
Polishing after biotreatment
Wanting to replace a troublesome sand filter, a mill in Germany replaced the unit with an Algas Microfilter fitted with a fine filter medium.
The resulting outlet quality met the tougher discharge standards and provided a trouble-free installation.
All of these installations, and many more, have provided a fast payback on capital varying from three months up to one year. More importantly, they helped the customers to solve problems that could have become, if not taken seriously, a challenge and a threat to success and survival in the future.
Len W. Dewhurst is sales manager Algas, Norway, len.dewhurst@algas.no; www.algas.no
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