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WATER MANAGEMENT: MAKING GOOD IN A NOT SO GOOD AREA
Water is the "Life Blood" of our industry and with the ever increasing changes in the world weather patterns, stricter legislation and more costs being applied to its abstraction and discharge, we have to take even better care of its re-use and treatment.
Legislation and necessity to improve discharge quality is however making many mills to look to improving the treatment of their waste water in order to meet discharge standards and where possible, reuse some of the treated effluent, back in the mill as process water and sometimes even fibre can be recovered.
With new and better methods of treatment will possibly come higher costs for some equipment, chemicals, service etc.
However, by better understanding the effluent treatment process there are areas of possibility to improve the efficiency and performance, save chemicals and in some cases valuable fibre. The re-use of treated water back in the mill as process water is also growing.
Microfilters have shown great promise in these areas of improving efficiency and savings.
A mill in the UK making fluting from mixed waste, suffered from the problem of floating sludge in their primary clarifier, fig.1
Fig. 1 Floating Sludge, a common problem
This was causing an overflow of fibre with the "clear" water to the bio plant, effecting efficiency there and passing the floating problem also onto the secondary clarifier.
Coupled with this, they had received an order from the British National River Authority to improve their discharge quality, reducing both TSS and COD levels.
In order to meet the new standards the mill had planned to increase aeration & oxygenation in their Aerobic plant and install a second, Secondary Clarifier but anticipated they would suffer the same sludge floating problem in the primary. They had also hoped to save both good fibre and clarified water from the primary but in the sedimentation basin, the fibre was rotting and the "clear" water was contaminated by the floating blanket of fibre.
After investigating the efficiency of Microfilters, the mill went ahead and installed two units in parallel treating primary effluent prior to the existing settling basin, fig. 2
Fig. 2. Microfilter v Settling - Only 5% Space
The immediate advantages of the working of the concept, fig. 3 were:
Fig. 3 Working Principle
The installation won the mill the British Environmental Solutions Award for Industry – Manufacturing Sector.
A nearby newsprint mill was planning to install a new 9m wide newsprint machine Fig. 4, using 100% ONP as the furnish.
Fig. 4 New Newsprint machine
Due to space limitations they had to consider a more modern alternative to the old, traditional large sedimentation concept for primary effluent treatment.
Evaluating the success of Case 1 and other Microfilter installations in Europe, they selected three Microfilters to provide primary treatment, Fig. 5 and a fourth to act as "Polishing/Police" filter after final bio sedimentation.
With a primary inlet flow of 30,000 l/min and consistencies of 3-4,000 mg/l of waste from all plant areas, DIP, stock prep waste treatment systems, through to general excess water flows the primary units, with the use of polymer addition produced consistent filtrate clarities of ca 150 mg/l were achieved.
From the "polishing" unit the requirement was < 30 mg/l but in this case the unit was operated without the use of polymer.
Fig. 5 Just Microfilters for Primary treatment
Benefits from the installation were:
A tissue mill in Germany, presented with ever increasing discharge costs from the municipality water treatment division, investigated methods of treating their effluent other than their existing sedimentation basin.
After evaluation trials with Microfilters they designed a plant that would reduce flows to 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 Fig, 7, treating effluent from a storage tank which was the original old settling basin.
Fig. 7 No pumps – gravity flow through system
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 between 1- 2.0%.
Fig. 8 Flow sheet of effluent system
With polymer added, the mix was fed into the Microfilters, the produced clear filtrate at 50 mg/l some of which was re-used and the remainder passed to sewer and the discharged solids at around 6-8% dropping onto a gravity dewatering table.
This stage increased the consistency to 15-20% and then was 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 dumping considerable costs.
A mill in Germany making packaging grades, had for years been treating the final outflow from the bio sedimentation unit with sand filters.
Satisfactorily initially, they 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, they selected to install 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.
Fig. 9 shows a filter being easily installed to polish the water after Bio sedimentation, in this case replacing a sand filter.
SEPARATE TREATMENT OF DIP SLUDGE REDUCES EFFLUENT LOADING
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.
Fig 10 shows an installation treating DIP Sludge at about 1 – 1.6-% inlet, thickening up to 6-8% which passed directly to belt presses. The filtrate at below 200 mg/l can be either returned to the DIP plant as dilution water or passed directly to the sewer.
In this case substantial 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.
Fig. 10 DIP sludge treatment
BACK TO BASICS
Whilst more effective and efficient methods of treating waste water are necessary, it is still even more important to plan to treat water flows further back in the process. This is in order to take full advantage of the immediate savings from the recovery of water, fibre, power, energy and chemicals.
"Savings" in the mill will result in less waste water to treat, containing less solids thereby reducing the size, capital investment and operating costs of effluent plant.
A recently developed computer programme can calculate from the input of information from the customer mill, the savings that are possible to achieve.
Details of water flow, solids content and stock preparation power in kW/hr/tonne, water and energy costs etc., in fact all losses or costs in the area of the "investments" in the water flows in the mill can be collated and those in individual area or for total savings can be presented.
Fig 11 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 US$ 300 per tonne and adding associated savings of water, power, energy, chemicals and sludge disposal costs, saved by better water management,, total savings of plus US$ 1.7 MILLION a year can be achieved
Fig.11 Savings – Hard to believe but true
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 payback achieved, for example, from capital invested to increase production
You have the figures, so just take a moment to calculate and decide for yourself.
WATER CAN NO LONGER BE TAKEN FOR GRANTED
POLLUTION PREVENTION PAYS