Low-effluent recycled paper mills

Transcription

1 Low-effluent recycled paper mills This paper, by BRUCE ALLENDER 1, GEOFF COVEY 1 and DENNIS SHORE 1 was presented at the 64th Appita Annual Conference and Exhibition, Melbourne, April 2010 ABSTRACT With increasing pressure on discharge quality and uncertainty of water supply in many parts of the world, the pressure to move towards effluent-free papermaking is increasing. The advantages of operating an effluent-free mill are that it requires no discharge licence, requires very little fresh water and can offer a product with good environmental credentials. Closure of a mill water system can result in a range of problems, the severity of which will depend on the nature of product being produced, the quality of the raw material and the degree of closure attempted. In most cases some additional treatment will be required before recycling the water. Commonly this will include at least one stage of biological treatment, but membrane processes, evaporation and other steps may also be required. Poorly applied, such processes can be very expensive and to be most cost effective the treatment process must be tailored to the needs of the mill and designed into the basic mill flow scheme. The types of problem that might be encountered are described and how these have been tackled in cases from mills employing very simple closure, through extensive biological treatment up to fully liquid effluent free mills. INTRODUCTION Most Paper Mills already reuse some of their water streams and treat final wastewater to reduce primarily suspended solids and biological oxygen demand (and in some cases additional factors such as colour) before discharge to a sewer or environmental receiving water. Mills have adopted various means to maximise reuse of effluent as process water, thereby reducing the volume discharged. In some cases this process has been taken to the extent that there is no liquid discharge at all. However there is likely to be some solid waste discharge solids from which the water has been removed in order to have a sustainable materials balance in the Mill systems. The advantages of operating a liquid effluent-free mill include having no liquid discharge licences, using little fresh water makeup and making paper products with improved environmental credentials. The cost and complexity of extra processing in-plant to achieve reusable water quality, is offset to some extent by having no liquid effluent discharge and licensing costs or compliance issues. But increasingly stringent regulatory requirements are an even greater driver towards zero-effluent operations for example the European Commission BAT Directives for the pulp and paper industry (1). However, to be most cost effective the treatment process must be designed into the mill unit processes. End of pipe treatment of all effluent to clean water is technically feasible, but commercially unrealistic. The key is to first treat and use water to the 1 Covey Consulting Pty Ltd, P O Box 99, Kew East, Victoria 3101, Australia quality required for the different process within the mill. While the emphasis continues to be on the water side of paper manufacture, the regulators and the broader community increasingly require a total site approach. All outputs (and inputs too) are considered; atmospheric emissions (visual, odour, toxic), solid wastes (landfill, recycling, fuel) as well as liquid discharges. Any consideration of an effluent free mill cannot be at the expense of these other outputs. CLOSING THE CYCLE Complete closed cycle operation means operating with no liquid discharges whatsoever. When this occurs the make up water requirement becomes about equal to the amount of evaporation occurring in the dryer section of the machine typically about 1.5 m 3 /t and the water leaving with the paper product (say 10% moisture). Therefore there is always some fresh make-up water. Some mills practice a partially closed system wherein there is substantial recycling, but still a small purge stream to effluent, at least periodically. When and how much is discharged varies with the mill and the operating circumstances. Mills that discharge effluents, albeit in reduced volumes, to municipal sewers rather than directly to receiving waters are low effluent, but not zero effluent operations someone else has to ultimately discharge the water. The advantages of reusing water are generally: To reduce effluent discharge fees and operating licence conditions, due to both quantity and quality of the water released. To reduce make-up water consumption to counter water charges, water scarcity and/or difficulties associated with poor water quality. To save energy costs in heating water circuits. Paper Mills usually practise various forms of water reuse, broadly in three categories. 1. Recycle of effluent without treatment, apart from physical settling of solids. 2. Recycle of effluent with biological treatment to reduce organic load, suspended solids, and some dissolved solids. 3. Recycle of effluent with biological treatment and removal of dissolved salts from the system. Closed cycle operations are where these processes are used to recycle all or almost all of the effluent. The cleaner the water required, the higher the treatment costs for capital and operations. But higher quality paper products and improved machine output is expected when clean process water is used. From a Paper Mill operating point of view, it is not necessary to subject all of the effluent to the same full treatment. Optimising the degree of treatment required at each stage can control costs. Different qualities of recovered water are produced for use for different purposes. The key is tailoring the quality of each water product to meet the minimum requirement for its use. Over the years, many Paper Mills have claimed to be operat- 186 Appita Journal Vol 63 No 3

2 RECYCLING WITH BIOLOGICAL TREATMENT AND INORGANIC REMOVAL When zero-effluent operation and minimum impact on product quality and machine performance is required, it is not sufficient to use biological treatment it is also necessary to remove inoring without liquid effluent discharge to the environment. But this includes Mills which are zero liquid effluent during socalled normal operation, with clean ups and discharges during mill shuts. RECYCLING WITHOUT TREATMENT Simply recycling effluent to the machine and letting contaminant levels build up until they are in equilibrium with losses from the water fed to the dryer section has been practiced by a number of mills since the 1970 s, with varying degrees of success. Managing large volumes of water entirely in-house is a further issue. Provision must be made for adequate surge capacity and storage when there are mill shuts or other operational disturbances (2). Mill that attempted this approach eventually reverted to discharging at least part of the effluent (usually via a suitable treatment system) to control the build-up of contaminants in the water (3). The advantages of water reuse outlined above, are countered by a number of problems associated with the build up of contaminants in recirculating water. Problem contaminants include; nutrients which stimulate growth of micro-organisms, salts, calcium, volatile fatty acids, suspended solids, anionic trash and secondary stickies. Operational and product problems vary with the type of mill and the installations, but the following difficulties are common: 1. Scaling and deposits at the wet-end. 2. Corrosion at various points in the machine and reduced equipment life. 3. Reduced effectiveness of papermaking chemicals and ph control. 4. Formation of foam and biological slime - often resulting in increased breakage frequency and sheet defects. 5. Reduced drainage necessitating reductions in operating speed. 6. Offensive odours in and around the mill 7. Suspended solids build up - plugging of showers, wires and felts. 8. Loss of runnability and productivity. 9. Higher operating temperatures 10. Dirt spots, slime holes and colour degradation of product as well as odours in the product. Depending on the product being made, contaminants can be incorporated into the final paper but often with a paper quality, appearance and performance penalties. Even in mills with no other treatment of recycled water, effective removal of suspended solids or filtration, and careful biocide use can reduce some of the problems related to plugging and some types of corrosion and deposits and allow on-going Paper Mill operations (2). The Green Bay Paper Mill has operated as such a near-zero effluent mill with minimal treatment for many years (4). RECYCLING WITH BIOLOGICAL TREAT- MENT Bacteria form slime and the growth of both can be controlled by: removing nutrients (but adequate removal is difficult); by increasing operating temperature above 45 C (but there will usually be some regions that cannot be maintained at such a high temperature); Or by the use of biocides and dispersants. Nutrients and volatile fatty acids are removed by biological treatment, using various well-established aerobic processes. Anaerobic digestion followed by an aerobic stage is the most effective. This is often called a kidney approach (5,6). There are various types of biological reactors that can be used, for example: Zulpich Paper Mill - two Up-flow Anaerobic Sludge Blanket reactors followed by carrousel aeration tanks and a clarifier. Julius Schulte Sohne Mill - Paques BV Internal Circulation anaerobic digesters, pumped air aerobic reactors and a final sand filter were used. Gissler and Pass Paper Mill trickling filer, aeration, sand filter and sedimentation cone. Lecoursonnois Mill floatation, anaerobic and aerobic reactors, decarbonisation. Importantly none of these are one-step approaches; with different contaminants in the wastewater being removed at specific multiple stages. Several German Mills claim to run sustainably using a kidney approach (7). Biological treatment also reduces odours associated with hydrogen sulfide formation from bacterial action on sulfates. The biogas formed in the anaerobic stage strips much of the hydrogen sulfide out of solution and so prevents it from reaching problem levels. The alkalinity resulting from bicarbonate formation in the biological treatment also causes the precipitation of much of the dissolved calcium and hence in reduced calcium-fouling problems. Whether or not an anaerobic stage is used, the sulfides will usually be oxidised in the aerobic stage. The effect of adding in-line biological treatment to a mill that had formerly used simple recycle with no treatment is shown in the following table for Zulpich Mill in Germany (8). Simple closure With in-line biological treatment COD mg/l Ca mg/l SO 4 mg/l Cl mg/l Acetic acid mg/l Propionic acid mg/l Butyric acid mg/l Lactate mg/l ph Conductivity ms/cm The addition of in-line biological treatment also reduced residual volatile fatty acids in the paper product by more than 70%. The above table shows that one deficiency of biological treatment is that it does not remove chloride (or other ions that are soluble at all ph s). Thus levels of chloride will remain high with resulting potential for corrosion and also interference with some process chemicals. This often necessitates further processing to reduce chloride levels. June July

3 ganics. High-grade paper products certainly need water carrying minimal dissolved chemicals. In principle the biological stage can be eliminated, and the filtered effluent sent directly to membrane processes or evaporators. However, this results in the production of a large quantity of high organics content liquid that must somehow be treated or disposed of. If membranes are used, fouling will be greatly increased and fluxes generally reduced. Once again the most efficient approach is multi-step, with as much treatment as possible before these final (and most costly) additional treatment phases. It is it not economic to do otherwise. The principle difficulty is in removal of chloride ions (and related species) that are highly corrosive, very soluble and cannot be precipitated by any practical reagent. Therefore the water must be removed from the biologically treated stream by membrane processes and/or evaporation to produce a clean water stream and a salt concentrate. This concentrate can be discharged as a low volume concentrated stream (difficult because municipal sewers are usually reluctant to accept such discharges), or else it must be evaporated to a solid product in a crystalliser. Whether evaporators or membranes (or both) are used, pretreatment to remove contaminants that would interfere with the operation is necessary. This pre-treatment will be much more onerous when membranes are used, and greater care is required in operating membranes than evaporators. However, the operating costs of membrane systems are lower than those of evaporators, and initial costs are also usually lower. The first mill to operate with zero-effluent and full impurity removal is the McKinley paper mill in New Mexico since its construction in 1994 (9,10). Although this mill is now sixteen years old, there is extensive data available and it remains a good example of the issues around zero-effluent operations - so will be described in some detail. Case Study: McKinley Mill There were several basic principles incorporated into the construction of McKinley Mill, including: 1. Design of the mill so that it would operate with zero discharge and use a particular treatment process. Thus care was taken in the selection of materials of construction for corrosion resistance and separation of the various water systems in the mill. 2. Not all of the water recycled was to be fully treated. This would be possible, but very expensive. Instead several partially treated streams of different quality are produced and used in applications where their quality is adequate. 3. Recognition that careful attention must be paid to the operation of water treatment plant or the quality of water returned to the paper machine will deteriorate and with it operating performance and product quality. And there is risk of expensive damage most obviously to the membranes, but also to other parts of the system. 4. In particular recognition that for reverse osmosis (RO) membranes attention must be paid to the concentration of various dissolved metals in the membrane influent to ensure that there is no damage caused by deposition of crystals in the membranes The system as used at McKinley is shown in Figure 1. Suspended solids are separated from the effluent by means of a rotating fabric disc screen and then by Dissolved Air Flotation. Fig. 1 Water treatment system at McKinley Mill, USA Some of this water is used for cleaning sprays on filters; the remainder is split between a sand filter and a Cyclic Activated Sludge System (CASS). The CASS (also referred to as a Sequential Batch Reactor - SBR) employs two batch reaction vessels to affect BOD and COD reduction using aerobic microorganisms. Aeration is by forced air and natural bacteria strains introduced with the waste paper are used to facilitate conversion of organic materials. The process ideally operates with non-filamentous bacteria that settle readily. Occasionally, levels of filamentous bacteria rise to the point where operation of downstream filtration is compromised. When this occurs, the DAF unit is temporarily switched to treating this stream, as it is very effective in removing filamentous bacteria. As soon as the system is brought back under control, the DAF reverts to its normal duty. The ph of the CASS decant is adjusted to below 6 to precipitate metal-organic complexes that would otherwise precipitate in the reverse osmosis stage and damage the membranes. After ph adjustment, the stream passes to a Continuous Micro Filter (CMF) that uses porous membranes to remove fine suspended solids, mainly residual bacteria and precipitated metal compounds. The rejects from this stage are sent back to the effluent feed tank (along with various other low consistency rejects streams). The CMF filtrate is very low in suspended solids, but still contains dissolved solids. The filtrate stream is split into two portions. One is sent to the Combined White Water Chest where it may be mixed with water from various other sources including sand filter filtrate and water that has bypassed the CMF stage. The second portion is sent to active carbon treatment and reverse osmosis. The principle function of the CMF is to prevent fouling of the RO membranes that will occur 188 Appita Journal Vol 63 No 3

4 if the suspended solids are above a critical level. The activated carbon system is regarded as optional, but is useful for removing residual, high molecular weight organics. The rejects from this stage are sent back to the effluent feed tank. The reverse osmosis stage uses a high pressure-drop (35-40 bar) across semi-permeable membranes to produce a permeate that is of comparable purity to the make-up water, and a concentrated solution of salts. This concentrated salt solution is passed to an evaporative crystalliser where most of the residual water is boiled off, condensed and transferred to the mill water system. At McKinley, the concentrate from the evaporative crystalliser sets to a solid on cooling below about 65 C. This product can be combined with other solid wastes and discharged. Depending on the composition of the salts, this may not occur in other installations, requiring instead crystals of solid to be filtered out for disposal. The various solids streams pass to a belt press where the solids content is raised prior to discharge of solids to landfill. Using this system McKinley achieved a BOD 5 reduction of in excess of 90% and a COD reduction of about 90% to produce a mill white water conductivity of ms/cm and a chloride level of about 1000 mg/l. However, this does require very careful operation, e.g. in the area of additives and of biocides (which must be effective at the machine, but not interfere with the biological treatment). Further, the plant is operated without odour complaints (it is a remote site) and with very few customer complaints related to slime-holes. Alternative equipment in a system The system at McKinley is a comprehensive example of a liquid effluent free paper mill, but variations on the equipment used there may present advantages in some other situations. Anaerobic and Aerobic Stages While the biological treatment at McKinley uses only aerobic digestion, as noted in earlier examples, European mills use anaerobic and aerobic digestion in sequence. The selection of a process will depend on local factors, but the general advantages of each are: Anaerobic treatment 1. Less sludge produced (important if there are solids disposal issues). 2. By-product methane produced (biogas). This can be used as a fuel to provide several percent of the mills energy needs, noting that there are potential but unlikely explosion issues. 3. Less energy required because there is no need to aerate, but need to operate at a higher temperature. 4. Smaller reactor volume required. 5. Inclined to generate odours, but if the biogas is collected, these odours can be scrubbed out with an overall reduction in odour. Aerobic treatment 1. Shorter start-up to develop biomass inventory. 2. Often better BOD and COD removal. 3. Control of ph less critical. 4. More tolerant of toxic shock. Combined anaerobic and aerobic treatment Anaerobic digestion followed by aerobic digestion offers most of the benefits of both systems: Because the bulk of the BOD/COD removal is done in the aerobic stage, much less aeration is required in the aerobic stage. The two types of microorganism working in series provide greater overall removal of BOD/COD than is practical with either type working alone. In some cases, considerable reduction in dissolved calcium is possible. Better quality water for direct reuse or feed to sensitive membrane filters Membranes and Evaporation A particular problem with organic polymer membranes is their fragility. They are easily damaged by chemical or mechanical means, and they can only be operated over limited ranges of temperature and ph. An alternative for the micro-filtration portion of the process is to use sintered stainless steel units (11). Expected advantages are the more robust construction, the ease of cleaning and the ability to concentrate the rejects stream to a paste. These should be considered if a full recycle cleaning system is to be used. Membrane stages do not have to be used, and multiple effect evaporators could concentrate all of the liquid from the biological stages. While this is probably easier to operate (as fouling in evaporators is usually easier to control than fouling in membranes) it has much higher operating costs and possibly higher capital costs CONCLUSION Pressure will continue to grow for mills to reduce the consumption of fresh water and discharges to the environment. In most instances this makes good business sense anyway. Standard packaging grades based on recycled paper are the most likely candidates for operation as effluent free mills because quality requirements are less demanding than high-end packaging and paper products. However, if this is done without a holistic approach to the mill systems and suitable treatment there are likely to be adverse consequences with respect to product quality and machine performance. Reuse of some process water is standard practise in paper mills, and mills have demonstrated that treatment of the recycled water to differing degrees can resolve these (which problems?) problems and that the level of treatment must be consistent with the degree of closure required. Operation with no liquid effluent at all is possible, but for this to be cost effective the treatment process must be fully integrated into the paper machine design. This is an opportunity in designing new mill installations. Retrofitting existing mills towards liquid effluent free operations are a much more challenging exercise. REFERENCES (1) European Commission, IPPC Reference Document on Best Available Technologies in the Pulp and Paper Industry, 475p (2001) doc/ppm_bref_1201.pdf Continued on page 194 June July

5 made three times on site and has passed HP certification as a recommended grade for printers using HP Indigo presses. The paper has the advantages listed below: 100% Electroink adhesion <1mm ink peel on score test Excellent ink transfer and blanket compatibility Extended shelf life with stable adhesion and optical properties Cost effective replacement of Sapphire technology with no negative attributes This is the first on-machine digital paper that has been made in Asia-Pacific that is certified for HP Indigo printing. This paper does not exhibit performance limitations or shelf-life concerns typical to papers treated with the polyethyleneimine (Sapphire) treatment. Additionally, a reduction in sheet whiteness yellowing does not occur. This should give rise to local and export supply opportunities for AP Shoalhaven. Plans are made for launching a 100% recycled digital grade of paper this year. Potential applications of this paper are UV coating, spot varnish, liquid lamination and clear foil application. ACKNOWLEDGMENTS Nalco would like to thank AP Shoalhaven for the opportunity to conduct a DIAMOND TM DIGITAL application and print trial. 1 HP, HP Indigo, Electroink and Sapphire are trademarks of Hewlett-Packard Corporation 2 DigiPrime is a trademark of Michelman Corporation 3 Some details have been withheld for confidentiality reasons REFERENCES (1) Webb, S, Aronhime, M, Forgacs, P, A Mechanism for the Adhesion of Indigo Electro-ink to Coated Papers, Research Disclosure, 511(2006), (2) Landa, Benzion, Liquid Developer Systems for Imaging on Transparent and Opaque Substrates, U.S. Patent 5,380,611. (1995) (3) Landa, Benzion et al, Toner and Liquid Composition using same, U.S. Patent 5,407,771. (1995) (4) Landa, Benzion et al, Toner Material and Method Utilizing same, U.S. Patent 6,979,523. (2005) (5) Landa, Benzion, Imaging System with Intermediate Transfer Members, U.S. Patent 5,410,392. (1995) (6) Landa, Benzion, Method for Fusing Developed Image, U.S. Patent 5,497,223. (1996) (7) Ben-Avraham et al, Liquid Toner and Method of Printing Using same, U.S. Patent 7,078,141. (2006) Continued from page 189 doc/ppm_bref_1201.pdf (2) Barton, D and Miner R, Experience at Recycled [-Fiber] Paperboard and Containerboard Mills Operating at or Near Zero Discharge, NCASI Technical Bulletin 733 (1) (1977) (3) Walraen, G. O et al, Closed Process Water Loop in NSSSC Corrugating Medium Manufacture, EPA-600/ , 71p, (1977) (4) Green Bay Packaging Ltd. Fact Sheet. WI (February ) (5) Webb, L, Kidney technology brings success, Pulp and Paper International, 44(4):28-32, (2002) (6) Alexandersson, T, Water Reuse in Paper Mills Measurements and Control Problems in Biological Treatment. Licentiate Thesis, Lund University, Lund, 126pp, (2003) (7) Hamm, U and Schabel, S. Effluent-free papermaking: Industrial experi- ences and latest developments in the German paper industry. Water Sc. Technol, 55(6): , (2007). (8) Habets, L. H. A and Knelissen, H. J, In Line biological water regeneration in a zero discharge recycle paper mill, Wat Sci Tech 35(2-3):41-48, (1997) (9) Norris, P.J, Water reclamation operations in a zero effluent mill, Tappi International Environmental Conference, Vancouver, Book 3, pp , (1998) (10) Aukia J-P, Managing a fully closed water cycle calls for expertise, proper biocide: Durango-Mckinley Paper is one of the few mills to operate a completely closed water cycle and was recognized for its success this spring by the EPA. Pulp Pap 79(11) pp (2005) (11) Filmer, J. Stainless steel cross-flow micro-filtration and its applicability to the pulp and paper industry Proceedings 60th Appita Annual Conference, 194 Appita Journal Vol 63 No 3