Membrane Technology Brochure

ENVIRONMENTALTECHNOLOGYBEST PRACTICEPROGRAMMENEW PRACTICE: New technology and techniques for profitable environmental improvementM E M B R A N E T E C H N O L O G YT U R N S E F F L U E N T I N T OC O S T S AV I N G SA NEW PRACTICE CASE STUDY AT KRONOSPAN LTDThis Case Study demonstrates the economic and environmental benefits of using membranetechnology to recover re-usable water and fibres from effluent generated during woodfibreboard manufacture. The manufacture of medium density fibreboard (MDF) at Kronospan Ltd’s site in North Walesgenerates large amounts of effluent with a high chemical oxygen demand (COD) and suspendedsolids content. Previously, this was tankered off site for treatment and disposal. However, thiswas expensive and interrupted production if the tanker was late. The Company, therefore,investigated various alternatives, finally installing a membrane-based system that allows virtually100% material recovery and re-use of treated water. This hybrid plant uses coagulation/flocculation followed by filtration to remove virtually all the suspended solids from the treatedfeed-water before it passes through reverse osmosis, cross-flow membranes. Kronospan optedfor a low-risk option, negotiating a lease-purchase agreement under which the supplier operatesthe plant for a monthly fee that is about half the cost of off-site effluent disposal. The benefits of using a membrane-based system at Kronospan Ltd include: n Net annual cost savings of over £250 000n Tankering of nearly 48 000 m3/yearof effluent avoidedn Mains water consumptionreduced by 44 000 m3/yearn Replacement of about480 tonnes/year of raw materials with recovered solidsNC259FINAL RESULTSMDF Production at KronospanThe process involves a number of simple, but large-scale,steps. Forest ‘thinnings’ and sawmill residues are de-barked and chipped before being washed to remove sand,grit, etc. The fibres are then softened in a pre-steamprocess. Excess water is removed by a screw press beforethe chips are ‘cooked’ at 70 - 80°C. The chips are then‘refined’ between two flat plates and passed throughheated cyclones to drive off most of the remainingmoisture. The light and aerated fibres are poured on to aconveyor and premixed resin (prepared in the site’s gluekitchen) is added. During the forming/pressing stage, theheated mix is passed between two moving steel belts tocompress it to the desired thickness. The formed boardsare then cut to the desired dimensions and stacked to cool.Water is used in large quantities for resin make-up, in chipwashing/refining, to raise steam and in air pollutioncontrol equipment. Most of the 47 800 m3/year of MDFeffluent is from the chip washing and refining stages. Thevariable moisture content of the wood changes both thequantity of steam required for softening and the volumeof effluent generated. Effluent quality (eg pH) also variesconsiderably. The effluent typically has a chemical oxygendemand (COD) of over 15 000 mg/litre and a suspendedsolids content of over 5 000 mg/litre. Organic componentsinclude celluloses, lignins and resin acids.Kronospan’s Previous Approach to Effluent DisposalThe high COD and suspended solids content of the effluentmean that it cannot be discharged to sewer withouttreatment. Evaporating the effluent in the site’s driers hadbeen tried, but proved too expensive in terms of energy andmaintenance requirements (capacity was also inadequate).The Company, therefore, began tankering effluent off siteat a minimum cost of £9.80/tonne. As well as the high cost,Kronospan was concerned about interrupted production ifthe tanker failed to turn up (the holding tanks being able tohold only a day’s worth of effluent). The Company,therefore, decided that it needed a low-risk, low-costsolution that provided maximum wastewater re-use andmaterial recovery.More Sustainable Effluent Treatment Options In 1993, Kronospan began evaluating various effluenttreatment options including conventional/biologicalsystems, mechanical systems and a membrane-based,reverse osmosis system1. Following pilot trials, most of thesystems were rejected as being either too costly orinadequate in terms of reliability or final effluent quality.However, the high technology membrane systemperformed well and offered the potential for waterrecirculation and material recovery, and hence significantcost savings. The proposed solution also allowed theCompany to meet some of its own environmentalimprovement goals and respond to increasing customerpressures. The only remaining problems were the capitalcost and the lack of experience in the UK of successfullyusing the technology with fibrous effluents. The Least-cost, Zero-risk Solution Adopted by KronospanTo minimise initial capital outlay, the plant was acquiredunder a lease-purchase agreement. Kronospan now pays afixed monthly fee to the equipment supplier to operatethe plant. To further reduce the risk to Kronospan, thesupplier-operator agreed to pay the costs of tankeringaway effluent if the plant failed to work satisfactorily.The plant, which was installed in June 1995, is a hybridsystem combining conventional physicochemical treatmentand high technology membrane systems (see Fig 1).The first stages of the treatment process (conventionalcoagulation/flocculation followed by filtration) removealmost 98% of the solids greater than 5 µm in diameter(typically 10 kg/m3), including colloidal material. ThisWater COD Suspended solids*(tonnes/day) (mg/litre) (mg/litre) (tonnes/day)Feed-water (holding tank) 129.6 16 220 5 870 0.77Filter cake 1.6 N/A N/A 0.75Filtrate (pre-membrane) 128.0 6 880 130 0.02Concentrate 7.0 Not measured 130 0.02Permeate 121.0 610 negligible negligible* Suspended solids measurements do not detect colloidal solids. An estimated 0.56 tonnes/day of other solids are removed from thewastewater in the filter cake. 1 An overview of membrane technologies is given in Good Practice Guide (GG54) Cost-effective Membrane Technologies for Minimising Wastes andEffluents, available free of charge through the Environment and Energy Helpline on freephone 0800 585794.Holding tank forwastewater fromMDF lineCoagulation/flocculation tankCarbonpolishing filterFiltrateContains 42% ofinfluent CODPermeateContains 3.8% ofinfluent CODReverse osmosis membrane modulesPolyFig 1 Flow diagram of effluent treatment plantTable 1 Average plant performance (1997/98)protects the membranes and reduces the need forbackwashing. The three-stage, cross-flow reverse osmosismembranes then remove most of the dissolved organics,thus reducing the total COD to less than 10% of the rawinfluent value. Suspended solids are reduced to virtuallyzero. To achieve the quality needed for re-use, themembrane permeate is ‘polished’ in a final carbon filter.The whole system is computer controlled. Averageperformance data for 1997/98 are given in Table 1.Performance has improved since then. COD levels in thepermeate are now typically as low as 200 mg/litre andrarely greater than 300 mg/litre.The spiral-wound, multi-layer, thin-film compositemembranes have a smooth finish to reduce surface build-up. They are chemically robust to allow high temperaturecleaning at low and high pH to remove surface build-upwhen it occurs. A valve protects the membranes fromhydraulic shock and thus prevents damage to end seals. Approximately 92% of the feed-water ends up as ‘clean’permeate and is used to provide some 60% of the boilerfeed water requirement. The ‘dirty’ concentrate containsuseful dissolved organics - including cellulose and lignins -and is added to the MDF resin binder. The filter presscake provides around 480 tonnes/year of dry solids as asubstitute feedstock for the wood fibreboard plant, thusallowing a corresponding reduction in wood use. Asshown in Fig 2, Kronospan has achieved zero effluentdischarge from its MDF manufacturing plant. Cost and Environmental BenefitsUnder the lease-purchase agreement2, Kronospan madean initial payment of £200 000 in February 1996 and afinal payment of £200 000 in February 1999. Thesupplier-operator charges a fee of £22 000/month, butthe net cost savings are over £250 000/year (see Table 2).The annual cash flow is always positive2 and the paybackperiod for the cost of the initial capital payment was lessthan ten months.The new technology has eliminated the need to tankereffluent off-site for disposal and reduced mains water useby 44 000 m3/year. All parts of the effluent are re-used inthe process; using the concentrate as an additive in theresin binder has actually improved product quality. Thesupplier-operator takes care of day-to-day operation andmaintenance, employment and training of operators. Theplant has recently been extended to take effluent from anew, higher capacity MDF line. Filter cake(45 - 50% dry solids)- recycled back intowood fibreboardproductsTreated water tank -recycled back toboiler feed waterConcentrate tank -recycled back toresin make-upKeyWaterSuspended solidsOther solidsMultimedia filter containing sand/anthracitePlate-type filter pressyelectrolyte addition (a few litres/day)Fig 2 Water mass balance for the MDF plant (1998 data)2 Information about investment appraisal and sources of finance is given in Good Practice Guide (GG82) Investing to Increase Profits and Reduce Wastes,available free of charge through the Environment and Energy Helpline on freephone 0800 585794. Water toraise steam(200 tonnes/day)Water in wood(459 tonnes/day)Mainswater(79 tonnes/day)ProcessWater in product(13 tonnes/day)Effluent(130 tonnes/day)Water evaporatedoff in driers(516 tonnes/day)MembranemodulesPermeate(121 tonnes/day)FilterpressFeedstockResinmake-upWater infilter cake(2 tonnes/day)Concentrate(7 tonnes/day)Filtrate (128 tonnes/day)Item Amount/year Cost savings (£/year)Avoidance of need for tankering effluent off site 47 800 m3 468 500Reduced mains water consumption 44 000 m3 29 480Replacement of raw materials with recycled fibre 480 tonnes 17 760Subtotal of cost savings 515 740Less cost of fees to supplier-operator of new treatment plant N/A 264 000Net annual cost savings 251 740Table 2 Savings summary for the new plant (1998 costs and production)THE ENVIRONMENTAL TECHNOLOGY BEST PRACTICE PROGRAMMEIS A GOVERNMENT PROGRAMME MANAGED BY AEA TECHNOLOGY PLC© Crown copyright. First printed March 2000.This material may be freely reproduced in its original form except for sale or advertising purposes.Printed on paper containing75% post-consumer waste.Kronospan LtdKronospan Ltd manufactures chipboard,medium density fibreboard (MDF), highdensity fibreboard (HDF), melamine-facedboard, sawn timber products and a range ofvalue-added products at its factory in Chirk,North Wales. The site, which employs over 500people, uses around 1 million tonnes ofprimary and recycled wood each year. TheCompany is part of the Austrian-ownedKronospan Group, which has 22 factories in 12European countries. Kronospan Ltd was oneof the members of the Dee WasteMinimisation Project. Host Company:Kronospan Ltd,Chirk,Wrexham,North WalesLL14 5NTMonitoring Company:ECOTEC Research and Consulting Ltd,28 - 34 Albert Street,BirminghamB4 7UDTel No: 0121 616 3643 Fax No: 0121 616 3699Mr M HiltonSupplier-operator:*ESMIL Process Systems,Westfield,London Road,High Wycombe,BuckinghamshireHP11 1HATel No: 01494 526155Fax No: 01494 474515Mr S FinnemoreFOR FURTHER INFORMATION PLEASE CONTACT THE ENVIRONMENT AND ENERGY HELPLINE ON 0800 585794world wide web: http://www.etbpp.gov.uk e-mail address: etbppenvhelp@aeat.co.uk * There may be other suppliers of similar equipment. Please consult your trade directories or contact the Environment andEnergy Helpline which may be able to give you more details on request.Comments from Kronospan LtdWe were looking for a sustainable and low-risk solution at minimum cost. This is what wehave got through the lease-purchasearrangement. Having the supplier to operatethe plant has proved cost-effective andtrouble-free. As well as the significant costsavings and environmental benefits, theproject has been the catalyst for furtherprofitable waste minimisation work and hasled to better understanding of the process ingeneral. The Group has also benefited: thesame technology being installed at factories inLuxembourg, Poland and Germany. “As well as the significant cost savings andenvironmental benefits, the project has been the catalystfor further profitable waste minimisation work ...”Mr T HackneyWorks DirectorKronospan Ltd