PRACTICAL WATER MANAGEMENT IN PAPER AND BOARD MILLS

Transcription

1 GG111 GUIDE ENVIRONMENTAL TECHNOLOGY BEST PRACTICE PROGRAMME PRACTICAL WATER MANAGEMENT IN PAPER AND BOARD MILLS GOOD PRACTICE: Proven technology and techniques for profitable environmental improvement

2 PRACTICAL WATER MANAGEMENT IN PAPER AND BOARD MILLS This Good Practice Guide was produced by the Environmental Technology Best Practice Programme Prepared with assistance from: Environmental Resources Management With particular acknowledgement for contributions from: Arjo Wiggins Fine Papers Ltd Aylesford Newsprint Ltd Inveresk plc The Paper Federation of Great Britain St Regis Paper Co Ltd Crown copyright. First printed February This material may be freely reproduced except for sale or advertising purposes. Printed on paper containing 75% post-consumer waste.

3 SUMMARY The UK paper industry is a major water user, and is under increasing pressure to improve the efficiency of its water consumption. This Good Practice Guide is intended to help companies assess and improve their management of water. Environmental Performance Guide (EG69) Water Use in UK Paper and Board Manufacture established that there are considerable variations in water consumption both within and between the various paper industry sectors. Most companies are likely to achieve significant cost savings by adopting best practice in this area, thereby improving their competitive position. Other benefits resulting from improved water management include: improvements in security of supply - a significant issue for mills in drought-affected areas; better relationships with regulators, employees and members of the public through the demonstration of sustainable water use policies. The first task for any company seeking to improve its management of water is to assess its current performance by either preparing water balances or undertaking a detailed water audit. In setting effective targets for water use, the company should take into account the wealth of experience that already exists among its staff and any limitations imposed by either plant or product. Modelling techniques can, where appropriate, be used to set targets, and companies should also consider running trials before making full-scale changes to plant operation. The steps that can be taken to reduce plant water consumption include: better overall management; design stage procedures (new plant/equipment); low-cost improvements, eg minimising hose use, production scheduling, leak detection and repair; process modifications such as optimising shower water use and re-using cooling water; process redesign, eg improving the quality and management of white water so that more can be recycled; total system closure with zero liquid effluent. This Guide includes several examples of UK best practice, including a detailed study of how one mill has almost halved its consumption of water in a four-year period by better management and by equipment and procedural modifications. It also includes a water use minimisation Action Plan.

4 CONTENTS Section Page 1 Introduction 1 2 Improving water management The potential The benefits 3 3 Assessing current performance Preparing water balances Staff resources required 7 4 Setting targets The importance of site experience Trials The value of modelling techniques Recognising the limitations 9 5 Improving water management and system design Overall water management The importance of education and training Design stage procedures 11 6 Low-cost improvement options Minimising temporary/accidental discharges Minimising hose use Individual equipment controls Production scheduling Leak detection and repair 14 7 Improvement through process modification Optimising shower water use Cooling water re-use Optimising sealing water use Alternative uses for white water Reconfiguring vacuum pump sealing water systems Installing dry vacuum systems Automatic tank cleaning systems 19 8 Improvement through process redesign Improving white water quality Improving white water management White water storage tank modification Recycling treated effluent 23 9 Overall system closure Concept Technologies for achieving zero liquid effluent 24 Continued overleaf

5 CONTENTS (continued) Section Page 10 Water management at Arjo Wiggins Buckland mill Water management drivers Commitment and involvement Management and monitoring Developing an understanding of the system Process improvements Problems and their resolution Possible future improvements Water use minimisation action plan 31 Appendix How to carry out a water audit in a paper mill 34

6 1 INTRODUCTION The UK paper industry is a major water user, and companies are under increasing pressure to reexamine and reduce their water consumption. The pressures are: 1 internal - associated with the need for companies to reduce their operating costs to remain competitive; external - associated with the security of water supply and the need to conserve water resources through careful use. These pressures are likely to increase for the foreseeable future. This Good Practice Guide demonstrates how companies can save money by reducing water consumption during the papermaking process. The information it contains is relevant to the main water-using operations/equipment including: pulping and stock preparation; de-inking; the wire, press and drier s of the paper machine; and the ancillary processes associated with these operations. This Guide can be used by production and technical staff to improve their water management practices. These practices range from the systematic management of all water used on a site - perhaps as part of an environmental management system (EMS) - to very specific measures such as reducing the water used for vacuum pump sealing or measuring flow rates accurately. 1

7 2 IMPROVING WATER MANAGEMENT 2.1 THE POTENTIAL 2 A recent survey has shown that UK paper mills vary widely in terms of water use per tonne of product (specific water consumption) (Fig 1). There are three main reasons for this: Different sectors of the industry have different water requirements. For example, the frequent grade changes associated with the production of fine quality writing papers incur higher levels of water use than the production of packaging materials. Different sites use different processing methods and equipment. A recently constructed, fully automated plant would, for instance, use less water than a year old plant producing the same product. Sites differ in the extent to which they have implemented systematic water-management and water-saving measures. There may, for instance, be a correlation with site water sources and supply costs. Where water is abstracted from rivers or boreholes, specific water costs are low compared with the cost of water from municipal systems, and this may have reduced the impetus for change. Packaging board (6 mills) Corrugated case materials (5 mills) Minimum Maximum Mean Newsprint (2 mills) 29 Printing and writing (10 mills) 32 Tissue (3 mills) 60 Speciality (8 mills) m 3 /tonne paper Fig 1 Specific water consumption range per sector Further details can be found in Environmental Performance Guide (EG69) Water Use in UK Paper and Board Manufacture available free of charge through the Environmental Helpline on However, while circumstances vary from mill to mill, the wide variations in specific water consumption suggest that there is considerable scope for improving water management throughout the industry. 2

8 2.2 THE BENEFITS The benefits of improved water management include: lower costs of water use; improved security of supply; ability to demonstrate compliance with regulatory requirements, such as those in Integrated Pollution Control (IPC) authorisations; improved relationships with regulators, employees, the general public and other stakeholders in the business. In addition, a water management and re-use plan can be an important component of other environmental strategies, including: 2 environmental management systems (EMS), the adoption of which is expected to become increasingly widespread throughout industry and commerce as companies endeavour to improve their environmental performance and demonstrate this to interested parties; international environmental management standards such as ISO 14001, which are already being adopted rapidly in the paper industry and which include a commitment to continuous improvement and the prevention of pollution. Arjo Wiggins Fine Papers Ltd s Buckland mill has recently implemented a systematic water conservation programme to reduce water consumption and drain losses. In a four-year period from 1991, the Company s average water consumption fell from 35 m 3 /tonne of product to approximately 18 m 3 /tonne. At the same time drain losses, expressed as the percentage of total suspended and dissolved solids compared with gross production, fell from 4.3% to approximately 2%. These reductions are the result of a series of step-by-step improvements which, to be effective, have required the total commitment of the staff. More details of this industry example are given in Section Lower costs of water use The average cost of raw water for paper manufacture varies from 3 pence/m 3 where supplies are abstracted from boreholes or watercourses to 50 pence/m 3 where the source is a municipal water supply. Given the large quantities required for paper manufacture, water supply costs can be a significant component of total production costs. Reducing the demand for raw water can therefore reduce costs significantly. Reducing water consumption is also likely to reduce the capital and/or operating costs associated with: the treatment of incoming water from boreholes and surface water sources; pump operation and maintenance; the installation of pipework, vessels, etc; on-site effluent treatment to meet required discharge standards; off-site effluent disposal. Further details of the costs associated with effluent treatment are given in Environmental Performance Guide (EG69) Water Use in UK Paper and Board Manufacture. In the case of an effluent treatment system, where the capital cost has been budgeted at 2 million, a 20% reduction in effluent flow can be expected to reduce these costs by There will also be reductions in operating costs such as pumping and chemical dosing. 3

9 2.2.2 Improved security of supply The growing demand for water in the UK is putting the country s water resources under increasing pressure. Some mills have already been asked by the Environment Agency (EA) to reduce the volume of water abstracted from surface watercourses, and it is quite possible that, in locations where water supplies are insufficient, paper mills and other large water users could be asked to cut back production. 2 By making significant reductions in their water use, mills should be able to improve the long-term security of their water supply, which is critical if their papermaking operation is to survive. For example, in some drought-stricken areas of the UK, manufacturing companies that could demonstrate a 20% reduction in water use were told that they would be exempted from possible interruptions in supply Compliance with regulatory requirements Mills that are authorised processes under the Integrated Pollution Control (IPC) regime are required to demonstrate that they comply with the concept of BATNEEC (best available techniques not entailing excessive cost) with respect to pollution control. 1 This involves: minimising water use; using appropriate techniques to reduce the risk of contaminating process or surface water; recycling water within or between processes. Even mills that are exempt from current UK IPC regulations may, in future, have to adhere to the EC Directive on Integrated Pollution Prevention and Control (Council Directive 96/61/EC). This IPPC Directive requires the use of best available techniques (BAT) to reduce pollution Improved relationships A water conservation programme can be used to improve important working relationships: with regulators - by showing that the company is working with the authorities to resolve problems of mutual concern; with staff - by improving levels of staff involvement and ownership: most practical suggestions and measures for reducing water consumption can be expected to come from staff who are operating the process on a daily basis; with the surrounding community - by demonstrating that water resources are being conserved for other users. After six months of below-average rainfall, a mill was asked by the Environment Agency to review its water use with a view to reducing by 50% the volume of surface water abstracted. The mill has implemented a series of measures, reducing its fresh water consumption over a one-year period from m 3 /day to m 3 /day. The site s effluent treatment plant has also improved its overall removal efficiencies. These positive achievements have allowed the mill to develop a good working relationship with the local regulators. 1 Chief Inspector s Guidance to Inspectors, Process Guidance Note IPR 6/9, Papermaking and Related Processes, Including Mechanical Pulping, Recycled Fibre and De-inking. 4

10 3 ASSESSING CURRENT PERFORMANCE 3.1 PREPARING WATER BALANCES Before a mill can determine the scope of any water conservation programme, it will need to establish its current level of performance by preparing water balances both for the site as a whole and for individual items of equipment. The following information will be needed for a full site water balance: metered fresh water intake volumes; metered effluent volumes; estimates of any other water inputs to the production process, eg from chemical additions and from pulp; estimates of water outputs that do not form part of the metered effluent, eg water evaporated from paper machine drying s and vacuum pump exhausts; estimates of the effect that stormwater discharges will have on metered effluent flow rates. 3 To obtain the relevant data, some preparatory work may be required: Additional monitoring of effluent flow rates. A review of design specifications to assess possible evaporation losses. A review of the drainage plans and diagrams to determine the contribution of surface runoff/storm water to effluent volumes. In many cases, significant volumes of storm water are discharged with the process effluent. The mill can use the data obtained to determine its specific water consumption. This provides a measure of the efficiency of water use. The same approach can be applied to individual items of equipment. This requires: metering the fresh water intake of individual machines; measuring the effluent flow rates from each machine, using appropriate flow measurement structures (eg weirs or flumes) in the effluent channels; a review of design specifications to identify the range of flow rates that can be expected. The need for additional metering can be minimised by using mass balances to estimate flow rates where appropriate. The information obtained can be used: To construct a water balance diagram, showing water use in different parts of the process (Fig 2). To provide a detailed breakdown of site water use (Fig 3), allowing staff to identify priorities for action in a water conservation programme. To facilitate checks on drain losses. This can be done on a shift basis and, provided the information is reported promptly, will allow the rapid identification of trends and unusually high losses, and ensure that corrective action is taken before significant increases in running costs occur. 5

11 River PM1 PM1 1.5 PM2 3.5 Cooling water 48 Cooling water Turbine pond 535 Turbine overflow 60 Condensate return PM2 PM3 Beater floor Effluent plant 3 Freshwater Broke/starch Unmetered flow Springwater Water balance (total flow to river total flow into mill) Cooling water to river = 5.0 Effluent flow to river = Evaporation = 6.5 TOTAL = Freshwater into mill = Springwater into mill = 4.4 Mutrator pumps = 0.7 Rainfall = 1.2 TOTAL = Difference = 4.8 Fig 2 Site water balance for a typical UK paper mill Application Vacuum pump Wire showers Disc thickener showers Pump gland sealing water UHLE box lubrication showers Sheet knock off UTM pulper showers Chemical carrying water Hoses Miscellaneous Felt cleaning Screen rejects dilution Refiner sealing water Total water use (%) Fig 3 An example of a breakdown of water use by percentage 6

12 3.2 STAFF RESOURCES REQUIRED Preparing a detailed water balance can require a significant commitment of staff time, which is always at a premium in the fast-moving production environment of a paper mill. Some mills have solved this problem by using university undergraduate or postgraduate students on placement to carry out the bulk of the initial assessment work. An alternative approach is to use outside contractors. Charles Turner & Co Ltd in north-west England produces approximately tonnes of tissue each year. The Company used two students from the Department of Paper Science at UMIST to carry out an initial site water and fibre balance over a three-month period. This involved preparing system process flow diagrams and measuring water flow rates and effluent fibre concentrations. Undertaking the water and fibre balance has allowed the Company to: 3 identify the main water-using processes within the mill, including some for which no records had been available; establish a benchmark for water use and identify opportunities for improvement; make an initial reduction in site water consumption of about 15%; initiate a second-stage project to reduce the water use of each paper machine. 7

13 4 SETTING TARGETS Fig 1 in Section 2.1 shows the considerable variation in water consumption within individual sectors of the paper industry. While site-specific factors may account for some of this variation, it should be realistic for most mills to set performance targets towards the lower end of the range for their sector. Table 1 lists suggested target ranges for effluent flow per tonne of production in different industry sectors. However, in determining its level of water re-use, each site will need to take into account both the quality of its incoming water and site process chemistry. 2 Actual effluent flow targets will, therefore, be site-specific. 4 Water consumption figures are generally m 3 /tonne higher than those for effluent flow, although this relationship will vary from mill to mill. Type of mill Target range for effluent generation (m 3 /air-dried tonne) Recycled fluting 0-3 Recycled multi-ply board 2-5 De-inked newsprint 8-10 De-inked tissue Printing and writing De-inked printing and writing Coated board 8-10 Lightweight coated paper Coated fine paper Table 1 Effluent flow targets for different sectors of the paper industry Several mills in the UK and abroad are moving towards a zero liquid effluent (ZLE) discharge target. While it is unlikely that every mill can achieve such a target in practice, a zero target may be useful as a means of driving forward a continuous reduction in water use. The concept of ZLE discharge is discussed in more detail in Section 9 of this Guide. 4.1 THE IMPORTANCE OF SITE EXPERIENCE Site staff often have first-hand experience of the tolerance of individual processes to variations in water quality that may have arisen during periods of abnormal operation. It is therefore important for companies to make full use of this experience when assessing the quality of water for re-use and setting targets. Brainstorming sessions may be an appropriate way of making the relevant information available. 4.2 TRIALS Any change in the pattern of water use has the potential to disrupt production, with serious consequences for company profitability. When a company does not already have a detailed understanding of the effects of individual water quality parameters on production, it is important for trials to be carried out before targets are set and full-scale changes made to patterns of water use. 8 2 There is, to date, little published information on water quality parameters for specific re-use applications, although the following guidelines have been published: Canadian Water Quality Guidelines, Canadian Council of Resource and Environment Ministers, March 1987; Water Use Reduction in the Pulp and Paper Industry 1994, Canadian Pulp and Paper Association, November 1994.

14 4.3 THE VALUE OF MODELLING TECHNIQUES Several computer-based techniques can be used to determine lower limit values for water conservation using process-specific parameters. These are briefly discussed below: Water pinch analysis uses computer modelling to optimise water re-use in complex manufacturing operations and has already been applied successfully in the chemical industry (see New Practice Case Study (NC55) Water Pinch Study Pays Major Dividends). However, although this methodology has been used in water conservation studies on newsprint production in the Netherlands, it has not yet been widely applied in the UK paper industry. Proprietary modelling programmes are available for paper production processes, some of which allow the dynamic modelling of mill systems. They can be used to predict the effects of specific process changes and are therefore very useful for investigating what if scenarios. Several UK mills have developed their own spreadsheet-based process models. These have been used to assess the effects of changing process parameters and to facilitate performance monitoring and reporting. 4.4 RECOGNISING THE LIMITATIONS 4 There may be certain factors that limit the potential for reducing water consumption: Product quality specifications may limit the extent to which water can be recycled. Food grade products, for instance, may have an upper limit for chloride ion concentrations, thereby limiting the maximum that can be tolerated in the mill s white water. Water recycling can increase the likelihood of slime build-up and machine deposits, although it may be possible to resolve the problem by varying the biocide and chemical addition programmes. Closing the white water system is likely to increase white water temperatures. Although this can have the positive effect of improving sheet drainage, it can also contribute to higher levels of biological activity and corrosion. This, in turn, may necessitate a review of construction materials and greater use of stainless steel or plastic components. Furthermore, where white water is used to seal liquid ring pumps, a higher white water temperature may reduce the level of vacuum produced by those pumps. Vacuum pump sealing systems are discussed in Section 7.5. A reduction in effluent volume can sometimes be associated with an increased level of pollutant concentration. However, there should be no increase in total pollutant load. Where effluent is treated on-site, treatment plant efficiency can be expected to improve. Where effluent is disposed of off-site, it may be possible, after discussion with regulators, to alter discharge consent limits, basing these limits on the actual pollutant load discharged rather than on pollutant concentration. A mill that reduced its fresh water consumption by more than 50% subsequently experienced higher white water temperatures and increased microbiological activity. By working closely with chemical suppliers the biocide dosing regime was adjusted to maintain counts at levels that are low enough to avoid operational problems. 9

15 5 IMPROVING WATER MANAGEMENT AND SYSTEM DESIGN 5.1 OVERALL WATER MANAGEMENT Long-term improvements in water conservation are likely only if measures are taken as part of an overall water management plan that can be integrated with other continuous improvement initiatives. The criteria for the success of a long-term water conservation programme have much in common with those needed in waste minimisation and quality improvement programmes. The methodology here is similar to that often used in waste minimisation campaigns (see Good Practice Guide (GG26) Saving Money Through Waste Minimisation: Reducing Water Use). This can be summarised as follows: 5 Obtain the commitment of senior management at an early stage, preferably to specific and achievable water reduction targets. This is essential to the long-term success of any water conservation programme and will involve the allocation of both staff and financial resources to that programme. Assemble a team that will implement the water conservation programme. This should include both management and production staff, particularly those operators and supervisors who are most familiar with the production processes. Identify a champion who will take responsibility for driving the programme forward to success. Identify opportunities for water conservation and assess their feasibility. Techniques that can be used at this stage include brainstorming, the comparison of design data with operational practice and comparisons with other, similar processes. Compare machines processing the same product mix. Prioritise water conservation opportunities, perhaps on the basis of expected financial payback. Implement the most attractive opportunities and assess their performance. Consider the effect of a reduction in water use on the performance of on-site water/effluent treatment plant. This not only demonstrates the programme s achievements, but helps to make sure that closing up the water loop does not give rise to other problems. Communicate the results achieved to the project team, the staff and other stakeholders. A number of mills use an annual environmental report to communicate improvements in environmental performance. Implement other, lower priority projects. It is particularly important to consider the possible effects of process change before implementation. This will help to determine monitoring requirements. Close monitoring of actual effects is essential when product specifications are likely to be affected. Methodologies based on the above principles have been used successfully in a large number of waste minimisation projects in the UK. Table 2 summarises some examples of management and organisational techniques that are being used successfully in UK paper mills. 10

16 Management technique Daily reporting of water consumption and drain losses for individual paper machines. Monitoring water use/drain losses over specific financial periods and setting targets for both. Checking pump/valve seals for leaks as part of routine engineering checks. Use tour checklists. Assessing, at the feasibility stage, the effects of each project on water consumption. Adopting this approach as a formal project management procedure. Introducing and reviewing standard operating procedures to improve water conservation. Using incentive/suggestion schemes. Benefit Allows rapid identification of non-routine conditions so that prompt remedial action can be taken. Particularly effective if reported at morning production meetings. Motivates staff to assess and improve their performance in these areas. Maintains staff interest. Allows quick response to leakage which, if unattended, may cause significant water losses. Facilitates the incorporation of water conservation measures at an early stage in a project, when changes in design are not too expensive. Procedural improvements can reduce effluent losses with very limited investment. For example, fibre losses to drain are often caused by operators dumping chests. This can be controlled by requiring supervisor authorisation. Encourages staff involvement and rewards positive contributions to the programme. Table 2 Practical management techniques 5 Mills can maximise the likelihood of achieving long-term reductions in water use by applying all the techniques listed in Table 2 as part of a water conservation programme. Where management resources are insufficient for a comprehensive programme, then the adoption of a more piecemeal approach can nevertheless achieve significant reductions in water use. 5.2 THE IMPORTANCE OF EDUCATION AND TRAINING The importance of education and training for all staff cannot be overstated. Only staff who are motivated and aware will be able to maintain a policy of continuous improvement in water conservation in the longer term. Raising staff awareness of water conservation issues could be included as part of environmental management system training. It is vitally important that education and training are used at an early stage in a water conservation programme: to raise and maintain staff awareness and understanding of the issues involved; to create commitment to the programme. Initial induction training should be followed up by regular briefings, articles in newsletters, etc. 5.3 DESIGN STAGE PROCEDURES Water use should be an essential consideration at the design stage of any project, whether this involves installing new papermaking capacity or replacing or refurbishing existing items of equipment. Suppliers of new paper machines should be asked to demonstrate those aspects of the design that minimise water consumption. There will often be significant variations between designs. 11

17 Opportunities for the cascade re-use of effluent should be assessed (ie the use of wastewater from one operation as the feed water for another operation that can tolerate a lower quality feed water). For example, it is standard practice to use paper machine white water as feed water for de-inking operations. Consideration should be given to the segregation of highly contaminated effluents from those which are more dilute. This will facilitate cost-effective treatment and possible re-use. For example, it is usual to separate coating effluents with a high concentration of suspended solids from more dilute effluent streams. Equipment replacement/refurbishment may provide opportunities for reducing water consumption. For instance, it may be possible to replace pumps that require sealing water with dry sealing pumps. A UK contractor has designed a packaging board mill with an initial specific water consumption of 7 m 3 /air-dried tonne (ADt) of product. This is 50% of the current average water consumption of packaging board mills in the UK. Once performance has been optimised after commissioning, the mill is expected to operate using as little as 4 m 3 /ADt of product. 5 12

18 6 LOW-COST IMPROVEMENT OPTIONS There are several techniques that, together, can significantly reduce water consumption with the minimum of capital investment. They are often referred to as good housekeeping measures, and many mills in the UK have already reduced their water consumption by adopting such measures. Good housekeeping also plays a vital role in increasing staff awareness of water use and losses, and in encouraging their involvement in water management. As processes are increasingly modified to close up water systems, it is expected that a greater proportion of water losses will be attributable to the actions of individual staff. 6.1 MINIMISING TEMPORARY/ACCIDENTAL DISCHARGES Temporary/accidental discharges arise from washing down equipment and floors and from overflowing tanks and vessels. In the absence of engineering controls such as alarms and high-level cut-out switches on tanks, only the motivation and awareness of operators and supervisors will reduce these losses. A significant increase in machine speeds at one mill resulted in frequent overflows from the trays and flumes positioned to collect water beneath the machine wire. The trays and flumes were found to be undersized for the new flow rates. Enlarging selected trays has eliminated these losses from the white water system MINIMISING HOSE USE Uncontrolled use of hoses can cause significant water losses, but hoses are often left unattended and running because the operator has been diverted to more urgent tasks. Various measures can be taken to conserve hose water consumption: fitting all hoses with triggers to ensure that they cannot be left running or leaking when unattended; reviewing hose and nozzle size: both are frequently oversized for their required duty, resulting in excessive water consumption; installing high-pressure, low-volume systems; reviewing the need for hoses in individual locations. 6.3 INDIVIDUAL EQUIPMENT CONTROLS Installing manually adjustable flow control valves and flow meters on individual items of equipment will allow operators to monitor and adjust individual flow rates and help to optimise water use. Another option may be to reduce mains water pressure, thereby reducing all water flow rates from the main water supply circuit. By providing separate shut-off valves, the flow control valves do not need to be readjusted every time a piece of equipment is isolated. 13

19 6.4 PRODUCTION SCHEDULING Careful production scheduling can minimise the amount of cleaning required when manufacturing different grades of product. In some cases it will be possible to use the white water from certain grades as make-up water for other grades. 6.5 LEAK DETECTION AND REPAIR Leak detection and repair is an essential part of good housekeeping. The process pipework systems in mills can be extremely complex, especially in older mills where there has been extensive modification and refurbishment. In some mills considerable leakage will occur through corroded pipes, pump seals and valves. Several measures can be implemented to reduce leakage: 6 Review pipework systems and remove redundant runs, prepare accurate process flow, pipework and instrumentation diagrams, and ensure that all pipework is clearly labelled and coded. It may be appropriate to employ an external contractor or student for this task. Install new pipework above ground to make leak detection easier. Incorporate regular visual checks for leaks from seals, valve glands, etc in standard operating procedures (Table 2). Scheduled maintenance shutdowns can be used to make more detailed system leakage checks. Incorrectly operating float valves are a frequent source of leaks from tanks. Leaks can be minimised by regularly checking tank level controls and automatic top-up systems to make sure they are operating correctly. Leak detection and repair can often be incorporated into existing preventive maintenance schedules. Alternatively, it may be more appropriate to use an external company to carry out a leak survey. A mill in south-west England carried out a flow survey of its process pipework system as part of a project to improve the performance of its effluent treatment plant. Clamp-on ultrasonic flow meters and data loggers were used to record flow rates in large diameter pipes. The results showed a significant leakage to drain via shut-off valves. This leakage could not be observed by operators and, without this survey, would probably have continued undetected. 14

20 7 IMPROVEMENT THROUGH PROCESS MODIFICATION There are several process modification options that can be implemented to reduce water consumption and effluent flows. 7.1 OPTIMISING SHOWER WATER USE Paper machine showers are one of the largest users of fresh water in a mill. Even modern, welldesigned showers can require 10 m 3 of water per tonne of product. The Environment Agency suggests that, where no steps have been taken to optimise water use, it is possible to reduce consumption by an order of magnitude. 3 Measures that can be taken include: adjusting the total number of shower nozzles, their positions, jet angles, and the distance between nozzles and the paper machine felt/wire to minimise water use; varying water temperatures and pressures to determine whether effective cleaning can be achieved at lower temperatures and flow rates; using different types of nozzle, eg flat, needle jet; using sprays intermittently, eg for 10 minutes/hour; using steam condenser cooling water in showers. It may be possible to use clarified white water in showers, although the feasibility of this approach needs to be considered on a case-bycase basis. In general, fresh water has always been used on felt sprays. Nevertheless, it may be possible to use clarified water for this purpose after additional filtration. High-pressure spray nozzles are prone to blockage by relatively small particles so particular care is needed when using clarified water. 7 Table 3 provides examples of where white water and fresh water are typically used in showers. Table 4 summarises the guidelines published by one equipment manufacturer for the limiting solids contents for shower duties. The data given are for guidance only, and individual mill experience should be used to determine the practical limits to water re-use for showers. Advice can also be sought from nozzle manufacturers. Fig 4 Optimising paper machine water sprays can significantly reduce fresh water consumption 3 Chief Inspector s Guidance to Inspectors, Process Guidance Note IPR 6/9, Papermaking and Related Processes, Including Mechanical Pulping, Recycled Fibre and De-inking. 15

21 Machine Typical Locations where Locations where consumption white water fresh water is (m 3 /tonne) may be used typically used Wire Breast roll Wire-cleaning shower Wire-turning and wire-return rolls (high pressure) Knock-off shower Couch suction box Trim knock-off shower Trimming Wire-cleaning shower Headbox Press 5-15 Cleaning of rolls Conditioning of felts Lubricating showers for felt suction box Lubricating showers for press roll suction box Fibre-recovery system 0-5 All positions Broke system 0-5 All positions Table 3 Types of water used in showers Solids content* (mg/litre) Recommended shower design Equivalent to filtered fresh water, no restrictions on use Can be used in fixed orifice nozzles of 1 mm diameter and larger Can be used in fixed orifice nozzles of 1.5 mm diameter and larger Can be used in fixed orifice nozzles of 3 mm diameter and larger Brush-type shower recommended >500 Purgeable shower recommended * Solids content of white water from which the long fibre has been removed in a saveall and the water polished using a 100-mesh screen. Source: Thermo Fibertek UK Ltd Table 4 The limiting solids content for shower duties A mill in north-west England installed a simple two-stage treatment system for clarifying white water. Polymer is used to cause flocculation and flotation of the solids. A spray arrangement then skims the flocs from the surface of the tank. This produces clarified white water with a suspended solids concentration in the range mg/litre. This water is used on machine wire and high-pressure washdown showers, and for sealing low-vacuum boxes. Intermittent high-pressure showers currently use fresh water. 7.2 COOLING WATER RE-USE Each mill is likely to have several individual cooling circuits, eg: steam turbine cooling; compressor cooling; winder brake cooling; steam condenser cooling; refiner gearbox cooling. 16

22 Although these may have been installed initially as once-through cooling systems, there are several ways in which cooling water use can be optimised: The collection of once-through cooling water for piping to process water storage tanks for re-use. Cooling water may be used in several different site locations, with collection involving the installation of several small sumps or tanks from which water is subsequently pumped to process water tanks. It may also be possible to re-use some cooling water directly for certain specific applications such as shower water. The higher temperature of used cooling water is an advantage in some shower applications. The conversion of once-through systems to closed-loop operation. This requires some form of temperature control using cooling towers, the periodic injection of fresh water in response to a temperature control system, or heat exchangers in which the heat lost from the cooling water is used to pre-heat other process streams. A UK mill using 200 m 3 /day of cooling water for a steam condenser has modified its original once-through system and piped the used cooling water to a process water storage tank for reuse. This relatively simple change has significantly reduced water consumption. Fig 5 shows a diagrammatic representation of a temperature-controlled closed-loop system. Temperature control valve Warm water Cool water Hot water To recovery 7 Fig 5 Temperature-controlled closed-loop recirculation system 7.3 OPTIMISING SEALING WATER USE There are large numbers of packed sealing glands in paper mills, for example, on pumps and agitators. Packed glands generally require water for lubrication and, in many instances, the systems installed use fresh water. Although the water flow to each gland is usually relatively low, the total volume can be significant because of the large number of glands involved. There are two possible water management options: Using clarified water for gland lubrication. This may involve a period of trials and monitoring to ensure that the quality of the clarified water is adequate. Replacing the packed glands with mechanical seals. If it is not possible to install mechanical seals, meters can be used to establish rates of water consumption and to identify opportunities for optimising water use in sealing duties. The introduction of mechanical seals on 70 pumps in a UK tissue mill reduced water consumption by 250 m 3 /day, equivalent to 5 m 3 /tonne of product. 17

23 7.4 ALTERNATIVE USES FOR WHITE WATER White water can be used as a feed to certain hoses, eg those used for general floor cleaning. Consideration needs to be given to water quality in this application. In mills with near-closed water systems, microbiological counts can be relatively high, and the use of white water for cleaning can cause a build-up of slime on surfaces. Another use for white water is to lubricate low-vacuum boxes on the paper machine. 7.5 RECONFIGURING VACUUM PUMP SEALING WATER SYSTEMS Vacuum is used in many parts of the papermaking process, primarily to assist de-watering on the wire and press s. Some of the applications are listed in Table 5, together with the level of vacuum required. Application Foil boxes (centrifugal fans have been used in this application) Suction boxes Rotary suction boxes (on heavier grades) Suction couches Vacuum boxes on pick-up, transfer and other press rolls Level of vacuum required Up to 7 kpa (27 inches water gauge) Up to 40 kpa (12 inches mercury gauge) Up to 68 kpa (20 inches mercury gauge) Up to 80 kpa (24 inches mercury gauge) Up to 74 kpa (22 inches mercury gauge) 7 Felt cleaning and blow boxes Up to 68 kpa (20 inches mercury gauge) Table 5 Likely vacuum applications in the paper industry 4 Liquid ring vacuum pumps are used for most vacuum applications. They are robust, relatively cheap, and are considered to have an appropriate operating characteristic for paper mill duties. Water is used for sealing. Additional training for operators will be required to ensure effective installation and maintenance of these devices. The level of vacuum that can be achieved by liquid ring vacuum pumps varies with the temperature of the sealing water, and this must be taken into account when considering sealing water recirculation. For lower levels of vacuum (ie less than 68 kpa or 20 inches mercury gauge) it may be possible to operate with higher sealing water temperatures. However, for higher vacuum applications lower sealing water temperatures may be required. There are several options for minimising water use in these systems: Cascading sealing water from high vacuum to low vacuum duties (Fig 6). Re-using the heated sealing water as process water, eg as a preferential feed for wire showers. In this case, it is usual to maintain the shower water temperature within 2 C of the stock temperature to improve performance. Using a closed-loop sealing water system with fresh water injection (Fig 5) or a cooling tower to control temperature. Heat exchangers can also be used to cool vacuum pump sealing water and pre-heat shower water. Using clarified white water in place of fresh water. This would depend on the quality of water that can be tolerated by the pumps and the effectiveness of any white water treatment carried out. More details are given in Good Practice Guide (GG101) Reducing Vacuum Costs, available free of charge through the Environmental Helpline on Energy Efficiency Best Practice Programme Good Practice Guide No. 83 Energy Efficient Liquid Ring Vacuum Pump Installations in the Paper Industry.

24 Cool water High vacuum pumps Low vacuum pumps Warm water Hot water (approximately 28 C warmer than incoming cool water) Fig 6 Cascade use of sealing water 7.6 INSTALLING DRY VACUUM SYSTEMS Dry vacuum systems (ie centrifugal fans) have already been introduced for certain applications, and their more widespread use could significantly reduce overall water consumption. However, effective pre-separation is needed to remove water to prevent damage to the fan blades. A new water-free, high-speed centrifugal pump has been developed, incorporating an advanced control system and a motor driven by a variable-frequency inverter. The one-piece motor rotor/impeller is carried in contactless, oil-free, active magnetic bearings, which means that no mechanical drive system or lubrication is required. The resulting level of control allows the pump to operate with characteristics similar to those of a liquid ring pump. There are significant savings in installation costs, energy consumption and space requirements, though capital costs will be higher. 7 The new pumps have been installed at three paper mills in Scandinavia and are also being evaluated by UK mills. 7.7 AUTOMATIC TANK CLEANING SYSTEMS Process tanks such as machine chests are traditionally open vessels and are usually cleaned manually by operators using hoses. Some mills have now installed automatic clean-in-place (CIP) systems that use sprayballs mounted at the top of the vessel. Cleaning systems of this type are widely used in the pharmaceuticals and food and drink industries. For enclosed vessels they may be the only practical method of ensuring consistent cleaning to the required standard. The use of CIP systems for tank cleaning allows water use to be controlled and, where appropriate, reduced. The automation of cleaning operations is an additional benefit. 19

25 8 IMPROVEMENT THROUGH PROCESS REDESIGN Process redesign to minimise water use may be a more capital-intensive option, but it can show acceptable financial returns and paybacks through improved fibre recovery, reduced water consumption and lower effluent treatment charges. 8.1 IMPROVING WHITE WATER QUALITY There are several proven techniques for improving white water quality. These involve the use of savealls such as: disc filters; drum filters; inclined screens; dissolved air flotation (DAF); gravity sedimentation. There are three main benefits of installing savealls: fibre recovery for re-use (historically usually the prime reason for their installation); the improvement in white water quality associated with the removal of suspended solids; the associated possibility of re-using white water in applications requiring a high water quality Disc filters Disc filters use a number of hollow discs to achieve a high surface area for filtration. They produce two qualities of filtrate. Cloudy filtrate is produced initially. This has a relatively high suspended solids content and can be re-used for dilution. Clear filtrate is produced once a mat of fibres has built up on the disc surface to act as a filter medium. The suspended solids concentrations of the two filtrates are a function of the type of furnish used in the mill. With a recycled fibre furnish, typical concentrations of 250 mg/litre can be achieved in the clear filtrate and 750 mg/litre in the cloudy filtrate Drum filters Drum filters use a rotating cylinder covered with a mesh that acts as the filtration medium. Depending on the furnish used at the mill, they can produce filtrates with suspended solids concentrations of mg/litre. Because of their design, drum filters can cope with considerable variations in inlet flow and consistency. For this reason they are often used on broke systems. One UK fine paper mill installed stock thickeners on a number of machines over a four-year period. These, combined with improvements to the broke and white water systems, produced cost savings in recovered fibre of more than /year (1995 prices). Fresh water consumption fell by 20%. 20

26 8.1.3 Inclined screens Inclined screens provide a simple separation system for the recovery of fibre and the clarification of white water. They are used successfully in many UK mills (Fig 7). Fig 7 Fibre recovery screen DAF/gravity sedimentation DAF and gravity sedimentation are widely used for effluent treatment and can also be used for fibre and water recovery. Polymeric flocculating agents are often used to improve the removal of suspended solids in both cases. With their assistance, DAF units can produce clarified effluent with suspended solids concentrations of less than 50 mg/litre. A tissue mill in north-west England installed three DAF units to recycle fibre and water from paper machines and a de-inking plant. The capital cost of the system was approximately 2 million, but the resulting fibre and water recovery gave a payback period on investment of only two years. Furthermore, the effluent produced had a suspended solids concentration of less than 30 ppm which improved compliance with the effluent discharge consent IMPROVING WHITE WATER MANAGEMENT Most of the fresh water inputs into a paper machine are associated with batch operations such as pulping. Fresh water inputs can be minimised by using white water for pulping, as long as sufficient white water storage has been provided. If an extended break on the paper machine coincides with filling the pulpers, there may be insufficient white water to meet pulper demand and fresh water will have to be used. However, it may be possible to increase white water storage volumes by modifying existing tanks for a relatively small outlay. Frequent overflows from a mill white water system indicated that the capacity of the white water storage tank (25 m 3 ) was insufficient. Modifications have now been made involving the conversion of a machine storage chest (capacity 25 m 3 ) for white water storage, thereby doubling the volume available for this purpose. 21