### GENERAL ANALYSIS OF WATER RECY CLE AT PULP AND PAPER MILLS

Barry Malmberg, National Council for Air and Stream Improvement, Inc. (NCASI) West Coast Regional Center, Corvallis, Oregon, USA

1.0 PURPOSE AND APPROACH
This study was undertaken to quantify the extent of water recycling within the United States pulp and paper industry. The industry has a long history of reducing water use, and progress has been achieved through extensive water recycling in combination with use of less water-intensive process equipment as well as water conservation practices. Since 1959 there has been a 69% reduction in the average volume of treated effluent discharges at pulp and paper mills within the United States (NCASI 1983, 2009a; Miner and Unwin 1991). The first TAPPI 1 monograph published in 1942 dealt exclusively with various aspects of water use and reuse within pulp and paper mills (Fleming et al. 1942).
Water reuse is a more difficult metric to quantify than effluent discharge because reuse deals with internal flows for which industry-wide statistics are not available and because the same volume of water may be used and reused for several different purposes within a mill before final discharge. To provide some idea of the extent of water recycling within the industry, this analysis compares water use at pulp and paper mills under current operating practices to use at hypo thetical mills operating with no water recycle but producing the same product. Pulp a nd paper mills have always been designed with a certain degree of water recycling, so the hypothetical completely open mill cases have been developed for purely comparative purposes. A century ago, Van Winkl e (1914) reported that paper mills required 400,000 gallons of water per ton of product; thus, water systems in m ills constructed a century ago may have approached the completely open water design. In addition to comparing current mill water use to hypothetical completely open mill cases, hypotheti cal modern mill cases designed for minimum use of water and employing best commercially available technology were developed and compared to the hypothetical open cases. This was done to provide some context for the state of water recycling under current operating practices compared to a hypothetical b est case scenario. It shou ld be stressed that the water use intensities for the hypoth etical modern mill cases were drawn from engineering studies and do not represent water use at actual, operating mills. There are many mill-specific factors (e.g., production of colored grades or multiple grades, multiple pulping lines, product basis weight, operational disturbances) that inhibit most mills from approaching or achieving the hypothetical modern water use intensities applied herein, so the hypothetical modern mill cases should be used for general comparisons only.

2.0 SUMMARY
US mills save over 53.9 billion cubic meters (m 3 ) of water annually by using current water recycling practices compared to water usage at completely open m ills. To put this number in perspective, it would require 3.3 million US football field-sized swimming pools 10 feet deep to equal 53.9 billion m 3 . If all US pulp and paper production was derived from modern mills equipped with best commercially available technology for water re duction, 2.6 billion m 3 (or about 5%) of additional process water savings would be realized on an annual basis relative to a completely open mill water system. This small difference in water savings between the hypotheti cal open and current mill water u se scenarios and the hypothetical open and hypothetical modern water use scenarios indicates a high degree of water recycling under current mill operations. Table 1 shows summary water use and reuse statistics by broad pulping category. Actual water use intensities were taken from NCASI Technical Bulletin No. 960, Water Profile of the United Sates Forest Products Industry (NCASI 2009a), and are valid for 2006. Production from 403

1 Technical Association of th e Pulp and Paper Industry

1mills, essentially 100% of the total US pulp and pape r production in 2006, is represented in Table 1. Calculations for water use at modern and open hypothetical mills are detailed herein.

Table 1. Water Use and Reuse in US Pulp and Paper Mills

Water Use (m 3 /admt) Recycle Ratio Water Savings compared to Open (billion m 3 )

Category Number of Mills Actual 2006 a Modern Hypothetical b Open Hypothetical Open/ Actual Open/Modern Hypothetical Actual 2006 Modern Hypothetical Bleached chemical 77 82.9 53.1 972 12 18 33.8 34.9

Unbleached chemical 38 44.1 17.6 511 12 29 12.1 12.8

Semi-chemical 10 32.5 32.5 87.5 2.7 2.7 0.22 0.22 Mechanical 20 59.2 15.8 209 3.5 13 0.75 0.97 Recycle - deinked 35 57.5 21.0 814 14 39 3.89 4.08

Recycle – non-deinked 110 7.29 2.08 187 26 90 2.23 2.30

Non-integrated
paper 113 58.0 10.0 187 3 19 0.94 1.29

Totals 403 53.9 56.5 a) Values are category production weighted means b) Values are for hypothetical mills designed for minimum use of water and employing best commercially available technology Water savings compared to the hypothetical open sce nario in Table 1 are calculated by taking the category difference in water use intensities betwee n the hypothetical open case and either the actual 2006 or modern hypothetical case and multiplying by 2006 pulp and paper category production amounts found in NCASI Technical Bulletin No. 960 (NCASI 2009a).

3.0 CHEMICAL AND SEMI-CHEMICAL CATEGORIES
Table 2 compares water use at a modern bleached kraft mill to that at a hypothetical bleached kraft mill with a completely open water system. The results are displayed graphically in Figure 1, and details on how the values were derived are provided in Sec tions 3.1 through 3.6. The modern mill uses approximately 5.5% of the water used in the open case, or a savings of nearly 920 m 3 per air-dried metric ton (admt) of product produced. Most water savings originate in water recycle and reuse within the bleach plant. Process water (i.e., water used direc tly within the process to manufacture and clean pulp product) and non-contact cooling water (i.e., water used for cooling duty that does not come into direct contact with process water) are considered in the analysis.
Water use for a hypothetical modern bleached kraft mill is assumed to be 53.1 m 3 /admt (Table 2). Water use for a hypothetical modern unbleached kraft mill is assumed to be 17.6 m 3 /admt, the sum for the modern case in Table 2 minus the bleach plant water use contribution. For the completely open cases 972 m 3 water/admt is used for bleached kraft mills and 511 m 3 water/admt is used for unbleached kraft mills. Water use differences between the modern hy pothetical and open hypothetical cases for bleached chemical dissolving and sulfite mills are assumed to be the same as for modern and open hypothetical kraft mill cases. Water use of 32.5 m 3 /admt at modern semi-chemical m ills is taken from average water use at semi-chemical pulp mills operating within the United States in 2006 (NCASI 2009a). Water use of 264 m 3 /admt at open semi-chemical mills is taken from reported historical usage at these types of facilities (Haynes 1966).

2Table 2. Comparison of Water Use by Department for Hypothetical Modern and Open Bleached and Unbleached Kraft Mills Hypothetical

Open Case Hypothetical Modern Case Department (m 3 /admt) (m 3 /admt) Bleach plant 450 35 Paper machine or pulp dryer 187 10 Screening 173 0 Cooling 109 5.5 Brownstock and/or oxygen delignification 50 0 Fresh water in causticizing 2.7 2.7

Bleached kraft sum 972 53.1 Unbleached kraft sum 511 17.6

0 100 200 300 400 500 600 700 800 900 1,000

Fully Open 2006 USWater Usage (m3/admt) Causticizing and Brownstock Washing
Cooling

Screening

Pulp dryer

Bleach plant

Average US bleached

kraft mill

Figure 1. Comparison of a Fully Open Bleached Kraft Mill and a Modern Kraft Mill

3.1 Chemical Pulp Bleaching
Most water in bleached chemical pulp mills is used in the bleach plant for washing the pulp between bleach stages (Histed, McCubbin, and Gleadow 1996). For older kraft pulp mills (1960s designs) bleach plant effluent can represent around 70% of total efflue nt, and for newer mills (1980s designs) bleach plant

3effluent can represent around 40% of total effluent (C arter and Gleadow 2001). Wash water used in the bleach plant is reduced by recirculating it (termed filtr ate) in some countercurrent fashion. Table 3 shows water savings from various filtrate recycling approaches for a five stage bleaching sequence. Problems associated with corrosion or scale buildup are th e primary deterrents to implementing direct countercurrent washing, and this configuration is not currently used within North America (Lindgren, Berlin, and Brelid 2002).

Table 3. Water Use for Various Filtrate Recycle Configurations in a Typical Five Stage Bleach Plant Effluent Discharge (m 3 /admt of pulp) Filtrate System Theoretical Actual Open 450 450 Internal circulation within stages 55 55 Jump-stage countercurrent 23-25 30-40 Split flow countercurrent 23-25 30-40 Split flow jump stage countercurrent 11-15 12-25 Direct countercurrent 13-15 15-25 [SOURCE: Histed, McCubbin, and Gleadow 1996]

Water use of 450 m 3 /admt of pulp is used for the completely open case. The jump stage countercurrent configuration is considered to be the modern case, and 35 m 3 /admt (the average of 30 and 40 m 3 /admt) is used when comparing water use within modern ble ach plants to the completely open configuration.
3.2 Brownstock Washing and Post-Oxygen Washing
Water is used in the brownstock washing area to clean pulp and collect organic material and spent chemicals for energy and chemical recovery. Hist orically, fresh water was used on all brownstock washers, and use of over 50 m 3 /admt unbleached pulp was not uncommon (Twitchell and Edwards 1974). Changes such as countercurrent recycling of water in brownstock washing, oxygen delignification, and use of post-oxygen presses with high consistency pulp discharge have reduced fresh water usage within the brownstock and oxygen deli gnification areas to around 4 m 3 /admt unbleached pulp (Twitchell and Edwards 1974; Edwards et al. 1976). Steam conden sates from the digester and evaporator areas, when properly treated, can replace fresh water in the brow nstock washing area for pulp washing, eliminating fresh water use within the brownstock and oxygen delignification areas entirely (Sebbas 1988).
For the modern hypothetical case all process water us ed in the brownstock/oxygen delignification area is assumed to be from stripped evaporator condensates; th at is, fresh water use is zero. For the open case 50 m 3 water/admt is used.
3.3 Screening
Screening removes knots, shives, dirt, and other c ontaminants from good pulp fibers to produce a more homogenous pulp stream suitable for paper or board production. Modern brownstock screening is accomplished with pressurized screens and has comple tely closed liquid circuits (Gullichsen and Fogelholm 1999). Screening and cleaning is most eff ective when carried out at low consistencies, and screening operations have traditionally been performed at consistencies varying from 0.5 to 1.5% solids. Fiber suspensions are diluted with circulating liqui d, screened, and then thickened back to medium consistency (10 to 15%). Because of the low consis tency dilution, large amount s of water are used at each screening step (between 63 and 210 m 3 /admt, depending upon screeni ng consistency and thickener consistency) (Edde and Sebbas-Bergstrom 1974). Between 1940 and 1972 brownstock screening was usually carried out in two steps, coarse screening before brownstock washing for knot removal and fine

4screening after brownstock washing to remove shives, fines, and other small contaminants (Boughner and Braid 1988).
Pressurized screens and a completely closed liquid circuit are assumed for the modern hypothetical case; that is, freshwater use is zero. For the open case 173 m 3 /admt is used, assuming a two step screening system for knot removal and fine screening with both screening systems operating at a consistency of 1.0% with subsequent thickening to 12.5%.
3.4 Causticizing
Process water is necessary within the causticizing depart ment (part of kraft chemical recovery) to dilute molten smelt from the recovery furnace to a target green liquor strength (typically measured as total titratable alkali, TTA). A higher TTA target requir es less dilution water. Dilution and wash water from lime mud washing (collectively known as weak wash ) and wash water from the dregs washing operation is typically the source of dilution water. Aver age wood pulping (or white) liquor properties for North American mills in 1976 were: TTA of 116 ±6 g/L as Na 2 O, active alkali (AA) of 66 ±6 s/L as Na 2 O, sulfidity of 21 ±5% (on a TTA basis), and a causticizing conversion 2 of 81 ±4%, where ± is one standard deviation from the mean (Kojo and Pöyry 1985). At these white liquor properties, approximately 2.7 m 3 water/admt of pulp is required for process wate r dilution within the causticizing department. There has been a trend toward using higher concentration white liquors due to increased energy savings in the causticizing, pulping, and evaporation areas, a nd this trend will reduce the amount of process dilution water needed within the causticizing department (Koj o 1979). Stripped condensates can also be used in the causticizing area for lime mud washing and dilution or as scrubbers, but this practice is not typical in North America or Europe because of total reduced sulfur (TRS) emissions limits (Sebbas 1988).
For both the hypothetical modern and hypothetical open cases 2.7 m 3 water/admt is used.
3.5 Papermaking and Pulp Drying
Water requirements in the paper machine area are larg e, with water discharges ranging from 6 to 43% of the hydraulic load to wastewater treatment plants (WWTPs) at integrated mills and practically 100% of the hydraulic load to WWTPs in non-integrated mills (Mahmood and Elliott 2007). Paper machines with open whitewater systems and centrifugal and vacuum pump seals supplied with fresh water can use over 100 m 3 /metric ton of finished product (English and Lindstr om 2001). Figure 2 shows that if 100% fresh water is used for paper machine or pulp dryer h eadbox dilution process water discharge will be even larger than 100 m 3 /metric ton of finished product, in the range of 178 m 3 /admt (174 m 3 /admt for headbox dilution plus 4.2 m 3 /admt excess water from press thickening). Modern paper machines use between 5 and 26 m 3 water/metric ton of finished product, and the difference relative to an open system is through increased recycling and reuse of whitewater in the paper mill. Replacing fresh shower water with recycled whitewater is a well established means of water reduction within the paper machine area (Francis and Wearing 1998; McGowan 2006).
Water use in liquid ring vacuum pump seal water applications is large, ranging from 4.5 to 14 m 3 /admt of production (ETSU 1993). Several prove n vacuum pump recirculation st rategies reduce the fresh water needed in vacuum pump seal water systems: (1 ) cascading seal water from high vacuum pumps to low vacuum pumps (45 to 50% savings in water use compar ed to an open system); (2) temperature controlled recirculation (up to 65% savings compared to an open system during winter months); (3) indirect heat exchange (60 to 95% savings compared to an open system); and (4) recirculating seal water through a cooling tower (over 90% savings compared to an op en system comprised of ~1% fresh water makeup for evaporative losses and a recirculation purge to pr event accumulation of contaminants) (Chandra 1997; English and Lindstrom 2001).

2 Causticizing conversion: C = (NaOH)/(Na 2 CO 3 + NaOH) all measured as equivalent Na 2 O

5Pulp furnish from high density storage Consistency: 14% Water: 5.5 m 3 /admt High Density Storage Mixing and Machine

Chests

Silo Paper Machine Wet End

Seal pit

and White
water tank Headbox Consistency: 0.5% Water: 179 m 3 /admt Mat leaving press Consistency: 40% Water: 1.3 m 3 /admt

Excess white water Water: 29.2 m 3 /admt Headbox dilution Water: 174 m 3 /admt Paper machine showers Water: 25 m 3 /admt

Figure 2. Water Flows within an “Open” Paper Machine Wet End

For the modern hypothetical case 10 m 3 water/admt is used, including pr ocess and cooling waters, and is representative of a paper machine with a high degree of whitewater recycle and an efficient fiber recovery system (Martin-Löf et al. 1973; NCASI 2009b). For the open case 187 m 3 water/admt is used (178 m 3 /admt for process water plus 9.25 m 3 /admt for cooling water). The analysis assumes that these same amounts of water are used in the modern and open cases for market pulp mills; that is, 10 m 3 /admt for the modern case and 187 m 3 /admt for the completely open case.
3.6 Cooling
Major cooling circuits within chemical pulp mills and their general temperature and volumetric flow characteristics were reviewed by NCASI (2008). The tw o largest cooling circuits are associated with the evaporator surface condenser and, for mills with a c ondensing steam turbine, the turbine dump condenser (Carter and Gleadow 2001). Table 4 shows the major c ooling circuits at chemical pulp mills and their associated specific volumetric flow rates.

Table 4. Major Cooling Circuits at Chemical Pulp Mills Cooling Tower Circuit Flow (m 3 /admt) Evaporator surface condenser 50-75 Turbine condenser 21-50 Vacuum pump seal water 4.5-14 Chlorine dioxide plant 7-15

In modern mills the cooling waters listed in Table 4 are typically reused by removing heat via direct contact cooling towers or indirect heat exchange , or by reusing heated cooling waters for process purposes. Cooling water flows associated with vacu um pump seal waters are considered in the papermaking and pulp drying section, so results here in only consider evaporator surface condensers, turbine condensers, and chlorine dioxide plants. By taking the av erage of entries where ranges are presented and not including the contribution of vacuum pulp seal water, the total flow from Table 4 is 109 m 3 /admt. For the modern bleached kraft hypothetical case a value of 5.5 m 3 /admt assumes that all flows (109 m 3 /admt) are recirculated through direct contact cooling towers and that the only effluent flow is a 5% bleed to control accumulation of contaminants and for evaporative loss makeup within the cooling

6circuit. For the hypothetical open bleached case a value of 109 m 3 /admt is used, assuming all cooling water flows are once through. For the mode rn unbleached hypothetical case a value of 4.9 m 3 /admt is used (5% of the evaporator surface condenser and turbine condenser flows in Table 4). For the hypothetical open unbleached case where a chlorine dioxide plant is not present a value of 98 m 3 /admt is used, assuming all cooling water flows are once through.

4.0 RECYCLE CATEGORIES
The primary water requirement within recycle pulp m ills is for ink separation and cleaning of secondary fiber. The two principle methods of ink removal ar e ink washing and dissolved air flotation. Water discharges for various types of ink washers are given in Table 5 (English 2001).

Table 5. Water Use for Ink Washers Washer Type Reject Flow (m 3 /admt) Gravity decker 75 Sidehill washer 68 Vario-split washer 65 Double-nip thickener 36 Inclined screw extractor 23 Screw press 16

Washer-only deinking plants with three to five washi ng stages in series are typically required to meet ink removal targets (Horacek and Kronlund 1980). A comp letely open three stage washing system using sidehill screens would use over 340 m 3 water/admt (Bliss, Guillermo, and Seifert 1994).
The large use of water and loss of energy and alkali in washer reject streams led to adaptation and development of dissolved air flotation for ink rem oval in the 1960s and to water recycle and reuse strategies using countercurrent washing and clarificat ion. Most new deinking mills in North America use a combination of flotation and washing, and many older mills have added fl otation units. Depending upon the final product, bleaching may be required to achieve necessary optical properties in finished pulp (Ackermann 2000). Deink bleach plants typically have one to three bleaching stages with inter-stage washing, but five stage bleach plants may be necessar y to achieve a final prod uct with high brightness (Singh 1987; Dessureault et al. 1994). Modern bl eaching systems have high consistency wash presses and high consistency bleach towers that minimize wa ter use, while older configurations may have low consistency washers and low consistency bleach towers. Historically, a deink plant producing fine paper or tissue and toweling may have required three bleach ing stages to achieve brightness targets (Kleinau 1958). An open three stage bleach plant with lo w consistency vacuum washers would require approximately 350 m 3 water/admt for inter-stage washing. A deink plant producing newsprint may require only one bleaching stage with open bleaching process water discharge of 87 m 3 /admt arising from dewatering of screening accepts from ~1% consistency to 30% consistency prior to entering the bleach retention tower.
Cooling water is used in recycle pulp mills for pump seal water, gearboxes, lubrication, and hydraulic oil recirculation units. Cooling water flows are not expect ed to be large in recycle mills or substantially different between the hypothetical open and hypothetical modern mill cases.
A modern, tightly closed flotation deinking plant without an ink washing step may have an effluent volume as low as 11 m 3 /admt (English 2001), but flows between 23 and 32 m 3 /admt are more typical (NCASI 1994). For the modern case 21 m 3 water/admt is used (11 m 3 /admt for deink plant plus 10 m 3 /admt for paper making), which includes water used in screening, cleaning, and flotation. For the open case deink mill producing fine paper or tissue and toweling 877 m 3 water/admt is used (340 m 3 /admt

7for pulping plus 350 m 3 /admt for bleaching plus 187 m 3 /admt for papermaking), which assumes an open three stage washing system, no dissolved air floatati on, and an open three stage bleach plant with low consistency vacuum washers. For the op en case deink mill producing deinked newsprint 614 m 3 water/admt is used (340 m 3 /admt for pulping plus 87 m 3 /admt for bleaching plus 187 m 3 /admt for papermaking), which assumes an open three stage wa shing system, no dissolved air floatation, and a single stage of bleaching with dewatering discharge . Modern non-deink plants producing packaging paper and board grades operate at or near zero proce ss effluent discharge (NCASI 1997, 1999). For the modern non-deink plant case 2.1 m 3 /admt process effluent discharge is used. For the open non-deink plant case 187 m 3 /admt for papermaking is used.

5.0 MECHANICAL CATEGORY

Most water use related to the manufacture of mechan ical pulp grades originates with the paper machine (Reside 2001). Table 6 illustrates differences in mi nimum and design effluent flows among the various types of mechanical pulp mills. In general, more wate r is needed in mechanical pulp mills with chemical pretreatment because of increased washing requirements.

Table 6. Minimum Effluent Flows for Mechanical Pulp Mills with Activated Sludge Treatment Pulp Mill Effluent (m 3 /admt) Process Design Minimum Stone groundwood 3.1 1.8 Refiner mechanical 3.9 2.3 Thermomechanical 6.5 3.8 Bleached chemi-thermomechanical 17.2 10.0

Cooling water is necessary within mechanical pulp mills to cool refiner motor windings, refiner and motor lube oil, and hydraulic drives and for miscellaneous purposes such as air conditioning. A typical cooling water flow with a completely open cooling water system may be around 15.0 m 3 /admt (Reside 2001). A fully closed cooling water system would discharge around 3.2 m 3 /admt because some of the cooling water from oil service cooling may become co ntaminated with oil and cannot be recirculated without further processing (Reside 2001).
For the modern hypothetical mechanical mill case a total water intake of 15.8 m 3 /admt is used, which includes all water used to produce lightweight-coa ted (LWC) printing paper using thermo-mechanical pulp produced onsite (Edelmann et al. 2000). For the open case a value of 209 m 3 /admt is used (6.5 m 3 /admt for pulping assuming a thermomechanical pulp mill from Table 6, 187 m 3 /admt for papermaking, and 15 m 3 /admt for cooling purposes).

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