Patent Publication Number: US-5290454-A

Title: Process for removal of suspended solids from pulp and paper mill effluents

Description:
BACKGROUND OF THE INVENTION 
     1. Field of invention 
     The present invention relates to removal of suspended solids, including resin and fatty acids from untreated or primary treated pulp mill effluents, including CTMP mill effluents. 
     The present invention will facilitate the treatment of pulp mill effluents by reducing or eliminating the input of suspended solids to the biological reactors and physical separation processes. 
     2. Description of Prior Art The presence of excessive amounts of total suspended solids (TSS) is detrimental to biological treatment of effluents, as has been observed in several treatment plants in Canada and Scandinavia. The suspended solids also interfere in physical separation processes employed in closed-cycle mill applications, namely evaporation, membrane filtration and freeze crystallization. 
     The principal methods for separation of solids from pulp mill effluents include: a) settling, b) flotation, and c) filtration. A number of studies have been performed on the conventional settling augmented by coagulation, in order to improve treatment performance. These studies included optimization of pH, temperature, retention time and the sequence of chemical addition. 
     O&#39;Brian and O&#39;Brien, in U.S. Pat. No. 4,115,188 discloses that acidification to pH &lt;4 followed by addition of calcium hydroxide to pH &gt;10 caused coagulation and precipitation of suspended solids from the paper mill waste water. Japanese patent JP 53,058,002, assigned to Sanyo Kokusaku Pulp, proposes acidification of the waste liquor and treatment with an anionactive polyacrylamide-based flocculent, to remove suspended solids. U.S. Pat. No. 3,959,129 describes a method for removing ink and suspended particles by treating the waste deinking effluent stream with ferric chloride and calcium hydroxide followed by vacuum filtration through a &lt;250 micron screen. Japanese patent JP 49,078,367 shows treatment of pulping waste effluent streams with magnesium chloride and calcium hydroxide to enhance settling rates. Japanese Patent 89,174,242 of Toyo Roshi KK, describes filtration of effluents through polyolefin, polyester and polyacryl fibers to remove suspended solids. 
     Kopylov, SU Pat. No. 981,233, describes a single cylinder flotation cell for removal of suspended solids from the pulp and paper industry effluents. Canzoneri, U.S. Pat. No. 4,782,789, proposed a double cell (concentric) process for removal of suspended solids. 
     Excess water from a paper machine vacuum filter savealls has been treated by a precoat filter to recover fibers (Lardieri, N.J. and J. W. Dappen, TAPPI. 50 (11), 125A (1967)). Pulp was used as the precoat medium and the recovered fibers, along with the precoat pulp, were bleached and used for tissue manufacture. 
     The above methods for treatment of waste streams from pulp and paper mills have several shortcomings, including (a) the use of expensive pulp as the filter medium in the filter, (b) no recycle of the precoat material or water, (c) limited removal of suspended solids, (d) large space requirements, (e) high capital costs, (f) production of dilute sludges that require dewatering before disposal, and (g) the need for chemical agents such as lime, alum and polyelectrolytes which, in addition to increasing the sludge volume, may be detrimental to the environment during sludge disposal. 
     SUMMARY OF THE INVENTION 
     In accordance with the invention there is provided a process for removal of suspended solids from a pulp mill effluent comprising: i) forming a mat of a non-biological sludge on a porous support, and ii) passing pulp mill effluent comprising an aqueous vehicle containing suspended solids through said mat whereby the suspended solids are retained in said mat and aqueous vehicle passes through said mat and said porous support. 
     The sludge is preferably a primary sludge, woodroom sludge, or a mixture of primary and secondary sludge i.e., a mixture of non-biological and biological sludge. 
     The suspended solids which are removed in accordance with the invention are those found in pulp mill effluents, including colloidal particles of resin and fatty acids; the pulp mill effluent may be the solids-containing effluent from a primary clarifier, a woodroom effluent, a pulp or paper mill effluent or mixtures thereof. 
     The mat is suitably formed on the support in a thickness of 0.5 to 25 mm with a specific filtration resistance of 4×10 12  to 1×10 13  m/kg. Such a mat is typically formed from a pulp mill sludge having a solids concentration of 3% to 5%, preferably about 4%, by weight. 
     When the present invention is practiced for treating effluents the total suspended solids level can be decreased by 85-98%. This high degree of removal is achieved particularly with CTMP mill effluents which typically have higher suspended solids level (500-1000 ppm), compared to kraft mill effluents (50-100 ppm). 
     Improved removal of suspended solids facilitates the subsequent secondary biological treatment. Further reduction of the suspended solids can be achieved by the use of suitable sand or multi-media filters. High removal of suspended solids is required for subsequent membrane treatment and/or other concentration techniques. 
     The process of the invention can be employed to decrease effluent resin and fatty acid content, as well as to lower biochemical oxygen demand, chemical oxygen demand and toxicity to fish, by removing solids which utilize oxygen and are toxic to fish. 
     The process may also be exploited to remove solids in biological system effluents in the pulp mill, and to remove solids from specific mill streams, for example, washing stage effluent. 
     The aqueous vehicle of the effluent, from which the suspended solids have been removed can be subjected to secondary treatment to produce water of high quality or purity for recycling within the pulp mill. 
     Suitable secondary treatments include the use of a sand or multi-media filter, which can be followed by separation processes such as use o ultra-filtration membranes, reverse osmosis, evaporation or freeze crystallization; in this way substantially all contaminants can be removed and clean water which is recovered can be recycled to the pulp mill, and the solids concentrate can be treated to recover pulping chemicals or burned and discarded. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS 
     The invention is further described by reference to the accompanying drawings in which: 
     FIG. 1 illustrates schematically an assembly including a drum filter for carrying out the process of the invention: 
     FIG. 2 illustrates schematically an experimental apparatus employed in a simulation of the process of the invention; 
     FIG. 3 illustrates schematically a first embodiment and apparatus of the invention, with woodroom and primary clarifiers upstream of a drum or belt filter; 
     FIG. 4 illustrates schematically a second embodiment and apparatus of the invention similar to that of FIG. 3, but without the primary clarifier; 
     FIG. 5 illustrates schematically a third embodiment and apparatus of the invention without a primary clarifier but with post-filtration and membrane treatment; and 
     FIG. 6 illustrates schematically a fourth embodiment and apparatus of the invention with a primary clarifier handling part of the effluent upstream of a drum filter. 
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS WITH REFERENCE TO THE DRAWINGS 
     With further reference to FIG. 1, there is shown an apparatus 10 for treating suspended solids which includes a drum 12 having a filter 14. 
     Drum 12 is mounted for rotation in a vat 16 having a feed inlet line 18 for effluent containing suspended solids and an effluent outline line 20. A line 36, which communicates with a resuspension tank 22 having a stirrer 24 provides the water produced in the mat formation step for mat resuspension. 
     A mat applicator 30 applies mill sludge fed from resuspension tank 22 along mat feed line 32, to the surface of filter 14 to form a mat filter maintained by means of vacuum or pressure (not shown). 
     A sludge doctor 26 removes the mat from filter 14 and recycles it along sludge line 28 to resuspension tank 22. A portion of the sludge in sludge line 28 is removed through purge line 34. 
     Drum 12 is divided into sectors A, B, C and D. 
     With further reference to FIG. 2, experimental apparatus 40 employed to simulate the process of the invention includes a filter device 42 having a screen 44, a source of mill sludge 46, and a suspended solids source 48. 
     A sludge feed line 50 communicates source 46 with filter device 42; and a suspended solids feedline 52 communicates source 48 with filter device 42. 
     Apparatus 40 further includes a treated effluent vessel 54 and a mat water vessel 56. 
     Effluent vessel 54 communicates with filter device 42 via effluent line 58 and mat water vessel 56 communicates with filter device 42 via mat water line 60. 
     A three-way valve 62 is disposed between filter device 42 and lines 58 and 60. 
     Vacuum line 64, having a pressure gauge and pressure control relief valve communicates via vessels 54 and 56 with filter device 42. 
     With further reference to FIGS. 3 to 6, there is shown different embodiments of treatment assemblies for use in the process of the invention. 
     For convenience the same integers are employed in the different figures for the same components. 
     With further reference to FIG. 3, a treatment assembly 70 includes a primary clarifier 72, a woodroom clarifier 74, a drum filter 76 and an optional secondary treatment station 78. It should be noted that the drum filter 76 embodies the filter drum, mat applicator, sludge doctor and the resuspension tank shown in FIG. 1. 
     Woodroom clarifier inlet line 80 communicates woodroom clarifier 74 with a pulp mill 84 and primary clarifier inlet line 82 communicates primary clarifier 72 with pulp mill 84. 
     Mat lines 86 and 88 communicate clarifiers 74 and 72 respectively with drum filter 76 to apply a mat of sludge on drum filter 76. 
     Suspended solids line 90 communicates with woodroom clarifier 74 and suspended solids line 92 communicates with primary clarifier 72 to feed suspension of suspended solids for treatment to the drum filter 76. 
     Effluent line 94 discharges treated effluent from drum filter 76 to the optional secondary treatment station 78 and effluent is recovered from outlet line 96. 
     An optional recycle line 98 communicates outlet line 96 with a water inlet line 102 which feeds pulp mill 84. 
     A screw press 100 assists in dewatering the mat removed from drum filter 76. 
     Pulp mill 84 further includes a wood inlet line 104 and a pulp line 106. 
     With further reference to FIG. 4, a treatment assembly 108 differs from assembly 70 of FIG. 3 in that it does not include the primary clarifier 72 and related components. 
     With further reference to FIG. 5, a treatment assembly 110 is similar to treatment assembly 108 of FIG. 4 but employs, in place of the secondary treatment station 78, a sand or multimedia filter 112, an ultra-filtration membrane 114 and reverse osmosis membrane 120. 
     A solids line 116 recycles solids from ultra-filtration membrane 114 and reverse osmosis membrane 120 to pulp mill 84. A filtrate line 124 recycle clean water from reverse osmosis membrane 120 to pulp mill 84. A backwash line 122 recycles a portion of clean water from reverse osmosis membrane 120 to filter 112, to clean the filter and the backwash line 118 feeds the filtered solids from filter 112 to drum filler 76 for formation of the mat. 
     With further reference to FIG. 6, a treatment assembly 126 is similar to assembly 70 of FIG. 3 but includes an effluent by-pass line 128, which feeds a portion of the suspended solids being fed to primary clarifier 72, directly to drum filter 76. 
     The proposed sludge-mat filtration process for removal of suspended solids can be effectively used for treating either combined mill effluents or individual wastewater streams such as the discharge stream from the screw-press in the pulp washing stage. 
     One embodiment of the present invention with the selected equipment proposed for achieving the removal of suspended solids from effluent streams is schematically shown in FIG. 1. The equipment configuration as shown consists of the rotating drum 12 having filter 14 defined by a perforated surface or a perforated covering belt. It should be clearly understood that various screens or belt filters can be used in place of the drum filter 14 described here by way of illustration. A vacuum, typically 100-700 mm Hg, is maintained inside the drum 12 by a suitable barometric leg or vacuum pump. An external pressure could be applied on the drum or belt filter instead of vacuum inside. 
     The process consists of the following steps: 
     1. Formation of the sludge mat on a drum or endless screen 14. 
     2. Filtration of the effluent. 
     3. Removal of the thickened sludge from the screen 14. 
     4. Purging a portion of the sludge for incineration/ landfill. 
     5. Addition of makeup sludge if required/available. 
     6. Resuspension of the dewatered sludge in the water used for mat formation in the previous step. 
     As shown in FIG. 1, the interior of the drum 12 is divided into 4 sectors. Step 1 is performed on the drum 12 at sector A in FIG. 1., step 2 at sector B and step 3 at sector C. Sector D represents a dead or lag zone where no specific operation takes place. This sector can be used for cleaning of the belt or the drum screen, if required. 
     The angle of each sector, diameter of the drum 12 and rotational speed will depend on the filterability of the effluent and the sludge characteristics. 
     Suitably the drum rotates at a relatively slow speed of 0.2 to 3 rpm, and preferably below 1 rpm. The mat is formed on sector A as shown in FIG. 1 and effluent in vat 16 is drawn into the mat with the aid of the vacuum applied to the interior of the drum, this vacuum also stabilizes the formed mat on the drum. The suspended solids are retained in the mat and the aqueous vehicle of the effluent passes through the mat and the screen. 
     At the end of each filtration phase B of the revolution of drum 12, the mat containing suspended solids from the effluent is removed by sludge doctor 26, whereafter a fresh mat is formed by applicator 30. 
     In tis way the mat is replaced after each revolution of drum 12, and of high filtering efficiency is maintained 
     One modification of the above process (steps 1-6) would entail (i) Suspending the mat (step 6) in a portion of the effluent and (ii) mixing the water filtered through during mat formulation (Step 1) with the feed to the drum. This modification could be used in cases when the total solids level of the water used for resuspending the mat increases very rapidly. 
     Another modification to the process may include addition of calcium chloride or other suitable settling aids, for example, to the effluent streato enhance the removal of suspended solids in the pretreatment stage by settling. It was observed in jar tests that the addition of calcium chlorides, at a dosage of 25 to 400 mg Ca/l to CTMP mill effluents, removed up to about 90% of the suspended solids upon subsequent settling, while simple setting without calcium chlorides addition remove only 60-70%. 
     Alternatively, filtration aids such as diatomaceous earth, lime mud, etc., can be used to augment removal of colloidal material. In such a process the filtration aid would not be removed after each cycle like the sludge mat, but would be removed and replaced when it becomes plugged, depending on usage. 
     In order to disclose more clearly the nature of the present invention, tho following examples illustrating the invention are given. 
     EXAMPLES 
     EXAMPLE 1 
     Experimental apparatus 40 to simulate the process consisted of a filtration screen 14, two separate liquid collection chambers 54 and 56 (one for water from mat formation and resuspension and another for collection of treated effluent), a three way valve 62, and appropriate vacuum source (see FIG. 2). The filter area used in the experiments was 1.26×10 -3  m 2 . 
     Combined CTMP mill effluents used in the experiments were obtained from two CTMP mills in Quebec, Canada. Mill I used softwood and Mill II used hardwood for pulping. The effluent was homogenized and brought to the required temperature in a constant temperature bath. Effluent aliquots of 50 ml were taken for each experiment. The amount of sludge used to form the filter mat was based on estimated fiber loss of 1-2% during the pulping process (oven dry basis). Combined primary sludges from the respective mills were used for formation of the mat. After the mat was formed 50 ml of the effluent sample was filtered through it. The mat was removed and resuspended in the mat formation water and formed again. The experiments were conducted at 20° C. and 60° C. 
     The experiments were conducted until steady-state conditions were reached, typically, 15-25 filtration cycles. This was determined by the time required for filtration of each aliquot (i.e., the filtration rate) to reach a plateau. Table I shows the steady-state filtration rates for the two mill effluents at different temperatures and the removal of total suspended solids. 
     
                       TABLE I                                                     
______________________________________                                    
Results of filtration experiment. Operating vacuum: 380 mm Hg.            
                       Total                                              
              Filtration                                                  
                       Suspended                                          
              Rate,    Solids                                             
Effluent  Temp, °C.                                                
                    L/m.sup.2 · h                                
                               Initial                                    
                                     Final                                
______________________________________                                    
Mill I    20        5730       660   100                                  
Mill II   20        1800       560    15                                  
Mill II   60        2200       560    5                                   
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     It can be seen in Table I that removal of more than 85% was obtained for CTMP mill effluents from Mill I at 20° C. and higher than 97-99% suspended solids removal was achieved for effluents from Mill II at 20° and 60° C., respectively. This clearly demonstrates that the process eliminates most or virtually all of the suspended solids and hence would improve the effluent treatment efficiency in the subsequent secondary treatment. 
     The process can be applied in pulp and paper mills for different scenarios. They are presented in the examples which follow. 
     
         ______________________________________                                    
Mat filtration processes for pulp mills:                                  
______________________________________                                    
Alternative I: with woodroom and primary                                  
               clarifiers before the filter.                              
Alternative II:                                                           
               without primary clarifiers.                                
Alternative III:                                                          
               without primary clarifier, with                            
               sand/multi-media filters and                               
               membrane treatment.                                        
Alternative IV:                                                           
               with new/existing primary                                  
               clarifier handling part of the                             
               effluent before mat filters.                               
______________________________________                                    
 
    
     EXAMPLE 2 
     The Alternative I for removal of effluent suspended solids by mat filtration, schematically shown in FIG. 3, was simulated on a laboratory scale. Effluent obtained from a hardwood pulping (Mill I) was used for these experiments (no woodroom clarifier sludge was included in these tests). The primary clarifier was replaced by a jar in which the effluent was stirred rapidly at 100 rpm for 2 minutes followed by slow stirring at 20 rpm for 20 minutes. The settleable solids were separated from the supernatant and used for formation of the mat. The supernatant was filtered through the sludge mat. The tests were conducted at 3 vacuum levels (305, 381 and 458 mm Hg). The results of the experiment are given in Table II. 
     The results show that if a primary clarifier followed by mat filtration is used, overall removal of suspended solids can be higher than 95%. No chemical addition is required. The effluent from this stage can be sent to the secondary treatment or for reuse in the mill. 
     
                       TABLE II                                                    
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Filtration experiments with simulated settling of effluents               
followed by mat filtration for removal of settleable solids.              
Stream                Filtration                                          
                               Suspended                                  
Number in Vacuum,     Rate,    Solids,                                    
FIG. 3    mm Hg       L/m.sup.2 · h                              
                               mg/L                                       
______________________________________                                    
1         --          --       539                                        
2         --          --       121                                        
3         305         1100     32                                         
3         381         1860     29                                         
3         458         1200     39                                         
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     EXAMPLE 3 
     The Alternative II, without a primary clarifier but including a woodroom clarifier, is presented in this example (see FIG. 4). In this embodiment the effluent from the pulp mill is sent directly to the filter. The solids removed by the mat filter are disposed of and the treated water is either recycled or sent to secondary treatment. This scheme would be particularly useful at mill sites where strict land restrictions apply. It could also be used in new mills with significant cost savings, because of the high capital required for clarifiers. 
     The experiments were initiated by using the settled sludge generated as in Example 2. The raw effluent as obtained from the Mill I. The filtration process was self sustaining with the solids removed during filtration. No addition of sludge was required. The results of the filtration experiments are given in Table III. 
     
                       TABLE III                                                   
______________________________________                                    
Filtration experiments with unsettled effluent.                           
Temperature 20° C.                                                 
Stream    Applied     Filtration                                          
                               Suspended                                  
Number in Vacuum,     Rate,    Solids,                                    
FIG. 4    mm Hg       L/m.sup.2 · h                              
                               mg/L                                       
______________________________________                                    
1         --          --       539                                        
2         305         493      8                                          
2         381         606      8                                          
2         458         404      8                                          
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     The results presented in Table III indicate that the separation efficiencies achieved by mat filtration of the effluent from the CTMP pulp mill were very high. However, the filtration rates were lower than those obtained with clarified effluents, as demonstrated in Examples 1 and 2. 
     As shown in FIG. 5 (Alternative III), high quality water for recycle and a concentrated stream for recovery can be obtained. This scheme includes the use of a sand or multimedia filter to reduce the TSS (&lt;8 ppm) of the inlet flow to the sensitive membrane treatment or other separation processes like evaporation or freeze crystallization. Additionally an ultra-filtration membrane and a reverse osmosis membrane are employed in the post treatment. 
     A hybrid of Alternatives I and II is shown in FIG. 6 where a portion of the effluent is settled and the rest is sent to the filtration stage directly. Stream 1 in this case could be divided according to the requirements of the overall process. 
     EXAMPLE 4 
     Pretreatment of the effluent with calcium chloride has a significant impact on the settling characteristics of the effluent in the primary clarifier. The process schematic considered in this example is shown by Alternative I in FIG. 3. Calcium chloride is added in the inlet to the primary clarifier (stream 1 in line 82). The sludge from the clarifier would be used for mat formation while the clarified effluent (stream 2 in line 92) would be used as the feed for the mat filtration to produce the clear filtrate (stream 3 in line 94). The results of the filtration experiment using the above configuration are given in Table IV. 
     
                       TABLE IV                                                    
______________________________________                                    
Filtration of presettled effluents with                                   
calcium addition in the settling                                          
tank. Calcium chloride level: 25 mg/L as Ca.                              
Stream                Filtration                                          
                               Suspended                                  
Number in Vacuum,     Rate,    Solids,                                    
FIG. 3    mm Hg       L/m.sup.2 · h                              
                               mg/L                                       
______________________________________                                    
1         --          --       539                                        
2         --          --       136                                        
3         381         1470      14                                        
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     The result from the above experiments show that the filtration rates with the addition of calcium chloride (1470 L/m 2  •h) are lower than those without calcium chloride (1860 L/m 2  •h) (see Table II). However the residual suspended solid level (14 mg/L) was lower than that without calcium chloride addition (29 mg/L). While the lower filtration rate would be less economical, the faster settling rate of the effluent solids in the primary clarifier could offset the slower filtration rates. 
     EXAMPLE 5 
     The magnitudes of the various operations of the proposed process for a mill that produces 500 tons of air dry pulp per day and has an effluent flow of 20 m 3  per ton of air dry pulp are shown in Table V. 
     
                       TABLE V                                                     
______________________________________                                    
Magnitudes of applications of the proposed process.                       
______________________________________                                    
Effluent volume to be treated                                             
                    m.sup.3 /h                                            
                             417                                          
Suspended solids level                                                    
                    mg/L     600                                          
Filtration rate (Example 1)                                               
                    L/m.sup.2 · h                                
                             5730                                         
Filtration area required                                                  
                    m.sup.2   73                                          
Sludge resuspension tank                                                  
                    m.sup.3   3                                           
volume                                                                    
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     When this process is used for the removal of suspended solids, the filtered water can be further processed in secondary treatment or can be reused in other operations at the mill, such as debarking drums. 
     EXAMPLE 6 
     In addition to an excellent removal of suspended solids, the process also removes substantial amounts of wood extractives in terms of resin and fatty acids (RFA) and turbidity. For instance, in filtration tests with total CTMP mill effluent, as per Alternative I (see FIG. 3), the initial RFA level of 15.1 mg/L was decreased to 1.4 mg/L upon primary sludge (PS) mat filtration, for a removal of 91% (refer to Table VI). Simultaneously, effluent turbidity was reduced from 220 to 103 N.T.U. 
     A comparable treatment of paper machine rich whitewater (RWW) reduced the RFA content from 8.9 to 2.7 mg/L (removal, 70%) and turbidity from 600 to 280 N.T.U. 
     Similarly, with CTMP screw press filtrate (SPF), the initial RFA level of 43 mg/L was reduced to 5.5 mg/L representing a removal of 87%. A laboratory solution of tall oil (TO), containing wood extractives, was reduced in RFA from 173 to 74 mg/L upon mat filtration. 
     
                       TABLE VI                                                    
______________________________________                                    
Removal of total suspended solids, resin and                              
fatty acids and turbidity.                                                
                        Filtration         Tur-                           
       Stream  Vacuum,  Rate,  TSS,  RFA,  bidity,                        
Effluent                                                                  
       No.     mm Hg    L/m.sup.2 h                                       
                               mg/L  mg/L  N.T.U.                         
______________________________________                                    
CTMP   2       --       --     167   15.1  220                            
CTMP   3       305      2700    38   1.4   103                            
RWW    2       --       --     540   8.9   600                            
RWW    3       305      1040    11   2.7   280                            
SPF    2       --       --     5679  43    ND                             
SPF    3       305      1680   974   5.5   ND                             
TO     2       --       --     ND    173   ND                             
TO     3       305      2140   ND    74    ND                             
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     EXAMPLE 7 
     This example demonstrates that the effectiveness of the mat filtration process results from the unique filtration capability of the primary sludge (PS) mat. In parallel tests using (i) PS mat and (ii) CTMP pulp mat, the PS mat removed 89% of TSS, 40% of RFA and effluent turbidity was decreased from 470 N.T.U. to 240 N.T.U. (see Table VII). 
     In contrast, the removal efficiency of CTMP pulp mat of a comparable solids charge was very low (6% and 0%, in terms of TSS and RFA) and the effluent turbidity was practically not affected by the filtration process. 
     
                       TABLE VII                                                   
______________________________________                                    
Efficiency comparison of mat filtration                                   
using (i) primary sludge mat and (ii) CTMP pulp mat.                      
                        Filtration         Tur-                           
       Stream  Vacuum,  Rate,  TSS,  RFA,  bidity,                        
Effluent                                                                  
       No.     mm Hg    L/m.sup.2 h                                       
                               mg/L  mg/L  N.T.U.                         
______________________________________                                    
CTMP   1       --       --     1388  27.3  430                            
CTMP   2       --       --     775   24.3  470                            
CTMP   3A.sup.1                                                           
               305      85900  727   25.9  490                            
CTMP   3B.sup.2                                                           
               305       552    82   14.5  240                            
______________________________________                                    
 .sup.1 CTMP pulp mat used.                                               
 .sup.2 Primary sludge mat used.                                          
 
    
     Streams 1, 2 and 3 are shown in FIG. 3, in which stream 2 is the feed stream to the drum filter and stream 3 is the discharge stream from the drum filter. Stream 1 is the stream prior to the primary clarifier. 
     It will be understood that the process is not limited to a drum filter and that various filtering devices or combinations of different devices can be employed; for example, the drum might be replaced by an endless horizontal belt filter with two feed boxes, one for the sludge mat and another for the pulp mill effluent. Separate suction boxes under the belt filter would collect the separate water streams.