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114  Membranes for Industrial Wastewater Recoverg arid Reuse

           BREF stipulates, amongst other things, permitted effluent loads and, for the first
           time, effluent flows for different kinds of pulp and paper mills. The directive also
           includes a list of candidate techniques and practises (under BAT, best available
           technology,  Tables 3.16 and  3.17) recognised  as being  able to  achieve  the
           mandatory effluent loads.
             The  effluent  loads  allowed  according  to  the  IPPC  directive  are  extremely
           rigorous.  In  particular,  target  levels for  nitrogen  and phosphorus cannot  be
           achieved  by  use  of  conventional  external  (end-of-pipe) biological  treatment
           alone. The allowed effluent flows, on the other hand, necessitate counter current
           water circulation systems and internal water recycling, also stated in the IPPC
           directive and aimed at minimising freshwater consumption.
             The focus of the internal water purification used today has been mainly on the
           removal of suspended solids by mechanical filtration (e.g. disc filter), flotation or
           chemical precipitation. Flotation (or flotation combined with sand filtration, so-
           called flotation filtration) has been successfully used to remove suspended solids
           and to recover fibres, fillers and fines from the white water. However, flotation
           tanks  generally  demand a lot  of  floor space and  the process is fairly energy
           intensive. The cost and the filtrate quality are very dependent on the chemicals
           used, which are usually expensive. Flotation is used to a large extent in mills
           using  recovered  paper  as  raw  material.  Chemical precipitation  is  also  used
           within the paper machine white water treatment system. The aim is to improve
           the first-pass retention in the wire section and, at the same time, the suspended
           solids content is reduced in the white water. However, chemicals are usually
           expensive  and  the  doses  required  can  be  large.  Moreover,  overdosing  of

           Table 3.16  BAT requirements for mechanical pulp mills (EIPPCB. 2001)




           Non-integratedCTMP   15-20   10-20   0.5-1   0.5-1   -   0.1-0.2   0.005-0.01
           mills (contribution of
           pulping only)
           Integratedmechanical   12-20   2-5   0.2-0.5   0.2-0.5   < 0.01  0.04-0.1   0.004-0.01
           pulp and paper mills
           (e.g. newsprint, LWC"
           and SCB paper mills)
           a LWC, lightweight coated SC, supercalendered.


           Table 3.17  BATdemands for pulp and paper mills using chemical pulp (EIPPCB, 2001)
           Papertype   Plow   COD   BOD       TSS     AOX     Tot N     Tot P
                     (m3/t)   (kdt)   (kdt)   (kg/t)   (kg/t)   (kg/t)   (kg/t)
           Uncoated   10-15   0.5-2   0.15-0.25   0.2-0.4   <0.005   0.003-0.01   0.05-0.2
           fine paper
           Coatedfine   10-15   0.5-1.5   0.15-0.25   0.2-0.4   <0.005   0.003-0.01   0.05-0.2
           paper
           Tissue    10-15   0.4-1.5   0.15-0.4   0.2-0.4   <0.01   0.003-0.01   0.05-0.25
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