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Paper Industry Wastes and Energy Generation From Wastes   89


                  2.  Pulping and bleaching
                     − Washing of chemically cooked pulp.
                     − Washing of pulp during bleaching.
                     − Pulp cleaning equipments.
                  3.  Stock preparation and paper machine
                     − Cleaning of pulp in cleaning equipment.
                     − Filtration for wire section of paper machine.
                     − Paper machine presses.
                  4.  Chemical recovery
                     − Foul condensate from evaporator and steam surface condenser.
                     − Boiler blowdown.
                     Besides above major sources of wastewater generation, there are fre-
                     quent leakages of black liquor from pump glands and its improper han-
                     dling, which contribute significant color and pollution to the stream.
              (ii)  Solid waste generation
                  In pulp and paper industry, solid wastes are generated from following
                  an operation;
                  −  Raw material handling.
                  −  Rejects from screening and centri-cleaners.
                  −  Primary and secondary sludges from wastewater treatment system.
                  −  Coal or boiler ash from steam and power generation.
                  −  Lime sludges from causticizing section of chemical recovery plant.
              (iii)  Air pollution
                  In pulp and paper industry, air pollution is caused due to odor emitting
                  reduced sulfur compounds such as hydrogen sulfide, methylmercaptan,
                  dimethyl sulfide, and particulate matter SO 2  and NO x  present in the
                  gases emitted by different process units.
                     Gaseous emission from pulp and paper mills can be broadly classi-
                  fied into the following categories:
                  •  Gases from digests.
                  •  Gases from multiple effect evaporators.
                  •  Gases from recovery.



              7.4  PRODUCTION OF ENERGY PRODUCTS FROM PAPER
              MILL WASTES

              The wastes developed from paper and pulp industry can be treated through
              several ways for developing energy or to convert it into some useful prod-
              ucts. Some important processes that are used already and processes in devel-
              opment for paper waste treatment are discussed in detail.
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