50 LIMNOLOGY, WATER SUPPLY AND WASTE DISPOSAL 



After the digester is brought up to pressure, therefore, it is relieved; that is, 

 the air and other accumulated gases are permitted to escape. This initial 

 "blowdown," as it is called, is usually condensed and its condensate is one 

 type of waste from a kraft mill. Similarly, at the completion of the second^ 

 or cooking, phase of the operation the digester is again relieved, these 

 "final blowdown" gases are condensed, and the condensate becomes waste. 



When the digesters are blown (or emptied) at the conclusion of the 

 cook, the pulp is blown to a pit and the spent liquor (or black liquor) is 

 collected and sent to the recovery plant. The pulp is washed by a series 

 of waste waters used in washing previous batches of pulp. The black liquor 

 concentration of these waste waters is increased with every batch of pulp 

 washed and, eventually, when the concentration is high enough, they are 

 sent to the recovery plant along with the concentrated black liquor. In 

 the final washing processes, however, the wash waters may be so dilute 

 that their chemicals cannot be recovered economically; for that reason, 

 they may be sent to the sewer. 



The spent black liquor and the concentrated wash waters are sub- 

 jected to evaporation under a vacuum to a consistency of approximately 

 50% solids. The vacuum is produced by a barometric leg and in its eflBu- 

 ent may be found in solution some of the noncondensable gases from the 

 process of black liquor evaporation. This effluent is a waste material. 



The concentrated black liquor containing most of the nonfibrous por- 

 tion of the original wood, as well as the spent chemicals, is then burned in 

 recovery furnaces; the chemicals and heat energy are thus recovered. 



When the black liquor from the blow pits is sent to the recovery plants, 

 it is usually stored in large receptacles for varying periods. As the material 

 stands in these tanks, a thick foamy material, called sulphate soap, rises 

 to its surface. Actually, it is composed of the sodium salts of resin and 

 fatty acids which were in the wood. Currently, this soap is recovered and 

 sold, but small amounts of it may be found in the dilute wash water. It 

 is interesting to note that, although eastern and southern woods have a 

 relatively high resin and fatty acid content, the woods of the Pacific North- 

 west contain very little. For that reason, the kraft mills in the northwest 

 are not ordinarily faced with a serious soap problem. 



In summary, it can be stated that the major stream polluting products 

 from the normal operation of a kraft mill are: (a) initial blowdown con- 

 densate, (6) final blowdown condensate, (c) evaporator condensate, and 

 (d) soaps and other material in the weak wash waters. 



It is the purpose of this paper to describe the possible effect of these 

 wastes on the aquatic environment and to consider measures which may 

 be employed to prevent their reaching the stream. 



Kraft Wastes and the Stream Environment 



The relief gases referred to above have been studied rather widely in 

 Sweden and Germany. It was demonstrated by Klason and Segerfelt 

 (1911) that up to 1000 gm of mercaptans may be produced per ton of wood 



