KRAFT PULPING WASTES AND AQUATIC ENVIRONMENT 53 



years has made its recovery especially profitable and it is expected that, 

 even after the scarcity ends, there will be a ready market for it. From 

 the stream pollution abatement point of view, the recovery of tall oil 

 removes essentially all the fatty and resin acids from the wastes. 



The digester relief condensates. It was noted above that these wastes 

 contained considerable amounts of turpentine. Formerly, most of this 

 material was wasted. In recent years, however, the economic advantage 

 of its recovery has become so apparent to mill operators that most mills 

 recover it and find a ready market for it. 



In addition to the turpentine, the digester relief condensates contain 

 sulphur compounds; in the present case, those of primary interest are the 

 sulphides and mercaptans. The amounts of these materials in these wastes, 

 which may be relatively large from the point of view of stream pollution, 

 are too small to be recovered to economic advantage. For that reason, not 

 a great deal of attention has been directed to their removal from the wastes. 

 The problem, therefore, is to provide a method by which these wastes 

 may either be removed or neutralized before they are passed to the stream. 



One of the complicating factors in removing the mercaptans and sul- 

 phides lies in their inherent odor characteristics. Anyone who has spent 

 much time around a kraft mill knows that there is, ordinarily, a typical 

 smell associated with the process. Many people think that this smell is 

 unpleasant, not to say foul, and there is some justification for this opinion. 

 The basic source of this smell is in the presence of the very substances 

 under consideration. In considering ways and means of eliminating these 

 compounds from the stream, attention must be given, therefore, to the 

 problem of air pollution as well. 



In conducting experiments on the toxicity of kraft mill waste com- 

 ponents to fish and other aquatic organisms, it was discovered that the 

 toxic properties of any given concentration may be significantly reduced 

 if a finely divided stream of air is passed through the solution. It is unlikely 

 that the phenomenon is caused by any other means than the action of the 

 air-flushing or washing the materials from the water, although it is possi- 

 ble that some oxidation occurs. This observation suggests the possibility 

 that the wastes in question could be detoxified simply by subjecting them 

 to a process of thorough aeration and/or agitation. 



The easiest and most economical method of achieving this end is the 

 one that has been and is now employed — namely, the use of the receiving 

 stream. In normal circumstances, if adequate volumes of water are avail- 

 able for dilution, the stream will absorb or dissipate these sulphur com- 

 pounds rapidly enough so that the danger to aquatic fauna is eliminated. 

 The possibility always exists, however, that, in periods of low flow and 

 high production, the absorbing capacity of the stream will be taxed beyond 

 a safe limit and, under those conditions, the need for some other method 

 of treatment becomes apparent. 



The problem is complicated by the relatively large amounts of water 

 in which the sulphur compounds may be carried. A treatment process 



