52 LIMNOLOGY, WATER SUPPLY AND WASTE DISPOSAL 



(mercaptans, sulphides) of these wastes may be toxic in concentrations 

 as low as 0.5 ppm. Hagman's results have been substantiated by Berg- 

 strom and Vallin (1937), who made a special study of the relief and evapor- 

 ator condensates described above. 



Several investigators have studied this problem in our own country. 

 Cole (1935) investigated the effects of whole black liquor on perch, blue- 

 gills, large-mouth black bass, and rock bass, and found that it would kill 

 these species in concentrations above 5000 ppm. Extrom and Farner (1943), 

 working on the premises of a typical kraft mill located on a medium-sized 

 stream, were able to calculate the concentration, under ordinary circum- 

 stances, of the wastes in question in the stream. Using these concentrations 

 in continuous flow experimental aquaria, they determined that bass, blue- 

 gills, and sunfish were not adversely affected by such concentrations of 

 black liquor and blowdown condensates. They found, however, that some 

 of the chemicals listed above, particularly the sulphur compounds and the 

 soaps, were present in the mill sewer in concentrations high enough to kill 

 fish. They concluded, therefore, that, if there was stream volume discharge 

 to adequately dilute the wastes, the stream environment would not be 

 adversely affected, but that it was possible (particularly at periods of low 

 stream flow) for the wastes to be present in dangerous concentrations. 



Generally speaking, therefore, it can be concluded that, of the com- 

 ponents found in a typical kraft pulp mill waste, the sulphur compounds 

 (particularly the sulphides and the mercaptans) and the resin and fatty 

 acids (and their sodium salts) are, potentially, the most dangerous to the 

 aquatic environment. 



Remedial Measures 



Studies on toxic effect of kraft pulp mill wastes on stream environments 

 have progressed to the point where a well-directed effort can be made to 

 materially abate such pollution. 



One of the most effective methods of abating kraft pulp mill pollution 

 embodies simply efficient and careful operation of the plant. It has been 

 pointed out that, normally, most of the spent liquor is recovered; indeed, 

 the economic feasibility of the kraft process depends on adequate chemical 

 and heat recovery. If the plant is efficiently operated at all times, the 

 loss to the sewer of harmful chemicals can be greatly reduced. However, 

 because of inadequately trained or careless operating personnel, or because 

 of unavoidable mechanical difficulties, it is not always possible to effect 

 top-rate performance. For that reason, it is desirable to examine additional 

 methods of abatement. For purposes of discussion, this can be done most 

 satisfactorily by considering each waste element separately. 



The soap problem. No well-operated mill permits the escape of much 

 of the soap material to the sewer. Formerly, it was burned along with 

 the rest of the black liquor and its chemicals and heat values were recov- 

 ered. More recently, however, its chief constituent, tall oil, has been sepa- 

 rated and marketed in considerable quantities. The fat scarcity in recent 



