Combining the stormwater and sewage systems for convenience and 

 economy is not usually acceptable. They are generally not capable of 

 handling the combined load during runoff surges. Also, the treatment 

 required for sewage may be quite different from that required for storm- 

 water. The best solution is to build separate systems to perform the two 

 functions and optimize each for its specific purpose. Modification of 

 existing systems to achieve separate stormwater treatment is difficult and 

 very expensive. In either expansion and rehabitation of old systems or 

 planning for new systems, substantial savings may be realized by designing 

 the separate stormwater system to treat only the initial, highly polluted 

 first flush increment of runoff— the first half inch or so which contains 

 most of the pollutants. For example, over 85 percent of the BOD may occur 

 in the first one-third to one inch of rainfall [112]. 



Storm runoff alone is often severe enough to drive the col i form 

 count above the national health standard of 70 MPN (most probable number 

 per 100 ml. of sample water). For example, on Long Island, New York, 

 local officials now believe that, despite plans for a billion-dollar 

 sewage treatment plant, productive shellfish beds will remain closed 

 because of bacterial contamination from urban runoff alone [113]. 



4.12.3 Industrial Sites 



Storm runoff from heavily industrialized sites, such as those occupied 

 by petroleum refineries, and petrochemical plants, contains quantities 

 of wastes not found in urban areas. In one study, runoff from industrial 

 areas contained 2,800 pounds of contaminants per road curb-mile. Residential 

 areas, on the other hand, contributed 1,200 and commercial areas 290 pounds 

 per curb-mile [71]. 



Significant amounts of toxic matter should not be allowed to run 

 directly to coastal waters, nor should such matter go into the sanitary 

 sewer system. The presence of toxic metals in industrial runoff poses a 

 special treatment problem, as their presence can reduce BOD removal by 

 killing bacteria in the activated sludge, anaerobic digestion, and 

 nitrification-dentrifi cation types of sewage treatment. Special manage- 

 ment is required for these sites, which may include separate collection and 

 treatment. The problems often can be mitigated by installing an on-site 

 treatment plant to remove contaminants. This may be required by the EPA 

 in some cases, including gas processing plants and oil refineries [114]. 



Economy in runoff treatment for an industrial site with an appropriate 

 land form and surface character may be accomplished through "land treatment" 

 by discharging runoff onto a specially designed vegetated slope [115]. The 

 soil and vegetation filter and recondition the water as it flows over the 

 slope to a collecting channel at its base. 



150 



