Figure 34. Advanced wastewater treatment showing the biological- 

 chemical method employed to activate sludge treatment with alum 

 addition and nitrification-dentrification (Source: Reference 101). 



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appropriate slopes (3 to 6 percent) and vegetation (e.g., canary grass), 

 130 acres (53 hectares) may handle the municipal wastes of 10,000 people 

 people [97]. Package plants, designed to treat the wastes of small 

 50- to 150-unit developments, may be particularly amenable to this 

 type of soil and vegetative restoration. 



4.10.5 Combined Systems 



Normally it is not practicable to combine stormwater with the 

 sanitary sewage system because plants are likely to be overwhelmed during 

 heavy storms with the result that volumes of untreated sewage overflow to 

 coastal waters. The additional plant capacity required to effectively 

 handle the large amounts of water received during peak storms is enormous 

 and entails great capital expense. 



Some pollutants in raw industrial wastewater that are discharged into 

 municipal collection lines may pass through municipal treatment facilities 

 unchecked. In some cases, industrial sewage so discharged contains 

 sufficient biocidal substances to reduce the efficiency of municipal 

 facilities to purify domestic sewage. 



To avoid any complications that might arise from trying to treat both 

 types of sewage with a combined facility, which could require costly 

 modifications of locally operated sewage treatment plants, dual systems should 

 be designed so that each type can be treated separately from the other. 



Safeguards should be instituted to ensure that raw or improperly treat- 

 ed industrial wastes are not accidentally discharged into municipal lines. 



139 



