in bays should occur only if land application, open ocean disposal, 

 and tertiary treatment cannot be proved applicable. 



4.10.3 Conventional Treatment Systems 



Primary and secondary levels of waste treatment are in common use. 

 In primary treatment, sol ids are usually screened or allowed to settle and 

 are removed from the water. 



In secondary treatment, wastes are further purified through the use of 

 biological processes (Figure 33). 



Figure 33. Components and processes of a secondary sewage treatment 

 plant (Source: Reference 92). 



DIGESTOR 



The sludge is pumped 



to digestion tank for 



stabilization. 



HOW A SEWAGE TREATMENT PLANT WORKS 



PRIMARY CLARIFIER 



Solids settle to the top 

 and bottom and are then 

 removed as sludge. 



TRICKLING FILTER 

 Liquid from settling tank , 

 trickles over rock filter 



CHLORINATOR 



Disinfects 



effuent. 



BAR SCREEN 



Removes large 

 objects such as 

 rags and sticks. 



GRIT REMOVER 



Sand and gravel 

 settle to bottom 

 and are removed. 



Unfortunately, secondary-treatment plants remove up to 90 percent of 

 the organic matter but less than half of the nitrate and phosphate 

 nutrients in sewage (Table 23). The effluent is discharged with nitrate 

 values as high as 15 to 50 ppm, which may be 1000 times greater than the 

 natural levels of healthy coastal waters [94]. Discharge of effluent with 

 such a high concentration of nitrate causes overfertilization of confined 

 and poorly flushed estuarine waters [95][96]. Resulting algae growth can 

 cause high turbidities, reduced dissolved oxygen, and the accumulation of 



136 



