TABLE 3-7 Typical Percentage Removal of Microorganisms by Conventional Treatment Processes

 

 

Secondary Treatment

 

Microorganism

Primary Treatment

Activated Sludge

Trickling Filter

Fecal coliforms

<10

0-99

85-99

Salmonella

0-15

70-99

85-99+

Mycobacterium tuberculosis

40-60

5-90

65-99

Shigella

15

80-90

85-99

Entamoeba histolytica

0-50

Limited

Limited

Helminth ova

50-98

Limited

60-75

Enteric viruses

Limited

75-99

0-85

 

SOURCE: Reprinted, with permission, from Crook, 1992. © 1992 by Academic Press, Inc.

vey of wastewater treatment plants using activated-sludge secondary treatment after disinfection found viruses averaging 10 to 130 PFU/100 liters in 40 to 100 percent of the samples (Rose and Gerba, 1990). In a similar survey in California, 67 percent of the samples taken from secondary wastewater treatment facilities following disinfection contained viruses at levels ranging from 2 to 200 PFU/100 liters (Asano et al., 1992). Other studies of secondary effluent report similar findings, ranging from 3.5 to 650 PFU/100 liters (Rose et al., 1996, 1997; Yanko, 1993). However, Irving (1982) reported levels of enteroviruses as high as 715,000 viral PFU/100 liters. Likewise, protozoa can survive secondary treatment and disinfection. Cryptosporidium oocysts have been reported in secondary effluent at a level of 140 oocysts/100 liters (Rose et al., 1996), while Giardia cysts were found to range from 440 to 2297 cysts/100 liters (Rose et al., 1996, 1997). Table 3-8 summarizes the reported levels of pathogenic and indicator microorganisms in secondary effluent. These data suggest that wastewater discharges are contributing enteric pathogens to ambient waters, many of which may be used downstream for drinking purposes. All planned potable reuse projects and demonstration studies in the United States have used treatment in addition to secondary treatment, and such additional treatment is essential for protecting against risks of microbiological contamination.

Ambient Waters

In indirect reuse (either planned or unplanned), reclaimed water is discharged to a natural system (surface water or ground water), where it typically spends a period of time before being further treated for use as



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