Typical Pathogen Levels in Wastewater Sludges

Anaerobically Untreated Digested

Viruses

2500-70,000 1.0 x 106

100-1000

Salmonella

Fecal coliforms

8000

Ascaris lumbricoides

200-1000

0-1000

The coarse-textured soils and high hydraulic loading rates used in SAT systems increase the risk of bacteria and virus transmission to groundwater aquifers. A considerable research effort, both in the laboratory and at operational systems, has focused on viral movement in SAT systems (Reed, 1979). The results of this work indicate minimal risk for the general case; movement can occur with very high viral concentrations if the wastewater is applied at very high loading rates on very coarse-textured soils. It is unlikely that all three factors will be present in the majority of cases. Chlorine disinfection prior to wastewater application in a SAT system is not recommended, as the chlorinated organic compounds formed represent a greater threat to the groundwater than does the potential transmission of a few bacteria or viruses.

3.4.4 Sludge Systems

As shown by the values in Table 3.10, the pathogen levels in raw and digested sludge can be quite high. The pathogen content of sludge is especially critical when the sludge is to be used in agricultural operations or when public exposure is a concern. The sludge utilization guidelines developed by the U.S. EPA are discussed in detail in Chapter 9. Sludge stabilization with earthworms (vermi-stabilization) is also described in Chapter 9, and some evidence suggests that a reduction in pathogenic bacteria occurs during the process. The freeze-dew-atering process will not kill pathogens but can reduce the concentration in the remaining sludge due to enhanced drainage upon thawing. The reed-bed drying concept can achieve significant pathogen reduction due to desiccation and the long detention time in the system. Pathogens are further reduced after sludge is land applied, by the same mechanisms discussed previously for land application of wastewater. There is little risk of transmission of sludge pathogens to groundwater or in runoff to surface waters if the criteria in Chapter 9 are used in system design.

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