Ground Surface Aspects

The major concerns relate to the potential for the contamination of surface vegetation or off-site runoff, as the persistence of bacteria or viruses on plant surfaces could then infect people or animals if the plants were consumed raw. To eliminate these risks, it is generally recommended in the United States that agricultural land treatment sites not be used to grow vegetables that may be eaten raw. The major risk is then to grazing animals on a pasture irrigated with wastewater. Typical criteria specify a period ranging from 1 to 3 weeks after sprinkling undisinfected effluent before allowing animals to graze. Systems of this type are divided into relatively small paddocks, and the animals are moved in rotation around the site. Control of runoff is a design requirement of SR and SAT land treatment systems (as described in Chapter 8), so these sources should present no pathogenic hazard. Runoff of the treated effluent is the design intention of overland flow systems, which typically can achieve about 90% removal of applied fecal coliforms. It is a site-specific decision by the regulatory agency regarding the need for final disinfection of treated OF runoff. Overland flow slopes also collect precipitation of any intensity that may happen to occur. The runoff from these rainfall events can be more intense than the design treatment rate, but the additional dilution provided results in equal or better water quality than the normal runoff.

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