Design Criteria and Parameters

The capacity of the soil to absorb liquid is proportional to the overall coefficient of permeability for the soil between the ground surface and the groundwater table as follows:


Q = gallons per minute (gpm) per acre K = overall coefficient of permeability, feet per minute (fpm)

S = degree of saturation of soil (near 1.0 for a steady-rate application)

The coefficient of permeability K depends on the soil characteristics (see Table 7.52.1).

The application rate depends on the soil structure, the land contour, the waste characteristics, the local evaporation-precipitation rate, and the supervision afforded the spray irrigation system. An efficient initial and design guide is the following schedule for the use of 1-acre spray plots:

1. Spray for 10 hr at 90 gpm; rest the plot for 2 weeks.

2. Spray for a second 10 hr; rest the plot for 2 weeks.

3. Spray for a third 10 hr, and discontinue use of plot after 30 hr dosage (162,000 gal) for approximately 90 days before restarting schedule.

The schedule is a guide; actual practice dictates which spray plots can receive more or less. Applying waste at too great a rate causes ponding and surface runoff. Ponding can result in anaerobic decomposition and thus cause odor problems. In addition, it renders the spray fields impass-

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