The best-documented system of overland filtration is a food industry installation in Paris, Texas. The soil structure is gray clay underlaid by red clay at varying depths that are highly erodable. Little infiltration occurs in this system which is laid on a land so contoured that the waste flows in a thin sheet across the surface. The treatment depends on the microbiological activity of soil organisms to purify the water as it travels across the field. The treated water is collected in terraces, and four or five sprinkler lines are laid out on a hillside as shown in Figure 7.52.5. This installation applies the wastewater at approximately 0.6 in/day or 3 in/5-day week.
Studies indicate that 175 ft of downhill slope provide effective purification. Sprinklers normally blanket an area 100 ft in diameter, and the downslope requirement is 50 ft beyond the perimeter of the sprinklers. For maximum efficiency, the degree of slope should be between 2 and 6%. Flatter slopes encourage puddling and subsequent anaerobic conditions, whereas the retention time on a steep slope is insufficient for complete treatment at normal application rates.
The primary grass in this system is Reed Canary, which yields a large quantity of high-quality hay, containing up to 23% crude protein with twice the mineral content of other good-quality hay. In feeding tests, cattle preferred the hay grown on the disposal site to other types of hay.
By relating potential evapotranspiration to the quality and quantity of the hay crop environmental engineers can predict the time of year or stage of growth when the highest-value hay can be harvested. They can use the relationship between potential evapotranspiration and soil trac-tionability to plan a hay harvest that least disrupts the disposal system's normal operation. Poor soil traction-
ability can interfere with a planned harvest, whereas in other areas with lighter soils, an optimum harvest can result in highest-crop value and equipment utilization. Two or more harvests of Reed Canary grass per growing season may be feasible.
This installation has achieved removal rates of up to 90% phosphorus and nitrogen in the wastewater. The subsequent reclaiming of most of these nutrients by the hay crop extends the finite capacity of the soil to store nutrients.
While the soil concentration of TDS and sodium is increasing at this site (Paris, Texas), it has not reached the point that is injurious to plants. Some signs exist that the rate of increase is lessening and a state of equilibrium is being approached. Nitrates percolating through the groundwater reserve are not expected to build to a harmful level.
Table 7.52.2 summarizes the treatment performance of the Paris overland flow system. Early observations indicated that while BOD and nitrogen removal were high— 99 and 90%, respectively—the phosphorus removal was low, about 45% (see Table 7.52.2). A change in the operating procedure that provided a longer rest period between applications, with no change in the total volume, increased the phosphorus removal to nearly 90% without affecting the BOD or nitrogen removal efficiency.
An analysis of groundwater samples showed that while mineral salts had increased over a 5-year period, the total accumulation was not critical and the rate of increase appeared to be dropping off. Based on the data accumulated to date, no significant disturbance to the soil structure is anticipated for 35 to 50 years, and an equilibrium stage (due to rainfall) will probably be reached sometime in the interim.
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