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NC State Extension

Wastewater Pretreatment for Improved Wastewater Treatment and Disposal to Soil

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E. Jerry Tyler
Small Scale Waste Management Project
Professor of Soil Science and Director of the Small Scale Waste Management Project(SSWMP)
University of Wisconsin-Madison,
1525 Observatory Drive,
Madison, WI 53706,
(608) 262-0853
FAX (608) 265-2595

Small Scale Waste Management Project (SSWMP),
1525 Observatory Drive,
Madison WI 53706,
(608) 265-6595
FAX (608) 265-2595
Web Page

Wastewater treatment prior to discharge to land is currently very popular. Sand filter and mechanical aeration units are common. Wastewater pollutants not treated by the land receiving the wastewater needs pretreatment. In many cases pretreatment is used to enable increased loading rates and smaller land areas.

All common domestic wastewater pollutants except pathogens can be used as a resource such as soil conditioners or plant nutrients. The wastewater resources must be placed in the soil so that they are utilized. If wastewater is applied below the root zone of plants, nitrates will likely pass to the groundwater as a pollutant and not be utilized as a plant nutrient. If wastewater is applied to the groundsurface pathogens will not be properly treated and there could easily be human contact. Wastewater treatment prior to land application could be used to treat pollutants unutilized or untreated on the land. Units offering either nitrogen removal or disinfection of bacteria and viruses are needed to meet treatment goals.

Treatment of wastewater prior to land application can enhance the infiltration and transmission of wastewater primarily be reducing the clogging of soil pores. Soil clogging results from the physical plugging of soil pores with cells and products of microbial growth such as slims and filaments. Removing energy and nutrient sources from the wastewater prior to land application reduces the microbial growth and clogging.

Enhanced transmission of wastewater into and through the soil reduces the area needed form infiltration. The reduction in area needed depends on soil characteristics. Areas needed for the infiltration of wastewaters in sandy soil can be reduced more than in clayey soils. Since clayey soil system reductions are less than sandy soil system, reductions are opposite desired by most people. If the pores in the clogging are similar to those in the natural soil no reduction is wastewater application area will be possible. In settings with groundwater at or near the wastewater infiltration area clogging may not be important and therefore pretreatment will have little influence on the wastewater acceptance rate.

There can be treatment and hydraulic benefits to pretreatment of domestic wastewater. Selecting pretreatment methods should be based on the goal of the entire system. The analysis should begin at the wastewater final discharge point and onsider the processes occurring at every point to where the pretreatment unit would be located. Pretreatment should accomplish that which the land will not.