Environmental Impact of Irrigating Lawns with Treated Domestic Wastewater

Article excerpt

Three lawn plots equipped for measuring and sampling runoff were used to observe the environmental impact resulting from daily irrigation with treated septic tank effluent. "Pop-up" sprinklers were used to apply septic tank effluent treated by an on-site recirculating sand filter to one plot and potable water to another. The third plot received no irrigation. All lawn-plot runoff during a nine-month period was measured, sampled and analyzed for quality. Coliform bacterial exposure risk was assessed for each plot on a quarterly basis.Greater quantities of each monitored pollutant (i.e., nitrogen, COD and coliform bacteria) moved off-site in runoff from the potable-water-irrigated plot than from either of the other two plots. Fecal and total coliform bacteria were found to be present on all lawn plots. The risk in summer of encountering a coliform bacterium as a result of contacting grass in the plot irrigated with treated wastewater appeared to be no greater than that associated with contacting grass in a conventionally managed lawn.Observation of single family residential units in the United States suggests that there are at least two specific requirements on almost everyone's list of essentials for an acceptable residence. One requirement is a means for discharging domestic wastewater and the other is an outside area containing living plants (i.e., a lawn and/or ornamental garden).In areas not served by sewer systems, soil plays an important role in satisfying both requirements. On-site domestic wastewater renovation systems have traditionally relied on the physical, chemical and biological properties of soil to remove waste components as wastewater passes through on its way to a groundwater reservoir. Plants growing in lawns and gardens also rely on soil for physical support as well as a reservoir from which water and essential nutrients can be drawn.Unfortunately, many sites where people desire and/or need to live have thin soils that require special efforts to meet the essential requirements of a lawn and facilities for waste water discharge. Effective waste water treatment systems that do not rely upon natural soil for cleaning wastewater, such as recirculating sand filters (1), can be implemented on such sites. However, they do not totally satisfy the requirement of a facility for wastewater discharge, since a means of discharging effluent from such systems must be provided. The requirement for a lawn on such sites can become a benefit at this point, because, on thin soils, water is often the factor limiting the establishment and maintenance of lawn plants. Discharge of treated wastewater through a lawn irrigation system, therefore, provides the means for satisfying two essential requirements that often limit use of sites with thin soils for residential development.However, concerns about potential negative impacts on the environment always accompany suggestions of lawn irrigation with treated wastewater. Concerns about potential health risks associated with human lawn contact and transport of contaminated runoff onto adjoining property and into streams serve to prevent widespread implementation of lawn irrigation with treated wastewater.Some of the concerns can be addressed by requiring subsurface irrigation. Treated wastewater subsurface irrigation systems are being implemented to some extent in some western states (2).The state of North Carolina permits the use of surface sprinklers for on-site irrigation of treated domestic wastewater and has issued rules and regulations governing such systems (3). Because of concerns about negative environmental impacts, buffer zones of 122 meters (400 feet) between inhabited residences and irrigation systems are required.A better understanding of risks coupled with efforts to develop design and operating criteria that will minimize risk may help identify a technology that can be readily accepted by society. The objective of the project described herein was to observe the impact of lawn runoff quality and bacterial exposure risk produced by daily lawn irrigation with treated domestic wastewater. …