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Published by the American Society of Agricultural and Biological Engineers, St. Joseph, Michigan

Citation:  Pp. 560-582 in On-Site Wastewater Treatment, Proc. Ninth Natl. Symp. on Individual and Small Community Sewage Systems (11-14 March 2001, Fort Worth, Texas, USA), ed. K. Mancl., St. Joseph, Mich. ASAE  701P0009.(doi:10.13031/2013.6064)
Authors:   R. M. Bohrer and J. C. Converse
Keywords:   Drip/Trickle irrigation, Soil, Fecal coliforms, Bacteria, Nitrate, Nutrient removal, Temperature, Wisconsin, Freezing, Cold climate

As the population increases, suitable land for wastewater treatment and dispersal decreases. Drip distribution of wastewater is becoming more popular as an alternative form of on-site wastewater dispersal. This method of dispersal distributes the wastewater in small uniform doses, allowing the soil system more opportunity to treat the wastewater before it reaches the groundwater, even on less suitable soil types. This protects the environment as well as human health. During the summer of 1999, soil samples were collected to a depth of 105 cm (42 in.) beneath six drip distribution sites in Wisconsin to evaluate the treatment performance of the soil. Three of the sites received septic tank effluent (STE), one site received recirculating gravel filter (RGF) effluent and two sites received effluent treated by aerobic treatment units (ATU). The soils at these sites ranged from coarse sand to clay loam. The depth of the driplines ranged from 10-50 cm (4-20 in.) below the ground surface. The systems receiving STE showed very low fecal coliforms at 45-60 cm (18-24 in.) below the dripline with no detects below 60 cm (24 in.). The systems with pretreatment showed even better results, both for the RGF, which was very heavily loaded, and the ATU systems. This could probably allow for a reduction in the separation distance to 45 cm (18 in.) for systems receiving STE and 30 cm (12 in.) if the effluent is aerobically pretreated to a fecal coliform level of <1,000 colonies/100 ml. The nitrate-nitrogen levels exiting the system at 105 cm (42 in.) below the dripline had median values in the range of 26-83 mg N/L of soil water, regardless of treatment type. However, these concentrations are similar to the background levels, which were unexplainably high. The ammonium-nitrogen levels at 105 cm (42 in.) were also similar to background levels. Temperatures were monitored at four of these sites from December 1, 1998 - March 30, 1999 and at five of these sites from December 1, 1999 - March 14, 2000. Although these were two mild winters for Wisconsin, air temperatures did fall below 0 C (32 F) for an extended period of time at each of the sites. In most cases, negative temperatures (Celsius) were also found in the soil at the depth of the dripline, as well as 10 cm (4 in.) below the dripline, often lasting a week at a time below 0 C (32 F). Soil temperatures at 10 cm (4 in.) below the depth of the dripline reached minimums of -1 C (30 F) in the southern portion of the state to -12 C (10 F) in the north. However, none of the systems studied encountered operational problems due the cold temperatures. With proper design and installation, drip distribution systems are an excellent alternative system for wastewater dispersal in cold climates.

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