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

Citation:  Pp. 261-270 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:10.13031/2013.6034)
Authors:   M. S. Wespetal
Keywords:   Mounds, Septic Systems, Experimental

Mound systems are used to overcome limitations imposed by seasonally saturated soils or bedrock. Mound systems are essentially a single pass, open bottom sand filter that provides both treatment and disposal of sewage tank effluent. Mound systems have proven to adequately treat and dispose of sewage in a reliable, cost-effective manner. However, mounds have been criticized, mainly by the public, for the large footprint required, poor aesthetics and the unusable lawn area resulting from the sharp rise in elevation. In addition, many existing sites with a seasonally saturated soil are too small to support the large footprint required for mound systems. To overcome these problems an alternative mound design is proposed that requires less footprint area and is more aesthetically pleasing. This vertical sidewall mound design is based on the perceived over-design of the absorption area and the additional unnecessary area needed to accommodate mound side-slopes. Evaluation of the standard mound design shows that the theory of pretreatment has not been applied. Specifically, the mound rock bed is designed on the development of a clogging mat and is sized at 5 cm/day (50 liters/meter 2 ) in Minnesota. As the effluent passes out of the rock bed and into the sand, the BOD and TSS levels are substantially reduced, therefore a biological clogging mat should not form at the absorption area at the sand/natural soil interface. However the sizing of the absorption area accounts for a the development of a clogging mat. An examination of the literature indicates that the absorption area for pretreated effluents is similar to what is required for a mound rock bed. Therefore, in theory, a mound with vertical sidewalls should hydraulically function. Two types of vertical sidewall mounds were constructed and observed in Minnesota by licensed professional designers and installers. These vertical side wall mounds are much smaller and have enhanced aesthetics. This paper presents the theory behind the vertical sidewall mound, design parameters, construction methods and observations of performance by licensed professionals in Minnesota.

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