Effluent Storage and Biomat Occurrence among Septic System Absorption Field Architectures in a Typic Fragiudult

Abstract

On-site wastewater treatment systems (OWTSs) are commonly used by households in areas of low population density to treat household wastewater and recycle it back to the environment. However, new absorption field products of differing architecture types have recently become available. A 3-yr field study was conducted in Bethel Heights, northwest Arkansas to assess several newer architecture types (i.e., chambers, polystyrene-aggregate, and gravel-less pipe) relative to the traditional pipe-and-gravel design under wet- and dry-soil conditions. Thirteen products of four different architecture types were installed in 46-cm-deep trenches in a Captina silt loam (fine-silty, siliceous, active, mesic Typic Fragiudult). Products were evaluated based on in-trench solution storage measured with an electronic water-level sensor approximately weekly from January 2009 through January 2012. Between May 2010 and January 2012, the thickness of any biomat formation was measured approximately weekly by insertion of a wooden dowel through in-trench monitoring ports. Architecture type alone did not affect ( > 0.05) in-trench solution storage. However, solution storage among individual products differed under wet- and dry-soil conditions ( < 0.05). When present, biomat thickness differed significantly ( < 0.05) among all four architecture types, ranging from 1.4 to 6.2 cm thick on average in the pipe-and-aggregate and polystyrene-aggregate types, respectively. Regression analyses showed that biomat thickness increased in three products, did not change in nine products, and decreased in one product over time. Results showed that several currently approved alternative products had similar in-trench solution storage but that several alternative products also had greater solution storage than that of the traditional pipe-and-gravel system. With no observed effluent surfacing, the soil morphology approach appears to be adequate and appropriately environmentally conservative for assigning typical single-family loading rates to alternative OWTS products and to the traditional pipe-and-gravel system.

Increased Effluent Dosage Effects on On-Site Wastewater Treatment Systems of Differing Architecture Type

“…Prater et al [15] extended the study initiated by Mathis et al [14] and summarized results of in-product storage after three continuous years. Biomat presence and thickness were also monitored after about 1.5 years of effluent dosing, where loading rates were based on soil morphology and the soil storage concept [16] and set at the maximum allowable loading rate by Arkansas regulations.…”

“…Beginning in 2012, research was conducted at the wastewater treatment facility in Bethel Heights, Arkansas (36˚14'12.74"N, 94˚7'56.81"W) as a continuation and extension of the studies conducted by Mathis et al [14] and Prater et al [15] that began in 2007. Bethel Heights had an estimated rural population of 2456 in 2013 [19].…”

“…The majority of the population has a septic tank that temporarily stores domestic wastewater that is pumped from each tank, approximately 460 connections, to the community wastewater treatment facility [14]. Raw, filtered effluent was used for this study as was used in the previous two studies [14] [15]. The soil at the study site is a Captina silt loam (fine-silty, siliceous, active, mesic Typic Fragiudults) with silty pedisediment over silty and clayey residuum weathered from cherty limestone and is moderately well-drained [20].…”

“…The soil around the BHWTF is representative of many areas throughout northwest Arkansas with a moderately well-drained silt-loam to clay-loam texture with a fluctuating seasonal water table and fragic soil properties in the top 35 to 65 cm [15]. Prater et al [15] extended the study initiated by Mathis et al [14] and summarized results of in-product storage after three continuous years.…”

“…Biomat presence and thickness were also monitored after about 1.5 years of effluent dosing, where loading rates were based on soil morphology and the soil storage concept [16] and set at the maximum allowable loading rate by Arkansas regulations. Prater et al [15] reported that none of the products experienced failure and biomat thickness increased in three products, did not change in nine products, and decreased over time in one product. However, there is speculation that some architecture types can handle increased loading beyond the present maximum rates, which in Arkansas are set equivalent to the standard 61-cm-wide, pipe-and-gravel product [17].…”