# Estimation of Saprolite Thickness Needed to Remove _E. coli_ from Wastewater
## 1 Introduction  
On-site wastewater management systems (OSWMS), commonly referred to as septic systems, are used to treat and dispose of sewage in areas not served by a municipal severe system. Approximately 20% of the households in the US, and one-half of those in North Carolina (N.C.), use these systems to manage their domestic sewage on-site [1]. A conventional OSWMS consists of a septic tank and a drainfield [2]. The septic tank provides primary treatment to the sewage by allowing solids to settle and go through anaerobic digestion. The liquid from the septic tank (referred to as wastewater) containing dissolved and suspended organic materials, microbial organisms (e.g., _Escherichia coli_ (_E. coli_)), and chemical pollutants are dispersed into the soil through a series of trenches in the drainfield [3]. In the unsaturated and aerated environment below the trenches, some pathogenic bacteria are removed through physical filtration, and the anaerobic bacteria typically die off in the aerobic soil environment [4; 5].  
In N.C. and many other states, most OSWMS are installed at sites with a suitable soil that is at least 60 cm thick [6]. However, in the Southeastern part of the United States (US), saprolite is being increasingly used for OSWMS where soils are thinner. Saprolite (commonly known as rotten rock) isovolumetrically-weathered bedrock that is porous and friable (similar to soil), but it has had some of its original minerals dissolved and removed, creating pores between the mineral grains that were in the original rock [7; 8]. The structure of saprolite is described as \"massive-rock controlled,\" meaning that the planar voids commonly found between the structural units of soil (i.e., soil peds) are absent in saprolite. Saprolite is found under nearly all soils in the Piedmont and Mountain regions of the Southeastern US and extends from the bottom of the soil column to solid bedrock [9].  
The thickness of saprolite materials is variable and can range from \(<\)1 m where bedrock is shallow to over 50 m thick where the bedrock is deep [9].  
Coliforms, such as _E. coli_, can be removed from soil water by: (1) attachment to soil particles, (2) consumption by larger soil microorganisms, or (3) straining by which coliforms are trapped in pore throats that are too small for the organisms to pass through [4]. [PERSON] et al. [10] showed that straining or filtering was an important mechanism responsible for removing colloid-size particles (\(<\)10 \(\upmu\)m diameter and including bacteria) from water moving through granular media. They noted that in their study, colloid straining occurred even for particles 0.45 \(\upmu\)m in diameter. Colloid straining was controlled by the size of the colloid and the pore sizes in the medium through which the colloid was passing through.  
Our previous research has shown that 60 cm of aerated, unsaturated sandy loam saprolite can effectively remove _E. coli_ from simulated wastewater [11]. Similar results have been found for soils of comparable texture [12; 13]. Saprolite materials with a sandy loam texture have pores small enough to filter or strain organisms such as _E. coli_. However, due to the presence of a significant amount of pores, which are \(>\)10 \(\upmu\)m in diameter, _E. coli_ may not entirely be removed by a 60 cm thick layer of sandy saprolite. Therefore, the main objective of this work was to develop a simple method for estimating the thickness of saprolite needed to filter _E. coli_ from wastewater for textures ranging from clay loam to sand. For this, we propose a unique method for determining the thickness of saprolite in use, which is required for the complete filtration of _E. coli_ and has a volume of pores smaller than 10 \(\upmu\)m.