24 November 2013

On Site Wastewater Disposal Systems: Soil Considerations

Posted by John Freeland

Source: Ministry for the Environment, Government of New Zealand. "The bugs in the soil have a very important role to play as they work to breakdown the nutrients and pathogens in the wastewater. "

Source: Ministry for the Environment, Government of New Zealand. “The bugs in the soil have a very important role to play as they work to breakdown the nutrients and pathogens in the wastewater. “

According to the U.S. Census Bureau, as of 1990 there were 24.67 million residences serviced by on-site waste disposal (OSWD) systems, representing 24.1 percent of the total number of households. The highest concentration of OSWD systems is found in the New England states where Maine, New Hampshire, and Vermont have about 50% of their households using them. That number has surely grown, but, unfortunately, 1990 data is the most up-to-date information I’ve found from the U.S. Census Bureau on this topic.

The Ocean Dumping Ban Act of 1988 (effective 12/31/1991) has likely led to increased use of OSWD systems. Alternative disposal facilities such as landfill and incinerators are expensive and often unpopular with the public. On-site waste disposal systems place the responsibility of sewage treatment entirely on the home or business owner instead of a publicly supported utility. OSWD systems have the economic advantage of not requiring frequent off-site transportation of sludge (Dunn and Leopold, 1978).

As population densities increase, the effectiveness of OSWD systems should come come under greater scrutiny for public health considerations.

Figure 1. Typical layout and cross-section for conventional OSWD systems (from Dunn and Leopold, 1978).

Structure and Function of OSWD Systems

Conventional on-site disposal systems (Figure 1) consist of three parts:
1. one or more collecting tanks that receive solids and raw wastewater from indoor plumbing;
2. distribution pipes that carry partially treated wastewater to the filter field, and
3. a filter field of soil that carries out the final waste treatment process.

Both anaerobic and aerobic conditions exist in properly functioning OSWDS. The collecting tank is buried under the topsoil and allows solids to settle out and be partially decomposed by anaerobic bacteria. The effluent leaving the collecting tank contains suspended and dissolved organic and inorganic solids, bacteria (including E. coli), and, perhaps, other pathogens.

Reneau et al. (1989) estimated 40% of the sludge volume, 60% of the biological oxygen demand (BOD), and 70% of the suspended solids were decomposed in the septic tank. Much of the organic N is converted to NH4+, a reduced form of nitrogen, which can later be converted to gaseous ammonia or oxidize to produce NO3-, soluble nitrate.

The distribution pipes, which have perforations, let the water flow slowly by gravity into the soil matrix. The soil needs to remain aerobic virtually all the time to facilitate further microbial decomposition of the remaining waste materials.

Some solids will be adsorbed onto soil particles where they will be slowly broken down. Other particles will be absorbed into interstitial spaces. The remaining wastewater infiltrates to the groundwater, is stored in the soil, or evapotranspires from the vegetated ground surface. Dissolved ions including nitrate, may enter the groundwater, even though the system has functioned properly (Dunn and Leopold, 1978).

The soil texture and depth to the seasonal high water table (SHWT) are of primary concern when selecting a site for an OSWD systems. Sandy soils enable water to percolate slowly in an aerated environment. If the soil is too coarse, however, water may travel too quickly to the water table without having sufficient time for microbial breakdown of waste.

Very coarse-textured soils lack adsorptive surface area and micropores needed to adequately filter the effluent. Fine-textured soils tend to have low hydraulic conductivities; water flows too slowly, and system backs up. Lack of aeration is another problem with fine-textured soil, limiting the activity of vital aerobic bacterial decomposers.

The more recent U.S. Soil Conservation Service county soil surveys and Web Soil Survey list soil suitability for OSWD system installations.

Depth to seasonal high water table (SHWT) must be far enough to prevent contamination of groundwater by inadequately-treated groundwater. The position of the SHWT is interpreted by soil classifiers using high and low chroma redoximorphic features, or mottles which are spots of rusty orange or greyish color that indicate iron mobilization in a chemical environment of alternating oxidation and reduction.
Exact criteria for minimum depth of the SHWT varies between states and between counties within states. Vepraskas at al. (1974) quoted Wisconsin regulations requiring at least 5 feet from the soil surface to the SHWT. The coarse-textured soils require greater depth to the water table because of their higher saturated hydrauli conductivity. Loamier soils provide slower percolation rates and higher adsorptive surface areas for fore efficient decomposition of wastes and therefore may require less depth to the SHWT.

OSWD Systems and Water Supply Wells
Figure 2. Conceptual diagram showing the interaction between a septic filter field and a water supply well. When the system is overloaded, increased hydraulic gradient between the groundwater mound and cone of depression presents a risk of well contamination.

cone of depression

Figure 2 illustrates a problem associated with over-pumping a supply well and overloading a filter field. A hydraulic gradient is produced by pumping a well and forming a cone of depression or zone of drawdown surrounding the well. This causes water to flow toward the well. Over-pumping a well creates a steeper, enhanced drawdown zone with a higher hydraulic gradient.

Simultaneous loading of the leach field sets up a groundwater mound and an even higher hydraulic head near the well. In this situation, according to Darcy’s Law, water will tend to flow down gradient from the saturated zone beneath the filter field toward the well. This is a potential source of water supply contamination.

Following World War II, many suburban residential subdivisions were built without adequate designs for septic systems. Often, the lots were too small and the areal density of the houses was high (Todd, 1988). With inadequate spacing and filtering designs, these neighborhoods run the risk of well contamination.

Clogging Mats
Incomplete decomposition in the filter field may result in the development of biological clogging mat; this often happens when loading rates exceed the decomposition rates. Usually, the mat begins to form at the proximal (nearest the collection tank outlet) end of the filter field, blocking portions of the filter field from receiving effluent. Clogging mats can cause effluent to saturate the soil and break out onto the surface. Saturated flow also causes rapid infiltration of untreated effluent to the underlying water table (Reneau et al. 1989).

Alternative Designs

Low Pressure Distribution Systems
To prevent this scenario, alternative OSWDS offer some advantages. The Low Pressure Distribution (LPD) system coordinates the rate and distribution of effleunt reaching the filter field. Periodically, “doses” of effluent are pumped from the collecting (septic) tank to the filter field. This method ensures the even distribution of waste throughout the entire filter field volume. Alternating the dosing with rest periods prevents overloading the system.

The Mound System
The “mound” system is another common alternative. Sand is brought in and stacked up to increase the depth to the SHWT and increase the volume of the filter field. Mounds enable septic systems to be built over wet soils or those shallow to bedrock.

The Soil Replacement System
Soil replacement consists of removing fine-textured soils and replacing them with sand (Reneau et al. 1989). Vepraskas et al. (1974) recommended soil replacement for a soil in Wisconsin that had a 3-foot thick silt cap with low hydraulic conductivity. Beneath the silt was a sandier soil. By digging out the silt and replacing it with sand, the filter field functioned properly.
In a comparative study including conventional, LPD, and mound designs, Cogger and Carlisle (1984) found the LPD system was the most effective in preventing off-site transportation of NO3-, NH 4+, total N, total P, and fecal coliform bacteria. The worst offsite contamination came from conventional and mound systems that were frequently or continuously saturated.

Design and site selection of safe, effective OSWDS requires an understanding of soil morpholgy and soil-water processes. Failure on the part of individuals or urban planners to address soil-water issues prior to building OSWDS can lead to real health risks. Individuals need to be aware of the limited capacity of their systems, and not exceed those limits by over-populating households and/or generating too much wastewater.

Soil classifiers need to be able to “read” landscapes and recognize recignize redoximorphic soil features, i.e., signs of saturation and reduction. Regulations designed to protect water quality are only as good as the extent to which they are enforced.

OSWD systems need periodic inspection and public agencies need the authority, expertise, and budgets necessary to manage the construction of proper OSWD systems and swiftly deal with failures. Because of the importance of controlled dosing and their demonstrated success, Low Pressure Distributed (LPD) systems appear safer than mound or conventional systems. In the long-run, LPD systems may pay for their initial higher cost by causing fewer maintenance problems.

Cogger, C.G. and B.L. Carlile. 1984. Field performance of conventional and alternative septic systems in wet soils. J. Environ. Qual. 13:137-142.
Dunn, T. and L.B. Leopold. 1978. Water in Environmental Planning. S.H. Freeman and Co., New York, NY.
Reneau, R.B. Jr., C. Hagedorn, and M.J. Degan. 1989. Fate and transport of biological and inorganic contaminants from on-site disposal of domestic wastewater. J. Environ. Qual. 18:135-144.
Todd, D.K. 1980. Groundwater Hydrology 2nd ed. Wiley & Sons, New York, NY.
Vepraskas, M.J. F.G. Baker, and J. Bouma. 1974. Soil mottling and drainage in a Mollic Hapludalf as related to suitability for septic tank construction. Soil Sci. Soc. Am. Proc. 38:497-501.