QUESTION: In previous articles in Pumper and Onsite Installer, soil sizing charts were mentioned that identify rapid perc rate, fine sandy soils as being comparable to loamy soils in sizing capacities and treatment capabilities. Our state of Nebraska is reviewing several possible regulation changes and I would like to get the addition of a fine sand classification included in these changes. Our regulatory group was not aware of any research data to support the assumptions that the fine sands and loamy soils have the same treatment characteristics.

Our current regulations require soils with perc rates faster than 5 minutes per inch be “modified” through the physical process of removal and replacement with a loamy sand liner, with the final objective being that this modified liner perc between the rates of 15-20 minutes per inch. This entire process is not only very time consuming and extremely unpredictable; it may be unnecessary if the soil performance assumptions above can be supported.

ANSWER: In essence, the question is: Why are loading rates used for the design and installation of soil treatment areas different for different types of sandy soils? The articles the questioner refers to were written by Dave Gustafson and myself for Onsite Installer three years ago and a recent Answer Man article.

 

BIOMAT 101

Before answering the question directly, let’s briefly review what happens when septic tank effluent is introduced into soil using gravity distribution.

Effluent flows from the septic tank into the soil treatment trench through a few of the 1/2-inch holes in the distribution pipe, then through the distribution media to the soil surface. When the effluent reaches the soil, a condition called biomat is created. The biomat is formed by anaerobic bacteria in the effluent and any finely sized organic suspended solids carried over from the septic tank. The bacteria secrete a sticky substance around the outside of soil and rock particles. The biomat develops along the trench bottom and ponds the effluent in the trench. As the liquid in the trench rises, biomat develops along the sidewalls.

The main result of biomat formation is dramatic slowing of the infiltration rate of the effluent into the soil, creating unsaturated flow conditions. This is ideal for growth of aerobic bacteria and other soil organisms that help with treatment of pathogens and other contaminants. This is good news, and explains why the biomat – when properly managed – is a necessary component of soil treatment capabilities. In addition, the biomat reaches equilibrium. If effluent quality is maintained, the biomat will have the same thickness and permeability over time.

This condition is referred to as the Long Term Acceptance Rate, or LTAR. The LTAR is related to soil texture class, soil structure and consistency to predict the loading rates in soils where the biomat is fully developed. Research on these relationships has been conducted since the early 1970s and in fact, the condition was recognized as early as the 1950s.

 

SANDY SOILS ARE DIFFERENT

These relationships are reflected and used in almost all current state codes dictating the design and installation of onsite sewage treatment systems. Sandy soils are the one exception.

Since the sandy soils particle size is larger (0.05-2.0 mm), the size of the pores are also larger, allowing effluent to move rapidly into and through the sand, often without forming a biomat. This rapid movement does not allow time for treatment. So in the early ’70s and again as recently as 2006, column studies were conducted looking at virus removal in sands under different loading rates. These studies showed good virus removal in 2 feet of sand if the loading rate does not exceed 1.2 gallons per square foot/day. This is the loading number for sands found in most of our codes today.

In the early- to mid-1980s, we conducted research looking at the hydraulic performance of a proprietary product designed to replace rock as the distribution media in soil treatment trenches. One of the research sites was on a sandy outwash plain. Here the sandy soils consisted of greater than 50 percent fine and very fine sand particle sizes.

One unexpected result of this study showed a thin biomat formed in the soil and this biomat was very effective at reducing the flow rate into soil. Over several years of study, the biomat showed an acceptance rate of 0.6-gallons/square foot/day. This was confirmed over the next several years and the loading rate was incorporated into the Minnesota state code.

Another interesting note on how these sandy soils react to septic tank effluent: If you conduct a percolation test, the rate would be in the range of 30 seconds to 3 minutes per inch; so you would not distinguish these soils from other types of sands on the basis of percolation rates.

 

THE BOTTOM LINE

So what about the treatment approaches in these soils? For the coarse and medium sandy soils, the only way to ensure the loading rate doesn’t exceed 1.2-gallons/square foot/day is to spread the effluent out evenly over the entire soil treatment area. This means that a low-pressure distribution system is used. This assures adequate time for treatment to occur.

In the “old days” we said gravity trenches were suitable for fine sands if the system was divided into four equal parts and loaded sequentially to quickly form a biomat. Recently, we have moved away from this approach because the biomat hasn’t formed as rapidly or as uniformally as we thought. Minnesota now requires pressure distribution in these sands as well.

Another question that arises: Can’t we add some finer textured material (sandy loam for example) to the trench to provide treatment? While this can be successful, it is time consuming and expensive. In addition, if you have too many fines (silt and clay size particles) in the material, you run the risk of reducing the acceptance rate below 0.6-gallons/square foot/day. So the most cost-effective approaches are dividing the system into four equal parts or using pressure distribution.