Time to Review Basics of Soil Sizing Factors and Organic Loading

Designing an effective residential septic system still comes down to the delicate balance of estimated daily sewage flows and long-term acceptance rates

During basic septic system classes, we stress trying to meet two goals that often work against each other during the designing and installing process: We (more particularly the homeowner) want the final soil dispersal and treatment area to accept all the water that exits the house, while at the same time treating effluent before it is released to the environment. Meeting these goals protects human health and the environment.

Our basic classes concentrate on single household (domestic) waste, although we briefly discuss some other business or commercial systems. In this context we discuss (and have reported in this column previously) household water use habits and how they affect determination of estimated daily sewage flows for residences and long-term acceptance rates, or LTARs, for soils that lead to establishing the size of the final soil dispersal and treatment area.


LTARs were determined in two ways and reflect the two goals discussed above. For sandy soils, the primary concern was how rapidly virus and bacteria could move through sand under saturated flow conditions. Research showed if a sandy soil was loaded at no more than 1.2 gallons per day per square foot, virus and bacteria would not move farther than 2 feet in the soil. As we look around the country at various state codes, we see most specified loading rates are either 1.0 or 1.2 gpd per square foot, consistent with the treatment concern for sands.

On the other end of the spectrum, when the soils have increasing amounts of clay, the numbers were determined by how much water could move through the soil after establishment of the biomat. It was determined that the saturated flow rate in clays was 1.0 centimeters/day, which is equivalent to a loading rate of 0.24 gpd per square foot. Most often, the number 0.2 gpd per square foot is used in state codes. As a side note, in older research articles, you will find the biomat referred to as a crust or clogging layer. The LTARs reflect a combination of the biomat and the soil texture and structure. In soils with high clay contents, how fast water moves through the soil is more determined by the soil itself, rather than the biomat.

Further, LTARs are based on a biomat that develops when typical residential septic tank effluent is applied to soil. Effluent with a biological oxygen demand of 150-170 milligrams per liter is considered typical. Effluent that consistently exceeds this level of what we call “organic loading” will result in a more resistant biomat and a lower LTAR. Maintaining the septic tank becomes very important to keep organic loading values within the accepted range.

As we have discussed in previous columns. Providing a “cleaner” effluent through additional pretreatment is desirable and can extend the life of systems and to a certain extent allow somewhat higher loading levels that would decrease the size of systems. Our concern about this approach is that what we have seen in codes is that decreasing the size of systems is applied across the board. So on the sandy end of the soil spectrum, you may not get the treatment of bacteria or viruses desired and on the clay end — since the soil becomes the controlling factor — the systems can end up undersized, resulting in premature hydraulic failure.


One of our previous articles on this topic led to this comment and observation by a reader:

“I assume your article is in the context of a single-family residential home; however, I would caution that the organic loading/sizing is becoming more and more of a critical factor in systems, especially commercial and community systems. We are finding through our management and sampling of clusters of homes on a community system that we are not getting the effluent quality we would expect from single-family homes even with adequate septic tank capacity. Commercial systems are a ‘whole different animal’ altogether with regard to what the organic content of the effluent may be. So, a word of caution to be sure to include organic loading as a factor in sizing, in addition to the hydraulic loading and soil type/texture criteria.”

We would agree with this comment; other establishments are a different animal and will require additional treatment. BOD levels from restaurants, bars, and any facility with kitchens and food preparation can be many times higher than 170 mg/L. Even multiple septic tanks in series as the only pretreatment usually will not bring the numbers down sufficiently. If the effluent has higher values, you will not get the same performance.

At the clay end of things, though, the soil is still the controlling factor; but more organic material can make it worse. Designers and regulators must recognize the importance of organic loading to the design numbers and criteria they are using. Cleaner effluent does not always mean the loading rate can be increased. And if they do not provide designs with adequate pretreatment of organic loading, the systems will be subject to premature failure.

The comment on sampling community systems is interesting. If they are not getting the necessary pretreatment using septic tanks, the tanks may in fact be too small or additional pretreatment may need to be added to bring the numbers down. We assume adequately sized tanks mean they meet the state code. To us, this means perhaps the code should be revisited based on their information and larger tanks or additional pretreatment required for community systems.


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