Exploring Hydraulic vs. Organic Loading in Residential Systems

As we see wastewater flows diminish, there will be some justified calls for changes in size requirements for soil treatment and dispersal components

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Up until now, wastewater quantity (flow), expressed as gallons per day, along with soil characteristics reflected in the long-term acceptance rate, expressed in gallons per day per square foot, was used to determine the size of residential systems. Septic systems are designed to accept and treat a finite amount of water, and if that amount is consistently exceeded, the system will eventually fail.

Last month we discussed a study showing a significant 22% reduction in average household water use. Overall water use is declining, primarily due to installation of low water-use fixtures and devices such as toilets and washing machines. The general assumption is that system sizing will or should change over time; at least sizing tables in our state and local codes should change along with these reductions.

While we support continually evaluating our codes to keep them up to date with technology and use changes, we want to throw out a caution: When changing sizing requirements, we need to factor organic loading to the system into the process.

All wastewater has two major components: a hydraulic or quantity component and an organic component. For our industry, reflected in state codes, the organic component is usually expressed as milligrams per liter of biological oxygen demand. Current LTAR’s are based on septic tank effluent with a BOD value of 170 mg/L.

Concentrated flows

Without getting too deep into the math involved, a maximum value for the mass loading to the soil treatment area can be determined and expressed in pounds per day per square foot of infiltrative surface, as indicated above on the average daily sewage flow and the BOD concentration of the septic tank effluent.

While flows (hydraulic) decrease, the amount of organic material in the wastewater does not. Concentrations of BOD in the effluent will likely increase. We do not know exactly how much the concentration will rise/change as flows are reduced; because with lower flows and similarly sized septic tanks, retention time in the tanks will increase, which may lead to some increased treatment.

To illustrate the potential problem, if concentration increases by approximately 20% while flow is reduced by 20% and the required infiltrative area is reduced at the bottom of the trench by 20%, the pounds per square foot of organic loading actually increases.

Since organic material in the wastewater is part of the formation of the biomat, which is factored into our estimated LTARs, any increase in organic loading at the infiltrative surface will result in a more resistant biomat and a reduced flow rate into the soil, affecting the capacity of the soil to hydraulically accept effluent. If this condition occurs continuously, the system will hydraulically fail.

The importance of both the hydraulic load and organic load to the system has been recognized for some time in the industry. It is the reason commercial establishments are treated differently than residences in terms of system treatment requirements.

A study of restaurants in Texas showed that the median concentration of BOD in septic tanks was about 1,500 mg/L versus our desired 170 mg/L for residential systems. A residential system with an estimated sewage flow of 600 gallons/day in a silt loam soil would require about 1,500 square feet of trench-bottom area. For a restaurant with an estimated sewage flow of 600 gallons/day and the 1,500 mg/L BOD, the area required to have a similar organic load to the infiltrative surface would be over 10,000 square feet.

Gather peak flow infoThis is the reason treatment systems for different commercial establishments have special considerations regarding pretreatment requirements and system sizing. Each type of business requires its own way of estimating both the hydraulic flow and the strength of the wastewater. Using average numbers from other parts of the country is useful, but not as useful as data collected on similar establishments in the area or, better yet, data collected from the system itself.

Organic load of wastewater is an important consideration when designing a commercial operation because the wastewater strength is usually greater than residential. The type of additional pretreatment before dispersal in the soil is dependent not only on the total flow, but also on the type of establishment and the water use patterns. The more information gathered about peak flow times, the better for selecting whether a media filter or aerobic treatment unit is the best choice. The goal is to reduce the organic loading to below residential strength.

As we look toward the future, we will probably continue to see reductions in hydraulic loading in our systems, not due to behavior change but merely because there will be increased use of water conserving devices. As this continues, there will be pressure to change the size requirements for the soil treatment and dispersal parts of our systems. While some reduction may be justified, it should not be done without considering both the hydraulic and organic loading parts of wastewater.


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