Here’s a Quick Review of Gravity Distribution to the Trench

Engineers call it delivering effluent in time and space. Simply put, soil must accept the wastewater sent through the septic tank.

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In the past few months, we’ve had a few questions related to hydraulic and organic loading of treatment areas through gravity and pressure distribution systems. We were going to jump right into a discussion about various ways effluent is delivered to sewage treatment and dispersal areas. But then we thought we should take a step back and talk about how systems operate under gravity and pressure distribution in a general way to highlight the differences.

This month we will discuss gravity distribution and next month pressure distribution within trenches. Whether distribution in the trench is by gravity or pressure, the objectives are the same: The soil needs to accept the amount of water the homeowner uses and provide treatment. To do this means accepting the water at the time it is delivered and to use the whole area of the system. Engineers call this “delivering effluent in time and space.”

Effluent is delivered to gravity trenches in one of two ways: either totally by gravity where flow out of the septic tank is delivered whenever there is a water-use event in the house; or in a pressure-dosed situation where effluent leaves the septic tank to a pump tank, is stored and then usually intermittently delivered in on-demand doses. A timer can be added to apply doses on a more regular timed basis through the day. This spreads the flow out more evenly from a time perspective, but does not affect what happens when the effluent is delivered to the trench.

Forming the biomat

In both cases, effluent is delivered to the pipe at the head of the trench. Depending on flow rate into the trench, effluent is delivered to the trench from just a few of the holes in the sewer pipe. This means all the effluent moves downward through the trench media to the infiltrative surface in a small area. This usually occurs in an area at the beginning of the trench. There are examples where the localized area can occur farther down the pipe depending on how the pipe was positioned, but the result is the same — effluent is delivered to a small area relative to the entire infiltrative surface available.

Effluent applied consistently in this one area results in increased biological activity and formation of what we call the biomat. Biomat is composed of the organic material from wastewater, bacteria and other organisms and their waste products. In the application area, biomat begins to reduce infiltration into the soil. Several things begin to happen: effluent spreads out over a larger infiltrative area developing biomat, and the soil becomes saturated in the area where flow moves through the biomat.

Unsaturated flow and presence of oxygen in soil pores is important for treatment to occur. Treatment includes the breakdown of organic material in the wastewater by aerobic soil organisms; reduction in pathogens (virus and bacteria) and tying up nutrients and other types of organic contaminants.

As more effluent is applied over time, the biomat continues to develop until it covers the entire trench bottom. Effluent then begins to pond above the biomat in the trench. At this point, the entire trench is being used to accept and treat the effluent. Up until the time biomat covers the entire infiltrative surface, the trench is not operating at its maximum treatment capacity. Areas where biomat is not developed accept effluent under saturated flow and treatment efficiency is reduced.

Plugging the pores 

How biomat is managed is critical for long-term system operation. Biomat forms in a biologically active area. Organic material is being added and broken down, so it will reach an equilibrium point as long as the effluent quality applied is maintained and effluent does not contain solids that won’t break down over time. So if hydraulic or organic loading from the septic tank does not increase and inorganic solids are kept at a minimum, the system will operate indefinitely.

Examples of solids that will not break down include everything from inorganic soil particles to small plastic particles contained in cleaning products. These will physically plug soil pores, and since they will not break down, they will permanently reduce the infiltration rate.

As we have discussed in the past, most soil-loading rates in use today assume the presence of a biomat and system size for an estimated daily sewage flow is determined by the biomat-influenced loading rate. A change in any parameter impacts the system size. A difference in soil type, organic load and water use all impact development of the biomat, long-term acceptance of effluent and treatment.

In gravity distribution, we depend on the soil with the biomat to distribute effluent over the area (space) of the system, not the distribution piping or the type of media in the trench. This is why installing trenches on the contour and level is critical to provide proper effluent distribution. 


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