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Septic Systems 101: The Septic Drainfield

In most modern septic systems wastewater is treated through use of a drainfield (also called a leach field or disposal field). There are many different setups and several variations on the drainfield. We will cover the purpose for and most common type of a drainfield in this section. However, should you have any questions about your drainfield or a particular setup please contact us- we’d be glad to answer any questions.

The primary function of a septic system drainfield is to purify and treat wastewater through soil absorption. This is done with a series of shallow and narrow trenches filled with a perforated pipe and a filtration media, such as gravel, or a man made product called EZ-FLOW™, that acts like gravel, or a series of chambers called Infiltrator chambers. The filtration media’s main purpose is to help maintain the structure of the trenches and provides an additional filtration layer.

Additionally, a very important part of a drainfield is the biomat. The biomat is living organism that forms as a drainfield is in use. This biomat is a black jelly-like organism that grows from the aerobic organisms that leave the tank with the effluent. A biomat is a very important part of a drainfield because it creates an additional filtration layer made of living organisms that can further breakdown any particles that enter the drainfield while also decreasing how fast wastewater permeates the ground.

The designs of the trenches and pipes have many variations using various pipe connections and layouts. The most common variation we use is called an equal distribution drainfield. This drainfield, in theory, sends equal amounts of effluent from the distribution box to a single pipe or multiple pipes connected to the distribution box. This helps keep the soil from becoming oversaturated as effluent enters the drainfield. The trick to an equal distribution drainfield is that the distribution box must remain level or effluent will be sent to pipes on the lower side, thus not equally distributing effluent.

Other common drainfield variations include a serial distribution drainfield and dual drainfields. In a serial distribution drainfield effluent flows from the distribution box to one pipe and trench then a second pipe and trench once the first once is fully used. This system works best on a sloped property as the first trench can become oversaturated if the ground is even. In a dual drainfield there are two series of trenches, however only one is ever in use. A valve must be turned to send effluent to the second drainfield. These systems work best in areas where a good site and soil conditions are present. The advantage of alternating drainfield trenches improves treatment of effluent and drainfield performance. The trick with this system is typically a system owner’s inability to remember to switch the valve regularly.

All of the above drainfield setups are designed to help wastewater to seep out of the pipe and into the soil where wastewater is naturally treated by the microorganisms living in the soil. Ultimately, each septic system is determined by the condition and amount of soil surrounding the system.