Septic System Components – Septic Tank

The Septic System Owners Manual / Septic System Components – Septic Tank

Overview

The primary functions of the septic tank are:

• to create a holding space where some of the solids can be separated from the liquids
• to break down solids through a biochemical process involving the bacteria
• to store the settled solids until pumping occurs.

In an anaerobic septic system, 40% of the solids are treated in the septic tank with the remaining 60% being treated in the drainfield.

Note: In a septic system retrofitted with an Aero-Stream® Remediation System, 90% of the solids are treated in the septic tank and only 10% are treated in the drainfield.

Septic Tank Design

The septic tank is commonly made of concrete, steel, fiberglass or plastic, is located underground, and ranges in size from 500 gallons for small septic systems to thousands of gallons for larger systems. The most common tank size range for a typical residential septic system is 1,000 to 1,500 gallons.

The septic tank can vary greatly in design, having a single treatment chamber or multiple chambers, depending on system requirements. The septic tank has one inlet in the treatment chamber and one outlet in the final chamber, with the outlet being at a lower height than the inlet to allow the effluent to flow in one direction only.

All chambers of a multi-chamber tank are connected by means of a single opening. The first (and possibly only) chamber of the septic tank is referred to as the treatment chamber. This is where the influent is received and the majority of the biochemical process occurs.

The second chamber (if one exists) and each additional chamber are referred to as settling chambers. Multi-chamber tanks are designed to act as a cascading system which increases holding time to more finely treat the effluent prior to being discharged to the drainfield.

What Goes On Inside

As the influent enters into an anaerobic septic tank, it separates into three distinct layers:

• the sludge layer
• the clear zone
• the scum layer.

All solids that are heavier than water settle at the bottom of the tank and make up the sludge layer. Because there is no oxygen at the bottom of the tank, the sludge layer supports anaerobic bacteria only. The anaerobic bacteria breakdown and digest the biodegradable solids of the sludge. During this process, the solids become lighter and migrate upwards to the middle of the tank or the clear zone.

The clear zone in an anaerobic septic tank is mostly grayish or brown, murky water containing fine and microscopic biodegradable and non-biodegradable materials in suspension. This layer contains mostly anaerobic bacteria, although there are some aerobic bacteria. As the clear zone discharges to the drainfield, it can carry some of the anaerobic bacteria with it. It is the anaerobic bacteria that discharge with the effluent that causes the build up of biomat in the drainfield.

The scum layer at the top of the tank contains greases, oils, soap films, and any other materials that are lighter than water. Both aerobic and anaerobic bacteria live in the scum layer, however, the anaerobic bacteria are dominant. As the bacteria digest the scum at the top of the tank, the digested waste of the bacteria becomes heavier than water and sinks to the bottom where it becomes sludge and is further treated.

To prevent any of the scum at the top of the tank from exiting the tank before being treated, the outlet baffle acts as a dam at the liquid surface level of the tank (if you recall, the top opening of the outlet baffle is located above the outlet of the chamber). With the outlet baffle properly installed, solids of any size and weight must pass through the clear zone as described above to discharge through the outlet baffle. This process is consistent throughout each septic tank chamber.

The Baffles

For additional treatment, an effluent filter is placed inside the outlet baffle. This can be a fine plastic mesh screen with slots or a bristle style filter similar to a large bristle brush. The filter prevents solids which are larger than the open area of the filter from leaving the tank. Also, bacteria can grow on the surface of the filter bristles further cleaning the effluent as it exits the tank.

Another advantage of using a filter exists during high liquid levels. In this condition, the water containing the scum could overflow the baffle dam and allow a significant amount of solids out of the tank into the absorption component. With a filter installed, the overflow would still pass through the filter and receive partial treatment from the bacteria growing on the bristles prior to flowing from the septic tank to a pumping chamber or drainfield.

The treatment chamber of the tank may or may not have an inlet baffle which is similar in appearance to the outlet baffle, but attached to the septic tank at the influent inlet. The purpose of the inlet baffle is to prevent surging during periods of peak water usage in the home, heavy rains, or spring thaws and also prevents solids from piling up on the scum layer. Without an inlet baffle, surging may cause the water level to instantaneously rise over the outlet baffle dam and allow solids to be discharged out of the tank chamber without being processed.