How Septic Tanks Filter Wastewater

All household wastewater flows into the septic tank through a main drainage pipe. Inside the tank, the contents are separated into three layers. Let’s look at these layers in greater detail.

• Scum Layer

This is the topmost layer that is composed of lighter materials like oils, fats, grease, and a few solids that are lighter than water. This layer plays a crucial role in the wastewater treatment process. It separates lighter materials from the effluent, thereby helping to prevent clogging of the outlet baffle and the pipes leading to the drainfield.

The scum layer, along with the sludge layer, helps in the initial clarification of wastewater. By trapping fats, oils, and greases, the scum layer ensures that these substances do not reach the drainfield, where they could cause clogging and reduce the soil's ability to absorb and filter effluent. If not contained in the septic tank, these floatable materials could cause surface scum or clogging in the drainfield.

Like the sludge layer, the scum layer is also a site for anaerobic bacterial activity. These bacteria help break down some of the organic materials within the scum. They reduce the volume of floatable waste and contribute to the overall treatment process. If the bottom of the scum layer is within 6 inches of the outlet baffle, or if the top of the scum layer is within 12 inches of the outlet baffle, it is time to schedule septic tank cleaning.

• Sludge Layer

The sludge is composed of heavier solids and organic matter that settle at the bottom of the tank. In other words, this layer consists of all the solid waste that the bacteria aren’t able to decompose. The sludge layer should not exceed one-third of the total tank depth. If the sludge level reaches this point, it is time to schedule septic tank pumping. It is important to monitor the sludge level in your septic tank to ensure the efficient operation of your septic system and prevent costly repairs.

The sludge layer harbors anaerobic bacteria that break down organic matter in the absence of oxygen. This digestion process reduces the volume of sludge by converting some of it into gases and simpler compounds. The digestion process aids in the further treatment of wastewater.

• Effluent Layer

The effluent layer is the portion of the wastewater that exits the septic tank through the outlet baffle and flows into the drainfield. It is partially treated at this stage and requires further treatment as it percolates through the soil in the drainfield. The soil acts as a natural filter, removing remaining pathogens, nutrients, and organic materials. This process is crucial for protecting groundwater and the environment.

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