What Are Sludge and Scum Layers in a Septic Tank?

Sludge is the layer of solids that settles to the bottom of a septic tank. Scum is the layer of oil, grease and light solids that floats on top. The partly treated liquid between them, called effluent, is what flows on to the drain field.

Explanation

The EPA explains that a septic tank holds wastewater long enough for solids to settle and form sludge while oil and grease float up as scum. Compartments and a T-shaped outlet keep both layers from leaving the tank and traveling into the drain field. Florida's rule describes a septic tank the same way: a watertight tank that separates solids from liquid, gives "limited digestion of organic matter" and stores solids (Rule 62-6.002(53), F.A.C.). For tanks made for use in Florida, the outlet's opening must sit at 30% to 40% of the liquid depth (Rule 62-6.013(2)(e)). The EPA also notes that greases and lighter solids float to the top.

Because the tank only partly breaks solids down, both layers grow over time. The EPA says to have the tank pumped when any of these is true:

• The bottom of the scum layer is within 6 inches of the bottom of the outlet.
• The top of the sludge layer is within 12 inches of the outlet.
• Sludge and scum together take up more than 25% of the liquid depth.

The EPA says household systems should be inspected at least every 3 years and tanks are typically pumped every 3 to 5 years. It also suggests writing down the sludge and scum levels the septic professional finds, so you can plan the next pump-out. The EPA says additives are not necessary for a septic system treating household wastewater and are not recommended; pumping is what removes the layers.

In Florida, Rule 62-6.010 says a pump-out removes the contents of every tank and compartment in their entirety. If the pumper leaves material in the tank for a reason, they must explain it to the owner in writing.

When to seek help

• You do not know when the tank was last pumped.
• Slow drains, gurgling or sewage backing up into the house.
• Your last report showed sludge or scum near the EPA limits.
• Damp spots, standing water or sewage odors over the tank or drain field.

Learn more in how often to pump a septic tank in Florida, 7 signs your septic tank is full and septic tank additives. Estimate your next date with the pumping interval calculator, see prices in the pumping cost guide, or compare septic pumping companies.

Related terms

• Septage
• Baffle
• Effluent filter
• Riser