Sludge 101

Understanding the solid and scum layers in a septic tank will help you explain necessary system maintenance to reluctant customers


I am new to pumping and am fascinated by the biological processes involved in an individual sewage treatment system. I have numerous questions, but will only submit one topic: I want to know what sludge really is. One online encyclopedia says sludge is sewage solids that quickly sink through the floating scum. Another source says sludge is the waste product of the enzymes as they do their job of decomposing waste material.

Further, does sludge consist of many fine particles if allowed to dry and be inspected? Is this material the cause of drainfield glazing?


There are many different kinds of sludge and the nature of the sludge depends upon the process that created it. Certainly, one can refer to any settled material from either bacterial activity or yeast activity as sludge. The consistency of the sludge will be quite different, however, from various kinds of activity.

In a municipal sewage treatment system, the bacterial activity is usually aerobic. This means the bacteria have oxygen for their activity. This sludge has a composition somewhat like soil. The City of Milwaukee added some nitrogen to the sludge from its municipal treatment plant and marketed it as Milorganite. So the “sludge” was basically organic matter with a little extra nitrogen.

I doubt there are reliable references to accurately describe the sludge that accumulates in the bottom of a septic tank. So I will describe septic tank sludge for you. Parts of household sewage that are heavier than water sink to the bottom when the sewage enters the septic tank. Anaerobic bacteria slowly decompose the solids to reduce their volume.


The bottom of the septic tank is where the sludge layer forms. As the sludge layer rises, there is less liquid volume in the septic tank and some particles may move through the tank and enter the soil absorption system. The septic tank should be cleaned before this happens. You must educate your customers about this situation.

If a garbage disposer is used and raw vegetables are introduced into the septic tank, these will not be easily broken down, and the sludge layer will accumulate faster. Materials like bones and coffee grounds, when put into the garbage disposer, add to the sludge layer because they do not break down by bacterial action. This is why I recommend homeowners with onsite sewage systems do not install a garbage disposer.

The sewage from a home will also likely contain soap or detergent scum. Scum is lighter than water and floats on the surface of the septic tank. Cooking oils and fats also float and become part of the scum layer. The homeowner should try to minimize the amount of cooking fat or oil entering the system. The scum layer also occupies part of the liquid volume of the septic tank. If the scum layer gets too thick, some of the scum can be washed out into the soil absorption system. Again, the septic tank needs to be cleaned and pumped before this happens.

Some onsite sewage treatment systems use an aerated tank to promote decomposition of sewage solids. The design usually calls for a settling tank in the sewer line ahead of the aerobic tank. The settling tank takes out the largest solids and basically serves as a septic tank. The aerobic action in the second tank does a better job of breaking down organic solids, but a residue in the aerobic tank must be periodically removed. And it is also called sludge.

In my opinion, if any of the sludge I have described were allowed to dry out, one would not be able to separate and inspect any fine particles.


The proper place for the recycling of sludge from a septic tank or an aerobic tank is on agricultural land. The sludge adds organic matter and nutrients, which are recycled to grow vegetation. The application of sludge must be done with proper procedures to ensure pathogens (disease-causing bacteria) are not “recycled.”

If a septic tank is cleaned periodically, there should be very little, if any, sludge or scum flowing out into the soil absorption field.

The “glazing” you refer to is likely a biological layer called the biomat. The biomat forms from suspended organic particles contained in the effluent from the septic tank. The biomat forms at the soil surface in a drainfield trench or leachfield.

The biomat is anaerobic on the trench side and aerobic on the soil side. The thickness of the biomat depends on the quality of the effluent from the sewage tank and the porosity of the soil. The biomat allows liquid to move through it. The biomat is a treatment layer that removes pathogens.

The biomat will always form in a soil absorption system. The rate of liquid movement through the biomat is called the Long Term Absorption Rate. The rate is dependent upon the texture of the soil. Larger absorption areas are needed in clay soils as compared to loam soils.

Onsite sewage treatment maintenance is essential to the continuing operation of an onsite sewage treatment system. This is the message you need to emphasize to your customers.


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