You Open the Tank and There’s No Sludge or Scum. What’s Next?

With no settling or floating solids present, it’s time to put on your detective hat, evaluate the reasons and take action.

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Recently a reader commented he was seeing numerous septic tanks lacking distinct layers of sludge and floating scum. The time frame coincided with more people staying at home because of the COVID-19 pandemic. 

Septic tanks operate to provide anaerobic (without oxygen) treatment of the wastewater from the house. When baffles are operating as they should, the tank slows down effluent movement and does a really good job of separating out the larger solids (settling to the bottom forming sludge) and oils, grease and soap forming the floating scum. Slowing the flow and allowing time for solids to settle is why it is important to have a properly sized tank for the expected flows. 

Anaerobic bacteria slowly break down the organic matter in the tank. This is an inefficient process, so while it somewhat removes larger solids and reduces organic loading, it does not remove nutrients or pathogens. Since organic matter is not totally broken down in the tank, it is expected that solids accumulate over time and need to be removed on a regular basis. This is also the reason additional pretreatment devices (media filters, aerobic treatment units) are needed to serve flows from restaurants and other establishments that generate a high-strength wastewater.

Where I live, a septic tank under continuous use needs to be (cleaned) pumped every 24 to 36 months. If use is periodic — lakeshore homes and cottages — with low additions of organic materials, the frequency of maintenance is reduced. The tank should be pumped when the scum and sludge make up 25% of the operating depth of the tank. 

Here is an example where the operating depth of the tank is 60 inches (from bottom of the outlet to the bottom of the tank). If the combined depth of scum and sludge is 15 inches or more, the tank should be pumped. As a side note, this is also a good time to check (inspect) to see that the baffles and tank are in sound operating condition and fix potential problems before they become bigger concerns.


A septic tank holds a wealth of information about the operation and performance of the onsite system. It can tell a lot about use habits within the house and be an indicator of problems downstream. This is why during servicing or inspection of the tank, it is important to open the manhole or remove part of the lid to be able to view the tank and its contents.

Follow these steps to evaluate septic tank performance:

 Determine the tank operating depth. As indicated above, operating depth is the depth from the invert of the outlet to the bottom of the tank. If the water level is above the invert, there is some type of backup in the system. This could be a plugged outlet (effluent screen) or a backup in the entire system.

 The tank should develop three separate layers — a scum layer on top, clear water in the middle and a sludge layer in the bottom. If the three layers are not present, the system is not operating like it should. Often it is because there is some type of upset to the biology of the tank; there is turbulence in the tank or baffles are missing.

 Upsets to the biology are usually the result of introduction of some type of bacteria-killing chemical. It could be certain medications if someone is undergoing some type of chemotherapy, or high blood pressure medications used to thin blood. In addition, overuse of anti-bacterial soaps and cleaners could affect bacteria in the tank. Only the homeowner can confirm whether one of these is happening in the house.

 Turbulence in the tank or lack of settling time caused by increased flows is another possible reason for a lack of these three distinct layers. In my system, I have a pump in the basement walkout area for the laundry and downstairs full bath. It pumps up to the septic tank from the basement. 

 Look for changes in usage if the tank doesn’t show the three layers. If the tank is exceeding expected flows, check with the homeowner for possible reasons. Kids may be moving back from college. A resident may have lost a job and is home 24/7. These changes may be compounded in homes with water softeners or other treatment devices where increased flows result in increased use of the devices.

 If there does not appear to be an in-house reason for increased flows, water treatment devices should be inspected to see they are operating more than they should and delivering water to the system even when there is not increased use. It could be due to stuck valves or defective timers. The area around the tank and the tank itself should be inspected to see if extra water is being added due to runoff, higher water table than normal, or that a septic tank is no longer watertight.


Over the long-term, a tank exhibiting no distinct layers may require changes in the treatment system including replacing the septic tank with one of larger capacity or one that is more watertight. Or depending on the size and location of the drainfield, additions to the soil treatment area may be required. Based on the description from the reader and the fact is happening in multiple tanks in his area, he should look at increased flows and lack of septic tank capacity as contributing factors. 


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