Septic tanks usually comprise two chambers and do not fully treat sewage. They are settlement tanks that require regular desludging. The maximum amount of sludge that a septic tank can store is approximately a third of its total volume.

Raw sewage flows into septic tanks, and solid matter or sludge settles to the bottom. Oil and grease from the sewage floats to the top, forming a layer of scum. Scum prevents oxygen from dissolving in the sewage and results in anaerobic digestion taking place. Raw sewage has to be retained for at least twenty-four hours for anaerobic digestion to breakdown the solid matter.

If desludging is not carried out, the sludge level may exceed maximum level. When this happens, sewage retention time decreases. This will result in an incomplete breakdown of sewage and thus, untreated sewage and sludge solids will be released from the septic tank into the drain. Where septic tanks have a filter bed after the settlement chamber, the filter beds will become choked. This too will result in untreated sewage and sludge solids being discharged into drains.