A standard septic system that was working properly and then not used for a period of time, such as 4 – 5 years, typically does not lose effectiveness or the ability to function properly once it is put back into use. Some of the older septic systems may even benefit from non-use by allowing some of the biomat, which is a biological clogging layer in the leachfield, to naturally break down and the soils rejuvenate. Septic systems with gravelless chambers used for leachfields are more prone to becoming less effective over time when they are not in use. This is because gravelless chambers are like long open tunnels underground that without wastewater flowing through them burrowing animals, like moles, can enter into them and begin to fill them with soil. Septic tanks, whether made from concrete or plastic, typically do not degrade or lose effectiveness when not in use. It is best to keep septic tanks full with water when they are not in use. This is because most empty septic tanks are not designed to withstand the exterior forces that are placed upon them from surrounding soil and groundwater when they are empty.