We get asked this a lot! It seems everyone has an opinion and nearly all are different. The answer can range from your neighbour who thinks the tank never needs to be emptied as it’s self cleaning to the Environment Agency that suggests a yearly clean out. Septic tanks do need to be emptied, they are not self cleaning; however the frequency depends on the number of folks using the tank and the size of the tank. A typical domestic tank will have a volume of around 2600 to 3000 litres. The average adult produces 110g of poo each day so in a full year will produce 40 kg of solid waste. Most human waste is of near neutral buoyancy so needs a couple of days residence time in the tank to drop down and accumulate as sludge. The maths shows that one person will produce around 1.5% solids in the tank in a full year. Our tests in the field show that when the solids are allowed to go above 8% then the digestive processes in the tank can lead to lower pH levels in the tank; this can cause problems in disposal. Most commercial treatment plants – the place your licensed contractor should take the waste – are designed to run best with a solids feed of around 5%. So at 8% you can get adverse reactions and 5% is the optimum for disposal. We think that these two solids levels give the upper and lower limits for time to empty the tank.So a family of four will produce around 160 kg of solids each year or 6% on a 2600 litre tank. In this case the tank should be emptied each year. Two people in the house should have the tank emptied every two years and one person can save a bit of cash and only have the tank emptied every four years. It is always best to choose a licensed contractor like Larkins to empty your tank as they will provide a paper trail for the disposal of the waste and make sure that it is disposed of properly. Contact us if you need more advice or would like to us to empty your septic tank.