Would you buy an expensive car and never change the oil? No! The same principal can be applied to your septic tank. Over time, solids build up in the tank. The number of people in the home, their lifestyles, garbage disposals, frequency of laundry, and multiple other factors determine how frequently a tank should be cleaned. When sludge builds up in the bottom of the tank and it reaches a level where it will be taken out into the absorption field, it begins to plug the soil. The level of scum and sludge can be determined by an inspection. When the solids are more than one third of the total liquid level, it is time for the tank to be cleaned. (See animation on About Septic Systems.)
Periodic removal of scum and sludge will prolong the life of your system. Solids cannot be removed from the tank through a 4 inch inspection port. The manholes and/or lids need to be removed for a thorough pumping.