Disadvantages of septic tank 1. Sometimes excess wasting of cooking oils and grease can fill up the upper portion of the septic tank and can cause the inlet drains to block. 2. Some people use a waste grinder to dispose of waste food will cause a rapid overload of the system and early failure. 3. Roots of a tree growing above the tank and shrubbery or the drain field may clog and or rupture them. 4. Excessive water in the septic tank due to some plumbing leakage may lead to an overload of the septic tank system.