The Influence of Sludge Accumulation Rate on Septic Tank Design

Sludge accumulation was determined in 28 septic tank systems of various sizes and configurations. Sludge accumulation rate in septic tanks declines steadily over time, with the rate significantly less after 12 months. Specific sludge accumulation per capita per day falls from 0.254 l/ca/d (92.7 l/ca/y) over the first 6 months of operation to 0.178 l/ca/d (64.9 l/ca/y) after 60 months. The mean sludge accumulation rate, irrespective of sludge age, was 0.234 l/ca/d (85.3 l/ca/y). Increasing the desludging interval significantly reduces the volume of sludge produced, and so the operational cost of the unit to the owner. Longer sludge ages result in much more stabilised sludges which do not need to be disposed of to a sewage treatment works. When empirical data is used to evaluate design, then a standard 27201 septic tank serving a population of 4 should require desludging every 5 years, while retaining a minimum hydraulic retention time of 2.0 d and a liquid settlement volume of 48% overall. There is an increasing carry over of solids from the primary chamber of some septic tanks with sludge age. Traditionally a short desludging interval has been recommended to prevent the possibility of excessive solids loss from tanks. However, there is potential for better designed systems to take advantage of longer sludge ages, with mutual benefits for owners and the environment.