Sludge accumulation was determined in 28 septic tank systems of variou
s sizes and configurations. Sludge accumulation rate in septic tanks d
eclines steadily over time, with the rate significantly less after 12
months. Specific sludge accumulation per capita per day falls from 0.2
54 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 rat
e, irrespective of sludge age, was 0.234 l/ca/d (85.3 l/ca/y). Increas
ing the desludging interval significantly reduces the volume of sludge
produced, and so the operational cost of the unit to the owner. Longe
r 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 retaini
ng 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 fr
om the primary chamber of some septic tanks with sludge age. Tradition
ally a short desludging interval has been recommended to prevent the p
ossibility of excessive solids loss from tanks. However, there is pote
ntial for better designed systems to take advantage of longer sludge a
ges, with mutual benefits for owners and the environment.