THE INFLUENCE OF SLUDGE ACCUMULATION RATE ON SEPTIC-TANK DESIGN

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.