Installation guide
13
De-Sludging
Bacteria and other micro-organisms present in the wastewater utilise the organic material as a food source, converting it into a
non-soluble mass. This non-soluble mass or floc is compromised of living micro-organisms, sewage particles, as well as inert (non-
biodegradable) material. As the process matures, the numbers of micro-organisms increase until there is an adequate biomass
to metabolise or digest all of the soluble organic material in the incoming sewage. At this point, competition for food results in the
dying (due to starvation) of organisms as new organisms are formed. These dying organisms in turn are metabolised, thereby
reducing the overall sludge volume.
The volume of solids will gradually increase due to the accumulation of inert remains of dead organisms (ash), combined with the
non-degradable material in the raw wastewater. As the solids increase, the mixed liquor (i.e. contents of the aeration chamber)
becomes thicker, developing an increasing dexter brown colour. Periodically, the excess solids must be de-sludged (wasted) from
the system in order to ensure continued plant efficiency.
DE-SLUDGING FREQUENCY
The rate at which the solids (biomass) accumulates in the system, and the subsequent rate at which the excess solids must be
removed, is dependent upon the total volume and strength (i.e. BOD) of the wastewater entering the plant. The typical residential
system will need to be de-sludged every 2-3 years.
DETERMINING DE-SLUDGING FREQUENCY
In order to ensure optimum treatment efficiency and effluent quality, it is necessary to maintain the level of aeration solids (MLSS)
within a suitable range (refer to Appendix C). A low level of solids in the aeration chamber (i.e. during plant start up) reduces the
result in poor settling during periods of hydraulic surges: or, in the development of septic conditions in the plant. In order to de-
termine when the system should be de-sludged it is necessary to perform a solid settlement test (30 minute) during each annual
service check.
PROCEDURE
1. Mark a large transparent jar into 10 equal portions.
2. While the compressor is running fill the jar with the liquid (MLSS) from the aeration chamber.
This sample should be collected at mid depth in the tank. Do not collect sample from within the draft tube.
3. Allow the sample to sit for 30 minutes.
4. Measure the volume of the settled sludge as a percentage of the total volume of the sample. Occasionally,
after the sample sits, a portion of the settled sludge may float to the top of the sample. If this occurs,
add together the volume of settled sludge and the volume of floating sludge.
5. Compare the percentage of settled sludge (i.e. sludge volume) to the figures given in the “Operational Control Chart”.
The optimum level of solid settlement is normally between 5-50%. Whenever the sludge volume exceeds 50%,
the plant should be de-sludged.