To determine the settling volume of sludge, operators will typically run a 30-minute settling test. But that only determines the settling volume. What about the sludge’s settling characteristics? Operators must find out how well the sludge is settling, if the sludge age is appropriate, if there are too many filamentous organisms present and if there are too many MLSS in the aeration basin. The way to find out the answers to these things is by creating a settling curve from the 30-minute settling test.
These settling curves are instructive tools on how the sludge is settling and can help with troubleshooting problems. During the recording period, the sludge should be observed to determine the settling characteristics. Does it form into a blanket or bed? Does it settle in a mass or do individual floc particles fall through the water, leaving a turbid effluent? Is there ashing or floating solids?
While running this test, you should record the settled sludge volume every 5 minutes for 30 minutes and then once more at the 40-, 50- and 60-minute marks. If operators record these volumes, they can then create the graph.
This graph shows a good settling sludge, indicative of good floc forming bacteria and low numbers of filamentous organisms. The environmental conditions that exist within this plant should be maintained.
This next graph shows a poor settling sludge. Poor settling can be caused by excessive filamentous organisms, slime bulking, or too many solids within the system (hindered settling). To determine if you have excessive filamentous organisms or hindered settling, a 50 percent diluted 30-minute settling test can be performed. If the curve improves, there are too many solids and wasting should be increased. If the curve does not improve, filamentous organisms are likely to blame. They need to be microscopically identified and the environmental conditions that contribute to filament growth should be altered accordingly.
This next graph shows sludge that “settles like a rock.” Sludge that settles this fast is usually an old sludge age and appears grainy and dark. Since old sludge is the likely problem, wasting rates need to be increased in order to increase the F/M ratio and decrease the sludge age.
The charting of the SSV is one way to determine how the plant is operating over a long period of time. The trend charts should be kept to give you a settled sludge volume that produces a good effluent so you have a SSV to return to when you have a plant up-set or other operational problem.