Modified Settleability Test for Activated Sludge Treatment

Modified Settleability Test for Activated Sludge Treatment
Figure 1. There are several versions of the settleability container available, from 1,000 mL beakers, 1.4-liter containers and 2,000 mL (2-liter) settleometers.

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One of the tried and true process control tests of the activated sludge treatment method, the settleability test has become an operator’s best friend. The 2-liter, wide-mouth container widely known as the settleometer, is a piece of equipment the treatment plant operator relies on just about every day. 

Running a settleability test gives the plant operator information about the settling characteristics of the biomass and the potential quality of the clarifier effluent. This article will discuss the basic settleability test and shed light upon a modification of that test called the diluted settleability test. 

Basic settleability test 

To perform the basic settleability test, a sample of the activated sludge mixed liquor suspended solids (MLSS) is collected from the process. This sample is best pulled from a well-mixed location after the last aeration tank but before the secondary clarifier. Depending on what tests will be run on the sample, at least 2 liters are needed for the standard settleometer container. 

There are several versions of the settleability container available (Figure 1), including 1,000 mL beakers, 1.4-liter containers and 2,000 mL (2-liter) settleometers. Of course, just about any clear container can be used to perform a settleability test. I’ve seen large pickle jars, 2-liter soda bottles and clear plastic measuring cups used as settleability containers. It doesn’t have to be fancy to give you a lot of information about your MLSS. 

The sample is poured into the settling container, or may have been collected in the jar itself; the sample is gently stirred and a timer or watch marks the start time. A widely accepted rule of thumb has been to record the top of the sludge blanket (SSV or settled sludge volume) as it settles every five minutes for the first 30 minutes. After the 30-minute mark, readings are taken every 10 minutes until 60 minutes have passed. Plotting the results on a piece of graph paper, on a form made for this purpose, or computer-based spreadsheet program allows the operator to view the results as a trend, a visual image of the settling process (Figure 2). 

When the MLSS settles well, it normally provides a very clear supernate above its blanket, which will soon become plant effluent. A well settling MLSS will first flocculate together, forming large floc particles during the first five minutes. Once this occurs, the slowly forming sludge blanket increases in density, slowly settling and compacting. As the blanket begins settling, it displaces the clearer liquid around it, forcing the clear liquid to rise above the sludge blanket. 

This is readily seen as ‘channels’ within the blanket. The sludge continues to settle; squeezing the liquid toward the top while it compacts and thickens near the bottom. If the results of this settling scenario are plotted, the operator notices that the trend line is a smoothly curved line, not too “L” shaped or too straight across. The operator will also notice that the five-minute reading is nearly double the 30-minute reading. For example, the five-minute reading is 900 mL and the 30-minute reading is 450 mL. 

When the sludge settles rapidly, to about 400 mL within the first five minutes, the liquid above the sludge can have a very turbid appearance. The term pin-floc or pinpoint floc is used to describe this condition and is normally indicative of a high sludge age, an over-oxidized MLSS. If the results of the settleability test are plotted on a graph, an “L” shaped line is seen. If the sludge settles slowly, the trend is a straight line, with little compaction. 

Normally, we find that a slow settling sludge can mean two things: a very young sludge age or a bulky, filament-laden MLSS. A young, under-oxidized MLSS is seen during a plant startup, excessive sludge wasting or the result of solids washout of secondary clarifiers. The supernate above the settling blanket commonly has a cloudy appearance, with large ‘straggler floc’ suspended in the liquid. If the liquid above the slow settling MLSS is very clear, the operator should inspect the MLSS further, looking for hair-like filaments within the sludge causing the slow settling. 

But what if neither of the above descriptions fit your situation? What if it seems that there is a dense MLSS, dark in color but settles slowly? Could there be just too much MLSS in the sample and in the treatment process? 

Modified settleability test 

If you've ever wondered if you have too much sludge in the process and need to waste some out, but don’t know how much is enough or if you've wondered if a waste rate change would make a difference or not, the modified, or diluted settleability test might be a big help. 

In an effort to simulate the change in settling brought about by a waste increase, the diluted settleability test can provide some valuable information in just 30 minutes. To perform the diluted settleability test, a few more settling containers are needed. Just like the basic settleability test, the containers do not have to be fancy or expensive. 

I like to use three containers in a diluted settleability test. The first container (we’ll call it #1), we fill to the top with 100 percent MLSS, so it’s essentially the same as the basic settleometer. In the second (#2) vessel, we fill it to about 75 percent MLSS and in the third container, fill to 50 percent with MLSS. Fill the #2 and #3 containers to the top with clear water collected from the secondary clarifier. 

Don’t use water from a garden hose or a sink faucet as the dilution water. Secondary clarifier liquid would have close to the same temperature as the MLSS and not contain residual chlorine to negatively affect the settling sludge. 

Once all three containers are filled to the top with liquid (Figure 3), give them all a gentle stir and set your stopwatch or timer. Collect readings at five-minute intervals as in the basic settleability test for the first 30 minutes, then every 10 minutes until 60 minutes have passed. Plot your results on graph paper, trending all three samples on the same graph for easy comparison. 

In the example discussed above, rather large dilutions were described. If the plant operator just wants to determine if a slight waste increase would be helpful, smaller dilutions could be made. I have used 10 percent and 20 percent dilutions to help decide if a waste increase was necessary. 

Interpreting the results 

As stated earlier, we are attempting to simulate the results of a waste sludge rate increase by performing dilutions on the #2 and #3 settling containers. The #2 container represents a 25 percent MLSS decrease; the #3 container would be a 50 percent decrease in MLSS. These results are for simulation only. Remember to make small changes in actual treatment plant waste rates to prevent shocking the biological system. I do not recommend wasting such large amounts of MLSS in one batch waste event. The diluted settleability results give you an idea of the reduction of MLSS on its settleability. 

If the result of the dilutions represent a great improvement in settleability from the 100 percent sample, then chances are very good you currently have a glutted system, or too much MLSS (Figure 4).

Figure 4. 

From these results, I like to decide which dilution looks the best, remix that sample and run an MLSS test on it. If you use a centrifuge for process control, run a spin-down on the selected sample MLSS. Read the sample MLSS directly as mg/L if you use a portable suspended solids meter. Once this is complete, then you have a target MLSS to use in process control calculations like SVI and sludge age. Remember to make small changes in waste sludge rates, as you change the biomass amounts, the floc characteristics will change. You might find you will not have to waste as much as originally thought in the original dilution test. 

If the result of the dilution test shows little improvement in settling, and the diluted solids just look thinner with not much change in settling, then further investigation is warranted (Figure 5).

Figure 5.

The quality of the supernate above the settling sludge will provide some answers. If the supernate is cloudy and septic, then young sludge conditions are present. An increase in MLSS (decrease waste rate) would be helpful. If the supernate is clear, then there may be filamentous conditions present. Get out your microscope and look for hair-like filaments extending from the floc and inhibiting the settling. Again, an increase in MLSS (decrease in wasting) might be helpful to allow the floc to build some density. 

The settleability test and its modified, diluted version can give the plant operator quite a bit of valuable information for day-to-day operation and decision making.

About the Author
Ron Trygar, Certified Environmental Trainer, is a senior training specialist, water and wastewater programs, at the University of Florida TREEO Center in Gainesville, Fla. He can be reached at 352/392-9570 or


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