The Settleability Test: Why is it so valuable?

The MVP (Most Valuable Process test) for wastewater process control is the 30-minute settleability test. This article will highlight theU.S. Water Services Corporation - Operator Training - Settleability Test 1 usefulness of this simple, easy and cheap test and explain how to execute a set test properly and what the test will show.

The set test is a crucial indicator of how the activated sludge flowing into the secondary clarifier should settle. Because the clarifier is influenced by hydraulics and detention time, the set test result will only be a close approximation to the current sludge blanket depth in the clarifier. However, because the set test is done in quiescent conditions, it becomes a troubleshooting tool for the Operator to determine if the plant is experiencing a hydraulic overload condition. This example can be seen when the Operator observes bulking solids going over the clarifier weir while the 30-minute settled sludge volume result is 300. Another reason for its usefulness is the short time frame for the test. The 30-minute time frame is especially ideal for Operators who have a daily route to complete. Most small package pants only have a 30-minute time requirement. Additionally, if, for some reason, the Operator only has a check that day and cannot stay 30 minutes, they could still run the test and record the 5-minute result. This is because, under healthy process conditions, the 5-minute reading should be double the 30-minute reading. For example, if the 30-minute result was 300 ML, the 5-minute reading was most likely 600. ByU.S. Water Services Corporation - Operator Training - Settleability Test 2 implementing the 5-minute reading with the 30-minute reading, the Operator can trend when it is time to increase or decrease the current waste rate. As the 5-minute reading goes higher, the 30-minute reading should also go up and vice versa. The idea is to have a more rounded L curve when the 5- and 30-minute readings are plotted on a chart. One can also use the 30-minute set test to calculate the sludge volume index (SVI). This mathematical calculation gives the Operator a better idea of how compact the sludge is settling. This SVI number can have a wide range and be different for every plant. Therefore, the target number for a specific plant should be the one that is calculated during optimum treatment. When testing for an SVI, a known mixed liquor suspended solids (MLSS) at the time of the 30-minute set test needs to be used as well. Below is the SVI formula:

Remember to use the SVI number as an indicator and trend the results. According to most textbooks, a result of less than 80 indicates an old, fast settling sludge, and a number at 300 or greater indicates a slow settling sludge, either indicating a young sludge age or one with an abundance of filaments. The likely SVI number for most plants during best treatment will be between 100-200. Lastly, the most crucial reason the set test is so valuable is the information it gives the Operator when there is a bulking sludge condition in the secondary clarifier. For instance, if the clarifier loses solids due to a bulking condition, the Operator can run a diluted settleability test. The easiest way to run this test is to fill the container with 50% clarifier supernate and 50% MLSS. It is important to use the clarifier liquid to match the temperature and avoid having any chemicals such as chlorine in the liquid. If the diluted test settles much better than the undiluted, then you can assume the blanket is bulking due to a glutted (too many solids) situation. The diluted sample simulates how the clarifier will settle if 50% of the solids are wasted out of the plant. The picture below shows the difference between an undiluted and diluted sample.

U.S. Water Services Corporation - Operator Training - Settleability Test 3Alternatively, suppose the bulking condition is due to young sludge (low MLSS) or filamentous bacteria. In that case, there will not be as much improvement in the diluted test compared to the undiluted sample.

Now that we know all the good reasons for running this easy, simple, cheap test, let us go over the essential steps to take and things to avoid. First, the container does not have to be an expensive lab container. Any decent-sized measuring cup (32 oz) will be sufficient.

Next, locate the last aeration zone before the clarifier and grab a sample 2-3 feet deep and 2-3 feet from the wall using a dipper. This will ensure a well-mixed representative sample. After pouring the sample into the container, use some paddle-type device to swirl or stir the sample gently.

It is important not to shake or agitate the sample as this will potentially shear the floc and give an incorrect settling result. It is also best to set the container on a quiet flat surface away from direct sunlight to avoid uneven settling. After setting the timer, visually observe the first 5 minutes of the test. Flocculation and sludge separation from the water should be evident within the first 5 minutes. The 5-minute reading should be recorded, and if the process is healthy, that reading will be close to double the 30-minute result. If that is not the case, then it is either a glutted system or slow settling due to filaments or low MLSS. If the sludge settles too fast in the first 5 minutes, that is a result of an underloaded (low F/M ratio) condition. In order to improve this condition, either increase wasting or add a carbon source to the influent.

In conclusion, this simple, easy, and cheap process control test gives an Operator much helpful information in a short amount of time. Therefore, I consider the 30-minute set test a precious tool for any wastewater operator.

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