Lesson 2:

Activated Sludge Process and Calculations



The settleometer test gives operators of wastewater treatment plants the ability to observe and measure the rate and characteristics of the separation of solids. This is essential for the operational control of the biological treatment process in which sludge is produced. This test is also used with aerobic digesters to determine the length of time required for the sludge to settle and concentrate. Protective latex gloves must be worn during the entire procedure; from the collecting of the sample to the running of the test.



Equipment needed for the test:



The two-liter direct settle-o-meter is a wide-bodied cylinder that is at least 10 centimeters in diameter. It can either be glass or plastic. This type of settle-o-meter, because of its wide mouth, closely resembles a clarifier. By using this type of settleometer, the results are more accurately estimated for sludge volume and settle-ability.




The settleometer which holds two-liters of the wastewater sample, has a scale that does not match the volume, instead, a ratio of the sludge to the total volume is used as the scale, which is measured in cc/liter.




Steps for doing Settleometer Test

There are four steps involved with this test:


The sample collection process is a relatively simple process. At least two and one-half (2 ½) of a representative sample should be collected in a plastic sample bottle and taken to the lab within 15 minutes. Remember that biological sludge is undergoing constant change. The accuracy of the test depends on looking at the sludge within the system. The shorter the holding time, the closer the results will be to the plants conditions.


Collecting Sample



The test procedures include:


After returning to the lab with your sample, it should be thoroughly mixed. The sample should be gently mixed with the paddle. Remember: Do not shake the sample. 

Mixing Gently


Pour your sample into the settleometer. The sample should be transferred quickly to make sure the sludge solids do not settle in the sample collection container.


Again mix the sample gently with the paddle in the settle-o-meter to make sure the sludge is completely mixed.

Slow the stirring with the paddle to slow the motion of the sludge sample. This step is to mimic the motionless condition at the beginning of the settling process. Once all the motion has stopped, remove the paddle.

At this point you are ready to observe the settling of the sludge and record your data.


For this lab, activated sludge, the settleometer should be read every 5 minutes for the first 30 minutes of the test and every 10 minutes during the next 30 minutes of your test. Your settle-o-meter test will run for 60 minutes.

During the first 5 minutes of the settling, you will need to observe and record your results for the following:


After you do your final readings of the settleometer test, the contents should be properly disposed and the gloves worn during testing disposed in the trash. With a clean pair of gloves on, the settleometer, paddle, and sample collection bottle must be properly cleaned and dried. This prevents contamination and residue transfer to other labs that are done with the equipment.




In this lesson we have seen that the activated sludge process is responsible for producing a high quality effluent. Also, that in this process, microorganisms are used as food for the organic contaminants, which produce this high quality effluent. Growing microorganisms is the basic principle behind the activated sludge process. This must occur for floc to form and settle to the bottom of the tanks. These settled solids that have resulted is the activated sludge. The food to microorganism ratio must be used when doing the calculations.




DEQ, Activated Sludge

NYWO, Activated Sludge Process and Calculations




Do Lab 1: Sludge Quality Evaluation. Also answer the following questions and email your response to hlong@me.vccs.edu.

  1. What must occur in the activated sludge process?
  2. What are the particles called that are formed during this process?
  3. What happens to the screened wastewater?
  4. List the components that are interrelated in the activated sludge process.
  5. What would be the number of pounds of microbes for the following equation: (Remember that food/microorganism ratio is usually .6)

    (300 mg/L) (8.34 lb/MG) (18,000GPD ÷ 1,000,000 G/MG)




Answer the questions in the Lesson 2 Quiz. When you have completed the quiz, print it or either email or fax it to the instructor. You may also take the quiz online and directly submit it into the database for a grade.

Harriett Long

3441 Mountain Empire Road

Big Stone Gap , VA 24219

(276-523-5459) Fax