Activated Sludge Process and Calculations
In this lesson we will answer the following questions:
- What occurs in the activated sludge process?
- What are the basic principles behind all activated sludge?
- What is floc?
- What happens to the screened wastewater?
- How do we do the calculations for activated sludge?
There is no set reading assignment for this lesson; however, you should use your textbook to help gain an understanding of the activated sludge process. There is also a lab for this lesson and you will need to set-up Wastewater Plant Visit and print the forms for the visits.
In every community it is to everyone's advantage for the wastewater to be treated in the most economical way. The activated sludge process is a process that has the advantage of producing a high quality effluent for a reasonable maintenance and operating costs.
The activated sludge process uses microorganisms to feed organic contaminants that are in wastewater to produce a high quality effluent. The basic principle behind all activated sludge processes is that as microorganisms grow, they form particles that clump together. These particles, which are referred to as floc, are allowed to settle to the bottom of the tank, which results in a relatively clear liquid free organic material and suspended solids.
The screened wastewater is mixed with varying amounts of recycled liquid that contains a high proportion of organisms that are taken from a secondary tank and becomes a product that is called mixed liquor. The next step for the mixture is to stir and inject it with large quantities of air to provide oxygen and keep the solids in suspension. After a period of time, the mixed liquor flows to a clarifier where it is allowed to settle. During this settling a portion of the bacteria is removed and the partially cleaned water flows on for additional treatment. The settled solids that resulted, the activated sludge, are then returned to the first tank to begin the process again.
Description of the process:
The basic activated sludge process consists of several interrelated components:
- The aeration tank where the biological reactions occur.
- An aeration source that provides oxygen and mixing.
- A tank, known as the clarifier, where solids settle and are separated from treated wastewater.
- A collecting means for the solids either to return them to the aeration tank, ( returned activated sludge , RAS ), or to remove them from the process ( waste activated sludge, WAS ).
Activated Sludge Process Calculation
The food/microorganism ratio is usually (.6) in a water treatment plant.
Let's do some food/microorganism ratio (f/m) calculations with the above information:
*Use standard conversions for these calculations. Refer back to Math Formulas if necessary
Take a sample of Return Activated Sludge (RAS):
- 100 grams of RAS, or sludge, dried is equal to 5 grams
- Cooked at 550°C yields 3.0 grams of sludge
- 2% of 100 grams, or 2 grams, of the sludge is microbes
To find the feed rate in pounds per minute:
At 1440 minutes in a day, the feed rate per minute is:
To find the feed rate in grams per minute:
Use the standard conversion here for pounds and grams.
Put a dish under the RAS that you know the tare weight of.
Do this for 1 minute. The grams/minute should be (789.05)
- If the feed rate is greater than calculated, lower the volume pumped.
- If the feed rate is lower than calculated, raise the volume pumped.
After the process levels out, increase the feed rate slightly and see what happens to the B.O.D. in the effluent.
- If the effluent B.O.D. rises, reverse the feed rate.
- If the effluent B.O.D. lowers, continue until the optimum condition exists.
When the B.O.D. in the effluent is the minimum, this is the optimum RAS
Age and Mix of Organism in Activated Sludge
Under a microscope at 400 power
Rotifers are the larger microorganisms in old sludge, a few are in the right age sludge, and none are in the young sludge.
Under a microscope at 400 power
All are a single cell type
The good settle ability comes from the sticky enzymes that help hold them together.
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:
- A two-liter direct-reading settleometer (glass or plastic)
- Stirring paddle
- Data and Graph curve sheets.
- Protective Latex Gloves
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:
- Collection of the sample
- Running the test
- Recording the results
- Clean up
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.
The test procedures include:
- Stirring the sample
- Pouring 2 liters of the sample in the settleometer
- Stirring again
- Observing the results
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.
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:
- Supernatant-The liquid between the sludge on the bottom and the scum on the surface of the settle-o-meter.
- Solids-The solids in the settle-o-meter that where the microorganisms feed on the wastewater.
- Floc-The clumps of bacteria and particles or coagulants and impurities that have come together and formed a cluster.
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 email@example.com.
- What must occur in the activated sludge process?
- What are the particles called that are formed during this process?
- What happens to the screened wastewater?
- List the components that are interrelated in the activated sludge process.
- 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.
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