Lab 16:
Mixed Liquor Volatile Suspended Solids

Reading Assignment

There is no reading assignment for this lab.  


Mixed liquor is a combination of sludge and water removed from the clarifier in the wastewater treatment process and reintroduced into an earlier phase of the treatment process.  The mixed liquor contains microorganisms which digest the wastes in the raw water.  This lab will show you how to test for MLSS and MLVSS in the mixed liquor. 

Mixed Liquor Suspended Solids (MLSS) is a test for the total suspended solids in a sample of mixed liquor.  This test is essentially the same as the test you performed for TSS in the last lab, except for the use of mixed liquor as the water sample.  In addition, the concentration of suspended solids found in the mixed liquor is typically much greater than that found in the raw or treated water.  MLSS concentrations are often greater than 1,000 mg/L, but should not exceed 4,000 mg/L.

MLVSS, or Mixed Liquor Volatile Suspended Solids, is a test for the amount of volatile suspended solids found in a sample of mixed liquor.  Volatile solids are those solids which are burnt up when a sample is heated to 550°C.  Most of the volatile solids in a sample of mixed liquor will consist of microorganisms and organic matter.  As a result, the volatile solids concentration of mixed liquor is approximately equal to the amount of microorganisms in the water and can be used to determine whether there are enough microorganisms present to digest the sludge.

In a wastewater treatment plant, operators should test for MLSS three times per week and for MLVSS once per week.  Both tests should use grab samples taken from the same location in the treatment plant. 

  • Dessicator
  • Drying oven, for operation at 103 to 105°C
  • Analytical balance, capable of weighing to 0.1 mg
  • Magnetic stirrer with TFE stirring bar
  • Wide-bore pipets
  • Graduated cylinder
  • Low-form beaker
  • Glass-fiber filter disks with organic binder
  • Filtration apparatus, which can be any one of the following:
    • Membrane filter funnel
    • Gooch crucible, 25 mL to 40 mL capacity, with Gooch crucible adapter
    • Filtration apparatus with reservoir and coarse fritted disk (40 to 60 um) as filter support
  • Filter flasks, of sufficient capacity for sample size selected
  • Vacuum pump
  • Tubing
  • Stop watch
  • Aluminum weighing dishes
  • Muffle furnace

  • Reagent-grade water

Laboratory Procedure

1. Collect a grab sample of mixed liquor. 

2. Measure the total suspended solids in your sample using the procedure outlined in Lab 11.  You will probably need to use a smaller sample volume, such as 5 mL.  Record the sample volume and the combined sample and filter weight in the Data section.  At least 10% of all samples should be analyzed in duplicate. 

3. Ignite the filter and the total suspended solids residue from step 1 in a muffle furnace at 550°C.  An ignition time of 15 to 20 minutes is usually sufficient for 200 mg residue.  However, when igniting more than one sample or when igniting heavier samples, the ignition time may need to be increased. 

4. Let the filter cool partially in the air until most of the heat has dissipated.  Then transfer the filter to a dessicator to cool the rest of the way to air temperature. 

5. Weigh the filter and record the weight in the Data section. 

6. Repeat the cycle of igniting, cooling, desiccating, and weighing until a constant weight is obtained or until the weight change is less than 4% or 0.5 mg, whichever is less. 

7. Calculate the volatile solids in the sample, as follows:

Formula for calculating volatile solids


A = Sample and filter weight from TSS test, mg
B = Sample and filter weight after ignition in muffle furnace, mg
8. If you have tested two samples, you can calculate the average volatile solids as follows:

Formula for calculating average volatile solids


C = Volatile solids of sample 1, mg/L
D = Volatile solids of sample 2, mg/L


Sample volume (mL)
Sample and filter weight from TSS test (mg)
Sample and filter weight after ignition in muffle furnace (mg)
Volatile Solids (mg/L)



Virtual Lab

The first virtual lab explains about filter preparation.  This virtual lab is applicable to TSS, MLSS, and MLVSS. 

The second virtual lab reviews MLSS testing.

The third virtual lab shows the procedure used for MLVSS testing


American Public Health Association, American Water Works Association, and Water Environment Federation.  1998.  Standard Methods for the Examination of Water and Wastewater.  American Public Health Association, Washington, D.C.