Mixed Liquor Suspended Solids/ Volatile Suspended Solids (MLSS/MLVSS)


1. Clean and thoroughly dry at least 2 ceramic crucibles. (These crucibles have to be able to withstand temperatures of up to 600°C)

2. Determine the mass of the empty crucible and record this as your starting mass.

3. Add a known volume of mixed liquor to each crucible. (make sure your samples are properly mixed and homogenised first)

4. Evaporate the liquid fraction by heating at 105°C for 1 hour.

5. Remove carefully and place into a desiccator to cool.

6. Determine the mass of the crucible + dried mixed liquor.

7. The difference between the starting mass and mass in Step 6 is your MLSS/sample volume. Calculate your MLSS/L:

MLSS (g/L) = (Mass difference (g))/(Sample volume (ml)) × 1000ml/(Sample Volume (ml))

8. Add your crucible containing the dried mixed liquor to a furnace set at 550°C, and heat for 15 minutes.

9. Cool your crucibles in a desiccator. (Cool your crucibles in a clean metal tray slightly before adding to the desiccator, as adding extremely hot samples to the desiccator will create a vacuum and result in you not being able to open the desiccator lid)

10. Calculate the final mass difference. This is the MLVSS/sample volume:

MLVSS (g/L) = ((Sample mass (step 6)-Sample mass (step 10))(g))/(Sample volume (ml)) × 1000ml/(Sample volume (ml))

11. Factor in your sample volume as above to calculate MLVSS. Notes: a. When dealing with wastewater it is most accurate to use a glass fiber filter and filtration apparatus with a known mixed liquor sample volume. It is then this filter which is weighed and heated. b. This method significantly shortens the time required to perform the test, but should be reported as a modification.


Omitting the filters results in a TSS or VSS measurement respectivley. This is more suitable for low biomass or controlled bioreactor samples as they have less inorganic particulates than a traditional environmental sample.