The really easy way...
The Standard Method for running the total suspended solids (TSS) test is a bit laborious and time-consuming. You need to use special filter paper, a vacuum pump, scale, and an oven. But, for those of you who have access to Hach’s DR 2800 spectrophotometer, you can very quickly determine the TSS of a mixed liquor sample, for example, with a result that is accurate enough to use for process control purposes. All you need is a 100-mL (or larger) graduated cylinder and, of course, a DR 2800.
The test procedure for the Hach DR 2800 is No. “630 Suspended Solids.” The measurement scale is 5 to 750 mg/L so for an MLSS sample you will need to do a dilution. I typical do a 10 to 1 just to keep it simple. Using a 100-mL graduated cylinder, add 90 mLs of tap water and then 10 mLs of MLSS. Zero the DR 2800 with a mL sample of tap water then run the test using a well-mixed sample of the dilute MLSS from the graduated cylinder. Then just multiply the reading on the instrument by 10 to obtain the MLSS concentration. That’s it, your done. The only time it took to run the test was getting the MLSS sample. The test is completed in just minutes once you get back to the laboratory.
I’ve found running TSS for mixed liquor this way to be sufficiently accurate for process control purposes such as making adjustments to the sludge wasting rate using the results from this test method. Unfortunately, I’ve found far too often that this test result is more reliable than the data generated running the standard method version of this test by personnel who haven’t really been trained properly. So if you are ever in doubt and you have a DR 5000, 2800, 2700, 2500, or 2400 instrument, just run this test and you’ll be all set. Please note that the Hach procedure for this suspended solids test suggests that you blend a 500 mL sample for two minutes. I have not found that to be necessary. Just get your MLSS sample from the effluent of the aeration basin, dilute it 10 to 1, and run the test.