Total Suspended Solids
Solids in wastewater can be classified as Total Solids (TS), Total Suspended Solids (TSS) and Total Dissolved Solids (TDS). TS represent all of the solids in a wastewater sample, after the water has been evaporated off. TDS represent all of the solids in a wastewater sample that has passed through a 2-micron (or smaller) filter, after the water has been evaporated off. TSS represent all of the solids in a wastewater sample that remain trapped on a 2-micron (or smaller) filter, which has had all water evaporated off. From the perspective of process control of wastewater treatment plants and NPDES permit monitoring, TSS is the parameter of most importance. The TSS procedure involves filtering known volumes of wastewater through pre-weighed glass fiber filters and then drying the filters at 103º – 105º C in a drying oven. The residue trapped on the filter is then weighed to determine the TSS concentration in mg/L.
Samples to be analyzed for Total Suspended Solids content should be collected in clean polypropylene or glass bottles (typically 500 – 1000 mL). It is important that the material in suspension does not adhere to the container walls. Analyze samples as soon as possible because of the impracticality of preserving samples. Refrigerate samples at 4º C up to the time of analysis to minimize microbiological decomposition of solids. Preferably, do not hold samples more than 24 hours, but in no case hold samples more than 7 days. Bring samples to room temperature before analysis, because changes in the sample temperature/density will affect volumetric measurements.
The filters used in the TSS test are specialized glass fiber filters with a nominal pour size of < 2 microns. Various companies supply these filters. These are a few examples: Whatman grade 934AH, Gelman type A/E, Millipore type AP40 and E-D Scientific Specialties grade 161. Other products that are demonstrated to give comparable results are allowable.
To prepare filters for use, insert the filter disk with the wrinkled (rough) side up in a filtration apparatus. Never handle the filter or aluminum dish by hand. Oil from your skin could cause an inaccurate test result. Always handle filters and aluminum dishes with forceps. Apply a vacuum and wash the filter with three successive 20 mL portions of reagent-grade water. Continue suction to remove all traces of water. Remove the filter from the filter apparatus and place it into an aluminum dish (known as a planchet). Dry filter and dish in an oven at 103 – 105º C for at least one hour.
If the filter will be used in the Volatile Suspended Solids test, ignite at 500 +/- 50º C for 15 minutes in a muffle furnace. Cool filter and dish in a desiccator and then weigh filter and dish on an analytical balance. Repeat cycle of drying and desiccating until a constant weight is obtained or until the weight change is less than 4% of the previous weighing or 0.5 mg, which ever is less.
The cleaning and complete drying of the filters is critical to obtaining accurate results with the TSS test. Rinsing the filters removes any loose debris from the filter. Verifying that the filters are completely dry before being used in the test prevents the introduction of error attributable to wet filters. It is important that good records be maintained that demonstrate that the filter rinsing and drying was carried out correctly.
Most laboratories prepare a week or even a months worth of filters at a time. Store prepared filters in a desiccator until needed for analysis.