Total Suspended Solids
Read Chapter 25 in Simplified Procedures for Water Examination .
The laboratory exercises you have performed up until this point have been primarily concerned with water treatment. This lab, and the rest of the labs in this course, are primarily used for testing wastewater and should be carried out in a wastewater treatment plant.
The primary purpose of wastewater treatment is to remove solids from water, so this lab will be concerned with testing for one of the types of solids found in water. There are a variety of terms referring to solids in wastewater, each of which is defined below:
- Total solids - all solids in water. Total solids are measured by evaporating all of the water out of a sample and weighing the solids which remain.
- Dissolved solids - solids which are dissolved in the water and would pass through a filter. Dissolved solids are measured by passing the sample water through a filter, then drying the water which passes through. The solids remaining after the filtered water is dried are the dissolved solids.
- Suspended solids - solids which are suspended in the water and would be caught by a filter. Suspended solids are measured by passing sample water through a filter. The solids caught by the filter, once dried, are the suspended solids.
- Settleable solids - suspended solids which would settle out of the water if given enough time. Settleable solids are measured by allowing the sample water to settle for fifteen minutes, then by recording the volume of solids which have settled to the bottom of the sample.
- Nonsettleable solids - suspended solids which are too small and light to settle out of the water, also known as colloidal solids. Nonsettleable solids are measured by subtracting the amount of settleable solids from the amount of suspended solids.
This lab focuses on the total suspended solids, which includes both settleable and nonsettleable solids. Total suspended solids should be tested at least five times per week using 24-hour, flow-proportioned composite samples. The test should be performed on both raw water (to determine the solids content of water entering the plant) and on finished water (to determine the efficiency of treatment at the plant.)
- 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
- Stop watch
- Aluminum weighing dishes
1. Prepare the glass-fiber filter disks (unless they are pre-prepared.)
Insert the filter disk with wrinkled side up in filtration apparatus. Apply vacuum and wash the disk with three successive 20 mL portions of reagent-grade water. Continue suction to remove all traces of water, turn vacuum off, and discard washings.
Remove the filter from the filtration apparatus and transfer to an inert aluminum weighing dish. If a Gooch crucible is used, remove the crucible and filter combination. Dry in an oven at 103 to 105°C for 1 hour.
Cool the filter in a desiccator to balance the temperature. Then weigh the filter and record the weight.
Repeat the above cycle of drying, cooling, desiccating, and weighing until a constant weight is obtained or until weight change is less than 4% of the previous weighing or 0.5 mg, whichever is less.
Store the filter in the desiccator until it is needed. You will need to prepare a filter disk for each sample you plan to test.
2. Select the filter and sample sizes.
Choose a sample volume which will yield between 2.5 and 200 mg of dried residue. If the volume filtered fails to meet the minimum yield, you will have to increase the sample size up to 1 L. If the complete filtration takes more than 10 minutes, you will have to increase the filter diameter or decrease the sample volume.
3. Analyze the sample.
Assemble the filtering apparatus, as shown above, and begin suction. Wet the filter with a small volume of reagent-grade water to seat it.
Stir the sample with a magnetic stirrer at a speed to shear larger particles, if practical, to obtain a more uniform particle size.
While stirring, pipet a measured volume onto the seated glass-fiber filter. For homogeneous samples, pipet from the approximate midpoint of the container but not in the vortex. Choose a point both middepth and midway between the wall and the vortex.
Use the stopwatch to measure the amount of time it takes for the sample water to flow through the filter. Remember that filtration should take no more than 10 minutes. If filtration takes too long, choose a smaller sample size or a larger filter and repeat the procedure. Record the filtration time.
Wash the filter with three successive 10 mL volumes of reagent-grade water, allowing complete drainage between washings. (This washes down solids which may have stuck to the glass on the upper filter holder and removes dissolved solids from the suspended solids captured by the filter. Samples with high dissolved solids may require additional washings.) Continue suction for about 3 minutes after filtration is complete.
Carefully remove the filter from the filtration apparatus and transfer it to an aluminum weighing dish as a support. Or remove the crucible and filter combination from the crucible adapter if a Gooch crucible is used.
Dry the filter for at least 1 hour at 103 to 105°C in an oven, cool in a dessicator to balance the temperature, and weigh. Repeat the cycle of drying, cooling, desiccating, and weighing until a constant weight is obtained or until the weight change is less than 4% of the previous weight or 0.5 mg, whichever is less.
At least 10% of all samples should be analyzed in duplicate. Duplicate determinations should agree within 5% of their average weight.
4. Calculate the concentration of total suspended solids in the sample using the following formula:
The weight of suspended solids is equal to:
(Dried residue + dish + filter) - (dish + filter)
5. If two samples were measured, then the average total suspended solids can be calculated as follows:
C = Total suspended solids of sample 1, mg/L
D = Total suspended solids of sample 2, mg/L
6. Calculate the total suspended solids in kilograms per day (KGD) at the plant, as follows:
Total suspended solids per day, KGD = (Average total suspended solids, mg/L) × (Flow, MGD) × 3.785