Government facilities, industrial manufacturing sites, and contract labs perform continuous effluent water analysis to meet quality standards for wastewater released into the environment. The measurement of total suspended solids (TSS), total dissolved solids (TDS), and volatile suspended solids (VSS) is an important aspect of wastewater quality testing.
Wastewater analysis procedures often involve preparing glass fiber filters for tests. Filter preparation requires a substantial amount of time, and labs testing wastewater samples are often operating at capacity. The Cytiva research and development team sought to determine how much time laboratory users spend preparing filters and how filters that are prepared in advance and ready to use could help save time.
How are TDS, TSS, and VSS defined and measured in wastewater samples?
- TDS: dry-weight of residue left in evaporation dish after filtrate is evaporated on steam bath or in a drying oven
- TSS: dry-weight of particles trapped by filter, often measured by gravimetric assessment
- VSS: mass loss from particles trapped by the filter following ignition at 550°C
What standards do labs follow when testing wastewater?
In the US, Canada, Singapore, and Latin America, wastewater testing labs follow the US EPA Standard Method 2540 parts C, D, and E. For TDS and TSS analysis, parts C and D of EPA 2540 require the filter to be binder-free glass fiber. The filter must be washed, dried, and weighed twice to show < 0.5 mg loss of mass before use. For VSS, part E of Standard Method 2540 requires the same washing steps; however, instead of drying, the filter is ignited at 550°C to remove all existing volatile solids. Gravimetry of the filtrate determines TDS, while the increase in filter mass indicates TSS. Ignition of the filter after TSS measurement provides an indication of VSS.
Other countries either use the EN 872 standard to guide the chemical analysis of wastewater, which has similar requirements to EPA 2540 parts C/D/E, or an adaptation of one of the two methods. Standard Method 2540 specifies a pre-prepared glass filter can be used, leaving open the possibility of using manufactured-prepared filters. EN 872 states only that the filters may be pre-washed and dried.
How did the research and development team test filter preparation methods?
TSS analysis for wastewater quality testing often involves gravimetric filtration under a vacuum. Filtration in EPA 2540 tests involve at least 10 steps, of which five are preparative. The research and development team mimicked the preparative steps usually conducted by laboratory filter users, as follows, recording the time taken for each:
Rinsing the filter under vacuum with three successive lots of 20 mL reagent grade water to remove any water-soluble material. If left in place, this material could affect subsequent gravimetric accuracy.
Maintaining the wet filter at 103°C to 105°C for 1 hour removes moisture. The user can then cool and weigh the filter, giving an initial measurement. Repeating the drying/cooling/weighing process provides the second filter measurement for comparison.
To continue, EPA 2540 requires the two measurements be within 4% or 0.5 mg of each other. Otherwise, the user needs to perform additional drying, cooling, and weighing cycles to make sure moisture is sufficiently removed.
For volatile solids analysis, the filter is ignited in a furnace at 550°C for 15 minutes, in place of drying in an oven.
Cool in a desiccator
A desiccator enables the filters to reach a balanced temperature without absorbing any moisture. A final weigh to 0.1 mg accuracy provides the pre-filtration mass of the filter. The filter is now ready for wastewater sampling.
How much time can laboratories save with filters that are ready to use?
For EPA 2540 parts D and E, these preparative steps are crucial for an accurate assessment of the wastewater. The breakdown of the total time spent on each step, as determined by the research and development team, is as follows:
- Pre-rinse: 1 min
- Pre-heating and drying at 103°C to 105°C: 77 min
- Pre-heating and igniting at 550°C: 35 min
- Second ignition: 15 min
- Cool and weigh twice: 40 min
Total: 168 minutes
Water testing labs perform large numbers of wastewater sample analyses each day. The research and development team’s investigation indicates that up to 168 minutes could be saved in each series of tests using filter that are prepared in advance of the wastewater analysis process.
What are the other benefits of ready-to-use filters for wastewater testing?
Following the wastewater analysis tests, Cytiva has developed a series of ready-to-use filters specifically designed to align to certain water quality testing requirements of standard methods (EPA 2540 parts D and E and EN872). The primary benefit of this approach is the time saved, freeing up capacity for additional tests or other laboratory work. Other benefits of ready-to-use filters include:
- Consistency: manufacturer-controlled product quality removes any batch-to-batch or person-to-person variation from washing and drying in the testing lab.
- Certification: all manufacturer-prepared products are certified to mass loss levels required by these standard methods.
- Convenience: filter products, such as Cytiva's ready-to-use filters, can be prepared to various stages of readiness to suit a wastewater testing lab’s workflow.
How are ready-to-use filters incorporated into the wastewater testing workflow?
Each pre-treated filter comes in an aluminum pan with its weight displayed. Each pan is also barcoded so that its unique filter weight can be scanned and downloaded. The user places the filter on the funnel of the vacuum filtration apparatus, sealing it by wetting. After filtering the sample, the user washes the filter with three aliquots of 10 mL reagent grade water and returns it to the aluminum pan. The filter, with pan, is dried at 103°C to 105°C for 1 hour before cooling and weighing.
The mass of TSS is the difference between the weight indicated on the pan and the measured weight. For VSS measurement, the user ignites the filter at 550°C for 15 minutes, cools, and re-weighs. The reduction in mass compared to the TSS measurement represents VSS.
For more information on ready-to-use filters for wastewater testing, download the Filtration for Water Testing brochure. For advice on using ready-to-use filters or to request a sample of our 934-AH RTU or GF/C RTU filters now.