Standard Operating Procedure (SOP) for TDS Testing in Wastewater (APHA Method 2540C)
1. Purpose:
To determine the concentration of total dissolved solids (TDS) in wastewater samples using the gravimetric method as specified in APHA Standard Method 2540C. TDS measures the mass of organic and inorganic substances dissolved in water that pass through a filter, typically after evaporation at 180°C.
2. Scope:
This SOP applies to the analysis of wastewater samples (industrial, municipal, or environmental) to quantify TDS for regulatory compliance, treatment efficiency, or environmental monitoring.
3. References
- APHA, AWWA, WEF. Standard Methods for the Examination of Water and Wastewater 24th Edition, Method 2540C: Total Dissolved Solids Dried at 180°C.
4. Definitions
TDS: Total Dissolved Solids, the residual mass of dissolved inorganic and organic substances in water passing through a filter (typically 1–2 µm) after evaporation at 180°C.
Gravimetric Method: A technique measuring the dry weight of residue left after evaporating the filtrate.
Filtrate: The liquid that passes through a filter during the filtration process.
5. Equipment and Materials
Analytical balance: Capable of weighing to 0.1 mg accuracy.
Glass fiber filter: Binder-free, 1–2 µm pore size (e.g., Whatman 934-AH or equivalent).
Filtration apparatus: Vacuum filtration system with filter holder.
Evaporating dish: Porcelain, platinum, or high-silica glass, pre-washed and dried.
Drying oven: Capable of maintaining 180 ± 2°C.
Desiccator: For cooling and storing evaporating dishes.
Reagent-grade water: For rinsing and blank preparation.
Pipettes and graduated cylinders: For accurate sample measurement.
Vacuum pump: For filtration
Forceps: For handling filters and dishes.
6. Safety Precautions
Wear personal protective equipment (PPE): lab coat, gloves, and safety glasses.
Handle hot evaporating dishes with care to avoid burns.
Ensure proper ventilation when working with wastewater samples to avoid exposure to volatile compounds.
Dispose of wastewater samples and residues per local regulations.
7. Sample Collection and Preservation
Collection: Collect wastewater samples in clean, non-contaminating containers (e.g., glass or polyethylene).
Preservation: Store samples at 4°C to minimize biological activity and analyze within 7 days.
Sample Volume: Typically 50–100 mL, depending on expected TDS concentration. Adjust volume to yield 2.5–200 mg residue.
8. Procedure:
The APHA 2540C method involves filtering the sample to separate dissolved solids, evaporating the filtrate, and weighing the residue. Below are the steps:
1. Preparation of Equipment:
- Wash the evaporating dish with reagent-grade water, dry in an oven at 180°C for 1 hour, cool in a desiccator, and weigh to 0.1 mg accuracy (record as W1).
- Prepare the glass fiber filter by rinsing with three 20 mL portions of reagent-grade water under vacuum to remove soluble material. Dry the filter at 103–105°C, cool in a desiccator, and weigh to ensure <0.5 mg mass loss between weightings.
2. Sample Filtration:
- Assemble the filtration apparatus with the pre-washed glass fiber filter.
- Mix the wastewater sample thoroughly to ensure homogeneity.
- Filter a known volume (e.g., 100 mL) of the sample through the glass fiber filter under vacuum. Collect the filtrate in a clean container.
3. Evaporation:
- Transfer the filtrate to the pre- weighed evaporating dish.
- Evaporate the filtrate to dryness on a steam bath or in a drying oven at 180°C. Ensure gradual heating to prevent splattering.
- Dry the residue for at least 1 hour at 180°C to remove all moisture.
4. Cooling and Weighing:
- Cool the evaporating dish in a desiccator to room temperature.
- Weigh the dish with the dried residue to 0.1 mg accuracy (record as W2).
5. Blank Preparation:
- Process a blank using 100 mL of reagent-grade water through the same filtration and evaporation steps to account for any background residue.
6. Calculation:
Calculate TDS concentration in mg/L using the formula:
TDS (mg/L)} = (W2 - W1) x 1000 /{Sample Volume (L)}
Where:
W1= Weight of the empty evaporating dish (mg)
W2 = Weight of the dish + dried residue (mg)
Sample Volume = Volume of filtered sample in liters (e.g., 0.1 L for 100 mL)
9. Quality Control
Blank: Run a reagent-grade water blank with each batch to ensure no contamination.
Duplicate Samples: Analyze at least one duplicate sample per batch to check precision (relative difference <5%).
Calibration: Verify the analytical balance calibration daily.
Filter Check: Ensure filter mass loss is <0.5 mg after washing and drying.
Interferences: Minimize turbidity interference by ensuring proper filtration. If high organic content is suspected, note potential bias in results.
10. Reporting
- Report TDS results in mg/L, rounded to the nearest whole number.
- Include details of sample volume, filter type, and any deviations from the standard method.
11. Flow Chart of TDS Testing Procedure
Below is a textual representation of the flow chart for the TDS testing process:
```
START
↓
Prepare Equipment
- Wash and dry evaporating dish at 180°C
- Rinse, dry, and weigh glass fiber filter (<0.5 mg mass loss)
↓
Collect and Prepare Sample
- Collect wastewater sample, store at 4°C
- Mix sample thoroughly
↓
Filtration
- Filter known volume (e.g., 100 mL) through glass fiber filter
- Collect filtrate in clean container
↓
Evaporation
- Transfer filtrate to pre-weighed evaporating dish
- Evaporate to dryness at 180°C (steam bath or oven)
- Dry residue for ≥1 hour at 180°C
↓
Cooling and Weighing
- Cool dish in desiccator
- Weigh dish + residue to 0.1 mg accuracy
↓
Blank Analysis
- Process reagent-grade water blank through same steps
↓
Calculation
- TDS (mg/L) = [(Weight of dish + residue) - (Weight of empty dish)] × 1000 / Sample Volume (L)
↓
Quality Control
- Check blank, duplicates, and balance calibration
- Verify filter preparation
↓
Report Results
- Report TDS in mg/L
- Include method details and any deviations
↓
END
```
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