About Volatile Suspended Solids (VSS)

Volatile Suspended Solids (VSS) represent the organic portion of suspended solids in water or wastewater that can be combusted at 550°C. This measurement is critical for assessing the amount of biologically active organic matter in water samples.

Calculation Formula

The VSS is calculated using the following formula:

VSS = (A3 weight) × 1,000,000 / Sample Volume

Where:

• A3 weight: Weight loss of the filter residue after burning at 550°C in a muffle furnace (mg)
• Sample Volume: Volume of the sample filtered (ml)

Importance of VSS

VSS is crucial for:

• Measuring the organic content in wastewater
• Assessing the biodegradability of suspended solids
• Determining the active biomass in biological treatment systems
• Calculating sludge activity and treatment efficiency
• Monitoring process performance in wastewater treatment plants

Key Differences Between TSS and VSS

• TSS (Total Suspended Solids): Measures all suspended solids (both organic and inorganic)
• VSS (Volatile Suspended Solids): Measures only the volatile (organic) portion that can be oxidized at 550°C
• The ratio of VSS/TSS indicates the organic fraction of the suspended solids

Typical VSS Values and Ranges

VSS concentrations vary depending on the water source:

• Raw Domestic Wastewater: 150-350 mg/L
• Primary Effluent: 80-200 mg/L
• Activated Sludge Mixed Liquor: 1,500-4,000 mg/L
• Final Effluent: 5-30 mg/L
• Surface Waters: 1-20 mg/L

Maximum and Minimum VSS Considerations

Maximum VSS Levels:

• In raw wastewater, VSS typically shouldn't exceed 70-80% of TSS
• In activated sludge systems, VSS > 5,000 mg/L may indicate sludge bulking issues
• High VSS/TSS ratios (>0.8) suggest predominantly organic solids

Minimum VSS Levels:

• In biological treatment systems, VSS < 1,000 mg/L may indicate insufficient biomass
• Low VSS/TSS ratios (<0.3) suggest predominantly inorganic solids
• In final effluents, VSS < 10 mg/L is typically desirable

Process Control Applications

VSS is used to:

• Determine the organic loading rate in treatment systems
• Calculate the Food to Microorganism (F/M) ratio
• Assess sludge age (Mean Cell Residence Time)
• Monitor sludge activity and health
• Optimize aeration and sludge wasting rates
• Evaluate treatment process efficiency

Testing Methodology

The standard method for VSS determination involves:

1. Filtering a known volume of sample through a pre-weighed glass fiber filter
2. Drying the filter at 103-105°C to determine Total Suspended Solids (TSS)
3. Igniting the filter at 550°C in a muffle furnace to burn off organic matter
4. Measuring the weight loss to determine VSS

Quality Control Considerations

• Ensure consistent furnace temperature (550±50°C)
• Use proper desiccation for cooling filters
• Account for filter blank corrections
• Maintain consistent ignition time (minimum 15 minutes)
• Use appropriate sample volumes to obtain measurable residues