ISO 11923:1997

Overview

ISO 11923:1997 specifies a standardized laboratory method for the determination of suspended solids in raw waters, waste waters and effluents by filtration through glass‑fibre filters. The method uses vacuum or pressure filtration, drying at 105 °C and gravimetric weighing. The practical lower reporting limit is approximately 2 mg/L; no upper limit is defined. The standard also notes key interferences (notably floating oil and other immiscible organic liquids) and sampling stability issues.

Key topics and requirements

• Principle: Filter a measured sample volume, dry the filter at 105 °C ± 2 °C, and determine the mass increase gravimetrically.
• Filters: Use borosilicate glass‑fibre filters containing no binders. Mass loss in a blank must be < 0.3 mg per filter; areic mass typically 50–100 g/m².
• Equipment: Vacuum or pressure filtration apparatus, drying oven (105 °C ± 2 °C), analytical balance with at least 0.1 mg resolution, suitable drying supports.
• Sample handling:
  • Prefer analysis within 4 hours of sampling; if stored, keep dark at < 8 °C (do not freeze).
  • Do not add chemical preservatives for suspended solids analysis.
  • Report any storage time > 4 hours in the test report.
• Procedure specifics:
  • Select sample volume so dry residue on filter is optimally 5–50 mg; results must be based on at least 2 mg dry residue. Avoid sample volumes > 1 L.
  • For samples with > 1 000 mg/L dissolved solids, use extended washing (three 50 mL washes).
  • Calculation: p (mg/L) = 1000 × (b − a) / V
    where b = filter mass after filtration (mg), a = filter mass before filtration (mg), V = sample volume (mL).
• Quality control: Run a control using the working reference cellulose suspension; recovery should be 90–110%.
• Interferences: Floating oil/immiscible organics can bias results; Annex B describes oil removal (ethanol then hexane) but requires reporting if used.
• Reporting: Include filter manufacturer/type, sample ID, date/place, any deviations, and result (below 2 mg/L reported as “below 2 mg/L”).

Applications and users

• Environmental and compliance monitoring of rivers, lakes, raw water intakes and wastewater effluents
• Wastewater treatment plant performance checks and process control
• Contract and regulatory testing laboratories, environmental consultants, research institutions, and water utilities
• Useful for sediment load measurements, turbidity correlation studies and routine operational monitoring

Related standards

• ISO 5667-2:1991 - Water quality - Sampling - Part 2: Guidance on sampling techniques
• ISO 6107-2 - Water quality - Terminology (referenced for definitions)

Keywords: ISO 11923, water quality, suspended solids, filtration, glass‑fibre filters, wastewater testing, effluent monitoring, gravimetric determination.