Overview
ISO 18749:2004 - "Water quality - Adsorption of substances on activated sludge - Batch test using specific analytical methods" - specifies a laboratory screening method to determine how much of a chemical is removed from wastewater by adsorption onto activated or primary sludge. The standard describes a controlled aqueous batch test (normally 24 h) using substance‑specific analytical methods to measure concentrations at the start, during and at the end of the test and to calculate the degree of adsorption, distribution coefficient and mass balance.
Key topics and technical requirements
- Scope and applicability: Applies to water‑soluble substances, or those that form stable suspensions/emulsions, that are not readily biodegradable, do not deflocculate sludge, are not removed by abiotic processes (stripping/foaming) and for which accurate analytical methods exist.
- Test system: A buffered inorganic test medium is prepared (stock solutions for phosphate buffer, Mg2+, Ca2+ and HCO3−) with adjustable hardness to match local wastewater conditions (example: 2.5 mmol/L hardness ≈ 80 mg/L Ca, 12 mg/L Mg).
- Activated sludge: Sampled from a functioning biological wastewater plant, washed and adjusted to a suspended solids concentration-typically 1.0 g ± 0.1 g SS/L. Sludge quality checks include floc structure, SVI (<150 mL/g) and suspended solids determination (e.g., ISO 11923).
- Test concentrations: Substance concentrations should simulate environmental exposure but be within analytical sensitivity (commonly 1–5 mg/L for substance‑specific analysis; 40 mg DOC/L for DOC analysis).
- Controls and validity: Use abiotic elimination controls, sterilized sludge or biodegradation tests to distinguish adsorption from other loss mechanisms. A reference compound (e.g., a water‑soluble dye such as Basic Violet 4) is recommended to check procedure and sludge adsorptivity.
- Measurements & calculations: Analytical determinations at defined time points are used to compute percent adsorption, distribution coefficient (sludge/aqueous ratio) and mass balance. The method warns that elimination measured may include flocculation, complexation or sedimentation unless appropriate controls are used.
- Safety: Activated sludge and sewage may contain pathogens; handle with appropriate biosafety measures. Toxic test compounds require care.
Applications and who uses it
- Environmental laboratories assessing adsorption behavior of chemicals in wastewater treatment plants.
- Regulatory bodies and consultants performing fate and risk assessments for industrial chemicals, pharmaceuticals, dyes and surfactants.
- Wastewater operators and R&D teams evaluating treatment performance, sludge management and potential need for anaerobic treatment for highly adsorbed substances.
Related standards
- ISO/TR 15462 - overview of biodegradation tests (biodegradability considerations)
- ISO 8192 - toxicity tests relevant to sludge health
- ISO 11923 - determination of suspended solids
ISO 18749 is a practical tool for laboratory adsorption screening, helping predict chemical fate in wastewater treatment and supporting compliance, environmental risk assessment and process design.