Overview
ISO 9887:1992 specifies the Semi-Continuous Activated Sludge (SCAS) method for evaluating the aerobic biodegradability of organic compounds in an aqueous medium. The procedure measures residual dissolved organic carbon (DOC) in the effluent of an activated sludge unit dosed daily with a test compound and compares it with a control unit receiving sewage only. The method is intended to assess ultimate or primary biodegradation under controlled laboratory conditions and is widely used for water quality and biodegradation testing.
Key topics and technical requirements
- Scope and applicability
- Applies to compounds that are soluble, non-volatile (negligible vapour pressure), not lost by foaming, not significantly adsorbable on glass/sludge, and non-inhibitory to test microorganisms at the chosen concentration.
- Principle
- Daily fill-and-draw (semi-continuous) dosing of SCAS units; DOC in effluents from test vs control units is used to calculate percentage biodegradation.
- Test conditions
- Incubation in the dark or diffused light at 20–25 °C.
- High microbial load: initial 1–4 g/L suspended solids.
- Effective sewage detention ~36 h with no deliberate sludge wasting (long sludge retention times).
- Concentration and duration
- Typical initial test concentration: ~20 mg/L DOC (from a concentrated stock).
- Test length is indeterminate but generally 12–26 weeks (continue until difference stabilizes over six consecutive measurements).
- Analytical and procedural requirements
- Measure DOC according to ISO 8245; samples clarified by centrifugation or 0.2–0.45 µm filtration.
- Frequent sampling early (daily if values change significantly), otherwise 2–3 times weekly.
- Use of domestic or synthetic sewage (detailed synthetic recipe provided).
- Safety and limitations
- Activated sludge and sewage may contain pathogens-use appropriate biosafety precautions.
- Pre-exposure/adapted inocula may be used where relevant; toxicity checks (e.g., ISO 8192) recommended.
Applications and users
ISO 9887 (SCAS method) is practical for:
- Environmental laboratories performing biodegradation screening and persistence studies.
- Chemical manufacturers conducting environmental fate assessments for regulatory dossiers.
- Wastewater treatment operators and process engineers evaluating removal efficiency of specific compounds.
- Consultants and regulators assessing water quality, potential environmental impact, and treatment requirements.
Keywords: ISO 9887, SCAS, semi-continuous activated sludge method, aerobic biodegradability, dissolved organic carbon, activated sludge, biodegradation testing, water quality.
Related standards
- ISO 7827 - DOC analysis method for ultimate biodegradability.
- ISO 8245 - Guidelines for total organic carbon (TOC) / DOC determination.
- ISO 8192 - Test for inhibition of oxygen consumption by activated sludge (toxicity screening).
- ISO 9408 / ISO 9439 - Alternative biodegradability methods (respirometric and CO2-evolution methods).
Annexes A and B of ISO 9887 provide example test results and typical control values for reference.