Overview
CEN ISO/TR 22707:2025 (ISO/TR 22707:2023) is an informational technical report on sludge recovery, recycling, treatment and disposal, focusing on processes and technologies for inorganic substance and nutrient recovery from sludge. It applies to sludge and products from urban wastewater collection systems, night soil and wastewater treatment plants for urban and similar industrial waters. Hazardous industrial sludge and dredged sludge are excluded.
This guidance document is descriptive (informative), prepared by ISO/TC 275, and summarizes current technologies, operational considerations and case studies to support sustainable sludge valorisation and nutrient recycling.
Key topics
- Scope and applicability
- Targets urban and similar treatment sludges; excludes hazardous and dredged sludges.
- Presents Annex A (sewage sludge composition examples) and Annex B (case studies).
- Methods of nutrient recovery
- Four principal approaches: whole use, cleaning, separation, and extraction - with advantages and trade-offs for nutrient retention, contaminant removal and product quality.
- Phosphorus recovery (major focus)
- Importance of phosphorus (sludge dry solids often >1.0%, up to ~5.0%).
- Technologies and process descriptions for:
- Struvite recovery (precipitation/crystallization from anaerobic digested sludge/filtrate; reaction fundamentals and product characteristics).
- Hydroxyapatite (HAP) recovery (calcium phosphate formation).
- Recovery from incineration ash (alkaline and acidic treatment routes).
- Sewage sludge slag treatment and other phosphorus extraction methods.
- Process diagrams, operating condition summaries and characteristics of recovered products are provided.
- Other nutrients and inorganics
- Nitrogen (including ammonia stripping), sulfur, potassium and metals - overview of recovery routes and considerations.
- Terminology and practical elements
- Definitions (e.g., struvite, centrate, seed crystal), process schematics and examples to guide technology selection.
Practical applications and users
This report is useful for:
- WWTP operators and utilities assessing nutrient recovery options and implementation pathways.
- Environmental and process engineers designing or specifying recovery units (struvite crystallizers, ash treatment lines, ammonium stripping).
- Technology vendors and consultants developing or demonstrating recovery systems.
- Policy makers and regulators wanting an overview of recovery technologies and environmental benefits.
- Fertilizer producers and circular-economy stakeholders exploring sludge-derived phosphorus and other recovered products.
Related guidance
- Published as ISO/TR 22707:2023, adopted by CEN as FprCEN ISO/TR 22707:2025.
- The document is informational (no normative references) and draws on national standards, guidance documents and industry practice; it is intended to be updated as technologies evolve.
Keywords: sludge recovery, nutrient recovery, phosphorus recovery, struvite, hydroxyapatite, wastewater treatment, sludge recycling, sludge valorisation, inorganic recovery.