GAC in Fluidized Bed – Process Evaluation at the WWTP Delémont and Surroundings
Summary
This report evaluates the full-scale performance of the GAC in fluidized bed (Carboplus) for micropollutant elimination at the Wastewater Treatment Plant (WWTP) in Delémont. Implemented to comply with the Swiss Water Protection Ordinance, the system utilizes micro-granular activated carbon (GAC) in fluidized bed reactors. The study monitored the system for approximately one year between late 2023 and late 2024, analyzing organic micropollutant removal (OMPs), sum parameters, activated carbon retention, and operational behavior under varying weather conditions. The system comprises three fluidized beds (CBP1, CBP2, CBP3) operated in parallel. Performance was benchmarked against regulatory targets and compared with a similar installation at WWTP Penthaz. Key findings are:
Micropollutant Removal Efficiency:
- Dry Weather: The process reliably achieved the regulatory target of >80% removal for indicator substances. Diclofenac concentrations are estimated to remain below the Swiss regulatory limit in the receiving river after dilution.
- Rain Events: Removal efficiency declined during rain events. This drop correlated with upflow velocities, likely due to reduced contact time and wastewater dilution.
- Injection Rates: Varying the GAC injection rate (approx. range 10.5–15 mg/L) showed no immediate correlation with removal efficiency possibly due to the long solids retention times (>200 days), which might buffer short-term dosing changes.
Activated Carbon Retention:
- Retention: The system demonstrated high retention of GAC (94–100%), on average meeting the new VSA recommended threshold of 96%. The mean effluent carbon concentration was 0.26 mg/L.
- Impact of Flow: Retention remained robust even during high flow velocities associated with rain events.
Operational Behavior:
- Bed Dynamics: Bed heights fluctuated, often increasing after rain due to the accumulation of total suspended solids (TSS), necessitating regular backwashing and particularly after strong rain events.
Practical recommendations:
Based on the project findings and on-site operational experience, the following parameters are recommended to consistently achieve a micropollutant removal efficiency of 80%, ensure sufficient Diclofenac elimination, and maintain adequate activated carbon retention (particularly during commissioning):
- Dosing: Maintain a dosage of at least 1.8 mg GAC per mg DOC (approx. 12–13 mg/L at typical DOC levels) to provide a safety margin for consistent removal.
- Hydraulics: Limiting hydraulic filter velocities to a maximum of 11–12 m/h seems to ensure sufficient contact time (approx. 6–8 min EBCT).
- Bed Management: Target a resting bed height of 1.6-1.8 m (expanded 2.0-2.7 m).
Conclusions The process is a mature technology capable of meeting regulatory standards. However, performance decreases during high-flow events. Small short-term dosing adjustments are ineffective for immediate process control likely due to system inertia. Future optimization could explore two-stage removal strategies, such as dosing grinded GAC into the biological treatment, further testing is needed to explore this possibility.