This deliverable outlines the key findings of the research performed by Aquafin and UGent during Task 2.3 (T2.3) within the WalNUT project, which focused on testing and optimizing a novel two-stage process for municipal wastewater treatment and nutrient recovery.

The system combines:

1. High-rate activated sludge in contact stabilisation mode (HRAS/CS) – an advanced biological treatment process that efficiently removes organic matter and improves the biomethane potential of the sewage sludge, but only partially removes nitrogen (N) and phosphorus (P), falling short of strict discharge requirements when used as standalone technology.
2. Adsorption/ion exchange – applied downstream of HRAS/CS to capture and recover nitrogen (ammonium form, NH₄⁺-N), addressing the main limitation of the HRAS/CS system.

Although both technologies have been studied individually, their integration for municipal wastewater treatment and nitrogen recovery was novel. T2.3 successfully demonstrated the technical feasibility of this combination. The HRAS/CS process was implemented by converting a conventional system (CAS/CS) into a high-rate one, by adapting both organic loading rate and sludge retention time. The obtained HRAS/CS system maintained a sufficient organic matter removal efficiency (> 75%) and allowed ammonium to pass through for subsequent recovery.

The adsorption component was studied using five different materials—two synthetic resins, two biochars, and zeolites. Zeolites (in their original ionic form) were selected for column tests based on their NH₄⁺-N removal efficiency and capacity. These tests helped to determine:

• Optimal influent flow rates (5–40 bed volumes per hour), with best performance at 5–10 BV/h.
• Regeneration potential using 3% KCl (potassium chloride), showing a decrease (> 20%) in treatment capacity after one cycle and requiring 30–40 BV of regenerant.

      Nitrogen recovery strategies, comparing three approaches:

• Harvesting adsorbent directly as solid bio-based fertiliser (BBF).
• Regenerating adsorbent and collecting the regenerant as liquid BBF.
• A combined approach yielding both solid and liquid BBFs

The production of a solid BBF offered the best approach (1.4 g NH₄⁺-N/kg), while the combination (both solid and liquid) allowed for better process and product optimisation.

D2.4: Report on HRAS/CS & adsorption/ion-exchange process

Conclusions and Next Steps

The objectives of Task 2.3 were achieved, demonstrating the feasibility of integrating HRAS/CS and adsorption into a two-stage process for urban wastewater treatment and nitrogen recovery. The HRAS/CS system successfully replaced a CAS/CS setup. Zeolites proved effective for ammonium removal and regeneration, with an average nitrogen removal efficiency of 92% and COD (chemical oxygen demand) removal of 83%, meeting European and Flemish discharge standards.

Further optimisation will aim to improve NH₄⁺-N recovery, reduce regenerant use, and enhance nutrient concentration. In the next phase (T3.3) of the WalNUT project, both solid and liquid BBF production methods will be scaled up (based on D2.4) for pilot testing, while ensuring the final treated water meets discharge regulations.. Expected outputs include ammonium and potassium-enriched zeolites and liquid BBFs from regenerant solutions. These products will be evaluated in work package 4 (WP4) for agricultural reuse

 

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