Evaluating the Hydraulic Performance of a GAC Filter Pilot Plant at Rya WWTP

Sammanfattning

The implementation of the revised urban wastewater treatment directive (UWWTD) introduced mandatory micropollutant abatement or quaternary treatment for many wastewater treatment plants (WWTPs) within the EU. Rya WWTP in Gothenburg, which is treating almost 880,000 PE of water, must find and implement a suitable option by 2045 at the latest. One alternative is granular activated carbon (GAC) which was previously pilot tested, but the results were inconclusive. Therefore, a new GAC pilot plant was commissioned and has been running since July of 2025. It has an EBCT of 22 min, a bed height of 2.7 m, filter area of 1 m2 and influent flow rate of 7.5 m3/h. The GAK pilot plant has exhibited deteriorating hydraulic performance over an extended period, with filter run times decreasing from an average of ca 12 hours to 3 hours with low TSS loads before backwashing. Therefore, this thesis was primarily aimed at evaluating, localising and combatting the poor hydraulic performance, and secondarily to evaluate the treatment performance in relation to the revised UWWTD, standard parameters and PFAS-24. A method was developed that successfully localised and quantified the pressure drop for GAC from different depths of the bed. By tracking changes in hydraulic resistance over time, it could be concluded that the main hydraulic bottleneck was localised to the top of the GAC bed. Following remediation of the problem, the filter run time improved to a mean of 24 h. However, despite more aggressive backwashing and mass balances that indicated TSS were overall removed from the GAC filter, the deterioration in the hydraulic performance resurged after three weeks. Particle size distribution (PSD) analysis confirmed stratification of the bed which can contribute to surface filtration and rapidly increasing pressure drop. Because mean influent TSS concentration and filter run time only showed weak to no correlation, along with a presence of biofilm in the GAC filter and on the particles, significant DO and BOD removal as well as indications of nitrification suggests the problem is associated with biofilm growth. The MP abatement performance was still in accordance with the UWWTD after ~12,000 treated bed volumes (BV) with 86% elimination based on the best combination of substances. However, treatment performance for PFAS-24 reached below 80% already at ~5,000 treated BV.