Ideal carbon utilisation in wastewater treatment for enhanced nutrient removal - Primary filtration with filter sludge fermentation as primary treatment for a novel biofilm process

Doktorsavhandling, 2025

Wastewater treatment is necessary to protect our receiving waters from eutrophication and oxygen deficiency, but requires space, energy and chemicals. A novel compact primary treatment for wastewater was tested, to enable resource efficient carbon utilisation for nitrogen and phosphorus removal and for methane production. Primary filtration and fermentation of filter primary sludge (FPS) at ambient temperature were studied at pilot scale to assess the efficiency of particle removal, and seasonal variation in volatile fatty acid (VFA) production by fermentation. A novel continuous biofilm process with bio-based biofilm support material was studied in combination with the primary treatment to understand how carbon management can impact enhanced biological removal of nitrogen and phosphorus.

Through the primary treatment, the VFA concentration in the wastewater could nearly be doubled. The seasonal variations in VFA production and distribution, and in the microbial community of FPS fermentation were considerable. Calculations and simulations indicated lower energy demand and volume requirement with the primary treatment, in addition to lower effluent nitrogen compared to conventional primary settling. The continuous biofilm process with two alternating moving bed biofilm reactors (MBBRs) resulted in mean nitrogen and phosphorus removals during the different operational periods of 70-85% and 57-82%, respectively, assuming complete particle removal. Aerobic and anoxic phosphate uptake was observed, signifying the presence of denitrifying polyphosphate accumulating organisms (PAOs). The FPS fermentation enables storage of the produced VFA[1]rich carbon source, and control of the dosage to biological nutrient removal. It was shown that VFA dosage was needed to enable high PAO activity with filtered influent, and that controlled dosage at low redox improved the phosphorus removal. The PAO activity decreased when the primary treatment was omitted. Microbial analysis showed high abundances of the PAOs Ca. Phosphoribacter, Ca. Accumulibacter and Tetrasphaera in the biofilm and in the effluent.

The importance of carbon management for enhanced biological nutrient removal was shown in this thesis work. The primary filtration and the addition of VFA from FPS fermentation could increase the enhanced biological removal of nitrogen and phosphorus, and reduce the needs for electricity, volume and chemicals in the wastewater treatment.