Performance of a full-scale anaerobic UASB digester treating blackwater from an urban city district in Helsingborg, Sweden

Abstract

The growing global population has increased the demand for circular sanitation solutions that enhance energy and nutrient recovery. Anaerobic digestion (AD) of blackwater (BW) offers a promising solution, but its application in full-scale UASB reactors remains unexplored. This study evaluated the performance of a full-scale UASB (50 m³) treating BW collected from a new city district (Oceanhamnen) located in Helsingborg, Sweden, over 130 weeks, applying two different organic loading rates (OLRs), focusing on COD removal, biogas production, and mass balances for COD, nitrogen, and phosphorus, and compared its performance with previous lab-scale studies. The full-scale UASB achieved its highest COD removal efficiency (88 %) and degree of methanisation (0.77 kgCOD-CH₄ kg⁻¹ COD_in) during the low organic loading rate (OLR) phase. In the higher OLR phase, COD removal dropped to 72 % due to sludge washout, although the degree of methanisation remained high (0.71 kg COD-CH₄ kg⁻¹ COD_in). Mass balance results showed 69 % of influent COD was converted into methane, while 28 % remained in the effluent, likely due to sludge washout. The decanted effluent contained 92 % of N and 82 % of P reflecting the low sludge production. Compared with lab-scale studies, the full-scale UASB achieved comparable COD removal and biogas yields despite challenges like sludge washout during the higher OLR. The results confirm that the full-scale application of blackwater treatment is suitable for circular sanitation solutions. Further research is needed to optimize solids retention and reduce effluent COD for sustainable full-scale operations, as well as optimizing the energy balance calculations.