Strategies for enhanced biological phosphorus removal in low-temperature environments - a case study in Finland

Abstract

Due to the limited and non-renewable nature of global phosphorus resources and the ineffi-cient use of phosphate fertilizers, the phosphorus crisis has received increasing attention. Alt-hough the primary objective of phosphorus removal in wastewater treatment plants (WWTP) is mainly to meet discharge standards and prevent eutrophication of surface waters, considering that the removed phosphorus has the potential to be reused as an alternative to mined phos-phate fertilizer, the wastewater treatment industry is considered one of the main hotspots for mitigating phosphorus depletion. Phosphorus removal in WWTPs commonly uses methods such as chemical precipitation and enhanced biological phosphorus removal (EBPR). These methods result in most of the phosphorus ending up in the residual sludge, which is subse-quently removed from the system to achieve phosphorus removal. When considering the re-covery of phosphorus from sludge, in terms of technical or economic considerations, EBPR presents more advantages compared to chemical precipitation. In Finland, the most commonly used phosphorus removal method is chemical precipitation, EBPR is used only as a partial solution in a few treatment plants, such as the Huittinen WWTP, which is the case study of this thesis. Due to the need to meet both nitrogen and phosphorus removal goals, maintaining stable EBPR operation year-round is challenging, especially in low-temperature environments. Driven by the need to control operational costs and meet future re-quirements for phosphorus recovery, the objective of this thesis is to integrate findings from the literature review with operational and water quality data from the Huittinen WWTP to explore strategies for maintaining stable biological phosphorus removal efficiencies throughout the year in Finland, with a special focus on winter time. Through a literature review, it was found that low temperatures have indirect impacts on EBPR. This is because nitrogen removal relies heavily on the activated sludge process, which becomes less efficient at lower temperatures. As a result, to maintain nitrogen removal efficien-cy, EBPR often has to compromise phosphorus removal, making it more reliant on chemical methods. By analyzing the operational parameters and water quality data of the Huittinen WWTP and comparing them with other successful EBPR cases in Nordic countries, several strategies for enhancing EBPR effectiveness have been identified. These strategies focus on improving nitrification efficiency in low-temperature environments, thereby reducing the propor-tion of aerobic tanks to increase the hydraulic retention time in anaerobic tanks, which will help improve the efficiency of EBPR. Specific measures include maximizing the mixed liquor sus-pended solids concentration in the biological reactors and using the secondary sedimentation tanks as post-denitrification tanks. Additionally, controlling the sludge return flow to maintain a deep anaerobic environment in the anaerobic tanks helps to create more space for biological phosphorus removal.