Biological phosphorus removal under cold-climate using Sequencing Batch Moving Bed Biofilm Reactor

Abstract

Biological phosphorus removal in cold-climate regions is historically challenged by suppressed kinetic rates and severe biomass washout during spring snowmelt events. This study evaluates the performance, resilience, and kinetic limitations of a pilot-scale Sequencing Batch Moving Bed Biofilm Reactor (SB-MBBR) for Enhanced Biological Phosphorus Removal (EBPR) un-der low temperatures (8.3–9.6°C) at the Fillan Wastewater Treatment Plant. The system's stabil-ity and treatment capacities were assessed across variable seasonal flows and two carbon sup-plementation strategies: a synthetic carbon source (SCS) and an on-site fermented internal car-bon source (ICS).
Cycle tracking showed SBR2 achieved effective EBPR (anaerobic release of 2.45 mg/L, aerobic uptake to 0.155 mg/L), while identical reactor SBR1 failed due to differences in the initial start-up operational strategy. Ex-situ testing confirmed biological activity was localized on the carriers (6.62 mgP/gVSS/h) rather than the suspended sludge (1.85 mgP/gVSS/h). During the snow-melt the system transitioned to an ICS. Following Monod kinetics, the ICS increased volumetric phosphorus removal from 1.77 to 3.32 gP/m3day, but higher background nutrient loading caused effluent accumulation due to fixed phase timers. Under the dual stress of snowmelt and ICS, surface-specific uptake rates temporarily dropped from 0.05 to 0.033 gP/m2day. This ki-netic suppression was driven by a 152% increase in biofilm mass, where ordinary heterotrophs temporarily covered the PAOs, causing diffusion limitations. However, kinetics fully recovered to 0.05 gP/m2day by May, demonstrating that the biofilm is a dynamic system capable of adap-tation. Cross-validated PLS modelling confirmed the system was bottlenecked by aerobic dis-solved oxygen and influent P load rather than cold thermodynamics. Benchmarking showed the SB-MBBR maintained high specific kinetics (0.0146 gP/gVSSday) during snowmelt while the parallel IFAS line collapsed to 0.0005 gP/gVSSday due to sludge washout. However, to con-sistently meet the 0.3 mg/L effluent target requires extending operational phase times.

Popular Abstract

Can microbes replace chemicals to clean phosphorus from cold northern wastewater?
Phosphorus pollution is suffocating the Baltic Sea. This project proves that by giving special-ized bacteria a protective home and feeding them recycled sludge can replace expensive chemi-cals to remove phosphorus from wastewater under harsh northern climates.
Every time a toilet flushes, wastewater carries phosphorus, a nutrient that presents an environ-mental paradox: it is essential for all life, yet when it enters aquatic ecosystems in excessive amounts it compromises water quality, a process known as eutrophication. Because the Baltic Sea remains one of the most heavily impacted marine environments globally, strict regulatory limits are forcing municipal treatment plants to find more efficient and sustainable removal methods. Northern Swedish treatment plants traditionally remove it by dosing expensive metal salts, a chemical-dependent process that generates heavy sludge that is difficult to process downstream. This thesis investigated a greener alternative: using specialized bacteria called phosphorus-accumulating organisms (PAOs) that naturally absorb phosphorus inside their cells under alternating anaerobic and aerobic conditions.
The primary challenge in northern environments is that cold temperatures lower biological ac-tivity, while spring snowmelt introduces diluted wastewater that can destabilize biological sys-tems. To address this, a pilot-scale Sequencing Batch Moving Bed Biofilm Reactor (SB-MBBR) was evaluated at the Fillan wastewater treatment plant in Sundsvall. In this setup, the bacteria grow attached to protective plastic carriers rather than floating freely. This physical an-chor shields the biomass from washing away during seasonal changes.
Operated below 10°C through winter and spring, the reactor demonstrated remarkable structural resilience. The health of the microscopic system was even visually apparent: a functional, active PAO biofilm maintained a light beige-brown colour, while non-functional bacteria in a parallel line turned a distinct grey black. Statistical modelling proved that the primary bottlenecks to treatment performance were not cold temperatures, but rather insufficient dissolved oxygen dur-ing the aerobic phase and low influent phosphorus loading. When optimized, the system main-tained stable kinetics and outclassed parallel hybrid technologies, which suffered severe effi-ciency drops as their suspended bacteria washed away during snowmelt.
However, a clear operational trade-off emerged when switching from controlled synthetic nutri-ents to a more sustainable, on-site fermented sludge. The unfermented, complex organics in this circular carbon source outpaced the bacteria's removal speed, leading to a build-up of phospho-rus in the effluent. This highlights that circular economy sustainability goals must be carefully integrated with strict treatment efficiency limits.
Ultimately, this research confirms that biological phosphorus removal is highly viable for cold northern climates. With targeted adjustments to aeration and cycle timing, fixed-film bioreactors offer a resilient, feasible platform to phase out chemical dependency in northern wastewater treatment.

• author: Panagiotou, Marina
• supervisor: Karin Jönsson; Linda Kanders; Malin Tuvesson
• organization: Chemical Engineering (M.Sc.Eng.)
• course: VVAM01 20261
• year: 2026
• type: H2 - Master's Degree (Two Years)
• subject: Technology and Engineering
• keywords: Cold-climate wastewater treatment, Enhanced Biological Phosphorus Removal, Sequencing Batch Moving Bed Biofilm Reactor, Internal Carbon Source, Water and environmental engineering
• language: English
• id: 9242649
• date added to LUP: 2026-06-24 08:56:59
• date last changed: 2026-06-24 08:56:59