Master thesis: Production of Internal Carbon Source through Sludge Fermentation

Background

Nitrogen removal, emission reduction, and climate neutrality are key goals for modern wastewater treatment plants. Stricter discharge requirements from the EU and Swedish authorities are setting increasingly higher demands for efficient and sustainable wastewater treatment processes. The project Kall-PN (funded by FORMAS and several northern Swedish water utilities) aims to develop and test technical solutions for biological nitrogen and phosphorus removal adapted to northern Swedish conditions, where low water temperatures and variable flows during snowmelt pose particular challenges. Historically, wastewater treatment plants in northern Sweden have been exempt from nitrogen removal requirements, partly because nitrogen removal has been considered difficult to achieve at low temperatures and high flows. However, with the new EU Urban Wastewater Directive, this is likely to change.

To achieve biological nitrogen removal, an external carbon source is required, i.e. an easily degradable organic substance that drives the denitrification process. Traditionally, liquid carbon sources such as methanol or ethanol are used. These provide good technical performance but have several disadvantages, as they are often fossil based, generate greenhouse gas emissions during production and transport, and have increasing costs. Their use also conflicts with the sector’s goals of improved resource efficiency and circular economy.

Kall-PN builds on experience from the earlier Kall-N project, in which efficient nitrogen reduction was achieved at temperatures as low as 5 °C using MBBR (Moving Bed Biofilm Reactor) technology. Within Kall-PN, the potential to produce and use an internal carbon source through fermentation of wastewater sludge is being explored. This could replace expensive chemicals and contribute to a more resource-efficient and climate-smart process. However, to achieve stable and efficient fermentation, operating parameters such as temperature, retention time, and loading rate must be optimized. It is also unclear what proportion of the sludge needs to be fermented to meet the carbon source demand for full-scale biological nitrogen and/or phosphorus removal.

About the master thesis

The master’s thesis focuses on evaluating operating conditions for sludge fermentation with the aim of producing an internal carbon source at the Fillan Wastewater Treatment Plant in Sundsvall, within the framework of the Kall-PN project. The work combines practical pilot-scale experiments with data analysis, providing a unique insight into the interplay between process design, resource efficiency, and climate impact.

The project may include the following components:

• Pilot trials in the fermentation facility at Fillan WWTP, under operating conditions selected based on previous small-scale laboratory experiments.
• Analysis of the produced carbon source, including composition, concentrations, and stability over time.
• Calculations of carbon source demand in the biological treatment process, based on operational data from Fillan WWTP and pilot-scale facilities. The study will also assess what fraction of the sludge flow must be fermented to meet the internal carbon source demand at full scale, for different process alternatives.

Calculations of carbon source demand in the biological treatment process, based on operational data from Fillan WWTP and pilot-scale facilities. The study will also assess what fraction of the sludge flow must be fermented to meet the internal carbon source demand at full scale, for different process alternatives.

The project is suitable for a student interested in wastewater treatment, biological processes, and practical process engineering. The work will be based at Fillan Wastewater Treatment Plant in Sundsvall. Experience in laboratory or pilot-scale work, or a particular interest in such, is considered an advantage.

Objectives

The purpose of the thesis is to evaluate selected operating parameters for sludge fermentation in cold climate conditions and to quantify how much of the sludge flow must be treated to meet the plant’s need for internal carbon source.

The work shall:

• Determine achievable concentrations and compositions of volatile fatty acids (VFAs) produced during primary sludge fermentation.
• Study the stability of the produced carbon source, i.e., its storage potential over time.
• Assess what proportion of the sludge flow must be fermented to achieve the desired carbon supply for full-scale biological nitrogen and/or phosphorus removal.
• Contribute to the development of climate-smart, resource-efficient, and self-sufficient wastewater treatment plants adapted to Nordic climates.

Location

The workplace will be at MittSverige Vatten & Avfall and the pilot at the Fillan wastewater treatment plant