Use of an internal carbon Source for climate-neutral water and wastewater services

For sufficient biological nitrogen and phosphorus removal to be achieved at municipal wastewater treatment plants, the addition of a readily biodegradable carbon source is often required. Chemicals are frequently purchased for this purpose, but it is also possible to produce a carbon source from the treatment plant’s own sludge. This project investigates how the production of an internal carbon source affects biogas generation at the treatment plant, as well as the direct nitrous oxide emissions from the treatment processes. The aim is to provide a basis for future process decisions that support resource-efficient and climate-neutral water and wastewater utilities.

Several life cycle assessment (LCA) studies have shown that external carbon sources account for a significant share of the carbon footprint of Swedish wastewater treatment plants. Rather than relying on externally purchased chemicals, a promising alternative is to produce an internal carbon source from the plant’s own sewage sludge and, potentially, other organic residual materials such as food waste. Although precipitation chemicals are not a major contributor to climate impact, rising costs and an increased focus on preparedness and resilience have made access to process chemicals more uncertain. This has increased interest in expanding the use of biological phosphorus removal (Bio-P), which also benefits from the availability of a readily biodegradable carbon source.

An internal carbon source can be produced through anaerobic fermentation. By adapting treatment processes for biological nitrogen and phosphorus removal, the need for externally purchased chemicals can be reduced or eliminated. Internal carbon production can be implemented as a pre-treatment step before anaerobic digestion, where the remaining organic material is further broken down to produce biogas. In this way, Swedish wastewater treatment plants can increase their degree of self-sufficiency in both energy and treatment chemicals.

This project makes use of data and samples generated through an ongoing project involving a pilot plant at the Fillan Wastewater Treatment Plant in Sundsvall (the Kall-PN project). To investigate the impact on biogas production, batch biochemical methane potential (BMP) tests are carried out in the laboratory, both with and without prior extraction of the internally produced carbon source. To assess how the co-production of an internal carbon source and biogas affects a wastewater treatment plant’s ability to achieve energy neutrality, operating economy, and climate performance, the project develops a range of case studies and calculation scenarios based on data from the Fillan WWTP, the Fillan pilot plant, and potentially additional participating treatment plants.

Experimental results on nitrous oxide production and reduction using internally produced carbon sources and external carbon sources from the ongoing FORMAS-funded Kall-PN project are used as input data to enhance existing wastewater treatment plant models. The project develops a generic denitrification model, calibrated for the specific carbon sources studied, enabling a model-based quantification of the climate impacts associated with each carbon source.

The project aims to achieve the following:

• Improved knowledge of how the co-production of an internal carbon source and biogas affects the overall climate footprint of a wastewater treatment plant and its ability to achieve energy neutrality, compared with the use of external carbon sources alone. This includes impacts on biogas production, nitrous oxide emissions from treatment processes, and operating costs.

• Enhanced tools for reducing nitrous oxide emissions from wastewater treatment processes through the further development of existing treatment plant models, taking into account the effects of different carbon sources on nitrous oxide emissions.