Research project Resource-Efficient Removal of Nutrients from Wastewater in Cold Climate

The availability of chemicals for phosphorus precipitation is also uncertain in future. Wastewater treatment plants in northern Sweden may need to be rebuilt from scratch to meet these challenges. This offers a possibility to choose efficient and resource-efficient process solutions that reduce dependency on chemicals, minimize climate impact and increase the possibilities for nutrient recovery.

The goal of the project is to design a resource-efficient wastewater treatment process with low climate impact adapted to Norrland conditions with periodically cold water temperatures. The project will focus on testing technical solutions for wastewater treatment from nitrogen and phosphorus, modeling these processes as well as optimization for resource efficiency, reduction of nitrous oxide emissions and removal of pharmaceutical residues. The project will also look at nutritional conditions in the Gulf of Bothnia to motivate for what level of treatment is reasonable to target. The process solution studied in the project consists of biological phosphorus and nitrogen removal in moving bed biofilm reactors (MBBR) combined with hydrolysis of primary sludge for internal carbon source production.

Researchers at Stockholm University are carrying out work package 1 of the project. The purpose of this work package is to study the effects of nitrogen input to the Gulf of Bothnia from wastewater treatment plants on the eutrophication situation in the Baltic Sea, in order to identify which treatment is justified from an environmental point of view. There are two main questions to be answered: 1) how large and spatially widespread is the direct impact in the form of deteriorated water quality from the nitrogen input and 2) how effective is it to reduce the nitrogen input from sewage treatment plants in the Gulf of Bothnia compared with other measures to reduce the input.

Question 1:

Recipient surveys already show elevated levels of nitrogen in the vicinity of treatment plant outlets. The focus here will be to analyse what the input of nitrogen means for eutrophication symptoms such as primary production, cyanobacterial blooms and oxygen deficiency using the Baltic Sea model BALTSEM. The parameterisations of the biogeochemical processes in BALTSEM have been improved, especially with a focus on the roles of dissolved organic matter and bacteria in nutrient cycling and regulation of the availability of nitrogen and phosphorus for primary production.

These are particularly important processes in the Gulf of Bothnia where previous generations of the mode has had difficulties in reproducing primary production. As conditions have changed in the Gulf of Bothnia in recent decades, and can be expected to change further in the future, the analysis will include both retrospective and prospective analyses to cover possible trajectories over the lifetime of new treatment plant installations.

Question 2:

Nitrogen inputs from wastewater treatment plants in the Gulf of Bothnia are not negligible compared to inputs from other activities in the Baltic Sea. In the HELCOM BSAP there are no reduction requirements for nitrogen and phosphorus in the Gulf of Bothnia, but this does not mean that reductions in the Gulf of Bothnia do not have positive effects in, for example, the Baltic Proper where there is a reduction requirement that has not yet been achieved. The reduction requirements in BSAP were calculated over 10 years ago with a much older version of BALTSEM, so we will make new calculations to study how a reduction in nitrogen input from the treatment plants in the Gulf of Bothnia can potentially contribute to replacing other possibly even more expensive measures.