Municipal wastewater treatment produces large amounts of sludge as a byproduct that presents both challenges and opportunities in a wider picture. In the Baltic Sea region around 4 million tonnes of municipal sludge, calculated as dry solids, is generated annually. One of the main environmental concerns in the Baltic Sea is the prevention of eutrophication caused by excess nutrients leading to algal blooms, oxygen depletion, and harm to the ecosystem. Municipal sludge is rich in phosphorus and nitrogen, and if not properly treated, these nutrients are likely to be released into water bodies. The sludge also contains contaminants like heavy metals and pharmaceuticals, which are difficult to remove with standard treatment methods. Therefore, effective sludge management is crucial to protect the environment and enhance sustainability.
Approximately 97% of the phosphorus and 9% of the nitrogen in municipal wastewater end up in the sludge during the treatment. Despite these high concentrations, around 50% of the phosphorus and nearly all the nitrogen in Europe remain unused because of regulations and limitations in recovery technologies. This unutilized portion could cover about 10% of Europe’s phosphorus needs and 2% of its nitrogen fertilizer requirements. Currently, the EU is almost entirely dependent on phosphorous imports while 30% of needed nitrogen fertilizers are imported. The vulnerability of this supply chain was highlighted in 2022 when fertilizer prices arose due to the Russian attack on Ukraine and disruptions in the energy market. Municipal sludge is a reliable and locally available resource offering a potential solution.
In addition to nutrients, sludge contains a substantial amount of organic carbon, which could be utilized for energy and materials. Around 65% of the carbon in municipal wastewater ends up in the sludge, which contains about 40% carbon in its dry solids. However, the use of carbon from sludges remains underdeveloped. Biogas production through anaerobic digestion is the most common method, but it exploits only a small portion of the carbon’s potential. Advanced technologies may allow the production of high-value carbon-based products, such as biochar for soil enrichment, activated carbon for water filtration, or even hard carbon for lithium-ion batteries.
Municipal sludge also contains inorganic compounds like iron and aluminum salts, which are used as coagulants and precipitation agents in wastewater treatment to remove solids and pollutants, as well as in the production of clean drinking water. In the Baltic Sea region, where protecting water quality is crucial, these chemicals are highly important. However, the mining and production of iron and aluminum salts are resource-intensive and have environmental impacts. Recovering these metals from sludge and reusing them in treatment processes follows the principles of a circular economy, reducing the need for raw materials and minimizing waste.
Despite these opportunities, a significant challenge remains in utilizing municipal sludge. Even when products derived from sludge are treated and deemed safe, they often face suspicions from consumers, farmers, and policymakers due to concerns of contaminants and long-term environmental impacts. This is particularly true in the Baltic Sea region, where environmental awareness is high. Overcoming this challenge will require more than just technological solutions. Transparent communication, rigorous safety standards, and thorough pilot actions to demonstrate the benefits of sludge-derived products are needed. Integrating sludge management into broader sustainability frameworks, such as the EU’s Green Deal or HELCOM’s Baltic Sea Action Plan, could provide the necessary policy support to encourage innovation and adoption. Efforts should be made to launch joint projects among the different institutions across the Baltic Sea region to demonstrate the environmental and economical benefits of municipal sludge valorization.
Eveliina Repo
Professor, Department of Separation Science, LUT University
Finland
