University of Tartu: Palette of solutions for nutrient recycling

Introduction and background

Nutrients such as phosphorus and nitrogen are vital to our current society, as modern food production depends heavily on fertilisation. Widespread use of fertilisers however has created one of large environmental issues we are facing – eutrophication. Similar to agricultural crops, phosphorus is also the main limiting nutrient for algae growth and the amount of phosphorus in the water systems has increased greatly due to human activities (runoff from fields, effluent from wastewater treatment plants (WWTPs), leakages). For past decades, many different actions have been undertaken to reduce the amounts of phosphorus entering the sea from different tributaries and the total loads of nutrients emitted to the Baltic Sea have been decreased significantly.

WWTPs have been one of the main point sources of phosphorus entering the environment. Today, advanced phosphorus treatment is mandatory for most medium sized and larger WWTPs in the region. Chemical precipitation and biological accumulation are used to take the phosphorus out of the wastewater streams and tie it into the sludge. This sludge is then treated in various ways and quite often the phosphorus potential there is lost – based on a key figure collection carried out in the IWAMA project, almost 60% of the treated sludge is not used directly. Most of it is either incinerated or accumulated at the WWTP and finally landfilled.

The main producers of phosphorus today are Morocco, the US and China. Both the EU and the Baltic Sea region import almost all of our phosphorus need, which leads to our food security being directly tied to foreign companies. At the same time, research has clearly shown that phosphorus is not used or treated sustainably in the area, resulting in large losses from the food chain. Three main areas of phosphorus losses have been identified: wastewater sludge, manure and organic waste.

Palette of solutions

BSR WATER as a platform project in the Baltic Sea region has been collection project results, network contacts and background knowledge on the issue to take this issue further. One of the main outputs of the BSR WATER project is the “Palette of solutions for nutrient recycling in the Baltic Sea region”, foreseen as an integral part of the HELCOM Regional Nutrient Recycling Strategy. The palette of solutions, is a compilation of most recent knowledge and practise on advanced and efficient solutions to recycle nutrients, and in particular phosphorus from wastewater.

While phosphorus is a recognized pollutant for the Baltic Sea region, it is also part of many organic molecules and is vital and irreplaceable for all lifeforms. Phosphorus is an essential and limited resource. The phosphorus demand and production in the world is steadily growing and the movement of phosphorus must be transformed from linear to circular in order to retain the safety of our economy and food. The efficient use of natural resources is also one of the main factors of becoming a sustainable society, therefore coupled with the environmental issues, reuse and recovery of phosphorus needs to be one of our main goals for the near future.

As wastewater is one of the main flows of nutrients, wastewater treatment plants should be viewed as a key location for nutrient recovery operations. Presently, phosphorus and nitrogen are removed from the wastewater, but in most cases the nutrient-rich sewage sludge is not used further due to possible problematic heavy metal and micropollutant concentrations. Many new and emerging technologies tackle this issue and aim toward extracting nutrients from these waste streams, while leaving the pollutants behind. These technologies are currently evaluated in various full, pilot and laboratory scale tests around the world to find most efficient parameters for operation, possible limiting components in the input streams and above all else the financial implications of the processes.

The palette of solutions document provides short overview on various topics:

• Nutrient sources coming in to the WWTPs and flow balance throughout the treatment
• Phosphorus recycling chemical and physical technologies
  • MAP and HAP crystallisation
  • Sorption processes
  • Wet-chemical leaching
  • Wet-oxidation and supercritical water oxidation
  • Thermo-electrical and thermo-chemical processes
  • Metallurgical processes
• Nitrogen recovery potential and technologies
• Economic feasibility of nutrient recovery
• Bioavailability and environmental risk of recycled products
• Current policy situation and the possible future

The palette of solutions includes general information about the global situation in nutrient production and use, main pathways of nutrients transportation to wastewater and technologies for integrating nutrient recycling from either liquid or sludge matrices into the treatment process. Downstream recovery methods from treated sludge or incineration ash are also discussed. For each of the main technology types, an example case of a full-scale or a pilot-scale application is included, with overall evaluations of potential environmental risk and financial feasibility rounding out the document. The palette of solutions itself is the final part of the document, summarizing all of the previous information and reforging it into a clear and concise format.

The palette of solutions for nutrient recycling in the Baltic Sea region is expected to be published by the end of 2020! We look forward to sharing our work with you!

Markus Raudkivi
University of Tartu