Municipal wastewater treatment sludge management has always been one of the most challenging waste management issues as it has both positive and negative sides. The positive side is that Municipal wastewater treatment sludge is one of the major sources with abundant nutrients, which are vitally needed by the plants – primary nitrogen and phosphorus. The negative side – presence of various toxic elements and pathogens, as well negative perception from the local communities due to stenches. EU Sludge directive (86/278/EEC) provides wide possibilities for the recycling of Municipal wastewater treatment sludge in agriculture, but despite this opportunity and the fact, that over the years toxic elements in Municipal wastewater treatment sludge have been substantially reduced due to implementation of much stricter environmental regulations, Municipal wastewater treatment sludge usage for fertilization of food crops has very diverse practices over the EU. Most of the EU members are tightening safety regulations for food production, therefore Municipal wastewater treatment sludge usage in food production chain is getting less attractive and less feasible solution for nutrient recycling.

On the other hand, nutrient recycling from nutrient rich waste in agriculture and forestry is one of the top priorities for circular economy. Because nutrient rich wastes, such as municipal water treatment sludge and biomass ashes, besides abundant nutrients contain a number of toxic elements, nutrient recycling from Municipal wastewater treatment sludge and biomass ashes in non-food crops such as biomass growing, could successfully become one of best practices all over the Europe and could successfully compete with landfilling and waste incineration – both economically and following the priorities in the hierarch of waste management.

Lithuania has quite a unique situation to become a successful demonstration platform for municipal wastewater treatment sludge recycling using circular economy model.

While implementing Lithuanian sludge management program, which was developed in 2006, and State strategic waste management plan, Lithuania invested hundreds of millions in into 13 large modern regional Municipal wastewater treatment sludge management centers, which had installed modern technologies for sludge thickening, anaerobic digestion, drying and final processing into Municipal wastewater treatment sludge digestate pallets. 6 smaller Municipal wastewater treatment sludge processing companies were upgraded as well, which after sludge thickening are composting sludge. As Municipal wastewater treatment sludge digestion and dry pallet production is installed in all the largest agglomerations all over Lithuania, 85% of total country’s sludge may be processed into fine dried MWTS digestate pallets. Total Lithuania’s annual MWTS production is about 40000 dmt.

While implementing complete modernization of all municipal wastewater treatment facilities, it was not decided how the fine processed sludge would be further used. Large cities do not have practices to use Municipal wastewater treatment sludge for nutrient recycling – typically Municipal wastewater treatment sludge has been stored in special facilities. There are no special Municipal wastewater treatment sludge incineration facilities in Lithuania as well. The only waste incineration plant in Klaipeda incinerates wet sludge. New waste incineration facility which will be build in Vilnius will not be able to incinerate sludge due to funding status (EU funding requirements) – only municipal waste could be incinerated. Another waste incineration facility in Kaunas is being build and will be able to incinerate sludge since 2020-2021.

Due to unsolved problem of the final usage of fine processed dried Municipal wastewater treatment sludge digestate, today modern municipal wastewater treatment facilities operate only at limited capacities and do not perform complete environmental functions as it was designed in modernization projects.

The other environmental issue that is emerging all over the EU and that is related to highly promoted usage of biomass in the renewable energy production – is rapid increase in another type of waste – biomass ashes. Biomass ashes are poised with similar dilemma as municipal wastewater treatment sludge - biomass ashes are abundant with certain nutrients (primarily potassium (K) and calcium (Ca), which are not so scare as phosphorus (P), concentration of which in biomass ashes is 10 times less than in MWTS) but they have significant concentrations of heavy metals (e.g. cadmium (Cd), concentration of which in biomass ashes may be several times higher than in Municipal wastewater treatment sludge). Due to high concentrations of cadmium (Cd) biomass ashes usage in agriculture is widely avoided on the EU level and large part of the biomass ashes are being landfilled. The countries with intensive usage of forest biomass in renewable energy production (e.g. Finland and Sweden) have developed practices to return biomass ashes to forest land, and biomass ashes are becoming a valuable soil improver especially in higher acidity organic forest soils, which are very scare with phosphorus as well.

In Lithuania, rapid shift from natural gas towards biomass-based district heating sector led to the situation that volume of biomass ashes approached MWTS volume. It is estimated that in 2016 appr. 37,000 t of biomass ashes were produced by biomass boilers and biomass based power plants (18,000 t by district heating system operators, 13,000 t by independent heat suppliers (private) to district heating network, and 6,000 t by industrial biomass boilers) and the number is expected to rise over 40,000 in the nearest years due to continuing biomass usage growth for renewable energy production.

Today the majority of municipal wastewater treatment sludge and biomass ashes in Lithuania are being used in food crop growing, without any environmental and safety control, thus posing a risk of contamination of food chain.

Our task is to develop a new model for nutrient from wastewater treatment sludge and biomass ashes reuse in non-food crop growing – in woody biomass plantations – that could ensure complete shift from risky and uncontrolled use in food chain.

To develop and demonstrate full-scale self-sustainable closed loop circular economy model for nutrient rich waste recycling for Vilnius (the largest city in Lithuania, with population of 540 thousand), the following actions and means will be employed:

- Mobilization of land within 60 km radius from Vilnius city for full-scale CE demonstration model: 900 ha of existing biomass plantations and 900 of new land will be mobilized to ensure full-scale nutrient rich waste recycling

- Environmental permitting for nutrient rich waste recycling: 100% of environmental permits and full legal compliance will be ensured for CE demonstration model operations;

- Establishing of missing biomass plantations: 900 ha of new biomass plantations will be established to ensure full scale nutrient rich waste recycling capabilities of CE demonstration model;

- Recycling of nutrients from dried MWTS digestate of Vilnius city: 100% of MWTS digestate from Vilnius wastewater treatment plant will be recycled for three consecutive years, each year on new land plots included in CE demonstration model;

- Recycling of nutrients from biomass ashes from renewable energy production of Vilnius city: calculated amount of biomass ashes, from combustion of biomass supplied from CE demonstration model annually, from renewable energy production for Vilnius city will be recycled for three consecutive years;

- Biomass supply and conversion into renewable energy for Vilnius city: biomass from CE demonstration model will be harvested and supplied to renewable energy production for Vilnius;

- Water, soil and biomass quality impact monitoring: comprehensive monitoring of impact of nutrient rich waste recycling in biomass plantations will be performed to ensure environmental sustainability of CE demonstration model and to develop recommendations for policy and regulatory improvement for nutrient rich waste recycling;

- LCA analysis, biomass yield improvement assessment and business planning: LCA analysis will be performed and, based on biomass yield improvement results, business plan will be developed to prepare for replicability of CE model in other regions of Lithuania and other EU countries

- Increase of public awareness on circular economy benefits: dissemination of project result within project target groups to promote circular economy models and to transfer results in other regions inside and outside of Lithuania.