Efficiency and factors for agricultural use of sludge in circular Bulgarian economy

The issue of utilization of sludge from wastewater treatment in agriculture is an important socio-economic and environmental problem in the European Union and Bulgaria. Its significance is determined by the fact that the amount of sludge formed is constantly growing, as the annual amount of sludge produced in Europe is 8.7 million dry matter, and in Bulgaria reaches 53 thousand tons of dry matter. One of the main ways to utilize sludge from wastewater treatment is its use as fertilizer in agriculture. It is becoming a topical issue along with the growing interest in effective “transformation of wastes into products” and their inclusion in supply chains and circular economy. In other countries, there are numerous studies on the effects, factors and efficiencies of sludge use in agriculture. Despite their relevance, in-depth studies of the diverse effects and critical factors of sludge utilization in Bulgarian agriculture are at an early stage. This paper presents the results of the first part of a large-scale study aimed at determining the socio-economic effects of sludge utilization in Bulgarian agriculture. First, an approach is presented to assess the multilateral effects, efficiencies and factors of sludge utilization in agriculture. Then the various factors stimulating and limiting the utilization of sludge in Bulgarian farms are identified. The results of a case study of a holding using sludge as fertilizer are then presented. Based on a qualitative analysis of regulations and institutional structure, and surveys with managers and experts of urban wastewater treatment plants, and farmers using and not-using sludge, the institutional, political, organizational, personal, educational, informational, social, economic, and environmental factors influencing the utilization of sludge in agriculture in two regions of the country (Sofia and Burgas) are identified. Impact factors are generally divided into two types: factors influencing the behavior of agents, and factors determining the type and extent of the effects of sludge use in agriculture. Research of this type is to continue and deepen to establish the economic, sectoral and regional specificities on the basis of more representative information from all participants and interested parties in the effective utilization of sludge in the country.