# Evaluation of Eco-Efficiency of Two Alternative Agricultural Biogas Plants
## 1 Introduction  
Implementation of circular economy is one of the priorities of the European Union (EU), which is reflected in the package of actions for this area presented in 2015 [1]. Two years later, the report on the measures undertaken was published [2]. In this context, the role of waste-to-energy in the circular economy was emphasised and the support for research and innovation, which contributes to bringing advanced energy-efficient technologies to the market, was declared [3]. The idea of the circular economy is based on closing the product life cycle through the use of waste in the consecutive production cycle (from \"cradle to cradle\").  
Waste-to-energy constitutes one of the elements of the circular economy. Considering the product life cycle, it is the last option after reuse, refurbishing, and recycling. Nevertheless, in the life cycle of many products (also at early stages), waste is generated which undergoes the electricity and heat recovery process. Waste-to-energy, in the context of processing agricultural residues and agri-food industry waste in agricultural biogas plants, is a part of energy recovery. This article contributes to the discussion on the effective use of this type of waste for the purposes of waste-to-energy in biogas plants.  
Many factors determine an efficient operation of a biogas plant. From the environmental and economic point of view, the feedstock of substrates and their transport is a very important issue, while the optimisation of these processes determines the benefits that may result from the waste-to-energy process. Therefore, it is worth stressing that the subject undertaken in this article is important not only in terms of searching for effective solutions for the circular economy, but also in the context of the necessity to diversify sources and methods of energy acquisition. In the case of Poland, the compliance with the EU requirements assuming 15% of the share of energy from renewable sources in the total gross energy consumption [4] will be possible mainly as a result of the development of the agricultural biogas plant market. In the draft document, _National Energy Policy until 2050_, biomass and biogas are recognised as the most important sources of renewable energy production [5]; Poland has considerable biomass resources provided by the extensively developed Polish agriculture and agri-food industry [6].  
The literature on this subject richly illustrates the environmental benefits arising from biogas production in biogas plants (e.g., [7; 8; 9; 10; 11]). However, a question arises as to what is the most optimum variant of the biogas plant, which would guarantee a more effective performance of this type of installation, both in terms of environmental and economic issues. In this paper, studies focusing on the comparative assessment of the eco-efficiency of the biogas production process are presented, depending on the type of the feedstock used, its transport and possibility to use generated heat. On their basis, a variant of agricultural biogas installation was selected, the implementation of which may be recommended within the construction of the circular economy, of which energy efficiency is one of the pillars. The indicator determined in this article may be used for the purposes of the assessment of the environmental and economic energy unit and may potentially represent the additional measure describing the energy management system in enterprises.