**1. Introduction**

Biowaste represents a significant component of MSW. In general, biowaste can be considered as a mixture of similar proportions of kitchen and garden waste from households. According to the definition in the EU's Waste Framework Directive (2008) [1], biowaste means "biodegradable garden and park waste, food and kitchen waste from households, restaurants, caterers and retail premises and comparable waste from food processing plants". Thanks to its properties, it is considered to be a renewable and sustainable source for energy production, and therefore its potential should be examined thoroughly.

There are two principal ways of biowaste collection, which also determines biowaste treatment systems. These collection systems are often running in parallel (see Figure 1). First, a dedicated infrastructure for biowaste collection and biowaste treatment is established. Biowaste is source-separated by citizens and handled as a specific stream (See Figure 1 left). Generally, the collection system is diverse, covering a range of options as traditional door-to-door or more sophisticated pneumatic underground system as reported in [2]. This stream ends in composting plants or digesters and is denoted as SEP-BIO later in the text. However, biowaste is also present in

residual municipal waste (RES), and we name this stream RES-BIO. Landfilling is the standard disposal method for RES in developing countries. In contrast, incineration with heat recovery (WtE) is preferred in countries with developed waste management systems. The benefits of WtE were evaluated in [3].

**Figure 1.** Two sources of biowaste and technologies considered in the analysis.

Depending on the segregation efficiency, the amount of biowaste treated as SEP-BIO or as RES-BIO varies.

The motivation for the investigations presented in this paper is to compare the environmental performance of different routes of waste streams commonly present in RES and treated by WtE. As follows from Figure 1, the main focus is on biowaste. However, the same approach can be applied to other components of RES (discussion on plastics separation and limited recycling options is a hot topic today).

Several studies with comparison of different biowaste treatment methods and biowaste management strategies have been published recently. Papers concerned with biowaste only are shortly reviewed first. Kong et al. [4] performed a comprehensive LCA confirming that the efforts to divert biowaste from landfilling to other ways of treatment (composting and fermentation) bring environmental benefits and reduce (GHG). Ardolino et al. [5] executed an LCA comparing the environmental impacts of different ways of utilisation of biogas produced in an anaerobic digestion plant. The biowaste-to-biomethane scenario, where biogas is upgraded to biomethane and used for transportation, provided higher benefits than traditional biogas treatment by burning in combined heat and power (CHP) unit and subsequent energy production.

LCA studies on residual waste (RES) are widespread. Laurent et al. [6] presented a comprehensive review of LCA studies in the waste management field. Nearly 100 papers dealing with mixed waste were identified. The majority of them is dedicated to RES from households. For example, Arena et al. [7] compared two options of thermal treatment of RES. Dong et al. [8] analysed the environmental performance of gasification and incineration technologies treating RES. The study was based on operational data from existing plants.

However, there are papers also dedicated to biowaste treatment, where biowaste is subject to thermal treatment with air excess. In this case, technologies processing SEP-BIO and RES are analysed together, and biowaste is only a part of the input to the WtE plant. Guereca et al. [9] performed an LCA analysis of biowaste management system for the city of Barcelona. Waste-to-energy was included in the current and proposed scenario. Pubule et al. [10] analysed an optimum solution for biowaste treatment in the Baltic States area. Incineration with and without energy delivery has been included as well. Thomsen et al. [11] carried out a comparative life cycle assessment (LCA) of diverting of the organic fraction of the household waste away from waste-to-energy (WtE) plant to manure-based and sludge-based biogas plants.

The results of the diversion showed a net increase in electricity production but a decrease in heat production. Greenhouse gases emissions (GHG) expressed as global warming potential (GWP) were reduced by 10%. Di Maria et al. [12] conducted a study on the sustainability of biowaste treatment in WtE facilities using a life cycle approach and the cumulative energy demand index. The case study indicated that the treatment of biowaste in WtE plants operated in CHP was more efficient in exploiting the energy content of waste for replacing primary energies than biowaste treatment in anaerobic digestion plants. In addition, the significance of CHP proved to be a critical factor for efficient and effective waste utilization in WtE. In general, the life cycle approach is currently a widely used and favourite tool for research in waste management. Zhou et al. [13] carried out a comprehensive review of LCA tools available for WtE and provided several recommendations for their applications. Mehta et al. [14] successfully applied this principle combined with economic analysis for the assessment of multiple waste management options in Mumbai, India.

Economic performance of a WtE plant treating RES as a mixture of several components was investigated in detail in [15]. In comparison to [16], where the economic model of WtE plant addressed one ton of RES, outcomes of [15] figured out contributions of individual components like paper, plastics, biowaste. A method of the marginal cost was applied. For example, biowaste marginal cost was 160 EUR/t, whereas average of all components, which is also the cost of RES treatment, was 100 EUR/t. For comparison, the cost of plastics was 290 EUR/t. Following the same logic, Ferdan et al. [17] presented an environmental impact of a WtE plant processing RES. The contribution of individual components to the overall performance of WtE was missing. While some LCA studies above focused on biowaste treated in WtE plants, the mechanism of contribution of biowaste treated in a mixture with other components in one processing facility was not sufficiently explained.

In this paper, three ways of ecologically suitable biowaste treatment are discussed—composting, fermentation, and incineration with energy recovery in a WtE plant. The article is concerned with the environmental performance, production of GHG, with a focus on biowaste. This paper aims to compare the environmental impact of the methods mentioned above using the GWP indicator. While GHG production of aerobic and anaerobic treatment of separately collected biowaste are reviewed for comparison reasons, the contribution of biowaste component during thermal treatment of RES in WtE is investigated in detail to cope with uncertainty as mentioned above. An approach inspired by marginal cost [15] is developed, explained, and tested through a case study. Once the contribution of biowaste is known, the influence of the energy-effectiveness of the WtE plant on GHG burdens and credits related to biowaste only is also analysed. Burdens related to the performance of WtE are mainly subject to biowaste content in the input waste [17]. For example, the study [18] analysed fossil-based CO2 emissions from 10 WtE plants in Austria. Credits are bound with form and amount of energy produced in WtE. Both credits and burdens are profoundly affected by WtE location:


Based on the information above, it is suggested that these parameters also influence the feasibility of diverting of biowaste component of RES from WtE treatment to other treatment methods. This paper further explores how the WtE plant operation mode (heat-oriented, power-oriented) influences the environmental performance of the plant. Using the results, the importance of this parameter for the trade-off of the environmental performance of several biowaste treatment methods is evaluated. The published papers concerned with a similar problem (e.g., [11]) did not consider such an aspect in their studies, although the necessity of such evaluation was indicated.

In addition, the detached effect of any component on the environmental performance of WtE plant, if known and described, would be beneficial for sophisticated modelling and simulation of waste management. Bing et al. [19] highlighted the need for holistic network flow models in waste management. An example of such a model is paper [20], where flows of several municipal fractions are optimised in one complex multiobjective problem. The task demands input data and hardware since it cannot be separated due to WtE processing of all the components of RES.

Section 2 describes the treatment methods—composting, fermentation, and thermal treatment in WtE—considered in this paper, together with the specifications of the chosen treatment plants. It also explains the proposed modelling approach based on marginal change. Section 3 presents the results obtained for each treatment method and a comparison of their environmental performance.
