**1. Introduction**

This paper pertains to municipal waste that may have a negative impact on the natural environment and human health [1–3]. Waste managemen<sup>t</sup> is a global problem originating from economic development [4–7]. Globalisation processes have contributed, through the systematic increase in the interlinkage between various markets and in numerous aspects of economic and social life, to the establishment of a new institutional order, new institutions and legal and economic solutions [8,9]. Such processes also promote increases in outlays on investment projects [10,11], boosting the level of innovation and competitiveness of economies [12–16]. The fact that in the last thirty years an economic convergence has occurred for the majority of countries is indicated in the literature [17,18]. The economic convergence processes contributed to permanent changes in consumers' attitudes and awareness, particularly in terms of sustainable consumption when considering concerns about energy consumption and the natural environment [19–23]. Energetic transformation embraces the majority of economies, including agriculture, in which use of biocomponents produced from organic waste plays a significant role [24,25]. This means that waste managemen<sup>t</sup> is equally important for big metropolitan areas and small rural communities [26,27]. An observed trend of the use of waste as a source of valuable raw materials

requires development of a functioning waste managemen<sup>t</sup> model, taking into consideration the possibilities and needs of a given region [28].

In the article, practical and theoretical aspects of the municipal waste elimination for the city of Koszalin based on an effective functioning waste managemen<sup>t</sup> system will be considered. The main objective of this paper is the presentation of the ERWP model to be used as a complementary waste managemen<sup>t</sup> system. The effectiveness of a system based on the ERWP model has been discussed in relation to Koszalin city (Poland).

It should be underlined that the volume of waste generated worldwide increases systematically, ye<sup>t</sup> a significant part of it is not covered by the system, thus causing quantifiable losses in the economy and natural environment [29,30]. In 2018 in the European Union, 2538 million t of municipal waste was generated, and 55% was recycled [31]. The remainder was put in landfills, causing their degradation to a variable degree [32]. The absence of proper industrial infrastructure for waste processing, mainly in the eastern EU countries, is still the main reason why 40 ÷ 45% of waste does not go to local waste treatment systems, resulting in wasting of valuable sources of recyclable and energetic materials [33,34]. This results in an increase in costs of economic system functioning that is different depending on the solutions adopted [35]. In Poland, almost 42% of generated municipal waste is directly put into landfills [36].

The ERWP model takes into account a number of conditions, among which the most important ones are: waste type, physicochemical properties, morphological composition and volume. Irrespective of the values of the above parameters, such systems are always based on the application of well-known unit processes, such as: screening, separation, biological treatment, dehydration, thermal transformation and storage. The proper compilation of processes that make up a complementary system depends on the adopted objective that is contingent upon financial and technical possibilities. Such an objective can be, for example, the recovery of valuable waste components within the material or organic or energetic recycling framework [37]. Ultimately, the objective can also be rendering such waste harmless through its storage in controlled conditions. However, taking into account waste composition and its physicochemical properties, this method is economically ineffective and, importantly, has a negative impact on the natural environment [38,39].

### **2. Potential of Municipal Waste to Energy Production—Review**

The introduction indicates the need for the construction of a waste managemen<sup>t</sup> system for any local governmen<sup>t</sup> unit. The main task of such a system is to use waste as a source of raw materials including raw materials for energy production. The volume of energy generated from waste depends, first and foremost, on the methods of processing, allowing for the recovery of energy in variable forms, generally in the form of heat generated in the combustion process [40,41]. Waste can be, as a source of energy, a significant element of the local energetic balance [42]. A good example here is the municipality of Copenhagen, which adopted in 2011 a strategy for development until 2025, which will eliminate the use of coal as the energetic raw material [43]. The share of energy generated from municipal waste in Copenhagen's energetic mix will finally be approximately 40%.

In 2018, 12.5 million t of mixed municipal waste was stored in Poland. Most of the waste, i.e., 9971.2 thousand t, was generated in households, which made up 83% of total generated waste. The remaining part of the waste, i.e., 1997.5 thousand t of household and commercial types, collected from the servicing of municipal infrastructure and entrepreneurs, amounted to 17%. Analysis of the morphological composition of waste delivered to plants using methane-biological processing technology (MBT), based on results of research work performed in 20 plants located in Poland, shows a grea<sup>t</sup> potential for broadly understood recovery [44]. Classification of particular waste components in terms of their use leads to distinguishing the following groups: Group No I—recyclable materials (glass, metals, synthetic materials qualified for recycling), Group No II—waste having high energetic value (paper, cardboard, textiles, composites and synthetic materials not qualified for recycling, as well as wood), Group No III—biodegradable waste (BIO1, BIO2, BIO3) and

Group No IV—waste classified as useless or dangerous. Percentage shares of particular waste components in mixed waste as well as in fractions separated with an 80–90 mm mesh screen, as average values obtained from research work performed in various plants in Poland, are shown in Table 1.


**Table 1.** Percentage shares of waste components and classification into four different groups.

(1) Own study.

Waste possessing values of recyclable materials (Group No I) recovered for mixed waste during the manual separation process can be obtained, as a rule, exclusively from the oversize fraction separated in, e.g., rotary drum sifters featuring mesh not less than 8 cm; the total content is 37.2%. A method of using Group No II, defined as RDF, considering its high calorific value of 18–24 MJ/kg, is combustion [44]. The application of screening and mechanical separation of mixed waste resulting in energetic raw material in RDF form allows, in the extreme case, for an increase in the calorific value from 8.4 to 25.0 MJ/kg [45]. A calorific value exceeding 11 MJ/kg guarantees, in principle, energetic efficiency of combustion or gasification exceeding 65%, which allows for classification of the process as energy recovery [46]. According to the International Energy Agency, the calorific value of waste to be used in combustion processes should not be, for process profitability, below 7942 kJ/kg [47]. In Germany, one of the biggest EU economies, the volume of RDF separated in MTP installations increased from 31% in 2006 [48] to 34.2% in 2017 [49]. A significant criterion for the application of the available techniques of thermal transformation of mixed municipal waste is relatively high humidity, which reduces their calorific value. The use of more advanced techniques such as, for example, pyrolysis or gasification, requires higher calorific value of waste; this is associated with a necessity to apply proper methods of batch preparation [50,51]. Unfortunately, pyrolytic installations of an industrial scale used for waste processing are unreliable, which has been proved by plants closing shortly after being put into operation. For example, THERMOSEL 2002 (opened)/2006 (closed), DBA 2001/2010, EDDITH 2002/2009 and Schwel-Brenn 1997/2000 [52,53].

Group No III comprises biodegradable waste, i.e., waste that can be subjected to biological gasification in the methane fermentation process. During decomposition of organic matter under controlled anaerobic conditions, biogas is generated, which contains flammable components, including methane. The share of biogas production in European Union countries makes up 136.6 million tons of oil equivalent [54]. The content of methane, depending on the raw material, is 50–75% and the calorific value is, on average, 22 MJ/m<sup>3</sup> [55,56]. In the case of the organic fraction separated from municipal waste, the yield of biogas volume in the plants that use the methane fermentation process in low

hydration conditions (DFI) reaches 339 m<sup>3</sup> CH4/Mg organic matter [57]. A possibility to generate biogas, resulting in a reduction in waste processing costs will be, in future, the primary factor deciding the selection of this biological waste processing method. Waste that is also suitable for the methane transformation process is sewage sludge (SS), originating from municipal wastewater treatment plants. In 2018 in Poland, 640,000 t of dry matter from sewage sludge was generated. This is equivalent to approx. 25 million tons of mechanically dehydrated sludge to 80% humidity on average [36]. The volume of methane generated during the fermentation process carried out in separate chambers in high-hydration conditions is 0.19–0.24 m3/kg organic matter [58]. The application of the thermal method for the disposal of sewage sludge requires a reduction in its humidity to at least 10%. This means that the consumption of energy for mechanical dehydration, then water evaporation from sludge with an initial humidity of 98%, is very high. Other sorts of waste, which due to their properties may constitute a raw material for energy generating processes in various forms, are generated in populated areas; they comprise, among other things, waste from selective collection, including biowaste, organic waste from green area cultivation and biodegradable waste from production and foodstuff processing as well as flammable packaging waste with low value as recyclable material.

The diversity of municipal waste means that optimisation of the system of energy generation from waste should take into account not just waste fuel properties but also its morphological features. This also means there is a necessity to apply various methods for the preparation of the raw material earmarked for the generation of energy in the form of heat, electricity and gas. Using (i) available techniques and (ii) applying the principle of cooperation between the waste generating and processing entities [59], an ERWP model was developed; it allows for assessment of the volume of energy generated from the processing of amassed waste independently of its volume, type and specific features. Empirical equations describing the impact of selected variables on the volume of generated energy and values of empirical indicators describing energetic efficiency of particular processes, achieved both in industrial plants as well as used in scientific research, were used for setting the energy balance. ERWP reflects the circular economy idea, which promotes the maximum usage of available raw materials in line with a rule that waste becomes a raw material for the next production cycle [60,61].

Waste-to-energy (WtE) plants are an integral part of the circular economy strategy in the treatment of non-recyclable waste. Waste with a high potential for thermal gasification or biogas production is converted into heat and electric energy. The ERWP model takes into account two of the six defined trends of the WtE strategy, i.e., more gasification plants offering commercial-scale operations and a push to use organic waste to replace natural gas [62].
