**5. Discussion**

In total, 3.044 t of biodegradable (BIO) waste was stored. In accordance with the earlier description, this waste originated from selective collection performed in individual households, collection from the gastronomic sector and green area cultivation. The quality of BIO waste justifies its delivery to BSAC provided that any wood waste that was not used for the production of wood chips as structure-forming material in any composting process was directed to the CSP [79]. As there is no accurate data to that effect, this stream of waste was omitted by putting 0 in Equation (6). In waste from selective collection, amounting to 100 tons/year, waste window and door woodwork as well as worn furniture make up 25%, which makes stream mass *mi*3 equal to 25 t/a.

In the Koszalin-Jamno wastewater treatment plant featuring throughput of 36,000 m3/d, approximately 71 tons of mechanically dehydrated sewage sludge (*mSS*) with 21% *dm* is produced during a day. The predisposed fermentation technique BOF, due to the high concentration of dry matter, is dry fermentation, which proceeds in low-hydration conditions below 85% [80]. This allows for a considerable reduction in reactor volumetric capacity compared to wet fermentation that is performed at a hydration level exceeding 94% [81]. Considering the composition and structure of BOF, this waste, being a coarsegrained raw material, requires preliminary preparation, i.e., comminution. The optimum degree of comminution of the organic fraction for the dry fermentation process should take into account the maximum share of granules with dimensions falling into the 20 to 40 mm interval [82]. The impact of comminution on the increase in fermentation gas yield is not clear-cut. The majority of the available sources of information indicate that the increase in the active surface of organic particles through material comminution facilitates access of microorganisms to nutritive substrates, thus improving the process conditions [83]. The ERWP model is based on the assumption that BOF arrives in its entirety to the DFI module of the GPP. The required content of water for the dry fermentation process, due to the low humidity of BOF at 28%, is secured by supplementation of mechanically dehydrated sewage sludge with 79% water content. To determine the mass of sewage sludge *mf*1 and *mf*2 directed to the methane fermentation process, Equations (1) and (2) were used, respectively. Based on these equations, it was found that 79 t of sewage sludge containing 95% water is directed to the wet fermentation process, whereas 28 tons of sewage sludge containing 81% water mixed with 20.3 tons of the organic fraction separated from mixed waste makes up a mass of daily charge into the dry fermentation process. The decrement of the dry mass of sewage sludge in the methane fermentation process is 31–35%, which causes a decrease in the heat of combustion on average by 22% [69]. The mass of post-fermenter *mi*1 amounting to 43.1 t was calculated using Equation (4).

The volume of gas generated in the GPP in both VFI and DFI modules was calculated using the unit indicators presented in Table 5. In the case of wet fermentation performed in the VFI module, an indicator of 60.6 dm3/kg *dm* was used as an average value of two empirical indicators given by [72,73]. In both cases, the fermentation process proceeded in mesophilic conditions and a similar period of charge was used in the HRT reactor. The generation of gas in the DFI dry co-fermentation process using a mixture of sewage sludge and

organic fraction separated from municipal waste (Group No III) was estimated using an indicator value of GPP = 265.0 dm3/kg *dm* [76]. The adopted indicator corresponds to the value attained on an industrial scale in a methane fermentation plant based on the LARAN technique employed in the municipality of Tychy (Poland), where the average daily production of gas in mesophilic conditions at HRT = 20 days is 266.3 dm3/kg *dm* [44]. A slightly lower gas yield obtained from a dry fermentation process amounting to 215.0 dm3/kg *dm* has been observed in laboratory conditions [73]. For the indicators assumed, the volume of gas generated in WFI and DFI modules is 239.37 and 5280.06 m3/d, respectively. The energy equivalent to the volume of gas generated in both modules of methane fermentation was calculated by assuming that the combustion heat was 22.5 MJ/m<sup>3</sup> [42] (Table 7).


**Table 7.** Mass and energy production using ERWP model.

To CSP generating thermal energy and electricity directed the remains from the dry fermentation process (*mi*1), Group No II (*mi*2) and door/window woodwork as well as waste furniture, jointly making a high-calorie raw material SRM (*mi*3). The content of organic matter in the waste stream *mi*1, determined as a weighted average in sewage sludge and BOF (Group No II) after the fermentation process, causing partial oxidation of *so*, was 60.56%. To calculate HHV for *mi*1 and *mi*2, Equations (7) and (8) were used, respectively. Contents of carbon and nitrogen in Group No II determined in the elementary analysis method are similar to the results of research published in numerous papers.For example, the share of carbon determined in the elementary analysis of C, N, O, H, P was 47.81% [48] and 44.72% [63]. HHV values calculated from regression equations or determined in the heat analysis indicated in the papers cited above were 23.19 and 19.50 MJ/kg *dm*, respectively whereas the HHV value found in my own research work, calculated from Equation (7), was 24.5 MJ/kg *dm.* The balance of raw materials mass and energy from their processing using the ERWP model's energy generation processes is presented in Table 7.
