## 3 Discussion and Conclusions  
The analysis conducted for the purposes of the present study demonstrates that it is possible to increase yeast production (\(\eta_{YP}\)) by directing an additional stream of process wastewater (\(nS_{4}\)) into the ES. This increase can be achieved by the application of the current area of agricultural fields (\(A_{AF}\)), which are located on the site of the yeast plant. At the same time (like \(\eta_{YP}\)), the same increase in the volume of organic fertilizer \(\eta_{FP}\) can be achieved. This fertilizer (\(F_{P}\)) is generated as a byproduct of the pre-treatment of wastewater at the ES. Theoretically, it is possible to obtain a significant increase in yeast production \(\eta_{YP}\) = 0.6057 (Figure 3, black line). However, if this level should be assumed, a complete neutralization of nitrogen in the ES needs to be achieved (theoretically increasing yeast production). Using the parameters of the actual reduction of nitrogen concentration in the ES of the analyzed factory, the maximum production increase was reached \(\eta_{YP}\) = 0.1027 (Figure 3, red line). This is the maximum increase in production obtained for \(n\) = 1 (i.e., where \(S_{4}\) is completely directed into the ES). In the analyzed plant, 6.46 m\({}^{3}\) of wastewater is generated per 1 Mg of yeast output (data from the factory). If we know the amount of process wastewater generated during actual production (Table 1), we can determine the annual production \(Y_{P}\), amounting to about 64,000 Mg. The increase in production (\(\eta_{YP}\)) at just over 0.1 corresponds to about 6500 Mg of yeast per year. This is the feasible value that can be derived from the existing area of AF and the ES located at the factory. In this case, there is no need to intervene in the existing AF-ES-WWTP system; thus, this is a beneficial solution for the operation of the yeast factory. As a result of the increase in the amount of process wastewater applied to the evaporator station (\(nS_{4}\)), the ES is faced with the need to apply greater volumes of energy to operate. However, the analyzed factory includes a boiler burning biomass. In this way, a reduction in the operating costs of the evaporator station is feasible. Figure 3: The increase in yeast production \(\eta_{YP}\) and fertilizer production \(\eta_{TP}\) versus the volume fraction \(n\) (the part of stream \(S_{4}\) directed to the ES). The dotted line represents the theoretical increase, and the red line is the real increase possible to achieve, considering the data from the analyzed factory (at current efficiency of the ES). In addition, the cost of using an ES can be compensated as a consequence of the increase in revenues resulting from the increase in yeast and fertilizer production (\(\eta_{\gamma p}\) and \(\eta_{\gamma p}\)). In addition, the increase in fertilizer production (\(\eta_{\gamma p}\)) will have a positive effect, not only on the productivity of the AF at the plant, but also on the fields of individual farmers locally. This fertilizer (\(F_{p}\)) can be used in large quantities to provide a significant enrichment of soils with organic substances. Concurrently, the possibility of the cultivation of energy crops (requiring a greater input of nitrogen) on the AF at the plant will offer the potential to deposit larger amount of process yeast wastewater in these fields. This alternative can lead to a theoretical further increase in the yeast (\(Y_{p}\)) and fertilizer (\(F_{p}\).) output. By contrast, the cultivation of energy plants located in the AF of the factory will lead to a decrease in the operating costs of the ES. Data curation, B.W. ; Investigation, P.P.W. and B.W. ; Methodology, P.P.W. ; Supervision, P.P.W. ; Analyzed the data, P.P.W. and B.W. Wrote the paper; P.P.W. and B.W. This research received no external funding. This work was created as a result of scientific internship of the authors in the Lesaffre Polska yeast factory (Wolczyn, Poland) and in the Poltava State Agrarian Academy (Poltava, Ukraine).