## 2 Analysis of the Potential for the Increase in Yeast Production and Results
\[S_{9}=S_{1}+S_{4}+S_{5}+S_{7}-S_{8}. \tag{1}\]0  
The nitrogen concentration in the \(S_{9}\) stream (after a part \(n\) of the \(S_{4}\) stream is directed to the ES) is described (based on Equations (3) and (18)) by  
\[S_{9}=S_{1}+S_{4}+S_{5}+S_{7}-S_{8}. \tag{1}\]1  
Taking into account Equations (4) and (19) (and omitting the \(S_{1}\) stream), the \(C_{9}\) nitrogen concentration, after directing the \(n\) part of the \(S_{4}\) stream to the ES, can be described as  
\[S_{9}=S_{1}+S_{4}+S_{5}+S_{7}-S_{8}. \tag{1}\]2  
The first part of this paper involved the determination of the theoretical increase in yeast production, on the basis of an assumption regarding complete nitrogen neutralization taking place in ES. This means that concentration expressed as \(C_{7}\) is equal to 0. Thus, Equation (20) can be written in the form  
\[S_{9}=S_{1}+S_{4}+S_{5}+S_{7}-S_{8}. \tag{1}\]3  
The mass of nitrogen \(m_{NAF}\), which is directed annually to the agricultural fields (AF), can be described as  
\[S_{9}=S_{1}+S_{4}+S_{5}+S_{7}-S_{8}. \tag{1}\]4  
It is possible to calculate the amount of nitrogen in the process wastewater stream (directed to the AF) per 1 Mg of the yeast production during the actual yeast production:  
\[S_{9}=S_{1}+S_{4}+S_{5}+S_{7}-S_{8}. \tag{1}\]5  
Taking into account Equations (22) and (23), we can calculate the total amount of nitrogen in the process wastewater stream (\(m_{NPYplus}\)) per 1 Mg of yeast production, during yeast production, after directing the \(nS_{4}\) stream to the ES:  
\[S_{9}=S_{1}+S_{4}+S_{5}+S_{7}-S_{8}. \tag{1}\]6  
where  
\(m_{NPYplus}\) is the total amount of nitrogen in the process wastewater stream (directed to the AF) per 1 Mg of yeast production, after directing the \(nS_{4}\) stream to the ES [kg/Mg]; and \(Y_{pplus}\) is the yeast production volume, after directing the \(nS_{4}\) stream to the ES [Mg/yr]. Based on Equations (16) and (22)-(24), it can be assumed that  
\[S_{9}=S_{1}+S_{4}+S_{5}+S_{7}-S_{8}. \tag{1}\]7  
Therefore,  
\[S_{9}=S_{1}+S_{4}+S_{5}+S_{7}-S_{8}. \tag{1}\]8  
Taking into account Equations (23) and (24), the ratio of production after directing the \(nS_{4}\) stream to the ES to the current production, can be described as  
\[S_{9}=S_{1}+S_{4}+S_{5}+S_{7}-S_{8}. \tag{1}\]9The increase in yeast production (\(\eta_{Yp}\)) after directing the \(nS_{4}\) stream to the ES can be described as  
\[S_{9}C_{9}=S_{1}C_{1}+S_{4}C_{4}+S_{5}C_{5}+S_{7}C_{7}-S_{8}C_{8}, \tag{2}\]0  
Therefore,  
\[S_{9}C_{9}=S_{1}C_{1}+S_{4}C_{4}+S_{5}C_{5}+S_{7}C_{7}-S_{8}C_{8}, \tag{2}\]1  
In addition, along with an increase in yeast production (\(\eta_{Yp}\)) after the stream \(nS_{4}\) is routed to ES, at the same time, an additional volume of pollutants (mainly nitrogen, but also potassium or organic substances) is derived as the organic fertilizer \(F_{p}\) (Figures 1B and 2--\(S_{13}\)). The increase in the amount of fertilizer \(\eta_{FP}\) is relative to the additional volume \(nS_{4}\) of the process wastewater that is routed to the ES, and is directly proportional to the increase in yeast production \(\eta_{Yp}\). The \(F_{p}\) organic fertilizer obtained in the ES has a dark brown color and a liquid-honey consistency. This fertilizer meets all standards for substances entering the soil [27; 28]. Moreover, this product is readily used by the local farmers as an organic fertilizer and as an animal feed additive [28; 29]. This fertilizer is introduced into the soil mainly during plowing. The increase in production of both \(Y_{p}\) and \(F_{p}\) was analyzed, assuming a constant surface area of the AF, as well as constant crops. In the yeast factory adopted in this analysis, the area of AF is equal to 1800 ha. Industrial and cereal crops (in the ratio of 3:2) are cultivated in these AF. On this basis, the possible increase in yeast and fertilizer production could be estimated on the basis of the relation in (29).