## 3 Results and Discussion
### Techno-Economic Analysis of Ethanol Fermentations  
Techno-economic analysis of ethanol fermentations at different nutritional contents was analyzed by calculating the economic yield (EY) and economic productivity (EP) from the experimental and predicted values. The major cost factors in the media used in fermentation are carbon, nitrogen and mineral sources [30]. Therefore, they were evaluated in economic analysis. The cost information of the components used in the fermentation medium and ethanol were taken from Sigma-Aldrich (www.sigmaaldrich.com) and given in Table 4. Besides, lower prices could be found for industrial grade ingredients at the commercial scale. Figure 4 shows the experimental and predicted EY and EP values for each nutritional composition. It is obvious that Medium E has indicated high EY (5.763 and 6.708 \(\varPhi\) of ethanol/\(\varPhi\) of nutrient for experimental and predicted, respectively) and EP (0.266 and 0.283 \(\varPhi\) of ethanol/\(\varPhi\) of nutrient.h for experimental and predicted, respectively) over Media A, B, C and D (Figure 4a,b). Media A and B gave very poor EY (0.367 and 0.432 \(\varPhi\) of ethanol/\(\varPhi\) of nutrient for experimental and 0.372 and 0.460 \(\varPhi\) of ethanol/\(\varPhi\) of nutrient for predicted, respectively) and EP (0.029 and 0.030 \(\varPhi\) of ethanol/\(\varPhi\) of nutrient.h for experimental and 0.031 and 0.028 \(\varPhi\) of ethanol/\(\varPhi\) of nutrient.h for predicted, respectively) values (Figure 4a,b). Media C and D also provided a significant increase in EY (1.115 and 2.858 \(\varPhi\) of ethanol/\(\varPhi\) of nutrient for experimental and 1.102 and 2.807 \(\varPhi\) of ethanol/\(\varPhi\) of nutrient for predicted, respectively) and EP (0.058 and 0.180 \(\varPhi\) of ethanol/\(\varPhi\) of nutrient.hfor experimental and 0.054 and 0.178 \(\upxi\) of ethanol/\(\upxi\) of nutrient.h for predicted, respectively) values (Figure 4a,b). Accordingly, in terms of both experimental and predicted results, Medium E resulted in maximum EY and EP in the biofilm reactor. Additionally, the experimental and predicted EY and EP values would ensure foresight in the selection of medium composition for the production of economic ethanol. To study the variation in EY and EP during the course of the fermentation, dynamic ethanol values yielded during the experimental and kinetic studies (Figure 3) were employed and the results were demonstrated on Figure 5. Based on the results, the experimental and predicted EY values showed a similar trend with the experimental and predicted ethanol production data. On the other hand, since the experimental and predicted EY values of Media A and B during the whole fermentation period were less than 1 \(\upxi\) of ethanol/\(\upxi\) of nutrient (Figure 5a,b), it \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Medium components** & **Cost/g (\(\upxi\))** & **Cost/g (\(\upxi\))** & **CAS Number** \\ \hline Carob pod & 0.000440 & 0.002621 & \\ Yeast Extract & 0.260000 & 1.548950 & 8013012 \\ (NH\({}_{4}\))\({}_{2}\)SO\({}_{4}\) & 0.093400 & 0.556431 & 7783202 \\ KH\({}_{2}\)PO\({}_{4}\) & 0.055600 & 0.331237 & 7778770 \\ MgSO\({}_{4}\)7H\({}_{2}\)O & 0.059600 & 0.355067 & 10034998 \\ CaCl\({}_{2}\)2H\({}_{2}\)O & 0.056200 & 0.334812 & 10035048 \\ \hline \hline \end{tabular} Online trading cost of chemicals used in the media as of 13 th February 2019 on Sigmaaldrich.com Online bull trading cost of carob pods (kibbled and broken) as of 13 th February 2019 on Alibaba.com Selling price of ethanol (CAS Number: 0000064175) = 0.03207 €/g = 0.1911 €/g on Sigmaaldrich.com 1 \(\upxi\) = 5.9575 € (As of 13 th February, 2019)  
\end{table} 
Table 4: Cost of nutrients used in different media compositions. Figure 4: Economic yield (EY, **a**) and economic productivity (EP, **b**) of ethanol on different nutritional compositions. indicates that ethanol production using Media A and B in a biofilm reactor is not economical.