**2.4 Environmental factors**

The environmental conditions in which plants grow play a dominant role in determining the mineral (impurity) contents of the feedstocks. These environmental factors include soil type, warm or cold weather conditions, rain or irrigation water supply, and location (latitude, longitude, or altitude). For example, switchgrass grown on clay soil has shown higher Si contents than that grown on sandy soils.

### **2.5 Type of industrial process**

The high level of impurities in the vinasse, given in **Figure 1**, is mainly due to the influence of the industrial process used. **Figure 2** shows a simplified schematic diagram of the integrated sugar-ethanol production process. The concentrations of impurities in the crushed sugarcane and byproducts, i.e., bagasse, filter cake, molasses, and vinasse, from the integrated process are as shown in **Figure 1**. Detailed descriptions of the integrated process and the fate of impurities in the process are available from Dirbeba et al. [8]. Here, the influence of the process steps on the concentrations of the major impurities in the various byproducts is described.

As seen from **Figure 1**, vinasse > filter cake > molasses > crushed cane > bagasse in terms of the total concentration of impurities. These variations in the concentration of impurities in the cane and byproducts from the integrated process arise mainly from the sugar and ethanol production process steps shown in **Figure 2**. The influences of the process units are summarized as follows: (1) The low total concentration of impurities in the bagasse is due to the leaching of most of the impurities from the crushed cane by the imbibition water added during the milling and extraction process step. (2) The raw juice treatment involves first heating the juice, then liming and sulfiting it with quick lime (CaO) and SO2 gas, respectively, and finally sedimenting and filtering the treated juice to remove soils, sediments, and other suspended solids in it as a filter cake. This and the removal of most of the CaO and SO2 added for the juice treatment with the filter cake as sulfites and phosphates of calcium make the

**Figure 2.** *Simplified schematic diagram of the integrated sugar-ethanol process.*

concentration of impurities in the filter cake considerably high. (3) Vinasse, the final byproduct from the integrated process, has the highest concentration of impurities. There are two main process-related causes for the high level of impurities in the vinasse: First, most of the impurities (ash-forming elements) left in the treated juice end up in the vinasse while the organic fractions in the juice are removed as products, sugar, and ethanol, leaving the vinasse concentrated with the impurities. Second, other chemicals such as H2SO4, UREA, and DAP are added during the molasses fermentation stage. These chemicals contain some impurities, S, N, and P, that are partly removed with the vinasse.

Another example of an industrial process where the process steps influence the concentration of impurities in the by-product is a pulp mill. In a pulp mill, the pulp is the main product and black liquor is the sidestream. As seen in **Figure 1**, the total concentration of impurities in the black liquor is very high, whereas the levels of the impurities in the input feedstocks, woody biomasses, for pulp mills are low. The high concentration of impurities in the black liquor is due to the addition of wood pulping chemicals, NaOH and Na2S, during the wood digestion process.
