# A Neural Network Model for Decision-Making with Application in Sewage Sludge Management
## 3 Materials and Methods  
The SS treatment is mainly carried out to reduce the sludge weight and volume, and to stabilize its biological part, leading to reduced disposal cost, environmental impact, and risk of human health problems. The main phases of a SS treatment system (SSTS) can be summarized in six steps (Figure 3). Many strengths and limitations are included for each alternative; the literature review provides an interesting indication of the range of values possibly associated with each unit process. In this section, only a synthetic overview of each alternative's benefits and limits is shown. If, on the one hand, multiple elements can lead the decision-maker to a particular solution, then, on the other hand, the studies provide for different cases a clear indication of the most effective sludge management strategy under the economic and environmental perspective. The strengths and the limitations of alternatives considered are independent of specific conditions related to the place where the plants are located. The research conducted mainly focuses on the decision recommended by studies rather than on the specific aspects that affected the decision-maker's choice. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Reference** & \multicolumn{3}{c}{**Dimension**} & \multicolumn{3}{c}{**MCDM**} & \multicolumn{1}{c}{**Method Features**} \\ \hline  & **Tec.** & **Ec.** & **Env.** & **Soc.** & & \\ \hline [PERSON] et al. [26] & \(\surd\) & \(\surd\) & \(\surd\) & \(\surd\) & \(\surd\) & Multi-attribute analysis \\ \hline [PERSON] et al. [8] & \(\surd\) & \(\surd\) & \(\surd\) & \(\surd\) & \(\surd\) & Multi-attribute analysis \\ \hline [PERSON] et al. [27] & \(\surd\) & \(\surd\) & \(\surd\) & \(\surd\) & \(\surd\) & DEMATEL \\ \hline \hline \end{tabular} Note: technological (Tec.), economic (Ec.), environmental (Env.), and social (Soc.).  
\end{table} 
Table 1: _Cont._  
Figure 3: Summary of the unit operations for each phase of the SSTS. The ANN model neglects the alternatives available to carry out the phases included in the boxes with the dashed line. The first one is thickening (Th\({}_{\text{h}}\)), which consists of increasing the solids concentration of the SS. The most commonly used thickening processes include sedimentation (Th\({}_{\text{1}}\)) or gravity thickening (Th\({}_{\text{2}}\)) or dissolved air flotation (Th\({}_{\text{3}}\)) or hybrid, i.e., Th\({}_{\text{2}}\) and gravity belt thickening (Th\({}_{\text{4}}\)) for primary and secondary sludge, respectively. The strengths and limitations of each alternative are shown in Table 2.  
Once the sludge is thickened, four options are identified for the stabilization, allowing us to reduce pathogens, eliminate odor, reduce organic matters, and prevent or inhibit future decomposition. The possible operations considered by the proposed model are lime stabilization (Sb\({}_{\text{1}}\)), aerobic digestion (Sb\({}_{\text{2}}\)), anaerobic digestion (Sb\({}_{\text{3}}\)), or hybrid (i.e., Sb\({}_{\text{3}}\) for primary and Sb\({}_{\text{2}}\) secondary sludge), Sb\({}_{\text{4}}\). Below are summarized the strengths and limitations of each described alternative (Table 3)  
The third step of the SS treatment consists of sludge dewatering through facilities such as drying beds (Dw\({}_{\text{1}}\)), filter press (Dw\({}_{\text{2}}\)), or belt press (Dw\({}_{\text{3}}\)), or through centrifugation (Dw\({}_{\text{4}}\)). The proposed ANN model does not consider the existing technologies related to