# What Advanced Treatments Can Be Used to Minimize the Production of Sewage Sludge in WWTPs?
## 4 Summary  
In the following review, advanced treatments for the minimization of SS production have been analysed. Chemical, biological, thermal and electrochemical processes have been reported (Table 5). Chemical treatments such as ozonation or the use of chlorine certainly have the advantage of representing a mature technology. However, if adopted in the water line, they can easily alter the balance within the biological sector. WO is one of the most interesting of the advanced chemical processes from the point of view of performance (in terms of sludge reduction). However, it should be remembered that it requires highly qualified personnel for its management. The Fenton and Photo-Fenton processes are still applied to the pilot scale only. The great disadvantage is that chemical sludge is inevitably produced, which must then be properly disposed of. TCH is currently adopted at the real scale, as pre-treatment before the anaerobic digestion in the sludge line, in order to maximize the stabilization of the SS. One of the main disadvantages is due to the cost for chemical reagents for enhancing the hydrolysis pre-treatment. Among the biological processes, interesting results are given, for example, by the application of the MMBRs in the water line that allow a significant reduction of the sludge produced compared to the CAS. For these systems, which are already widely adopted at the real scale, fouling remains the main problem. On the other hand, despite having shown significant results, especially if applied in the sludge line, the \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Initial DM** & **Final DM [\%]** & **Time Dim** & **Pressure [kPa]** & **Voltage [V]** & **Energy Consumption [kWh kgwater-1]** & **References** \\ \hline
16.6 & 40 & 7.5 & 100 & 20 & 0.065 & [121] \\
20–25 & 40–50 & 10–14 & 25.5 & 30–40 & 0.06–0.18 & [125] \\
22 & 37.5 & 30 & n.a. & 15 & n.a. & [128] \\  & 40 & & & 25 & n.a. & \\  & 46 & & & 35 & n.a. & \\ n.a. & 40 & 16 & n.a. & 25 & n.a. & [129] \\  & 45 & & & 35 & n.a. & \\
18 & 40 & 25 & 300 & 10–20 & 0.123 & [122] \\
9–10 & 40–50 & 120 & 500 & n.a. & n.a. & [124] \\ \hline \hline \end{tabular} DM: Dry matter; n.a.: Not available.  
\end{table} 
Table 4: Several applications of electro-osmosis processes. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Technique**} & \multicolumn{3}{c}{**Use**} & **Maturity of Technology** & **Effect** \\ \cline{3-5}  & & **Water Line** & **Sludge Line** & **[Scale]** & **Effect** \\ \hline \multirow{4}{*}{**Chemical**} & (O\({}_{3}\)) & X & X & Real & (B) \\  & WO & & X & Real & (B) (C) \\  & Fenton & X & X & Laboratory & (B) \\  & TCH [1] & X & X & Laboratory (in water line) & (B) \\  & & & & Real (in sludge line) & \\ \hline \multirow{4}{*}{**Biological**} & MMBR & X & & Real & (A) \\  & TMBR & X & X & Real & In the water line (A) \\  & GSS & X & & Real & (A) \\  & BP & X & X & Real (in water line) & (B) \\  & & & & Laboratory (in sludge line) & \\ \hline \multirow{2}{*}{**Thermal**} & Combustion & & X & Real & (B) (C) \\  & Pyrosis & & X & Laboratory & (B) (C) \\  & Gasification & & X & Laboratory & (B) (C) \\ \hline
**Electrochemical** & Electro-Osmosis & & X & Laboratory & (C) \\ \hline \hline \end{tabular} (A): reduction of the production of sludge in water line; (B) reduction of the fraction of VS; (C) dewatering. (1): In the sludge line, it must be coupled with anaerobic digestion.  
\end{table} 
Table 5: Uses and yields of advanced treatments and technology for the minimization of SS and maturity of technology [7; 11; 21; 25; 45; 82; 83; 115; 123; 130; 131]. TMBRs do not yet have a high diffusion. Finally, among the advanced biological processes, the BP is presented. This has given interesting results in terms of minimization yields of SS but nevertheless there are not yet a significant number of applications at the real scale (mostly examples exist on a pilot scale). The research has shown increasing interest in recent years for thermal processes.