# What Advanced Treatments Can Be Used to Minimize the Production of Sewage Sludge in WWTPs?
## 3 Advanced Treatments and Technologies
### Chemical Treatments
#### 3.1.4 Thermochemical Hydrolysis (TCH)  
Generally, anaerobic digestion is considered to consist of three steps: hydrolysis, acidification and methanogenesis [45]. In order to promote the dissolution of organic particles and the hydrolysis of organic matters, various pre-treatment methods have been [45; 46; 47]. The TCH pre-treatment of sludges for anaerobic digestion increases biogas production, enhances volatile solids reduction, and improves dewaterability of digested sludge [48]. TCH is commonly applied in sludge pre-treatment as it is easy to control, stable in performance, and flexible during operation [47]. It is possible to operate both in the water line (with a reactor arranged on the sludge recirculation) and in the sludge line. The alkaline reagents are the most effective ones [25]. The commonly alkali chemicals used in thermo-alkaline pre-treatment are NaOH, KOH, Ca(OH)\({}_{2}\) and Mg(OH)\({}_{2}\)[45]. As reported by [PERSON] and [PERSON] [34], in fact, the drawbacks of acidic pre-treatment are: (i) bad odour generation, (ii) possible corrosion of equipment and (iii) potential inhibition to the bacteria involved in digestion. However, in contrast to the alkaline pre-treatment, acidic reagents (e.g., H\({}_{2}\)SO\({}_{4}\)) improve the dewaterability of the SS. While the application of the sludge line is decidedly widespread [45; 47; 48], the application of the water line is not very widely applied at the real scale [25]. In terms of performance, [PERSON] et al. [47] found a higher increase of soluble chemical oxygen demand (SCOD) concentration in the sludge supernatant after TCH pre-treatment, and the biodegradability of sludge was enhanced greatly. In Table 1, several applications of chemical treatments are reported. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Process** & **Time [min]** & **Operative Conditions** & **Results** & **Ref. ** \\ \hline \multirow{3}{*}{**O\({}_{3}\)**} & 10 & O\({}_{3}\) = 268 mg L\({}^{-1}\) & SSR = 30–35\% & [49] \\ \cline{2-5}  & 1–5 & O\({}_{3}\) = 300 mg g\({}_{\text{CCD}}\)\({}^{-1}\) & SRR = 25–30\% & [50] \\ \hline \multirow{3}{*}{**Chlorine**} & n.a. & CIO\({}_{2}\) = 2 mg g\({}_{\text{STS}}\)\({}^{-1}\) & SRR = 28\% & [51] \\ \cline{2-5}  & 120 & T = 200–250 \({}^{\circ}\)C; & & \\  & 120 & P\({}_{\text{O2}}\) = 8-25 atm; & n.a. & [52] \\  & & initial TSS = 8\% & & \\ \cline{2-5}  & 60 & T = 200 \({}^{\circ}\)C; & & \\  & 60 & pH = 2; & CO\({}_{\text{envonal}}\) = 50–70\%; & \\  & & P\({}_{\text{O2}}\) = 30 bar & & \\ \cline{2-5}  & 300 & +thermal hydrolysis; & CO\({}_{\text{envonal}}\) = 58\%; & \\  & 300 & T = 180 \({}^{\circ}\)C; & VSS\({}_{\text{envonal}}\) = 20–40\% & [54] \\  & & pH = 3.3 & & \\ \cline{2-5}  & 30 & T = 240 \({}^{\circ}\)C; & & \\  & & P\({}_{\text{O2}}\) = 60 bar; & & \\ \cline{2-5}  & 60 & T = 240 \({}^{\circ}\)C & TSS\({}_{\text{envonal}}\) = 85\%; & \\ \hline \multirow{3}{*}{**WO**} & \multirow{3}{*}{n.a.} & +thermophilic anaerobic digestion; & \\  & & Fe\({}^{2+}\) = 0.067 g g\({}_{\text{H2O}}\)\({}^{-1}\); & & The DM values decreased by 28.2\%. & \\  & & H\({}_{\text{O2}}\) = 60 g kg\({}_{\text{SEM}}\)\({}^{-1}\); & & The VS values decreased by 26.8\%. & [34] \\  & & pH = 3 & & \\ \cline{2-5}  & \multirow{3}{*}{60} & Fe\({}^{2+}\) = 200 mg L\({}^{-1}\); & & \\  & 60 & H\({}_{\text{O2}}\) = 8 g L\({}^{-1}\); & & 2213 mg L\({}^{-1}\) & [34] \\  & & pH = 3 & & \\ \cline{2-5}  & \multirow{3}{*}{n.a.}