# Thermal Hydrolysis of Sewage Sludge: A Case Study of a WWTP in Burgos, Spain
## 1 Introduction
\begin{table}
\begin{tabular}{c c c c} \hline
**Process** & **Characteristics** & **Results** & **References** \\ \hline \multirow{6}{*}{Advanced thermal hydrolysis process} & Secondary sludge & Better dewaterability when H\({}_{2}\)O\({}_{2}\) is & \\  & Direct live steam injection & added to the thermal hydrolysis & \\  & Temperature range: 86–164 \({}^{\circ}\)C & process & [2] \\  & Maximum pressure: 8.3 bar & Sludge solubilisation: 10–70\% & \\  & Time: 21–34 min & Organic removal: 40–60\% & \\  & H\({}_{2}\)O\({}_{2}\) & & \\ \hline \multirow{6}{*}{Thermal hydrolysis} & Mixed sludge (33\% primary–67\% & \\  & secondary) & Increase in methane yield: 70\% & \\  & Temperature range: 110–187 \({}^{\circ}\)C & Increase in biodegradability: 70\% & \\  & Time: 20–40 min & & \\ \hline \multirow{2}{*}{Thermal hydrolysis (review of several studies)} & Sewage sludge & VS removal: 38–49.5\% & \\  & Temperature range: 100–200 \({}^{\circ}\)C & Increase in methane production: & \\  & & 29–57\% & \\ \hline \multirow{6}{*}{Low and high temperature} & Secondary sludge & \multirow{2}{*}{COD solubilisation: 9 times increase} & \\  & Low temperature: 60–90 \({}^{\circ}\)C & & \\  & Time: 1–72 h & Ammonia: 2–3 times increase & \\  & High temperature: 120–180 \({}^{\circ}\)C & Biogas: 0–16.5\% increase & \\  & Time: 15–180 min & & \\ \hline Thermal hydrolysis & Sewage sludge & \multirow{2}{*}{Methane yield: 43\% increase} & [5] \\  & Energy applied: 488–2700 kJ/L & & \\ \hline \multirow{6}{*}{Thermal hydrolysis with recuperative thickening} & Sewage sludge & \multirow{2}{*}{Reported no increase in methane yield} & \\  & Temperature: 165 \({}^{\circ}\)C & & \\ \cline{1-1}  & Pressure: 6 bar & & \\ \cline{1-1}  & Time: 30 min & 100\% increase in throughput rates & \\ \cline{1-1}  & Thickening of digested sludge and recycling solids back to digestion & & \\ \hline \end{tabular}
\end{table} 
Table 1: Results reported in the literature regarding the application of thermal hydrolysis processes for enhancing anaerobic digestion. acid solutions) has been reported to effectively enhance the thermal hydrolysis of waste activated sludge (WAS) [13]. On the contrary, when the severity of treatment is increased over a certain range, as it would be by increasing the temperature above 190 \({}^{\circ}\)C or increasing treatment time, Maillard reactions may occur, which cause the formation of toxic materials [16; 17]. Soluble recalcitrant compounds associated with these reactions are produced from the polymerisation of carbonyl groups from the reducing sugars with amino acids [18; 19]. These dark coloured compounds can compromise the stability of downstream biological processes and water discharge limits. The presence of these compounds was studied by [PERSON] et al. [20], who reported that a small increase in water effluent COD concentration may be associated with the presence of recalcitrant substances derived from THP. Table 2 presents a list of technologies that are commercially available. Some of these processes operate under batch mode; recent developments allow operation under continuous conditions. The intermediate thermal hydrolysis process (ITHP) consists of a THP unit installed between two digestion stages. The first reactor treats thickened sludge, which, after digestion, is subsequently submitted to thermal hydrolysis and then sent to a second digestion stage. Greater production of biogas is thus expected, thanks to the higher rate of volatile solid removal (VSR) attained [9]. The advantage of ITHP over THP is that the easily biodegradable components of the substrate are firstly converted into biogas during the first digestion stage, thus greatly reducing the volume of sludge to be submitted to hydrolysis and treating only recalcitrant compounds by ITHP. This saves on capital investment costs [28; 29] since much smaller hydrolysis reactors and boilers are needed [30]. A Cambi ITHP pilot plant was installed in Basingstoke (United Kingdom) [31].