# Thermal Hydrolysis of Sewage Sludge: A Case Study of a WWTP in Burgos, Spain
## 1 Introduction  
Mesophilic anaerobic digestion is considered the most widely applied technology for the treatment of organic wastes, and it is an essential component of modern wastewater treatment plants (WWTPs). Digestion is considered a useful process for generating renewable energy and provides a means to alleviate problems associated with low access to energy [1]. This technology transforms sewage sludge into biogas and reduces the total amount of biosolids needing final disposal. Another important benefit of this process relies on the destruction of most pathogens present in sludge and the stabilisation of organic matter [2]. The stabilisation of sludge by anaerobic digestion significantly reduces transport costs thanks to the decrease in sludge volume, which positively impacts greenhouse gas emissions. Some other environmental benefits worth mentioning include odour reduction, nutrient recycling and soil conservation if land disposal is selected as the final management option [3]. There exist several treatments for improving the partial degradation of organics to facilitate the hydrolysis stage. The conventional thermal hydrolysis process (THP) has been widely used in WWTP to enhance digestion performance, resulting in higher methane yields thanks to the disruption of microbial cell walls. THP consists of heating the sludge to high temperatures, generally in the range of 140 to 180 \({}^{\circ}\)C, with a treatment time span of about 30 to 60 min, applying high pressure (between 6 and 11 bars). There are more than 82 full-scale, THP-operating WWTPs [4]. During the thermal degradation of sludge, proteins are partially degraded, giving rise to the production of ammonia (NH4+) and, thus, reaching high concentrations of this compound in the pretreated sludge and thedigestate [5]. The higher volatile solid (VS) content of sludge fed into the digestion system and the higher degradation rate attained cause a further increase in the ammonia content (total ammonia nitrogen and free ammonia nitrogen) of the reactor liqueur [6]. Digesters treating thermal-hydrolysed sludge can reach values in the range of 2400 to 3500 mg N-NH4\({}^{+}\)/L [7,8]. The increase in the temperature of sludge during thermal pretreatment guarantees that biosolids obtained will be free of pathogens, with a stabilisation degree of about 60% (measured as COD conversion into biogas) [9]. Table 1 shows the results reported by different authors regarding the enhancement obtained when thermal pretreatment is applied to sewage sludge. About 40% more biogas can be obtained with half the residence time in the reactor when compared with conventional digestion [10]. The addition of chemical compounds aids in the removal of organic matter and sludge solubilisation. At laboratory-scale, several studies have reported on different ways for performing hydrolysis. However, on a commercial scale, this process is usually performed by the addition of direct live steam injection. In THP, thermal energy is typically provided by direct injection of saturated steam until it reaches the desired temperature and pressure. Afterwards, sludge is submitted to a sudden decrease in pressure (expansion), thus reducing its temperature, although some configurations use standard heat exchangers for decreasing temperature to operating digestion conditions [14]. The application of THP at low temperatures (between 60-90 \({}^{\circ}\)C) has also been reported in the literature. [PERSON] and coworkers [15] indicated that biogas production was not significantly increased unless alkaline conditions were also established during the pretreatment assay. Thus, the addition of chemical substances (alkaline or