## 1 Introduction  
High temperature and pressure treatments are technologies useful for converting organic materials leading to their stabilization and accumulation of carbon content. Hydrothermal carbonization is performed in high-pressure reactors where the increase in temperature causes a disruption of bonds and the formation of entirely different compounds where carbon is accumulated in a solid form. The process involves a series of reactions such as hydrolysis, dehydration and decarboxylation, where gaseous and water-soluble products are released, in addition to water itself and a solid char [1]. The water generated in the process seems to become a problem for large-scale implementation of this technology due to the high volume needed if the substrate has a low moisture content. In addition, a liquid by-product stream is derived from the process, with high organic content and low energetic value. Some authors have proposed the recirculation of this process water into consecutive batches to reduce water consumption of the hydrothermal carbonization (HTC) technology [2], but if wastes already contain high humidity, this option would be unsuitable. Thermal hydrolysis is an alternative technology where high temperature and pressure conditions are applied. This technology has been implemented as a pre-treatment of sewage sludge before anaerobic digestion. This way, the biodegradability of the substrate is increased and bioreactor performance enhanced. Several reports have evaluated the benefits of thermal hydrolysis, studying the temperature range (120-170 \({}^{\circ}\)C) [3; 4; 5], the effect on solubilization and different types of substrates [6; 7; 8; 9] along with the effect on methane production. An increase in organic matter recalcitrance with the severity of the process has been demonstrated [10; 11; 12], which may adversely affect the outcome of a subsequent digestion step. The benefits of any of these processes are mainly associated with the reduction attained in the amount of waste and the capacity to stabilize an organic material that otherwise could cause pollution problems related to its uncontrolled degradation and offensive odor. However, the energy demand of these thermal treatments and high installation costs of running these systems under high pressure makes it imperative to find ways to increase efficiency and attain lower installation costs. There are different commercial technologies available, widely used in sewage sludge pre-treatment, including technologies developed by Cambi, Veolia Water Technologies, Haarslev, DMT Environmental Technology and Eliquo Water and Energy BV [13; 14; 15; 16; 17]. There are many reports published of experimental work in the scientific literature [5; 18; 19; 20; 21], but the number of manuscripts dealing with large-scale results is much lower. The study of [PERSON] and co-workers [22] presents results from a pilot plant at the Hengelo Wastewater Treatment Plant, The Netherlands, indicating a significant reduction in volatile solid removal. A full-scale study was described by [PERSON] and co-workers [23] after three years operating a thermal hydrolysis plant located in Hamar, Norway. These authors indicated a great conversion into biogas of the organic matter contained in sludge and a pathogen-free digestate. The analysis performed by [PERSON] and [PERSON] [24] in the United Kingdom used data from 66 conventional wastewater treatment plants (WWTP), six of them with thermal pre-treatment units followed by conventional mesophilic digestion. Data were recorded daily for periods of 2-7 years. They indicated similar biogas yields from mesophilic digestion after sludge pre-treatment, whereas higher biogas production was attained if the digestion stage proceeded under thermophilic conditions. Even though this finding may seem disappointing, the decrease in the amount of sludge obtained still gives a positive balance.