## 1 Introduction
However, this option is not always feasible based on the geographical location of the plant and the performance of the digestion process [23]. Biogas composition affects engine efficiency in different ways. [PERSON] and [PERSON] [24] studied an engine in dual fuel mode with diesel as pilot fuel and biogas as the main fuel for which different methane compositions were evaluated. These authors suggested that exhaust gas temperature and air-fuel ratio were the key engine parameters affected, with the latter exhibiting a marginal increase with methane enrichment. However, when evaluated as a single fuel in compression engines, the presence of CO\({}_{2}\) showed detrimental engine performance with increasing content. [PERSON] and [PERSON] [25] indicated that increasing levels of CO\({}_{2}\) in biogas leads to a decrease in the brake thermal efficiency, whereas the flame initiation and combustion durations were increased. Nitric oxide (NOx) emissions were reduced, but as a detrimental effect, these authors reported an increase in hydrocarbon and carbon monoxide (CO) emissions. On the other hand, the high CO\({}_{2}\) content in biogas gives it a greater resistance to detonation, the latter being a phenomenon that, if it occurs during combustion, adversely affects the efficiency, emissions and durability of engines [26]. Hydrogen sulfide (H\({}_{2}\)S) is another common component of biogas. Its presence may have negative consequences to human health and impacts equipment durability [27]. Therefore, it is usually removed by different methods such as reaction with iron oxides or by biological techniques [28; 29]. Turbocharging is an efficient technology available for increasing the effectiveness of engines and output power [30]. Turbochargers are widely used in internal combustion reciprocating engines, as they increase power thanks to the compression effect, allowing a greater mass of air to flow into the cylinder whilst recovering a proportion of the exhaust gas energy [31; 32]. Since cylinder capacity is increased, keeping size constant, this action is known as downsizing [33]. The basis of turbocharging is to increase engine power by introducing a greater amount of air and fuel into the cylinders, as the air-fuel ratio is restricted to a very narrow range to achieve complete combustion. Standard petrol engines operate with compression ratios between 6:1 and 10:1 [34], to avoid self-ignition of the air-fuel mixture. Increasing the compression ratio to 16-18 improves the efficiency and power output. This can be attained when biogas is used as fuel due to its good detonation resistance [35]. The use of biogas using a dual-fuel turbocharged engine was evaluated by [PERSON] et al. [36]. These authors concluded that the presence of biogas did not adversely affect brake thermal efficiency. The present manuscript aimed to analyze the energy balance of turbocharged biogas engines operating at the Burgos wastewater treatment plant (WWTP), where thermal hydrolysis sludge pre-treatment units are installed. Biogas is used for producing electricity and thermal energy. The energy efficiency of the plant was evaluated considering the addition of a co-substrate to enhance biogas production and analyze its effect on the energy balance. The engine thermal circuits were studied to establish the efficiency of the global energy system and its improvement. A sensitivity analysis of the main operating parameters of the engines and cooling circuits was carried out. The analysis performed considered the operating point of the WWTP and assumed the production of biogas from thermally pre-treated sludge as a baseline scenario. A comparison regarding electricity production was made if a co-substrate was available to enhance digestion performance and take advantage of the high treatment capacity of the plant. It was essential to consider the hypothesis that the characteristics of the co-substrate regarding digestibility could prevent the system from reaching energy autarky.