Fernández-Polanco, D.1 and Fdz-Polanco, F.2
A number of pre-treatments have been proposed to overcome the main limitation of anaerobic digestion processes: the hydrolysis or solubilization step. Among them, thermal hydrolysis is becoming the pre-treatment of choice due to its techno-economic benefits. The main drawback of this booming technology is a high energy intensity.
While the yield improvements reported by commercial processes are all comparable, the key differentiator lies in their specific energy consumption. A thorough analysis of the key parameters impacting energy consumption, and their interactions, is carried out by applying the R-curve methodology and pinch analysis. To ensure energy self-sufficiency, the conventional, preheating limited technologies need to operate at higher concentrations with the associated drawbacks in terms of concentration equipment and polyelectrolyte usage. Second generation thermal hydrolysis technologies recover 100% of the process vapours and can achieve energy self-sufficiency at lower concentrations.
Advanced anaerobic digestion, energy efficiency, energy self-sufficiency, pinch technology, R-curve methodology, sludge management, thermal hydrolysis.
The global kinetics of anaerobic digestion, an established technology to generate biogas from organic wastes such as sewage sludge, are limited by the hydrolysis stage. A wealth of literature (Ariunbaatar et al. 2014; Carrère et al. 2010; Pérez-Elvira et al. 2006) discusses the merits of the biological, chemical, physical and thermal pre-treatments suggested to overcome it. Among them, thermal hydrolysis (TH) is becoming prominent because of the undisputable benefits it offers (Sridhar et al. 2015) such as improved biogas yields, reduced biosolid volumes, EPA Class A pasteurised biosolids and increased digesters loading rates.
The main disadvantage of thermal hydrolysis is a significant energy consumption (Cano et al. 2015), mostly in the form of steam required for sludge heating. In order to optimize the energy integration of this pre-treatment within the wastewater treatment plant (WWTP) and ensure its energy self-sufficiency, two thermodynamic tools have been used: the R-curve methodology and pinch technology (also referred to as process integration, heat integration and pinch analysis). Both are widely used within the process industries to analyse energy systems, but have been rarely applied to the water sector because of its relative simplicity. Now, with the additional layer of complexity added by the installation of thermal hydrolysis, they become useful optimisation tools for WWTPs also.