Implementation of Thermal Hydrolysis technology to achieve Advanced Anaerobic Digestion
Introduction
Anaerobic digestion (AD) is an established and widely used biological process where microorganisms break down organic waste in the absence of oxygen, producing biogas primarily composed of methane (CH4) and carbon dioxide (CO2). Conventional AD systems face challenges related to slow biodegradation that can be summarized in one key insight: the limiting step of the AD mechanisms is the hydrolysis (solubilization) of the solid organic waste.
To overcome this limitation and achieve Advanced AD, several pre-treatments have been tested. Among them, Thermal Hydrolysis (TH), that subjects organic waste to high pressures and temperatures, is the pre-treatment of choice due to its technical and economic benefits.
Conventional Anaerobic Digestion
The first step in standard AD is the hydrolysis of complex organic matter, that is broken down by enzymes into simpler compounds that can undergo the subsequent stages leading to the methanogenic microorganisms producing biogas. The performance of conventional AD systems is limited by several factors:
•Slow hydrolysis rate: the initial solubilisation of complex organic materials is a slow process, limiting the overall throughput and efficiency of the digestion system.
•Lower biogas yield: the slow hydrolysis and subsequent methanogenesis stages result in lower biogas yields.
•Long retention times: conventional AD requires long retention times to process waste, leading to larger and more expensive digesters.
Advanced Anaerobic Digestion
To overcome the limitations of conventional AD, a TH pre-treatment can be implemented. By using heat (typically between 140°C to 160°C) and high pressure to the waste before entering the digesters, it provides several key advantages:
•Enhanced hydrolysis: TH accelerates the waste breakdown, resulting in faster and more efficient conversion of organic matter into simpler compounds. This enhances the overall efficiency of the subsequent AD process.
•Increased biogas yield: TH improves biogas yields by up to 30-50% compared to conventional AD systems, due to the improved availability of biodegradable material.
•Better digestate quality: TH yields a pathogen-digestate suitable for agricultural application even under the most stringent regulations.
•Reduced biosolid volume: due to the improved dewaterability of the hydrolysed waste.
•Shorter Hydraulic Retention Times (HRT) and higher Organic Loading Rates (OLR): leading to smaller digesters that significantly reduce the required capital costs.