Knowledge Base

Anaerobic digestion (AD) is a biological process in which microorganisms break down organic material in the absence of oxygen, producing biogas and digestate. The biogas consists primarily of methane (CH4, approximately 50% to 60%) and carbon dioxide (CO₂). The digestate is the nutrient-rich residue that remains after digestion. AD is the core technology behind HoSt’s biogas plants, converting organic waste streams into renewable energy and biofertilizer.

How does the anaerobic digestion process work?
Anaerobic digestion takes place inside a sealed digester tank (reactor), where micro-organisms break down organic material in a controlled, oxygen-free environment. The conversion happens over four consecutive stages:

• Hydrolysis: complex organic matter (carbohydrates, proteins and fats) is broken down into simpler, soluble molecules such as sugars, amino acids and fatty acids.
• Acidogenesis: these simpler molecules are broken down further into volatile fatty acids, alcohols, and gases such as CO₂ and hydrogen.
• Acetogenesis: the products of acidogenesis are converted mainly into acetic acid, releasing further CO₂ and hydrogen.
• Methanogenesis: methane-producing micro-organisms convert the acetic acid, CO_2, and hydrogen into biogas.

These stages take place at either mesophilic (around 40°C) or thermophilic (around 55°C) temperatures. The bacteria inside the digester require a stable supply of heat and proper mixing to maximize biogas generation.