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.

What are the outputs of anaerobic digestion?
Anaerobic digestion produces two valuable outputs. The first is biogas, composed primarily of methane and CO₂. It can be used to generate electricity and heat or purified into biomethane (also called renewable natural gas or green gas) for injection into the gas grid or use as transport fuel. The second is digestate: a nutrient-rich residue that remains after digestion. Digestate separation splits this residue into a solid (thick fraction) and a liquid (thin fraction). Both fractions can be post-processed and applied as an organic fertilizer on agricultural land, closing the nutrient cycle. The thick fraction can, with additional post-treatment, also be processed into compost. In some configurations, the CO₂ captured during biogas upgrading can also be recovered and liquefied for industrial or food-grade use.

What is the difference between anaerobic digestion and composting?
Both anaerobic digestion and composting process organic material into useful outputs, but they work in fundamentally different ways. Composting is an aerobic process: it requires oxygen, takes place in open or semi-open systems, and produces compost as its primary output. Anaerobic digestion takes place without oxygen in a sealed tank, and its primary output is biogas (an energy carrier), alongside digestate. This makes AD the more energy-productive route: organic waste becomes a source of renewable electricity, heat, or gas, rather than solely a soil amendment. The thick fraction of the digestate can, with additional post-treatment, also be turned into compost, so the two routes are not mutually exclusive.

For organizations looking to extract energy value from organic waste, anaerobic digestion is the more powerful option: composting does not capture the energy locked in organic material, while AD does. The biogas produced can replace fossil fuels across heat, power, and transport applications, and the digestate produced by AD is comparable in fertilizing value to compost, retaining the key nutrients (nitrogen, phosphorus, and potassium) from the input material. AD therefore delivers a dual return: renewable energy and a circular nutrient product, from waste streams that would otherwise require costly disposal.