Anaerobic Digestion Flashcards

Flashcards on Anaerobic Digestion

Anaerobic digestion (AD)

A microbial process which breaks down organic matter into simpler chemical components in the absence of oxygen, resulting in the production of biogas (60% CH4, 40% CO2) together with liquid & solid digestate.

Key components in Anaerobic Digestion

Volatile acids, Alcohols, Hydrogen, Carbon dioxide, Acetic acid, Methane, Amino acids, Simple sugars, Fatty acids, Carbohydrates, Lipid, Complex organic compounds, and Protein.

Hydrolysis

Breaks down complex molecules during anaerobic digestion.

Acidogenesis

Produces VFA’s i.e. (propionic, butyric, acetic) NH3 , CO2, H2, H2S with pH 5.5-6.5 during anaerobic digestion.

Acetogenesis

Acetic acid, CO2 & H2 production during anaerobic digestion.

Methanogenesis

CH4 production during anaerobic digestion and is sensitive to pH (ideal pH 7.2-8).

Mesophilic digestion

37-41°C, residence time 15-30 days, more robust & tolerant but lower gas production. Larger digestion tanks required & sanitisation is a separate process.

Thermophilic digestion

~55°C, residence time 12-14 days, Higher CH4 production, faster waste throughput, better pathogen kill but requires more expensive technology, greater energy input and a higher degree of operation & monitoring.

‘Low rate’ AD systems

Long hydraulic retention time (HRT), used for slurries & solid waste systems (Batch, Plug-flow (7-22% TS), Continuously Stirred Tank Reactor (CSTR) with 7-14% TS).

‘High rate’ AD systems

Short (HRT), used for dilute feedstocks e.g. wastewater treatment with <1% solids (Anaerobic Filter, Fluidised Bed, Upflow Anaerobic Sludge Bed (UASB)/ Expanded Granular Sludge Bed (EGSB) (0.5-6% TS)).

Benefits of Anaerobic Digestion

Reduces GHG emissions, produces renewable heat, power & transport fuel, recycles nutrients back to land, reduces air & diffuse water pollution, reduces amount of waste sent to landfill.

Substrates for Anaerobic Digestion

Cattle and pig (manure & slurry), Poultry manure, Silage effluent, Industrial waste, Sewage sludge, Organic fraction of MSW, Energy crops.

Output uses for Biogas

Heat only, electricity (IC engine), CHP (>80% energy efficiency), transport fuel & gas to grid (require gas upgrading).

Biogas upgrading technologies

Pressure swing adsorption, membrane separation, water scrubbing, chemical absorption, physical absorption, and cryogenic separation.

Financial support for AD in the UK

ROCs (Renewable Obligation Certificates) introduced in 2002 and Feed In Tariffs (FIT) introduced in April 2010 for <5MWe to provide financial support.

Use of Liquid digestate

Fertiliser for crop growth (Nitrogen (mainly NH4+) more readily available to plants but high potential for losses as NH3).

Use of Solid digestate

Low nutrient content - soil conditioner can be aerobically composted to stabilise & increase value.