An introduction to anaerobic digestion

Martin Wager explains the process, applications and benefits of anaerobic digestion (AD)

A revenue-boosting method of generating renewable energy is via anaerobic digestion (AD). This is the controlled breakdown of organic matter in a closed digester vessel. The air supply is restricted to stimulate anaerobic decomposition (unlike composting, which takes place in the presence of air). This process of decomposition, accelerated in the presence of bacteria, produces a methanerich biogas, which can then be turned into heat and electricity via a Combined Heat and Power (CHP) unit.

Anaerobic digestion (AD) is an attractive proposition for suitable sites, especially for those organisations with organic waste and residues from manufacturing processes (such as food and drink) that are uneconomic to reuse or recycle. Instead, these waste products make good feedstock for AD.

How anaerobic digestion works
Anaerobic digestion is a four-stage process in which the organic feedstock (containing proteins, carbohydrates and fats) is broken down by bacterial microorganisms in a sealed, heated digester tank. Typically, this takes 60 days, but it is dependent on the composition of the feedstock and the process parameters, e.g. digester temperatures.

There are four stages:
Stage 1: Hydrolysis, where the feedstock is broken down into simple sugars, amino acids and fatty acids
Stage 2: Acidogenesis, which produces volatile fatty acids, alcohols, CO₂, hydrogen sulphide and ammonia
Stage 3: Acetogenesis, which produces acetic acid, CO₂ and hydrogen
Stage 4: Methanogenesis, which converts the acetic acid, CO₂ and hydrogen into biogas (comprising 50-60 per cent methane and approx 40 per cent CO₂ and trace compounds.

Digestate
Some feedstocks produce an additional process by-product called digestate. This is indigestible feedstock (90–95 per cent original volume) containing nutrient-rich materials and exhausted microorganisms with high contents of nitrogen, phosphorus and potassium.

Typical uses for solid fibre and liquid digestate are soil conditioners and biofertilisers, or it may be treated and used as animal feed.

Using biogas for CHP
The primary use of biogas is in the generation of renewable energy, as it can be used as the combustion fuel in a Combined Heat and Power (CHP) unit.

CHP, also known as cogeneration, is the simultaneous generation of renewable electrical power and heat energy (and cooling, if required).

Electricity is used on-site to power production line equipment, site lighting, etc. Any excess electricity can be exported and sold to local customers or back to the local grid. Renewable heat energy is used in the site’s manufacturing processes, heating buildings, and warming the digester. Excess heat may be exported and sold to other local heat users.

Biomethane
If the biogas output is greater than required on-site, any excess capacity can be upgraded to biomethane – to the same quality as natural gas. Alternatively, the biomethane can be compressed for use as transport fuel.

What equipment is necessary for anaerobic digestion?
Equipment specifications will depend on the intended use of the AD plant, i.e. whether the process will produce highquality digestate and/or is biomethane production viable. The main digestion equipment list comprises: a primary tank, digester tank, gas bag, flare and auxiliary equipment. Generation equipment includes a CHP unit, generally supplied as a packaged system, and an optional heating boiler.

The benefits of anaerobic digestion
Generating on-site renewable electrical and heat energy from biogas via AD can mean energy self-sufficiency, with the benefits of increased reliability and security of supply.

The combination of biogas production by AD and energy generation with CHP will result in significant energy and cost savings. CHP is around 30 per cent more efficient compared with conventional energy from fossil fuels.

The potential energy savings and increased profit margins that biogas production will produce are wholly dependent on the amount of renewable energy that can be generated from the feedstock used, and whether the generated energy will be utilised on-site or exported.

AD and CHP used for renewable energy generation is considered carbon neutral when biogas is used as the combustion fuel. This is because the methane is completely consumed and the amount of carbon in the CO₂ exhaust gas emissions is the same as that originally absorbed during photosynthesis in the natural carbon cycle.

Carbon savings will also be made by reducing or even eliminating the greenhouse gas emissions associated with transportation of food waste to landfill sites.

Financial incentives and income
Feed-in Tariffs (FiTs)/Contract for Difference (CfD), the Renewable Heat Incentive (RHI) and Renewable Obligation Certificates (ROCs) complement renewable energy savings with valuable revenue streams. These tariffs can present a fixed income for the life of the installation, based on the rate at time of construction. However, FiTs and the RHI are subject to a biannual degression, which can affect new projects.

Other incomes and incentives might be derived from selling high-quality digestate as a biofertiliser, or the production and sale of biomethane for injection into the natural gas grid under the provision of the RHI tariff. Alternatively, it may be compressed and sold as transport fuel, which is eligible for Road Transport Fuel Certificates.

Is AD and biogas right for you?
You will need to check that you have the following:

1. A constant feedstock of good composition and quantity
2. Space on-site for equipment – with good access for any storage and transport.
3. A potential market for digestate – to be sold as biofertiliser.
4. A potential market for biomethane – if you are able to produce more biogas.
5. Finance for biogas – capital investment, or install under a hire purchase agreement.

Environmental planning and permitting
Planning permission is normally required for most new AD installations. Approval will be concerned with how the development may impact on the local environment and community. It may be necessary to undertake an Environmental Impact Assessment (EIA).

The Environmental Permitting Regulations specify criteria for whether a permit or exemption is required for the operation of a new AD plant and the use of digestate. Some sites may be exempt from environmental permitting if their control of waste meets the criteria.

ENER-G Combine d Power Ltd
Martin Wager is Business Development Manager for ENER-G Combined Power Ltd. The UK headquartered manufacturing and engineering business is a global specialist in combined heat and power (CHP), biogas generation and anaerobic digestion. With over 30 years’ experience, more than 2625 units manufactured, some 5900 CHP running years logged and nearly two million tonnes of CO₂ saved by its customers, ENER-G is the largest provider of small scale CHP units in Europe.

For further information please visit: energ.co.uk/chp