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Anaerobic Digestion is an Energy Solution in Sustainability.

Anaerobic Digestion is the digestion of carbohydrate by anaerobic bacteria in the absence of oxygen to produce

biogas.

Biogas produced from this process predominantly contains methane and carbon dioxide generally in the

proportions of 50-60% methane and 40-50% carbon dioxide dependent on the feedstock.

The biogas so produced is the source of energy fuel for either electricity generation or direct heat use or for road

traffic fuel conversion.

The digestible carbohydrate suitable for anaerobic digestion is found in all living and waste growing materials.

Carbohydrates which are unsuitable are of high lignin content from plant material and bone from animal origins.

The anaerobic digestion will utilise and convert between 40 and 80% of the carbon content of the feedstock with

the residue being available for oxidation under natural circumstances or deliberate combustion.

In practice suitable feedstocks for anaerobic digestion are purpose grown or are derived from the waste stream.

Purpose grown or purpose harvested crops provide the most reliable and energy efficient feedstock for anaerobic

digestion provided that the material has not been sourced from crops beyond annual maturity.

Waste stream derived feedstock can be in the form of green and food waste or the energy content of municipal

sourced waste.

Sewage from domestic sources has a potential value from the anaerobic digestion of undigested human waste and

AD in the sewage stream should be considered as part of the sewage treatment solution.

In agriculture animal slurry (waste) having been for the most part anaerobically digested does not yield

substantial biogas unless it is supported with included biocrops.

Certain organic wastes will provide very high gas yields, such as abattoir waste. Inconsistencies and operational

problems detract from these targeted feedstocks and the economic benefit is more often found in the process

solution value rather than the energy produced.

Sustainability of inputs to the process is a serious consideration. Resource protection and reuse are very

important to support sustainable biocrop production.

Successful sustainable management of biocrop production is key to longevity of a project. This management

responsibly will consider fertility and water usage.

Waste creation is inefficiency and inevitably an efficient human race, surviving population growth, will target

waste reduction significantly. Sustainable sources of waste will not exist and must be regarded as a diminishing

resource.

Sewage is of course a sustainable resource which will increase with population growth and therefore a secure

source of feedstock for anaerobic digestion.

Anaerobic Digestion Technology

For Anaerobic digestion to take place the activity has to be supported above 36 degrees centigrade. Optimised

temperature for complete activity is 37.5 deg C. Higher temperatures will accelerate the process but the digestion

is significantly incomplete.

Two different process approaches have developed.

1. Wet AD which enables the feedstock to become a pump able product which will include added water. This

has developed into process methods which are either batch digestion or continuous flow digestion.

The plant operation involves considerable pumping agitation and feedstock pre preparation all of which is

parasitic in energy demand and prone to mechanical failure and maintenance down time.

Post digestion digestate disposal, use and treatment are further considerations and process efficiency and

purpose will be targeted considerations.

Choice of wet technology logically will depend

a) on the nature of the feedstock primarily being wet and pump able

b) on the particle size and nature of the feedstock

c) the impurity content, type and nature

d) the potential for digestion interference in the feedstock content

e) the project lifetime

The Wet process is most suitable for the treatment of domestic sewage and historically this has been the most

frequent application. Post digestion digestate management has always been a problem and ingenuity to develop

solutions in this area which treat and efficiently recover resource content is part of essential sustainability.

2. Dry AD is based on digestion being completed through batch treatment and involves the feedstock being

consigned to sealed heated chambers which are exhausted of air content. The content is inoculated with

anaerobic bacteria which rapidly breed and digest the contents. Biogas is produced and extracted and

utilised. When the gas production ceases the digestion is complete.

The feedstock for this is flexible and it is not restricted in particle size. But digestion interruptive content is critical.

Dry AD applied to a batch process is able to mitigate this risk on the assumption that should the feedstock fail to

digest this is identified and confined to one chamber within the unit.

Dry AD as an energy solution is best designed around at least 10 phase filled digestion chambers to enable energy

generation balance.

The operational process of dry digestion is mechanised use of loading shovels which vastly differs from the wet

process of pumping and agitation.

The feedstock can vary from wet and dry crops to municipally sourced waste.

The post digestion digestate treatment will depend on the feedstock and will only require pasteurisation when

food waste, desiccated sewage sludge and municipally sourced waste are used.

There is continuous flow dry AD processes designed but the application has to specific to a feedstock and be

prepared for mechanical breakdown and maintenance downtime.

Assessment of technology type for a project must consider

1. Feedstock

2. Location

3. Digestate disposal purpose and value

Feedstock

a) Feedstock’s which suit wet digestion are

Sewage

Food processing sludges

Abattoir wastes

Vegetable packing wastes.

b) Feedstock’s which suit dry anaerobic digestion are all green materials and crops which have been

produced in the same growing season.

Food wastes

Municipal sourced wastes

Green wastes

Desiccated sewage sludge

Animal manures

In all processes feedstock particle size should be as small as process economy allows enabling maximum surface

activity during digestion.

Location of a project must consider

a) Feedstock proximity

b) Energy use proximity

c) Energy choice and purpose

d) Climatic conditions

e) Digestate storage and conditions for disposal

Digestate disposal, purpose and value.

a) Wet digestate is best applied to land use as a fertility support. For this to be effective the wet digestate

must have its bacterial activity stopped and the bacteria killed off.

b) Wet digestate has a significant content of soluble plant nutrient which has to be carefully applied to land

use avoiding fertility wastage and potential pollution

c) Refined wet digestate is suitable fertility support for algae production and low grade hydroponics

d) Wet digestate can be processed to provide an element of vegetable fibre suitable for inclusion in growing

medium products.

e) Dry digestate is very suitable for growing medium inclusion

f) Dry digestate can be further processed for creation of biomass fuels achieving complete recycling of 100%

of the input carbon

g) Dry digestate is very valuable as a fertility support in agriculture and when regularly incorporated under

crops contributes to short term organic fraction support in the soil

h) Fossil fuel derived fertilizers are proving to be very expensive worldwide and the value of the digestate

produced and used as fertilizer is potentially £30 GBP per tonne.

Phil Buckley

Tel: +44 (0)7833 762 488 E-mail: phil@water-neutral.com

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