Method comparisons

Anaerobic digestion: Method comparisons

Within the field of anaerobic digestion there are a number of different methods for producing biogas. The first part of this page compares the two main methods. The second part of the page compares the 3-step Aikan with other methods that are optimized for handling of solid organic waste.

Main methods: Wet AD and Dry AD

Anaerobic digestion turns organic waste into useful biogas and fertilizer in an environment with little or no oxygen. The two main methods used have become known as dry anaerobic digestion (Dry AD) and wet anaerobic digestion (Wet AD).

The main difference between to two methods relates to the form of the solid waste. Dry AD handles organic waste more or less as it is, using only simple mechanical sorting and with digestion taking place from waste in its solid form. Wet AD requires ¬that the waste be turned into a homogenous pulp that can be pumped while being processed.

Both Wet AD and Dry AD take place in a wet environment, so for new-comers the terminology can be a little confusing.

Comparison of Wet AD and Dry AD treatment of solid organic waste

Wet AD

Dry AD

Feedstock

Clean organic waste fractions that can be made pumpable

Any kind of solid organic waste

Flexibility in processing capacity

Low

High

Tolerance to impurities

Low

High

Full internal recirculation of liquid digestate at plant

Not possible

Possible
(only Aikan does this)

CAPEX and OPEX per tonnes of solid waste

Higher

Lower

Possible end products

Biogas, sludge, digestate

Biogas, compost, digestate

Typical use

Farm biogas plants

Plants that treat segreated or non-segretaged Municipal Solid Waste (MSW)

Both types of anaerobic digestion rely on the same organic processes: hydrolysis, methane generation and composting of the solid fraction. The Wet AD and Dry AD approaches differ with regards to how waste is processed and what qualities of waste can be treated.

Historically, attempts have been made to apply the Wet AD method to Municipal Solid Waste (MSW). Many of these attempts have failed due to problems with high levels of impurities in MSW.

Aikan Technology uses the Dry AD method in a unique 3-step process which allows for the solid waste to remain untouched inside the process module during methanation and composting.


Comparison of Aikan Technology and other Dry AD systems

The following table shows a comparison between Aikan Technology and other Dry AD processing methods currently available on the market.

Comparison of 3-step and 1-step processes

  

Aikan Technology

Other Dry AD systems

Comment

Method

3-step batch processing, integrated Dry AD and IVC (in-vessel composting)

1-step batch processing

1-step continuous processing

Use of processing module

Hydrolysis and composting

Biogas production

Biogas production

 

Use of gas tank

Biogas production and biogas storage

Biogas storage

Biogas storage

Methane % in biogas

Approx. 70%

55%

Approx. 60%

Difference up to 15 percentage point. High methane percentage makes it easier and cheaper to connect to existing natural gas infrastructure or bottle distribution systems.

Traceability

Possible because of batch processing

Not possible because of need for inoculation with already processed waste

Not possible

Composting

Integrated

Not integrated

Not integrated

In an Aikan system waste remains inside the closed processing module while composting.

Compost sanitization

Integrated, complete sanitization is well documented

Not integrated

Not integrated

Safety procedures when opening the process modules

No special instructions (no gas production in the processing modules. Modules remain closed until composting is completed).

Strict opening procedures due to explosion risk (gas production takes place in the processing modules).

(n.a.)

Less operational risk with integrated composting.

Amount of produced energy used for processing

10%

20%

(n.a)

Aeration integrated in process modules (Aikan) uses less energy than external agitated aeration.