Proceedings

Abstract:

Biogas production via Anaerobic Digestion (AD) is primarily limited by the rate-limiting stage of
hydrolysis, in which high molecular weight substrates are cracked. Dewatering is also a common
requirement in AD processes, both for feedstock and final solids. These processes are constrained by
multicellular clusters (inc. organisms), agglomerations, cells and subcellular structures.
Various methods have been developed to accelerate these process steps which address differing
feedstock issues in different ways. Matching method to feedstock, scale and process is essential as is
ensuring a commercial return.  Pulsed Electric Fields (PEF) is the application of an electric field in pulses across biological cells. PEF can be used to create permanent pores in cells, but also to lead to their complete disintegration. By opening the cell structure microbial and enzymatic access is provided to organic compounds within the cells that would otherwise not be available, thus increasing digestion. It also disables pathogens and facilitates dewatering.

This paper outlines the background and history of PEF, its use in lysing cells and application to
Anaerobic Digestion positioning it against other pre-treatment methods. PEF is illustrated as both an
effective pre-treatment method, but also as complementary to other methods.
Energy consumption is a major factor. Conventional PEF consumes less energy than most other
methods. A recent development is the use of precise high power square waveform pulses that both
increase effectiveness and substantially reduce specific power consumption leading to substantial
increase in the return on energy invested.

Introduction:

Anaerobic Digestion (AD) comprises four main decomposition stages. The last three (acidogenenis,
acetogenesis and methanogenesis) are not inherently rate limiting. However the first stage, hydrolysis,
which converts feedstock into a form ready for the next stages is usually rate limiting. Complete
degradation of the feedstock also minimise encapsulated water and eases dewatering.
Feedstock generally comprise biological structures (cells, fibres) which themselves are formed from
mainly polymeric compounds. The first stage of AD both breaks down the structures and hydrolyses
the compounds making them available for acidogenesis. Hydrolysis itself can be the primary method of
achieving structure decomposition; mechanical, chemical and enzymatic methods are also used. The
pre-eminent method for accelerating this step is Thermal Hydrolysis, however this is capital intensive and thus mostly only economic in large scale plants. Other methods have been developed including
enzymes, steam explosion, ultrasound and cavitation. With the exception of enzymatic treatment, most
methods involve means of employing energy to disrupt the feedstock.

Key Factors
Ultimately any solution has to deliver a business benefit. The primary requirements are risk
minimisation, maximising financial return and meeting regulatory requirements; these are followed by
process stability, ease of operation, process flexibility etc. The financial case predominates, albeit
balanced against risk.