Inverted Phase Fermentation for Digestion Intensification

Hu, R.2, Tyrrel, S.2, Le, M.S.1 and Cartmell, E.2, 1United Utilities PLC, 2Cranfield University, UK


Inverted Phase Fermentation (IPF) is a sludge thickening process that operates on the basis
of flotation with nascent carbon dioxide. The biogas given off during the initial stages of
fermentation causes the sludge to concentrate in a top layer known as the solid phase. With
up to 11% dry solids (DS) the solid phase enables the digestion process to be intensified by
up to 300% compared to the un-thickened feed. Since no polymer is used, the solid phase is
less viscous and is expected to be more easily blended in the digesters than belt thickened
sludge. In addition to the sludge thickening effect, IPF also provides improved pathogen
destruction and a carbon source at the same time.

The application of IPF should allow the operation of existing digestion assets to be intensified
to operate with organic loading rates up to 4.9 kg of VS/m3 d without any significant
modifications or increased risk of foaming. Benefits to the operators would include alleviation
of any hydraulic overloads; increased digestion capacity; and greater availability of biogas for
power generation or conversion to biomethane for transport. Other technology benefits
include the elimination of process emissions from secondary digesters; and a free carbon
source for biological nutrient removal applications.

This paper reports the findings of an initial study of the effect of sludge source, temperature,
initial dry solid content and fermentation time on process performance.

Sludge thickening; Intensification; Fermentation; VFA production; E. coli reduction; Viscosity

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