Proceedings

The effects of steam explosion and hydrolysing time on the digestibility of sewage sludge in anaerobic digestion

Barua, R., Lee, K., Mills, N., Ouki, S., Thorpe, R., Thames Water Innovation, Centre for Environmental Strategy, University of Surrey

(free)

Abstract

Thermal hydrolysis process (THP) is a sludge pre-treatment process that provides improved anaerobic digestion performance. THP heats the sludge with steam at high pressure and temperature to produce more digestible sludge by hydrolysis. Currently there is a debate over whether steam explosion or hydrolysis time is the more dominant mechanism. This research paper investigates how steam explosion pressure influences the overall sludge digestion process. Analyses were carried out to ascertain the effects on biogas production, total COD, soluble COD, dry solid and volatile matter contents. The main objective of the project is to identifying the best THP conditions to optimise the performance on the overall anaerobic digestion process.

Keywords Advanced Anaerobic Digestion, Steam Explosion, Thermal Hydrolysis, Anaerobic Digestion, Hydrolysing Time, Extent of Hydrolysis, Optimising Anaerobic Digestion, Optimising Thermal Hydrolysis Process Background

This paper presents some preliminary results from the first of a four year collaborative research project between Thames Water and the Centre for Environmental Strategy at the University of Surrey; employing an engineering doctorate student, the primary author of this paper.

Introduction

In order to optimise anaerobic digestion, various methods have been used to pre-treat the sludge. Sludge treatment methods such as pre-treatment and co-treatment usually means processes which are combined with main biological treatment methods such as anaerobic or aerobic digestion. The primary objectives of these treatment methods are usually to increase bio-gas production, pathogen removal, odour removal, reduction in sludge volume and mass and reducing sludge viscosity for easier pumping and handling.

One of the most developed variants of anaerobic digestion process pre-treatment is thermal hydrolysis at high temperature. Generally, in thermal hydrolysis process (THP) sludge is treated with high pressure saturated steam at 7-8 bars followed by sudden pressure release. This causes the sludge at high pressure to undergo an explosive decompression. Generally the THP process runs for 30-40 minutes at temperature around 160-170. The most commonly used THP process has been developed by Cambi which uses three stages to hydrolyse sludge. During first stage, the sludge is heated in a pulper. The pre-heated sludge is then introduced in the reactor where it is heated to about 165 and pressurised at 7bar for 30 minutes.

Hydrolysing at high pressure and temperature disintegrate organic macromolecules and floc particles so the resulting sludge is less viscous, more soluble, easier to pump and degrade in a conventional digester (Veolia 2011).

The higher biodegradability of the thermally hydrolysed sludge results in increased production of biogas. Biogas produced from anaerobic digestion consists of about 65% methane, 25% carbon dioxide, 4% oxygen and 6% other gases. The increase in methane production in anaerobic digestion process is advantageous, since it can be used in a CHP engine to produce energy and lower the operational cost of the plant. The increase of methane production resulting from this process has been linked to sludge COD solubilisation by linear correlations. Which means the more soluble the sludge is, the better it will be in producing methane. But research by Dwyer et al. (2008) has found that while increasing temperature above 150 increased solubility, no increase in methane production was observed. At excessively high temperature of 170-190  the sludge biodegradability decreased, despite of having high solubilisation of sludge. The increased production of non-biodegradable soluble material was responsible for the increase in solubility, which did not result in high gas production. (Dwyer et al., 2008).

It has been also assumed that the THP increases sludge biodegradability due to steam explosion. There is limited amount of published literature addressing the effects of steam explosion on the extent of sewage sludge hydrolysis in THP process. This paper intends to present some preliminary results looking at the effect of how the steam explosion under different pressure conditions can affect the digestion stage of sewage sludge.

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