Comparison of the performance of anaerobic digestion of thermally hydrolysed sludge during thermophilic and mesophilic anaerobic digestion

Shana, A.1, Perez, E.R.1, Perrault, A.1, Mills, N.1, Fountain, P.1, Gurton. L.1, Tilahun. D.1, Asaadi, M.2, and Gauchard, N.2,

1Thames Water, UK, 2 AD Technologies Ltd, UK



Thermophilic anaerobic digestion process is predominantly used as a pre-treatment process based on a shorter Hydraulic retention time (HRT) in USA, Germany and elsewhere mainly to pasteurise sludge. Standalone longer HRT thermophilic digestion process has also been used at the lower end of the thermophilic temperature range but due to increased heat requirements of the process, its use has been limited. Recently, the use of Thermal Hydrolysis Process (THP) to pre-treat digester feed has significantly increased resulting in the availability of significant amount of low grade heat which has to be removed before to ensure effective operation of Mesophilic Anaerobic Digestion (MAD).

In this paper, the performance of thermophilic AD of THP pre-treated sludge was investigated in comparison with THP+MAD.


The results obtained from laboratory scale experiments showed that the thermophilic anaerobic digestion process achieved an overall average VSr range of 50 – 66% in comparison with the mesophilic anaerobic digestion process which provided a VSr range of 48- 54%. As a result, the overall biogas production from the thermophilic anaerobic digestion process was found to be in excess of 420 m3/tonne dry solids (TDS) fed, compared with 409 m3/tds feed from mesophilic anaerobic digestion process.


THP, mesophilic, thermophilic, anaerobic digestion, bacteria, sewage sludge


Anaerobic digestion is one of the oldest wastewater treatment process unit operations and is an essential tool for protecting the environment and public health. It is a well-known fact, that the anaerobic digestion process converts potential pollutant organic substrates to economically useful by-products such as biogas and digestate. It also offers public health (sanitation) benefit by eliminating odour, killing pathogens and preventing the spread of diseases.

Anaerobic digestion processes are classified into three types based on the digestion temperature ranges used. These are known as psychrophilic, mesophilic and thermophilic digestion temperature ranges. The psychrophilic temperature ranges from 0 – 25oC, mesophilic from 25 – 45oC and thermophilic from 45 – 70oC (Shamskhorzani and McKinney, 1990; Aoki and Kawase, 1991 cited in Aitken and Mullennix, 1992; and Shana, 2015). Psychrophilic digestion (cold digestion) takes place when sludge is intentionally stored in storage tanks or when unintentionally stored in primary settlement tanks.

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