Proceedings

Comparing the performance of Thermophilic and Mesophilic anaerobic digestion of THP Pre-treated Sewage Sludge

Shana, A., Gurton, L. and Perez, E.R. Thames Water, UK

(free)

Abstract:
Robust anaerobic digestion process performance depends on digester temperature, substrate quality,
organic loading rate, retention time and the availability of skilled operator involved. This applies to both
thermophilic anaerobic digestion (THAD) and mesophilic anaerobic digestion (MAD) processes.
Historically, THAD process is predominantly used as a pre-treatment process based on a shorter
hydraulic retention time (HRT). A standalone longer HRT THAD process had also been used at the
lower end of the thermophilic temperature range, but due to increased heat requirements of the process,
its use had been limited. Recently, however, the use of Thermal Hydrolysis Process (THP) to pre-treat
sludge feed has significantly increased, resulting in the availability of significant amount of low grade
heat which has to be removed to ensure the effective operation of the MAD process.
In this paper, the performance of THAD of THP pre-treated sludge was investigated in comparison with
that of THP+MAD process, used as a control, within temperature ranges of 45 – 56 oC and 42 oC
respectively.

The results obtained from laboratory scale experiments showed that the THAD process achieved an
overall long-term average VSr range of 50 – 55%, overall biogas production of 420 m3/tonne dry solids
(TDS).

Introduction:
Anaerobic digestion is one of the oldest wastewater treatment process unit operations and is an
essential tool for converting potential pollutant organic wastes to useful by-products and protecting the
environment and public health.

It is now recognised, that anaerobic digestion process converts potential pollutant organic substrates
to economically useful by-products such as biogas and digestate.  This offsets some of the expenditure invested to treat the wastewater. It also offers a public health (sanitation) benefit by eliminating odour, killing pathogens and preventing the spread of diseases. This particularly applies to industrialised countries of Europe and some parts of America.  A retrospective view and current state on the use of low and high temperature range thermophilic anaerobic digestion of thermally hydrolysed sewage sludge in comparison with mesophilic anaerobic digestion was given in Shana et al. (2016).

Therefore, no additional extensive literature review was included in the present paper, but if needed a
reader is guided to an up-to-date literature review on current topic in Shana et al. (2016).

This paper is an extension to the previous paper by Shana et al. (2016) that reported thermophilic
anaerobic digestion of thermally hydrolysed sludge in the temperature range of 46 – 50 oC, whereas
the current paper adds data obtained from 52 – 56 oC.

The most important operational factors that can significantly affect the robustness of an anaerobic
digestion process performance are digester temperature, substrate quality, organic loading rate,
retention time and operator’s skill. The most important of all is the operator’s skill. An operator is the
one who makes operational changes, i.e., controls and maintains the required digester temperature,
organic loading rate, and retention time as well as keeps the sludge well mixed.

The operator skill attributes such as knowledge of anaerobic digestion process, attitude, ownership and
team work spirt are essential ingredients for a successful anaerobic digestion process and the quality
of the data obtained.

Poor operational skill (an external factor) can often be taken for inadequate anaerobic digester
performance process caused by the malfunctioning of bacteria population in a digester or poor quality
substrate being fed. The lack of knowledge on anaerobic digestion process, inconsistencies in digester
feeding regime, indifference or lack of ownership, and constant reactive work by untrained operators
can cause significant damage to anaerobic digestion process.

Anaerobic digestion process is a biotechnology that requires the leadership of skilled technical
personnel and should be only run by a well-trained and proactive minded team of technicians. Even the
involvement of a single untrained operator in a team, if allowed to run the anaerobic digestion process,
can cause a prolonged setback to sustainable anaerobic digestion process performance.

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