Three Phase Separation of Thermal Hydrolysed Waste Activated Sludge

Luning, L., van de Ven, M., Edens, J. and Traksel, D., Sustec (part of DMT), Netherlands



Thermal hydrolysis process (THP) has proven to be a valuable process to optimize anaerobic digestion
by increasing the biogas production from waste activated sludge (WAS) and by substantially increasing
the capacity of digester plants. Sustec BV (part of DMT) is continuously developing its continuous THP
(cTHP) technology TurboTec®. Within these developments, Sustec BV discovered the capability of
WAS to be separated into three different phases with direct dewatering at high temperatures (>100 °C)
after cTHP. The produced (gelatinous) intermediate, formed during this thermal treatment without
chemical additions, mainly consists of polysaccharides and proteins. The intermediate composition
seems to have similarities with the produced alginate out of Nereda® sludge. However, in contrast to
the formation of alginate out of Nereda® sludge, this recovery of potential valuable materials can be
applied on all thermal hydrolysed WAS. At this moment, further research is being done on the exact
composition of the produced intermediate and its potential applications


Waste water can be considered as a significant potential source for renewable energy and raw
materials. Water authorities such as the Dutch Waterboards are committed to striving for the
transformation of waste water treatment plants (WWTP’s) into “energy and resource factories”. This
objective is also reflected in the European project: “full scale demonstration of energy positive sewage
treatment plant concepts towards market penetration”, (Powerstep). Next to improvements in the water
line, specifically the sludge line has great potential for recovery of renewable energy, nutrients, valuable
resources and las but not least cost reduction.

Mesophilic anaerobic digestion (MAD) is applied at WWTP’s to convert surplus sludge into stabilised
biosolids and biogas. The digester performance can be strongly enhanced by applying a thermal
hydrolysis process (THP) as a pre-treatment prior to digestion. In a THP pre-treatment the sludge is
treated at a high temperature (140 – 160 °C) and high pressure (4 – 6 bar). The advantages of applying
THP are more biogas production, increased volatile solids (VS) reduction, shorter digestion retention
times and an increased total solids (TS) content of the final sludge cake. By THP it is also possible to
apply higher loadings to the digester, resulting in more capacity in the same volume. These effects
result in substantial lower costs for the sludge treatment, while at the same time more renewable energy
is produced.

The potential of the THP-process was recognised almost a decade ago by the Dutch company Sustec
BV, when it decided to develop a practical process of its own. The activities led from lab experiments
to determine overall process parameters, to pilot plant development and operation and finally to fullscale
design, construction, and operation.

Developing the basic ideas into a working process has led to the typical set-up of the TurboTec® process
as illustrated in Figure 1. In the Netherlands two full-scale references at WWTP Venlo and Apeldoorn are nowadays operational and working according to this principle, while two additional plants are in the
design stage.

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