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alternative to anaerobic digestion or a FIGURE 5 COMPARISONS OF THERMAL ENERGY EFFICIENCY BETWEEN A TRI-GENERATION SYSTEM AND A CHP SYSTEM
complement. In the latter case, digested
sludge serves as feedstock for HTC. The
advantages of HTC are nearly complete
conversion of the organic matter, very
good dewaterability of the resulting
sludge and improved energy balance.
HTC is a recent technology on the
market, even though the process has
been known for over a hundred years.

New technologies to boost biogas generation
Application of ultrasonic pre-treatment FIGURE 6 SCHEMATIC DESIGN OF COMBINED HEAT AND POWER SYSTEM AT THE ALICE HO MIU LING NETHERSOLE
of sludge for anaerobic digestion HOSPITAL IN TAI PO (WSP, 2018)
Other studies are being carried out to
boost biogas generation from sewage
sludge by adopting new technologies
and processes. The new technologies
and processes include ultrasonic
pre-treatment of sludge to enhance
anaerobic sludge digestion.
Ultrasound disintegration is a very
effective mechanical pre-treatment
method on surplus activated sludge to
boost biogas production and enhance
the biodegradability of the sewage
sludge by cavitation.
The basic goal of the ultrasound
technique is to weaken the bacterial cell
wall and to make intracellular matter
available for subsequent degradation.
The results show a significant increase
of biogas, from 2951 to 3736m³/day,
between pre- and in-trial periods.
Figure 7 shows that high biogas
production values were achieved
while using the Ultrawaves high-
power ultrasound (HPUS) system as a
sludge pre-treatment during anaerobic FIGURE 7 ANAEROBIC DIGESTION PERFORMANCE AT THE SHEK WU HUI SEWAGE TREATMENT WORKS DURING OPERATION USING THE
digestion. To achieve maximum gas ULTRAWAVES HIGH-POWER ULTRASOUND (HPUS) SYSTEM
production, the percentage treatment
volumes of feed and biomass were
chosen as dynamic variables and the
optimisation of these variables was
carried out using the response surface
methodology (RSM) technique.

Way forward
Installation of additional CHP
generators and gas-turbine systems
To maximise the use of biogas generated
during the sludge treatment process,
DSD is installing additional 800kW
and 600kW CHP generators and gas-
turbine systems at its sewage treatment
works at Sha Tin, Tai Po and Yuen Long
respectively. Once completed, it is
anticipated that up to 5.4 megawatts of
electricity could be generated.
absorption chiller to generate chilled applications, such as using biogas as
Conclusions water. To maximise the utilisation of a natural gas substitute in private gas
The thermal energy efficiency of the heat and electricity, thermal hydrolysis networks and the application of fuel
original CHP system improved by and thermophilic digestion (with a cell technology are also being explored.
adopting the tri-generation system temperature over 50°C) are proposed as This study raises the idea that it might
in the summer, as the waste heat this increases biogas yields and reduces be a viable business plan for a private
can be used as a source to drive the biosolid volume. Other potential biogas entity to operate the co-digestion of

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