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FIGURE 3 PROCESS FLOW OF FOOD WASTE AND SEWAGE SLUDGE ANAEROBIC CO-DIGESTION
Method 2: Thermal hydrolysis process for
anaerobic digestion
Anaerobic digestion is limited by the
hydrolysis (or solubilisation) stage. To
overcome this limitation, different types
of pre-treatment have been suggested:
biological (enzymatic), chemical
(ozonisation, acid or alkaline hydrolysis),
physical (ultrasonic, pulses, high-pressure
homogenisation, centrifuge) and thermal
(thermal hydrolysis). An abundance of
literature discusses the relative merits
of each technology. To maximise the
utilisation of heat and electricity, thermal
hydrolysis and thermophilic digestion
(with a temperature over 50°C) are
proposed as the pre-treatment of choice
due to their techno-economic benefits, FIGURE 4 COMBINED HEAT AND POWER (CHP) GENERATORS AT A SEWAGE TREATMENT WORKS
increased biogas yields and reduced
biosolid volume.

Strategies to maximise electricity
utilisation efficiency
On-grid connection for CHP generating
systems or micro-turbines
According to an agreement with
supplier China Light and Power (CLP),
power generated by CHP or via micro-
turbine is fed directly into the electrical
switchboard of the water heater house.
The electrical energy produced is
supplied to electrical appliances and
equipment such as lighting, ventilation
fans, motors, etc., within the water
heater house, with a backup power
supply from CLP. Therefore, the
reliability of the electricity supply is
increased whether the micro-turbine is
in operation or not.

Gas purification and enrichment
Natural gas substitute system is also used to heat up the However, it is recognised that its
Upgrading biogas to biomethane and hospital’s potable and non-potable hot related investment costs are still high,
injecting it into the natural gas grid is water supply systems. so commercial deployment is yet to
an efficient way of integrating biogas be achieved. Energy input to a MCFC
into the energy sector. Biomethane can Option 2: Using biogas as fuel or feedstock at is equivalent to two per cent of the
then be used as a substitute for natural gas production plants electricity generated, with electrical
gas and as a transportation fuel. Subject to the location of gas sources, and thermal efficiencies of 50 and
biogas could be used as a fuel or 40 per cent respectively. In addition,
Option 1: Blending biogas directly into the feedstock for the Tai Po gas production the operating requirements and
gas networks plant belonging to the Hong Kong & performance of food waste and sewage
The biogas generated at the Tai Po China Gas Company, for example using sludge co-digestion - which can turn
sewage treatment works could be biogas generated at the nearby Tai Po food waste into useful energy, increase
blended directly into the gas networks sewage treatment works. According to the yield of biogas and reduce the
for use by gas customers in applications preliminary information, the energy burden of landfills - are being studied in
such as gas water heaters, burners, etc. efficiency of the biogas could reach 90 the context of Hong Kong’s environment.
The energy efficiency of the biogas per cent, depending on its applications.
CHP system at the Alice Ho Miu Ling Digestate (hydrothermal carbonisation
Nethersole Hospital in Tai Po is nearly Fuel cell technology of sewage sludge)
87 per cent. The CHP plant produces Fuel cell technology is in an advanced Hydrothermal carbonisation (HTC) is a
electricity and generates waste heat stage of development. Due to its high thermochemical process that converts
at a temperature of 500°C from the efficiency, low emissions and multiple liquid biomass into so-called biocoal,
generator’s exhaust, which goes to a fuel applications, the molten carbonate which can be used as a solid fuel or soil
waste heat boiler to produce steam. fuel cell (MCFC) is considered one conditioner. In the context of sewage
Waste heat from the generator’s cooling of the most promising technologies. sludge treatment, HTC may be an

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