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igem news – yppc

MAXIMISING THE RECOVERY

OF BIOGAS ENERGY

ANAEROBIC CO-DIGESTION OF SEWAGE SLUDGE WITH FOOD WASTE

By Crystal Chan, Electrical & Mechanical Engineer, Electrical and FIGURE 1 THE DRAGER SAMPLING TUBE WAS USED TO COLLECT
Mechanical Projects Division, Drainage Services Department, HKSAR GAS SAMPLES FROM ANAEROBIC DIGESTER NO 3 AT THE TAI PO
SEWAGE TREATMENT WORKS
and YPPC 2020 finalist

Current problems and remediation rest is mostly carbon dioxide). The
Issues of municipal solid waste and Drager sampling tube gas detector was
limited landfill space used to collect gas samples from the
According to the United Nations, anaerobic digester at the Tai Po sewage
the global volume of food wastage treatment works (Figure 1).
is estimated as 1.6 billion tonnes of
‘primary product equivalents’. Total Strategic framework of biogas utilisation
food waste for the edible part of this With a view to enhancing biogas
amounts to 1.3 billion tonnes. In Hong utilisation and economic performance
Kong, food waste is a major constituent in a CHP system, a strategic framework
(31 per cent) of the municipal solid was proposed (Figure 2). Combined
waste disposed of at landfills. Disposal heat and power (CHP), gas purification
of biodegradable food waste at landfills and enrichment and digestate are the
is unsustainable and environmentally three methods for biogas utilisation.
undesirable as it depletes the limited The use of tri-generation systems
landfill space, creates odour nuisance, and thermal hydrolysis is being
generates leachate and landfill gases and investigated, and other potential biogas
squanders the useful organic contents. applications such as using biogas as
a natural gas substitute in private gas
Anaerobic co-digestion of food waste and networks and fuel cell technology are
sewage sludge also being explored. system efficiencies of 60 to 80 per cent,
The synergy effect of a significant compared to 50 per cent for conventional
increase in biogas production Combined heat and power (CHP) technologies (i.e., purchased utility
To tackle the issues of food waste and technologies electricity and an on-site boiler).
limited landfill space in Hong Kong, Sewage treatment is an energy-intensive
implementation of anaerobic co-digestion endeavour that demands mostly electrical Problems of biogas utilisation: Low
of sewage sludge and food waste is a power. An obvious way to reduce power utilisation efficiency and imbalance in
panacea. The results of feasibility studies consumption is to generate it internally thermal energy utilisation
into anaerobic co-digestion demonstrate from the digester biogas. This not only In order to make full use of the available
a significant 30 per cent increase in reduces the operating costs, but also biogas and improve the efficiency of
biogas production and a 30 per cent utilises a renewable energy source and energy utilisation, combined heat and
reduction in sludge volume. displaces fossil fuels. While other options power (CHP) and micro-turbine systems
In other words, food waste is such as fuel cells and steam turbines are currently implemented in large
considered a valuable renewable exist, gas engines, gas turbines and sewage treatment plants, providing
energy source in the process of micro-turbines are the most common an effective solution for generating
maximising biogas energy. To increase CHP technologies used to convert electricity by burning biogas and
the usage of renewable energy, this anaerobic digester gas to electrical producing hot water as a by-product.
paper focuses on effective utilisation power and process heat. However, the low heating demand
of biogas to achieve energy neutrality of digesters is one major problem of
of sewage treatment plant and to Advantages and disadvantages CHP systems and a large amount of
minimise carbon footprints. of using CHP thermal energy has to be released into
Gas engines, gas turbines and micro- the environment, which results in
Biogas composition and process for turbines are discussed in Table 1, significant energy waste.
generation of power and their relative advantages and
Sludge, a by-product of the sewage disadvantages are compared. Strategies to maximise heat
treatment process, produces biogas By using waste heat recovery utilisation efficiency
during anaerobic digestion. Biogas technology to capture wasted heat Method 1: Tri-generation system
is a form of renewable energy which associated with electricity production, Tri-generation system for retrofitting
contains 55-65 per cent methane (the CHP systems typically achieve total existing reciprocating CHP

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