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Combined heat and power generating system in Tai Po Sewage Treatment Works
At present, RE installations in DSD''s plants generate over 28 million kilowatt-hours of electricity per annum,constituting around 9% of DSD''s annual energy demand. In terms of efficiency, DSD''s current annual REcontribution is equivalent to the energy consumption of 5,600 households and can reduce about 19,600 tonnesof carbon dioxide (CO2) emission a year.
Over the years, DSD has endeavoured to utilise the open space in STWs and SPSs to install photovoltaic (PV) system. As at end March 2019, DSD has installed PV panels at 15 STWs and 13 SPSs to harness solar energy by maximising the use of the plants'' space. The major plants include Sha Tin STW, Yuen Long STW, Shek Wu Hui STW and Stonecutters Island STW. Notably, the Solar Farm at Siu Ho Wan STW commissioned at the end of 2016, comprises over 4,200 units of polycrystalline PV panels which can generate as much as 1.1 million kilowatt-hours of electricity annual, making it the largest PV system among the Hong Kong SAR Government facilities at present. In 2018-19, the total PV system of DSD is generating about 1.12 million kilowatt-hours of electricity.
DSD will continue to extend the use of solar power in its facilities and install more PV panels at various pumping stations and STWs. The total generation capacity of the new systems is about 0.9 megawatts, including the installation of thin-film PV panels above the sedimentation tanks at Stonecutters Island STW, fully utilising space to generate energy. It will become the largest thin-film solar system in Hong Kong, with a generation capacity of over 0.5 megawatts.
PV Installations at Sha Tin Sewage Treatment Works
Hydro-turbine system at Stonecutters Island Sewage Treatment Works
Sludge, a byproduct of sewage treatment process, produces biogas during anaerobic digestion. Biogas is a formof renewable energy which contains 65% methane (the remaining components mainly being carbon dioxide).DSD utilises combined heat and power (CHP) generators and gas-turbines that run on biogas to generateelectricity and heat for in-house use. In 2018-19, the total electricity generation capacity of five CHP generatorsat Sha Tin STW, Tai Po STW and Shek Wu Hui STW is about 3.6 megawatts, while two gas-turbines at Sha TinSTW and Yuen Long STW have a total electrical generating capacity of 280 kilowatts. This year, the total energygenerated by biogas at our STWs amounted to about 27 million kilowatt-hours.
A 30 kilowatt micro-turbine generator atYuen Long Sewage Treatment Works
A 1.4-megawatt combinedheat and power generatorat Sha Tin SewageTreatment Works
A new biogas-fueled combined heatand power generator at Sha Tin SewageTreatment Works
To maximise the use of biogas generated during the sludge treatment process, we are installing additional CHPgenerators and gas-turbine systems at our STWs such as Sha Tin STW, Tai Po STW and Yuen Long STW. Oncecompleted, it is anticipated that up to 5.4 megawatts of electricity can be generated. In addition, a tri-generationsystem, which generates heat, cooling and power has been installed at Sha Tin STW to increase the use of RE.
To harness the synergy effects of the food waste and sludge co-digestion process, DSD has as worked in closecollaboration with the Environmental Protection Department (EPD) to conduct a "Food Waste/Sludge AnaerobicCo-digestion Trial Scheme" in Tai Po STW. DSD is also planning to implement similar scheme at other suitablesewage treatment works on a larger scale.
Food waste is the largest component of municipal solid waste in Hong Kong 2018, 11,428 tonnes of municipal solid waste were disposed of at landfillseach day, 3,565 tonnes of which were food waste. Sludge, a by-product ofsewage treatment process is also solid waste. Currently, around 1,100 tonnesof sludge are taken to T • Park for incineration each day. On the one hand, foodwaste and sludge are solid waste; on the other hand, they are rich in energyresources.
Source: Hong Kong 2018 Waste Statistics – At a glance, Environmental Protection Department
Anaerobic digestion refers to the microbiological breakdown of organic matter in the absence of oxygen. Inthis process, the odour and pathogens of waste and organic matter will be reduced, with biogas, a renewableenergy source, generated as byproduct. Currently there are two Organic Resources Recovery Centres in HongKong adopted anaerobic digestion to treat food waste: Organic Resources Recovery Centre Phase 1 in Siu HoWan is currently in operation and Organic Resources Recovery Centre Phase 2 in Sha Ling is expected to becommissioned in 2022. These two centres are targeting 200 tonnes and 300 tonnes of food waste per dayfor treatment respectively and the digestate from these centres will then be further treated and turned intocompost.
Currently, the sludge generated in the DSD''s secondary STWs is treated in the anaerobic digesters of the plant the anaerobic digestion process, the volume of sludge is reduced and biogas is produced for generatingelectricity and heat. In some countries, digested sludge will undergo further treatment for use as compost,while in Hong Kong the digested sludge is dewatered and then transported to T • Park for use as fuel in theincineration process, realising the concept of waste-of-energy. Heat produced from the incineration process isrecovered and converted into electricity.
Anaerobic co-digestion of food waste with sludge, hereinafter referred to as co-digestion, has been successfullyapplied overseas. Compared with anaerobic mono-digestion of food waste and sludge, co-digestion reducedsolids content and increases the production of biogas, which can be used to generate energy and supplementthe STWs'' energy consumption. With sufficient food waste and sludge, co-digestion has the potential to producesufficient biogas to meet the energy demand of STWs. The Government is exploring the use of co-digestion inexisting STWs to extend the network of organic resources recovery facilities, hence raising Hong Kong''s foodwaste treatment capacity.
The 2016 Policy Address announced that a co-digestion trial scheme will be conducted at Tai Po STW from 2019onwards, treating 50 tonnes of food waste and sludge everyday. The trial scheme includes the construction ofa food waste pre-treatment plant with a treatment capacity of 50 tonnes, located at the Shuen Wan LeachatePre-treatment Works, adjacent to Tai Po STW and an operation trial scheme. The co-digestion trial scheme iscollaboratively run by the DSD and EPD.
Installation of new mixer inside the sludge digester designated for food waste and sludgeco-digestion at Tai Po Sewage Treatment Works
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