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Conference 2012 - Efficient Energy Management in the Water and Waste Industry
Page 1 of 12
Utilisation of Biogas at Shatin Sewage Treatment Works
FUNG Wing Cheong and YEUNG Tak Kuai
Electrical and Mechanical Projects Division
Drainage Services Department
Abstract A significant amount of energy is consumed in the wastewater
treatment. For years, designers and operators have been working hard to reduce the
energy input. Global climate change makes the issue become more pressing than
ever. Incorporation of anaerobic digestion in a wastewater treatment plant not only
reduces the sludge to be disposed of, biogas is also produced as by-product of the
process which is a source of energy. Power and heat generation from biogas has
been practiced for nearly 30 years in Shatin Sewage Treatment Works. Through the
case of Shatin Sewage Treatment Works, this paper will illustrate how biogas could be
used to reduce the energy bill. The design considerations of the replacement of the
old dual fuel generator with the new gas engine will also be presented in this paper.
1. Introduction
The Shatin Sewage Treatment Works (Shatin STW) is the largest secondary sewage
treatment works in Hong Kong. It occupies 28 hectares of land and serves population
of 600,000 in Shatin and Ma On Shan Districts, treating 250,000 m3 of sewage per
day. The Shatin STW was commissioned in 1982 with the treatment capacity of
100,000 m3 per day. After the Stage III Extension recently, the treatment capacity has
increased to 340,000 m3 per day.
The Shatin STW is one of the leaders in the application of renewable energy in Hong
Kong. There are six dual fuel generators supplying electricity and hot water for the
use of the sewage treatment plant itself. The dual fuel engines are powered by the
biogas produced in the anaerobic digestion process. After nearly 30 years in
operation, a new generation of power generation engine is to replace the old dual fuel
engine. This paper reviews the experience of using biogas for power generation and
shares the experience in the design of the new combined heat and power generator in
the Shatin STW.
2. Biogas Production and Characteristics
2.1 Production of Biogas in Shatin Sewage Treatment Works
Conference 2012 - Efficient Energy Management in the Water and Waste Industry
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The Shatin STW is a typical secondary sewage treatment plant incorporated with
biological nutrient removal (see Figure 1). Sewage entering STW is firstly screened
and degritted. It is then diverted to primary sedimentation tanks, in which around
50% of suspended solids are removed. The removed suspended solids are known as
primary sludge.
The primarily treated sewage is then fed into the aeration tanks where microorganisms
absorb or adsorb majority of the organic pollutants remained after primary treatment.
Microorganisms forming the activated sludge grow in the aeration tank.
Liquid-solids separation is done in the final sedimentation tanks. Removal of the
surplus activated sludge maintains the required population of microorganisms in the
treatment process. It is thickened and digested in the anaerobic digester together
with primary sludge.
Figure 1 Flow Diagram of Shatin Sewage Treatment Works
Removal of the organic pollutants, i.e. biochemical oxygen demand or chemical
oxygen demands, is achieved mainly through the removal of primary sludge and
surplus activated sludge from sewage. Mesophilic anaerobic digestion is adopted in
the Shatin STW and the organic matters are decomposed into simple chemicals.
Anaerobic digestion involves a series of processes, such as hydrolysis, acidogenesis,
Conference 2012 - Efficient Energy Management in the Water and Waste Industry
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methanogenesis. Microorganisms break down the biodegradable materials in the
absence of oxygen and, eventually, methane is produced.
The major environmental factors in the anaerobic digestion process are (1) retention
time, (2) temperature, (3) alkalinity, (4) pH and (5) the presence of inhibitory
substances. Among them, the presence of sulphate has greater impact on the
production of biogas in the Shatin STW than in many other places. Sea water toilet
flushing brings in a large amount of sulphate in sewage and sludge. During anaerobic
digestion process, sulfate is reduced to sulfide. The sulfide formed is an inhibitor to
anaerobic digestion. High hydrogen sulfide content (H2S gas) in biogas can adversely
affect the operation of downstream combustion equipment. It is important to control
the H2S concentration in the biogas because combustion will lead to the oxidation of
sulphide into sulphur dioxide which will become sulphuric acid. Sulphuric acid will
damage the combustion equipment. In order to tackle this problem, the Shatin STW
has been using ferric chloride (FeCl3) to control sulfide. FeCl3 dosing can suppress H2S
in biogas to a level suitable for the generators downstream. FeCl3 solution is added to
the digestion tanks and sulphide is precipitated resulting less hydrogen sulphide in the
sludge and hence in the biogas.
Between January 2009 and February 2012, the monthly production rate of biogas
ranges from 304,518m3 to 561,917m
3. Besides the fluctuation in biogas production,
there are also some degree of variation in the gas composition, which can be seen in
the following table:
Compositions Monthly Average
Methane, CH4 49-65 %
Carbon dioxide, CO2 36-48 %
Hydrogen sulphide, H2S 100 – 4705 ppm
3. Existing Dual Fuel Generators
Methane is a flammable gas and its combustion gives out heat. Instead of
flaring off, there are six dual fuel generators installed in the Shatin STW
producing electricity from biogas. Each generator has a four-stroke
compression ignition engine, which operates mainly on biogas fuel with a small
percentage of diesel as pilot fuel to initiate combustion or may operate with
diesel only. The normal power rating is 1.12MW when biogas is used. An
11kV internal grid has been built in Shatin STW to receive the power from the
Conference 2012 - Efficient Energy Management in the Water and Waste Industry
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dual fuel engine and the power supply company. Normally, two dual fuel
generators run in parallel operation for the Shatin STW to consume all the
biogas.
In addition to electricity generation, the system has facilities to recover heat
from the charged air, lube oil, jacket water and flue exhaust of the dual fuel
generator. Water is recirculated to supply the recovered heat to the sludge
digestion heating system in Shatin STW. External heat is required to keep
the digester at mesophlic range in winter.
After nearly 30 years in operation, equipment aging becomes apparent. The
number of breakdown increases and more diesel fuel is consumed. On the
other hand, the energy efficiency of internal combustion engine has increased
in the past few decades. For improving the energy and environmental
performance, the Drainage Services Department has decided to start replacing
these dual fuel generators.
4. Project of Biogas fueled Combined Heat and Power (CHP) Generator
4.1 Selection of CHP generator
As a start, one dual fuel engine was selected for replacement. The design
work was started in early 2010. Because of the previous experience, the
combined heat and power (CHP) generator set was selected. The CHP
generator consists of a lean burn gas engine, an alternator and an integrated
heat recovery system.
The advantages of the CHP generator include:
1. The four-stroke internal combustion engine using lean burn gas
combustion with turbo-charge and after-cool system making the
� diesel is no longer necessary as combustion is spark ignited,
� lower combustion temperatures and hence lower NOx formation from
the air-fuel mixtures,
� efficiency is improved from higher compression ratio for
combustion,
2. The tailor- made heat exchanging system is an integral part of the CHP
generator.
Conference 2012 - Efficient Energy Management in the Water and Waste Industry
Page 5 of 12
The new CHP generator is expected to satisfy the following requirements:
1. The electricity output of the CHP genset shall connect with the existing
electricity distribution system in the Shatin STW. The heat recovery
system should connect with the existing hot water system which supplies
heat to the anaerobic digesters.
2. The CHP genset shall be designed for continuous operation. Together with
not more than one dual fuel genset, the CHP genset should be able to cope
with the change in biogas production rate as well as the anticipated
fluctuation in gas composition. Flare-off should be avoided as far as
possible.
3. The CHP genset is normally run at above 50% of full load to meet the
exhaust requirements. Sufficient load is therefore required to be
connected with the CHP genset to enable continuous and stable operation.
4. The genset should be installed within the space left by the old dual fuel
genset.
Two ratings of CHP genset, i.e. 1MW and 1.4 MW, were considered and the
particulars of the gensets are as follows.
Based on the gas consumption of the CHP generators and the dual fuel
generator and the historical gas production data and characteristics, it can be
shown that the 1.4 MW is the best fit to the biogas gas production in the Shatin
STW. With this combination, a new 1.4 MW CHP generator running with one
existing dual fuel generator could fully utilize the biogas in most of the time
resulting very little chance of operating more than two generators or flaring off
the biogas. The energy efficiency of the 1.4MW generator is also higher.
In 2011, the biogas production monthly average has a 6% increase from 2010, which
is still well within the operation range of the selected 1.4 MW scheme.
1 MW 1.4 MW
Fuel gas LHV in kWh/Nm³ 5.3 5.3
Loading in % 100% 75% 50% 100% 75% 50%
Gas volume Nm³/h 502 386 271 637 491 346
Electrical output in kWe 1064 798 530 1416 1.062 706
Electrical efficiency in % 40.0 39.0 36.9 42.0 40.8 38.5
Conference 2012 - Efficient Energy Management in the Water and Waste Industry
Page 6 of 12
Graph 1 - Biogas production against generators consumption
4.2 Biogas Treatments and Conditioning
Compared with the existing dual fuel generator, the new CHP generator has
more stringent requirements on the biogas quality.
Currently, in the Shatin STW, ferric chloride is dosed to the anaerobic digester to
control hydrogen sulphide. Since the new CHP generator cannot tolerate too much
hydrogen sulphide (maximum 200 ppm), a separate biogas desulphurization system is
required to ensure the limit of hydrogen sulphide (H2S) in biogas not to be exceeded.
Iron sponge is being used for other CHP system in the Drainage Services Department.
However, the high hydrogen sulphide concentration and high gas flow would require a
lot of space and frequent replacement of the media. A system using chemical and
biological process for sulphide removal is selected. The system consists of scrubber,
biological reactor and sulphur separator as shown in Figure 2.
Hydrogen sulphide is absorbed in the scrubber where the pH of the absorbent is
maintained between 7.9 - 8.9. The absorbed hydrogen sulphide is oxidized in the
subsequent biological reactor where majority of the sulphide is reduced to elemental
sulphur. Sulphur will settle and leave the system. The system should be able to
1.4 MW CHP at full
load + 1DF
1.0 MW CHP at full
load + 1DF
1.4MW CHP at 50%
load
1.0 MW CHP at 50%
load
CHP = CHP
generator
DF = dual fuel
generator
0500010000150002000025000
Apr-2009 Apr-2009 Apr-2009 May-2009 May-2009 Jun-2009 Jun-2009 Jul-2009 Jul-2009 Aug-2009 Aug-2009 Sep-2009 Sep-2009 Sep-2009 Oct-2009 Oct-2009 Nov-2009 Nov-2009 Dec-2009 Dec-2009 Jan-2010 Jan-2010 Feb-2010 Feb-2010 Mar-2010 Mar-2010 Mar-2010Gas consumed by df genset (m3) Total gas production (m3)
Conference 2012 - Efficient Energy Management in the Water and Waste Industry
Page 7 of 12
maintain H2S in the biogas not more than 150 ppm.
Figure 2 Block diagram of biogas desulphurization system for the CHP generator
After treatment in the desulphurization system, the biogas is further conditioned by
downstream units. Chiller, separator with demister and air blast cooler are required
to keep the relative humidity in the biogas not more than 50% at the temperature
below 40oC. Activated carbon is required to remove siloxane which could damage
the engine. A gas booster is to maintain the feed gas pressure at 300 mbar to the
engine. The conditioning system is indicated in Figure 3.
Figure 3 The Conditioning System for Biogas Treatment
Conference 2012 - Efficient Energy Management in the Water and Waste Industry
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4.3 Integration of CHP generator into the Electric Power System
The CHP generator will connect with the existing electricity distribution system
(Figure 4). The CHP generator is designed for three operation modes:
(a) running alone,
(b) parallel with existing generators and
(c) future connection with the grid of power supply company
Parallel operation of the generator enables all the biogas to be consumed for heat
and power generation for most of the time. There is more than enough electrical
load to use up the generated power. Large machinery includes air blowers
supplying air for the activated sludge process and the pumps in the sewage
treatment process. The CHP generator will have automatic voltage regulator and
governor to adjust the generator output to match with the load demand.
Connection with the grid of power supply company will further enhance the
operation flexibility of the CHP generator.
Figure 4 The existing dual fuel generator no. 2 will be replaced by the CHP generator
4.4 Connecting the CHP generator into the Heat Recovery System
The CHP generator will have an integrated heat recovery system (Figure 5).
Conference 2012 - Efficient Energy Management in the Water and Waste Industry
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Through the hot water recirculation system, it recovers heat from:
1. air/fuel mixture
2. lube oil
3. engine jacket water.
4. exhaust gas.
The recoverable thermal output (RTO) of the CHP generator is 1408kW. Through
the main heat exchanger, the recovered heat is transferred to the existing hot water
re-circulation system for sludge digestion heating system through the downstream
main heat exchange.
Since the heat demand of the digester may be less than the RTO or the heat recovery
system is shut down for maintenance, a radiator is provided to reject the heat to the
atmosphere so that power generation is not affected.
@ the tested figure shows slightly higher amount of recoverable heat
Figure 5 The heat exchanger system of CHP generator connecting the existing system
4.5 Energy and Environmental Benefits
4.5.1 Electricity and Thermal Energy
The new CHP generator will significant improve the energy performance of the power
Conference 2012 - Efficient Energy Management in the Water and Waste Industry
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generation system in the Shatin STW. The energy efficiency of the new CHP
generator and the existing dual fuel generator are compared in the table below. It
can be seen that significant improvement is expected from the new generator. The
maximum combined energy efficiency is around 83%.
Electrical Energy Efficiency Improvement
New CHP generator #41.7% Dual fuel generator by biogas 33.77 % 23.47%
Thermal Energy Efficiency Improvement
New CHP generator #41.8% Dual fuel generator by biogas 28.30 % 47.69%
# based on biogas Lower Heat Value at 6.5 kWh/Nm³ under full electric load
4.5.2 Estimation on Power Generation and Heat Recovery
It is estimated that the CHP generator will produce 8.5M kWh electricity each year
assuming its availability is 80%. The produced electricity is equal to 23.6% of the
annual electricity consumption (36M kWh) in Shatin STW.
4.5.3 Green House Gas CO2-e Emission
The production of electricity and heat recovery can lead to reduction of greenhouse
gas emission though not directly. With the utilisation of biogas for power generation,
less power is imported from the power supply company and therefore there would be
less greenhouse gas emission offsite.
Year Electricity Generated
by Biogas (in kWh)
Thermal Energy Recovery
from Biogas (in kWh)
Avoidance of Green
House Gas CO2-e
Emission (in Tonne)
2011 8,871,086 7,433,871 8,805
5. Conclusions
Biogas being a by-product in the anaerobic sludge digestion press, nowadays, is a
valuable source of renewable energy at the sewage treatment works. With proper
treatment, the biogas could be used for power generation in the sewage treatment
works. In Hong Kong seawater flushing increases hydrogen sulphide concentration
in the biogas. The new CHP generator requires a higher quality of biogas for use.
The H2S concentration in the biogas should be below 200ppm and the relative
humidity at 50%. Siloxanes should be removed to safeguard the engine against
abrasion.
Conference 2012 - Efficient Energy Management in the Water and Waste Industry
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Utilisation of biogas at the Shatin STW leads to 7,400,000 kWh heat recovery and
8,900,000 kWh power generation each year. It significantly contributes to the
energy efficiency of the sewage treatment works. The new CHP generator is
expected to further improve its energy performance.
Because of heat and power generation at the Shatin STW, the avoidance of
greenhouse gas CO2-e emission is around 8,800 tonnes each year. It indirectly
reduces greenhouse gas emission from Hong Kong.
Conference 2012 - Efficient Energy Management in the Water and Waste Industry
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References
i. Metcalf & Eddy, Inc., George Tchobanoglous, Franklin L. Burton and H. David
Stensel, Wastewater Engineering Treatment and Reuse - Fourth Edition
ii. Approved EIA report EIA-022/1999 for “Sha Tin Sewage Treatment Works,
Stage III Extension - Environmental Impact Assessment Study”
iii. http://en.wikipedia.org/wiki/Anaerobic_digestion
iv. Operations record data from Shatin Sewage Treatment Works
v. Technical Description - Cogeneration Unit JMS 420 GS-B.L- B125
vi. http://www.dieselnet.com/standards/eu/hd.php
vii. http://www.dieselnet.com/standards/de/taluft.php
viii. http://en.wikipedia.org/wiki/Lean_burn#Toyota_lean_burn_engines
ix. EMSD and EPD, Guideline to Account for and Report on Greenhouse Gas
Emissions and Removals for Buildings
x. https://www.clponline.com.hk/ourenvironment/measureourimpact/carbonfootpri
nt/pages/default.aspx?lang=en