Improving energy self-sufficiency in municipal wastewater treatment
plant using renewable energies
J.Kanga, H.J. Yanga, Y.S.Ana, D.S. Kima, W.K. Kimb, D.E. Parkc, K.J. Chaea*
a
R&BD Center, Kolon Global Corp., 199-5 Jeondae-Ri, Pogok-Eup, Cheoin-Gu, Yongin, Korea.
b
R&D Center, Halla E&C Corp., 7-19 Sincheon-Dong, Songpa-Gu, Seoul, Korea.
c
Smart IT Center, Kolon Benit, 41-1 Smart Tower, Joongang-Dong, Gwacheon-Si, Korea.
[email protected]
[email protected]
[email protected]
[email protected]
[email protected]
[email protected]
[email protected]* (corresponding author)
Abstract: Increasing energy prices and concerns about global warming address the need to improve
energy self-sufficiency in municipal wastewater treatment plants (WWTPs). This outline paper presents
renewable energy production and energy self-sufficiency scenarios in a municipal WWTP (30,000 m3d1
) located in Yongin, South Korea. It was assumed that 1) a 100 kW of solar photovoltaic (PV) systems
are installed in the rooftop spaces in the existing buildings, 2) a 10 kW small hydropower plant is
installed in the discharge outlet, and 3) a 25 Refrigeration ton (RT) heat pump is installed to recover
thermal energy from treated wastewater. The energy production by PV and small hydropower were
estimated by the RETScreen software while thermal energy recovery from treated wastewater was
manually calculated using site-specific and heat pump specific conditions. Our results showed that a
total of 486 MWh of energy production was expected when proposed systems are properly installed.
The energy self-sufficiency defined as the percent ratio of renewable energy production and/or savings
to the energy consumption was estimated to be 7.6% at maximum which can vary with energy
consumption in the WWTP. Strategic energy mining specific to site conditions is necessary to
successfully implement renewable energy facilities.
Keywords: energy self-sufficiency, heat pump, hydropower, renewable energy, photovoltaics,
wastewater treatment plant
Introduction
Municipal wastewater treatment plants (WWTPs) are among the most energyintensive facilities, representing a nexus between energy and water (Stillwell et al.,
2009). Collecting, treating, and discharging municipal wastewater to acceptable water
quality standards requires energy, mostly electricity, but also as natural gas or other
fuels. According to the Korea Ministry of Environment (MOE), WWTPs alone
comprise 0.5% of national energy consumption and the portion of WWTPs in total
energy consumption becomes larger within local city and community government
(MOE, 2010). Moreover, increasing population, strict discharge limits, aging structure,
and possible future standard for removing pharmaceuticals and health care products in
water are likely to increase energy consumption in WWTPs.
Recent emphasis on global warming and climate change has brought an energy
paradigm shift in WWTPs from energy consumer to producer, which is an emerging
concept in the power market (Greenfield and Batstone, 2005). As a part of response to
these global needs, the Korea MOE announced a commitment toward energy selfsufficiency in municipal WWTPs by 50 % until the year 2030 (MOE, 2010). The
energy self-sufficiency is defined as the percent ratio of renewable energy production
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and/or energy savings to energy consumption within WWTPs. There are two basic
approaches described by the MOE to achieve the energy self-sufficiency: energy
savings by improving efficiency in unit processes, and energy mining for renewable
and/or unused energy (collectively referred to as “renewable” hereafter) in WWTPs.
In general, the most energy-intensive components in wastewater treatment are
pumping stations for sewer and activated sludge aeration powered by fans and motors,
accounting for 21% and 40%, respectively, in overall energy use (MOE, 2010).
Therefore, improved pumping and optimized aeration are the main components that
can significantly reduce the overall energy use by 3 to 6% (Stillwell et al., 2010).
Renewable and unused energies applicable to WWTPs include solar photovoltaic
(PV), small hydropower, wind power, biogas production, and heat recovery from
wastewater using heat pump. Among these sources, the availability of biogas
production, wastewater heat, and small hydropower is site-specific while PV and wind
power are relatively universal energy sources. From the 50% of energy selfsufficiency target set by the Korea MOE, solar PV was the single most influential
energy source accounting for 23% followed by biogas production (16.4%). Overall
achieving the 50% of energy self-sufficiency in municipal WWTPs in Korea is
translated to 907 GWh/yr of energy production and 558,000 CO2 ton/yr of greenhouse
gas emission.
This study presents the methodology for energy mining in a municipal WWTP as a
preliminary study for a field-scale demonstration project. The objectives of this study
were 1) to estimate annual renewable energy production using site-specific conditions,
and 2) to determine the energy self-sufficiency scenarios depending on energy
production as well as energy consumption in the testing WWTP.
Methodology
Site description and energy mining in WWTP
A municipal WWTP was selected for a field-scale demonstration project (Figure 1).
The WWTP is located in Yongin, South Korea and it was accommodated with soccer
field and walking trail for general public. The WWTP employs B3 (Bio Best Bacillus)
treatment process that is a modified activated sludge process adopted in Korea. Most
treatment facilities were placed underground below the soccer field. From our
preliminary investigations, it was determined that energy production from biogas and
wind power cannot be employed due to the absence of anaerobic digester and lack of
wind at the site. Therefore we considered PV, small hydropower, and thermal energy
from wastewater as viable energy sources in the testing WWTP.
2
Figure 1. Site layout showing potential locations for renewable energy production.
Solar energy is one of the most benign electric generation resources without
air or water emissions. Unlike other renewable resources such as hydroelectric and
wind power, solar power has a location advantage due to the universal availability of
sunlight. Rooftop in facility buildings has been used for PV installation while
sedimentation and/or aeration basins in WWTPs could be additional locations for the
PV installation. In this study, we assumed to install PV systems in the rooftop spaces
in two facility buildings (Figure 2). The available spaces in these two buildings were
determined to a total of 817 m2 that can accommodate 96 kW of mono-crystalline
silicon (Si) solar modules. Additionally, we considered installing building integrated
PV (BIPV) in the administration building, which can replace current windows with
amorphous and transparent Si thin-film modules. The transparent thin-film BIPV
modules are known to reduce energy cost and greenhouse gas emission while
improving light diffusion and aesthetic appearance (Ruther et al., 2003; Maurus et al.,
2004; An et al., 2010). A total of 100 kW systems (PV 96 kW + BIPV 4 kW) were
assumed to be installed in the testing WWTP.
Figure 2. Top-view of fixed PV systems that will be installed in the testing WWTP.
Small hydropower is a collective term defined as 10,000kW or less capacity while
mini (100 to 1000 kW), micro (5 to 100 kW), and pico (<5 kW) hydropower plants
are further classified by design capacity. In general, small rivers and agricultural
reservoirs are the main resources for the hydropower, but drinking water supply
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facilities and WWTPs are also viewed as viable resources due to recent emphasis on
developing small hydropower in existing public facilities. In this study, effluent
discharge area of the WWTP was selected as a location for installing small
hydropower plant as it offers the highest head between the upstream and downstream
within the facilities (Figure 3). A flow rate of water (Q) and a head (H) are essential
two elements in designing hydropower plant. Field leveling revealed that the gross
head (Hg) from the upstream to downstream level is 4.6 m. Using effluent flow data
for the past five years (2007 to 2011), we constructed a flow duration curve (FDC) for
the effluent and determined a design flow for the hydraulic turbine.
Figure 3. Photo of effluent discharge outlet and the results of field leveling
It is well-known that wastewater could a resource of thermal energy that can be used
for heating and cooling buildings with heat pump. Compared with other traditional
sources of energy for heat pump (e.g., groundwater, geothermal heat, outdoor heat),
wastewater from local residential drainage systems exhibits relatively high
temperature because it comes from warm sources such as dishwashers, showers, and
warm industrial plants. In general, the energy potential of treated wastewater is known
to be higher than that of raw wastewater in a WWTP due to minimal heat exchange
interference by solid matters (Schmid, 2008). Ideally, the energy in the treated
wastewater can be further used for the heating of the digester tank and lowtemperature sludge drying. However, such a large energy potential of wastewater
cannot be used in many WWTPs because many plants do not have anaerobic digester
and/or they are located outside residential areas, in which no demand for the heating
and cooling are available. In this study, heating and cooling in the administration
building was assumed to be a sole demand for heating and cooling after surveying the
thermal energy use in the WWTP. We also assumed that treated wastewater is routed
from the end of UV disinfection facility to the administration building through a preexisting recycling water line that was a source of deforming water in the aeration
basins. Figure 4 shows a schematic of wastewater heat recovery system that will be
implemented in the testing WWTP.
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Figure 4. A schematic of wastewater heat recovery system
Estimation of renewable energy production
Among the selected three energy sources, two involves with electric energy
production (PV and small hydropower) while thermal energy production is available
from treated effluent. The RETScreen Clean Energy Project Analysis Software
(shortened hereafter to RETScreen) was used to simulate electric energy production
while thermal energy production was manually calculated according to site-specific
conditions.
RETScreen is capable of assessing a total of 10 renewable energy technologies in
terms of the feasibility of clean energy projects; wind energy, small hydropower, PV,
ground-source heat pumps, combined heat & power, biomass heating, solar air heating,
solar water heating, passive solar heating, refrigeration (Natural Resources Canada,
2003). Our analysis considered renewable energy production and potential energy
savings only; the economic analysis of the proposed energy production in the WWTP,
while highly relevant, was not included in this study.
Photovoltaic
RETScreen integrates a number of databases to assist the site assessor, including a
global database of climatic conditions obtained from 4,700 ground-based stations and
NASA's satellite data (Natural Resources Canada, 2003). We selected Suwon, Korea
for a climate data location. Other input parameters used for simulating electric energy
production by PV are presented in Table 1. A 100 kW of solar systems (PV 96 kW and
BIPV 4 kW) were simulated and it was assumed that the proposed PV system feeds
electrical energy directly into the electric utility grid.
Table 1. Properties of photovoltaics and inverter used for RETScreen simulation
PV type
PV power
PV efficiency
Inverter
Inverter
capacity (kW)
(%)
efficiency (%)
capacity (W)
Mono-Si
96
14.9
95
100
A-Si
4
6.1
95
10
5
Small hydropower
RETScreen provide a Small Hydro Model that can evaluate the energy production,
life-cycle costs and greenhouse gas emissions reduction for central-grid, isolated-grid
and off-grid small hydro projects, ranging in size from multi-turbine small and mini
hydro installations to single-turbine micro hydro systems (RETScreen 2003). We
assumed that a water reservoir is installed in the discharge area of the WWTP and the
produced power is fed to central grid. Other input parameters used for simulating
electric energy production by small hydropower are presented in Table 2.
Table 2. Properties of turbine and generator used for RETScreen simulation
Turbine
Hydraulic
Generator
Turbine type
Gross head (m)
efficiency (%)
losses (%)
efficiency (%)
Propeller
81
4.6
5
98
Thermal energy from treated wastewater
Theoretically the thermal energy reserve from treated wastewater is estimated by:
E = W ⅹ Cp ⅹ dt
(1)
where
E = thermal energy reserve (kcal)
Cp = specific heat of wastewater (kcal / kg °C)
dt = temperature that can be extracted (°C)
The recoverable energy for heating and cooling depends on the coefficient of
performance (COP) for cooling and heating as follows:
Ec = E ⅹ [COPc/(COPc + 1)]
(2)
Eh = E ⅹ [COPh/(COPh + 1)]
(3)
where
Ec = recoverable energy for cooling (kcal)
COPc = coefficient of performance for cooling
Eh = recoverable energy for heating (kcal)
COPh = coefficient of performance for heating
For the calculation of the recoverable energy, it was assumed that we use a heat
exchanger that extract 5°C from the warm water along with a commercial heat pump
that meets performance presented in Table 3. Based on these varying COPs, we
calculated the recoverable energy for cooling and heating. The survey on the thermal
energy use in the administration building of the WWTP suggested that the capacity of
heat pump needs to be designed at a capacity of 25 refrigeration ton (RT). We
calculated the net energy production of the proposed heat recovery system by
subtracting input energy from the produced thermal energy.
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Table 3. Coefficient of performance for cooling and heating in a heat pump as a
function of temperature of source water
Temperature of Cooling/Heating Input energy
COP
source water
capacity
(°C)
(kW)
(kW)
Cooling
15 to 25
107
20
5.46
25 to 32
103
23
4.42
Heating
5 to 10
106
30
3.53
10 to 15
130
32
4.12
Results and Discussion
Estimation of renewable energy production
Photovoltaics
Designing a proper installation for PV array is a key factor in yielding
maximum solar energy production. Figure 5 shows the simulated solar energy
production by Azimuth when installing 96 kW of fixed type, mono-crystalline Si solar
modules. Azimuth is the array’s east-west orientation in degrees. The preferred
orientation should be facing the equator, where the azimuth angle is 0° (South) in the
Northern Hemisphere, which is the case for South Korea. In northern hemisphere,
between the latitudes of 23 and 90, the sun is always in the south (Jackson, 2007).
Therefore, the modules on an array faced to south can get the most out of the sun’s
energy. Figure 6 shows the simulated power production by the slope of solar
collectors. The greatest power production was expected at a slope of 37.5°, which was
in the range of latitude in the simulated area (Suwon, Korea). The installation of BIPV
(4 kW) in the administration building was estimated to produce 4.3 MWh/yr,
assuming its tiling slope at 90° (vertical) and its azimuth at -45° (i.e., 45 degrees west
from the exact northward direction). Installing a total of 100 kW of proposed PV
systems was estimated to produce 155 MWh/yr at maximum.
Figure 5. Annual power production of 96 kW fixed PV varying in the array orientation
(azimuth)
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Figure 6. Annual energy production by PV varying in the slope of solar collectors
Small Hydropower
The effluent flow rate in the discharge outlet showed seasonal variations
(Figure 7). Overall there was an increase in the effluent flow during summer while
decreased flow rate was observed during winter. Diurnal variations were also obvious,
showing a decrease in the flow rate after midnight through early morning, which
reflects decreased sewer water coming into the WWTP (Figure 8). These results
suggest substantial variations in the flow rates, which should be considered in
determining a design flow of the small hydropower plant.
Figure 7. Seasonal variations in the effluent flow rate
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Figure 8. Diurnal variations in the effluent flow rate
We selected the design flow rate at 0.35 m3/s based on the FDC plotted using
the effluent flow rate data for the past five years (Figure 9). A FDC is a graphical
representation of the percentage of time in the historical record that a flow of any
given magnitude has been equaled or exceeded. The effluent flow rate in Figure 9
showed a relatively steady flow compared to typical FDC for river that often shows a
high flow for a short time (Natural Resource Canada, 2004). The design flow for a
small hydropower was determined to be a flow rate (0.35 m3/s) at which the variation
of available flow became steady in the FDC.
Figure 9. The flow duration curve at the discharge outlet of the testing WWTP
In this study, it was assumed that a propeller turbine is installed at the
discharge outlet and available power was simulated by RETScreen after selecting
turbine type, design flow, and gross head as input parameters (Table 2). The
RETScreen determined the power capacity of the turbine to be at 10 kW with the
given conditions. Assuming 96% of system availability (i.e., approximately 15 days of
downtime in a year), 5% of hydraulic loss in the water passages, and a 97% efficiency
in generator, annual power production was estimated to be 57 MWh with a capacity
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factor at 72% (i.e., the ratio of the average power produced by the hydro plant over a
year to its rated power capacity).
Thermal energy from treated wastewater
The temperature data of treated wastewater showed seasonal variations ranging from
12 to 28 °C with a yearly mean temperature at 20°C (Figure 10). The differences
between water and atmospheric temperature varied with seasons with maximum
difference during winter time up to 26°C and minimum difference during summer
time down to 0.4 °C. These temperature differences indicate the potential of thermal
energy recovery for cooling during hot seasons (May to September) and heating
during cold seasons (October to April).
Figure 10. Temperature of treated wastewater and available energy for heating and
cooling (year 2010 data).
For the calculation of potential energy savings by the thermal energy recovery, a 25RT
of heat pump with an input power (25 kW) of the heat pump compressor was assumed
to be installed in the administration building. Annual energy recovery was estimated
to be 660 Gcal with an assumption of year-around heat pump operation. Input energy
required to operation the heat pump was estimated to be 219 KWh/yr. These results
are translated to the net energy savings by 548 MWh/yr. However, in reality, the
heating and cooling in the administration building may not be necessary year-around.
Assuming a 50% of the system operation (i.e., 12-h operation year around), the net
energy savings came down to 274 MWh/yr.
Energy self-sufficiency in the testing WWTP
The energy consumption in the testing WWTP showed an increase over time with a
six-year average at 6,428 MWh/yr (Figure 11). The increased energy consumption
was attributable to an increase in service population and more strict discharge limit on
nutrients (N and P) (personal communications with operation manager). Using the
simulated renewable energy production for PV and small hydropower along with a
hypothetical estimation of thermal energy recovery from treated wastewater, we
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calculated expected energy self-sufficiency in the testing WWTP (Table 4). Including
thermal energy, the energy self-sufficiency was estimated to be 7.6 % when
considering the six-year average energy consumption as a denominator. The expected
greenhouse gas (GHG) emission reduction was estimated to be 260 ton ton CO2 /yr,
which is equivalent to 60 cars not in used (assuming 4.4 ton CO2/yr per car). As a
single energy source, the thermal energy recovery from wastewater appeared to be the
most influential source in overall energy self-sufficiency. If the energy consumption
for 2010 is used, the energy self-sufficiency decreased down to 6.4%.
Figure 11. The energy consumption in the testing WWTP
Table 1. Hypothetical estimation of energy self-sufficiency and greenhouse gas
reduction by employing renewable energies in the testing site
Electrical
Thermal
Photovoltaics Hydropower
Treated water
Total
Design capacity
100 kW
10 kW
25 RT
Average energy
6428
consumption
(MWh/yr)
Expected energy
155
57
274
486
production
(MWh/yr)
Energy self2.4
0.9
4.3
7.6
sufficiency
(%)a
GHG emission
99
32
129
23
reduction
(ton CO2 /yr)b
a
Energy self-sufficiency (%) = (Expected energy production/total energy
consumption)ⅹ100.
Greenhouse gas(GHG) reduction = electrical energy replaced ⅹ GHG emission
factor. The GHG for individual energy production were adapted from MOE (2010):
b
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photovoltaics 0.6405 ton CO2/MWh, hydropower 0.5649 ton CO2/MWh, geothermal
(wastewater heat recovery) 0.4707 ton CO2/MWh.
Conclusions
The estimation of renewable energy production in a WWTP (100 kW solar PV, 10 kW
small hydropower, and 25 RT heat pump) suggested that about 7 % of energy selfsufficiency is achievable when properly installed and managed. The concept of energy
sufficiency proposed by the Korea MOE is a quantitative index that can measure the
efforts of renewable energy implementations in the municipal WWTPs in a nationwide scale. If this self-sufficiency concept is applied to a single WWTP (e.g., 50%
self-sufficiency), it would require strategic energy mining plan specific to the WWTP.
Our study indicated that implementing PV in a large-scale would be a general solution
to increase the energy self-sufficiency, but there could be some limitations in available
spaces particularly for the WWTPs located in urban areas. In this case, thermal energy
recovery from wastewater could be a viable source because there could be more
thermal energy demand from the urban areas with a greater proximity.
Acknowledgement
This project was supported by Korea Ministry of Environment as "Global Top
Project"(Project No.: GT-11-B-01-010-0).
References
An, Y., Kim, S., Lee S., Song J., Hwang, S., and Yoon J. (2010) Case study on 5 kWp
transparent thin-film BIPV system. Journal of Korean Solar Energy Society 30(4),
29-35.
Greenfield, P.F. & Batstone, D.J. (2005) Anaerobic digestion: Impact of future
greenhouse gases mitigation policies on methane generation and usage. Water Sci.
Technol. 52(1-2), 39-47.
Jackson, F. (2007). Planning and Installing Photovoltaic Systems: A Guide for
Installers, Architects and Engineers. Earthscan, Washington DC.
Maurus, H., Schmid, M. Blersch, B., Lechner, P. & Schade, H. (2004) PV for
buildings: Benefits and experiences with amorphous silicon in BIPV applications.
Refocus 5(6), 22-27.
Ministry of Environment (MOE), Republic of Korea, (2010) Strategic Plan for the
energy self-sufficiency in the municipal wastewater treatment plan (Korean
language).
Natural Resources Canada (2003). Clean Energy Project Analysis: RETScreen
Engineering & Cases Textbook – Small Hydro Project Analysis. (Catalogue no.:
M39-98/2003E-PDF). Accessed from the World Wide Web (5/31/2011):
http://www.retscreen.net/download.php/ang/109/0/textbook_hydro.pdf.
Natural Resources Canada (2004). Micro-Hydropower Systems: A Buyer’s Guide.
Accessed from the World Wide Web (5/31/2012):
http://canmetenergy.nrcan.gc.ca/sites/canmetenergy.nrcan.gc.ca/files/files/pubs/bu
yersguidehydroeng.pdf.
Ruther, R., da Silva, A.J.G., Montenegro, A.A., Salamoni, I.T., Kratzenberg, M. &
Araujo, R.G. (2003) Assessment of thin-film technologies most suited for BIPV
12
applications in Brazil: the PETROBRAS 44 kWp project. 3rd World Conference
on Photovoltaic Energy Convention, May 11-18, 2003. Osaka, Japan.
Schmid, F. (2008) Sewage Water: Interesting Heat Source for Heat Pumps and
Chillers. Proceedings of the 9th IEA Heat Pump Conference. Number 5.22.
Stillwell, A.S., Hoppock, D.C., & Webber, M.E. (2010) Energy Recovery from
Wastewater Treatment Plants in the United States: A Case Study of the EnergyWater Nexus. Sustainability 2(4), 945-962.
Stillwell, A.S., King, C.W., Webber, M.E., Duncan, I.J. & Hardberger, A. (2009)
Energy-Water Nexus in Texas. University of Texas at Austin / Environmental
Defense Fund. Accessed from the World Wide Web (5/31/2012):
http://www.edf.org/documents/9479_Energy-WaterNexusinTexasApr2009.pdf.
Wett, B., Buchauer, K., and Fimml, C. (2007) Energy self-sufficiency as a feasible
concept for wastewater treatment systems. IWA Leading-Edge Conference on
Water and Wastewater Treatment, Singapore.
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