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Heat recovery from wastewater: Assessing the potential in northern areas

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Lauri Mikkonen
Lauri Mikkonen
Lauri Mikkonen
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Jaakko Rämö
Jaakko Rämö
Jaakko Rämö
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Riitta L Keiski
Riitta L Keiski
Riitta L Keiski
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Eva Pongrácz at University of Oulu
Eva Pongrácz
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1 Introduction Wastewater from the industry and municipalities always contain a certain amount of heat. The temperature of discharged wastewater can be considerably higher and more stable compared to the sourroundigs. Swedish studies have shown that decreasing the temperature of wastewater by one degree by recovering heat can bring 720 GWh of energy savings annu-ally. However, this energy potential is often unused due to the lack of awareness and proper technology. Consequently, heat is discarded to the environment and the embedded energy is wasted. Heat recovery from wastewater could be a considerable source of energy at water utilities. Recovered heat could be utilised for warming the buildings of the water utility or heat-ing water. Processes, such as anaerobic digestion and sludge drying could also benefit from the recovered heat. Furthermore, heat is often being transferred into a district heating system. The use of heat pump technology could also increase the overall efficiency of the heat recov-ery system by being able to provide cooling energy during the summer season (Tekes, 2013).
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161
WATER RESEARCH
at the University of Oulu
Heat recovery from wastewater:
Assessing the potential in northern areas
Lauri Mikkonen1*, Jaakko Rämö2, Riitta L. Keiski3 and Eva Pongrácz1
1University of Oulu, Thule Institute,
Centre of Northern Environmental Technology (NorTech Oulu),
FI-90014 University of Oulu, P.O.Box 7300
2University of Oulu, Thule Institute,
FI-90014 University of Oulu, P.O.Box 7300
3University of Oulu, Department of Process and Environmental Engineering,
Heat and Mass Transfer Process Laboratory,
FI-90014 University of Oulu, P.O.Box 4300
1 Introduction
Wastewater from the industry and municipalities always contain a certain amount of heat.
The temperature of discharged wastewater can be considerably higher and more stable
compared to the sourroundigs. Swedish studies have shown that decreasing the temperature
of wastewater by one degree by recovering heat can bring 720 GWh of energy savings annu-
ally. However, this energy potential is often unused due to the lack of awareness and proper
technology. Consequently, heat is discarded to the environment and the embedded energy is
wasted. Heat recovery from wastewater could be a considerable source of energy at water
utilities. Recovered heat could be utilised for warming the buildings of the water utility or heat-
ing water. Processes, such as anaerobic digestion and sludge drying could also benet from
the recovered heat. Furthermore, heat is often being transferred into a district heating system.
The use of heat pump technology could also increase the overall efciency of the heat recov-
ery system by being able to provide cooling energy during the summer season (Tekes, 2013).
2 Objectives of the research
This research is performed as a part of the Water Asset Renewable Energy Solutions Northern
Periphery project. The aim of the project is to nd and explore hidden, renewable energy
potential in the water utilities in the Northern Periphery. In this rst stage, we assess the vi-
ability of heat recovery from wastewater in northern areas.
3 Heat recovery from wastewater in northern areas
A heat recovery system can be implemented in different stages from its source to the water
utilities as we can see from Figure 1. Heat can be recovered immediately after the wastewater
is produced (Figure 1a). These applications are often very small-scale and the recovered heat is
used for pre-heating domestic hot water. In addition, there are applications in which the heat
recovery system has been installed in a sewer (Figure 1b). With this option, the heat transfer
area can be relatively large. Conventionally, heat recovery systems are installed at the waste-
water treatment plants (Figure 1c) after the wastewater is treated in order to avoid fouling
of the heat exchanger. According to Wanner et al. (2004), a restriction with recovering heat
before the water treatment process is that the temperature drop can affect the biological water
treatment processes, such as nitrication, resulting in a less efcient purication (Tekes, 2013).
*Corresponding author, E-mail: lauri.mikkonen@oulu.
162
Figure 1 Options for placing a heat recovery system (based on Eavag, 2013).
Heat can be recovered from wastewater by using either a heat recovery system or a heat
pump. The rst system is generally a heat exchanger in which wastewater ows through the
exchanger containing another uid in addition to the wastewater. Heat is being transferred to
the other uid having lower temperature than that of the wastewater. This kind of system is
often installed in smaller scale applications, such as in buildings or inside sewer pipes. Larger
scale systems often include a heat pump, which is more efcient, even though investment
costs of the technology are considerably higher compared to heat recovery systems (Meg-
gers et al., 2010).
In heat pump systems, a working uid having a temperature lower than the wastewater re-
ceives heat from the wastewater. Heat is recovered by an evaporator, which is basically a heat
exchanger. The uid is moved by a compressor, which is also raising the pressure and tempera-
ture of the working uid. Compressed uid is led to a condenser, where heat is rejected for
usage. The rejected heat is not only sensible, but also latent, since a phase change from gas to
liquid is often related. After the condenser, the cooled uid is owing to an expansion valve,
where the pressure and temperature of the working uid are decreased into a level that heat
can be transferred from wastewater to the uid again. What makes a heat pump often more
efcient compared to a heat recovery system is that the energy output is considerably higher
compared to the electricity consumption of the compressor. In other words, the coefcient
of performance (COP) is higher than two. Furthermore, the heat pump systems often have
an advantage of being able to operate the other way around, that is, producing cooling energy
(Meggers et al., 2010).
The main factors affecting the energy potential in wastewater is the amount of discharge
and the temperature of the wastewater. The amount of wastewater discharge is strongly
depended on the amount of citizens. The temperature, however, depends on a number of
factors, mostly on the outside and ground temperatures, the length of the sewage pipe and
the retention time of wastewater in the sewer. The more the temperature can be allowed to
decrease by the utility, the more heating energy can be extracted. However, there are already
a few operating wastewater heat recovery processes in northern areas, so this technology is
viable also in cold climates (Tekes, 2013).
163
WATER RESEARCH
at the University of Oulu
3.1 Heat recovery plant in Hammarbyverket, Stockholm
Hammarbyverket in Stockholm, Sweden, is the largest wastewater heat recovery plant in the
world, receiving a wastewater discharge of 4000 – 18 000 m3/h. The plant has installed seven
heat pumps with the total capacity of 225 MW. These heat pumps are recovering heat from
treated wastewater, producing 1235 GWh of heat annually. The produced heat is used to
warm up 95 000 residential buildings. In order to maximise production, the plant produces also
cooling energy for the district cooling network. In addition, a small, 315 kW turbine is installed
in order to produce electricity from the hot side stream (Fortum, 2013).
The temperature of the incoming wastewater from the wastewater treatment plant in Ham-
marbyverket varies between 7 and 22 °C. After the heat pump system, the temperature of
the efuent is 1 – 5 °C. Even though the heat pump system can raise the temperature up to
80 °C, two bio-oil and two electricity boilers are installed in order to increase the temperature
up to 120 °C during peak consumption hours and colder periods (Fortum, 2013).
3.2 Heat recovery plant in Lapua
Lapua waterworks in Finland has installed a heat pump system in 2012 with the nominal power
capacity of 120 kW. The system recovers heat from wastewater after the treatment process,
allowing the temperature to drop to 3 °C. The produced heat is utilised to warm the buildings
of the treatment utility. There is no long-term experience of the installed system yet, but it
was estimated that the system will be able to save 20 000 euros annually. With an investment
of 45 000 euros, this means that the payback period is 2 – 3 years (Tekes, 2013).
3.3 Heat recovery plant in Sandvika, Oslo
In Sandvika, Norway, heat is recovered from wastewater by using a heat exchanger in the
main sewer of the community. A heat pump system has been installed in order to get the
maximum benet from the system. Also in this case, the wastewater stream is treated before
the heat is recovered with three heat pumps having nominal capacities of 20 MW for heating
and 18 MW for cooling energy. The average discharge through the sewer is 3 000 l/s and the
temperature of the wastewater is allowed to decrease to 4 °C. The system is estimated to
decrease CO2 equivalent emissions by 6 000 tons annually. The wastewater heat recovery
system supplies energy for ofces and residential buildings, satisfying more than 50 % of their
energy consumption (Tekes, 2013).
4. Conclusions
Heat recovery from wastewater can provide a signicant potential for providing heating energy
for end-users in northern areas. Even though there is a number of larger scale applications in
use, there is still a potential for lower than megawatt scale applications. At the moment, the
two main technologies for heat recovery from wastewater include a heat recovery system
and heat pumps, where the latter can also provide cooling energy. In most cases, especially in
the northern areas, the heat recovery system is installed after the wastewater is treated, in
order to avoid fouling and decreased efciency of biological wastewater treatment processes.
Acknowledgements
This research is performed within the Water Asset Renewable Energy Solutions (WARES)
project. The authors acknowledge the funding of Northern Periphery Programme provided
for the WARES project.
164
References
Eavag 2013. Process Engineering. Heat recovery from wastewater. [Online] Available at:
http://www.eawag.ch/forschung/eng/schwerpunkte/abwasser/waermerueckgewinnung/
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European Commission. Directive 2009/28/EC of European Parliament of the Council of 23
April 2009 on the of energy promotion of the use from renewable sources and
amending and subsequently repealing Directives 2001/77/EC and 2003/30/EC.
Fortum 2013. Hammarbyverket. Värmeproduktion och kraftvärme. [Online] Available at:
http://www.fortum.com/countries/se/om-fortum/energi-och-produktion/
varmeproduktion-och-kraftvarme/varmeproduktion/hammarbyverket/pages/
default.aspx
Meggers, F and H, Leibundgut (2010) The potential of wastewater heat and exergy:
Decentralized high-temperature recovery with a heat pump. Zurich. Elsevier. Tekes
(2013) Lämpöenergiaa jätevedestä – katsaus nykytilanteeseen ja mahdollisuuksiin.
[Online] Available at: http://www.tekes.//gateway/PTARGS_0_201_403_994_
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Wanner,O; Vassileios, P; Clavadetscher, P; Hansruedi, S (2004) Effect of heat recovery from
raw wastewater on nitrication and nitrogen removal in activated sludge plants.
Elsevier. Zurich.
... This heat is used to heat 95 000 residential buildings. 9 Finland has also installed a heat pump system at its Lapua WWTW to recover heat from treated wastewater. The Lapua WWTW scheme is a 120 kW system and uses the recovered heat to heat the buildings of the WWTW, with estimated annual savings of €20 000 and a payback period of two to three years. ...
... The Lapua WWTW scheme is a 120 kW system and uses the recovered heat to heat the buildings of the WWTW, with estimated annual savings of €20 000 and a payback period of two to three years. 9 Scotland is currently delivering the UK's first treated wastewater-based district heat network in Stirling, which will provide low-carbon heat to several public buildings, 10 whereas in England a £120 million development is underway to heat two commercial greenhouses in Norwich and Bury St Edmunds, East Anglia, using recovered heat from the discharges of two WWTW. 11 An alternative method of wastewater heat recovery is to extract heat from sewers prior to entering WWTW. ...
... 12 Similarly, in Sandvika, Norway, a heat exchanger in the main community sewer extracts heat which covers more than 50% of the energy consumption of the supplied office and residential buildings. 9 Scotland implemented the UK's first sewer heat recovery system in December 2015 at Borders College, Galashiels. The system now provides ∼95% of the heat demand of the college campus buildings. ...
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... Also, the quantity of wastewater is the largest after the treatment plant and therefore has a substantial amount of thermal energy-in Austria the approximate thermal energy potential was calculated to be 3200 GWh/a (Kretschmer et al., 2016). Swedish studies have shown that decreasing the temperature of wastewater by one degree via recovering heat can bring 720 GWh of energy savings annually (Mikkonen et al., 2014). It is estimated that there are over 500 wastewater heat pump systems worldwide with thermal ratings between 10 kW-20 MW (Hepbasli et al., 2014), for instance in Germany, Switzerland, and the Scandinavian countries (Seybold, 2013). ...
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