with modeling tools
by Jorge A. Elías Maxil
1. Water temperature at the end of the sewer depends more on the soil temperature and lateral discharges downstream a heat extraction point, than on the extracted heat at the extraction point (This thesis)
2. The accuracy of the stochastic heat model decreases with the number of households combined into one connection of a pipe, and increases with the number of connections in the sewer network (This thesis).
3. In steady state conditions, neglecting the heat transfer processes related to the air in small diameter pipes give similar results as when they are considered (This thesis). 4. In unsteady-state conditions, local losses and stored water in the sewer are the main
sources of inaccuracy in the prediction of heat in the sewerage (This thesis)
5. 80% of energy used in the urban water cycle is for water heating. Therefore, the transi-tion to energy-optimized cities must consider methods to increase energy recovery from water.
6. As the fraction of energy needed to heat water increases with the energy-efficiency of a building, it is essential to reduce the hot water demand (Adapted from Meggers and Leibundgut, 2011 Energy and Buildings, 43, 879-886).
7. Even though the warmest wastewater, the shortest distance to the user and the largest portion of the sewer are found in minor pipes, the lack of small-scale demonstration plants has impeded feasibility studies on heat recovery in minor sewer systems. 8. People supplied with low quality drinking water often do not realize that a bad service
is more expensive than an adequate one, because bottled water is not paid at once eve-ry month, and the payment of the supply and disposal services are conceived as a tax. 9. Completing a PhD is comparable to making a film. The researcher must find a good
story to tell, communicate it to the staff, manage resources to make action scenes, and editing them in the order that the story demands.
10. In analogy to the proverb: “When you sweep the stairs, you start at the top”, the leader of a country who truly wants to fight against corruption and impunity must be the first good example.
These propositions are considered opposable and defendable and as such have been ap-proved by the promotors Prof. dr. ir. L.C. Rietveld and Prof. dr. ir. J.P. van der Hoek.
van Jorge A. Elías Maxil
1. De watertemperatuur aan het einde van het riool hangt meer af van de bodemtemperatuur en laterale warmteafvoer stroomafwaarts van een warmteterugwinningspunt, dan van de hoeveelheid onttrokken warmte bij het winningspunt (Dit proefschrift).
2. De nauwkeurigheid van het stochastische warmtemodel neemt af met het aantal huishoudens gecombineerd aangesloten aan een pijp, en neemt toe met het aantal aansluitingen aan het rioleringsnetwerk (Dit proefschrift).
3. In stationaire toestand geeft, in leidingen met een kleine diameter, het verwaarlozen van de warmteoverdrachtsprocessen naar de lucht vergelijkbare resultaten als wanneer ze wèl in beschouwing worden genomen (Dit proefschrift).
4. In niet-stationaire toestand zijn lokale verliezen en de berging van water in het riool de belangrijkste bronnen van onnauwkeurigheid in de voorspelling van de warmte in de rioolleiding (Dit proefschrift).
5. 80% van het energiegebruik in de stedelijke watercyclus betreft het verwarmen van water. Daarom moeten voor de transitie naar energie-efficiënte steden methoden worden overwogen die terugwinning van energie uit water verhogen.
6. Aangezien de fractie van energie, die nodig is om water te verwarmen, stijgt met het meer energie-efficiënt zijn van een gebouw, is het essentieel om de vraag naar warm water te verminderen (Naar Meggers and Leibundgut, 2011 Energy and Buildings, 43, 879-886).
7. Alhoewel het warmste afvalwater, de kortste afstand tot de gebruiker, en het grootste deel van het rioleringsnetwerk te vinden zijn in de rioolsystemen met kleine diameters, het ontbreken van kleinschalige demonstratie-installaties hebben tot nu toe haalbaarheidsstudies met betrekking tot warmteterugwinning uuit kleine rioolstelsels belemmerd.
8. Mensen voorzien van een lage kwaliteit geleverd drinkwater realiseren zich vaak niet dat een slechte drinkwatervoorziening duurder is dan een adequate, aangezien gebotteld water niet maandelijks betaald wordt, en de betalingen voor de levering van drinkwater en afvoer van afvalwater ervaren worden als belastingen.
9. Het voltooien van een doctoraat is vergelijkbaar met het maken van een film. De onderzoeker moet een goed verhaal vinden om te vertellen, dit delen met het personeel, de middelen beheren om de actiescènes te maken en ze bewerken in de volgorde die het verhaal eist.
10. In analogie met het spreekwoord: "Als je de trap veegt, begin je aan de top", moet de leider van een land die echt wil vechten tegen corruptie en straffeloosheid het eerste goede voorbeeld zijn.
Deze stellingen worden opponeerbaar en verdedigbaar geacht en zijn als zodanig door de promotoren Prof. dr ir. L.C. Rietveld en Prof. Dr. ir. J. P. van der Hoek goedgekeurd.
Heat modeling of
wastewater in sewer
networks
Determination of thermal
energy content from sewage
Heat modeling of wastewater in sewer
networks
Determination of urban potential for heat
recovery from wastewater with modeling tools
Proefschrift
er verkrijging van de graad van doctor aan de Technische Universiteit Delft,
op gezag van de RectorMagnificus Prof. Ir. K. Ch. A. M. Luyben, voorzitter van het College voor Promoties,
in het openbaar te verdedigen op 7 April 2015 om 12:30 uur
door
Jorge Armando ELIAS MAXIL
Master in Environmental Engineering geboren te San Juan Huactzinco, Tlaxcala, Mexico
Rector Magnificus, voorzitter
Prof. dr. ir. L. Rietveld, Technische Universiteit Delft, promotor Prof. dr. ir. J. P. van der Hoek, Technische Universiteit Delft, promotor Prof. dr. A. J. M. van Wijk Technische Universiteit Delft
Dr.ir. J.H.G. Vreeburg Wageningen University
Prof. dr. ir. J. A, M. H. Hofman University of Bath
Prof. dr. ir. S. Tait University of Sheffield
Prof. dr. ir. F. H. L. R. Clemens Technische Universiteit Delft Prof. dr. ir W. S. J. Uijttewaal Technische Universiteit Delft
Jan Hofman heeft als begeleider in belangrijke mate aan de totstandkoming van het proefschrift bijgedragen
Keywords: Wastewater, Heat recovery, Energy, Sewer networks
Printed by: IPSKAMP drukkers
Cover by: J. A. Elías Maxil
ISBN: 978-94-6186-450-5
Copyright © 2015 by J.A. Elias Maxil
An electronic version of this dissertation is available at
Que se eduque a los hijos del labrador y del barrendero como a los del más rico hacendado.
Nomenclature ... i 1 Introduction ... 1–1 1.1 Thesis outline ... 1–5 2 Energy in the Urban Water Cycle ... 2–1 2.1 Introduction ... 2–2 2.2 Energy in the urban water cycle ... 2–3 2.3 Actions to reduce the energy expenditure in the UWC ... 2–7 2.4 Thermal energy in the UWC ... 2–13 2.5 Factors influencing heat reclamation projects in the UWC ... 2–24 2.6 Discussion and recommendations ... 2–29 2.7 Conclusions ... 2–31 3 A Bottom-up Approach to Estimate Dry Weather Flow in Small Sewer Networks ... 3–1
3.1 Introduction ... 3–2 3.2 Stochastic modeling ... 3–3 3.3 Materials and methods ... 3–5 3.4 Results and discussion ... 3–10 3.5 Conclusions ... 3–17 4 A reduced model for water temperature in sewer networks ... 4-1 4.1 Introduction ... 4-2 4.2 Methods ... 4-4 4.3 Results ... 4-16 4.4 Conclusions ... 4-26
5.1 Introduction ... 5–2 5.2 Model description ... 5–4 5.3 Methods ... 5–7 5.4 Results and discussion ... 5–14 5.5 Conclusions ... 5–23 6 Modeling the implications of heat recovery from wastewater in sewer networks ... 6–1
6.1 Introduction ... 6–2 6.2 Methods ... 6–4 6.3 Results and discussion ... 6–10 6.4 Conclusions ... 6–17 Conclusion and recommendations ... 7–1 7
7.1 Conclusions ... 7–2 7.2 Final remarks and recommendations ... 7–6
Summary ... v
Samenvatting ... ix
List of publications ... xiii
Acknowledgements ... xv
N
Nomenclature
Variables
Symbol Parameter Unit
a Thermal diffusivity m2 s-1
𝑬̇𝒓 Radial temperature transfer into a control volume m3 k s-1
𝑬̇𝜽 Tangential temperature transfer into a control volume m3 k s-1
F Frequency of use [-]
HTP Heat transfer performance of a heat exchanger J s-1
I Intensity of discharge L s-1
j Mass flux per unit area kg m-2
j’ Mass flux per volumetric unit kg m-3
M Equivalent appliances [-]
N Number of end-users [-]
Nu Nusselt number [-]
PC Pump capacity L s-1
Pr Prandtl number [-]
Q� Heat flow rate J s-1
q̇ Heat flux per unit area J s-1 m-2
q̇’ Heat flux per volumetric unit J s-1 m-3
Re Reynolds number [-]
T Temperature ℃
t Time s
u Velocity m s-1
V Volume m3
V� Volumetric flow rate m3s-1
v̇ Discharge per unit of length m2s-1
X Fraction water vapor kg kg-1
Y Given measured variable [-]
Ŷ Given modeled (estimated) variable [-]
Z Duration (water demand | discharge) s
α Heat transfer coefficient J s-1 m-2 K-1
N
Symbol Parameter Unit
τ Discharge/Water demand time of appearance s
Constants
Symbol Parameter Unit
b Air exchange coefficient [-]
C Chezy coefficient m1/2 s-1
cp Heat capacity J kg-1 K-1
fF Fouling factor J s-1 m-2 K-1
g Gravitational force 9.81 m2s-1
K Thickness factor [-]
kP Correction factor for plunging flow [-]
ks Strickler friction coefficient m1/3 s-1
kΛ Correction factor for free surface [-]
p Pressure mbar
rCOD COD degradation rate mgCOD m-3 s-1
λ Thermal conductivity J s-1 m-1 K-1
ξ Head loss coefficient [-]
ρ Density kg m-3
φ Ambient relative humidity [-]
Geometric variables
Symbol Parameter Unit
A Cross sectional area of pipe m2
A’ Lateral area to the pipe m2
B Width of water surface m
D Nominal diameter m
h Depth m
L Length m
m Tangential sectors [-]
n Radial layers [-]
r Radial distance/coordinate from the center of the pipe m
RW Hydraulic radius m
So Sewer slope m/m
N
Symbol Parameter Unit
δ Thickness m
θ Tangential coordinate of the pipe rad
Θ Angle between inflow and outflow pipes rad
Λ Surface area m2
Ω Perimeter (wet | headspace) m
Subscripts
Symbol Parameter
0 Condition at time = 0
amb Ambient
c Cold water
cond Conduction (heat transfer)
conv Convection (heat transfer)
DW Drinking water |water demand
eq Equivalent
h Hot water
HX Heat exchanger
in Entering | Input value
inf Value at boundary condition
L Air at headspace
m Measured (Observed) readings
mh Manhole
out Leaving | Output (Result)
P Pipe
Pw Pipe wall
R Residence time
S Soil
s Simulated (estimated) data
sat Saturation conditions
W water WW Wastewater ζ Condensation ν Evaporation r Radial direction θ Tangential direction
N
Abbreviations
Abbreviation Parameter
BOD Biochemical oxygen demand
cap Capita
CFD Cumulative frequency distributions
COD Chemical oxygen demand
COP Coefficient of performance
d Index of agreement
DTS Distributed temperature sensor
DW Drinking water
DWF Dry weather flow
HX Heat exchanger
ME Mean error
MSE Mean squared error
NE Northeast
PVC Polyvinyl chloride, pipe material
R2 Coefficient of determination
RMSE Root mean squared error
RP(s) Rectangular pulse(s)
SF Safety factor
SRT Sludge retention time
SW Southwest
UWC Urban water cycle
WWTP Wastewater treatment plant
1
Introduction
actors like fossil fuel dependency and climate change have encouraged different programs in urban zones around the world to make a better use of resources such as water and energy. The Intergovernmental Panel on Climate Change estimated that in order to prevent some impacts due to climate change (increase of global temperature, more frequent extreme weather events), the greenhouse emissions (measured as equivalents of CO2)must be reduced to at least 80% (related to the emission in 1990) by 2050 (Solomon et al., 2007). The domestic sector is one of the main consumers of energy; nearly one fourth of the total energy consumption of the European Union (24.6% corresponding to 12,000 PJ) was assigned to this sector in 2007 (EEA, 2010).
Table 1-1 depicts the “total final energy consumption” (TFC) of the Netherlands and USA, which embraces all the energy consumed in one year mainly for transportation, agriculture, industry, commerce and public services, and at the residential sector (residential final consumption or RFC). The energy con-sumption by waterworks (mostly electrical energy to treat, transport and dis-tribute water, and to transport and treat wastewater) is also listed, as well as the water-related energy consumption in the residential sector, which is the energy that the end-users consume while using water at houses. It can be no-ticed that the energy spent by the waterworks represents about 0.6-1% of the TFC or 3.7-5.4% of the RFC while the energy that households need for heating (mostly water and space heating) is 17% to 40% of the RFC.
1
The, supplied drinking water is mainly used for personal hygiene, washing and food preparation (Geudens, 2010). These activities, in many cases, require hot water, making water heating the highest energy consuming operation with more than 80% of the total consumption of all water-related activities in the residential sector (Blom et al., 2010, Leerdam et al., 2010). Similar proportions have been found in other countries with different weather conditions (Kenway et al., 2013, Cheng, 2002, Daigger, 2009). Most of the households and buildings use electricity or fossil fuels to heat water, which are considered as high quality energy sources because they can be converted into kinetic energy or high temperature heat. The energy quality of the water after its use cannot be used to produce movement. Nevertheless, it is warmer than cold tap, and with a similar energetic quality of the heat needed for space heating (Meggers and Leibundgut, 2011).
Table 1-1: Total energy consumption, at the domestic sector, and in water-related activi-ties in Netherlands and USA. Units: MJ cap-1 a-1. Sources: (IEA, 2013, IEA, 2014, Blom et
al., 2010, Leerdam et al., 2010, Daigger, 2009).
Netherlands USA Total final consumption (TFC) 153,173 192,163 Residential final consumption (RFC) 25,788 33,951
Consumption by waterworks 942 0.6% of TFC 1,847 1.0% of TFC 3.7% of RFC 5.4% of RFC Water-related energy consumption
by the residential sector 4,400
2.9% of TFC
13,600 7.1% of TFC 17.1% of RFC 40.1% of RFC In order to reduce the greenhouse gas emissions, the member states of the European Union have set a target to substitute 20% of the total energy con-sumption with energy from renewable sources by 2020 (EU, 2013). Recently the European Commission published a communication in which a policy framework is proposed for climate and energy in the period from 2020 to 2030 (European Commission, 2014). This includes the establishment of tar-gets to reduce the greenhouse gas emissions by 40% compared to 1990, the increase of the renewable energy share to 27%, and the realization of 30% energy savings. In the Netherlands, Amsterdam launched the so-called “Am-sterdam Climate Program”, a set of actions aimed to reduce CO2 emissions up
1
(City_of_Amsterdam, 2009). Waternet, the city’s water utility, being responsi-ble for the operation of the entire water cycle of the city, started the program “Energy from Water” to assess innovative techniques whose application make the urban water cycle (UWC) more sustainable (Mol et al., 2010) and become climate neutral by 2020 (van der Hoek, 2011).
Some studies suggest that in order to reduce the high quality energy consump-tion in houses, it is preferable to maximize the low quality energy streams, rather than to increase the insulation between the household and its sur-roundings (Meggers et al., 2012). For water heating and space heating this can be performed with heat pumps that transfer heat from a low temperature source to a higher temperature location, using a low amount of high quality energy (Holland et al., 1999).
Temperatures close to 60℃ are enough to operate a centralized hot water supply system (Torío and Schmidt, 2010) and heat recovery from sewage for space heating has already been used in Germany and Switzerland showing to be a reliable technology. However this technology has not been wide-spread in other countries because of some technical restrictions such as:
1. Published literature regarding successful case studies, mention that the installation and operation of such heat recovery systems is consid-ered to be only economically feasible when the heat demand of a heat recovery system is more than approximately 1 MW (Schmid, 2008), which is enough to supply more than 1000 households of energy for space heating1.
2. A minimum requirement for the flow in the sewer of 10 Ls-1 at the heat
extraction point (Deiss, 2007, Berg, 2009, Biesalski, 2010), has limited these installations to few points at main pipes, which constitute only a minor fraction of sewer networks.
3. A maximum sewage cooling of 0.5 °C when heat is extracted before the wastewater treatment plant (DWA, 2009, Buri and Kobel, 2004) to avoid that the biological nitrogen removal in wastewater treatment
1 Considering 222 days of operation per year, average heat demand of 220 MJ m-2
(Cholewa and Siuta-Olcha, 2010), a household of 70 m-2, and 25% of energy lost in
1
plants can be affected by cold water at the influent (Wanner et al., 2005).
4. Short distances between the heat extraction point and the customer (Schmid, 2008), to minimize losses in the transportation of heat. Nor-mally, the warmest wastewater is found immediately downstream the household where the volume of wastewater is small and intermittent. 5. The lack of knowledge about the amount of energy that can be
ob-tained from sewage in different sewer networks (Cipolla and Maglion-ico, 2014).
The use of heat recovery systems, located, relatively close to the end-user of the heat, promote the use of these technology in cities (Narita and Maekawa, 1991). At the same time, there is a need to acquire knowledge about the ener-gy that can be obtained from the sewage without compromising the operation of wastewater treatment plants.
The main objective of the thesis is to present an approach to assess the ener-gy content (flow and temperature) at any point of the sewer system. The ap-proach should result in a tool for sewer network managers and designers to control the number, size, and operation mode of heat recovery installations, and to avoid negative effects on the influent of wastewater treatment plants, without the need of extensive monitoring campaigns.
The research has the following specific objectives:
1. To obtain an overview on the energy expenditures in the urban water cycle and to make an inventory of state-of-the-art technology for the recovery of heat from water.
2. The integration of a stochastic model for wastewater discharge predic-tion in households to a hydraulic model to model wastewater dis-charge in pipes. To calculate the diurnal flow rate in sewer systems. 3. To develop and test a reduced model to calculate the temperature
change of wastewater in pipes.
4. To combine the stochastic model to calculate wastewater discharge to the reduced model; and to calculate the temperature change in pipes while being validated for a small sewer network.
5. To demonstrate the application of the combined model in large-scale situations, and to demonstrate its added value in assessing the
tem-1
perature at the end of the sewer system when the sewage temperature is droped (to the level allowed by some guidelines (DWA, 2009, Buri and Kobel, 2004) or higher temperature drops) at some points of the sewer system.
Figure 1-1 depicts the approach of the performed research. During the re-search work, the stochastic model for water demand prediction SIMDEUM® (Blokker et al., 2010) was adapted to model the sewage discharge from hous-es, and the output was used in the urban drainage software Sobek™ to simu-late the wastewater flow in a sewer network. In addition, a model to estimate the temperature loss in pipes was developed and integrated into the urban drainage software Sobek™. Both sub-models (sewage discharge and tempera-ture in pipes) were adapted to stochastically calculate the heat (flow and tem-perature) at any given point in the sewer network. The final model was tested in a small and a complete scale sewer network. Scenarios were built to evalu-ate the consequences of heat extraction points (to imitevalu-ate the operation of HRS) in the complete scale sewer network.
1.1 Thesis outline
hapter 2 reviews and summarizes state-of-the-art measures applied in different parts of the world to reduce the energy consumption related to urban water. Based on a literature review, the energy balance in the urban water cycle in some regions of the world is presented. Examples of measures to reduce the consumption of energy are described. The emphasis is on technologies and case studies to recover energy from urban water, as well as some influencing factors on the deployment of these technologies.
Chapter 3 describes and demonstrates a method to model intermittent charges in small sewers by linking a stochastic model for wastewater dis-charge (based on a stochastic model for drinking water demand) to a hydrau-lic sewer model to predict the attenuation of the discharges and its impact on the arrival time to a defined spot of a sewer network.
1
Figure 1-1: Followed approach to calculate heat (flow and temperature) in sewer pipes with subsequent performance tests in small and complete sewer networks. Chapter 4 compiles the published knowledge of modeling wastewater tem-perature in partially-filled sewer pipes, and presents a reduced model that takes into consideration the most significant interactions governing heat transfer from water to the surroundings. In addition, the model is bench-marked with the model called TEMPEST to calculate water temperature in partially-filled pipes.
Chapter 5 presents a method to predict the available heat in sewer networks by assembling a stochastic model predicting sewage discharge from house-holds. The performance of the model is demonstrated by comparing its results to experimental measurements of flow and temperature obtained from the small sewer network. An analysis to simplify the model for scale-up purposes is also shown.
In Chapter 6, the developed approach for heat modeling is used to simulate the temperature and sewage flow in a complete-scale situation. The results of a monitoring campaign carried out in Amsterdam were used not only to assess the temperature behavior in potential places to install heat recovery systems,
Stochastic water demand modeling (SIMDEUM) Stochastic wastewater discharge modeling Conversion Temperature model in pipes Urban drainage
model (Sobek) & validationIntegration
Development, integration & validation Small sewer network Development & validation Full network Validation & use
1
but also to calibrate and validate the model in a complete sewer network . An example of the use of the model is given, as well as the comparison of the sto-chastic model against a proportional method to estimate flow rate in the sew-er system.
Chapter 7 provides the general conclusions of this research and further evalu-ates the added value of the developed model for the recovery of heat from sewer networks. Suggestions are made for future research and applications.
References
BERG, H. 2009. Wärmegewinnung aus Abwasser und Grundwasser. In: E.V., D. V. D. G.-U. W. (ed.). Bonn, Germany: Ingenieurbüro H. Berg & Partner GmbH.
BIESALSKI, M. 2010. Moderne Wärmetauschertechnologie aus Baden-Württemberg - ein Exportprodukt Kongress Wärmegewinnung aus Ab-wasser Stuttgart. Stuttgart: Ministerium Fur Umwelt, Naturschutx und Verker. Baden-Wurttemberg.
BLOKKER, E. J. M., VREEBURG, J. H. G. & VAN DIJK, J. C. 2010. Simulating resi-dential water demand with a stochastic end-use model. Journal of Water Resources Planning and Management, 136, 19-26.
BLOM, J. J., TELKAMP, P., G.F.J. SUKKAR & WIT, G. J. D. 2010. Energie in de Waterketen. In: WATERBEHEER, S. T. O. (ed.). Netherlands: Stichting Toegepast Onderzoek Waterbeheer
BURI, R. & KOBEL, B. 2004. Wärmenutzung aus Abwasser–Leitfaden für Inhaber, Betreiber und Planer von Abwasserreinigungsanlagen und Kanalisationen. Bern: Bundesamt für Energie BFE. URL: http://www.bfe.admin.ch/infrastrukturanlagen/[Stand3.3.2009].
CIPOLLA, S. S. & MAGLIONICO, M. 2014. Heat recovery from urban wastewater: Analysis of the variability of flow rate and temperature. Energy and Buildings, 69, 122-130.
CITY_OF_AMSTERDAM 2009. Amsterdam: a different energy 2040 Energy Strategy. In: CITY OF AMSTERDAM, K. (ed.) Electronic. Amsterdam Netherlands: Klimaatbureau.
CHENG, C. L. 2002. Study of the inter-relationship between water use and en-ergy conservation for a building. Enen-ergy and Buildings, 34, 261-266.
1
CHOLEWA, T. & SIUTA-OLCHA, A. 2010. Experimental investigations of a de-centralized system for heating and hot water generation in a residential building. Energy and Buildings, 42, 183-188.
DAIGGER, G. T. 2009. Evolving urban water and residuals management para-digms: Water reclamation and reuse, decentralization, and resource re-covery. Water Environment Research, 81, 809-823.
DEISS, C. 2007. Energiequelle zum Heizen und Kühlen - Grösste Abwasserenergienutzungsanlage in der Schweiz. GWA. Gas, Wasser, Abwasser., 6, 413-420.
DWA 2009. 114: Energie aus Abwasser–Wärme-und Lageenergie. In: GERMAN ASSOCIATION FOR WATER, W. A. W. D. (ed.) 6th ed.: Deutsche Ver-einigung für Wasserwirtschaft, Abwasser und Abfall e.V.
EEA. 2010. Final energy consumption by sector (CSI 027/ENER 016) [Online]. Copenhagen: European Environment Agency. Available:
http://www.eea.europa.eu/data-and-maps/indicators/final-energy-consumption-by-sector-1/final-energy-consumption-by-sector-5 [Ac-cessed 30 September 2010 2010].
EU 2013. Renewable energy progress report. In: COUNCIL, C. T. T. E. P., COM-MITTEE, E. E. A. S. & REGIONS, C. O. T. (eds.). Brussels: European Com-mission.
GEUDENS, P. J. J. G. 2010. Dutch Drinking Water Statistics 2008. The water cycle from source to tap. Rijswik: Association of Dutch Water Companies (VEWIN).
HOLLAND, F. A., SIQUEIROS, J., SANTOYO, S., HEARD, C. L. & SANTOYO, E. R. 1999. Water Purification Using Heat Pumps, New York, E & FN SPON. IEA 2013. Key World Energy Statistics. In: AGENCY, I. E. (ed.). Paris, France:
OECD, IEA.
IEA. 2014. Energy Balnaces. Energy Balance flow [Online]. International
Ener-gy Agency. Available: http://www.iea.org/statistics/topics/energybalances/ [Accessed
Sep-tember 2014 2014].
KENWAY, S. J., SCHEIDEGGER, R., LARSEN, T. A., LANT, P. & BADER, H.-P. 2013. Water-related energy in households: A model designed to understand the current state and simulate possible measures. Energy and Buildings, 58, 378-389.
LEERDAM, R. V., ROEST, K., GRAAFF, M. D. & HOFMAN, J. 2010. De waterketen als energiebron. In: INSTITUTE, K. W. R. (ed.). Nieuwegein: KWR.
1
MEGGERS, F. & LEIBUNDGUT, H. 2011. The potential of wastewater heat and exergy: Decentralized high-temperature recovery with a heat pump. En-ergy and Buildings, 43, 879-886.
MEGGERS, F., RITTER, V., GOFFIN, P., BAETSCHMANN, M. & LEIBUNDGUT, H. 2012. Low exergy building systems implementation. Energy, 41, 48-55. MOL, S. S. M., KORNMAN, J. M., KERPERSHOEK, A. J. & HELM, A. W. C. V. D.
2010. Opportunities for public water utilities in the market of energy from water. In: IWA, ed. IWA Water and Energy, 10-12 November 2010 2010 Amsterdam, The Netherlands. IWA.
NARITA, K. & MAEKAWA, T. 1991. Energy recycling system for urban waste heat. Energy and Buildings, 16, 553-560.
SCHMID, F. Sewage water: interesting heat source for heat pumps and chillers. 9th International IEA Heat Pump Conference, Switzerland. Paper, 2008. 1-12.
SOLOMON, S., D. QIN, MANNING, M., CHEN, Z., MARQUIS, M., AVERYT, K. B., TIGNOR, M. & MILLER, H. L. 2007. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Cli-mate Change. In: CHANGE, I. P. O. C. (ed.) CliCli-mate Change 2007: The Physical Science Basis. UK: IPCC.
TORÍO, H. & SCHMIDT, D. 2010. Development of system concepts for improv-ing the performance of a waste heat district heatimprov-ing network with exer-gy analysis. Enerexer-gy and Buildings, 42, 1601-1609.
VAN DER HOEK, J. P. 2011. Energy from the water cycle: a promising combina-tion to operate climate neutral. Water Practice and Technology, 6.
WANNER, O., PANAGIOTIDIS, V., CLAVADETSCHER, P. & SIEGRIST, H. 2005. Effect of heat recovery from raw wastewater on nitrification and nitro-gen removal in activated sludge plants. Water Research, 39, 4725-4734.
*This chapter is based on:
Elías-Maxil, J. A., van der Hoek J. P., Hofman, J. & Rietveld, L. 2014. Energy in the urban water cycle: Actions to reduce the total expenditure of fossil fuels with emphasis on heat reclamation from urban water. Renewable and Sustainable Energy Reviews, 30, 808-820.
2
Energy in the urban water cycle*
In the urban water cycle, water supply, transportation, treatment and dis-posal are services that consume a considerable amount of energy. This pa-per reviews and summarizes state-of-the-art measures applied in different parts of the world to reduce the energy consumption related to urban water. Based on a literature review, an overview of the energy balance in the urban water cycle in some regions of the world is presented. The balance shows that water heating is the largest energy expenditure with approximately 80% of the total primary energy demand in the residential sector of the cycle, while the remaining 20% of energy is spent by waterworks on pumping and treatment. Examples of measures to reduce the consumption of energy are presented according to a philosophy of actions in order to achieve energy ef-ficient processes. The emphasis is on technologies and case studies to recov-er the enrecov-ergy from urban watrecov-er, as well as some factors that influence the deployment of the technologies.2
2.1 Introduction
he water cycle of an urban zone starts with the abstraction of water, followed by its transportation, treatment and distribution to the end-user. Once water has been used, water is collected, often mixed with rainwater run-off and transported to a wastewater treatment plant. Treated water is, in some cases, reclaimed for reuse, but most frequently discharged to the surface water. After this last stage, water comes into contact with the nat-ural water cycle and eventually, it will return to the urban zone closing the urban water cycle (UWC). The UWC thus comprises water catchment and withdrawal, drinking water treatment, distribution, water use, wastewater treatment, recycling and rain water (Mitchell et al., 2001) and can be divided into three main categories: before, during and after use. The initial and final characteristics of water differ in each stage; so does the type of energy re-quired during its handling. Figure 2-1 shows the main energy types involved in each stage of the main categories. Negative and positive signs correspond to energy expenditure or gain respectively. For drinking water production ener-gy is mainly required for pumping. In addition, there are (indirect) enerener-gy burdens in the production of chemicals required for the treatment. During water use energy is used for water heating, while during wastewater treat-ment thermal and electrical energy can be recovered from the chemical ener-gy (biogas from digestion of organic compounds) in wastewater.
In order to make urban zones more sustainable, infrastructural changes aim-ing at energy neutral activities are required. A concept that has been developed to strive to zero net energy consumption is called the “Trias Energetica” -a brief description c-an be seen -at www.triasenergetica.com or Entrop and Brouwers (2010). It suggests three directions that have to be followed, suc-cessively or in parallel, to promote energy savings and avoid the use of fossil fuels. These directions are: (1) Reduction of energy demand by avoiding waste and implementation of energy-saving measures (prevention). (2) Replace-ment of fossil fuels by renewable energy sources (RES) whenever it is possi-ble. (3) Efficient use of fossil fuel and its reuse (efficiency).
This paper describes state of the art measures, applied in different parts of the world, to reduce the energy consumption in the UWC. Section 2 starts with an inventory and compares information on the energy balance in the UWC to
T
2
identify the largest energy expenditures. The third section addresses some examples where the energy expenditures in the UWC were reduced. Section 4 provides a description of technologies and case studies related to efficient use of fossil fuels. The fifth section describes factors that influence the feasibility of heat recovery systems. Discussion and conclusions from the review are pre-sented in the last sections of this chapter.
Figure 2-1: Energy involved in different stages of the UWC. The main categories according to water use are denoted by dotted squares.
2.2 Energy in the urban water cycle
2.2.1 Energy consumption for drinking water production
ifferent factors such as the distance from the water source to the con-sumer, water abundance, initial quality, and required treatment for use determine the overall energy expenditure per volume unit of wa-ter in the UWC (Horne et al., 2008). In the first stage of the UWC, water must be abstracted and transported for human use. In this stage electricity is the main energy source. In addition, there is an indirect energy use for the pro-duction of chemicals. Water Treatment Water Use Wastewater Treatment Sea Water Surface Water Ground Water Precipitation Catchment and abstraction Supply Collection/ Drainage Disposal Evaporation Recycling and Redistribution 5 9 1 2 3 4 10 8 6 Thermal Energy Electrical Energy Drinking Water Tap Water Waste Water 7
D
2
A life cycle assessment-based research was performed by Racoviceanu et al. (2007) for the operation of the water facilities in Toronto. The authors found out that electrical energy needed for pumping (including raw water pumping, pumping during treatment and treated water pumping) accounted for 94% of the total energy burden and for 90% of the total CO2 emissions, in water
facili-ties while the energy burden related to the production of chemicals and trans-portation (of chemicals) accounted for only 5%. The city spent 1.4 PJ of elec-trical energy on water pumping in 1998; which accounted for 0.75% of the total supplied energy to the city (Cuddihy et al., 2005). Depending on the availability of water sources, the share of the required energy for water supply can increase. For example, 5 % of the total electricity use in California is re-quired for water supply to the state, where 4.3 % is used for transportation and the remaining 0.7 % is used for treatment and distribution (McMahon and Price, 2011).
2.2.2 Energy consumption during water consumption
In the state of California, although the electricity expenditure for water supply is the largest electricity consumer, the electricity used for water heating in the same state reaches 14% of the total consumption in the residential, commer-cial and industry sectors (McMahon and Price, 2011). The purposes of water use in the residential and commercial sector are limited; most of them are related to cleaning purposes. Applications can be divided into eight main ac-tivities of which toilet flushing and showering represent the largest tap water expenditure in households, the latter usually being heated before its use (Blokker et al., 2010). According to results shown in a survey in the USA, ap-proximately 150 L cap-1 of water were heated from 13 ℃ (average tap water
temperature) to 40 ℃ (Hendron and Burch, 2008). Taking into account that in the USA 40% of the residential water is heated with electricity, the efficiency factor of electric (0.9) and gas heaters (0.82), and the efficiency factor to con-vert primary energy to thermal energy by electricity (ACEEE, 2011), the pri-mary energy consumed for water heating amounts to 13,000 MJ per capita per year.
In Netherlands, this value amounts to 4400 MJ cap-1 a-1 considering that the
average temperature of discharged wastewater from household is 27 ℃ (NEAA et al., 2005, Geudens, 2010, Hofman et al., 2011). In both countries showering represents the activity that uses most of the heated water.
2
2.2.3 Energy consumption after use
Once wastewater leaves a household, the heat carried with the water diffuses to its surroundings and, when the temperature of the soil is lower (as it nor-mally is), thermal energy in the water decreases. In order to dispose the wastewater, it must be collected and pumped to the wastewater treatment plant (WWTP). The share of energy consumption in this stage varies according to factors such as level difference in urban zones, precipitation, proximity to the WWTP, type of sewer (combined or separated) and population density. In Sydney, the percentage of energy consumed in sewer systems is close to 7% of the total energy spent in the UWC (Lundie et al., 2004), while in Netherlands, the percentage is approximately 10% (Blom et al., 2010b).
In the WWTP, aeration in aerobic treatment processes can account for 1.8 MJ m-3 to more than 3.6 MJ m-3 of electricity depending on the type of biological
system (Kennedy and Tsuchihashi, 2005). A study of almost 1,000 wastewater treatment plants in Japan focused on determining the specific energy con-sumption for wastewater treatment. The range of energy concon-sumption was 1.0 to 7.5 MJ m-3 and in most of the cases, biological treatment represented half of
the total operating costs (Liu et al., 2004).
Biogas is the most common form of energy that is recovered from wastewater. The potential energy harvested from biogas recovery in wastewater treatment plants is estimated to be about 360 MJ (100 kW h) of electricity and 777 MJ of heat produced with the help of combined heat and power units for every 45,000 inhabitants with an average water consumption of 380 L cap-1 day-1
(EPA, 2007).
2.2.4 Comparison of energy balances for different case studies
Table 2-1 presents the comparison of energy use in the UWC for different re-gions. Water before and after consumption requires about 20% of the total primary energy in the UWC. In the case of the USA and Netherlands, the pri-mary energy used for water heating in the residential sector accounts for more than 80% of the total consumption. Similar percentages have been found in Taiwan where 84.5 % of the energy spent in common buildings is used for water heating in showers and food preparation. The rest (which corresponds to 8.46 kJ m-3 of primary energy) is spent on treatment and transportation of
2
studies (Plappally and Lienhard V., 2012, Hofman et al., 2011, Leerdam et al., 2010) which also suggest that heating is the most important energy input in the UWC. The generation of energy is highly related to the emission of green-house gases which is considered as an accelerating factor for climate change.
Table 2-1: Primary energy consumption in stages of the domestic urban water system. delivered energy was converted into primary energy taking into account conversion ratios (ES, 2011, Racoviceanu et al., 2007, Widén et al., 2009, Blom et al., 2010a, Leerdam
et al., 2010, Daigger, 2009, Lassaux et al., 2007, Prevedello and AQUAWAL, 2012).
Category Place Netherlands USA Walloon
region a1 Toronto Sydney b1
Population, million 16.6 296 3.4 2.5 4.4 Water
consump-tion, m3∙cap-1∙a-1 46.7 121.5 45.8 91.2 108.4
Before wa-ter con-sumption, MJ∙cap-1∙a-1 Catchment 301 474 123 177 [-] Treatment 55 107
Supply and
distri-bution 60 843 137 404 348
During water consumption,
MJ∙cap-1∙a-1 4 400 13 600 [-] [-] [-] After water consump-tion, MJ∙cap -1∙a-1 Collection and transportation * 105 530 0a2 380 74 wastewater treat-ment 476 176 434
a1 Population was obtained from http://statistiques.wallonie.be (2009), leakages and water
ex-ports to other regions were taken into account.
a2 Energy for collection was considered negligible since most of the sewer is driven by gravity. b1 Water consumption per capita was obtained from www.sydneywater.com.au (2010).
* Collection and transportation of domestic wastewater, industrial wastewater and rainwater The primary energy per capita consumed in the UWC (Table 2-1) multiplied by an emission factor corresponding to the type of delivered energy, results in the amount of emitted greenhouse gases. It must be considered that biogas (mainly methane and carbon dioxide) is produced during the biological treat-ment of wastewater. In the Netherlands, assuming that almost all biogas is completely flared, every person produces annually 324 kgCO2-eq. by using the
UWC (Table 2-2). This amount represents 3.14 % of the total CO2 emission per
capita (UNDP, 2011). Frijns et al. (2008) estimated that the contribution of CO2 emissions from waterworks and the overall UWC were 0.8% and 3.3%
respectively. In regions where the gas is not flared the amount of emitted me-thane would contribute to the greenhouse emission even more, since meme-thane has 21 to 23 times higher greenhouse gas effects than CO2 (EPA, 2011).
2
Table 2-2: Annual eq. CO2 emitted per person by domestic water use. Factors correspond
to CO2 emission by electrical production, hot water heating and biogas burning in Neth-erlands (Vreuls and Zijlema, 2009, Leerdam et al., 2010).
Category Stage Primary Energy, MJ cap-1 a-1 Type of Energy Emission Fac-tor, kg CO2 eq. MJ-1 kg CO2 eq. cap-1 a-1 Before Con-sumption
Water Catchment 301 Electricity 0.0689 21 Water Treatment
Water supply and
distribution 60 Electricity 0.0689 4 During
Con-sumption Water Use 4400 Natural Gas 0.0567 249
After Con-sumption Collection and transportation of wastewater and other water 105 Electricity 0.0689 7 wastewater treat-ment 476 Electricity 0.0689 33 Biogas Production
during treatment 110 Biogas 0.0842 9 The numbers in Table 2-2 do not consider gases produced in the sewer net-work which are not captured or treated in most places in the world. For the case of gases produced inside sewer networks, Guisasola et al. (2008) have determined that the emission of methane in sewers could have a comparable greenhouse gas effect to that produced in WWTPs. Measurement results of greenhouse gas production in sewer systems of Amsterdam suggest a similar conclusion (de Graaff et al., 2012). Furthermore, an experiment that traced the carbon isotopes in four treatment plants in Australia, showed that fossil organic carbon contributes for 4 to 7% to the total carbon balance from do-mestic wastewater (Law et al., 2013). According to Washington et al. (2009), in order to mitigate half of the effects of Climate Change, a global reduction of 70 % in the emission of greenhouse gases must be achieved by the year 2100, which implies that activities involved in the UWC must be improved.
2.3 Actions to reduce the energy expenditure in the UWC
2.3.1 Prevention of energy use
n agreement with Sala and Serra (2004), the reduction of energy demand would begin with the protection of water sources, enforcement of water savings and water reuse, since the energy expenditure needed to pump or
I
2
heat water is related to the amount of handled water. On a household level, there are documented cases where the use of water-efficient appliances has reduced water demand in urban zones. After four years of implementing a water reduction program in one of the largest counties in Florida, USA, the single change of any hot water demanding appliance (shower head or wash machine) reduced the water demand by 10.9 and 14.5% respectively (Lee et al., 2011). In Sydney, Australia, the results of a large program where changes of head showers, installation of tap flow regulators, improvements in toilets, detection of leakages and water consumption advice were performed in ap-proximately 200,000 households, indicate that the reduction of water con-sumption reaches 12% for indoor use (Turner et al., 2005).
On an urban zone level, demand management could reduce the volume of wa-ter consumed in a region and subsequently decrease the energy needed to operate water facilities without major changes in infrastructure.
Vairavamoorthy et al. (2008) enumerate some techniques to manage the de-mand in distribution networks and, with the help of a case study of a distribu-tion network in India, they conclude that, by following the guidelines for de-mand management, it was possible to improve the supply for consumers without increasing the volume of water to be supplied. Minimization of leak-ages could help to improve water conservation. Kumar and Karney (2007)
estimate that 13% of the water loss by leakage could represent about 10 to 20% energy losses in pumping and treatment.
2.3.2 Use of renewable energy sources
The use of RES in the UWC relates to two different concepts that are often con-fused. The first concept concerns the use of energy from renewable sources to substitute energy from fossil fuels during the UWC operation. Green power purchase by waterworks is one type of practice (and often the easiest to im-plement) to substitute energy from fossil fuels (EPA, 2012). In most cases, the use of renewable energy is realized by buying certificates that guarantee that part of the electricity supplied by the grid was produced from RES.
Energy from RES can also be used directly when water is used or treated. So-lar collectors or heaters are an example of technology that has substituted energy from fossil fuels in the UWC. Balaras et al. (2000) estimate that, de-pending on the ambient conditions of a region, the use of solar collectors could
2
reduce the energy consumption in buildings by 60 to 80% compared to elec-tric heaters. Michopoulos et al. (2009) investigated electricity savings when renewable energy sources were applied for ambient heating and hot water production in a conventional house located in northern Greece. With solar collectors, there was overall electricity saving of 54% during a year period. The use of renewable energy sources for water heating has not widely been adopted. Leidl and Lubitz (2009) consider that the implementation of heating technologies in households has been hindered by insufficient awareness, lack of experience and insufficient information on the performance. The authors further suggest that, in addition, financial incentives are needed to facilitate a large scale roll-out of renewable heating systems in medium-sized North American communities.
In water treatment, much effort has been put in research on the desalination process where energy consumption in large reverse osmosis treatment facili-ties can reach up to 22 MJm-3 (2-6 kWh·m-3) (Subramani et al., 2011).
Mathioulakis et al. (2007) have discussed the characteristics of different desal-ination processes driven by renewable energy sources suggesting that amongst other solar-driven processes (such as dehumidification, reverse os-mosis and electro-dialysis with photovoltaic and wind power), membrane distillation seems to be an adequate technology that can be implemented in warm climates and at locations with waste heat sources such as cooling water (often warm brackish water) from industrial sources. Nevertheless, mem-brane distillation has not been widely applied in practice yet. In some docu-mented cases of full scale applications, the major reported problems relate to the membrane (fouling) and plant lifetime (Charcosset, 2009). Although ther-mal desalination is a higher energy consuming treatment process, this tech-nology has become a suitable option for decentralized facilities in remote loca-tions of high solar intensity (Shannon et al., 2008). Solar photo-catalysis for advanced water treatment and disinfection is also a technology currently un-der development, suitable to substitute fossil energy sources (Blanco et al., 2009). However, the high price and low yield of delivered energy compared to conventional energy sources is still a barrier for applying these technologies on a large scale. Besides, the fluctuating generation profile of renewable ener-gy sources such as wind and sun makes it difficult to use them for base-load supply and, in most cases, energy storage facilities have to be considered dur-ing the design of such systems.
2
The second concept visualizes the UWC as a source of renewable energy since water and wastewater are a carrier of diverse types of energy (Frijns et al., 2012). Potential energy from the UWC, for example, can be recovered with hydropower. The effluent of a WWTP in Warendorf, Germany was used for this purpose. A water wheel (4.83 m diameter, 1.5 m width, 40 plates) was installed in the effluent of a municipal WWTP to drive a generator to produce electricity. Annually, the energy output varied between 30 and 144 GJ (DWA, 2009). Ramos et al. (2010) mentioned that the efficiency of using pumps as turbines to regenerate electricity could be near to 85%. However, these sys-tems can only be installed in places where there is a level difference between the stream and the turbine and enough flow (at least 5m of head and a flow rate of 2m3 s-1) to make a project economically feasible.
So far, the most important energy source, contained in wastewater, comes in the form of biogas. When biogas is recovered from anaerobic digestion of sludge, electricity and heat can be reclaimed by means of combined heat and power units (CHP). The heating value of biogas (23.3 MJm-3 at normal
condi-tions (IEA, 2007) can produce electricity and heat with a combined efficiency close to 95%. This is the reason why this technology is probably the most wide spread method to harvest electricity and heat simultaneously. The average energy yield of anaerobic treatment is 13.5 MJkg-1 and, when this energy is
converted into electricity and heat, the electricity output (assuming a conver-sion efficiency of 40%) is 5.4 MJ (1.5 kWh) (Van Lier, 2008). Horne et al.
(2008) showed several case studies where energy was recovered applying
CHP units in the USA.
The success of biogas production as an alternative method to reduce fossil fuel consumption is linked to the variety of usable sources for its production, the capability to produce it in small and large quantities and the availability to use the types of produced energy (heat, steam, electricity, hydrogen) in different applications (Weiland, 2010). One example of the flexibility of the sludge di-gestion process is described by Schwarzenbeck et al. (2008). The authors doc-umented a case in Germany where a WWTP is using its digesters to treat skimmed fat from a dairy together with the activated sludge produced in the aerobic treatment process. Before the treatment of the fat, the WWTP was purchasing more than 80% of the energy with one digester, but with the
addi-2
tion of a second digester and the treatment of fat, the plant produced 100% of its own energy needs and sells the surplus (approximately 10%).
After biogas production, the digested sludge volume is reduced. In the EU, approximately 10 million tons of dried solids are produced annually. The pro-duction of biogas reduces this volume up to 60% (Appels et al., 2008). The incineration of dried sludge obtained from the anaerobic digestion is another way to recover energy from wastewater. In Amsterdam, the use of the heat produced by the incinerated sludge avoids the use of 1 million N m3 of natural
gas per year (van der Hoek et al., 2011).
One technology that promises direct electricity generation during the treat-ment of wastewater is the fuel cell, especially the microbial fuel cell. The cells can be conceived as bioreactors that can utilize the electric current generated when microorganisms, in anaerobic conditions, catalyze organic compounds.
Aelterman et al. (2006) demonstrated on laboratory scale a production of 58 W m-3 (288 kJ) of electricity, when microbial fuel cells were applied to treat
wastewater. In relation to the quality of the produced effluent, microbial fuel cells have been able to reduce 8% of initial chemical oxygen demand at a hy-draulic retention time of 33 h (Liu et al., 2004), although a large fraction of organic matter was removed without the subsequent production of energy. Because the electricity that can be extracted from microbial fuel cells is still lower than the amount that can be extracted with combined heat and power,
Pham et al. (2006) consider that the future of anaerobic digestion and micro-bial fuel cells will be complementary. The former will be able to treat highly polluted wastewater at temperatures near 30 ℃ whereas microbial fuel cells will be able to treat water with low concentrations and at low temperatures. Microbial fuel cells have not been developed commercially, mainly because of scale-up and operational problems. The required area for the electrodes still poses a technical problem when fitting in small reactors. The materials of the electrodes have decreased significantly in price although they are still high. In addition, there are problems in continuous-mode operation and changes in substrate and temperature affecting the process (Logan, 2010).
In total, the chemical energy that can be extracted from wastewater repre-sents 0.25 to 0.5% of the total primary energy consumption in developed countries, even though its utilization could reduce the carbon footprint of
2
WWTPs if this energy was reused (Svardal and Kroiss, 2011). Funamizu et al. (2001) estimate that the chemical energy converted to work obtained in a WWTP in Japan, represents near one fifth of the total energy contained in wastewater if heat is taken into account. The remaining energy stays in form of sensible heat. Ways to recover sensible heat will be considered in detail in the following section.
2.3.3 Efficient use of fossil fuels
Automation and control of water treatment facilities is the state of the art method to improve energy efficiency, and nutrients removal without affecting the effluent quality. In conventional WWTPs, aeration is the most energy de-manding operation. For 35 days Ingildsen et al. (2002) tested a dynamic con-trol in a WWTP where the aeration of the mixed liquor was concon-trolled by an online monitoring of ammonium in the mixed liquor tank and the effluent from the clarifiers. Although it was not possible to quantify with accuracy the energy saving because of technical problems during the test, it was shown that energy for aeration could be reduced from 5 to 15%. A similar optimization technique based on aeration control by ammonium concentration was put into practice in two WWTPs in Switzerland. In those cases, other site-specific au-tomation and control strategies were applied together with the control of aer-ation. After the implementation, it was estimated that energy savings ranging from 16 to 25% could be achieved (Rieger et al., 2012). In another study,
Baroni et al. (2006) developed and patented a fuzzy logic control system for the aeration in biological treatment. The controller was installed in one of four tanks of a WWTP and tested for one year. The remaining three tanks were operated as usual (with a proportional integral derivative control). The new control could save 4% of the global energy consumption in aeration (up to 15,841 kWh day-1) In addition, Lutz (2005) estimates that the replacement of
adjustable speed drivers in pumping stations and other energy saving pro-grams in WWTPs can save 5% of electrical energy.
In a global survey with more than 15 case studies, the Global Water Research Coalition and the United Kingdom Water Industry Research identified niches for energy efficiency practices and technologies in water and wastewater facil-ities. They concluded that pumping is the main energy-demanding operation with more than 80% of the total consumption for water treatment and supply, while aeration was the operation that consumed most of the energy in
2
wastewater treatment (50-60%). In wastewater treatment, pumping account-ed for 30% of the energy expenditures (Brandt et al., 2011). For water treat-ment and supply they identified 5 to7% potential improvetreat-ments in pumping optimization, 3 to 7% energy savings with pump replacements, and 5 to 30% of improvement with the optimization of current operational techniques. In wastewater, 25% of energy savings can be reached with improvements in the aerobic treatment; a good control of the process could save 50% of energy consumption. They also found improvement opportunities in buildings inside treatment plants (15% energy savings).
2.4 Thermal energy in the UWC
n a typical US household, after the air conditioning, the second largest en-ergy consumer is water heating with approximately 20% of the total con-sumption (ACEEE, 2011). Hofman et al. (2011) estimate that 40% of the total energy losses in modern Dutch houses are represented by hot wastewater leaving the houses. A simulation carried out in order to calculate the benefits of installing heat recovery systems in the metropolitan area of Tokyo with the aim to recover heat from all water sources available in the city, showed potential energy savings of 41 PJ annually in the year 2,000 (Narita and Maekawa, 1991). Some actions to improve the energy balance for heating in the UWC deal with the installation of heat recovery systems for space heat-ing. This chapter introduces equipment, case studies and considerations of heat recovery installations in the UWC.
2.4.1 Equipment used in heat recovery systems
heat recovery system (HRS) is a combination of equipment such as heat exchangers, heat pumps and thermal storage. They can be applied in sewers, and drinking and industrial water supply networks (Mol et al., 2010).
Heat exchangers
First, there are heat exchanger types such as shower heat exchangers, which have the distinguishable characteristic that they recover the waste heat al-most at the same place where the heat is needed. Shower heat exchangers have a flat shape and are placed beneath the drainage of a single shower. They
I
2
can also be installed inside basements or service rooms. They can vary from a few centimeters long and 1 cm of diameter, to 1.8 m long and 7.5 cm diameter. These devices are commonly made of copper and arranged in spiral tubes. A common characteristic is that both do not have movable parts and the main aspects affecting the efficiency are the place where it is installed (near the drainage, the heater and the tap) and fouling. Optimal places are limited in already existing buildings (Goorskey, 2004).
A second type of heat exchanger comprises larger devices that work as sewers and, at the same time, are applied to recover heat. These devices are common-ly coupled to heat pumps. These heat exchangers can be shell and tube heat exchangers, spiral tube heat exchangers or plate heat exchangers mounted on pre-built pipes or pits which can be placed in existing networks. Some of these heat exchangers are equipped with a mechanical screener to remove solids and avoid fouling due to the solid fraction of water and the formation of bio-films (Wanner, 2009), which are the major problems reported in all kind of heat exchangers (Zogg, 2008).
Figure 2-2 presents the main configuration of heat exchangers utilized in wa-ter networks. Especially for sewer-pipe heat exchangers, different models have been developed. In accordance with the cross section of the pipe, they can be round, oval or rectangular (Rometsch et al., 2004). Advantages of heat recovery from sewers are the relative proximity of the energy source to the consumers, the widespread network in cities and the heat quality that can be found in wastewater when there is a high density of houses nearby. Normally, the warmest wastewater is found immediately downstream the household where the volume of wastewater is small and intermittent. The installation and operation of such heat recovery systems is considered to be economically feasible from 80 kJ s-1 of supplied heat (Schmid, 2008).
Heat pumps
Heat pumps are devices that transfer heat from a lower temperature to a higher temperature level. Depending on the principle of operation, there are three types of heat pumps: vapor absorption and soli-gas sorption heat pumps, reversible chemical reaction heat pumps, and vapor compression heat pumps (Wongsuwan et al., 2001).
2
Instantaneous Heater Shower Head Shower HX Drainage HX Pit HX Pipe HX Tap Water Wastewater Warm Water Warm WaterFigure 2-2: Heat exchanger types for water networks according to the site of installation. (HX: Heat exchanger).
In the first two types of heat pumps, a set of substances that make a reversible (chemical or phase-change) reaction in two stages is required. In the stage called production, the reaction releases heat while in the regeneration stage, one part of the heat pump needs an energy input in order to store heat that will be released again in a new production stage (Wongsuwan et al., 2001). These devices are used as chillers in industry (Hughes, 1997), but there are few publications regarding the use of chemical heat pumps to recover heat from the UWC. Ajah et al. (2008) compared chemical and mechanical heat pumps to recover low quality heat from industrial wastewater with similar characteristics to domestic wastewater. The comparison took into account economic, technical, energetic, environmental, and safety aspects. The space needed to install a chemical heat pump was the major constraint in the study compared to the feasibility of a mechanical heat pump.
Mechanical heat pumps are devices comprised by two heat exchangers, a compressor and an expansion valve. They are based on the Carnot cycle where the entropy of a compressed gas or refrigerant is higher causing an increase in temperature. When the compressed refrigerant is warmer, a fluid (water for heating) can interchange the heat inside of a heat exchanger to lower the re-frigerant’s temperature and then, the refrigerant is expanded causing a further
2
temperature decrease of the fluid. The colder refrigerant can hence inter-change heat with a heat source (wastewater or another type of waste heat) to become warmer before it is compressed again to end the thermodynamic cy-cle (Figure 2-3). Classifications of mechanical heat pumps depend on the heat source, the method of expansion and the type of engine: air-source, ground-source, direct-expansion solar-assisted, integrated solar-assisted, gas engine and multi-function heat pumps (Hepbasli and Kalinci, 2009). Mechanical heat pumps are devices most frequently found in heat recovery systems from wastewater. Therefore, in the following, the text will only refer to these types of devices
Figure 2-3: Basic parts and thermodynamic cycle of a heat pump. Adapted from (Holland et al., 1999).
The coefficient of performance (COP) of heat pumps is estimated as the useful energy with respect to the employed energy (COP=Q/W). The COP is dimen-sionless, Q is the useful heat (depending on the purpose, it can be for heating or cooling), W is the electrical energy utilized by the compressor. If the system considers other equipment, such as additional pumps for water conveyance to the heat exchangers, the required energy is added to the denominator.
Most of the reports about mechanical heat pumps for building heating come from Europe. In Zürich, the city hall, a swimming pool, the city administration and an already installed district heating network were equipped with heat pumps from 1938 to 1943 with power ranges from 100 to 5860 kJ s-1. The
heat sources were water from rivers, lakes and industry (Zogg, 2008). Alt-hough heat pumps have been utilized for some decades, not many develop-ments have occurred. Alterations in the compression stage (in multi-stages
Condensation Evaporation Pressure Temperature Evaporation Condensation Low Quality Heat (2) (1) Work Compresion-Expansion Device (Heat Pump) Higher Quality Heat
2
and scroll compressors) and the ejector system have improved their global efficiency to nearly 35% compared to the firsts commercially available heat pumps (Chua et al., 2010). In general, heat pumps are a reliable technology; commercial information published by Mueller (2009) mention one facility in Luzern that has been extracting heat from the sewage for more than 28 years without changing important parts.
Thermal energy storage
Thermal energy storage technology can be applied in heat extraction from water to match the heat requirements fluctuations (mostly annually) to the daily fluctuation of heat available from water in households. The working principle is based on the heat capacity of materials (mainly water). The stor-age can be placed above the ground or in the underground.
Above the ground, an insulated tank can be used to store thermal energy re-covered from the sewage. Funamizu et al. (2001) explain the use of a storage tank to balance the heat demand of office buildings in a wastewater treatment plant in Japan. Underground thermal energy storage is divided into aquifer thermal energy storage (ATES) and borehole thermal energy storage (BTES). The difference between ATES and BTES is that the second makes use of an arrangement of heat exchangers in such a way that only one well works as the heat source and sink; they are normally applied in smaller projects than ATES, when the soil has a relatively high resistance and can use a coolant to carry the heat in and out the well. An ATES has two wells (a warm and a cold well) separated from each other. It is an open system where the wells are separated by a barrier or screen (aquifers can be located at the same depth with a dis-tance between them or in different depths and closer) (Andersson, 2007a). The design considerations of such systems include the required heat inter-change, the anticipated levels of stratification in the storage vessels, the heat transfer between the stored water and the environment and the level of insu-lation of the system (Baker, 2008). The heat or cold, stored in thermal storage systems, can be stable in time lapses of months. Therefore, they can be used for seasonal storage. In winter, when heating is needed, heat is extracted from the water. The extracted heat is transferred and boosted using a heat pump (Nielsen et al., 2003). In a long term period, the heat extracted from under-ground thermal energy storage must be similar to the heat carried into the
