Assessment of energy consumpton and carbon footprnt n urban water systems: two case studes from Portugal Asha Mamade 1,2, Dála Lourero 1, Helena Alegre 1, Dída Covas 2 1 CERIS, Insttuto Superor Técnco, Unversdade de Lsboa, Lsbon, Portugal 2 Natonal Cvl Engneerng Laboratory, Lsbon, Portugal Correspondence to: Asha Mamade (asha.mamade@tecnco.ulsboa.pt) Abstract. The am of ths paper s to present an assessment of energy consumpton n the man stages of the urban water cycle water abstracton, treatment, dstrbuton and wastewater collecton and treatment and to calculate ndrect greenhouse gas (GHG) emssons assocated wth electrcty uses. The man nnovaton s the use of tme-dependent emssons that provde more accurate estmates than prevous methodologes. Global values for two water utltes n Portugal show that the yearly carbon footprnt per servced populaton ranges from 24-30 kg CO 2e n water supply and s approxmately 24 kg CO 2e n wastewater. A rankng of crtcal subsystems n the case-study s carred out usng GHG emssons (sustanablty ndcator) and other energy effcency ndcators. Results hghlght the mportance of usng dfferent performance ndcators for dentfyng crtcal areas n terms of water and energy losses and envronmental sustanablty. 1. Introducton Urban water systems are accountable for a consderable amount of energy consumpton. Energy assocated wth the stages of the urban water cycle s varable n tme and space. For example, the energy assocated wth pumpng of 1 m 3 of water to the treatment plant s 0.09 kw h n Melbourne, Australa, and 2.3 kw h n Southern Calforna (Olsson, 2011). Ths dsparty n energy consumpton s due to varous factors such as dfferences n topography, nfrastructure condton, dstance from water source, treatment levels and type of consumers [1]. Due to the water-energy nexus, most energy effcency solutons reduce both water and energy losses [2]. For nstance, reducng overflows n servce tanks reduces water losses, whch also reduces energy consumpton as less water needs to be treated and pumped. Recent studes have been rasng awareness to the carbon footprnt assocated to urban water systems: nearly 5% (290 mllon tons) of total annual GHG emssons n the U.S.A. orgnate n the water sector [1]. Therefore, despte the mportance of assessng energy consumpton for effcency purposes, t s also hghly relevant to assess GHG emssons for envronmental sustanablty purposes snce emssons account for the source of the energy consumpton. Energy provded by a renewable source s more envronmentally sustanable than energy from a non-renewable source. Accordng to the EU 2020 clmate and energy package, a set of legslaton has been created to ensure three key targets: 20% cut n GHG emssons (from 1990 levels), 20% of renewable energy and 20% of energy effcency mprovement [3]. Assessng energy consumpton and related GHG emssons also contrbutes to the 2030 Agenda for Sustanable Development, partcularly wth the targets defned for goals 6, 9 and 11 [4].
The am of ths paper s to present an assessment of energy consumpton n the man stages of the urban water cycle and to calculate ndrect greenhouse gas (GHG) emssons related to water pumpng and other electrcty uses n order to dscuss the mportance of dfferent performance ndcators for dentfyng crtcal areas n terms of water and energy losses. Most studes calculate emssons based on fxed electrcty mx emsson factors [5, 6]. The man nnovaton compared to prevous studes s the use of tme-dependent emssons that provde more accurate estmates than prevous approaches. 2. Methodology Ths paper follows a three-step methodology. The frst step s to assess energy consumpton n the man stages of the urban water cycle. Ths s carred out by collectng monthly electrc blls referrng to raw water pumpng, water treatment, water dstrbuton, wastewater collecton and wastewater treatment. A smlar analyss has been appled by [7]. The second step s to estmate hourly GHG emssons assocated wth water pumpng. Only ndrect emssons from energy consumpton were analysed n the scope of ths work. Drect emssons from the treatment process and other ndrect emssons resultng from other actvtes (e.g., dscharge of the effluent or untreated sewage to rvers) have not been consdered. GHG emssons can be obtaned by the energy provder. The followng procedure has been adopted to calculate hourly emssons: 1 Calculaton of hourly total emsson factors n g CO 2e/kWh ( HEF ): HEF n s1 s EG s1 s, EG EF s, s (1) where EGs, s the total energy generaton (kwh) from source s n hour and EF s the emsson factor (g CO 2e/kWh) from source s. 2 Calculaton of daly emsson load dagram for each month ( DEF ): the average value of HEF from the total number of days of a gven month s computed for each hour. 3 Calculaton of hourly electrcty load dagram for each month n kwh ( ELD t, ): s ELD t, Et n n d h, t (2) where E t s the monthly energy consumpton (kwh) under tarff t, nd s the number of days n the month and n ht, s the number of hours of tarff t n a day. It should be noted that energy tarffs usually vary from summer to wnter.
4 Calculaton of hourly emssons ( GHG ) n kg CO 2e : GHG DEF ELD t, (3) 1000 Fnally, the thrd step s to dvde the system n analyss areas and to calculate performance ndcators to fnd crtcal areas for nterventon. For ths purpose, two ndcators have been calculated: G1 (tons CO 2e) emssons n the urban water cycle (sustanablty ndcator) E3 (-) rato between energy consumpton from water supply and mnmum energy (energy effcency ndcator for water supply systems). The ndcator E3 represents a rato of the theoretc energy n excess that s suppled to the system n comparson to the mnmum energy requred. It should be as low as possble, though t s always postve, snce the total energy suppled ncludes headlosses. The ndcator E3 has already proven to be a useful energy effcency ndcator to prortse analyss areas [8-10]. 3. Results 3.1. Assessment of energy consumpton n the urban water cycle Two case-studes from real water utltes have been analysed. The frst belongs to a rural area wth 20108 households and a total network length of 1140 km, 670 km for water supply and 470 km for wastewater. There are 10 pumpng statons n the water supply and 69 n wastewater. The second belongs to an urban area that supples water to 35652 households and a total network length of 1240 km, 870 km for water supply and 470 km for wastewater. There are 28 pumpng statons n the water supply and 144 n wastewater. Fgure 1 shows the dstrbuton of energy consumpton n each stage of the urban water cycle. Rural Urban 22% 12% 16% 16% 56% 29% 22% 6% 21% Raw water pumpng Water dstrbuton Wastewater treatment Water treatment Wastewater collecton Raw water pumpng Water dstrbuton Wastewater treatment Water treatment Wastewater collecton Fgure 1. Energy consumpton per stage of the urban water cycle for the rural and urban water utltes n Portugal
Jan 2010 Mar 2010 May 2010 Jul 2010 Sep 2010 Nov 2010 Jan 2011 Mar 2011 May 2011 Jul 2011 Sep 2011 Nov 2011 Jan 2012 Mar 2012 May 2012 Jul 2012 Sep 2012 Nov 2012 Jan 2013 Mar 2013 May 2013 Jul 2013 Sep 2013 Nov 2013 Jan 2014 Mar 2014 May 2014 Jul 2014 Sep 2014 Nov 2014 Jan 2015 Mar 2015 May 2015 Jul 2015 Sep 2015 Nov 2015 Total energy consumpton (%) Volume of abstracted water (m 3 ) In the rural system water s abstracted from a groundwater source wth good water qualty, therefore water treatment has neglgble energy consumpton (only chlorne s added). Raw water pumpng accounts for 56% of total energy consumpton the most energy-ntensve stage n ths system. In the urban system ths stage s less energy ntensve (16%): surface water s abstracted and treated representng 36% of total energy consumpton. Water dstrbuton n the rural system only represents 6% of total energy, whle n the urban system t s more than the trple 21%. Ths can be explaned by the hgher number of pumpng statons n the urban system. Smlarly, whle wastewater collecton represents 16% of total energy n the rural system, the percentage almost doubles (29%) n the urban system. On the contrary, wastewater treatment accounts for 22% n the rural system and s much lower n the urban system 12%. These results hghlght how energy consumpton s varable n tme and space and how a prelmnary assessment mght be mportant for focusng on the most crtcal stages. For example, whle n the rural system t s worth lookng to the raw water pumpng stage, n the urban system t may be more useful to deepen the understandng on the wastewater collecton stage snce t represents one thrd of total energy consumpton. Fgure 2 shows the fve-year monthly varatons of energy consumpton n each stage of the urban water cycle for the rural system. 100% 90% 80% 70% 60% 50% 40% 30% 20% 10% 0% 800,000 700,000 600,000 500,000 400,000 300,000 200,000 Tme (months) Raw water pumpng Water dstrbuton Wastewater collecton Wastewater treatment Abstracted water Fgure 2. Monthly energy consumpton per stage of the urban water cycle for the rural system from Jan 2010 to Dec 2015 Summer peaks (August) that were regstered n the volume of abstracted water (Fgure 2) are typcally concdent wth energy peaks from raw water pumpng and water dstrbuton. These two stages show a smlar behavour throughout the years contrarly to wastewater collecton and treatment. Ths can be due to not all wastewater beng treated by the utlty.
3.2. Assessment of carbon footprnt n the urban water cycle Accordng to the procedure for assessng GHG emssons presented n the methodology, the frst step was to calculate hourly emsson factors. Three years of hourly energy generaton were obtaned from the Natonal Energy Provder (REN). To llustrate the procedure, Fgure 3 (a) presents the daly emsson factors obtaned (DEF), and the medan, 10 th and 90 th percentle for two groups of months. Group 1 ncludes hourly emssons from January to Aprl showng smooth curves wth the lowest values due to a hgher percentage of renewable energes. Group 2 ncludes months from May to December wth slght varatons throughout the day and hgher emsson values, meanng that the energy generaton n these months has a consderable proporton of coal and fuel gas. Fgure 3 (b) depcts the energy load dagram for raw water pumpng stage for the same three years used for calculatng GHG emssons for the rural system. Summer and Wnter medans are also presented. The two seasons have been defned accordng to the energy tarffs, where summer s from Aprl to October and wnter ncludes the remanng months. Fgure 3. (a) Hourly emsson factors n 2013 grouped n months and (b) Energy load dagram from raw water pumpng for the rural system for 2010, 2011 and 2013 grouped by season. The energy tarff s dvded n four levels, as presented n Table 1. It s observed from Fgure 3 (b) that the utlty s not takng full advantage of the lowest energy tarffs, snce energy consumpton s lower from 02h-06h (Off-Peak 1 tarff), although t tres to decrease consumpton n some peak hours lke md-day and 20h. Table 1. Results of performance ndcator comparson Tarff type Off-Peak 1 Off-Peak 2 Peak 1 Peak 2 Prce lowest low hgh hghest Wnter (Jan-Mar; Nov-Dec) Summer (Apr-Oct) 02h-06h 02h-06h 06h-08h 22h-02h 06h-08h 22h-02h 08h-09h 10h30-18h 20h30-22h 08h-10h30 13h-19h30 21h-22h 09h-10h30 18h-20h30 10h30-13h 19h30-21h
Monthly GHG emssons can be calculated as the product between hourly values dsplayed n Fgure 3 (a) and Fgure 3 (b). Fgure 4 presents the results for daly GHG emsson curves for each stage n the urban water cycle. Snce the hourly emsson curves have rather smooth varatons, the emsson curves follow the behavour of the energy load dagram. Ths can be seen by comparng the behavour of the energy load dagram (Fgure 3 (b)) and the GHG emsson curve for raw water pumpng n Fgure 4 (a). Hourly emsson curves for water supply reflect water consumpton patterns wth lttle consderaton of the energy tarff due to the lmted storage capacty of the water supply system. Contrarly, wastewater emsson curves reflect the energy tarffs.e., are hgher for the lowest energy rates. Fgure 4. Rural system hourly emsson curves grouped by season for the years 2010, 2011 and 2013: (a) raw water pumpng; (b) water dstrbuton; (c) wastewater collecton and (d) wastewater treatment In the rural system, medan values for GHG emssons range from 25 to 100 kgco 2e n raw water pumpng, 5 to 10 kgco 2e n water dstrbuton, 5 to 30 kgco 2e n wastewater collecton and 15 to 35 kgco 2e n wastewater treatment. Except for wastewater collecton, emssons tend to double n the summer season, probably due to the holday season and the return of expats. For the same reason, changes n the behavour of the emsson curves for raw water pumpng and water dstrbuton are also observed. Ths
can be attrbuted to the changes n the water consumpton pattern that occur n the summer [11], meanng that electrcty, GHG emssons and water consumpton behave smlarly n these stages (although water dstrbuton havng much lower emssons). Wastewater collecton curves are very smlar n the wnter and summer and there s clearly an effort for pumpng water n off-peak hours. Smlar curves for summer and wnter may ndcate the presence of unwanted nflows. 3.3. Comparson wth other studes Results for carbon footprnt have been calculated and compared wth prevous studes. Table 2 summarses emssons per capta and per servced populaton for water supply (raw water pumpng, water treatment and water dstrbuton) and wastewater (wastewater collecton and wastewater treatment). Per capta emssons range from 36 and 43 kg CO 2e/nhabtant and are hgher for the urban system. When comparng the results n terms of servced populaton, results are hgher for the rural system and range from 24-30 kg CO 2e n water supply and s approxmately 24 kg CO 2e n wastewater. Despte havng the same order of magntude, these values are much hgher than prevous studes: n Mexco and Peru there s no energy s consumed for wastewater collecton and n Thaland there are no values for water supply. The emsson factors for Mexco and Thaland are much hgher than the ones for Portugal and Peru, meanng that energy s produced wth a hgher carbon footprnt n the former countres. Table 2. Emssons per capta and per populaton servced for water supply and wastewater and comparson wth other studes (2013) Water Locaton Wastewater Total Water supply Wastewater Emsson factor supply kg CO2e/ servced Unts kg CO2e/nhab/year kg CO2e/kWh populaton/year Rural system 15.2 20.3 35.6 30.9 24.3 0.24 Urban system 22.7 20.7 43.4 24.2 23.5 0.24 Mexco [12] 13.4 3.4 16.7 14.1 8.6 0.44 Peru [13] 8.3 1.0 9.3 9.8 1.5 0.23 Thaland [14] - 1.0 - - 6.9 0.47 Table 3 shows total emsson ntensty per stage calculated for each system usng hourly emsson factors and results obtaned from prevous studes. Smlar values have been underlned, although ntenstes and emsson factors vary wdely among stages and locatons. Table 3. Emssons per stage and comparson wth other studes (2013) Locaton Raw water pumpng Water treatment Water dstrbuton Wastewater collecton Wastewater treatment Emsson factor Unts kg CO2e/m3/year kg CO2e/kWh Rural system 0.117 0 0.013 0.223 0.310 0.24 Urban system 0.061 0.063 0.089 0.120 0.047 0.24 Turn [5] 0.014 0.051 0.160 NA 0.222 0.53 Oslo [5] 0 0.032 0.008 NA 0.270 0.05
3.4. Performance ndcators comparson The rural system has been dvded n three sub-systems and two performance ndcators (G1 and E3) have been calculated to fnd the crtcal sub-system. G1 (tons CO 2e) refers to the GHG emssons assocated wth the urban water cycle. E3 s a dmensonless energy effcency ndcator whch refers to the rato between energy consumpton from water supply and mnmum requred energy. Table 4 shows the results obtaned for three sub-systems. Despte sub-system 2 provdng 5.5 tmes the mnmum requred energy, sub-system 3 has hgher GHG emssons. Table 4. Results of performance ndcator comparson for the rural system 4. Conclusons and future work Sub-system G1 E3 Unts tons CO2e - 1 148 3.5 2 160 5.5 3 670 2.7 The current research work amed at assessng energy consumpton and related GHG emssons n the urban water cycle. Results for a rural and an urban system have been presented, showng that the amount of energy consumpton n each stage wdely vares n tme and space and showng how a prelmnary assessment mght be mportant for focusng on the most crtcal stages. For example, whle n the rural system t was worth lookng to the raw water pumpng stage, n the urban area t mght be more useful to deepen the understandng on the wastewater collecton stage, snce t represented one thrd of total energy consumpton. Fve-year monthly varaton of energy consumpton was cross-analysed wth abstracted water, showng the smlartes between water and energy consumpton patterns. Tme-dependent emsson factors have been calculated for three years where data from the energy provder was avalable showng smooth varatons throughout the day, wth hgher values for summer months and lower values for Aprl and March. Emsson curves have been analysed for each stage of the water cycle and were found to follow a smlar behavour of energy load dagrams. Wastewater collecton s the stage where energy s used only n off-peak hours for a lower energy bll. Results of carbon footprnt per capta, per servced populaton and per m 3 were compared wth prevous studes, showng a smlar order of magntude but a hgh varablty among the dfferent locatons and studes. Fnally, a comparson between an energy effcency performance ndcator (E3) and a sustanablty ndcator (G1), has shown that despte some areas provdng lower energy n excess, they may have hgher carbon footprnt worth lookng wth hgher detal. For future work, a mult-objectve optmzaton method wth tme-dependent emssons could be used for achevng optmzed solutons n terms of cost and emssons. Wthn the water-energy nexus, ths work also hghlghts the mportance of water utltes workng together wth energy utltes, as ths wll enable hgher jont benefts as far as sustanablty s concerned.
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