Estimating Water, Energy, and Carbon Footprints of Residential Swimming Pools

14 Estimating Water, Energy, and Carbon Footprints of Residential Swimming Pools Tyler Gallion, Tyler Harrison, Robert Hulverson, Kiril Hristovski COLLEGE OF T ECHNOLOGY AND INNOVATION, ARIZONA STATE UNIVERSITY, MESA, AZ, USA

1. Introduction Providing access to clean water represents a pressing concern in the wake of the expectations that, in the near future, projected population growth and water demand will exceed the easily accessible fresh water resources.1,2 Historically limited to countries of the developing world, this pertinent issue necessitates a broader recognition among the developing countries if the growing water demands for domestic, agricultural, and industrial applications are to be met. The US National Academy of Engineering, together with other organizations, such as the United Nations, has recognized the seriousness of this upcoming problem and raised the alarm by identifying it as one of the grand challenges that will need to be addressed if a sustainable future on a global scale is to be achieved.3,4 While the technological innovation paves the way for actuating traditionally unsuitable water resources into new avenues of fresh water supply, the increasing demands for potable water also necessitate reexamination of the existing potable water management practices.5,6 In arid environments, where water is a scarce commodity, it becomes imperative to adequately allocate and manage this resource, especially if the community exhibits rapid growth and diverts large portions of this resource to fulfill agricultural needs.7 Consequently, understanding temporal water balances and the water losses resulting from continuous or intermittent use of recreational facilities represents a prerequisite for proper development and employment of prudent and sustainable water management practices. Residential swimming pools, which are becoming a growing common feature of the communities in the developed world, often represent a small subsystem in a typical municipal or regional water management system. While often considered a luxury in the regions with cooler climates, they represent a necessity for the residents of the warmer regions needing to escape the hot weather. With increases in the quality of life, the number of residential swimming pools steadily grows in the metropolitan areas, Water Reclamation and Sustainability. http://dx.doi.org/10.1016/B978-0-12-411645-0.00014-6 Copyright © 2014 Elsevier Inc. All rights reserved.

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especially in the arid regions of the Southwestern United States. Unfortunately, only sporadic and unrealistic data exist that elucidates the environmental implications associated with consumption of water by residential swimming pools. In warm and dry climates like Arizona’s desert environment where the number of residential swimming pools is estimated in the hundreds of thousands, such data often might prove to be an important component in the overall water management scheme. In the absence of credible data, development of simple models to estimate, examine, and predict the environmental implications of residential swimming pools might serve as a prudent tool that could aid water management professionals to properly plan and allocate the necessary water resources for domestic, industrial, agricultural, and recreational purposes. This chapter demonstrates the development and application of such a model as a tool to assess and compare water, energy, and carbon footprints originating from residential swimming pools. To demonstrate applicability and dependability of this model as an assessment, planning, and management tool, realistic scenarios were employed to estimate water, energy, and carbon footprints of residential swimming pools located in Maricopa County (Phoenix, Arizona, metropolitan area). The model considered three different commercially available swimming pool sizes and a model pool representative of the residential swimming pools in Maricopa County (Phoenix model pool) as described by Forrest and Williams.8 These estimates were occasionally compared with similar estimates for residential swimming pools operated in colder climates (e.g., New York City) to emphasize the impact of climate and operational factors on these footprints.

2. Methodology 2.1

Description of the Water Balance Model

A simplified conceptual water balance model, used as a basis for estimating the water, energy, and carbon footprints of a swimming pool system, is illustrated in Figure 1 and can be summarized by the mass balance relationship presented in Eqn (1) Accumulation ¼ Inputs  Outputs  Transformations

(1)

Since no water in the pool is being created or destroyed, there is no transformation; so without transformation, it is possible to simplify Eqn (1) into Eqn (2). In mathematical

FIGURE 1 Conceptual depiction of the water balance model used for estimating water, energy, and carbon footprints of residential swimming pools.

Chapter 14 • Estimating Water, Energy, and Carbon Footprints 345

kg , Eqn (2) can l be rewritten as Eqn (3), which corresponds to the depiction of the system (i.e., swimming pools) in Figure 1.

terms, considering that r ¼ m/V, and the estimated density of water r ¼ 1

Accumulation ¼ Input  Output

(2)

A ¼ QFill þ QTop þ Qprec  QBack  QEvap  Qdrain

(3)

where A ¼ is the indefinitely small change of water volume in the pool over an indefinitely small time period. Integration using boundary conditions from V0 ¼ 0 to Vf ¼ VPool and from t0 ¼ 0 to tf ¼ t, transforms this relationship into a simple linear 3 1 relationship A ¼ VPool t [L] [T] . Based on literature review and personal communications with pool manufacturers in Arizona, three pool volumes were (VPool) selected as representative of small, medium, and large residential pools.9,10 These are presented in Table 1. Additionally, a value that represents the average pool volume of a typical residential swimming pool was used to calculate the average water, energy, and carbon footprint of a residential swimming pool in Maricopa County, Arizona, and extrapolate the cumulative values for these parameters for the Phoenix metropolitan area.8 QFill represents the water inflow necessary to fill an empty residential swimming pool over a period of time. Similarly, after integration, this relationship can be described as QFill ¼ VtPool  fFill [L]3[T]1; where tfill is the time necessary to fill a residential swimming Fill pool with volume VPool at a frequency fFill (fFill ¼ 0.2 fill/year; once in 5 years). Similarly, Qdrain represents the water loss necessary to drain an already-filled residential swimming pool over a period of time. Similarly, after integration, this relationPool ship can be described as Qdrain ¼ Vtdrain  fdrain [L]3[T]1; where tdrain is the time necessary to drain a residential swimming pool with volume VPool into the sewer system at a frequency fdrain (fdrain ¼ 0.2 drains/year; once in 5 years). Qprecip represents water entering the pool as a result of atmospheric precipitation. For Pool the purposes of the model, this parameter was calculated as QPrecip ¼ tAPrecip  IPrecip , where dV dt

APool represents the surface area of the residential swimming pool [L]2, IPrecip represents the average rainfall intensity [L]3[L]2 [T]1; and tPrecip represents the average precipitation time. Monthly average values were used for the purposes of the model. QBack represents water loss from the system due to backwash of the swimming pool filtration system. The backwash was calculated as QBack ¼ QPump  tBackwash  fBackwash, Table 1

Sizes of Typical Residential Swimming Pools Used in the Model estimates

Swimming Pool Size Smalla Mediuma Largea Phoenix model poolb a

Swimming Pool Volume (m3)

Swimming Pool Depth (m)

Swimming Pool Area (m2)

15.0 59.6 108 62.6

1.3 1.5 1.5 1.5

11.5 39.7 72.0 41.1

Values obtained from Refs 9,10. bSwimming pool size representative of average pool size in the Phoenix metro area as described by Forrest and Williams.8

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Table 2

Operational Parameters Used as Model Inputs

Operational Parameter

Descriptor

Value

Units

References

Backwash rate Backwash time period Backwash frequency

QBack tBack fBackwash

238 4 0.25

l/min min Cycles/week

8 10 10

where QPump represents the effective flow rate of the pump [L]3[T]1; tBackwash represents the time period of backwash [T], and fBackwash represents the frequency of backwash [T]1. Table 2 summarizes the parameters that were used in the model calculations. w QEvap represents the rate of evaporation, and it was calculated as QEvap ¼ r tp Evap , water where rwater ¼ 1 kg/l and tEvap is the time period of evaporation [T]. The parameter wp represents a mass evaporation of water expressed in kilograms per second, and it is estimated based on the empirical relationship provided by Eqn (4)11,12: wp ¼


A pw  pa ð0:089 þ 0:782vÞ Y

(4)

where A is the area of the pool surface (square meters); Y is the latent heat required to change water to vapor at a surface water temperature (kilojoules per kilogram); pw is the saturation water vapor pressure taken at surface water temperature (kilopascals); pa is the saturation water pressure at air dew point (kilopascals); and v is the air velocity over the water surface (meters per second), which was estimated as 10% of the average wind velocity because most of the swimming pools are located in urban fenced areas and the wind velocity is measured at heights (10 m) extending beyond the height of a brick fence (w2 m). The model describing the swimming pool system could be operated in two scenarios: (1) filling the pool initially, where the mass balance model equation could be simplified to Eqn (6), which assumes that all the other water inputs and outputs (e.g., evaporation) are negligible because of the short period of time; and (2) operating the system in steady state where the accumulation does not change (i.e., the volume of the pool is constant). The two operating conditions are described in Eqns (5) and (6). Non steady state : Steady state :

0¼

dv ¼ QFill dt

dV ¼ QTop þ QPrec þ QFill  QBack  QEvap  Qdrain dt

(5) (6)

Considering that (1) the non-steady state described in Eqn (5) is conducted only at the initial filling of the swimming pool, and (2) (QFill  tFill þ Qdrain  tdrain) ¼ 0 when water is replaced in the swimming pool, the overall water footprint was calculated by operating the model under steady state conditions.

Chapter 14 • Estimating Water, Energy, and Carbon Footprints 347

2.2

Description of the Energy Footprint Model

The energy requirements for the collection, treatment, and distribution of water and collection and treatment of wastewater were considered to estimate the energy footprint. Equation (7) summarizes the model used to estimate the energy footprint.   E ¼ VWater  fWater þ VSewage  fSewage  tE

(7)

where E is the energy ([M] [L]2[T]2) needed to provide the collection, treatment, and distribution of a given volume of water and a given volume of wastewater as estimated by the water balance model over a time horizon tE [T]. Equation (8) summarizes the relationship used to estimate the volume of water considered for the energy footprint calculations. Similarly, Eqn (9) summarizes the volume of wastewater considered for the wastewater calculations, assuming the entire wastewater is discharged into a sewer system and conveyed to a wastewater treatment plant.   VWater ¼ QTop  tTop þ QFill  tFill þ QBack  tBack þ QEvap  tEvap þ Qdrain  tdrain

(8)

VSewage ¼ ðQBack  tBack þ Qdrain  tdrain Þ

(9)

fWater is the conversion factor for energy used in water collection, treatment, and distribution; fSewage is the conversion factor for energy used in wastewater collection, treatment, and discharge. According to Hallin and the California Energy Commission,13,14 the values for fWater could range from 800 to 2700 kWh/MGal for treated and distributed water; while depending on the water source, water supplying and conveyance to the water treatment plant could range from 0 to 16,000 kWh/MGal. Table 3 summarizes the most commonly used estimates for each fraction constituting fWater. Similarly, Table 3 shows that values for fSewage could range from 1100 to 5000 kWh/MGal for collected, treated, and discharged wastewater. Considering Table 3 Energy Consumption for Municipal Water Supply, Treatment, and Distribution and Wastewater Treatment and Discharge (kilowatt hour/million gallon)

Water Processing Stage Extraction (supply & conveyance) Treatment Distribution Wastewater treatment and discharge fWater fSewage a

Surface Waterb

Central Arizona Project Waterb

Maximum Value Reported by CECa

610c

120c

4700f

16000

100 700 1100

10c 1130c,d 1640e,f

80c 1130c,d 1640e,f

80c 1130c,d 530f

1500 1200 5000

800 1100

1750 1640

1330 1640

5,910 530

19000 5000

Minimum Reported by CECa

Groundwaterb

0

Estimates by the California Energy Commission (CEC).14 bEstimates for Phoenix Active Management Area.8 cFrom Ref. 15. dFrom Ref. 16. eFrom Ref. 17. fFrom Ref. 18.

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the average fWater values (fWater-average ¼ 2997 kWh/MGal) and fSewage values (fSewageaverage ¼ 1270 kWh/MGal) for the Phoenix water management area are within this range, the minimum, maximum, and average f values were considered in the model estimates.

2.3

Description of the Carbon Footprint Model

Energy footprint estimates were used to assess the carbon footprint for the different modeled scenarios. Equation (10) summarizes the model for estimating carbon footprint, using different sources for generating electrical energy. RC ¼


E  fCOAL  CCOAL þ fHYDRO  CHYDRO þ fGAS  CGAS þ fNUCLEAR  CNUCLEAR þ fPV  CPV tE

(10)

where • •

•

•

•

•

RC is rate of generation of carbon dioxide to produce energy E [M] [L]2[T]2 over a time horizon tE [T]. fCOAL is the fraction contributed from generation of energy via thermal coal power plant energy generation. For the state of Arizona, fCOAL ¼ 0.39 because 39% of the energy is generated from coal power plants.19 CCOAL is a conversion factor that describes the amount of carbon dioxide generated per kilowatt hour during the process of generating electrical energy by coal power plants. A value of 0.964 kg carbon dioxide equivalents (CO2e)/kWh was used for the model estimates.20 fHYDRO is the fraction contributed from generation of energy via a hydroelectric route. For the state of Arizona, fHYDRO ¼ 0.07.19 CHYDRO is a conversion factor that describes the amount of carbon dioxide generated per kilowatt hour during the process of generating electrical energy by hydroelectric power. For purposes of the modeling, a value of 0 kg CO2e/kWh was considered.20 fNUCLEAR is the fraction contributed from generation of energy by thermonuclear power plants. For the state of Arizona, fNUCLEAR ¼ 0.37.19 CNUCLEAR is a conversion factor that describes the amount of carbon dioxide generated per kilowatt hour during the process of generating electrical energy by nuclear power plants. For purposes of the modeling, a value of 0.066 kg CO2e/kWh was considered.20 fGAS is the fraction contributed from generation of energy via a natural gas power plant energy generation route. For the state of Arizona,19 fGAS is 0.15. CGAS is a conversion factor that describes the amount of carbon dioxide generated per kilowatt hour during the process of generating electrical energy by natural gas power plants. A value of 0.443 kg CO2e/kWh was used for the model estimates.20 fPV is the fraction contributed from generation of energy via solar energy. For the state of Arizona, fPV ¼ 0.02.19 CGAS is a conversion factor that describes the amount of carbon dioxide generated per kilowatt hour during the process of generating electrical energy by natural gas power plants. A value of 0.032 kg CO2e/kWh was used for the model estimates.20

Chapter 14 • Estimating Water, Energy, and Carbon Footprints 349

2.4

Description of the Water Footprint Model

The overall water footprint, which incorporates water for energy generation in addition to the water used for maintaining the residential swimming pool, is estimated using the energy footprint expressed in Eqn (7). Similar to Eqn (10), the water footprint is described by Eqn (11). Rw ¼


E  fCOAL  WCOAL þ fHYDRO  WHYDRO þ fGAS  WGAS þ fNUCLEAR  WNUCLEAR tE

(11)

where • •

•

•

•

Rw is rate of water utilization to produce energy E [M] [L]2[T]2 over time horizon tE [T]. WCOAL is a conversion factor that describes the amount of water needed to generate kilowatt hour of electrical energy by coal power plants. A median value of 2.56  103 m3/kWh (687 Gal/MWh) for generic coal power plant was used for the model estimates, although the value for this conversion factor depends on the type of the cooling system of the power plant and could range from 4 to 1110 Gal/ MWh.21,22 WHYDRO is a conversion factor that describes the amount of water needed to generate kilowatt hour of electrical energy by hydroelectric power plants. A median value of 1.70  102 m3/kWh (w4490 Gal/MWh) was used for the model estimates, although the value for this conversion factor could range from 1425 to 18,000 Gal/ MWh.1,21,23 WNUCLEAR is a conversion factor that describes the amount of water needed to generate kilowatt hour of electrical energy by nuclear power plants. A median value of 2.54  103 m3/kWh (w672 Gal/MWh) was used for the model estimates, although the value for this conversion factor could range from 100 to 845 Gal/ MWh.21 WGAS is a conversion factor that describes the amount of water needed to generate kilowatt hour of electrical energy by gas power plants. A median value of 1.49  103 m3/kWh (w393 Gal/MWh) was used for the model estimates, although a value ranging from 0 to 1170 Gal/MWh for this conversion factor could be used.1,21,24

3. Results and Discussion 3.1

Direct Water Consumption

Figure 2 summarizes the direct water consumption ranges estimated for the three different model swimming pool sizes and the Phoenix model pool as described. Based on the model estimates, presented in the inset of the figure, the annual direct water consumption for the Maricopa County could range from 38 m3 for small residential pools to

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FIGURE 2 Estimated ranges for direct water consumption of the modeled pool size systems located in Maricopa County, Arizona. The error bars represent direct water consumption values for large and small pools.

176 m3 for large ones. A medium-sized pool would require about 103 m3 of water per year. This value is very similar to the value of about 106 m3/year obtained for the representative Phoenix-size pools. Residential swimming pools require the largest quantity of water during the period from May to July, when the direct water consumption increases as much as 31 m3 for large pools and 6 m3 for small pools. The model estimates suggest that medium-sized pools consume about 18 m3 of water per month during this period. The direct water consumption significantly decreases during the winter months. During the period from November to March, the model water consumption estimates range between 2 and 3 m3 for small, and between 7 and 12 m3 for large nonwinterized residential swimming pools. This is to be expected because evaporation comprises the majority of the consumed water from nonwinterized residential swimming pools. The evaporation is encouraged by the Maricopa County’s desert environment, which is characterized by relatively warm winters and low air humidity. Figure 3 summarizes the percent of water supplied by the utilities that is lost to evaporation from the modeled residential swimming pools. The model estimates suggest that large swimming pools could lose between 84% and 97% of the supplied water as a result of evaporation during the winter and summer months, respectively. Such high

Chapter 14 • Estimating Water, Energy, and Carbon Footprints 351

FIGURE 3 Percentage of the total utility supplied water that is lost to evaporation for the modeled residential swimming pools located in Maricopa County, Arizona. The error bars represent values for evaporation from large and small pools.

values are not surprising for Maricopa County because winters are relatively warm and dry, and summers, with the exception of the rainy season in the latter part of summer, are very hot and dry. The effects that humidity increases have on water evaporation become evident during the month of August when the average monthly humidity jumps from about 20% to more than 38%. During the same period, the monthly average temperature exhibits a negligible change. Figure 2 clearly illustrates the effect of humidity on swimming pool water evaporation. In June, the peak water loss to evaporation could reach as high as 31 and 6 m3 for large and small pools, respectively. This water loss is reduced by a factor of three during the month of August because of humidity increases. During August, the water consumption resembles the one exhibited in the winter months, where both the humidity and temperature are low. The model estimates suggest that small swimming pools could lose somewhere between 45% and 80% of the supplied water during the winter and summer months, respectively. Interestingly, the loss to evaporation from large pools does not change as rapidly as it does for small pools exposed to the same seasonal weather variations. For

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WATER RECLAMATION AND SUSTAINABILITY

large pools, the percent of water loss to evaporation increases from winter to summer about 15%, while this value is about 45% for small pools for the same period. Figure 3 depicts the greater effect that seasonal weather conditions have on small pools. As the winter season progresses into summer, the lower error bar values, describing the water loss from small pools, decrease more rapidly than the upper error bar values, which describe the water loss from large pools. This counterintuitive pattern could be explained if the influences of swimming pool size and surface area on the water loss are examined. Although larger swimming pools exhibit overall large surface areas from which larger quantities of water could be evaporated, they also contain a larger quantity of water than the small swimming pools. In contrast, the small swimming pools are typically shallower and exhibit greater specific surface area. The water in the smaller pools warms up much faster than the one in the larger pools when exposed to the increased solar radiation and air temperature during the summer season. This increase in water temperature results in faster rates of evaporation through the greater specific surface area, which results in the loss of a greater percentage of the overall water volume. The modeled water consumption estimates are based on a scenario where residential swimming pools are uncovered and typically filled with water throughout the year in an extreme weather environment like the one that exists in the Arizona desert. In contrast, residential swimming pools located in parts of the country with colder climates are typically operated from mid-May to mid-September. In this scenario, the majority of the water consumption occurs during the month of May, when the swimming pool is filled with water. For example, the model estimates for the same type of small pools, but located in the City of New York and operated from mid-May to mid-September under the same conditions as the pools located in Arizona’s Maricopa County, is about 15 m3 for the month of May only. Similarly, the water consumption for large pools is only for the month of May and generally corresponds to the size of a large residential swimming pool. Water consumption during the other months of the season is negligible when rainfall is considered an input. Through the mid-September to mid-May period, the residential swimming pools are drained and winterized, so no evaporative losses exist. Although pool winterization might adversely affect the pool deterioration and add to its overall management costs, it definitely reduces the annual water consumption.

3.2

Energy Footprint

Figure 4 summarizes the estimated energy footprints for the three different pool sizes and the Phoenix model pool size described by Forrest and Williams.8 The estimated annual energy footprints of the modeled residential swimming pools could range between 2400 and 2800 kWh/year for small and large pools, respectively, and they could be as high as 20% of the typical annual household energy footprint in Arizona.25 The Phoenix model pool consumes about 2500 kWh/year. This value is slightly lower than the values of 3500 to 3900 kWh/year reported by Forrest and Williams.8 The difference between the reported and modeled values primarily stems from the different model inputs

Chapter 14 • Estimating Water, Energy, and Carbon Footprints 353

FIGURE 4 Estimated ranges for the energy footprint of the modeled pool size systems located in Maricopa County, Arizona. The upper error bars represent estimated maximum energy footprint for large residential swimming pools, and lower error bars represent estimated minimum energy footprint for small residential swimming pools.

that were used to estimate the energy footprint of the recirculation pump. Forrest and Williams use 9.2 h/day for the summer and 4.8 h/day for the winter as a standard operating cycle for the pool’s recirculation pump. Discussions with professionals in the residential swimming pool business, suggested that the typical operating period for recirculation pumps in the Phoenix metro area ranges between 7 and 8 h for the summer season and 2 and 3 h during the winter, so the higher values reported by Forrest and Williams were not used as model inputs because they were considered slightly higher than the realistic values.10 Furthermore, the Forrest–Williams model also considers the energy footprint of pool chemicals, which additionally contributes to the high reported values. They estimate that pool chemicals used to maintain the pool water quality could contribute up to 500 additional kilowatt hour to the annual energy footprint of a swimming pool. Depending on the size of the pool, hours of seasonal operation, backwash cycle, pump power, water consumption, and energy required to supply and treat the water, the

354

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monthly energy footprint of small swimming pools in Arizona could range from 110 to 300 kWh/month. This footprint is only slightly higher for large swimming pools (from 130 to 340 kWh/month) because most of the energy footprint originates from the daily operation of the recirculation pump necessary to maintain uniform free chlorine residual and removal of particulates. Ignoring the energy footprint originating from the manufacturing and use of pool chemicals, only about 2–3% of the energy footprint results from the energy required to convey, treat, and distribute water. This leaves over 97% of the energy footprint associated with the daily needs to operate the recirculation pump. Considering that the recirculation pump is operated longer during the summer, it is reasonable to expect that energy consumption will peak over the summer period. The energy footprint originating for residential pools located in the cooler and more humid parts of the United States and operated only from mid-May to mid-September (e.g., New York residential swimming pool scenario) could be estimated to range between 1100 and 1400 kWh/year. Interestingly, contribution of the water consumption to the energy footprint of residential swimming pools could be as high as 25% for large pools because of the annual winterization of the pools, which requires draining and refilling the pools once per year. However, when compared to the annual energy footprints for residential swimming pools located in the desert environment of Arizona, the energy footprints for pools located in regions with cooler climates are significantly lower.

3.3

Carbon Footprint

Figure 5 illustrates the estimated carbon footprint for the modeled residential swimming pools located in Maricopa County and expressed as carbon dioxide equivalents. The carbon footprint exhibited relatively small variability as a function of the pool size. The annual carbon footprint ranged from 1350 to 1450 kg CO2e/year for small and large residential swimming pools, respectively. These estimates could represent as much as 45 or 10% of the carbon footprint generated by a family vehicle or typical US household, respectively.26 Specifically, the annual estimated carbon footprint for the Phoenix model residential pool was w1380 kg CO2e/year, which corresponds to the lower boundary value reported by Forrest and Williams8 for the same type of residential swimming pool. The difference in values could be attributed to the facts that Forrest and Williams used (1) a composite factor of 0.582 kg CO2e/kWh to describe the carbon emissions originating from different power generation sources; (2) longer summer and winter operation periods for the recirculation pump, which is the greatest carbon footprint contributor; and (3) estimates for the carbon footprint originating from the utilization of pool chemicals. The estimates from this model correspond well with the reported values when these factors are taken into consideration, suggesting a carbon footprint of about 1800 kg CO2e/year for the Phoenix model swimming pools. The highest carbon footprint is exhibited by the residential swimming pools during the months of June and July, when the summer season is at its peak and the temperature and humidity reach their highest and lowest values, respectively. During this period, the

Chapter 14 • Estimating Water, Energy, and Carbon Footprints 355

FIGURE 5 Estimated ranges for the carbon footprint of the modeled pool size systems located in Maricopa County, Arizona. The upper error bars represent estimated maximum energy footprint for large residential swimming pools, and lower error bars represent estimated minimum energy footprint for small residential swimming pools.

carbon footprint could reach 170  5 kg CO2e/month regardless of the size of the swimming pool. During the winter period, the carbon footprint decreases as much as 60% from the peak summer values, primarily because of the reduced length of the circulation pump’s operation cycle. The model estimated values of 65  3 kg CO2e/month for the months of January and December. Interestingly, the model estimates that a New York residential swimming pool exhibits the same monthly carbon footprint (65  5 kg CO2e/month) for the period between June and August. The annual carbon footprint for a New York residential swimming pool is estimated to range between 250 and 300 kg CO2e/year, which corresponds well with the reported values by Forrest and Williams8 when the carbon footprint attributed to pool chemicals is not considered. The residential swimming pools’ contribution to the overall carbon footprint of a community becomes significant when the cumulative effects are considered. Considering a conservative estimate of about 286,0008 residential swimming pools located in Maricopa County, the total annual carbon footprint for Maricopa County

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could be as high as 395 million kg CO2e/year. Furthermore, it is estimated8 that the number of residential swimming pools in Maricopa County will increase to a conservative estimate of approximately 470,000 by 2025. In that case, the projected annual carbon footprint for Maricopa County originating from the use of residential swimming pools would increase to 649 million kg CO2e/year. By all accounts, a carbon footprint of this magnitude represents a significant amount from a community that is already experiencing the global warming effects.

3.4

Water Footprint

Estimated ranges of the water footprint are presented in Figure 6, which appears almost identical to Figure 2 because water consumption is the largest contributor to the water footprint of the modeled swimming pools. The estimated contribution of the energy footprint to the water footprint for the modeled residential swimming pools ranged between 0.3 m3/month for the winter months to 1.1 m3 for the summer months. It did

FIGURE 6 Estimated ranges for the carbon footprint of the modeled pool size systems located in Maricopa County, Arizona. The upper error bars represent estimated maximum energy footprint for large residential swimming pools, and lower error bars represent estimated minimum energy footprint for small residential swimming pools.

Chapter 14 • Estimating Water, Energy, and Carbon Footprints 357

not significantly depend on the size of the modeled pool. Energy footprint contributions to the water footprint are slightly higher during the summer period because of the longer recirculation pump operating cycles. On average, the energy contributions to the water footprint range between 5 and 10 m3/year for the modeled residential swimming pools. The annual water footprint ranges from 45 to 185 m3/year for small and large residential swimming pools, respectively. The annual water footprint for medium-sized pools is estimated to be approximately 115 m3/year. Although the annual water footprint of a typical Phoenix residential swimming pool might be considered small, the total projected water footprint for the Phoenix metro area stemming from recreational use of residential swimming pools appears colossal. Considering the conservative estimate of about 286,0008 residential swimming pools located in Maricopa County, the total water footprint stemming from recreational use of residential swimming pools was 32.8  106 m3 (w26,540 acre-foot) in 2007. This volume of water represents approximately 45% of the Canyon Lake capacity, which is one of the major water sources for the Phoenix metro area. If the 2025 estimates for the number of swimming pools are considered, the projected residential swimming pool water footprint could be estimated to be 53.9  106 m3 (w43,700 acre-foot), which represents approximately 75% of the Canyon Lake capacity. To maintain these residential swimming pools on a monthly basis in 2025, estimated 2.3  106 m3 (w1850 acre-foot) of water for the winter period and 9.0  106 m3 (w7300 acre-foot) for the peak summer period will be required. Allocation of such large water quantities for recreational purposes could be viewed as an imprudent and unsustainable practice in the arid southwestern part of the United States, especially in perspective of the declining Colorado River watershed.27

4. Conclusions The model described in this chapter demonstrates its applicability and enhances the assessment and decision-making process. Although this model builds on a relatively simple water balance concept, it provides invaluable estimates. The modeling approach demonstrated for the extreme climate scenario of the Phoenix metro area could be applied to estimate and compare water footprints of swimming pools located in different climates and environmental conditions. The realistic estimates could serve as a basis for cost–benefit analysis, which often represents pivot point in prudent water resources management practices. The performance capabilities of this relatively simple model could be expanded by increasing the swimming pools’ complexity and incorporating other factors that contribute to pool’s water, energy, and carbon footprint such as misters or pool chemicals. Correlating the model estimates with data obtained with real water consumption measurements would further validate and increase the accuracy of the model predictions. However, regardless of the complexity levels, development and utilization of simple models to assess and predict water consumption proves to be an invaluable instrument that should be part of a versatile water management toolkit.

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