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follow instructions and answer questions a to j. Detailed procedures needed.

CEE 350 Assignment 4

You have the option to work on this project alone, or in groups of 2 or 3. If work is done in a group, each
member of the group should submit the complete assignment on Canvas.

A luxurious suburb region in Omaha, Nebraska will be transformed into an ecocity with initial funding
from Berkshire Hathway Inc. The plan of the investor is to attract new talents to the Midwest to employ
in a technology hub for 5G and green energy. Note that in all the calculations below no pipe leakage will
be considered, thus water is conserved in the system.

There are 500 homes in the subdivision. The estimated family size is 5 people. Total lands of the
subdivision cover 1.32 km2, with the following breakdown of land use.

Land type Area (m3) and attributes

Single family homes 500 homes, 1000 m2 (~1/4 acre) footprint on soils
with low infiltration capacity:
150 m2 roof,
25 m2 paved driveway, and
825 m2 yard in good condition >75% grass.

Streets and roads (paved, with
curbs and storm sewer)

200,000 m2

Commercial and business 125,000 m2 (85% impervious)

Parks and greenbelts,>75%
grass, same as yards.

500,000 m2

The frequency distribution of rainfall depth for a mean number of observed storms of 55 in a year is
given below (MAP is mean annual precipitation):

# of storms 55

MAP (mm) 789.25

rainfall (mm) Fraction Cum. (mm)

5 0.4 11

10 0.2 110

15 0.15 123.7

20 0.05 55

25 0.05 68.75

30 0.04 66

35 0.02 38.5

40 0.03 66

45 0.05 123.75

50 0.01 27.5

Cumulative: 1 789.25

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0.45

0
1
5

0
2
5

0
3
5

0
4
5

0 Rainfall depth (mm)

Water requirements:

• According to recent statistics a person in Lincoln, NE uses 320 L/(cap-day). This amount excludes
irrigation of yards.

• Irrigation requirement of yards is 3 mm/d for a period of 60 days in the summer. Individual
homeowners use this from the grid before the ecocity planning. Irrigation water in the parks are
not part of ecocity calculations.

• Storm runoff in the common areas (schools, parks, roads, parking lots) are not included in the
fee because they are subject to river discharge without treatment. Runoff from common areas
can be captured and reused in homes with centralized treatment for this community.

• Runoff from individual yards can also be captured for reuse purposes with treatment.

• There is a creek that runs year-around with a flow discharge of 40 m3/h inside the development
area. The Department of Ecology only allows the use of half of this creek discharge in order to
leave sufficient flow for aquatic life.

Energy requirements:

• Energy needed for using water from the grid is 0.9 kWh/m3.

• Energy needed for wastewater removal and treatment by the city of Lincoln grid is
0.75kWh/m3. This amount is charged to each household regardless if the water is used in the
house (and returned as wastewater) or used for watering plants (and not returned). The volume
of overland flow from house roofs connected to the sewer system by downspouts is also subject
to this rate.

• Estimated energy for pumping water from the creek and filtering is 0.51 kWh/m3.

• Energy for pumping and filtrating of roof runoff for irrigation is 0.25 kWh/m3.

• Estimated energy cost of treatment of runoff water from driveways and roofs for residential
use is 0.65 kWh/m3.

• Energy cost of wastewater treatment is 1.2 kWh/m3 (for full recover and reuse).

Cost of water:

• Cost of metered residential water from the grid is $2.00 per unit (a unit water is 748 gallons)
applied as a uniform rate.

• Cost of residential wastewater discharge into the grid is $2.45 per unit.

Carbon equivalent of energy: Nebraska obtains 63% of its net electricity generation from coal,
15% from nuclear power, and 14% from wind. The rest is generated from hydropower (4%) and
natural gas (3%). Hydropower, nuclear and wind are considered green energy for this project.
You can use the table below for CO2 release per kWh energy production.

Task: The investor wants to develop a concept towards ecocity that:

• Reduce the current use of water from the grid by half

• Reduce its current CO2 footprint in half or more (if possible)

• Per-capita water reduction in houses should not exceed 15% of the current use.

• Reduce the cost of current energy cost in US$ by half. However, if the cost reduction cannot be
met, the client still wants to see your scenarios that can reduce large amounts of emissions.

• Xeriscape is acceptable, however the client wants you to look into option of roof runoff for
irrigation and keep yards at least for a certain fraction of the landscape.

They want to know if only using the runoff water in the land and partly from the creek can achieve this.
You are asked to plan a water use strategy for this goal. You can suggest any changes on the driveway
material or convert loan to xeriscape (see the related limitation below) that will not require irrigation.
You will need to pay attention to the limitations listed above. If the proposed goal of the investor
cannot be achieved, develop a solution that will come closest to what they ask for. Answer the
following questions which will also help to organize your ideas.

Before development:

a) Calculate the total annual water requirement for the city before development. (Answer:
366,250 m3/yr) (1 point)

b) Calculate the cost (in US$) of water use and wastewater disposal (and their sum) for the city.
(Answer: 626,600 dollars/yr) (1 point)

c) Estimate the energy use per capita per day in kWh/cap-day that includes both using the water
from the grid and wastewater (including irrigation water although not returned). (Answer: 0.71
kWh/(cap-day)) (1 point)

d) Estimate the total CO2 emission in kg/year from this suburban region before development.
(Answer: 404,150 kg/yr) (1 point)

For development consider the scenarios below. Report your results in a table that will include the name
of the scenario, Water use m3/year; Emission in kg CO2 /year; Cost in US$. You will develop two plots
with the data you will generate for different scenarios. You will plot Water use (m3/year) in x-axis in
both plots. One of the plots will use kg CO2/year and the other plot $/year on y-axis. You will explore if
your figure looks like Novotny (2011) hypothesis. Please address many alternatives as you can and plot
your options.

e) Reduce water use in houses 15% (1 point)
f) Reduce water use 15%, keep existing yard and supplement irrigation water with roof runoff (1

point)

g) Reduce water use 15% and convert all lawns to xeriscape (1 point)
h) Reduce water use 15%, keep existing yard and supplement irrigation water with roof runoff (1

point)

i) Reduce water use 15%, convert all lawns to xeriscape, treat and use runoff from roof and
driveways in residences (1 point)

j) One additional scenario of your choice (1 point)

Water and energy link in the cities of the future – achieving

net zero carbon and pollution emissions footprint

V. Novotny

ABSTRACT

This article discusses the link between water conservation, reclamation, reuse and energy use as

related to the goal of achieving the net zero carbon emission footprint in future sustainable cities.

It defines sustainable ecocities and outlines quantitatively steps towards the reduction of energy

use due to water and used water flows, management and limits in linear and closed loop water/

stormwater/wastewater management systems. The three phase water energy nexus diagram may

have a minimum inflection point beyond which reduction of water demand may not result in a

reduction of energy and carbon emissions. Hence, water conservation is the best alternative

solution to water shortages and minimizing the carbon footprint. A marginal water/energy chart is

developed and proposed to assist planners in developing future ecocities and retrofitting older

communities to achieve sustainability.

Key words 9999 one planet living criteria, greenhouse gases emissions, water conservation, water

reclamation, LEED criteria, green development, water demand, energy use, carbon footprint

INTRODUCTION

Goals

The Cities of the Future or Ecocities represent a major
paradigm shift in the way new cities will be built or older
ones retrofitted to achieve a change from the current unsus-
tainable status to sustainability. A working definition of an
ecocity and the goal of future new urban developments as
well as retrofitting the old ones is as follows (Register 1985;
Novotny et al. 2010):

An ecocity is a city or a part thereof that balances social,
economic and environmental factors (triple bottom line)
to achieve sustainable development. A sustainable city or
ecocity is a city designed with consideration of environ-
mental impact, inhabited by people dedicated to minimi-
zation of required inputs of energy, water and food, and
waste output of heat, air pollution – CO2, methane, and
water pollution. Ideally, a sustainable city powers itself
with renewable sources of energy, creates the smallest
possible ecological footprint, and produces the lowest
quantity of pollution possible. It also uses land efficiently,

composts used materials, recycles or converts waste-to-
energy. If such practices are adapted, overall contribution
of the city to climate change will be none or minimal
below the resiliency threshold. Urban (green) infrastruc-
ture; resilient and hydrologically and ecologically func-
tioning landscape and water resources will constitute one
system.

The current criteria and guidelines used for ecocity certifica-
tion are LEED (Leadership in Energy and Environment
Design) of the US Green Building Council (2005, 2007) and
One Planet Living (OPL) by the World Wildlife Fund (2008).
OPL criteria, the National Science and Technology Council
(NSTC) (2008) recommendations and governments of several
countries (e.g., Great Britain) call for achieving net zero
greenhouse gas (GHG) emissions.

Figure 1 shows the possible paths towards the net zero
GHG emissions goal. Current scientific research quoted in
the NSTC (2008) report indicates 60 to 70% of energy
reductions in buildings in cities can be achieved with more
efficient appliances such as better water and space heaters,

V. Novotny (corresponding author)
Department of Civil and Environmental

Engineering,
Northeastern University,
Boston,
MA 02415,
USA.
E-mail: novotny@coe.neu.edu

doi: 10.2166/wst.2011.031

184 & IWA Publishing 2010 Water Science & Technology 9999 63.1 9999 2011

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heat pumps, significant reduction of water demand by water
conservation and other improvements. NSTC also estimated
that 30 to 40% of energy can be produced by renewable
sources, including heat recovery from used water or extracted
from the ground and groundwater.

The green net-zero GHG and pollution emissions mea-
sures in the ecocity developments and retrofits include
(Novotny et al. 2010):

� passive architectural features for heating and cooling;
� renewable energy sources (solar, wind, extracted from

used water and stormwater);
� water conservation and reuse, addressing the entire water

(hydrologic) cycle within the development, including rain-
water harvesting and storage;

� distributed stormwater and used (waste) water manage-
ment to enable efficient used water and reuse and renew-
able energy production;

� xeriscape of the surroundings that reduces or eliminates
irrigation also collects and stores runoff from precipitation;

� energy efficient appliances (e.g., water heaters), treat-
ment (e.g., reverse osmosis) and machinery (e.g., pumps,
aerators);

� connecting to off-site renewable energy sources such as
solar power plants and wind farms;

� organic solids management for energy recovery;
� connection to low or no GHG net emissions heat/cooling

sources such as heat recovered from used water or from
the ground;

� smart metering of energy and water use providing flex-
ibility between the sources of water and energy; and

� sensors and cyber infrastructure for smart real time
control.

WATER AND ENERGY NEXUS – THE HYPOTHESIS

Figure 2 presents the possible relationship of water demand
reduction leading to a closed urban water cycle and energy.
This article suggests a hypothesis that there is a minimum
inflection point beyond which further reduction of water use
will increase energy demand. Consequently, the relationship
has three phases: (1) Water conservation phase in which
energy and GHG emissions reduction is proportional to the
reduction of water use; (2) Inflection phases in which addi-
tional and substitute sources of water are brought in, treated
and used; and (3) Phase in which energy use is rising while
water demand of the development is reduced by used water
reclamation and multiple reuse. In the water conservation
phase, energy use and GHG emission reduction by reducing
water demand are achieved by using more efficient appli-
ances, xeriscape (reducing irrigation) and plugging the leaks
and losses. These measures do not require a large amount of
extra energy, hence, the energy use reduction is directly
proportional to the reduction of the water demand. However,
several current ecocities are located or being planned in areas
with meager water resources which necessitates using desa-
linated, brackish and reclaimed water. To further close the
water cycle, energy demanding water reclamation processes
are needed such as micro and nanofiltration and reverse
osmosis. Consequently, larger dependence on renewable
zero carbon energy sources (wind, solar, geothermal, energy
recovery from used water organic solids) will ensue. The
recycle systems cannot be fully closed to prevent accumula-
tion of nondegradable potentially harmful compounds that

Figure 1 9999 A path to achieving the net zero energy goals (NSTC 2008). Subscribers to the
online version of Water Science and Technology can access the colour version of

this figure from http://www.iwaponline.com/wst

Figure 2 9999 Relation of water related energy use to water demand of the development.

185 V. Novotny 9999 Water and energy link in the cities of the future Water Science & Technology 9999 63.1 9999 2011

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may pass reverse osmosis and other high degree treatment
processes.

Water conservation and its effect

In the US, buildings consume 40% of the energy of which
22% is residential and 18% commercial, respectively. Indus-
tries consume 32% and transportation 28%, respectively
(NSTC 2008). Providing treated water and disposal of waste-
water represents about 3% of the energy use. However, within
the buildings, 8% of the energy use is for water related
processes such as cooking, wet cleaning, and water heating.
A percent or more is needed to pump and transport water and
wastewater.

The US Department of Energy (2000) published estimates
of carbon equivalent of energy produced by fossil fuel power
plants as

� 0.96 kg of CO2/kW-hour produced by coal fired power
plants

� 0.89 kg of CO2/kW-hour produced by oil fired power
plants

� 0.60 kg of CO2/kW-hour produced by natural gas power
plants

Because 30% of energy is produced by processes that do
not emit substantial quantities of GHG (nuclear, hydropower
and other renewables), a weighted average of the CO2 will be
considered in this analysis which is

0.61 kg of CO2 emitted per kW-hour of energy produced

The Energy Information Administration (2009) documen-
ted the total energy production in the US in 2007 was
4,157 TWh (4,157�109 kWh) which represented about
2.516 billons tons of CO2 emitted. Using the 3% estimate
for providing and treating water, ‘‘water share’’ of the energy
use is 124.7 TWh and 75.5 million tons of CO2 were emitted
as a result of providing clean and disposing polluted water,
plus an additional 200 million tons of CO2 for hot water
heating, cooking and boiling, and wet cleaning.

Phase I – Water conservation – Linear reduction

The first phase of the water – energy nexus is a linear or near
linear nexus relationship between water conservation and
energy reduction, The building and community water use
systems range from linear systems in which water is extracted
from the source, brought to the city where its is polluted, then
transferred to a treatment plant where it is treated and
discharged into a receiving water body, to closed loop systems

reclaiming and reusing water. It will be subsequently shown
that a 100% closed system is potentially possible on a space
station but unrealistic in cities. Table 1 shows the per capita
volumes and proportions of the daily water use in a typical
US single family home. The left part of the table is based on
the AWWA RF (1999) study as reported by Heaney et al.
(2000). On the right side are the estimates of water savings
used by the AWWA RF study and by the Pacific Institute
(Gleick et al. 2003) study for California. The table shows high
water use in the US of 550 L/cap-day, which is much higher
than in most other developed countries. After implementing
mostly common sense water conservation measures (for
details see Novotny et al. 2010), the US use can be reduced
to less than 200 L/capita-day, still high but comparable
to European values. The largest water use is for lawn irriga-
tion that can be reduced or eliminated by xeriscape land-
scaping using native plants and landscaping not requiring
water.

Reducing water use by conservation will not require
extra energy. It also does not have to be in a closed system
but it works best if it is done in a distributed urban manage-
ment system which provides ecological flow to urban
streams (restored or daylighted) and allows energy and
water reclamation from used water. In 2007, 55 billions m3

of water was used by the population of 301.3 million in the
US. Using the US EPA estimate of 3% energy use for water
would result in the unit energy use of 2.26 KWh/m3 attrib-
uted to water. Corresponding carbon emission is of
1.37 kg CO2/m

3. Most of the water conservation reduction
in Table 1 can be achieved by more efficient appliances
(water saving shower heads, toilets, laundry wash machines,
etc) and xeriscape. Hence for each cubic metre saved,
energy in the ideal average household would be reduced
by the above amount. This is the linear Phase I of Figure 2.
The water saving potential shown in Table 1 is 65%
reduction.

In addition to emissions by power plants producing
energy for water, CO2 is also emitted in the biological treat-
ment process that oxidizes organic matter. Changing to
anaerobic treatment saves the energy and allows to recover
biogas and nutrients (Verstraete et al. 2009)

Phase II – Inflection

In the inflection phase, a city is looking for additional sources
of water or brings in sources that have worse quality, will
require more treatment and/or have to be pumped from long
distances or from deep geological layers. Many cities in the
southwest US cannot meet the water demand using relatively

186 V. Novotny 9999 Water and energy link in the cities of the future Water Science & Technology 9999 63.1 9999 2011

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inexpensive sources of water and/or may be located on
receiving water bodies that require a higher degree of treat-
ment. For example, pumping 1 m3 of water from a depth of
500 m with a pump that has an overall efficiency of 80% will
require work of W¼gVH¼9,819�1�5000/0.8¼6,131,125
J¼1.7 kW-hrs (g¼specific density of water in N/m3) and
will result in 1 kg of additional CO2 emissions. Many water
short communities are pumping higher salinity water as deep
as 1000 m.

Low energy demanding sources of water are rainwater
harvesting (negligible pumping energy needs) and stormwater
(some pumping and treatment).

Phase III – Increasing energy demand and CO2 emissions

In the increasing phase, tapping on higher salinity water
sources (brackish sea or groundwater) is supplemented with
water reuse that requires a two or three step high efficiency
treatment (Figure 3). Table 2 presents energy and CO2
emissions.

In activated sludge processes, for each mole of oxygen
consumed in the aeration process, one mole of carbon
dioxide is emitted. Hence CO2 emitted¼(12þ2�16)/
(2�16)¼1.37 O2 consumed. For example, if the BOD5
concentration in used water is 300 mg/L¼0.3 kg/m3 then

the CO2 emission in aeration unit removing 95% of BOD5
will be

CO2 emitted ðkg=m3Þ¼ 1:4ðBODultimate=BOD5Þ � 0:95

� 0:3ðkg BOD5=m3Þ
� 1:37ðCO2 emitted=O2 consumedÞ

¼ 0:53 kg=m3 of CO2 emitted:

This value should be added to the CO2 emissions due to
the energy use listed in Table 2. However, some may claim
this CO2 emission component does not originate from burn-
ing fossil fuel and should be counted as a neutral carbon
footprint as it should for methane burning from sludge diges-
tion or biofuel production.

Planners of water frugal ecocities in Qingdao (China) and
Masdar (UAE) consider a fully closed loop similar to that
shown on Figure 3. The Qingdao double loop (Fraker 2008)
was modified to avoid direct potable reuse. The numbers on
the plot represent daily water use in L/person-day living in
the cluster of the ecocity. The Qingdao ecocity cluster has
about 1500 to 2000 inhabitants proposed to live in several
highrise and medium height buildings. The figure shows the
total water use in the cluster as 130 L/capita-day but the
municipal grid supplies only 50 L/capita-day. It is assumed

Table 1 9999 Indoor and outdoor water use in a single family home in 12 monitored cities in North America

Water use

Without water conservation* With water conservation

L/cap-day Percent L/cap-day Percent

Faucets 35 14.7 35 25.8

Drinking water and cooling 3.6 1.2 2.0 1.5

Showers 42 17.8 21 15.4

Bath and hot tubs 6.8 2.0 6.0 4.4

Laundry 54 22.6 40 29.4

Dish washers 3.0 1.4 3.0 2.2

Toilets 63 26.4 14 10.3

Leaks 30 12.6 15 11.0

Total indoor 238 100 136 100

Outdoor 313 132 60** 44

Total 551 232 196 144

Adapted from AWWA RF (1999); Heaney et al. (2000) and Asano et al. (2007)

**Reflects converting from lawn to xeriscape using native plants and ground covers with no irrigation. Water use is for swimming pools, watering flowers and vegetable gardens.

187 V. Novotny 9999 Water and energy link in the cities of the future Water Science & Technology 9999 63.1 9999 2011

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maximum water saving practices are implemented in the
cluster ecoblock. The water reclamation and reuse is carried
in a double loop consisting of black and grey water reclama-
tion and reuse. Black water flow includes water from toilets,
kitchen sinks and dishwashers. The subsurface flow wetland
treatment is assumed to emit minimal quantities of carbon
dioxide and nitric oxide, both GHGs. In addition to providing
water to inhabitants, the double loop system also provides
some ecological flow to the surface water bodies within the
ecocity and garden irrigation. It can be seen that 50 L/capita-

day water input from the municipal grid is not sufficient to
sustain the total demand of 140 L/cap–day during dry
weather. Rainwater harvesting and stormwater capture and
infiltration (via pervious pavements and infiltration raingar-
dens) is needed to supplement the dry weather flow (Novotny
& Novotny 2009). Hence, one can consider the 50 L/cap-day
as the minimum inflow from the grid and 130 L/cap-day as
the optimal water demand after implementing a suite of water
conservation measures.

The ATERR is a generic anaerobic treatment unit that
produces biogas. In the original Qingdao system proposal
sequencing batch reactors were proposed. Verstraete et al.
(2009) suggested anaerobic upflow sludge blanket reactor
combined with a septic tank. In this application, PS reactor
is optional. The Qingdao ecoblock also saves energy by
passive heating and cooling, producing energy by solar
panels, voltaics, and wind turbines. It will also produce biogas
from digested sludge and organic solids harvested from the
wetland, fallen leaves and gardens. In the overall scheme, the
planners claim the ecoblock to have a net zero carbon
emission footprint. The Qingdao ecoblock concept is now
being implemented in Tianjin Ecocity 150 km southwest of
Beijing (Harrison Fraker, personal communication).

Energy (CO2) balance for an ecoblock

An ecoblock or a cluster is a semiautonomous water/storm-
water/used subdivision or a part of a city that manages water
in a semi closed water cycle and produces energy to achieve
the net zero carbon footprint. At this time, there is no

Figure 3 9999 Mass balance of flows in L/cap-day for the system in a closed water cycle ecocity
on a dry day. Legend: SFW-subsurface flow wetland; PS – primary settler with

solids removal; ATERR –anaerobic treatment and energy recovery reactor; MF-

membrane filter; SF-sand- filter; NF-nanofilter; RO-reverse osmosis; UV-

ultraviolet disinfection; O3 – ozone addition; X – water loss by evaporation, IRRF –

integrated resource recovery facility.

Table 2 9999 Energy use of treated volume of municipal used (waste) water and corresponding CO2 emissions. Raw data from Asano et al. (2007) and from Novotny et al. (2010)

Treatment process

Energy use kw-hr/m3 (CO2 emissions kg/m3)

Daily flow volume of treated used water (m3/day)

10,000 25,000 450,000

Activated sludge without nitrification and filtration 0.55 (0.33) 0.38 (0.23) 0.28 (0.17)

Membrane bioreactor with nitrification 0.83 (0.51) 0.72 (0.44) 0.64 (0.37)

Reverse osmosis desalination

Brackish water (TDS 1–2.5 g/L) 1.5 (0.91)–2.5 (1.52)

Sea water 5 (3.05)–15 (9.15)

Ozonization (ozone produced from air)

Filtered nitrified effluent 0.24 (0.15)–0.4 (0.24)

Desalination by evaporation (using waste heat) B25 (15.25)

188 V. Novotny 9999 Water and energy link in the cities of the future Water Science & Technology 9999 63.1 9999 2011

guideline that would establish the size of the ecoblock. The
Qingdao ecoblock would contain 1500–2000 inhabitants out
of the total 40,000 living in the (future) ecocity. In this
illustrative analysis the starting reference point of water
and energy use is the alternative with no water conservation
and open linear no reuse water management system. The
city and the ecoblock would be located in south-western US.
The freshwater source (groundwater and nearby stream) is
mined and is unsustainable. The illustrative assumptions are:
Total population 100,000

Original water demand 500 L/cap-day

Sustainable water available from fresh
water source

100 L/ cap-day

Sustainable rainwater and stormwater
reclamation

20 L/cap-day

Sustainable brackish groundwater
(TDS 1500 mg/l)

30 L/cap-day

Maximum water conservation limit 200 L/cap-day

Because the sustainable water is available only to satisfy
150 L/cap-day demand, water use must be reduced by water
by conservation and reuse.

Wastewater treatment includes activated sludge process
with nitrification. Reuse will be done by filtration of the
effluent, followed by reverse osmosis and ozonization.
Reused water will not be available for potable use.

Calculations

A marginal water/energy nexus chart has been prepared and
presented on Figure 4 for carbon emissions. Marginal carbon/
energy is the carbon emission per one extra m3 of water
demand reduction.

Current unsustainable : water use 0:5 m3=cap-day

Total wateruse 0:5 � 100; 000 ¼ 50; 000 m3=day
Marginal energy use 2:26 kWh=m3 � 0:5 m3=cap-day

¼ 1:16 kWh=cap-day
Carbon emissions 0:61ðkg of CO2=kWhÞ� 2:26

¼ 1:37 kg of CO2=m3

Total carbon emissions 50; 000 � 1:37
¼ 69; 3580 kg of CO2=day

Reduction to 200 L/cap-day (60% reduction) or
20,000 m3/day can be achieved solely by water conservation
but the water use is still unsustainable and the available
sources cannot provide enough water. At 100 L/cap-day of

water available from the fresh treated water supplying grid,
additional water will originate from rainwater/stormwater
(20), sustainable brackish water (30) and reuse (50) to
provide 200 L/cap-day of water. Rainwater/ stormwater use
will require storage, pumping and filtration which will result
in estimated carbon emissions of 0.1 kg of CO2/m

3. Brackish
water has to be pumped (1.6 kWh/m3¼1 kg CO2/m3 if
pumping depth is 500 m) and treated by reverse osmosis
and UV/ozonization (1.7 kg CO2/m

3). Reuse will approxi-
mately emit 2.0 kg of CO2/m

3.
At 100 l/cap-day of fresh water availability from the grid

the marginal kg CO2/m
3 emissions become (0.1 [freshwater]

�1.37þ0.02 [rain]�0.1þ0.03 [brackish]�2.7þ0.05
[reuse]�2)/0.2¼1.6 kg CO2/m3.

The total carbon emissions at 200 L/cap-day demand and
100 L/cap-day fresh water availability from the grid will be
1.6 kg CO2/m

3�20,000 m3¼32,000 kg CO2/day. The mar-
ginal kg of CO2/m

3 and the total CO2 based on additional
calculations are plotted on Figure 4.

CONCLUSIONS

Water and energy uses are intertwined and represent a
significant portion of the total carbon emissions reaching
the environment. Water conservation is the best alternative
solution to a water availability problem because it does not
increase carbon emissions. Hence, it should be maximized.

Figure 4 9999 Water energy nexus chart that includes total and marginal carbon emissions
related to water demand reductions by water conservation, additional sources

and recycle.

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from 500 L/per cap to 200 L/per cap
marginal CO2 emission stays at 1.37 kg/CO2/m3 water because this saving won’t require any extra energy and therefore CO2 release.

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Furthermore, energy can be extracted from used water by
heat pumps for a carbon credit. A common water to water
heat pump provides 4–5 times more energy than it uses. The
extracted heat can be used to warm water in the buildings or
generate carbon emission free energy. If water conservation
can accomplish the water use reduction goals without reuse it
will be done best in a linear distributed water management
system whereby highly treated effluents, after heat energy is
extracted, provide ecological flow to the receiving water and
water for downstream uses. Such a system has been imple-
mented in Hammarby Sj+ostad in Stockholm (Novotny &
Novotny 2009).

Reuse with high efficiency solids and pollutant removals
(e.g., microfiltration and reverse osmosis) in a closed cycle
(e.g., Masdar in UAE or Orange County in US) requires
more energy because of the energy requirement in the treat-
ment process and double or triple cycle reuse (i.e., the water
is reclaimed and treated more than once). This leads to a
higher marginal carbon emission rate and higher total energy
use. In order to stay sustainable the extra energy has to be
provided by renewable energy sources as it is indeed done in
Masdar or was proposed in Qingdao. Methane production in
the treatment and recycle process, if burned, is carbon
neutral.

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