quantitative engineering questions

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: [email protected]

doi: 10.2166/wst.2011.031

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

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

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

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

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.

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

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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Fraker, H., Jr. 2008 The Ecoblock-China Sustainable Neighborhood Project, Power point presentation – Connected Urban Develop- ment Conference, September 24, 2008, Amsterdam, http://bie. berkeley.edu/ecoblocks.

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Register, R. 1985 Ecocities. In Content 8: Living with the Land Context Institute, Langley, WA, USA (http://www.context.org/iclib/ic08/ Register.htm)

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USGBC 2005 Green Building Rating System for new Construction & Major Renovations, Version 2.2, US Green Building Council, Washington, DC., http://www.usgbc.org.

USGBC 2007 LEED for Neighbourhood Development Rating System, Pilot version, US Green Building Council, Washington, DC., http://www.usgbc.org.

Verstraete, W., Van de Caveye, P. & Diamantis, V. 2009 Maximum use of resources present in domestic ‘‘used water’’, Bioresource Technol. 100, 5537–5545.

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190 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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