Download Urban water-cycle energy use and greenhouse gas emissions

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Urban water-cycle energy use and greenhouse
gas emissions
JOAN O PPENHEIME R, M O H A M M A D B A D RU ZZAMAN , R O BY N MC G U C K I N , AN D J O S EP H G . J AC AN G EL O
http://dx.doi.org/10.5942/jawwa.2014.106.0017
The water–energy nexus has motivated water industry
professionals to maintain water performance goals with less
energy input and fewer greenhouse gas (GHG) emissions.
This article investigates protocols and methodologies, as
well as tools for energy and GHG emissions control and
tracking, that are adaptable to or specifically developed for
use within the urban water cycle. Although there are many
operational energy-minimizing strategies available, real-time
or near-real-time tools for monitoring and reporting energy
use and GHG emissions are not yet widely developed for
the water sector. Additional study is warranted to obtain a
better understanding of process-specific emission factors and
real-time measurement and control of activity levels
associated with emissions. It may also be beneficial for the
water industry to consider a continually updated
compendium of GHG emission estimation methodologies.
The urban water cycle represents the movement of water
into, within, and out of defined urban boundaries that
contain the engineered systems for potable water production, wastewater processing, and stormwater management.
GHG emissions occur indirectly from energy use and also
occur directly from certain processes within the urban
water cycle. A key feature of sustainable urban development is minimization of the carbon footprint of the urban
water cycle in order to curtail global changes in climate.
Water quality criteria and reporting requirements are
established for urban water cycle segments through use of
codified regulations, guidelines, industry standards, and
uniform practices. Corresponding requirements for energy
and GHG emission minimization are lacking because
reporting is largely voluntary. Thus the objective of this
article is to assess the availability of relevant and appropriate tools for managing segments of the urban water cycle
for energy minimization and GHG emissions accounting.
aspects within the urban water cycle. Also, because emission reporting usually is not mandated, utilities frequently
lack the monitoring tools for accurate isolation and
assessment of the activity data.
Six GHGs are required for monitoring on a national
scale by the signatory countries to the Kyoto Protocol.
Within the urban water cycle, only carbon dioxide (CO2),
methane (CH4), and nitrous oxide (N2O) are considered
relevant. The global warming potential of CH4 and N2O
are higher than CO2 and demonstrate the importance of
monitoring for these gases even when these emissions
occur at much lower levels than CO2. Emission ownership (direct or indirect) is also tracked to prevent double
accounting. Within the urban water cycle, emissions
associated with power for pumping or treatment processes are considered scope 1 (direct ownership) when
onsite fuel combustion is used for its generation; processes are considered scope 2 (indirect ownership) when
grid electricity is used. Scope 1 direct-process emissions
of CH4 and N2O occur in wastewater treatment facilities
and to a lesser degree from water treatment processes
using activated carbon or ozone generated from air. Scope
3 emissions (indirect ownership) include emissions associated with product life-cycle activities before and after
use onsite, and outsourced activities. Facility-owned
vehicles are direct ownership (scope 1) and contractorowned vehicles and employee business travel emissions
are indirect ownership (scope 3).
GHG REPORTING FRAMEWORK FOR THE URBAN
WATER CYCLE
GHG emissions reporting for any sector derives from
a protocol that provides an organizational framework for
the emissions sources and a methodology that defines the
quantification process. Tools provide software-driven
algorithms that translate input parameters to the output
variables needed to report in accordance with a prescribed methodology. Most rely on calculating an emission rate in accordance with the following equation:
Emission Rate = Emission Factor × Activity Data
(1)
The difficulty comes in how the emission factor is
characterized for use with site estimations of the activity
level. High-level approaches showing greatest global
harmonization rely on gross averages that lack the detail
needed for application to many design and operational
URBAN WATER CYCLE ENERGY-DEMAND
MINIMIZATION STRATEGIES
The greatest opportunity for energy minimization in
the potable water sector lies in selecting source waters
with lower embedded energy. This embedded energy
results from requirements for pumping, treatment, and
distribution before a given quantity of water can be used
at a specific location. Energy intensities of 380–2,800
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kW·h/mil gal have been reported for potable water, with
85% attributed to pumping in systems that are not gravity
fed and values as high as 9,200 kW·h/mil gal for pumping
over 2,000-ft mountains in California. Typical energy
intensities of 670–4,600 kW·h/mil gal have been reported
for wastewater treatment process facilities, and 6,000
kW·h/mil gal when membrane bioreactors are used in place
of activated sludge or extended aeration. Low-impact
development technologies that reduce urban stormwater
runoff and pollution loads contribute to urban water-cycle
energy minimization by augmenting local water supplies
with use of less energy for treatment and distribution.
Energy inefficiency in pumping system design can be curtailed by reducing oversizing for performance unknowns
and by less restriction of operational expenditure improvements because of capital expenditure costs.
On an individual facility basis, potable water systems
can achieve additional energy-demand reduction through
four key strategies:
• implementing water-use efficiency programs
• selecting water sources with lower embedded energy
• optimizing distribution system energy efficiency
• increased reliance on energy obtained from renew able fuel sources
The wastewater industry, on an individual facility basis,
can achieve additional energy-demand reduction through
five key strategies:
• minimizing collection system infiltration and inflow
• modifying activated sludge aeration systems
• optimizing ultraviolet disinfection systems
• capitalizing on combined heat and power installation
opportunities
• capturing additional latent energy in digested biosolids
TOOLS FOR GHG EMISSION MANAGEMENT BY CONTROL
OF ENERGY USE OR DIRECT PROCESS EMISSIONS
Several energy-fficiency tools are applicable for use in the
urban water cycle. Energy-use survey data have been statistically assessed and benchmarking tools have been used
for evaluating energy performance at water and wastewater
facilities. Examples are the US Environmental Protection
Agency’s Energy Star®, the Water Research Foundation’s
energy benchmarking, the New York State Energy Research
and Development Authority, and the Canadian National
Water and Wastewater Benchmarking Initiative. There are
pump- and motor-efficiency evaluation tools (e.g., the US
Department of Energy’s pump-system assessment tool and
MotorMaster) as well as GHG emission-activity factor
spreadsheets for different energy mix alternatives (such as
the Pacific Institute’s Water to Air Model). There are also
commercial energy management tools that provide automated or near real-time control for improved energy efficiency of water facility operations. Such systems use de­­
mand forecasting and hydraulic modeling to reduce
distribution system water pumping, improve pumping
system efficiencies, or provide real-time relative energy-use
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data through system submetering. Although the energy
reductions achieved simultaneously reduce carbon emissions, these tools do not use carbon emissions as a control
parameter and often focus more on energy cost savings
through the use of lower electricity-tariff periods.
Two processes at water facilities generate direct GHG
emissions. N2O is produced when air (not oxygen) is used
to generate ozone for oxidation/disinfection with a
reported emission factor of 0.00011 kg/m3 N2O (UKWIR,
2005). CO2, CH4, and N2O are produced from the fuel
combusted to heat the furnace for the granular activated
carbon (GAC) regeneration process when GAC is used
for treatment. These GHG emissions can be assessed from
direct measurement of the consumed fuel or estimated
(Huxley et al, 2009). Direct emissions of CH4 and N2O
are a larger concern for wastewater facilities, and models
have been developed and are being further refined to better estimate or depict these emission factors. Processes
that are not at steady-state require mechanistic process
models with dynamic simulators to capture real-time
rather than average process emissions.
Scope 3 emissions occurring outside the boundary of
the utility are less emphasized in government enterprise
reporting than in commercial product-level reporting. For
this reason, many urban water cycle agencies that voluntarily report carbon emissions focus on scope 1 and 2
categories (McGuckin et al, 2013). Scope 3 accounting
can be difficult to assess because of a dearth of appropriate emissions factors for water facility construction or the
manufacturing of water utility supplies.
CONCLUSIONS
Comprehensive GHG emissions accounting and control
methodologies for the urban water cycle are not fully
developed. This overview presents a snapshot of available
tools for energy and GHG emissions management. Tackling GHG reductions through energy minimization can
be difficult because of the need to congruently optimize
for water quality, operational needs, and asset management. Achieving reduction of direct-process GHG emissions requires enhanced process-modeling tools that
provide more accurate GHG emissions factor estimates.
REFERENCES
McGuckin, R.; Oppenheimer, J.; Badruzzaman, M.; Contrerras, A.; &
Jacangelo, J., 2013. Toolbox for Water Utility Energy and
Greenhosuse Gas Emission Management. Water Research
Foundation, Denver.
UKWIR (UK Water Industry Research), 2005. Workbook for Quantifying
Greenhouse Gas Emissions, 05/CL/01/3. Superseded by UKWIR,
2007/2008. Carbon Accounting in the UK Water Industry:
Methodology for Estimating Operational Emissions, 08/CL/01/5.
Corresponding author: Joan Oppenheimer is principal
scientist for MWH, 618 Michillinda Ave., Ste. 200,
Arcadia, CA 91107 USA; joan.oppenheimer@
mwhglobal.com.
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