Project Update:
Domestic Wastewater Cooling Technology
Feasibility Analysis
NOVEMBER 14, 2018
Barbara Bennett
Surface Water Assessor
Jon Erickson
Senior Review Engineer
Water Quality Control Division Presenters:

 
Presentation Overview
- Impetus and Background (Barbara)
- Feasibility Study Methodology and General Results (Jon)
- Energy Recovery and Reuse Case Study (CDM Smith/Avon)
- Additional Details and Results (Jon)
-
Next Steps (Barbara)
-
Questions and Discussion

 
Feasibility Study Impetus
- In 2015, dischargers working on DSV proposals for temperature
ran into hurdles
- Recognized the need for technical guidance on alternatives
analyses for temperature
- Also identified need to address concerns related to energy
demand from cooling technologies

 
Scope of Feasibility Study
- Identify temperature reduction alternatives
- Consider technologic, economic, and
environmental
feasibility
- Administer questionnaire to other states
- Prepare user guide:
Applicability and limitations of each technology
Planning level cost estimates
Generalized environmental impacts
Considerations for temperature related DSV proposals
- Beyond scope:
Assess extent or cumulative statewide impacts (lack of sufficient data)
Making policy decisions regarding significance of environmental impacts

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Final Deliverable
General contents of the document:
1. Background and Purpose
2. Basics of Heat Transfer
3. General User Guidance
4. Innovative, Hybrid, and
Combination Approaches
Appendix:
Basic Cooling Technology Fact Sheets
with
detailed example calculations
and other supplemental information

 
Methodology of Evaluation
- Contractor support for sizing, cost, and energy use estimates
- Used hypothetical case study as an example
- Dischargers to warm water streams appear most likely to be affected
- Scenario: wintertime cooling at a 5 MGD facility with a 2
o
C reduction
- Conservative ambient environmental conditions were assumed
- Selected to be easily scalable and adaptable
- Technologies were categorized to isolate heat transfer pathways

 
Estimating Costs and Impacts
- Each technology was sized to meet the hypothetical design condition
- Vendor quotes and equipment cost information was obtained
- Energy use was estimated based on the identified equipment
- Assumed electricity is purchased from the grid
leads to i
ndirect impacts
associated with power generation
- Region-specific multipliers applied to electric use values, USEPA eGRID2016
greenhouse gas emissions: 1,376.8 lbCO
2
e/MWh
- Water loss and PM
10
calculations are
direct impacts
from the plant site
estimated based on technology specific calculations
- Other waste issues qualitatively identified, e.g. refrigerant disposal

 
Technologies Evaluated in this Study
Natural Heat Flow
1. Heat exchanger using colder surface water/shallow groundwater
2. Blending using deep groundwater
3. Ground loop exchanger/geothermal cooling
Evaporative cooling
4. Passive cooling pond
5. Spray pond
6. Once-through cooling tower
Mechanical cooling*
7. Air-cooled chiller using the vapor-compression refrigeration cycle
Innovative, Hybrid, or
Combination Approaches
A. Chiller with closed-loop cooling tower or other cooling water source*
B. Use of high efficiency motors and energy efficient designs
C. Alternate electric sources
D. Shade options for wastewater lagoons
E. Additional combinations and hybrid approaches
F. Energy recovery and reuse*
G. Absorption refrigeration*
H. Electricity from waste heat and other potential advancements
* Alternate or modified mechanical cooling processes are addressed as innovative, hybrid, or combination type approaches

 
Generalized Findings
Technology
Capital Cost Energy Usage
GHG SO
x
NO
x
Emissions
Onsite PM
10
Emissions
Water Loss
1. Heat Exchanger Using
Surface Water
$
2. Blending with Deep
Groundwater
$$
3. Ground loop exchanger/
geothermal cooling
$$$$
4. Passive Cooling Pond
$$$
5. Spray Pond
$$
6. Cooling Tower (once
through)
$$
7. Chiller (air-cooled)
$$

 
Tools and Examples in the User Guide
Section 4: Innovative, Hybrid, and Combination Approaches includes
quantitative tools to evaluate these options in more detail:
F. Energy Recovery and Reuse
Efficiency ratings, including COP
Mass and energy balances
Comparison of solids handling processes
G. Absorption Refrigeration
Example comparison of mechanical cooling options for heat recovery and reuse
H. Electricity from Waste Heat and Other Potential Advancements
Analysis of heat pump to heat engine actual vs. theoretical efficiency

 
Coefficient of Performance
A dynamic tool for evaluating energy use!
Most general form:
???????????????????????????????????????????? ???????? ????????????????????????????????????????????
=
???????????????????????? ???????? ???????????????????????????? ???????????????????????? ????????????????????????????????????????????
???????????????????????? ???????? ???????????????????????? ????????????????????
When cooling:
????????????
????????????????????????????
=
When heating:
????
????, ????????????????????????????????????????
????????????
????????????????????????????
=
????
????,???????????????????????????????????? ℎ????????????
????
????????????????????

 
Evaluating Performance with COPs
Design basis air-cooled chiller performance ratings
Chilled Water
Supply
Temperature
(
o
C)
Air Temperature on Condenser (°C)
25
40
Capacity
(kW)
COP
Capacity
(kW)
COP
5.0
894
3.7
835
2.4
6.0
919
3.7
856
2.5
7.0
945
3.8
877
2.5
8.0
970
3.9
899
2.6
9.0
996
4.0
921
2.7
10.0
1023
4.0
944
2.7
Alternate water-cooled chiller performance ratings
Chilled Water
Supply
Temperature
(
o
C)
Water Temperature on Condenser (°C)
40
50
Capacity
(kW)
COP
Capacity
(kW)
COP
5.0
930
4.4
837
3.2
6.0
962
4.5
866
3.3
7.0
994
4.7
896
3.4
8.0
1026
4.8
925
3.6
9.0
1058
4.9
955
3.7
10.0
1090
5.1
984
3.8

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Energy recovery and reuse
could involve heating and
cooling at the same time.
From an energy balance:
????
????,???????????????????????????????????? ℎ????????????
=
????
????????????????????
+
????
????,???????????????????????????? ????????????????
Therefore, in an ideal system:
????????????
????????????????????????????
=
????????????
????????????????????????????
1
or
????????????
????????????????????????????
=
????????????
????????????????????????????
+ 1
Since listed COP values can include energy
inputs and losses associated with pumping,
controls, or other appurtenances, the
relationships above will not be exact in
actual installations, but may still be a
reasonable approximation since the
majority of energy goes into the refrigerant
as work (pressurization) or heat lost to
friction.

 
Case Study

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November 14, 2018
Town of Avon’s Heat Recovery System Overview
Colorado Roadmap Workgroup Meeting
Tim Rynders, P.E.
Rynderst@cdmsmith.com
Siri Roman, P.E.
sroman@erwsd.org

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Avon Community Heat Recovery Facility
A Colorado “New Energy Community” Project

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Project History (2008-9)
?
Urban renewal plans seeking to attract private
investment and densify commercial area
?
Town searching for “green” energy sources
?
Climate Action Plan
?
Set goals for 20-50% reduction in E and GHC
?
80% reduction by 2050.
?
Resulted in conservation efforts and switch to wind-
power
?
Pool heating = major E user and GHG emitter
?
Avon WWTF anticipating effluent temperature
limitations

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Collaboration of Town and ERWSD
?
Town Goals
?
Move ahead with urban renewal plans
that include a snowmelt system
?
Obtain a readily accessible, long-term
source of heat energy with low GHG
emissions
?
Reduce energy consumption and GHG
emissions in Recreation Center
?
ERWSD
?
Obtain low cost/low GHG source of
heat for space heating
?
Reduce temperature in discharged
plant effluent

 
Fun “Facts” / Pop Quiz
?
How much of wastewater’s embedded energy is thermal?
(vs. chemical, i.e. carbon for biogas)
?
How much of a conventional WWTP’s energy could
theoretically be offset with heat recovery?
19

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Fun “Facts” / Pop Quiz
?
How much of wastewater’s embedded energy is thermal?
(vs. chemical, i.e. carbon for biogas)
?
How much of a conventional WWTP’s energy use could
theoretically be offset with heat recovery?
20

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Fun “Facts” / Pop Quiz
?
How much of wastewater’s embedded energy is thermal?
(vs. chemical, i.e. carbon for biogas)
?
How much of a conventional WWTP’s energy use could
theoretically be offset with heat recovery?
21

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Fun “Facts” / Pop Quiz
?
How much of wastewater’s embedded energy is thermal?
(vs. chemical, i.e. carbon for biogas)
?
How much of a conventional WWTP’s energy use could
theoretically be offset with heat recovery?
22

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Avon Community Heat Recovery System
Initial Concept
Recreation Center
Main St Snow Melt
(Future)
Transit Center
Heat Recovery Building
ERWSD’s
Avon WWTP
Interface and Connection
To Main Street Project
8” Heat Recovery Loop
Supply And Return
Located in Same Trench
Heat Distribution
Building
Nottingham Lake

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Community Heat Recovery System –
Initial Installation 2009

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Heat Pump Building
On ERWSD Property –
Adjacent to diversion manhole
and lift station

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Heat Distribution Building

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Heat Distribution Building

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Town of Avon Heat Recovery Recent Expansion

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Photos of Renovated New Town Hall

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Evaluation of Expansion Impact
New Avon Town Hall
30

 
SHR System Implementation Costs
Primary Heat Recovery “Backbone” of 3.5 MMBH
$3.5M construction base bid (GE Johnson)
$1.5M grant from DOLA’s New Energy Communities Initiative in 2010
Design, Construction, Startup and Training Services $750k
Doubling capacity of system to 7 MMBH ~$500k-1M estimate.
Only heat pump building affected.
Does not include cost for new connection pipelines and heat transfer equipment at user
Heat Recovery Expansion to 15,000 sf New Town Hall just completed in October
~$120k construction
Additional 5-10% of heat pumps current capacity
31

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Preliminary Engineering Analysis
1.
WW Effluent Source Heat Pump System
?
Highest Capital Cost
?
Lowest Carbon Footprint
2.
Ground Source (Geothermal) Heat Pump
System
?
Low Maintenance due to low clean fluid
?
Highest Carbon Footprint
3.
Hybrid WW Effluent Heat Pump with NG
boiler System
?
Low Carbon Footprint
?
Moderate Capital Cost

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Load Forecasting
1.
Peak and Annual Loads
1.
Measured if available
2.
Energy Use Intensity Method
2.
Load Diversity
Ratio of peak on system vs. peak at buildings
The more buildings connected, the higher the
diversity (lower factor)
Typical load diversity ranges from 0.7 to 0.9
3.
Load Duration Curve
Hourly or daily distribution of loads ranked from high
to low
30% of Peak Demand yields 70% of
annual output

 
Key Project Anecdotes
?
Low grade heat still very effective at heating buildings
?
Brush Cleaning add-on needed in heat pumps to reduce
fouling for WW effluent systems is critical to maintain
reliability and efficiency
?
Lack of incentive/driver for additional users to tie-in has
delayed making full use of installed system.

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Pre-Heat Recovery
Post-Heat Recovery

 
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11
12
13
14
15
16
17
18
19
20
21
22
Degrees C
Effluent DM Temperature Comparison (Pre-Heat Recovery vs Post-Heat Recovery)
Avon Pre Heat Recovery
Avon Post Heat Recovery

 
0.00
0.10
0.20
0.30
0.40
0.50
0.60
0.70
0.80
0.90
1.00
1.10
1.20
Effluent Heat Reduction in DM Temperatures (Pre-Heat Recovery minus Post-Heat Recovery)
Difference in DM
Degrees
C
Minimum
0
25th percentile
0.1
50th percentile
0.2
75th percentile
0.4
Maximum
1.1
Average
0.2
Reductions in DM effluent temperatures range from 0 to 1.1° C with average and median reductions of 0.2 °C

 
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11
12
13
14
15
16
17
18
19
20
21
22
Degrees C
Weekly Average Temperature Comparison (Pre-Heat Recovery vs Post-Heat Recovery)
Avon
-
Pre Heat Recovery
Avon Post
-
Heat Recovery

 
0.00
0.10
0.20
0.30
0.40
0.50
0.60
0.70
0.80
0.90
Effluent Heat Reduction in WATs (Pre-Heat Recovery minus Post-Heat Recovery)
Difference in WAT
Degrees
C
Minimum
0
25th percentile 0.3
50th percentile 0.4
75th percentile 0.5
Maximum
0.8
Average
0.4
Reductions in WAT effluent temperatures range from 0 to 0.8 ° C with average and median reductions of 0.4 °C

 
0
2
4
6
8
10
12
14
16
18
20
22
24
Degrees C
Downstream DM (Post Heat Recovery)
Downstream DM (Pre Heat Recovery)
Acute Standard
Downstream DM Eagle River Temperatures Pre-Heat Recovery vs Post-Heat Recovery
Minimum
0
25th percentile
0
50th percentile
0
75th percentile 0.01
Maximum
0.04
Average
0
Effluent temperature reductions under actual and future estimated conditions (HR at full capacity) have little to no impact on
temperatures in the Eagle River below the WWTF primarily due to large stream flow to effluent flow ratios

 
0
2
4
6
8
10
12
14
16
18
20
22
24
Degrees
C
Downstream DM Eagle River Temperature (Pre-Heat Recovery vs Post-Heat Recovery with 2.4 Deg C Reduction)
Downstream DM (Pre-Heat Recovery)
Downstream DM (Pre-Heat Recovery Less 2.4 Deg C)
Acute Standard
Minimum
0
25th percentile 0.01
50th percentile 0.04
75th percentile 0.07
Maximum
0.29
Average
0.05
Effluent temperature reductions under actual and future estimated conditions (HR at full capacity) have little to no impact on
temperatures in the Eagle River below the WWTF primarily due to large stream flow to effluent flow ratios

 
0
2
4
6
8
10
12
14
16
18
Degrees C
Downstream WAT (Post Heat Recovery)
Downstream WAT (Pre Heat Recovery)
Chronic Standard
Downstream WAT Eagle River Temperatures (Pre-Heat Recovery vs Post-Heat Recovery)
Minimum
0
25th percentile
0
50th percentile 0.01
75th percentile 0.01
Maximum
0.03
Average
0.01

 
0
2
4
6
8
10
12
14
16
18
Degrees C
Downstream WAT Eagle River Temperature (Pre-Heat Recovery vs Post-Heat Recovery with 2.4 Deg C Reduction)
Downstream WAT (Pre-Heat Recovery)
Downstream WAT (Pre-Heat Recovery Less 2.4 Deg C)
Chronic Standard
Minimum
0
25th percentile 0.01
50th percentile 0.04
75th percentile 0.07
Maximum
0.17
Average
0.05

 
?
Shows State Water Quality Commission
proactive steps to control heat discharges to
the Eagle River
?
Potential to reduce effluent temperature by
2.4 deg C (4.4 deg F) when operated at max
design build out load.
?
Could help WWTP to meet chronic effluent
limit temperature standards
Collaboration of Town and ERWSD

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Thank You
Siri Roman, P.E.
sroman@erwsd.org

 
Additional Details and Results
Section 4 provides considerations for reducing environmental impacts
and find sustainable solutions.
A. Chiller with Closed Loop Cooling Tower or Other Cooling Water Source
B. Use of High Efficiency Motors and Energy Efficient Designs
C. Alternate Electric Sources
D. Shade Options for Wastewater Lagoons
E. Other Combinations and Hybrids
F. Energy Recovery and Reuse
G. Absorption Refrigeration
H. Electricity from Waste Heat and Other Potential Advancements

 
Energy Recovery & Mechanical Cooling
5 MGD Example Facility
with CHP Process
Comparison of Mechanical Refrigeration Options
Vapor Compression
Refrigeration
Absorption
Refrigeration
Cooling Load
1830 kW
Capacity
1830 kW
1830 kW
Current Electric use
500 kW
COP
cooling
3.3
1.5
Electricity from CHP*
130 kW
Electricity requirement
555 kW
-
Usable Heat from CHP*
146 kW
Heat requirement
-
1,220 kW
Fossil fuels for electricity
1,300 kW
-
Renewable electricity use
120 kW
-
Heat available for recovery
2,300 kW
2,900? kW
Example: Combined heat and power (CHP) potential for cooling from literature values
*Based on low to medium strength waste,
high strength waste will boost CHP production

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Vapor
Compression
Chiller
e
-
Waste Sludge to Digestion
Activated Sludge Treatment
Preliminary/
Primary
Treatment
Pumping
Facilities
Disinfection
Treated Effluent
Power supply for pumps, mixers, blowers, and other equipment
Normal power draw:
500 kW
Chiller power draw:
555 to 1,665 kW
Total facility power draw
during cooling:
1,055 to 2,165 kW
COP
cooling
= 3.3
Amount of heat
available for reuse:
2,300 to 7,000 kW
1,830 to 5,550 kW
of excess heat
Absorption
Chiller
(alternate)
COP
cooling
= 1.5
Amount of heat
available for reuse?
2,900 to 8,800 kW
Natural gas or coal for electricity
: 210,000 kW
Residential electric use: 62,330 kW
Commercial electric use: 84,564 kW
Industrial electric use: 59,040 kW
WWTF normal draw: 1,174 kW
WWTF chiller: 1,300 to 3,920 kW
27,800 households
5 million gallons
per day of domestic
wastewater
5.5% grid loss
Electric supply requirements
Residential electric use: 27,860 kW
Commercial electric use: 37,834 kW
Industrial electric use: 26,400 kW
WWTF normal draw: 528 kW
WWTF chiller: 586 to 1,760 kW
35% conversion
efficiency
Direct use natural gas or propane
: 253,600 kW
Residential space heating: 33,400 kW
Residential water heating: 14,500 kW
Commercial use: 34,200 kW
Industrial use: 171,500 kW
Transportation
218,230 kW
1,830 to 5,550 kW
of excess heat in
the domestic
wastewater
0 kW of
excess heat!
Renewable electricity generation
: 20,000 kW
Wind energy: 16,120 kW
Hydro energy: 2,800 kW
Solar: 900 kW
Natural gas use:
1,220 to 3,660 kW
22% of electricity in
Colorado is supplied
from renewable sources
Absorption Refrigeration
may be a useful
alternative that could
save on fossil fuel use
compared to tradition
vapor compression
Where is
the best
bang for
the buck?
Effluent Cooling and Energy Recovery and Reuse Opportunities at Wastewater Treatment Facilities
A Holistic Evaluation of Facility-wide and Community-wide Energy Uses for a Hypothetical Community in Colorado

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This is all so complicated…
Q: Where do I start?
A: That depends on your interests.
Data, tools, and technical
information on a variety of energy
use topics is provided throughout
the user guide.
There is also a specific section
with guidance for DSV proposals

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How do the costs apply to my facility?
An “MGD*
o
C” of water can represent a
rate of energy use. It’s about 183 kW.
The example in this guidance has a
value of 10 MGD*
o
C. As a ball park
estimate, the costs for a specific site
can be estimated from the example
cost:
???????????????????????????? ????????????????
10
∗ ???????????????? ???????????????? ???????????? ∗ ???????????????? ∆????
(
????
????)
In general, the
capital cost
of each
alternative primarily scales with
the magnitude of the temperature
reduction and flow rate. The
operational cost
of each
alternative primarily scales with
the magnitude of the temperature
reduction, flow rate, and duration
(i.e. months of cooling operations).
However, many other factors could
significantly affect costs depending
on site specific conditions.

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Cost scalability is also
addressed
Details on cost estimates are
included for each basic technology

 
User Guidance for Alternative Analyses
Step 1. Establish baseline conditions
Step 2. Develop list of alternatives
Step 3. Evaluate technologic feasibility
Step 4. Evaluate economic feasibility*
Step 5. Evaluate environmental feasibility*
Step 6. Rank and select alternatives
Step 7. Develop alternate effluent limits
Section 3.D of the user guide
provides tips and information
regarding the data and analysis
that should be provided to support
each step in a DSV proposal.
The Fact Sheets in the Appendix
and the information on Innovative,
Hybrid, and Combination
Approaches in Section 4 provides
example calculations that can be
adapted to site-specific situations.

 
Discharger Specific Variances (DSVs)
- DSVs are temporary, facility-specific water quality standards
- Colorado adopted current provisions in 2010, became effective in 2013
- EPA adopted framework in 2015, generally consistent
- WQCC Regulation 31, Section 31.7 (4):
Variances to numeric standards are authorized only where a comprehensive
alternatives analysis demonstrates that there are
no feasible alternatives
that
would allow for the regulated activity to proceed without a discharge that
exceeds water quality-based effluent limits.

 
Next Steps
- Follow the methods and apply to a site-specific scenario
- Carefully evaluate site-specific baseline conditions
- Use fact sheet calculations as a template
- Coordinate with division, EPA and stakeholders on environmental
feasibility
- Interest in technical/engineering working group on utilizing this user
guide for site-specific application?

 
Questions and Discussion
“I have never let my schooling interfere with my education.”
-
Mark Twain
Barbara Bennett
Surface Water Assessor
Water Quality Control Division | Colorado Dept. of Public Health and Environment
P 303-692-3577
barbara.bennett@state.co.us
Jon A. Erickson, P.E.
Senior Review Engineer
Water Quality Control Division | Colorado Dept. of Public Health and Environment
P 303-692-3593
jon.erickson@state.co.us
CDPHE Presenter Contact Information:

 
Potential Discussion Slides
“If you tell the truth [and have backup slides], you don’t have to remember anything.”
-
Mark Twain, annotated

 
DSV – Feasibility Tests
Limits of Technology: Demonstration that attaining the water quality standard is not
feasible because, as applied to the point source discharge, pollutant removal techniques
are not available or it is
technologically infeasible
to meet the standard;
Economics: Demonstration that attaining the water quality standard is not feasible
because meeting the standard, as applied to the point source discharge, will cause
substantial and widespread adverse social and economic impacts
in the area where
the discharge is located. Considerations include such factors as the cost and
affordability of pollutant removal techniques; or
Other Consequences: Human caused conditions or sources of pollution prevent the
attainment of the use and cannot be remedied or would
cause more environmental
damage to correct
than to leave in place.

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Potential Compliance Problem
Jan
Feb
Mar
Apr
May
Jun
Jul
Aug
Sep
Oct
Nov
Dec

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Source Control/Other Mitigation Options
A lot of heat in domestic wastewater comes from residential water use. How much
comes from industrial, commercial, or retail sources?
Potential Source Control Options:
- Outreach and education to residential users
- Evaluate heat loads from industrial, commercial, and retail sources
- Consider implementing voluntary or mandatory controls for select users
- Options at WWTF are limited (more on this in solar shade section)
Other non-technologic options (alternate discharge locations, consolidation, etc.)
are highly site-specific and beyond scope of feasibility study.
Encouraging residents to minimize
hot water use alone may not achieve
compliance, but it can be easily
incorporated into any DSV proposal

 
Evaporative Cooling
What is the wet bulb (WB) temperature?
The lowest temperature that can be reached by evaporating water into the air.
-
Measured by placing a wetted muslin sock on a thermometer with air blowing over it.
- Combines the ambient air temperature (aka dry bulb (DB) temperature) and the
relative humidity into one factor that shows the limit to which evaporation can be
used for cooling.
Note:
When relative humidity = 100%, wet bulb = dry bulb
When relative humidity < 100%, wet bulb < dry bulb
Therefore: wet bulb ≤ dry bulb

 
Natural Heat Flow
Heat energy transfers from areas of high temperature to areas of low temperature
Hot Substance
(Heat Source)
Cold Substance
(Heat Sink)
Q
(heat flux)
Q = U
overall
* Area * (T
source
-T
sink
)
U
overall
= overall heat transfer coefficient, varies based on substance properties

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Natural Heat Flow
Example: Heat Exchanger – allows natural heat flow but keeps fluids separate
The outlet wastewater
temperature can never
get below the
inlet cooling water
temperature

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Evaporative Cooling
Energy is taken up and stored in gaseous water molecules
Liquid Water
(molecules in lower energy state)
Gaseous Water
(molecules in higher energy state)
In order to change phase
(i.e. evaporate),
water molecules will
“steal” energy from
nearby molecules to get
to the higher energy
state
Q (heat of vaporization)
Q = ∆H
vap
* M
evap
∆H
vap
= heat of vaporization
(latent heat)
M
evap
= Mass of water evaporated

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Evaporative Cooling
Example: Once-through Cooling Tower – optimization of air to water contact
The outlet wastewater
temperature can never
get below the
ambient wet bulb
temperature

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Evaporative Cooling
Jan
Feb
Mar
Apr
May
Jun
Jul
Aug
Sep
Oct
Nov
Dec

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Mechanical Cooling
Heat energy is transferred from areas of low temperature to areas of higher
temperature (Trying to cheat nature)
Hot Substance
(Heat Sink)
Cold Substance
(Heat Source)
Q
(heat flux)
External Energy Input
Q = COP
chiller
* W
external
COP
chiller
= coefficient of performance, varies based on technology and heat sink temperature
W
external
= external energy input

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Mechanical Cooling
Example: Air-cooled Chiller (vapor compression) – electricity used to drive process
The efficiency of a
chiller is affected by the
temperature of the
receiving sink,
it would only approach
zero in extreme
conditions
Heat is taken up by
evaporating refrigerant
Heat is dumped when refrigerant
is compressed to a liquid

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Chiller with Closed-Loop Cooling Tower
More efficient chiller
allows smaller compressor
and less electric use
Cooling tower allows chiller to
discharge heat near wet bulb
temperature, increasing efficiency
(i.e. higher COP)
At large heat loads, the
combination of a smaller
chiller and smaller cooling
tower can be more cost
effective than a stand-alone
chiller or cooling tower.
Blow-down discharge, make-up water,
and chemicals to prevent scale,
corrosion, or biogrowth will be required

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Minimizing Electric Use Impacts
Impact of electric use is based on two components:
Impact = Amount of electricity used * Impact per unit electric use
Two ways to minimize impact: reduce use or find alternate source with less impact:
1.
Use of High Efficiency Motors and Energy Efficient Designs
2.
Alternate Electric Sources
These are not standalone alternatives,
but rather a hybrid or combination of other technologies
Adding high efficiency motors
and solar generators to an air-
cooled chiller can significantly
minimize environmental
impacts, however, it will
increase capital costs
(operating costs may go down).

 
Solar Shades
Minimizing incoming solar radiation while maintaining as much outgoing long-wave (e.g.
infrared) radiation as possible may have a measureable impact on effluent temperatures
Energy Transfer Phenomena
Temperature change
o
C/day
Short-wave radiation [net incoming solar radiation]
+ 0.5 to 2.5
Long-wave radiation [net outgoing blackbody radiation]
-0.5 to 1.0
Sensible heat [heat transferred to the air and through aeration]
+/- 0.5 to 3.5
Evaporation
-0.5 to 2.5
Process energy [energy released from biological reactions]
+ 0.5 to 2.0
Mechanical energy [heat from blower inefficiency/friction losses]
+ < .1
Geothermal energy [heat transferred to ground through basin walls]
+/- <0.05
Precipitation, rain/snow at surface
+/- <0.2
Typical Range of Contributions, Temperature Changes in Treatment Plants
Reproduced from la Cour Jansen et al., 1992

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Other Combinations and Hybrids
Countless combinations and hybrids of the basic heat transfer mechanisms exist.
Example:
Pipe-in-pond
cooling is …
a heat exchanger, in a
passive cooling pond,
with possible blending
Q
1
Q
2
Q
1
= Q
2
+ Q
3
Q
3
Unless there is colder
water flowing through
this pond, it will need to
be as big as a standalone
passive cooling pond.

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Vapor
Compression
Chiller
e
-
Waste Sludge to Digestion
Activated Sludge Treatment
Sludge Heater
80% Efficiency
Preliminary/
Primary
Treatment
Pumping
Facilities
Disinfection
Treated Effluent
Power supply for pumps, mixers, blowers, and other equipment
Normal power draw:
500 kW
Chiller power draw:
555 to 1,665 kW
Total facility power draw
during cooling:
1,055 to 2,165 kW
COP
cooling
= 3.3
Amount of heat
available for reuse:
2,300 to 7,000 kW
1,830 to 5,550 kW
of excess heat
Absorption
Chiller
(alternate)
COP
cooling
= 1.5
Amount of heat
available for reuse?
2,900 to 8,800 kW
Natural gas or coal for electricity
: 210,000 kW
Residential electric use: 62,330 kW
Commercial electric use: 84,564 kW
Industrial electric use: 59,040 kW
WWTF normal draw: 1,174 kW
WWTF chiller: 1,300 to 3,920 kW
27,800 households
5 million gallons
per day of domestic
wastewater
5.5% grid loss
Electric supply requirements
Residential electric use: 27,860 kW
Commercial electric use: 37,834 kW
Industrial electric use: 26,400 kW
WWTF normal draw: 528 kW
WWTF chiller: 586 to 1,760 kW
35% conversion
efficiency
Direct use natural gas or propane
: 253,600 kW
Residential space heating: 33,400 kW
Residential water heating: 14,500 kW
Commercial use: 34,200 kW
Industrial use: 171,500 kW
Transportation
218,230 kW
174 kW if pre-thickened
1,183 kW without pre-thickening
1,830 to 5,550 kW
of excess heat in
the domestic
wastewater
0 kW of
excess heat!
Renewable electricity generation
: 20,000 kW
Wind energy: 16,120 kW
Hydro energy: 2,800 kW
Solar: 900 kW
Summary of Basic Cooling Alternatives for a Hypothetical 5 MGD Facility
Overall COPc
Applicability
Range
Allows heat
Recovery?
Proven reliability
for WWTFs?
No action/status quo
0
N/A
N/A
N/A
Heat exchanger with external
cooling water
52
Limited
Unlikely
Yes, when
available
Blending with deep groundwater
3.5
Limited
Unlikely
Yes, when
available
Geothermal loop exchanger
293
Limited
Unlikely
Unknown
Passive Cooling Pond
Infinite
Moderate
No
Unknown
Spray Cooling Pond
36
Moderate
No
Unknown
Cooling Tower
31
Moderate
No
Yes
Vapor Compression Chiller
3.3
Broad
Yes
Yes
Absorption Chiller
1.5
Moderate
Yes
Unknown
Natural gas use:
1,220 to 3,660 kW
WWTF Normal power draw:
500 kW
WWTF Total Power draw when
chilling 2
o
C:
1,055 kW
WWTF Total Power draw when
chilling 6
o
C:
2,165 kW
8 loads per day of
dilute sludge
Less than 2 loads per
day if dewatered
Effluent Cooling and Energy Recovery and Reuse Opportunities at Wastewater Treatment Facilities
A Holistic Evaluation of Facility-wide and Community-wide Energy Uses for a Hypothetical Community in Colorado
Operating at:
∆2
o
C
Operating at:
∆6
o
C
22% of electricity in
Colorado is supplied
from renewable sources
Anaerobic Digester
Biogas can be
wasted via
direct flare
e
-
Combined Heat and Power
Co-generation System
130 kW of
electricity
146 kW of
usable heat
Heat lost
to atmosphere
23 kW
90 kW of
waste heat
366 kW of
waste heat
Digester Decant:
56 kW of excess heat if pre-thickened
864 kW of excess heat without pre-thickening
Digested Sludge:
59 kW of excess heat
Heat from
reaction
CHP production can
be optimized up to
2x
these values!
Absorption Refrigeration
may be a useful
alternative that could
save on fossil fuel use
compared to tradition
vapor compression
Influent Sludge Heating Requirement
115 kW required if pre-thickened
923 kW required without pre-thickening
Sludge
Dewatering
Return streams
can have high
heat and nutrient
loads
???????????????????????????????????????????? ???????? ????????????????????????????????????????????
(????????????) =
???????????????????????? ???????? ???????????????????????? ????????????????????????????????????????????
???????????????????????? ???????? ???????????????????????? ????????????????????
Vapor compression chillers can run on renewable electricity
Absorption chillers can run on waste heat

 
Since the
energy balance between these
inputs must hold under actual or theoretical conditions, the
theoretical and actual heating and cooling COP values can be related as follows:
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????,???????????????????????????????????? ℎ????????????
=
????
????????????????????????????????
∗ ????
????????????????????
(???????????????????? )
+
????
????,???????????????????????????? ????????????????
????????????
????????????????????????
????????????????????????????
=
????????
,????????????????????????????
(????????????????????
????????????????
)
This can be rearranged to give:
????
????,???????????????????????????? ????????????????
=
????????????
????????????????????????
????????????????????????????
∗ ????
????????????????????
(???????????????????? )
This term can now be combined with the heating COP as follows.
????????????
????????????????????????
????????????????????????????
=
????
????,???????????????????????????????????? ℎ????????????
????
????????????????????
(???????????????????? )
=
????
????????????????????????????????
∗ ????
????
????????????
(????????????????????
)
+
)
????
????,???????????????????????????? ????????????????
By substituting for
????
????,???????????????????????????? ????????????????
:
????????????
????????????????????????
????????????????????????????
=
????
????,???????????????????????????????????? ℎ????????????
????
????????????????????
(???????????????????? )
=
????
????????????????????????????????
????
????????????????????
(???????????????????? )
+
????????????
????????????????????????
????
????????????????????
(????????????????????
)
????????????????????????????
∗ ????
????????????????????
(???????????????????? )
????????????
????????????????????????
????????????????????????????
=
????
????????????????????????????????
+
????????????
????????????????????????
????????????????????????????
Or alternatively:
????????????
????????????????????????
????????????????????????????
=
????????????
????????????????????????
????????????????????????????
− ????
????????????????????????????????
and
????????????
????????????????????????
????????????????????????????
=
????????????
????????????????????????
????????????????????????????
+
????
????????????????????????????????

 
Summary of Conclusions from Feasibility Study
Compared with other unconventional pollutants for domestic wastewater treatment, such as arsenic, chloride and
selenium, there are many technologically feasible options for reducing heat from effluent. Further, compared with
common treatment for other pollutants (e.g. ammonia, nitrate), the cost to achieve compliance is comparatively
low. The water loss associated with evaporative cooling is similar in magnitude to the water loss associated with
other common wastewater treatment technologies.
The energy consumption varies greatly among the alternatives. Some of the alternatives, such as passive cooling
ponds, generally do not require any additional inputs of energy. For the technologies that consume significant
electricity, there are strategies available for optimizing energy efficiency, recovering heat and energy, and
producing renewable energy onsite that allow for temperature control to be integrated into a larger plan to
reduce overall demand for electricity.
The feasibility of cooling domestic wastewater is intimately related to the energy implications associated with the
various cooling alternatives and may be an important consideration in the water-energy nexus. There are
significant trade-offs between available options and constraints and opportunities may also vary widely from site
to site. The following diagram illustrates how many of these factors are related and shows where key topics are
addressed in this document. This user guide provides a toolbox to develop a holistic view of energy use as part of
an efficient facility-wide energy management strategy. There are numerous ways to optimize energy efficiency
and reduce the costs associated with electricity, and the division recommends that facilities consider energy
management strategies as part of their long-term facility planning.
The conclusions represented in the hypothetical scenario are based upon generally conservative assumptions, and
should conservatively scale upward (in magnitude of heat reduction or the volume of the discharge) and should be
a conservative estimate of the local conditions in most Colorado locations.