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EPA Review Notice
i ' ' '
This report has been reviewed by the U.S. Environmental Protection
Agency, and approved for publication. Approval does not signify that the
contents necessarily reflect the views and policy of the agency, nor does
mention of trade names or commercial products constitute endorsement
or recommendation for use.
* r ' . . f .
This document is available to the public through the National Technical
Information Service, Springfield, Virginia 22161.
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-------
CONTENTS
Acknowledgements ........ . ........... ............ . ____ ........ m
Forward . . .... ....... . ____ ......... ____ . ..... .................... v
Introduction . . ........ . .............. . . . . .' ____ . ________ . ....... fmm, 1
Background * Basics of Biogas Generation and Use * In-Plant Applications for
Biogas * Precautions for Use of Unscrubbed Biogas
County Sanitation Districts of Orange County ....... . ........ .... 9
Facility Description * Description of the Technologies * Process Modifications *
Pretreatment Program Effects pn Energy Conservation * Benefits of the
Energy Conservation Program
City of Los Angeles Hyperion Wastewater Treatment Plant ........ 17
Facility Description * Energy Recovery from Biogas * Energy Recovery from
Biosolids * Process Modifications * Benefits of the Energy Conservation
Program
Sunnyvale Water Pollution Control Plant ... .... . . . . , . . ......... _____ 2?
Facility Description * Description of the Technologies * Operation and
: Maintenance * Landfill Gas Production * Biosolids Dewatering
Sanford Big Buffalo Creek WWTP, North Carolina ........... ..... as
Facility Description * Energy Conservation Audit * Description of the
Technologies * Process Modifications * Financial Benefits
Seattle Metro Renton Water Reclamation Plant ...... . . . . ..... -..".'..-, 43
Facility Description * Energy Recovery from Biogas * The Metro Therm
Program * Applicability to Other Systems * Benefits of the Energy
Conservation Program
Other Promising Technologies . . . ____ . ........... . ' ...... ... ____ . . 53
Anaerobic Wastewater Treatment * Lake County Southeast Geysers Effluent
Pipeline * Biomass-Enhanced Digester Gas Production
Factors that Contribute to Success ......... ______ . . ____ ____ . . . ____ 59
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The Influence of Financial Factors 61
Biosolids: Onsite Use versus Offsite Reuse * Biogas: Onsite Use versus Offsite
Sale * Energy from Effluent: Purchase versus Contractual Equipment
Conclusions '.. 63
Resources - - 65
-------
Acknowledgements
This report was prepared by Science Applications International Corporation under National
Renewable Energy Laboratory Subcontract No. YAE-3-13480-01 for the U.S. Department
of Energy, and Contract No. 68-C8-0066, WA No. C-4-73 (M) with the U.S.
Environmental Protection Agency.
We thank the staff and management of each of the wastewater treatment plants involved in
this study for cheerfully providing information and graphics.
111
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IV
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Foreword
Public support for water quality
improvement has placed
increasing demands on
wastewater treatment plants in the years
since passage of the Clean Water Act in
1972. The public's expectations and the
resulting new environmental legislation
(at national, state, and local levels) have
led to new programs and increased
expenditures.
As a result, WWTP managers continually
tackle issues associated with broadening
environmental concerns. These concerns
include aquatic habitat protection,
wastewater reclamation, air quality issues,
industrial waste disposal, biosolids reuse,
and others up to and including global
climate change. Many plant managers are
dealing with all these issues and the
corollary need for funding.
The premise of this document is that
WWTPs can address environmental
mandates in an integrated framework
based on energy conservation, through
the use of renewable resources. As the
examples presented herein show, activities
-that conserve energy also reduce pollution
and costs. Energy conservation is a
'particularly appropriate goal for WWTPs,
which exist to reduce pollution.
WWTPs are among the few community
institutions that are efficiently designed to
manage renewable resources.
Conventionally, renewable resources are
considered to include water, air and soil,
wild and domesticated organisms, forests,
rangelands, cultivated land, marine and
freshwater ecosystems that support
fisheries, and other aspects of the natural
environment. However, human ability to
manage these scattered and generally
poorly understood resources is in most
respects very limited. In contrast,
WWTPs have collection systems to
convey the resource to a single point.
Treatment processes then separate solids
from the water fraction, producing
different resource streams for reuse.
Many plants now profitably obtain
methane for in-plant energy production
from the biosolids fraction. Examples of
such facilities are discussed in this
document. However, some plants are
moving forward to generate energy from
a combination of landfill gas and digester
gas (as seen in Sunnyvale, CA) or
production of digester gas for offsite sale
(Seattle Metro), or biosolids oxidation to
produce energy for onsite and offsite uses
(Los Angeles' Hyperion plant). Creative
WWTPs are also solving community
waste disposal problems by placing high-
strength biowastes into anaerobic
digesters. These facilities benefit from the
resulting increased production of
methane.
Energy can also be obtained from
wastewater effluent, as demonstrated by
Seattle Metro and The Boeing Company.
By using Seattle Metro's effluent for
cooling via heat exchangers, instead of
building cooling towers, Boeing has
conserved potable water and preserved
the City viewscape. Any WWTP faced
with building pipelines for water
reclamation purposes can explore this use
of effluent. The potential for energy
-------
conservation by using effluent in heat
exchangers is enormous; the U.S.
Department of Energy has estimated that
space heating and cooling account for 34
percent of commercial energy usage and
45 percent of residential usage. Great
community benefit would be obtained
even if only a small part of this usage
were defrayed.
By integrating wastewater treatment with
energy conservation, the WWTPs
described in this document have met the
challenges of new environmental
regulations. These facilities have
achieved benefits in cost savings while
enhancing their ability to comply with
regulations. Their activities illustrate
highly effective pollution prevention
strategies.
VI
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Introduction
The U.S. Environmental
Protection Agency (EPA) and
the National Renewable
Energy Laboratory (NREL) for the
U.S. Department of Energy (DOE)
funded a study to document energy
conservation activities and their effects
on operation costs, regulatory
compliance, and process optimization
at several wastewater treatment plants
(WWTPs).
Beginning in the mid-1970's, industry and
government has perceived an increasing need
for energy conservation efforts. While water
conservation has long been a goal, recent
initiatives requiring municipal pollution
prevention programs support the need to seek
innovative solutions that address both
concerns in a holistic manner.
The purpose of this report is to review the
efforts of wastewater treatment facilities
that use residuals as fuels. Case histories
are presented for facilities that have taken
measures to reduce energy consumption
during wastewater treatment. Most of the
WWTPs discussed in this report have
retrofitted existing facilities to achieve
energy conservation. The case studies of
energy conservation measures found no
effects on the facilities' ability to comply
with NPDES permits. Indeed, energy
conservation activities enhance
environmental compliance in several
ways.
Background
Studies conducted previously by DOE
identified the wastewater treatment
processes with the highest energy usage.
These processes exhibit the greatest
potential for energy savings, and include
activated sludge, biosolids dewaterihg and
conditioning, biosolids incineration,
aerobic digestion, advanced wastewater
treatment, and use of aeration ponds.
Anaerobic digestion uses comparatively
small amounts of energy, but also shows
great potential for energy savings because
its energy requirements are easily reduced
through the use of biogas for heating, the
technology to do so is commercially
available, and the economics is almost
always favorable.
A survey conducted by the Illinois
Association of Wastewater Agencies
found that the annual energy costs .for
wastewater treatment plants in Illinois
ranged from 20 to 35 percent of 1990
operation and maintenance (O&M) costs:
In comparison to this figure, the County
Sanitation Districts of Orange County,
which has implemented a comprehensive
energy conservation program, expects to
spend only 6 percent of its total O&M
budget on energy during fiscal year 1993-94.
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Residuals Use and Energy Conservation
The DOE studies found thatWWTP
managers' primary concern is to meet
discharge requirements. Energy
conservation, when considered at all, is
often of secondary importance. Now,
marry WWTP managers are finding that
energy conservation and use of residuals
as fuels can actually enhance
environmental compliance. The
experiences of some of these facilities are
presented as examples to other agencies
considering whether to implement such
technologies.
Basics of Biogas Generation and Use
Anaerobic digestion is one of the most
widely used processes of wastewater
biosolids stabilization. The process
involves bacterial decomposition of the
organic constituents of the biosolids in the
absence of oxygen. The products of
anaerobic digestion, apart from solids,
include water and a gas composed of
methane, carbon dioxide, hydrogen
sulfide, and other minor gaseous
compounds. This "biogas" has a heat
value of approximately 550 Btu/ft3, about
60 percent of the heat value of natural
gas.
ป
Biogas may be used either off-site or
within the plant to improve energy
efficiency of wastewater treatment
processes. Both possibilities should be
considered when designing new treatment
facilities or upgrading existing ones.
Local objectives and conditions, however,
will decide the use made of biogas at a
particular plant.
In-pliant uses are those that result in the
biogas being consumed completely within
the wastewater treatment plant, either as
primary or backup fuel. Uses include
fueling boilers in process heating
operations and space heating and cooling,,
engine-driven machinery, engine
generators for electricity generation,
solid s incinerators, boilers for
pasteurization of digested biosolids, gas
fired biosolids dryers, arid generation of
electricity by steam turbines and fuel cells.
Figure 1 provides a schematic of in-plant
uses. These uses are described in detail in
the next section.
Use of waste heat recovery increases
energy efficiency in the system, and is of
particular value whenever in-plant use
involves the operation of equipment not
primarily designed to produce heat (i.e.,
engines, incinerators, turbines, etc.). As
the case histories in this study
demonstrate, fuel energy efficiency can be
increased from 30 to 70 percent by
recovering heat for process or space
heating/cooling requirements. Recovery
of biogas should always be supplemented
with waste gas burners, or flares, to
ensure that excess gas is controlled with
the smallest environmental impact.
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Residuals Use and Energy Conservation
Offsite., biogas can be used to create
either energy or chemicals that are sold
for use external to the plant. There are
many potential offsite uses for biogas, as
indicated in the schematic in Figure 2.
The case study presented below of Seattle
Metro's Renton Reclamation Plant
describes one such use. Generally, it is
less practical to process biogas for offsite
uses if the gas can be used in the plant.
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Figure 1: Onsite uses for biogas
-------
-------
Residuals Use and Energy Conservation .
In-PIant Applications for Biogas
Biogas use can result in significant energy
savings. Production depends on plant
wastewater flows and suspended solids
loading, rather than on warm weather or
other outside variables, as long as the
digester environment is uniform.
The five most adaptable in-plant uses for
biogas are as a fuel for (1) generating heat
for treatment processes, (2) generating
heat for space heating and cooling, (3)
powering engines used to drive equipment
directly, (4) powering engines used with
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-------
Residuals Use and Energy Conservation
Process Heating
A plant that uses anaerobic digestion for
biosolids stabilization should include a
process-heating system that can maintain
the contents of the digesters at their
optimum temperature (usually 95ฐ F).
Such a system should maintain boiler
temperatures above 212ฐ F, and hot water
in the biosolids heat exchanger should not
be allowed to rise above 160ฐ F. At
temperatures more than 160ฐ F the -
biosolids heat exchanger may cake with
biosolids, which quickly ruins the system's
heat transfer coefficient. Other uses of
process heat include chlorine and sulfur
dioxide evaporation and raw biosolids and
scum preheating.
Space Heating
The use of space heating can be expanded
effectively to include space cooling.
When combined with absorptive
refrigeration units, the hot water
produced with the biogas can be arranged
to produce chilled water, which can then
be piped around the plant for space and
equipment cooling. Often such space
cooling can increase savings by
eliminating the need for excessive
ventilation.
Direct Engine Drives
Direct engine-driven equipment usually is
employed in plants whose major
horsepower demands are required only
during peak flow or load conditions, for
example, raw wastewater pumps, effluent
pumps, and aeration blowers. The use of
direct engine-driven equipment eliminates
the need for standby electric power to
operate this equipment during periods of
peak load. The electric power company,
in turn, can make this peaking power
available to someone else. Any type of
treatment plant can use direct engine-
driven equipment.
Indirect Engine Drives
Indirect engine-driven equipment provides
the designer with an exceptionally flexible
system. It can be used (1) to reduce peak
demands of major equipment that is
remote from the source of fuel and
maintenance, (2) to drive both local and
remote equipment, (3) to achieve
operational speed variability of remote
major equipment, and (4) to use engine
generators as both indirect engine drivers
and general-purpose electrical generators.
The extra flexibility obtained by using
indirect engine-driven equipment may be
the difference between efficient and
inefficient use of biogas.
General Purpose Power Generation
As more plants are modified or enlarged
to include secondary treatment processes,
efficient use of biogas will require greater
use of in-plant, general-purpose power
generation. Biogas production from
plants involving secondary treatment .can
be sufficient to provide up to 60 to 80
percent of the plant's total power needs,
depending on the actual treatment
processes involved. In those plants with
minimal process pumping, biogas may
provide nearly all of the power needs.
Engines for generating plant power
usually operate at slower speeds and
-------
Residuals Use and Energy Conservation
lower mean effective pressures. Such
heavy-duty engines can generate power
reliably for many years.
Precautions for Use of Unscrubbed
Biogas
Biogas contains 60 to 70 percent
methane, 30 to 40 percent carbon dioxide,
up to Yz percent hydrogen sulfide and
other inert gases and water vapor. Many
WWTPs clean up the biogas before use to
remove contaminants. Sunnyvale, for
instance, uses simple baffle plate
condensers to remove moisture from
biogas. Biogas from Hyperion's
anaerobic digesters contains 60 to 100
ppm of hydrogen sulfide, which would
produce unacceptable emissions when the
gas is burned. Therefore, Hyperion treats
the biogas in a Stretford unit to reduce
the sulfur content to less than 40 ppm of
hydrogen sulfide. Seattle Metro removes
carbon dioxide from biogas produced at
the Renton WWTP before sale to the
local gas utility for offsite use. Biogas
which does not meet the standard of 99
percent purity is rejected by the utility.
Depending on local factors and the final
use intended for the biogas, scrubbing is
not always necessary. However, certain
precautions should be considered in the
event that biogas is used without
scrubbing. Any boiler or engine using
unscrubbed biogas must be operated at
temperatures above 212ฐ F. Unless the
combustion temperature is maintained at a
high level, exhaust temperatures will not
be sufficient to maintain non-condensing
conditions within the collection and
discharge conduits. The carbon dioxide
and hydrogen sulfide in the spent biogas
becomes acidic and extremely corrosive
when combined with water. Exhaust
condensation must be eliminated from
equipment fueled by unscrubbed biogas.
Blending biogas with a gas having lower
hydrogen sulfide content can reduce the
corrosivity concerns associated with
unscrubbed biogas.
Biogas heat recovery systems must be
isolated from each other. The upsets
(production rate changes) of one system
must never be allowed to affect the
operation of another. This isolation can
best be accomplished by using separate
steam condensers to transfer the boiler or
engine heat into a common hot-water-
circulation system. The system provides a
flexible method of transferring heat
throughout the plant. Using individual
secondary parallel heat loops to points of
need assures that the final supply of hot
water is at optimum temperature.
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Residuals Use and Energy Conservation
County Sanitation
Districts of Orange
County
This section discusses the energy
programs implemented at the two
wastewater treatment plants operated by
the County Sanitation Districts of Orange
County. -. . .
Facility Description
The County Sanitation Districts of
Orange County (CSDOC) provides
wastewater treatment for a population of
about 2.1 million people. CSDOC
operates two treatment plants, with a
combined average wastewater flow of
about 23 5 MOD. Each plant uses
advanced primary treatment with ferric
chloride and anionic polymer addition in
the primary basins. About 50 percent of
the plants' flow receives secondary
treatment. The plants discharge to the
ocean through a common outfall which
has a 301(h) waiver.
PSDOC has carried out various energy
conservation techniques for several years.
For instance, the facility uses bipgas to
heat the digesters and to fuel some
engines that run pumps and blowers.
However, the recovery system did not
have the capacity to use all the gas
produced by the digesters, and the excess
was burned off. m 1989, CSDOC
codified formal energy conservation plans
in the "2020 Vision Plan."
The 2020 Vision Plan incorporates a
variety of energy conservation activities,
including lighting, building heating and
cooling, and generation of electricity
onsite.
In June 1993 CSDOC put the Central
Power Generation System (Central Gen)
on-line. Central Gen incorporates state-
of-the-art techniques to reclaim energy
from biogas. This system has been
installed at both treatment plants.
Currently, CSDOC does not purchase any
electricity, as all of its electricity needs are
supplied by onsite manufacture of energy
from a combination of biogas and natural
gas. CSDOC projects that by the year
2010 enough biogas will be produced to
completely fuel all the generators.
Other aspects of CSDOC's energy
conservation program include improving
operator skills, motivating and training
operators to be "energy aware," providing
computerized power management data,
optimizing equipment for maximum
efficiency, and providing management
technical skills, support, and funding.
CSDOC has an energy conservation
committee to review existing measures
and propose new possibilities for savings.
Operation of processes at the treatment
plants is aggressive. CSDOC has
implemented a lighting conservation
program and a summer peak savings program.
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Residuals Use and Energy Conservation
Description of the Technologies:
Central Power Generation System
Central Gen consists of a total of eight
internal combustion engines fueled by
both biogas and natural gas. The engines
drive generators to produce electricity
that is then used to operate the treatment
plants. These engines were specifically
designed to reduce emissions from the
engine exhaust and to use all the gas
produced by the digesters. Power output
is 5 megawatts at the Fountain Valley .
plant (Plant 1) and 7 megawatts at the
Huntington Beach plant (Plant 2).
Plant 2 has the greater energy demand (8
megawatts), due mainly to the presence of
the outfall pumping station at this plant.
Plant 1 uses about 4 megawatts. Now, all
biogas from Plant 1 is exported via
pipeline to Plant 2 for use, and the Plant 1
Central Gen operates entirely on natural
gas.
The three engine generators installed at
CSDOC's Plant 1 are Cooper Bessemer
Model LSVB-12SGC. The five engine
generators installed at CSDOC's Plant 2
are Cooper Bessemer Model LSVB-
16SGC. Plant 1 engines are rated at
2,500 kilowatts each, and those at Plant 2
are rated at 3,000 kilowatts. At 7,200
Btu/horsepower, the engines are highly
efficient.
The engine units consist of an electrical
generator, a spark ignition gas-fueled
internal combustion engine, engine
cooling equipment with automatic and
manual controls, and engine exhaust and
jacket water heat recovery equipment and
controls. All engines are the stratified
combustion charge type, with separate
precombustion chambers designed to
reduce exhaust pollutant emissions. The
generators' design efficiency is rated at a
minimum of 96 5 percent at rated
conditions.
Each engine has a fuel-injection system
suitable for accommodating biogas and
natural gas. A fuel gas cutoff valve and
totalizing flowmeter are provided for both
fuels and each engine. The engines can
use either biogas, natural gas, or any
combination of the two fuel types. The
engine fuel control system can rapidly and
automatically adjust the fuel/air ratio in
response to changes in engine load or fuel
heating value. The engine design enables
the fuel control system to accomplish
these adjustments in a manner that does
not reduce engine efficiency or result in
greater pollutant emissions, even at a fuel
value fluctuation rate of up to plus or
minus 100 Btu per cubic foot per minute.
Three-stage biogas filters to remove oil,
water mist, and solids are installed on the
engine fuel supply piping. The three
stages consist of: (1) mechanical
centrifugal separation, (2) separation by
coalescing and entrainment, and (3) final
filtration through a porous-fiberglass
medium. These filters are designed to
remove 99 percent of all dispersed liquid,
five microns and larger, and a minimum of
98 percent of all solids, one micron and
larger. A differential pressure gauge is
present to indicate when cleaning or
replacement of the filters is necessary.
10
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Residuals Use and Energy Conservation
Each engine generator unit has an
electronic governing system for automatic
synchronization, load sharing, and load
regulation. An air fuel ratio controller is
also present on each engine to
continuously monitor the air fuel ratio.
Systems that use exhaust sensors can be
susceptible to damage by components of
biogas, so CSDOC specified that control
of the air fuel ratio must be maintained by
monitoring air manifold temperature and
pressure and engine load instead. Engines
are also supplied with various protective
and safety devices and monitoring and
measuring devices to ensure safe and
efficient operation. Equipment vendors
and a consulting firm provided operator
training for Central Gen.
Description of the Technologies;
Waste Heat Recovery
The facility uses engine heat to heat the
digesters and for some heating and
cooling needs of buildings. The ability to
recover and use "waste" heat gives
Central Gen greater thermal efficiency
than that of Southern California Edison
(60% compared to 30%). Each engine
generator has a minimum recoverable
thermal output at rated load as follows:
% JfssSf f <% * *
f^ ff-m'ff ~f',f ** ***W' &*.
Engine exhaust:
4.39
5.27
Engine jacket water
1.90
2.30
Total
6.29
7.57
The jacket water heat recovery system
transfers heat to a plant-wide circulating
pressurized hot water system. The
exhaust heat recovery system is designed
to reduce the engine exhaust gas
temperature to a minimum of 3 80ฐ F
while generating 125 psig dry saturated
steam.
Process Modifications: Advanced
Primary Treatment
Application of advanced primary
treatment (APT) at both plants has
increased solids and BOD removal in the
primaries. This resulted in an increase in
biogas production, because the energy
content of the solids recovered from the
primaries is greater than that for solids
recovered from secondary treatment. In
APT, chemicals are added to the primary
settling facilities. Currently, ferric
chloride and polymer are added for about
12 to 13 hours daily. The facility has
conducted experiments with chemical
addition on a continuous 24-hour per day
basis, and found it to be a cost-effective
means to increase biogas production.
Central Gen has more than adequate
capacity to use all the biogas produced by
11
-------
Residuals Use and Energy Conservation
the facility, and as more biogas is
produced, less natural gas needs to be
purchased. Plant staff estimates that
biogas production increases between 12
and 18 percent because of APT. The
lower figure of 12 percent is gained with
16 hours per day APT at 20 mg/L ferric
chloride and 0.15 mg/L polymer. The
higher figure of 18 percent is obtained
with increased chemical addition (ferric
chloride at 30 mg/L and polymer at 0.22
mg/L).
APT has reduced the need for secondary
treatment, resulting in energy savings.
Before APT, the primary treatment
process removed about 65 percent of
total suspended solids; -with APT the
plants achieve 80 percent removal.
Increasing the amount of primary solids
sent to the anaerobic digesters results in
increased biogas production, equivalent to
3,000 kilowatts.
Another benefit achieved through APT is
reduction of the amount of biosolids that
must be disposed offsite. Less biomass is
produced in the secondary process.
Therefore, less biosolids must be hauled
offsite, resulting in reduced vehicular
emissions and conservation of
nonrenewable fuels.
Other Modifications
Besides advanced primary treatment,
CSDOC has implemented other process
changes designed to reduce energy
consumption. These include the
following:
Dissolved air flotation (D AF)
process reductions
DAF fan turned off
Transformer turned off
Reduced operation of aerators
Dewatering fan turned off
Elimination of scrubber
recirculation pumps associated
with obsolete scrubbers.
Lighting energy conservation
Use of gravity feed reduced the need for
pumping, and the facility realized
substantial energy savings by insulating
the digester domes.
Prellreatment Program Effects on
Energy Conservation
Imposing mass-based limits on BOD
discharges from industrial users has
contributed to the Districts' ability to
reduce its energy use. In the past, the
plant observed dramatic increases in
influent BOD during the food processing
season. One industry alone discharged up
to 70,000 pounds of BOD per day over
the two to three month season. CSDOC
now limits discharges from food
processing industries to 10,000 pounds
per day average, and 15,000 pounds per
day maximum BOD. Plant staff has
calculated the total reduction in BOD
discharged by industry to be equivalent to
12 MGD of secondary treatment on
average, peaking at up to 50 MGD of
secondary treatment for several weeks at
a time. The staff estimates that energy
use is reduced by 500 kilowatts per year
by these efforts.
12
-------
Residuals Use and Energy Conservation
Benefits of the Energy Conservation
Program: Air Emissions Reductions
CSDOC cites concerns with meeting air
emissions requirements as one of two
factors driving their energy conservation
efforts. Southern California air
regulations are among the most stringent
in the country. Both CSDOC and
Hyperion are subject to local regulations
promulgated by the South Coast Air
Quality Management District
(SCAQMD). SCAQMD regulates
emissions of sulfur dioxides from
stationary source internal combustion
engines, and sets limits on the allowable
content of sulfur in gaseous fuels.
SCAQMD also requires wastewater
treatment plants to develop risk
assessments, and bases influent volume
allowances on the results of the risk
assessments.
Substitution of biogas for natural gas has
enhanced the CSDOC plants' ability to
meet air quality requirements. Because
biogas has a heat value approximately
one-half that of natural gas (LEW = 550
for biogas compared to 950 Btu for
natural gas), biogas burns more slowly
and more completely. Ferric chloride is
added to the digesters to control sulfides
and odor, and the gas is chilled to
condense out water vapor.
The following table shows the maximum
emission characteristics of the Cooper
Bessemer engine generators installed at
the CSDOC plants.
~ / , ' v ', ' ^ ' ' - * 'x ,,-'''.
, ', , ,--.. -^'S,'- *- ซ "^ ' S"
'"> . ' ' ' ,' , :
- ' ; - % , -x^", ,sv3ftilttf*at "- --' :
Oxides of nitrogen
Carbon monoxide
Nonmethane hydrocarbons
Partfculates (em/dry stdL cubic foot
ffetuial&ksr '^" 1
:ate/Bfr4ir" - -. j
1.0
3.0
0.75
-
f S" %" Jsfee**: %
0.9
3.0
0.3
0.00026
CSDOC specified parameters for the
engine generators' performance in the
contract with the supplier. Performance
parameters included exhaust emissions,
generator output, and engine fuel
consumption. Penalties for
noncompliance with these parameters
were specified in the contract.
Financial Benefits
CSDOC cited high power costs as a
factor that drove the decision to install
Central Gen. The $65 million cost for
Central Gen and all associated projects
will be recovered in about seven years
because of the savings achieved by this
project.
13
-------
Residuals Use and Energy Conservation
Before construction of Central Gen,
CSDOC calculated the savings resulting
from its existing energy conservation
program in fiscal year 1991-1992. During
this year the facility reduced electrical
power purchases by biogas fueling of
engines, process changes, lighting energy
conservation, peak load shifting, and
reduction of loadings to the secondary
process. CSDOC estimated the total
savings from these programs at
$4,101,800. Flow decreased by 16
percent from June 1991 to June 1992 due
to the drought, and this contributed about
12 percent of this savings. Over the
approximately 30 years that CSDOC has
been using biogas as a fuel, approximately
$2 million per year has been saved.
The following table summarizes energy conservation savings realized in fiscal year 1991-92.
l^i^P^^/^j^n M4' > " r
l^liifl'l JC4*^^s*vW*'s ' --'
teซl&t;^^i/sVri^ v- '- -- v
Use of biogas to power pumps
Advanced primary treatment
Other process changes (DAF, blower, etc.)
Source control BOD reduction
Water conservation (pumping costs from
6/91 to 6/92)
California Energy Coalition rebates
Lighting conservation
Peak load shifting
TOTAL
,\- * "'V' '*-ป
'^' \ *i'?V)i/ ' * ''
%'5;&l^*i**ป4''
-s'* ^3a**av H
2,625
1.700
792
500
500
126
.
j. f*. "* ffft f f
: ',"><.', "', "/ ' '"&
I jE&ta*te*.Aiinaal,,
h' ^SajrtHgj^ -"'I
$1,464,000
1.200.000
569.100
350,000
315,000
40,000
88,500
75,200
S4.101.800
With Central Gen on-line and able to fully use the biogas produced, the calculated savings
in 1993-94 are substantial. The plant staff estimates sayings totaling 12,630 kilowatts,
worth about $8,850,000.
14
-------
Residuals Use and Energy Conservation
The following table provides a breakdown of components of the savings.
;\^^:^;;^Vv '*;
f " -%#'V , r"- , ?;S?vH',v
Blogas power production:
Normal plant operation
Additional eas from APT operation
Reduced secondary treatment due to APT
Other process chances (DAF, blower, etc.)
Source control BOD reduction
Water conservation
Lighting conservation
TOTAL
-;fe'^;V
"'^SS^^-
7^00
1,000
,700
1,000
500
500
126
12,630
/ -aSjjftiBHtedt Atoraasii ,
I'-V.^fJT ,, *
1^200,000
350,000
315,000
88,500
$8^50,800
* Savings are calculated at $0.08 per kilowatt hour
15
-------
16
-------
Residuals Use and Energy Conservation
City of Los Angeles
Hyperion Wastewater
Treatment Plant
Facility Description
The Hyperion Wastewater Treatment
Plant receives an average daily flow of
320 to 400 MOD (the lower flows
reflecting recent water conservation
efforts). Upstream wastewater
reclamation plants discharge biosolids to
Hyperion, resulting in an influent
wastestream containing 360 to 400 ppm
of total suspended solids. About 190
MGD receives secondary treatment by
activated sludge. The facility currently
discharges partial secondary-treated
wastewater under a consent decree:
however, construction is underway to
provide full secondary treatment.
The Hyperion Energy Recover System
(HERS) came on-line in 1987. HERS
generates energy from biosolids using two
distinct methods:
, 1. Biogas from anaerobic digestion
fuels three gas turbines. Each
turbine has the capacity to
produce 4,500 kilowatts of
electricity. Waste heat from the
turbines is fed to heat recovery
boilers to make high pressure
steam. Generators driven by two
turbines use the steam to produce
more electricity.
2. Biosolids from the digesters are
dehydrated and the powder is
burned in a fluid bed gasification
multi-stage combustion chamber.
About 20 percent of the total
biosolids produced are burned in
this process. Ash from this
combustion process is currently
used in an offsite cement
manufacturing process,
Hyperion's total average electrical
production is 20 megawatts.
The City estimates that HERS saves $12
million in electricity costs per year.
Energy Recovery from Biogas
Biogas provides approximately 80 percent
of the energy produced onsite.
Hyperion's anaerobic digesters produce an
average 7.5 million cubic feet per day of
biogas. Under normal operating
conditions, all of the biogas is captured
and used to either generate electricity (via
gas or steam generators) or to make
steam for heating purposes in the plant.
Hyperion's biogas has a fuel value of 600-
650 Btu/cubic foot. Figure 3 is a
schematic of the distribution of the daily
gas production. The schematic also
shows where natural gas is introduced to
augment the fuel supply.
17
-------
Residuals Use and Energy Conservation
Iron compounds are added upstream of
the primary settling basins and to the
digesters to control hydrogen sulfide, at
an annual cost of $1.5 million. Even so,
biogas contains 60 to 100 ppm of
hydrogen sulfide. The high sulfide
content may result from sulfur bacteria in
the collection system acting on the
biosolids produced by upstream water
reclamation plants. Increasing the amount
of iron added to the process tanks is not
economically feasible, so biogas is usually
treated in a Stretford desulfurization unit
to produce a product with a content of
less than 40 ppm of hydrogen sulfide. To
pass it through the Stretford unit, the gas
is subjected to "intermediate" pressure
(40 psi) as it comes off the digesters. The
Stretford unit produces about 50 to 60
pounds of sulfur daily. The annual cost of
operating the unit is $20,000.
After desulfurization, the boilers can
directly use the biogas as fuel to produce
steam for digester heating and biosolids
drying. However, most of the gas is
further pressurized, mixed with natural
gas, land used to power three gas turbines,
each with a capacity of 4,500 kilowatts of
electricity.
Waste heat from the turbines is fed to
heat recovery boilers to make high
pressure steam. Generators driven by
steam turbines use the steam to produce
more electricity. By using this "combined
cycle" approach to produce power from
both gas and steam turbines, the plant
increases its net electrical production by
50 percent over that of a conventional
"simple cycle" power plant (a plant that
uses only one kind of generator). The
fluid bed gasification combustion
chambers which had originally been
designed only to burn solids have been
modified so that they can use biogas as
fuel. Therefore, even when the Carver-
Greeinfield process is down, the gasifiers
can be used to produce steam to power
the steam turbine generators.
18
-------
Residuals Use and Energy Conservation
Figure 3: Schematic of the distribution of daily gas production
HYPERION TREATMENT PLANT
Avg< Daily Gas Distribution
6.1 MCFD
(M2
SCF
0.6 MCFD
_EL
Oxldlzer
0Oo O ฐ. o
ฐ0 o00 0ฐ
7.0 MCFD
Digesters
O.2 MCFD
Flares
0.5 MCFD -
110}
uero
0.6 MCFD
Aux Boiler
0.6 MCFD
Natural Gas
1.6 MCFO
0.5 MCFD
"GTG gas includes natural gas
1.1 MCFD
0.1 MCFD
Domestic
Scattergood
Generating Station
19
-------
Residuals Use and Energy Conservation
The following table shows the amounts of biogas used for each activity:
^f^ti^^t?1^ ^^ ^ ^ft ^ '""* * ' "ฐ '-'' '
V
^> S"* j. "" "^^ > f s "^ ^ "* 5/^ ^ /' ^ S ^ * S ff f ' % W '
Flares
Plant steam
Fluid bed gas afterburners
Fume incinerator
Digester heating
Gas turbine generators
Total biogas production
,~\$' ^ v>% $?"ฃ' -} *
i^1" %"? ' ff * "v" < ^ ซ ' . -
4fff ^*"*'^^tt)aปซ.,/v' //;
;- {ซAtc feet per day> :
200,000
800,000
600,000
0-100,000
800,000 to 1 million
4.1 minion
7 million
* Values are approximate and reflect production during July 1993.
The facility currently uses about 600,000
cubic feet per day of natural gas to
supplement biogas production. This
figure represents about 8 percent of the
total amount of gas burned at Hyperion.
Energy Recovery from Biosolids
Biosolids from the digesters are
dehydrated in one of three trains of a
Carver-Greenfield process, and/or in one
of two steam dryers. The resulting
powder is burned in a three-train fluid bed
gasification/multi-stage combustion
chamber to produce steam. This process
provides, on average, about 20 percent of
the total energy generated onsite. The
HERS solids handling schematic is
presented in Figure 4. The facility has
recently added two new rotary disc steam
dryers to increase biosolids drying
capacity, and thus, energy recovery
capacity.
Digested biosolids are removed from the
digesters and screened; polymer is added
and the screened biosolids are directed
into centrifuges. Solids cake comes out
of the centrifuges with a solids content of
23 to 24 percent A carrier oil transports
cake to the steam-heated drying pathway
where water is evaporated. The Carver-
Greenfield drying system currently
processes 230 to 240 tons of wet
biosofids per day.
**
Approximately one pound of dry powder
is obtained for each 4.3 pounds of steam
fed to the dryers. The powder has an
energy content of 5,500 to 6,000 Btus per
ton, depending on the amount of oil in it.
On average, the dryers produce about 45
tons per day of powder. During July
1993, the facility produced 840 tons of
dry powder and 18,170 gallons of sludge
oil. Efowever, an average two of the
20
-------
Residuals Use and Energy Conservation
three powder combustion trains were
down throughout the month; therefore,
the facility only burned 545 tons of
powder during July 1993.
Dried biosolids are fed into the fluid bed
gasifiers along with a controlled amount
of air. No additional fuel is necessary to
sustain the pyrolization that occurs here.
Additional burn occurs with controlled air
addition in the two afterburners. Hue gas
from the system is passed through heat
recovery boilers to produce steam, which
in turn drives generators to produce up to
10 megawatts of electricity at design
loads. The net power generated is 200
kilowatts per ton of powder.
Powder from the Carver-Greenfield
process must be transported and stored
under nitrogen to prevent autogenic
combustion. The dryers use several
chemicals, including antifbam, antiscale
and dispersant. The total cost of
chemicals for the drying process
(including nitrogen and the oil for cake
transport) is about $35,000 per month.
About 75 percent of the cake (800 wet
tons per day) is hauled offsite for land
application, but ultimately the plant
expects offsite disposal to decrease to
approximately 50 percent. An additional
200 wet tons of solids daily will be
generated beginning in January 1998 as
Hyperion achieves full secondary
treatment. The plant staff expects gas
production to increase by 50 percent over
current levels, to about 12 million cubic
feet per day. By installing two new steam
dryers, the facility will obtain an
additional daily capacity of 350 wet tons
of biosolids. Two new boilers and two 16
megawatt steam turbines will also be
added, bringing the total rated capacity of
the power generation facilities to about 55
megawatts.
Process Modifications
In advanced primary treatment, ferric
chloride and polymer are added to the
primary tanks to improve solids settling.
As a result, primary treatment removal
efficiencies are routinely 85 percent for
total suspended solids and 50 to 55
percent for BOD.
Hyperion has carried out several
modifications designed to increase the
efficiency of energy use at the plant,
including both demand side and
generation side changes. These include:
Reduction of the number of
blowers in aeration tanks
Optimizing loadings to
centrifuges, which have various
design loadings
Minimization of the use of flares
Retrofitting the fluidized bed
gasifiers for use of biogas
Optimizing the effluent pumping
plant.
21
-------
C3
E
-------
Residuals Use and Energy Conservation
Hyperion currently operates three
digesters as two-stage digesters in a
series, and has plans to operate all the
digesters in this manner. This mode of
operation allows reduction of the
retention time while increasing the
destruction of pathogens and production
ofbiogas. The facility has plans to install
egg-shaped digesters as future capacity
becomes necessary, as they expect the
egg shape will allow for better mixing and
require less cleaning.
Modifications are planned to increase the
efficiency of the drying process. The
facility intends to install rotary-disc steam
dryers to supplement the existing steam
dryer system. Rotary disc dryers use
steam-fed discs which rotate within a
large vessel containing dewatered
biosolids cake. The discs conduct heat to
the cake, raising its temperature to the
boiling point of water and evaporating
most of the moisture.
Modifications to the existing combustion
facilities are planned to enable other plant
residuals to be treated. Grit and
screenings may be fed through the
'process to eliminate odors and reduce the
.amount of material that must be disposed
of at landfills. Screenings, which include
a high organic content, are expected to
add to energy generation capacity.
Benefits of the Energy Conservation
Program: Air Emissions Reductions
Hyperion is able to meet stringent local
regulations promulgated by the South
Coast Air Quality Management District
(SCAQMG). SCAQMD regulates air
emissions through health risk assessments.
Hyperion's staff has the technical
expertise to perform these risk
assessments onsite. Staff can experiment
with ways of reducing the identified risks.
Compared to a traditional power plant,
biosolids burning is a cleaner process,
emitting only about 50 percent of the
nitrous oxides that would be expected
from a comparably sized natural gas-fired
plant.. Hyperion staff has found that
burning biogas in the gas turbines results
in lower nitrous oxides emissions than
burning natural gas, because the higher
level of carbon monoxide in the biogas
serves as a sink. Secondary oxidation of
carbon monoxide yields carbon dioxide.
Thermal oxidizers fueled by biogas
control fumes from the drying processes.
Financial Benefits
In July 1993, the power plant produced
11,312,000 kilowatt hours of electricity,
equivalent to about $837,000. Hyperion's
steam generators and gas generators have
a total combined electrical generation
capacity of 25.2 megawatts; however, 17
to 20 megawatts is the normal operating
rate. About 1 megawatt is exported for
sale; the plant uses the remainder onsite.
HERS reportedly saves Hyperion about
$12 million per year hi electricity costs.
23
-------
Residuals Use and Energy Conservation
The following table summarizes the operating costs at Hyperion's biosolids drying facility
during July 1993:
Labor - 59 employees
Chemicals
Utilities
IVTaintm
Total Gross Operating Cost
Energy Production
Total Net Operating Cost
S210.614
47,036
80,620
46389
5384,659
-64,617
$320,042
These costs are based on an estimated value of $62 per ton of powder and $0.69 per gallon
of sludge oil.
Over the period 1992 through 1993,
monthly electricity purchases ranged from
less than zero (when the facility receives a
credit for producing more electricity than
, can be used onsite) to about $460,000.
As an example, in July 1993 Hyperion
consumed an average 389 megawatt
hours daily, and generated an average
365, for a total daily shortfall of 24
megawatt hours. The total cost for
energy during July 1993 was $865,000.
To supplement its onsite production and
make up for the shortfall, the facility
bought electricity at a total value of
$202,000. Thus, Hyperion generated
over 75 percent of the needed energy
onsite during this month.
24
-------
Residuals Use and Energy Conservation
The following table provides a breakdown of electrical usage at the plant in July 1993.
4 . f ^ % *ts> % -. -. f*f
s X A. -*ป%*<, ^_ "^
: .p %-" vx^* *.-v- ^vwv\ $f ,- j. f sv.-, %J.
i"'<^ 5V^>*ซ*^v^/;' 'si,,-.- ,
'%^ f"v"$S 'V' ' '.f'Zr.fJ* ^ ,#. f~- , , .,, sฃf
mmmmj^mmfi^fmfmf^^^^^^^^^^^^^^^m^^^^
^^^^^^^^^^^^^^i^BM^Mi^^B^MMBBBB
Secondary Treatment
Buildlnes/Faclllties
Biosolids Dewaterine
Primary Treatment
Coeeneration
Biosolids Combustion
Digesters
Dehydration
Total
>i % ^'f^"-.
' ' / ' '
f ,t ^^y;\f/>, f , ,4,
'< , */ ''$&& '* ซ, i
{' - , ''-, ,<\. -' '
S298T000
$129,000
S123.000
$80.000
$77.000
$62,000 -
S61,000
$35,000
$865000
:"V ^ >'-ซ",,
- "FซW3tiปgC:ซr:::
^^MltlMlH^,
"'*' '^tปgป *f
34.3
15.0
14.2
9.2
8.9
7.2
7.1
4.0
ioo
The value of the electrical production
from burning biosolids does not presently
cover the costs of processing the biosolids
onsite. As an example, during My 1993
the Hyperion power plant produced
electricity equivalent to $837,000. On
average, 20 percent of the facility's
electricity generation comes from burning
biosolids. Thus, burning biosolids
produced electricity worth $167,000 in
July 1993. During this month Hyperion
processed 4,493 tons of solids cake
through the drying facility, at a (gross)
cost of $384,700. The net cost of
handling the biosolids onsite was
$217,700 for the month. At $35 per ton,
it would have cost only $157,300 to send
the 4,493 tons of solids cake offsite,
saving about $61,000 over costs to
process the biosolids onsite.
However, the economics of biosolids
handling at Hyperion will change with the
planned additions of dryers and other
energy recovery equipment that can
handle more biosolids. These changes are
expected to make the process competitive
with ofisite management. The HERS
staff estimates that with two drying trains
operating, 5 to 7 MW of electricity
(worth about $3 million) could be
exported. Costs to process biosolids
onsite should fall as low as $109 per dry
ton, compared to $ 132 for offsite
management.
25
-------
Residuals Use and Energy Conservation
Power generation varies based on needs
for equipment maintenance and repair. In
three of the 12 months in fiscal year 1992-
1993, HERS generated more power than
Hyperion consumed. Figure 5 contrasts
the power generation by HERS with
Hyperion's power consumption hi the
period from August 1992 through July
1993.
The gas turbines require a major overhaul
every 10,000 to 12,000 hours (1 to 1.5
years). This schedule is more frequent
than what would be required for a larger
sized turbine. Thus, hi this respect,
HERS does not achieve the economy of
scale that would be seen at a conventional
power plant.
14
12
KWH (Million*)
Aug Sep Oct Nov Dee Jan Fett Mar Apr May Jun Jul
I 92 I 93
Power Generation
Power Consumption
Figure 5: Power generation versus consumption at Hyperion
26
-------
Residuals Use and Energy Conservation
Sunnyvale Water
Pollution Control Plant
The Sunnyvale Water Pollution Control
Plant (WPCP), in California incorporated
use of biogas in its original plant
construction in 1956, and has been
successfully carrying out energy
conservation ever since. Recently, the
City has implemented or planned some
unique new methods to increase energy
recovery and further the pollution
prevention and water conservation goals
of the plant. These innovative energy
recovery options include transfer of
suspended solids biomass harvested from
the oxidation pond effluent to the
digesters to increase gas production, and
plans to extend the energy recovery
operation to the use of gas from the
adjacent municipal landfill. Sunnyvale
expects to be able to meet 100 percent of
the plant's energy demands through use of
a combination of landfill gas and biogas.
Facility Description
The original 7.5 MGD primary plant was
designed to service a population of
1O,000 and to provide separate treatment
for a seasonal cannery load of 4.0 MGD.
The plant was equipped with two 55-ft-
diameter anaerobic digesters and two
biosolids drying lagoons. Biogas
produced by the anaerobic digestion
process was collected and piped to
operate three engines, each of which
drove a 100-hp raw wastewater pump and
a 50-hp pre-aeration blower. Engine-
driven pumps were used because they
could cope with the great range between
minimum and maximum flow rates (1 to
50 MGD) and could provide the flexibility
required to operate separate domestic and
seasonal wastewater treatment systems.
This flexibility eliminated the need for
intermittent pumping and large wet wells.
For the first few years of operation, the
pump engines operated on biogas 20 to
40 percent of the time. The facility used
waste heat from the engines to produce
steam for digester heating and for space
heating of the plant's main building.
In the early 1960's Sunnyvale's population
increased by 500 percent to 60,000
people. Plant expansions in 1965 and
1968 increased the treatment plant's
capacity to 15 MGD, incorporating
primary and secondary wastewater
treatment. These expansions included a
third 55-ft-dianieter anaerobic digester
and a 440-acre oxidation pond with a
four-pump circulation pumping station
and a remote three-engine-generator
facility to provide power for the pumps.
The three engine-generators use either
natural or biogas for fuel.
Also hi 1968, the plant's solids handling
facilities were improved with the addition
of a third biosolids lagoon and a hot water
reservoir system to replace the original
direct steam injection and heating system.
After this improvement, the biogas supply
provided an estimated 50 percent of the
engine fuel and plant-heatin requirements.
The City increased the plant capacity and
constructed a fourth 70-ft. diameter
anaerobic digester in 1972.
27
-------
Residuals Use and Energy Conservation
In 1978, due to substantial upgrading of
effluent discharge regulationsincluding
the ammonia removal requirements-
upgrades were made to add fixed growth
reactors (FGRs), air flotation units
(AFTs), dual media niters, and breakpoint
chlorination and dechlorination
equipment. As part of this construction,
the facility modified the electrical
distribution system to allow the
circulation pump engine-generators for
the oxidation ponds to be used to
supplement general electrical power
needs. Currently, aeration of the
oxidation ponds is done only on an as-
needed basis.
Sunnyvale increased treatment capacity to
22.5 MGD when the population exceeded
100,000, with a final upgrade to 30.0
MGD in the early 1980's. Seasonal
treatment capacity for cannery discharges
was no longer needed when canneries
were relocated out of the service area in
1983. Due to water conservation
activities by domestic, commercial and
industrial users, the annual average
influent to the plant in 1992-1993 was
13.4 MGD.
*
Description of the Technologies
The energy recovery system at the WPCP
combines the use of biogas as an engine
generator fuel and boiler fuel, and uses
heat recovery from engine-cooling and
exhaust stack systems to supplement plant
energy requirements. The components of
the energy recovery system are discussed
below.
Biogas Production and Use
A design goal for the original Sunnyvale ,
wastewater treatment plant was to make
maximum use of biogas. This objective
has remained an important consideration
in each of the subsequent plant
modifications. The 1956 plant included
two digesters; in the 1960's three gas-
fueled engine-generators were added to
the plant to power recirculating pumps for
the. oxidation ponds. The remote power-
generating facility was provided because
the recirculation pumps are approximately
one mile from the digesters. A full
parallel electrical distribution board is
present so that any or all of the plant
electrical circuits can selectively use
power generated either within the plant or
commercially.
Digesters are operated at 100ฐ Fahrenheit
as completely mixed primary units. Each
digester is equipped with four gas tubes
that run from the floating dome top to the
bottom of the digester. The tubes
facilitate agitation and mixing. Baffle
plate condensers are used to remove
moisture from the biogas. Sunnyvale has
some gas storage capability at the tops of
the digesters, and at present has no plans
to add external gas storage.
Currently, a blend of biogas and natural
gas powers three 110 kilowatt
"enginators," or engine generators, which
together produce 330 kilowatts of power.
Natural gas is purchased from the local
supplier and blended with air to lower the
heating value to about 550 Btu, so that it
is equivalent to that of biogas. The
biogas piping system joins with the
28
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Residuals Use and Energy Conservation
natural gas piping system, and the two
gases are blended to maintain a constant
flow to the pump and generator engines
and to maintain adequate pressure in the
gas header. The biogas piping system
associated with each of the four digesters
is equipped with a flow meter, flame trap
and pressure relief valve. Headers are set
to maintain eight inches of water column
pressure. If the water column exceeds
eight inches, the excess gas is flared
through pressure relief valves which are
automatic and set to maintain the optimal
header pressure. The flares can be
operated manually if the automatic system
fails. .
Recent plant data show that biogas
production for the 12 months between
December 1991 and November 1992
averaged 172,000 cubic feet per day. The
monthly average biogas production varied
from a low of 126,000 cubic feet per day
in July, to a high of 235,000 cubic feet per
day in November. The blend of biogas
and natural gas meets roughly 30 percent
of the plant's 1,000 kilowatt energy
demand.
'In 1964, total gas consumption was
.approximately 60 million Btu per day in
1964, of which only 1 million Btu was
supplied by natural gas. Li 1976, total
consumption averaged 107 million Btu
per day, of which approximately 22
million Btu was supplied by natural gas.
Over this period, the use of biogas
reduced Sunnyvale's daily natural gas
consumption on average by 60 million
Btu. This is equivalent to the daily
natural gas use of 150 typical American
households. Figures from 1991-1992
show that biogas production at
Sunnyvale has continued to increase,
averaging about 95 million Btu per day.
This increase occurred despite the loss of
the canning wastestream, which
contributed to increases in gas production
before the early 1980's.
Increases in biogas production since the
early 1980's are largely attributable to two
activities. First, Sunnyvale conducted
studies that concluded that suspended
solids removed from the oxidation pond
effluent by the AFTs could be fed to the
digesters. Approximately 30 percent of
the solids removed by the AFTs are
directed to the digesters. The plant
recycles the remaining 70 percent of the
solids to the ponds. Sunnyvale calculates
that the energy which could be obtained
from digestion of these solids is close to
that obtained from primary biosolids. The
City estimates that gas production will
increase a further 25 percent when all of
these solids are sent to the digesters, to
approximately 224,000 cubic feet per day.
Expressed in thermal units, estimated
future biogas production is 5.1 million
Btu per hour.
In the second effort at increasing gas
production, Sunnyvale abandoned the use
of alum for coagulation in the AFTs, and
substituted polymer. Elimination of alum
reduced the toxicity of metal inhibition
and has allowed for increased gas
production. The dependability of gas
production and the available digester
capacity has increased. In addition, the
polymer is an organic compound which
contributes to the energy recovered from
digestion.
29
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Residuals Use and Energy Conservation.
Waste Heat Recovery
An important design feature of the
Sunnyvale plant is the use of waste heat
from the gas-fueled engines to provide
both process heat for the digestion and
chlorination systems, and space heating
for various buildings at the treatment
plant. Currently, waste heat is recovered
in three systems: (1) pump-engine heat
recovery, (2) generator-engine heat
recovery and (3) stack heat recovery.
These systems may be supplemented as
required by the low-pressure, gas-fired
steam boiler; however, typically more
heat is obtained from the heat recovery
systems than is actually needed in the
plant. Heat from all sources is converted
into hot water for use throughout the
plant. Presently, the plant does not use
excess heat for cooling needs.
All engines operate on high-temperature
ebullient cooling (212ฐ to 220ฐ F).
Cooling water circulates through the
engines by convection and the lifting
action of steam bubbles. The main pump-
engine heat recovery system reclaims the
waste heat from both the engine's cooling
system and the engine's exhaust-silencing
system. The system operates at a slight
positive pressure (5 to 7 Ib/in2), and the
temperature of the circulating cooling
water leaving the engine is always above
212*F. Heat is recovered from the
system by transferring it from low-
pressure steam to hot water in a
condenser heat exchanger. Excess heat is
discharged as steam to the atmosphere
through a pressure relief valve.
The generator-engine heat recovery
system operates at atmospheric pressure;
therefore, the temperature of the cooling
water leaving the engine is 212ฐ F.
Operation at atmospheric pressure is
much simpler than operation at higher
pressures since the open steam discharge
pipe from the condenser acts to provide
both pressure and vacuum relief.
Atmospheric pressure operation
eliminates the ability to recover the waste
heat from the generator-engines1 exhaust
silencers. However, this heat is not
needed for use in the plant. When the
heat exchanger of the generator-engine
condenser cannot cope with all the heat
recovered, the excess is discharged to the
atmosphere as steam.
Isolation of the engine-cooling system
from the hot-water-heating system
assures the integrity of each system. Hot
water is piped throughout the plant as
part of a recirculating heat reservoir
system. Secondary heat loops, which
operate in parallel with the main
circulation system, are equipped with
their own blending valve and circulating
pump and are provided to satisfy process
and space heating requirements. The
main heat reservoir and the secondary
loops for chlorine evaporation and space
heating operate between 180ฐ F and 210ฐ
F, while the secondary heat loops for
biosolids heating are maintained between
140ฐFandl60ปF.
Operation and Maintenance
The original plant influent pumps were
designed to pump a minimum flow in dry
weather of 1.0 MOD and a peak flow in
storms of 50 MOD. During the past 20
30
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Residuals Use and Energy Conservation
years, the City has reduced infiltration
into the sewer systems such that minimum
flows are now 6 MOD while peak flows
are only 32 MOD. Three dual-fuel
engines each drive a 100-hp raw
wastewater pump and a 50-hp pre-
aeration blower. During installation in
1956, the pump engines used dual
suction-type carburetors. The weight of
the digester covers maintained two inches
or more of water column pressure in the
digester system. Engine fuel was changed
from biogas to natural gas when the
pressure in the biogas system fell below
two inches and reverted to biogas when
the biogas pressure built up to four
inches. Waste gas burners came on when
biogas pressure built up to 8.5 inches of
water column pressure.
In 1969, the City installed three 330
kilowatt-capacity engine generators to
provide power for the four 60-hp pond
recirculation pumps. The carburetors on
the engine generators were designed to
use the same fuel system as the main
pump engines. However, booster gas
compressors were installed to supplement
natural system pressure. These
compressors supplied gas to each engine
carburetor at a much higher working
pressure. Problems occurred almost at
once with this fuel system. Despite good
maintenance, the gas compressors tended
to draw air around the shaft packing,
causing operational problems with the
carburetors and with the control of the
digesters. The plant abandoned use of the
booster gas compressors due to these
operational problems and phasing out of
the original carburetors by the
manufacturer. The facility increased gas
, piping sizes and installed new single,
positive pressure carburetors on all six
engines.
The new carburetors operate as follows:
the fuel supply is switched from biogas to
natural gas when the pressure in the
biogas system falls to two inches of water
column pressure. The fuel supply returns
to biogas when the pressure in the biogas
system increases to six inches of water
column pressure. This system maintains
at least two inches of water column
pressure within the biogas system. As
long as this minimum pressure is
maintained, there is no danger of air being
drawn into the digester system.
Sunnyvale has not made any efforts to
upgrade to energy efficient engines
because of other facilities' experiences
that such engines are not successful in the
long run. However, other energy efficient
equipment installed at the plant has
proven successful. Special chlorine
injectors are used to supply chlorine into
the flow system, providing a cost savings
of approximately $20,000 per year. The
propellers associated with the main sewer
pump system have been coated with a
coating that reduces drag and increases
water flow and pump efficiency.
Sunnyvale uses a preventive maintenance
schedule which is designed to identify
potential problems before they occur. A
positive feature of the system has been the
low maintenance requirement over the
years of operation. The three engine
generators essentially run full-time. The
main engines running time is more
variable, but works out to about one and
31
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Residuals Use and Energy Conservation
one-half engine on full-time.
The main engines installed in the 1950's
and the engine generators installed in the
1960's are still in use today. These
engines have been through a complete
overhaul and several rebuilds, and are in
good operating condition. Engine failure
has never been a problem or prevented
the plant from providing treatment. The
main pump engines are scheduled for
overhaul every six years, based on the
number of running hours. Plant staff
recondition the engine generators every
four years.
Other Conservation and Pollution
Prevention Activities
Sunnyvale is currently working on several
other energy conservation activities
including: constructing a 1.6 megawatt
power generation facility that will use
methane gas from the adjacent landfill,
combined with anaerobic biogas from the
WPCP to fuel engines and generators that
supply electricity to the WPCP, a $14
million water reclamation project, and
construction of a tile dewatering facility.
i
. Landfill Gas Production
The Sunnyvale WPCP is located next to
the municipal landfill. The landfill has
received its final load of solid waste, and
was closed on October 1, 1993. Landfill
gas (LFG) is produced by bacterial
decomposition of the organic portion of
refuse in the absence of oxygen. Once
begun, the rate of decomposition reaches
a peak within a few years, then gradually
declines as the decomposable organic
material is depleted. In inactive landfills
such as Sunnyvale, the production of LFG
is dependent on the portion of previously
disposed refuse which has yet to be
converted to LFG.
LFG is a mixture of methane and carbon
dioxide, with trace contaminants. The
concentration of methane in undiluted
LFG has been measured between 55
percent and 65 percent at the Sunnyvale
landfill. Trace contaminants in LFG can
affect: engines primarily due to the
presence of chlorine (carried in
compounds such as trichloroethylene),
which produces hydrochloric acid during
fuel combustion. An advantage to LFG
as a generator fuel is its much lower
hydrogen sulfide concentration compared
with that of biogas. The concentration of
hydrogen sulfide in Sunnyvale's biogas
averages 1,270 parts per million, but
when blended with LFG will result hi a
reduced concentration that should lower
emissions and improve equipment
longevity.
To meet Bay Area Air Quality
Management District (BAAQMD)
regulations, at present all LFG is flared to
the atmosphere. The proposed energy
conservation project will collect LFG and
use it together with biogas from the
WPCP anaerobic digesters to fuel engines
and generators that supply the WPCP
with electricity. All of the energy needs
of the WPCP will be met through a
combination of these sources. The City
expects that LFG will also meet some
energy demands of the new solid waste
transfer station next to the WPCP. The
collection potential for LFG in 1095 is
32
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Residuals Use and Energy Conservation
estimated to be 1.2 million cubic feet per
day. The City estimates that present
biogas energy production at the WPCP
represents only one tenth of the energy
available from LFG.
LFG collection and use will have to be
conducted in compliance with BAAQMD
Rule 8-34. LFG not used as fuel must be
burned or otherwise treated in compliance
with the LFG system BAAQMD
Operating Permit in effect at the time.
The City expects that LFG generated by
the landfill will decline during the 20-year
life of the proposed power generation
facility, due to gradual and continuing
depletion of organic material in the
landfill. Despite this decline, the City
estimates that 100 percent of the energy
demand of the Sunnyvale wastewater
treatment plant, all of the power for the
water reclamation facility (discussed
below), and some power for the municipal
waste transfer station will be met through
use of LFG and biogas. The City projects
savings in reduced purchases of electricity
to be $826,400 in FY 94-95.
As part of this project, the plant will be
fitted with two new 800-kilowatt low
emission lean burn engine generators, at
an estimated cost of $1.5 million. The
total cost of the LFG project is estimated
at $4.47 million. The project has received
a grant from the California Energy
Commission for $500,000. At the
$826,400 annual savings in electrical
costs, project payback is anticipated in
approximately six years.
33
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Residuals Use and Energy Conservation
Biosolids Dewatering and Drying Bed
System
Sunnyvale WPCP is converting its
original biosolids drying beds to a screen-
type biosolids drying system. The new
drying system will be made up of two-
inch thick tiles, with fine slits to allow
water to pass through to the drainage
system. Polymer will be added to
biosolids as it comes off the digesters;
mixing will occur in the transfer line to
the biosolids beds. The tiles will be laid
across the bottom of the biosolids drying
bed and will induce separation as solids
are captured on the surface and liquid
drains through the slits in the tiles. This
system is expected to reduce biosolids
volume to 18 percent (by weight) of its
original total volume.
The City selected tile screening for
dewatering its biosolids based on cost and
applicability to the biosolids'
characteristics and final reuse. The cost
of installing the tile dewatering system is
about half what a belt press of comparable
capacity would cost. Operation and
maintenance costs for the tile dewatering
system are low; two pumps and a grinder:
are the only energy expenditures
associated with this dewatering system.
The dewatered biosolids will be used as
final cover for nearby municipal landfills.
34
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Residuals Use and Energy Conservation
Sanford Big Buffalo
Creek WWTP, North
Carolina
FacUity Description
The Big Buffalo Creek (BBC) WWTP
provides wastewater treatment for a
population of approximately 17,000
people. The plant has an average influent
flow of about 3.52 MOD, and a design
peak flow of 6.8 MGD. During major
rainfall events inflow and infiltration (I &
I) may cause the flow to peak at 12
MGD. The facility was constructed in
1973 and then upgraded from 1989 to
1992. BBC is a tertiary facility with
mechanical bar screening and grit
removal, extended aeration, secondary
clarification, mixed media filtration, and
aerobic sludge digestion. Effluent is
chlorinated before discharge to the Deep
River.
History of the Energy Conservation
Program
During the late 1970's several U.S. oil
companies violated price controls. Due
to the subsequent litigation by the U.S.
Government against the oil companies,
certain companies were assessed and paid
large settlements. The monies were
dispersed, through a U.S. Department of
Energy grant to the individual states.
During the years of 1983 to 1986, the
North Carolina Department of Economic
and Community Development, Energy
Division, used part of the grant to
conduct on-site energy audits of 15
wastewater treatment plants and three
water treatment plants.
BBC has carried out several energy
conservation actions since 1985, many as
a direct result of the energy audit. The
audit found that the plant components
which consumed the major power were
extended aeration (70%), influent
pumping (17%), aerobic digestion (5%),
sludge pumping (3%), and small
miscellaneous uses (5%).
35
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Residuals Use and Energy Conservation
The energy audit made the following recommendations;:
Alternative A-l: A sluice gate should be installed to limit the excess storm water
received at the WWTP during rainfall events. The excess flow should be bypassed
to the receiving stream rather than being treated. This action would reduce
wastewater pumping, return activated sludge pumping, chlorine usage, and aerobic
digester supernatant pumping. The estimated installation cost was $7,000 and the
estimated annual savings $1,200. The calculated payback was six years.
Result: A sluice gate was installed, however, the excess volume was backed up in
the collection system rather than bypassed. The influent was then treated as a steady
flow. In a recent upgrade the sluice gate was replaced with a "Beck" valve which
automatically adjusts to return part of the influent flow to the influent wet well to
maintain a constant head level and therefore constant pump operation. Continual
pumping at a stable head conserves energy by eliminating electrical surges.
Alternative A-2: A low head hydro-power producing system (turbine) should be
installed on the discharge. This would result in the generation of 6 kilowatts of
electrical power at a flow of 2 MGD. The estimated installation cost was $61,000
and the estimated annual savings $4,400. The calculated payback was 13.8 years.
Result: The WWTP did not act on this recommendation.
Alternative A-3: A microprocessor-based energy management system should be
installed which would control selected equipment to reduce power demand levels.
The estimated installation cost was $ 15,500 and the estimated annual savings
$12,000. The calculated payback was 1.3 yeans.
Result: A process control system was installed which reduced the power demand of
the extended aeration process. This action is addressed in greater detail under
Alternative C-l below.
ป " '/-,*
Alternative A-4: The laboratory building should have storm windows installed,
walls insulated, and an HVAC control installed. The payback was over 10 years and
the energy audit calculated that the expense could not be justified.
Result: The WWTP enacted some of these recommendations during the plant
upgrade. ,
Alternative B-l: This alternative had four options. The first three options are based
on the field tests which showed influent pump No. 2 to be the least efficient. Option
one recommended the replacement of the influent pump station No. 2 pump impeller
36
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Residuals Use and Energy Conservation
with a smaller impeller. Option one had an estimated installation cost of $1,600 and
an estimated annual savings of $340. The calculated payback was 4.6 years.
Option two recommended that pump No. 2 be used only during times of excessive
storm water events. This option had no payback. Option three recommended the
replacement of pump No. 2 with a variable speed energy efficient pump.
Result The first option was selected by the WWTP and the impeller size was
reduced. The result was that more than one pump operated at a time. The impeller
size reduction proved to be beneficial during dry weather, however, during wet
weather the pump cycled at a rapid rate which resulted in increased energy costs. ,
During the plant upgrade the pumps were replaced with high efficiency winding
pump motors.
Alternative B-2: Archimedes screw pumps are used for the return activated sludge
(RAS). The audit recommended .that the aeration basin mixed liquor suspended
solids level be reduced from 6,000 mg/L to 4,000 mg/L to reduce the volume of
RAS to be pumped. The estimated annual savings was $2,500.
Result The screw pumps were replaced with centrifugal pumps during the upgrade.
Alternative B-3: The energy audit studied the feasibility of replacing the pump
impellers at the waste activated sludge (WAS) pumping station. The audit
concluded that this action was not justifiable.
Result No action was taken. However, during the plant upgrade the pump station
was replaced. , '
Alternative C-l: In comparison to other extended aeration facilities the WWTP
consumed a higher amount of energy (2.1 kilowatts) per pound of BOD5 stabilized.
Additionally, the aeration process was found to consume more energy than any
other plant component. It was recommended that a microprocessor-based process
control system be installed. The system should be capable of process control, load
management, preventive maintenance reporting, records management, and alarm
monitoring. The process control should be based on the aeration basin dissolved
oxygen (DO) content which should be monitored continually. The estimated
installation cost was $31,500 and the estimated annual savings $29,000. The
calculated payback was 1.1 years. The audit also proposed to operate only one of
the two aeration basins and to operate process control according to mean cell
residence time (MCRT).
37
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Residuals Use and Energy Conservation
Result: A process control system was installed which monitored and controlled the aeration
according to DO, low flow, and high flow conditions. One of the aeration basins was
removed from service and is currently used for biosolids storage.
Alternative C-2: The audit studied the feasibility of replacing the mechanical
aerators with diffused aeration.
Result: The payback was more than 15 years and the energy audit concluded that
the action was not justified.
BBC considers the process control system
for automated aeration monitoring and
control to be its most successful energy
conservation mechanism. The control
system automatically reduces the aeration
basin DO content to the lowest level
which will still achieve optimum
wastewater stabilization. Other aspects of
BBC's energy conservation program
include:
A time of use on-peak/off-peak
load management system
Upgrade of pump motors to high
efficiency windings and low
voltage starters
Addition of recirculation to the
influent pump station to achieve a
constant electrical load
Replacement of the mercury vapor
lighting with sodium lighting
Use of energy efficient windows in
the operations building,
Recent pump upgrades at two lift
stations.
38
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Residuals Use and Energy Conservation
A summary of the BBC electrical usage and cost before energy conservation is shown in the
following table (taken from the original energy audit report).
" , \~3,C- ~, ,V ^ *""", '
- ' ,' ' S^ ' ' ป , ' , *
" SHXING^ PERIOD I982-S3 . -
. . . ? . % A ^ '"
198210/16-11/15
198211/16-12/15
198212/16-1/15
19831/16-2/15
19832/16-3/15
19833/16-4/15
19834/16-5/15
19835/16-6/15
19836/16-7/15
19837/16-8/15
19838/16-9/15
19839/16-10/15
TOTALS
12 MONTH AVERAGE
2 YEAR AVERAGE
' '-" *.*;, V^\?';,v
% ^'f ' ซ' 'ป,S;j 's. '?',ฃ, *
" ? nraEMRMHbr^ ,' - ;
^ .ซ- ป ^ - < sซ.^ ,
180,000
181,500
160,750
199,000
191,000
216,500
218.000
193.500
205,500
186,000
184,000
205.000
2321.250
193,437
197,812
I BILLING
I^IHBM&KB -
:' -rt^w
618
390
380
370
385
480
480 ,
465
460
450
430
455
5363
447
454
> ''
v- \'
- - COST
' JIMVI-
9,772
7^81
6,278
7,724
7.569
8^80
8340
8,006
8,636
9,091
8,900
9,681
$101,058
S8.422
S8 755
'Average power cost (based on kwh) = $0.04
Average cost/MG treated .=$117
Average kwh/MG treated = $2695
KwMb BOD stabilized = 2.1
The plant has also improved operators' skills through involvement with energy conservation
equipment installation contractors. The involvement developed a working interest in the
energy saving equipment and motivated the operators to become more energy-aware.
39
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Residuals Use and Energy Conservation
Description of the Technologies
The process control system consists of an
Andover controller unit which
communicates with a laptop computer
(386 microprocessor). The unit is
accessible to the operational staff and
chief operator. The microprocessor is
connected to a modem which allows the
chief operator to monitor and adjust
parameters from his home. The system
controls the extended aeration basin
aerators according to DO, high flow, and
low flow. DO information is obtained
from a permanent self-cleaning, DO probe
which is located toward the effluent end
of the extended aeration basin. The
facility staff anticipates that the probe will
require replacement in the future at a cost
of $1,200 to $1,500.
Target DO in the aeration basin is 1 mg/L
to 4 mg/L. Energy is conserved through
reduced operation of the four 100
horsepower, low speed, mechanical
aerators. Previously, the DO level was
collected manually with less frequency
which could result in excess aeration.
The system has an approximate five to ten
minute delay which requires a stable DO
before adjusting the aeration through
control of the aerators. The delay is to
eliminate short ofFon cycles of the
aerators. The delay is automatically
overridden by the low DO mode as
necessary to start additional aerators.
The plant staff conducts a manual check
of the aeration process DO content three
times daily at four locations in the basin.
This manual collection of DO readings is
with an independent meter to assure no
malfunction of the controller system has
occurred.
The system monitors flows from many
locations in the wastewater plant. If the
high flow exceeds a preset volume of
approximately 8.0 MGD the final aerator
in the aeration basin is shut off. This
allows the mixed liquor suspended solids
to settle out and be stored in the aeration
basin during excess flows. This action
conserves electricity and greatly reduces
the effluent suspended solid level during
high flow events. When the flow returns
to normal the aerator is started and again
suspends the solids. The flow control
also has a delay to eliminate short cycle of
the aerator. During low flows, if the
process is stable, .the process control
system continues to operate from the DO
input:. However, the system alternates the
aerators in service. Regular operation of
all the aerators should extend their life.
Other major processes are also operated
by the process control system. The
system monitors the tertiary filters for
flow rate to determine optimal timing for
backwashing. The aerobic digester has
two 100 horsepower mechanical aerators.
Aeration was controlled by the process
control system before the WWTP
upgrade, but was not tied into the system
after the upgrade. The biosolids storage
basin is not automatically controlled by
the process control system, however,
following a manual start, the controller
operates the four aerators as mixers. The
process control system also can graph and
print any variable, generate daily reports,
and generate histories of variables.
40
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Residuals Use and Energy Conservation
In response to the energy audit concern of
inflow and infiltration-induced high flows,
a sluice gate was installed to achieve flow
equalization during rainfall events. The
gate caused the excess flow volume to
backup water in approximately four miles
of the collection system. This reduced the
surge and allowed a constant volume to
be pumped during the storm event, which
reduced the electrical consumption.
During the facility upgrade the sluice gate
was removed and the influent pump
station was modified. An Allen Bradley
controller was added to the influent pump
station. Also^ an automatic "Beck" valve
was installed to maintain a constant head,
of approximately ten feet, in the influent
pump wet well. The valve uses a sonic
meter to detect the head in the influent
wet well and then recirculates a variable
volume of the flow back to the wet well.
This allows the influent pumps to run
continuously, in a steady state, and
achieves a constant electrical pump load.
It does not result in a reduced RAS
pumping,'reduced chlorine usage, or
reduced aerobic digester supernatant
pumping, as recommended in the study.
During the facility upgrade, many pump
motors were replaced with motors which
have high efficiency windings and low
voltage starters. The Gasters Creek
Pump Station pumps were replaced with
high efficiency, higher capacity centrifugal
pumps. The Little Buffalo Creek Pump
Station pumps were replaced with high
efficiency submersible pumps. The RAS
screw pumps were replaced with
centrifugal pumps. The original RAS
pump station was then placed into service
as the WAS pumping station. The screw
pump belt drives, which experienced
some slippage, were replaced with direct
drive units to conserve energy. The plant
also replaced the mercury vapor yard
lighting with energy efficient sodium
vapor lighting, and installed energy
efficient windows in the operations
building.
Process Modifications
The process control system has saved
energy, improved the aeration process
and reduced the effluent suspended solids.
From October 1981 to October 1983 the
annual average effluent parameters were
BOD5 = 12.5 mg/L, TSS = 26.5 mg/L,
NH^ = 0.72 mg/L, and DO = 8.2mg/L.
Currently the annual average effluent
parameters are BOD, = 8.23 mg/L, TSS =
16.3 mg/L, NH3N= 0.54 mg/L, and DO =
7.13 mg/L. This is likely the result of
maintaining a uniform DO in the extended
aeration basin, maintaining a DO which is
optimum for stabilization, and retaining
solids during high flows. The increased
solids increased the loading to the aerobic
digester by 15 to 25 percent.
Another process modification which has
saved energy and improved the effluent
quality is the removal of one. aeration
basin from service. The aeration basins
were designed to treat 10 MOD, while the
average flow was 4.56 MOD. Use of a
single aeration basin allowed operators to
match the flow volume with the design.
The MCRT was reduced, which
conserved energy through less pumping.
This reduction should also improve the
effluent suspended solids through a
reduction of pin floe.
41
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Residuals Use and Energy Conservation
Financial Benefits
BBC staff found that actual installation
costs for the implementation of the energy
audit recommendations were close to
estimated costs. Actual payback time for
the process control system was less than
the 1.1 years originally estimated.
An operating budget increase has been
unnecessary over the past five years.
StaiFbelieves that energy savings have
contributed greatly to stable operating
costs. The two-year average monthly
electrical cost during 1982-83 was $8,755
(at$0.044 per kilowatt hour). Monthly
electrical costs averaging $4,200 over the
period July 1993 to April 1994 reflect the
effects of energy conservation measures
on electrical costs at the BBC plant.
42
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Residuals Use and Energy Conservation
Seattle Metro Renton
Water Reclamation Plant
Facility Description
Unlike the other wastewater treatment
plants in this study, the Seattle Metro
East Division Reclamation Plant at
Renton does not use its biogas onsite for
heating and/or cooling. Instead, Metro
has worked out relationships with local
utilities that have made it more cost-
effective to sell the gas for ofisite use arid
replace its potential in-plant use with
electrically operated heat pumps that
remove heat from effluent. The
economics that make this feasible depend
on the low prices for electricity in the
Seattle area, and grants and other
assistance from the electric utility. Metro
also has developed a unique program,
called Metro 77ier/n, which uses effluent
for offsite heating and cooling of
buildings at privately owned facilities.
The Renton plant treats about 66 MGD of
wastewater. The plant is undergoing
expansion, due to be completed in 1996,
which will increase its current design
capacity of 72 MGD to 108 MGD. Plant
processes consist of primary settling,
aeration, secondary settling, chlorination,
and dechlorination. Biosolids are treated
in dissolved air flotation thickeners,
followed by anaerobic digestion and belt
filtration. In 1986, a 12-mile effluent
pipeline to Puget Sound was completed.
Pipeline construction included eight reuse
taps spaced along its length (Figure 6).
Effluent is discharged two miles offshore
in 580 feet of water.
Seattle Metro has undertaken several
energy conservation activities at its
Renton plant, including insulating the
digesters, recovering waste heat from
blowers, using energy efficient motors
and variable speed drives, and installing
motion detectors to control lighting in
conference rooms.
Energy Recovery from Biogas
The Renton plant's four anaerobic
digesters generate 1.2 million standard
cubic feet per day of biogas. The facility
scrubs the biogas to remove carbon
dioxide, and sells the resulting 99 percent
pure methane to the local gas utility.
Metro receives approximately $1,100 per
day for the scrubbed gas. The biogas
potential for onsite heating use is replaced
with four 600-horsepower electrically-
operated heat pumps. These heat pumps
supply 135 degree water to a closed loop
system that meets 90 percent of building
heat requirements, and also maintains ten
million gallons of biosolids in four
digesters at 96 degrees. The cooler water
that has passed through the heat
exchangers is used in the gas scrubber
unit to increase its efficiency.
43
-------
44
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Residuals Use and Energy Conservation
Figure 6: Location of Renton's 12-mile effluent pipeline
r. T- L
Beacon
~~mr --'
MERCER
ISLAND
Boeing':.,,
' Field * ' "
MICHIGAN ST^ *
Fauntiorpy^ _ . .' _.
OXBOW IMTERCHANGE
MARGINAL WAYS
43RDAVESBRIDGE
EAST-DIVISION
RECLAMATION
-PLANTAT .. '.'
\SEA-TAC ;'--
AIRPORT I
BOEING
LONGACRES
TuKtvlla J pARK
MetroTherm/Rause
Tap Location
Customer
Forcemain
Treatment
Plant
-------
-------
Residuals Use and Energy Conservation
The heat pumps produce four times more
heat than would be obtained per watt of
power consumed by directly converting
electricity to heat (3.4 Btus are obtained
per watt hour). Metro anticipates that the
efficiency will decrease when it changes
from the current refrigerant (R12) to a
new refrigerant (134A) that does not
contain chlorofluorocarbons, because the
134A refrigerant is not as efficient in heat
transfer.,
Advantage of Cold Water for Biqgas
Scrubbing
\
The carbon dioxide scrubber consists of a
vessel into which secondary effluent is
injected under 300 psig. Digester gas is
fed into the vessel, and during contact
Between the gas and the effluent, pressure
forces the carbon dioxide into solution in
the water. Cleaned methane gas is drawn
off. To achieve maximum efficiency,
cooled effluent that has passed through
the heat pumps is used in the scrubber,
since cooler water can hold more gas in
solution.
The heat pumps drop the temperature of
'the effluent flowing through them by 10
.degrees Fahrenheit at a flow rate of 960
gallons per minute. This chilled water is
fed into the digester gas scrubber. Metro
has found that savings can be achieved by
operating a heat pump solely to produce
chilled water to ensure that the digester
gas is adequately cleaned to
specifications. Without chilled water,
summer heat conditions would cause
reduced scrubber efficiency resulting in
wasting some gas that does not meet sale
specifications. '
The MetroTherm Program
The plant's effluent is available for use in
a unique program called MetroTherm.
MekoUterm is designed to provide
treated wastewater effluent for heating
and cooling of buildings, both at the
wastewater treatment plant and offsite at
privately owned facilities. Taps in the
effluent pipeline were placed to allow
facilities to draw from and return effluent
to the pipe.
In 1982, the State of Washington began a
"District Heating and Cooling" (DHC)
program to encourage communities to
develop centralized hot water production
to serve various energy needs. The
Washington State Energy Office (WSEO)
implements this program to provide
project guidance, marketing support and
funding sources for development of
centralized energy. WSEO has provided
grants and assistance and will continue to
provide support to Metro with a $25,000
grant and $25,000 in services in 1994.
Metro also received grants from the
Bonneville Power Administration (BPA),
which provided funding for initial
feasibility studies that determined
placement for the effluent pipeline taps.
Facilities can use effluent in three modes:
heating and cooling, cooling only, or
heating only, depending on individual
customer needs and efficiencies
associated with each site. A heat pump or
heat exchanger and a compatible heating
or cooling system is necessary to use the
effluent (see Figures 7-9). The
connection between the effluent and the
facility occurs indirectly, through a heat "
47
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Residuals Use and Energy Conservation
exchanger, so there is no possibility of
adding pollutants to the effluent, Metro's
intention is that heat exchangers will be
owned and operated by each participating
facility.
The economics of using MeteoTherm
generally will favor new construction
having large and continuous heating and
cooling requirements and located near the
effluent pipeline. Seattle Metro has
entered into a demonstration project with
The Boeing Company that will provide
effluent for cooling Boeing's new training
facilities located near the Renton plant.
Eventually, Metro envisions some
facilities taking heat from the pipeline and
others returning heat to the effluent,
yielding an unlimited potential for energy
reclamation.
Plate and
Frame Heat
Exchanger
Chilled
Water
Hot
I Water
Supply
i
Condenser #1 (+)
Condenser #2 (+)
S^_/--i_
SeT<ฃ/
CUSTOMER
Combined
Heating and
Cooling
Option
METRO
Effluent Pipeline
Figure 7: Combined heating and cooling option
48
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Residuals Use and Energy Conservation
Figure 8: Heating only option
Heating
only
Option
Chlted
^, WWปr
V Return
CUSTOMER
METRO
Cooling-
only
Option
Effluent Pipeline
Figure 9: Cooling only option
49
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Residuals Use and Energy Conservation
The Boeing Company Project
The Boeing Company is constructing a
Customer Services Training Center near
the Renton wastewater treatment plant,
and is participating in a demonstration
project -with Seattle Metro to use effluent
from the Renton plant to cool its facilities.
During the demonstration period, both
conventional cooling (via cooling towers)
and MetroTherm cooling will be used.
Boeing will operate these two systems
simultaneously to collect data on
performance and costs. The
demonstration project was designed to
commence in August 1994. Boeing
makes a good subject for the
demonstration.prqject in part because it is
incorporating MetroTherm cooling into
new construction, where it is most cost-
effective to install, and because the
Boeing training center will operate 24
hours per day. As a continuous
operation, the center's cooling needs are
also continuous, but peak period
electricity costs are reduced through use
ofMetroTherm.
Boeing received a S1.2 million grant from
' Puget Sound Power and Light Company
to participate hi the demonstration
program. Although costs and savings that
will result from use of the MetroTTrem
facilities will not be fully known until
completion of the demonstration
program, Boeing expects to achieve
benefits in several other areas. Thus, an
aesthetic benefit will result from use of
MetToTherm, as Boeing can avoid
building and operating additional cooling
towers on the site. This will conserve
potable water. In addition, pollution
prevention benefits will be realized in that
chemicals will not be necessary for
cooling towers and boilers.
Applicability to Other Systems
Use of effluent for onsite heating and
cooling purposes could be economically
feasible for many wastewater treatment
plants. Facilities that do not use
anaerobic biosolids digestion and thus
have no onsite fuel production could use
effluent heat pumps for building heating
and cooling requirements.
Seattle Metro is unique in the siting of its
effluent pipeline. However, more
WWTPs are building pipelines as part of
water reclamation projects. These
pipelines could be designed for the dual
purpose of water reclamation and energy
reclamation. Industries located near
treatment plants should also be able to
take advantage of effluent heating and
cooling. Areas having high electricity
costs would provide a more favorable
environment for such opportunities, due
to the higher financial incentive.
Financial Benefits of the Energy
Conservation Program
Metro received a $400,000 grant from
Puget Sound Power and Light Company
to defray nearly half the $900,000 (1987
dollars) cost of the heat pumps. The
capital costs have been recovered through
Metro's sewer rates and bonds.
In 1992, the heat pumps operated for a
total of 9,200 hours. The electricity cost
(at 2.5 to 3 cents per kilowatt hour) was
50
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Residuals Use and Energy Conservation
approximately $105,000. The cost of
maintenance on the four heat pumps
totaled $30,000 for the year. The total
heat production was 55 trillion Btus. The
following table summarizes this
information, and contrasts it with the sale
price ($410,000) and Btu value of the
digester gas.
: -*ปV \'f ** <' ^' < O -
...: S^ ~sr-- {feat ^ - ,
DIeesterzas
Heat pumps (4)
r'l^ฃ$hi
-------
52
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Residuals Use and Energy Conservation
Other Promising ,
Technologies
Anaerobic Wastewater Treatment
Anaerobic wastewater treatment is
sometimes called "upflow anaerobic
sludge blanket" (UASB) or as anaerobic
upflow ("ANFLOW"). TheANFLOW
process has been successfully proved for
treatment of domestic wastewater at
WWTPs in Oak Ridge and Knoxvffle,
Tennessee, in pilot studies conducted by
Oak Ridge National Laboratory and the
cities (funded by the Department of
Energy). Anaerobic wastewater
treatment is most often used as a
pretreatment process, with effluent being
directed into a conventional aerated
treatment process such as activated
sludge or trickling filtration for polishing.
This technology is most appropriate for
WWTPs receiving less than 1 MOD and
for pretreatment of high-strength
industrial wastestreams.
In the anaerobic upflow process,
wastewater influent is drawn off the inlet
'of the primary clarifier and directed into a
.bioreactor. In the ANFLOW system, the
bioreactor is a 24,000-gallon cone-bottom
tank that contains a plastic or ceramic
filter medium. The UASB process uses a
sludge blanket instead of a constructed
filter, and tanks are sized as necessary.
Wastewater enters near the bottom of the
bioreactor and flows upward through the
filter medium. Effluent is discharged near
the top of the bioreactor and sludge can
be removed from the bottom. Bacteria on
the filter or in the sludge blanket consume
the organic material in the wastewater,
producing methane gas that bubbles to the
top and is collected. Bioreactor effluent
typically receives additional treatment to
meet surface water discharge standards,
although effluent from some industrial
facilities that discharge to WWTPs may
not require additional treatment.
In the early 1980's, Anheuser Busch
began developmental work on this
technology, which was not widely used
then for treatment of food processing
wastewater. Brewery wastewater is
readily biodegradable and free of toxics,
but its BOD/COD content is very high.
In 1991, Anheuser-Busch modified
existing aerobic wastewater treatment
processes to incorporate UASB at
breweries in Jacksonville, Florida and
Baldwinsville, New York. These
facilities generate wastewater with highly
variable flow, BOD and solids loadings,
pH, and temperature. Therefore,
screening, equalization and pH and
temperature control are necessary to
reduce the impact on the UASB process.
Ferric chloride is added to the reactors to
control odors.
Anaerobic wastewater treatment has
many advantages over aerobic treatment.
Estimates based on data from the
53
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Residuals Use and Energy Conservation
Tennessee pilot study indicate that an
ANFLOW system would use
approximately 45 percent of the energy
required by an activated sludge system for
a design flow of 50,000 gallons per day,
and would use approximately 30 percent
of the energy required by a 1 MOD
activated sludge plant. Anheuser-Busch
reports a 75 percent reduction in energy
consumption with the UASB process on-
line. UASB reduces energy consumption
because anaerobic treatment requires less
energy than aerobic treatment and
produces energy through methane
generation.
Methane recovery from gases collected in
the bioreactor's vapor space is 70 to 75
percent. This compares very favorably to
methane recovery from anaerobic
digesters, which typically produce only 55
to 60 percent.
Anaerobic wastewater treatment produces
relatively small amounts of biosolids,
reducing the costs and energy
requirements associated with their
disposal. The ANFLOW pilot plant
produced only about 25 percent of the
solids that would be produced by an
activated sludge process.
Anaerobic treatment produces gases
which consist mostly of methane. The
methane is captured and used to replace
nonrenewable fuels. In contrast,
activated sludge and other aerobic
processes produce only carbon dioxide
gas, which is vented to the atmosphere
and contributes to the potential for global
warming. Anheuser-Busch calculates
that an anaerobic process treating
100,000 pounds of BOD per day would
produce 40 percent less CO2 than an
aerobic process. This works out to a
reduction of 14,000 tons of CO2 per
year.
Nutrient addition is frequently required
for aerobic treatment of high-strength
food processing wastestreams because
typically such wastestreams do not
contain nitrates and phosphates adequate
to support the biological growth
necessary to consume the BOD load,
Anheuser-Busch found that nutrient
addition was not necessary for UASB
treatment, which produces less biomass
growth and thus has a lower nutrient
requirement than aerobic treatment.
Finally, Anheuser-Busch has shown that
treatment costs are considerably lower
with the UASB process. Before installing
UASB, the cost to treat this wastestream
was $0.076 per pound of BOD. With the
anaerobic process, costs dropped to
$0.019 per pound. Costs savings were
realized in residuals handling, reduced
need for aerobic treatment, and through
biogas recovery. Construction costs are
about half as great.
The DOE-funded ANFLOW study
concluded that ANFLOW is more energy-
efficient than conventional aerobic
processes, and can be a net energy
producer. Depending on what associated
processes are required to meet effluent
discharge limits and depending on costs of
biosolids disposal, it is possible that an
ANFLOW secondary treatment plant
might approach energy independence.
Although the most optimal operating
54
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Residuals Use and Energy Conservation
temperature range for methanogenic
organisms is 85 to 1GGT, ANFLOW could
operate effectively at temperatures as low
as 70ฐ. Influent of lower temperature would
probably need to be adjusted, however.
Lake Coaaty Southeast Geysers Effluent
Pipeline Project
About 30 years ago the large California
utility, Pacific Gas
and Electric
(PG&E), opened a
geothermal energy
plant in Lake
County, California.
This facility, known
as the Geysers, is the
nation's largest
geothermal resource
area, with over 1,000
MW of installed
power plant capacity.
However, since the ,
.mid-1980's,
production from the
Geysers has been .
declining at a rate of
about 6 percent
annually, due to the
declining amount of
natural steam.
enhanced environmental protection resulting
from a more desirable means of wastewater
disposal, and retention and creation of jobs
in the community.
The project is the world's first system that
will convert wastewater effluent into
geothermal steam, and, in turn, electricity
for community residents and businesses. It
is also unique in the public/private
Figure 10: Locations of geological formations containing "hot
rock." ,
Source: San Jose Mercury News
Lake County designed an effluent pipeline
project to partially remedy the problem by
supplying treated wastewater effluent for
injection into the steam reservoir, thereby
augmenting naturally-occurring steam
extracted for power generation. The project
is expected to produce three major benefits:
sustainment of geothermal generation,
partnership created for its implementation.
Besides Lake County, participants include
PG&E, Northern California Power Agency
(a consortium of twelve municipal electric
utilities), Calpine Corporation (a geothermal
development company), the California
Energy Commission, and the U.S.
55
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Residuals Use and Energy Conservation
Departments of Energy and Interior.
These participants are sharing in the $40
million construction cost of the project.
This cost includes associated wastewater
treatment plant improvements.
Although the southeast Geysers project is
the first in the nation, large parts of the
western United States have been found to
contain geologic formations of shallow
hot rock (Figure 10). These areas have
potential for development as geothermal
energy sources. WWTPs are located in
population centers which could use the
energy that would be obtained through
injection of wastewater effluent and
recovery of steam.
The southeast Geysers project will consist
of a 26-mile, 24-inch diameter buried
pipeline that will cany 7.8 MGD of
secondary-treated effluent from two Lake
County WWTPs to the Geysers
geothermal steamfield. The effluent will
be injected to a depth of approximately
7,000 feet. Pipeline operation and
maintenance is estimated at $2 million
annually.
Depending on steam recovery rates for
the injected effluent, the project is
expected to produce an additional 70 MW
of generating capacity for existing
geothermal power plants at the Geysers.
This will equate to as much as 825,000
megawatt-hours of clean, lowcost energy
annually. Construction should commence
in early 1995, with the project becoming
operational in 1996.
Biomass-Enhanced Digester Gas
Production
Several WWTPs in California have
successfully augmented production of
biogas by adding biomass directly to the
anaerobic digesters.
South Bayside System Authority (SBSA),
operates a tertiary WWTP in Redwood
City. In 1986, SBSA began a
demonstration program to find out the
effects of adding plant scum and grease
trap wastes to one of its two digesters.
The scum and grease wastes were added
only to Digester 1, while Digester 2 was
maintained as a control. Both digesters
continued to receive the same volumes of
solids from the gravity thickener. SBSA
kept records on the volume of wastes
received and the amount of gas generated,
and also various operating conditions of
each digester.
SBSA found that excellent digester
mixing (turnover rate = 8.5 times daily)
and long detention times (40 days)
probably contribute to the ability to
accept large volumes of grease. Grease
loadings were increased as the
demonstration project progressed,
reaching 730,215 gallons per year in 1993
for Digester 1. SBSA believes that this
figure does not represent the maximum
loading for the digester. SBSA calculated
that each gallon of grease introduced to
the digester results in the production of
about 20 cubic feet of biogas. When the
digesters were cleaned, no significant
difference was found in the contents of
the control versus the digester that
received grease wastes.
57
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Residuals Use and Energy Conservation
SBSA now accepts grease trap wastes into an anaerobic digester. By avoiding
and septic wastes from a large geographic the secondary treatment process, none of
area beyond its service area. This the energy inherent in the wastes is lost
program provides an environmentally and there is no chance of adversely
beneficial disposal option for waste affecting the secondary process. No
haulers. Instead of conventional disposal effects on effluent quality have been
into a designated area of the collection observed because of the demonstration
system, these wastes are placed directly project.
58
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Residuals Use and Energy Conservation
Factors that Contribute
toSuccess
i . '
The facilities in these case studies have
been highly successful in carrying out
various types of energy conservation
activities. Orange County Sanitation
Districts, Hyperion, and Sanford's Big
Buffalo Creek WWTP analyzed the
factors that have contributed to the
success of their programs. Facilities
considering implementing similar energy
programs should benefit from reviewing
the factors that go into the achievement of
a successful program.
The facilities in these case studies
identified the primary factors that have
contributed to their success as follows:
1) The design of CSDOC's two adjacent
wastewater treatment plants provides
considerable flexibility in treatment
options. For instance, operators can
divert flow from Plant 1 to Plant 2.
Secondary treatment is flow equalized,
and can be adjusted to maximize
treatment. Advanced primary treatment
allows solids removal to be maximized in
the primary clarifiers, reducing the
loading to secondary processes and giving
the plants greater effective capacity. This
allows experimentation with energy
conservation activities without risking
NPDES or air permit noncompliance.
The design and operating criteria at
Sanford's Bjg Buffalo Creek WWTP also
provide for flexibility. The parallel design
of the extended aeration basins allows
easy removal of one basin from service
and matching of average daily flow to the
basin design volume. This alleviates
underloading and subsequent sludge aging
and pin floe which can cause deterioration
of secondary clarifiers effluent. The
process control system allows operators
to be instantly aware of factors which
affect the wastewater treatment process.
The system's automatic response achieves
optimum treatment in the most energy
efficient manner. The ability to equalize
the flow through the automatic valve at
the influent pump station eliminates pump
cycling and reduces the electrical demand.
This equalization creates a steady state in
the extended aeration process, which
improves treatment.
2) CSDOC and Sanford cite their
effective programs to control incoming
pollutants. CSDOC was one of the first
facilities in California to establish loading-
based limits for industrial users for both
toxics and conventional pollutants.
Industrial users are limited to discharging
10,000 pounds of BOD per day each. At
present, CSDOC is studying the feasibility
of having industrial users convert soluble
BOD to solids before discharge to the
sewer. Lower BOD loads to the plant
mean lower treatment costs.
59
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Residuals Use and Energy Conservation
3) Hyperion identified staff expertise as
most important to the success of their
energy recovery operations. The HERS
system is the most technically complex of
the facilities in these case studies.
Hyperion has assembled a diverse .and
competent staff whose backgrounds and
training are in power generation.
Additional support is provided by the
trained plant operators and
instrumentation staff whose primary
responsibilities are in wastewater
treatment
CSDOC and Sanford also identified the
importance of management and staff
training, interest, and technical expertise
to successfully carry out energy
conservation without risking '
noncompliance with permit requirements.
Their staffs have a genuine interest in
energy saving actions in addition to
expertise in wastewater operations.
4) Although CSDOC is a public agency,
it is operated similarly to a business
enterprise with managers having certain
goals to achieve in cost savings and other
areas. This management attitude provides
a strong motivation for energy
conservation.
5) Sanford cites the value of a
comprehensive energy audit as an
essential tool for cost-effective energy
conservation.
60
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Residuals Use and Energy Conservation
The Influence of
Financial Factors
The wastewater treatment plants included
in this study provide several good
examples of the factors that should be
considered in making decisions regarding
the use of biogas and other renewable
energy technologies.
Biosoiids: Onsite Use versus Ofisite
Reuse
Unlike the other facilities in this study,
Hyperion recovers energy from biosoiids
by drying and oxidizing the digested
solids. This activity augments Hyperion's
total electricity generation by 20 percent.
At present, the cost to prepare the
biosoiids for burning is greater than the
value of the electricity subsequently
generated by using the biosoiids for
energy.
However, under other scenarios the cost
balance changes to favor onsite
processing of biosoiids, as follows:
1) If the cost of electricity purchased
.from the public power company were to
increase by 45 percent or more, the onsite
option becomes more economical.
2) If the cost to dispose of biosoiids
ofFsite were to at least double, it becomes
more cost effective to process the
biosoiids onsite.
3) Recent estimates by Hyperion staff
show that the addition of steam dryers
lowers the cost of onsite biosoiids
processing to $109 per dry ton, compared
to $132 for ofFsite management.
Biogasi Onsite Use versus Ofisite Sale
Biogas is typically used onsite by
wastewater treatment plants hi one or
both of two ways: 1) to generate
electricity, and 2) to provide heat for
digesters and buildings. The low cost for
electrical power in the Seattle area means
that using biogas to generate electricity is
not particularly attractive. TheRenton
plant obtains electricity at an average cost
of about $0.025 per kilowatt hour. In
comparison, electrical costs for WWTPs
in Southern California average $0.08 per
kilowatt hour. Thus, the payback period
for installation of engine generators that
use biogas as fuel would be about three
times longer in the Seattle area, or around
20 years.
The other potential for in-plant use of
biogas is to generate heat for facilities and
for the anaerobic digesters. Metro has
replaced biogas for this use with the
electrically operated heat pumps. A grant
was received to defray about half the
purchase cost of the heat pumps, and this
contributed to the attractiveness of this
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Residuals Use and Energy Conservation
option. At electricity costs about three
times Metro's (that is, about 7.5 cents per
kilowatt hour), the cost of replacing
biogas with heat pump technology is
probably about even in terms of operating
and maintenance costs, all other factors
being equal. If the initial purchase cost of
the heat pumps must be borne by the
facility, as opposed to receiving a grant or
subsidy, the benefit decreases further.
Metro's low electricity costs result in a
low operating cost for heat pumps.
Treatment plants capable of producing
biogas should consider the capital and
operation costs for engine generators that
can use biogas as fuel versus the capital
and operating costs of heat pumps. Other
WWTPs may not be subject to the
conditions which favorMetro's use of
heat pumps.
Facilities located in areas where they pay
more than approximately 7.5 cents per
kilowatt hour may find that using digester
gas onsite is the more cost-effective
option. A WWTP considering the choice
of using the gas onsite versus selling it to
a utility might select a different option.
For instance, depending on the
circumstances, it might be more cost
effective to use part of the biogas
production for onsite heating. The
remainder would be available for sale at
(with all other factors being equal) about
33 percent of the income that would be
received from sale of all the gas. This
option would avoid the capital cost and
operation and maintenance costs for heat
pumps.
Energy from Effluent: Purchase
versus Contractual Equipment
Seattle Metro's M&troTherm program is
currently based on the premise that heat
exchangers will be owned and operated
by each participating business that uses
effluent for heating or cooling purposes.
Another option for such energy recovery
programs would be for the WWTP or an
outside party to provide, operate, and
maintain the heat exchangers, perhaps on
a rental or contractual basis.
This would address three concerns from a
potential customer's viewpoint:
The customer may have no
expertise in the operation or
maintenance of heat exchangers;
The customer may not want to or
be able to bear the capital costs of
purchasing a heat exchanger unit;
The customer may not wish to
commit to purchase of a heat
exchanger system without
knowing how well it will work for
his particular needs.
By providing a second option to potential
customers, one not involving outright
purchase and operation of the heat
exchanger units, the WWTP could attract
businesses who otherwise may not have
considered using the effluent energy
recovery program.
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Residuals Use and Energy Conservation
Conclusions
These case studies show that many
options for energy recovery or
conservation are available for wastewater
treatment plants. The options selected by
a particular plant should be based on site-
specific considerations, and these will
vary from facility to facility.
Some options are in more widespread use
than others. For instance, energy
recovery from biogas is universally cost
effective and has gained widespread
acceptance. The technology exists to
allow full use of biogas, and the extra
costs of incorporating this energy source
into a system are small. The payback
period for installation of biogas energy
recovery at plants having anaerobic
digesters is short, typically less than six
years. Recovery and use of biogas
accomplish energy conservation and
pollution prevention goals, and also cost
savings, making this an obvious choice for
application in all treatment plants that
employ anaerobic digestion for
stabilization of wastewater biosolids.
t
Other energy conservation and municipal
pollution prevention activities can be
integrated with use of biogas, as
demonstrated by the Sunnyvale WPCP,
including collection and use of landfill
gas, recovery of waste heat, water
reclamation, and municipal water
conservation. Often, wastewater
treatment plants are located near
municipal landfills, and could potentially
develop the landfill gas as an additional
energy source. Advantages lie not only in
the cost savings from energy recovery
from the landfill gas, but also in meeting
regulatory and safety concerns posed by
landfill gas emissions.
Energy conservation is considered a
worthwhile goal because it conserves
natural resources. The examples of
CSDOC and Hyperion suggest that
reductions in energy use can also lead to
increased ability to comply with air
emissions regulations. Carbon dioxide is
a "greenhouse gas" which is released by
all wastewater treatment and biosolids
management processes. Converting
biosolids to fuel achieves substantial
benefit from the wastes before carbon
dioxide is ultimately released. In addition,
nonrenewable energy sources are replaced
by renewable energy from wastewater.
The experiences of these facilities show
that actions which enhance process
efficiency, such as advanced primary
treatment, ban simultaneously result in
increased energy recovery. There is no
evidence that energy conservation efforts
have in any way adversely affected
receiving water quality.
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Residuals Use and Energy Conservation
The energy conservation potential of for application of this technology are
effluent heating and cooling has been increasing. Water reclamation projects
explored to date by only a few facilities. should be designed not only to reclaim
However, with more plants incorporating water as a valuable resource, but also to
water reclamation, leading to pipeline take advantage of any opportunities to
construction through commercial and substitute effluent heating and/or cooling
residential areas, potential opportunities for nonrenewable energy sources.
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Residuals Use and Energy Conservation
Resources
Pierson, F.W. and C.V. Pearson. 1982.
Energy from municipal waste:
Assessment of energy conservation and
recovery in municipal wastewater
treatment Argonne National Laboratory,
Argonne, H. NITS No. DE85-004826.
Miller, Williams & Works. 1984. Energy
Audit: Buffelo Creek Wastewater
Treatment Facility, City of Sanford, NC.
Prepared for the North Carolina
Department of Commerce, Energy
Division.
The Washington State Energy Office has
literature and computer programs
available pertaining to district heating.
WSEO can be contacted at the following
address:
Washington State Energy Office
District Heating and Cooling Program
809 Legion Way SJE.
Olympia, Washington 98504
(206)586-5000
Seattle Metro can provide information
regarding the MetroTherm Program, and
can be contacted as follows:
MetroTTzemf Program
Water Pollution Control Department,
MS. 130
821 Second Avenue
Seattle, WA 98104
(206)689-3184
Additional information on use of
geothermai energy is available as follows:
Mark Bellinger
Energy and Resource Manager
Lake County Sanitation District
Lakeport,CA
(707)263-2273
65
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hing existing data sources.
tad completing md reviewing the collection of infonnation.
Suite 1204, Arlington, VA 22202-4302, and to the Office of
REPORT DOCUMENTATION PAGE
Form Approved
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1. AGENCY USE ONLY (Leave
blank)
2. REPORT DATE
June 1995
3. REPORT TYPE AND DATES COVERED
Final subcontract report
4. TITLE AND SUBTITLE
Case Studies in Residual Use and Energy Conservation at Wastewater Treatment Plants
Final Report ' ' '
6.AUTHOR(S)
Dianne Stewart
5. FUNDING NUMBERS
(QYAE-3-13480-1
(TA)WM51.1010
7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES)
Science Applications International Corporation . . .
5150 El Camion Real
Los Altos, California 94022
8. PERFORMING
ORGANIZATION
REPORT NUMBER
DE95009216
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National Renewable Energy Laboratory
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Golden, CO 80401-3393
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AGENCY REPORT
NUMBER
NREL/TP-430-7974
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12b. DISTRIBUTION CODE
UC-1414
13. ABSTRACT (Max/mum 200 words)
By integrating wastewater treatment with energy conservation, the waste water treatment plants described in this report have met the
challenges of new environmental regulations. These facilities have achieved benefits in cost savings while enhancing their ability to comply
with regulations. Their activities illustrate highly effective pollution prevention strategies.
14. SUBJECT TERMS
ป
wastewater treatment plants, effluent, heating and cooling, pollution prevention
15. NUMBER OF PAGES
60 pages
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A03
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