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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

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                             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

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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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                                                                                                 606B-21
                                         Figure 1:  Onsite uses for biogas

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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
                                  generators to drive remote equipment,
                                  and (5) powering engines used with
                                  generators to produce general purpose
                                  electrical power.                  "
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                                                                         621B-02
                              Figure!:  Offsite uses for biogas

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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

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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

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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

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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

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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

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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

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16

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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

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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

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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

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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

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        C3
        E
        
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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

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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

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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

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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

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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

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Residuals Use and Energy Conservation
     In 1978, due to substantial upgrading of
     effluent discharge regulations—including
     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

-------
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

-------
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

-------
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

-------
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

-------
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
                                           61

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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.
                                            62

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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.
63

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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.
                                             64

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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

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  Final subcontract report
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 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
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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
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