United States
Environmental Protection
Agency
Municipal Environmental Research EPA-600/2-78-1 49
Laboratory August 1978
Cincinnati OH 45268
Research and Development
Total Energy
Consumption
for Municipal
Wastewater
Treatment
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EPA-600/2-78-149
August 1978
TOTAL ENERGY CONSUMPTION FOR MUNICIPAL WASTEWATER TREATMENT
by
Robert Smith
Wastewater Research Division
Municipal Environmental Research Laboratory
Cincinnati, Ohio 45268
MUNICIPAL ENVIRONMENTAL RESEARCH LABORATORY
OFFICE OF RESEARCH AND DEVELOPMENT
U.S. ENVIRONMENTAL PROTECTION AGENCY
CINCINNATI, OHIO 45268
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DISCLAIMER
This report has been reviewed by the Municipal Environmental Research
Laboratory, U.S. Environmental Protection Agency, and approved for publica-
tion. Mention of trade names or commercial products does not constitute
endorsement or recommendation for use.
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FOREWORD
The Environmental Protection Agency was created because of increasing
public and government concern about the dangers of pollution to the health
and welfare of the American people. Noxious air, foul water, and spoiled
land are tragic testimony to the deterioration of our natural environment.
The complexity of that environment and the interplay between its components
require a concentrated and integrated attack on the problem.
Research and development is that necessary first step in problem
solution and it involves defining the problem, measuring its impact, and
searching for solutions. The Municipal Environmental Research Laboratory
develops new and improved technology and systems for the prevention, treat-
ment, and management of wastewater and solid and hazardous waste pollutant
discharges from municipal and community sources, for the preservation and
treatment of public drinking water supplies, and to minimize the adverse
economic, social, health, and aesthetic effects of pollution. This publi-
cation is one of the products of that research; a most vital communications
link between the researcher and the user community.
The increasing cost of energy and the potential for shortages of
fossil fuels in the future has created a keen interest in methods available
for minimizing energy consumption by conservation or recovery of energy
from waste materials. A vital part of this initiative is to quantify
energy used for all forms of human activity. This report attempts to
quantify the total energy used in collecting and treating municipal waste-
water. An effort is also made to show the effectiveness of measures which
might be used to conserve energy and to recover it from the residuals
produced at the wastewater treatment plant.
Francis T. Mayo, Director
Municipal Environmental Research
Laboratory
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ABSTRACT
Quantities of all forms of energy consumed for collection and treat-
ment of municipal wastewater are estimated. Heat energy is equated to
electrical energy by a conversion factor of 10,500 Btu/kwh. Total energy
consumption, expressed as kwh/mg of wastewater treated, ranges from 2300-
3700 kwh/mg. Energy used for construction of the treatment plant and the
sewerage system represents 35-55% of the total energy consumed. The
remainder used for plant operation is predominately (65-75%) electrical
energy. The use of high efficiency aeration devices combined with good
maintenance practices appears to offer the best opportunity for conserva-
tion of energy within the plant. Recovery of energy from the sludge
produced at the plant can be accomplished by anaerobically digesting the
sludge and using the digester gas as fuel for internal combustion engines.
In large plants, when the sludge is sufficiently dewatered, it is also
possible to recover energy by incinerating the dewatered sludge with
production of steam in a waste heat boiler. The steam can then be used
within the plant or expanded through a steam turbine to produce mechanical
or electrical energy.
This report covers the period January 1, 1977 to January 1, 1978, and
work was completed as of January 1, 1978.
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TABLE OF CONTENTS
Foreword i i -j
Abstract iv
Figures yi
Tables vii
Abbreviations and Symbols viii
1. Introduction ..... 1
2. Conclusions 3
3. Electrical Energy for Plant Operation- 6
4. Energy for Sewer and Plant Construction 11
5. Energy for Manufacture of Process Chemicals 18
6. Heat Energy Used at the Plant 21
Anaerobic digester heating 21
Space heating 22
Heat pumps 27
Auxiliary fuel for sludge incineration 29
Trucking of sludge to land disposal 33
7. Potential for Energy Recovery 34
References • • • • • 41
„ ............ 42
....... ..... 43
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FIGURES
Number Page
1 Relationship between community size and (A) total miles of
street; (B) street (sewer) miles per 10,000 acres; and
(C) total miles of sewer 19
2 Space heating requirements for wastewater treatment plants
versus plant size. Solid lines represent estimates by
Voegtle, circled points are typical plant data, squared
point represents Ely, Minnesota tertiary plant, dashed
lines show estimates based on building volume and 4 Btu
per year cu ft per annual degree-days 29
3 Normalized space heating requirements for wastewater
treatment plants in terms of Btu/yr per cu ft of heated
space per annual degree-days versus thousands of annual
degree-days 33
4 Heated space in wastewater treatment plants versus plant
size. Circled points are typical secondary plants,
squared points are estimates by George S. Russell,
triangular point is Ely, Minnesota tertiary plant 35
5 Auxiliary fuel (Rf = gal fuel oil/ton DVS) shown by dashed
lines and excess air ratio (E = air supplied for com-
bustion/stoichiometric amount) shown by solid lines for
holding incinerator flue gas temperature at any set point
as functions of heat available for combustion products.. 41
6 Amount of heat (BTU/lb DVS) recoverable in the waste heat .
boiler. Solid lines show heat recoverable from the DVS
alone. Dashed lines show total recoverable heat from
both DVS and auxiliary fuel. Flue gas set point tempera-
ture is 1100 F. Afterburner used to raise flue gas
temperature to 1400 F. Boiler exit temperatures are
500°F and 350 F 49
7 Excess air ratio shown by solid line to hold incinerator flue
gas temperature at 1100 F. Auxiliary fuel reguired is shown
by dashed lines for combustion of DVS at 1100 F and for
raising the gas temperature to 1400 F with an afterburner 51
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TABLES
Number page
1 Estimated Total Energy Budget for Municipal Wastewater
Treatments 5
2 Estimated Operating Energy Budget for Municipal Waste-
water Treatment PI ants 6
3 Estimated Electrical Energy Consumption for Operation of
Municipal Wastewater Treatment Processes: Kilowatt-Hours
Sizes (mgd = millions of gallons treated/day) 8
4 Energy Consumption for Production of Building Materials.... 16
5 Itemized Energy Consumed in Construction of 1 mgd Trickling
Filter Wastewater Treatment Plant in Terms of Btu's
and Equivalent Electrical Energy in Kilowatt-Hours 17
6 Itemized Energy Consumed in Construction of 35,000 ft
(6.652 miles) of Municipal Sewer in Terms of Btu's
and Equivalent Electrical Energy in Kilowatt-Hours....... 18
7 Estimates of Heated Volume and Heat Used for Space
Heating in Wastewater Treatment Plants 31
8 Theoretical Steam Rates (Ibs/kwh) 52
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ABBREVIATIONS AND SYMBOLS
ABBREVIATIONS
Btu -- British thermal unit (1 Btu = 1055 joules)
kwh -- kilowatt-hour
mg -- million gallons (1 mg = 3785 cu meters)
DVS -- dry volatile solids
mgd -- million gallons per day (1 mgd = 0.0438 cu m/sec)
1C -- internal combustion (engines)
scf -- standard cubic foot
psig -- pounds per square inch gage (1 psig = 0.0703 kg/sq cm)
bbl — barrel (1 bbl - 42 gallons = 0.15987 cu m)
ton — 2000 Ib (1 ton = 907.2 kg)
cu yd -- cubic yard (1 cu yd = 0.7646 cu m)
hp-hr -- horsepower-hour (1 hp-hr = 0.746 kwh)
SYMBOLS
MGD -- millions of gallons per day
TDH -- total dynamic heat, ft or meters of water
AEF -- aeration efficiency, percent of supplied air
dissolved in water or pounds of oxygen dissolved
per kwh or hp-hr utilized
SRT -- sludge retention time (days) defined as the mass (Ib)
of activated sludge held in the process divided by
the wasting rate (lb/day)
vm
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SECTION 1
INTRODUCTION
Since the national energy awareness was heightened by the Arab oil
embargo between October 1973 and March 1974, much has been written about
consumption of energy in municipal wastewater treatment plants and the
potential for conservation or recovery of energy. Energy used for operating
wastewater treatment plants is known to be small compared to other national
energy consuming activities. For example, electrical energy used in plants
has been estimated (1) at 1-2% of the average residential consumption and
only about 0.17% (2) of the total national energy use. Although these
comparisons might suggest that energy usage, and thus the importance of
conservation and recovery, in wastewater treatment plants is relatively
trivial, this is definitely not true in terms of the annual plant operating
budget. The likelihood of substantial increases in the cost of energy
over the useful life of the plant and the possibility of local shortages of
some forms of energy are factors which should not be neglected in planning
the processing scheme to be used at the plant.
Although much has been written about energy consumption for collection
and treatment of municipal wastewater, there is a need to clearly delineate
the total and operating energy budgets and to quantify the potential for
conservation and recovery of all forms of energy. This report will begin
by estimating the magnitude of the major consumptive uses of energy such
as energy for construction of plants and collection systems, electrical
energy for plant operations, energy for off-site manufacture of chemicals
used in plant operation, heat for plant buildings, and the consumption of
heat energy for processes such as anaerobic digestion and incineration.
Using the total or operating energy budget as a basis, the potential
for energy conservation or energy recovery within the plant will then be
examined in order that the relative value and importance of alternative
energy saving measures might be clarified and put into perspective.
Although energy occurs naturally in many forms, the energy consumed for
operating wastewater treatment plants is primarily hydraulic energy for
pumping water or air and heat energy used by processes such as anaerobic
digestion or incineration of sludge. The efficiency of pumps and blowers
is usually in the range of 70-80% so that mechanical energy can be converted
to hydraulic energy with no more than about 30% loss. Similarly, mechanical
and electrical energy can be converted from one form to the other with a
loss of less than 10%. On the other hand, the conversion of heat energy to
mechanical energy necessitates the wasting of roughly 2/3 of the heat energy,
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For example, if electrical energy is converted to heat energy, one kilowatt-
hour will generate about 3413 Btu of heat. However, if heat energy is used
to generate electrical energy in a modern coal fired power plant, about
10,500 Btu of heat energy is needed to generate one kilowatt-hour. To
clarify the discussion of energy consumption, all energy forms will be con-
verted to electrical energy and expressed as kilowatt-hour per million
gallons of water treated.
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SECTION 2
CONCLUSIONS
Estimates of 1 total energy and 2 energy used for plant operation are
shown in Tables 1 and 2 respectively, for activated sludge treatment of
municipal wastewater. Heat energy has been converted to electrical energy
by assuming that one kwh is equivalent to 10,500 Btu.
It can be seen from Table 1 that roughly one-half of the total energy
consumed is associated with construction of the treatment plant and the
sewage collection system. Electrical energy used at the plant accounts for
another 30-35% of the total energy usage. The remainder is distributed
among energy for production of chemicals, digester heating, building heat,
auxiliary fuel for sludge incineration, and fuel for trucks hauling dewatered
sludge to the land disposal site. Auxiliary fuel for incineration of sludge
represents about 10% of the total energy budget. If dewatered sludge is
trucked to land disposal, the energy consumed is usually small compared to
energy used for incineration. On the other hand, if liquid sludge is trucked
to land disposal, the energy used can be equivalent or even greater than the
energy used for incineration.
Energy used to operate the treatment plant includes electrical energy
and heat energy for digester heating, building heat, sludge incineration,
and hauling of sludge to land disposal. Electrical energy represents 65-75%
of the energy used for plant operation. Building heat is estimated at only
5-10% of the operating energy.
Energy can be conserved or recovered within the treatment plant in
various ways. For example, energy used for supplying air to the activated
sludge process can be reduced significantly by the use of efficient aeration
devices and proper maintenance. As much as 80% of the building heat used
for ventilating work spaces can be recovered by the use of energy wheels.
The anaerobic digestion process produces methane gas which, when used as a
fuel for internal combustion engines, can recover about 573 kwh/mg for use
in operating the plant. Heat from the water jackets of the internal combus-
tion engines can be used to heat the anaerobic digesters. In very large
plants it is also possible to incinerate all of the sludge produced and
recover part of the heat of combustion in a waste heat boiler. Steam from
the waste heat boiler can then be used in a steam turbine to produce
mechanical or electrical energy for use in the plant. The analysis made of
this system shows that energy recovery is competitive with anaerobic diges-
tion only in large plants and only when the moisture content of the incine-
rated sludge is low.
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TABLE 1. ESTIMATED TOTAL ENERGY BUDGET FOR MUNICIPAL WASTEWATER TREATMENT PLANTS*
1.
2.
3.
4.
5.
6.
7.
8.
Sewer Construction
Plant Construction
Electrical Energy
Chemicals
Digester Heating
Building Heat
Sludge Hauling
Sludge Incineration
Total Energy Consumption
1 mgd
1352 (36.4%)
758 (20.4%)
1100 (29.6%)
158 (4.3%)
168 (4.5%)
160 (4.3%)
20 (0.5%)
3716
10 mgd
777 (29.4%)
363 (13.7%)
893 (33.7%)
158 (6.0%)
168 (6.3%)
59 (2.2%)
230 (8.7%)
2648
100 mgd
642 (28.0%)
174 (7.6%)
835 (36.4%)
158 (6.9%)
168 (7.3%)
86 (3.8%)
230 (10.0%)
2293
*In terms of kilowatt-hours per million gallons of wastewater treated. Estimates are based on
activated sludge plants with anaerobic digestion. Sludge disposal is by incineration in the
10 mgd and 100 mgd si^s and by hauling dewatered sludge 40 miles one-way to land spreading
at the 1 mgd size.
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TABLE 2. ESTIMATED OPERATING ENERGY BUDGET FOR MUNICIPAL WASTEWATER TREATMENT PLANTS
1. Electrical Energy
2. Digester Heating
3, Building Heat
4. Sludge'Hauling
en
5. Sludge Incineration
1 mgd
Used Recoverable
10 mgd
Used Recoverable
100 mgd
Used Recoverable
1100
168
160
20
573
152
106
893
168
59
230
573
152
39
835
168
86
230
573
152
57
1448
831
1350
764
1319
782
*In terms of kilowatt-hours per million gallons of wastewater treated. Estimates are based on activated
sludge plants with anaerobic digestion. Sludge disposal is by incineration at the 10 mgd and 100 mgd
sizes and by hauling dewatered sludge 40 miles one-way to land spreading at the 1 mgd size. Energy
recovery is by generation of electrical power with 1C engine using digester gas as fuel. The waste
heat from the 1C engines is used to heat the digesters. Energy wheels are utilized to conserve
building heat.
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SECTION 3
ELECTRICAL ENERGY FOR PLANT OPERATION
Electrical energy is used in wastewater treatment plants for pumping
fluids such as water, air, or sludge, and for other mechanical chores. A
detailed accounting of electrical energy usage is given in reference 1, but
approximate estimates for individual processes are shown in Table 3. Total
electrical power consumption is approximately expressed as follows:
kwh/mg = 390 MGD~°'15 (primary plants) (1)
kwh/mg = 700 MGD~°'12 (high rate trickling filter plants) (2)
kwh/mg = 1100 MGD~°'06 (activated sludge plants) (3)
Thus, it can be seen that at the 1 mgd size, the trickling filter plant
requires 80% more energy than the primary plant, and the activated sludge
plant requires roughly three times the energy of the primary plant. The
economy of scale is greatest for the primary plant which at 100 mgd utilizes
only one-half the energy per gallon treated used at the 1 mgd size. This
ratio between unit energy consumption at 1 mgd and 100 mgd is 1.74 for the
trickling filter plant and 1.32 for the activated sludge plant.
In activated sludge plants, about 57% of the electrical energy is
consumed in supplying diffused air, about 20% in pumping the influent and
return streams, and another 6% for mixing and heating the anaerobic
digester. Thus, about 83% of the total electrical energy consumption is
associated with three processes. Since the diffused air system appears
to offer the greatest potential for' conservation of electrical energy, this
process will be discussed in greater detail. Adiabatic work for compressing
atmospheric air (14.7 psia - 70 F) to gage pressures in the range 6-10 psig
can be approximated (within about %%) by the following linear relationship:
Btu/lb = 2.2 + 1.825 GP (4)
GP = compressor exit pressure, psig
If the average adiabatic efficiency of the air compressor is taken as 75%
and the efficiency of the electric drive motor as 95%, the two coefficients
in equation 4 can be divided by 0.7125 to find a relationship for pump work.
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TABLE 3. ESTIMATED ELECTRICAL ENERGY CONSUMPTION FOR OPERATION OF MUNICIPAL WASTEWATER
TREATMENT PROCESSES: KILOWATT-HOURS PER MILLION GALLONS TREATED FOR THREE
PLANT SIZES (mgd = millions of gallons treated/day)
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
13.
14.
15.
Preliminary Treatment
Influent Pumping (30 ft TDH)*
Primary Sedimentation
Recirculation Pumping
a. Trickling Filters (Qr/Q - 3.0)
b. Activated Sludge (Q /Q = 0.5)
Diffused Air Aeration (AEF = 6%)
Mechanical Aeration (2 Ib 02/hp-hr)
Final Sedimentation
Chlorination
Sludge Pumping
Gravity Thickening
Air Flotation Thickening
Anaerobic Digestion
Vacuum Filtration
Incineration
Lights and Misc. Power
1 mgd
18.5
153.0
30.6
183.0
45.0
532.0
404.0
30.6
0.7
2.7
10.2
70.0
123.6
58.5
65.0
57.0
10 mgd
6.6
145.1
12.2
174.0
42.3
532.0
404.0
12.2
0.7
2.7
2.0
60.8
45.6
34.6
28.7
21.0
100 mgd
2.5
129.3
7.3
155.2
31.3
532.0
404.0
7.3
2.7
2.7
0.4
46.9
19.1
36.4
25.9
24.0
*TDH = total dynamic pumping head, Qr/Q = return flow rate/average daily plant flow rate.
AEF = aeration efficiency in percent for diffused air and Ib 02/hp-hr for mechanical aeration,
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For diffused air equipment, aeration efficiency (AEF) is defined as the
mass of air dissolved in the aeration basin contents divided by the mass of
air supplied to the air diffusers. Aeration efficiency is then expressed as
a percentage. Since atmospheric air is 23.2% oxygen by mass and 3413 Btu's
are equivalent to one kilowatt-hour, the electrical power consumed in deliver-
ing one pound of oxygen can be expressed as a linear function of compressor
exit pressure and aeration efficiency as follows:
kwh/lb 02 = (0.39 + 0.318 GP)/AEF (5)
Thus, from equation 5, electrical power consumption for diffused air can
be estimated as 0.49 kwh/lb Op delivered when the aeration efficiency is
6% and the gage pressure at tne compressor exit is 8 psig. It can be seen
that electrical power consumption for diffused air is inversely proportional
to aeration efficiency and also very dependent on the compressor exit
pressure.
The amount of oxygen consumed in the activated sludge process depends
on the BOD concentration of primary effluent as well as the operating sludge
retention time (SRT). For conventional activated sludge, a rule-of-thumb of
one pound of oxygen per pound of BOD entering the process is commonly used.
Thus, if the BOD concentration of primary effluent is 130 mg/1, the oxygen
consumed in the process can be estimated as about 1083 Ib Op/mg. If 0.49
kwh/lb Op is used for compressing the air, the power consumption can there-
fore be estimated at about 531 kwh/mg. This is roughly equivalent to
1 scf per gallon of water treated which is another commonly used rule-of-
thumb. A more detailed discussion of oxygen consumption is given on pages
12 and 13.
Although the field efficiency of mechanical aerators varies with the
size of the aerator drive motor and the spatial relationship between the
aeration basin and the rotor of the mechanical aerator, a generally accepted
average value for efficiency is 2.0 Ib Op/hp-hr or 2.68 Ib Op/kwh. Thus,
electrical power consumption for mechanical aerators can be estimated at
about 0.37 kwh/lb Op delivered or about 404 kwh/mg, using the assumptions
mentioned above. It can be seen from equation 2 that when the compressor
exit pressure is 8 psig, a diffused air system will consume the same amount
of electrical power as a mechanical aerator system when the aeration
efficiency is 7.87%.
Three principal methods are available for conserving electrical energy
used for aeration; first choosing the most efficient aeration equipment and
providing the necessary maintenance; second, providing for automatic control
of the dissolved oxygen concentration in the aeration basin; and third,
operating the process at a sludge retention time which maximizes sludge
production and minimizes oxygen demand.
The importance of selecting efficient aeration equipment and providing
adequate maintenance was demonstrated in an important set of experiments
conducted at the Jones Island Treatment Plant in Milwaukee, Wisconsin by
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Leary, Ernest, and Katz (3-4). Aeration efficiency of various kinds of
aeration equipment was measured under operating conditions.
The method used was to collect samples of the gas discharged from the
water surface. From the volume of the gas and the nitrogen content of the
gas, the amount of air supplied per unit of water surface was computed.
This was checked against the measured mass of air supplied. From the oxygen
content of the gas, the aeration efficiency was calculated. The carbon
dioxide concentration in the off-gas was used as a check. The experiments
were performed on the East Plant which is designed to treat 115 mgd of
wastewater. Each aerator was designed for a two-pass flow, and each pass
had a length of 370 ft, a width of 22 ft, and a depth of 15 ft.
When the experimental work started, the East Plant was equipped with
ceramic tube diffusers which were placed on spiral flow. The field
efficiency in 1962 was measured as 5.4%, but this dropped to 3.2% in 1963
and then increased to 5.1% in 1964 as a result of a program of cleaning
and refurbishing the diffusers.
During the same period, the 85 mgd West Plant was operated with an
average oxygen consumption of 0.8 Ib 02/lb BOD removed. The West Plant is
equipped with ridge and furrow aerators distributed uniformly along the
bottom of the aerator. The aeration efficiency of the ridge and furror
diffusers was measured in 1961 as 10.7%, and this decreased to 7.9% in 1962.
By cleaning and refurbishing the diffusers, the aeration efficiency was
increased to 14.2%, as measured in the 1964 measurements. These experiments
show dramatically the conservation of electrical power which can be achieved
by the use of efficient aeration equipment and proper maintenance policies.
In municipal wastewater treatment plants, the production of wastewater
varies in volume and strength in a reasonably predictable diurnal pattern.
Thus, the demand for oxygen also varies an dissolved oxygen probes
installed in the aeration basin can be used to match the oxygen supplied
to the process with the demand. When diffused air is used, the amount of
air supplied must be modulated by control of a valve on the compressor
inlet. When mechanical aerators are used, the oxygen supplied is controlled
by varying the submergence of the rotor blades by means of variables weirs.
Dissolved oxygen control has been demonstrated in hundreds of plants and
can cut the power consumption (5-6) by approximately 20%.
The amount of oxygen used in the activated sludge process is known to
depend on the amount of BOD synthesized to microorganisms and the amount of
sludge destroyed in the process by endogenous respiration. This can be
seen by recognizing that the mass of COD removed in the process must equal
the mass of oxygen consumed in the process. For example, if one pound of
BOD, equivalent to 1.5 Ib of COD, is synthesized to 0.65 Ib of microorganisms
having a COD equivalent of 1.42 lb-COD/lb microorganisms, the net COD removal
will be 1.5 - (0.65 x 1.42) or 0.58 Ib COD. Thus, when the synthesized
microorganisms are removed from the process, at once the oxygen usage will
be 0.58 Ib 0^/1b BOD removed. If the microorganisms remain in the
process to be destroyed by endogenous respiration, the mass of microorganisms
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wasted from the process will be reduced by a maximum of 81% of their original
mass when the sludge retention time (SRT) is very long. Sludge retention
time (days) is defined as the mass (Ib) of microorganisms held in the
process divided by the rate at which they are wasted from the process (Ib
per day). Thus, at very long SRT values, the oxygen consumption can be as
much as 1.5 - (0.19 x 0.65 x 1.42) or about 1.3 Ib 02/lb BOD removed.
If the endogenous respiration rate is estimated at lz.5% per day, the
theoretical relationship between oxygen consumption and SRT can be written
as follows:
Ib 02/lb BOD removed = 0.58 + 0.0935/(0.125 + 1/SRT)
Since operating SRT values normally range from 1-10 days with about 5 days
correlating to the conventional activated sludge process, it can be seen
that oxygen usage will vary from about 0.66 Ib 0?/lb BOD removed at a
1-day SRT to about 1.0 Ib 0?/lb BOD removed at a 10-day SRT. Oxygen
usage at the 5-day SRT will be about 0.87 Ib 02/lb BOD removed. Therefore,
use of the short detention time "modified" activated sludge process (1-day
SRT) can be expected to utilize about 25% less oxygen than the conventional
5-day SRT process.
To summarize, a reduction of 50% in electrical power consumption for
supplying diffused air to the activated sludge process is not an unreasonable
goal. This would correlate to a reduction of about 30% in total electrical
energy usage at the plant.
10
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SECTION 4
ENERGY FOR PLANT AND SEWER CONSTRUCTION
Energy consumed in construction of treatment plants and collection
sewers for municipal wastewater represents a significant fraction of the
total energy budget. The most widely used reference for estimating energy
used in construction is the input-output analysis performed at the Center
for Advanced Computation of the University of Illinois. The most recent
report is CAC Document No. 140 (7) which estimates energy consumed in New
Public Utilities Construction as 86,929 Btu/1963 construction dollar. In
1963, construction cost for a 1 mgd activated sludge treatment plant was
about $500,000. Thus, energy for construction would be estimated as 43.5
billion Btu. If construction energy is allocated per volumetric unit of
water treated and the useful life of the plant is taken as 20 years, the
construction energy can be expressed as 6 million Btu/mg or 567 kwh/mg,
assuming 10,500 Btu/kwh. Since electrical power consumption in a 1 mgd
activated sludge plant has been estimated at about 1100 kwh/mg (1), it can
be seen that plant construction energy is roughly one-half the electrical
energy used in operating the plant.
The energy consumed in plant and sewer construction can also be
estimated by summing the energy used in the manufacture of the materials and
the energy used for on-site work such as excavation. A pamphlet (8)
entitled "Sewer and Sewage Treatment Plant Construction Cost Index,"
published by FWPCA, gives the rationale for development of the EPA plant and
sewer indexes. This pamphlet also contains a detailed breakdown of the
materials and labor components for construction of a 1 mgd trickling filter
plant and a one million dollar sewer project involving the laying of 35,000
ft of sewer. This information was used together with estimates of energy
for the manufacture of building materials to make estimates of energy consumed
in building sewers and plants. Estimates of energy utilized for the manu-
facture of building materials, taken from references (9) and (10), are shown
in Table 4. Table 5 shows a computation of the energy consumed in construct-
ing a 1 mgd trickling filter plant. Table 6 shows a similar computation for
the energy consumed in constructing 35,000 ft of sewer. The cost of con-
structing the 1 mgd trickling filter plant was given as $470,000 in 1963
dollars. Therefore, the energy consumption was 124,000 Btu per 1963 dollar.
Similarly, the cost of the sewer project for laying 35,000 ft of sewer was
1.03 million dollars in 1963 and the corresponding energy consumption was
42,000 Btu/1963 dollar. These estimates agree reasonably well with the
86,929 Btu/1963 dollar given in the CAC report.
11
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TABLE 4. ENERGY CONSUMPTION FOR PRODUCTION OF BUILDING MATERIALS
1. Iron and Steel Shipped: 31 million Btu/ton
2. Cement Production: 1.22 million Btu/bbl
3. Concrete: 500 Ib. cement/cu. yd. = 1.33 bbl/cu. yd.
Ready Mix = 0.272 million Btu/cu. yd.
1.22 x 1.33 + 0.272 = 1.89 million Btu/cu. yd.
4. Brick: 8000 Btu/brick (7.5" x 3.5" x 2.25")
5. Vitrified Clay Pipe: 5100 Btu/lb
6. Lime: 3770 Btu/lb.
7. Glass: 17 million Btu/ton
8. Forging: 35 million Btu/ton
9. Foundry: 13 million Btu/ton
10. Die Casting: 12,000 Btu/lb.
11. Rubber: 13,000 Btu/lb.
12. Industrial Chemicals: 6240 Btu/lb.
13. Petroleum Products: 590,000 Btu/bbl
12
-------
TABLE 5. ITEMIZED ENERGY CONSUMED IN CONSTRUCTION OF 1 mgd TRICKLING FILTER WASTEWATER TREATMENT
PLANT IN TERMS OF Btu's AND EQUIVALENT ELECTRICAL ENERGY IN KILOWATT-HOURS
1. Total Iron and Steel: 1688 tons 0 31 million Btu/ton 52.3 billion Btu 4.98 million kw-hr
2. Ready Mix Concrete: 1794 cu. yd. @ 1.89 million Btu/cu. yd. 3.4 billion Btu 0.32 million kw-hr
3. Vitrified Clay Pipe: 748 ft. - 126.8 ton @ 5100 Btu/lb. 1.3 billion Btu 0.12 million kw-hr
4. Concrete Pipe: 1952 ft. - 434.3 ton @ 422 Btu/lb, 0.4 billion Btu 0.04 million kw-hr
5. Concrete Block: 9884 - 30 Ib ea. @ 422 Btu/lb. 0.1 billion Btu 0.01 million kw-hr
6. Brick: 62,000 @ 8000 Btu/brick 0.5 billion Btu 0.05 million kw-hr
7. Excavation & Backfill: 3500 cu. yd. @ 15,000 Btu/cu. yd. .05 billion Btu .05 million kw-hr
Total Energy for Treatment Plant Construction 58.05 billion Btu 5,53 million kw-hr
-------
TABLE 6. ITEMIZED ENERGY CONSUMED IN CONSTRUCTION OF 35,000 FT. (6.652 MILES) OF MUNICIPAL
SEWER IN TERMS OF Btu's AND EQUIVALENT ELECTRICAL ENERGY IN KILOWATT-HOURS
1. Total Iron and Steel: 1067 tons @ 31 million Btu/ton 33.1 billion Btu 3.15 million kw-hr
2. Ready Mix Concrete: 2632 Cu. yr. @ 1.89 million Btu/cu.yd 5.0 billion Btu 0.47 million kw-hr
3. Fuel Usage: 13,950 equip, hrs. @ 150,000 Btu/hr. 2.1 billion Btu 0.20 million kw-hr
4. Vitrified Clay Pipe: 10,414 ft. - 32 Ib/ft @ 5100 Btu/lb 1.7 billion Btu 0.16 million kw-hr
5. Concrete Pipe: 25,000 ft. - 66 Ib/ft @ 422 Btu/lb 0.7 billion Btu 0.07 million kw-hr
6. Concrete Brick: 76,000 ft. - 5.9 lb. ea. @ 422 Btu/lb 0.2 billion Btu 0.02 million kw-hr
7. Concrete Block: 6800 - 30 lb. ea. @ 422 Btu/lb 0.1 billion Btu 0.01 million kw-hr
Total Energy for Sewer Construction 42.9 billion Btu 4.08 million kw-hr
-------
3000
1000-
3000
00
LU
100 —
0
3 10 100
COMMUNITY POPULATION
000
OUSANDS)
:igure 1. Relationship between community size
and (A) total miles of street:
(B) street (sewer) miles per 10,000 acres;
and (C) total miles of sewer.
15
-------
The cost of constructing plants increases with plant size at a slope
on log-log paper of about 0.65-0.7- Thus, the cost of energy consumed in
building plants might be expressed as follows:
Energy Consumption, billions of Btu = 58.1 MGD°'68 (6)
Energy consumed in the construction of sewers is more likely to be directly
proportional to the length of the sewer. A relationship for estimating
the length of sewer as a function of community population (11) is shown
by the dashed line (C) in Figure 1. Estimates of the length of street as a
function of community size are also shown in Figure 1 by the solid line (A).
It can be seen that these relationships are similar, but the street length
is probably more reliable as an estimate of sewer length. Therefore, if a
population of 10,000 is taken as equivalent to 1 mgd of municipal wastewater
volume flow, a 1 mgd plant will be served by about 40 miles of sewer, a 10
mgd plant by 230 miles, and a 100 mgd plant by about 1900 miles.
If the energy for laying sewers is taken as 42.9 million Btu/35,000
ft or .1.226 million Btu/ft (6.46 billion Btu/mile), it can be seen that for
a community of 10,000 persons (1 mgd), the energy for construction will be
259 billion Btu for the sewer and 58.1 billion Btu for the plant, making a
total of 317 billion Btu or 30.2 million kwh. If the useful life of the
sewer is taken as 50 years (18,250 days) and the useful life of the plant
as 20 years (7300 days), the energy consumption per day will be 1352 kwh/mg
(10,500 Btu/kwh) for the sewer and 758 kwh/mg for the plant, making a total
of 2110 kwh/mg.
Since the electrical power consumed in a 1 mgd activated sludge plant
has been estimated at 1100 kwh/mg, it follows that the energy consumed in
constructing the plant and sewer system is roughly twice the electrical
energy used.
For the 10 mgd plant with 230 miles of sewer, the energy consumed for
constructing sewers will be 7766 kwh/day or 777 kwh/mg. Similarly, the
energy used for constructing the plant will be 3628 kwh/day or 363 kwh/mg.
The total for the 10 mgd plant is, therefore, 1140 kwh/mg or about 128% of
the 893 kwh/mg electrical energy used at the plant.
For the 100 mgd plant with 1900 miles of sewer, the energy for construc-
ting sewers will be 642 kwh/mg and the energy for constructing the plant will
be 174 kwh/mg, for a total of 816 kwh/mg or roughly the same as the 835 kwh/mg
daily electrical energy used at the plant.
16
-------
SECTION 5
ENERGY FOR MANUFACTURE OF PROCESS CHEMICALS
Energy consumed in the production of chemicals can represent a
significant fraction of the total energy used in the treatment of municipal
wastewater. Estimates available in literature differ for a number of
reasons. For example, production processes used often produce not one
but two or more chemicals, raising the question of how the production energy
should be apportioned between the products. Also, some processes sucK as
that used for production of sulfuric acid naturally produce rather than
consume heat. This heat energy, under the proper circumstances, can be
recovered for other uses. These problems will be considered in the following
discussion for individual chemicals. Some chemical producing processes, such
as that for lime, use primarily heat energy while others, such as that used
for production of chlorine, use primarily electrical energy. A conversion
factor of 10,500 Btu/kwh will be used here. This value is a commonly used
average conversion factor used for coal fired steam power plants or station-
ary diesel engines.
The Department of Commerce and the Department of Energy are conducting
a voluntary industrial energy program (9) which contains some of the best
estimates for overall energy used in the production of various manufactured
products. The National Lime Association reports that in 1975 an average of
3770 Btu were consumed for the production of each pound of lime. This is
equivalent to 0.36 kwh/lb of lime.
The electrolytic process for producing chlorine from sodium chloride
also produces caustic soda (NaOH). A small amount of additional energy is
required for concentrating the caustic soda solution. The Ford Foundation
report (10) estimates the total energy used in producing chlorine and caustic
soda as 2.216 kwh/lb of total product. Since the products are roughly 47%
chlorine and 43% caustic soda, a reasonable estimate for chlorine production
is 1.04 kwh/lb of chlorine. In a report written for the Canadian Ministry of
the Environment, Zarnett (12) took a similar approach in working with the
estimates provided by Shreve (13) and estimated 1.21 kwh/lb of chlorine
produced. The IR&T report (14) on energy use by various industrial chemicals
showed that in 1971 energy used in the alkalies and chlorine industry averaged
2.05 kwh/lb. The corresponding value for 1973 was 1.98 kwh/lb. Data from
Diamond Shamrock on the efficiency of their Sanilec system for in-plant
production of chlorine shows that chlorine can be produced for 2 kwh/lb under
ideal conditions. Thus, the consumption of energy for production of chlorine
ranges from 1-2 kwh/lb, depending on the assumptions made.
17
-------
Dry alum (17-1% Al^CL) is made by mixing aluminum ore (Bauxite) with
sulfuric acid. Heat is required to concentrate the clarified aluminum
sulfate solution in open steam-heated evaporators. Shreve (13) estimated
that 640 Ib of coal and 29 kwh are needed to produce one ton of alum.
Using 11,790 Btu/lb of coal, Zarnett (12) computed 7.85 million Btu/ton of
alum. This is equivalent to 0.374 kwh/lb of alum. The energy required to
produce the sulfuric acid (1140 Ib/ton of alum) is negligible as will be
shown later.
Ferric chloride is produced by treating spent pickling liquor with
chlorine. When sulfuric or hydrochloric acid is used to clean steel, the
spent solutions are primarily ferrous chloride (FeClp) or ferrous sulfate
(FeSOa). Since the effectiveness of either iron salt is dependent on the
amount of ferric iron present, the stoichiometric amount of chlorine needed
to oxidize ferrous iron to ferric iron is best expressed as 0.635 Ib
chlorine/lb iron. Thus, whether the pickling liquor is ferrous chloride or
ferrous sulfate, the amount of chlorine used^is 0.635 times 0.3443 Ib iron
per Ib of equivalent ferric chloride or 0.22"lb chlorine per Ib of
equivalent ferric chloride. If the energy for production of chlorine is
taken as 2 kwh/lb chlorine, the energy for production of ferric chloride or
its equivalent ferrous sulfate is 0.44 kwh/lb.
Pure oxygen can be produced at the plant site by the cryogenic process
or by pressure swing adsorption (PSA). The commercial separation of oxygen
from atmospheric air has large amounts of nitrogen gas as a by-product. The
Ford Foundation report (10) estimated the energy used for production of
oxygen as 19 kwh/1000 cu. ft., in terms of 1958 technology. This is equiva-
lent to 0.23 kwh/lb. Culp/Wesner/Culp (2) interviewed a number of oxygen
generation equipment suppliers and averaged their estimates to 0.25 kwh/lb.
An EPA in-house study (15) estimated the power consumption for generation as
0.23 kwh/lb for PSA generation and 0.133 kwh/lb for cryogenic generation.
However, a significant amount of energy is also needed for dissolving the
oxygen in the activated sludge aeration basin. The total energy consumption
for generation and dissolution was estimated as 0.382 kwh/lb for PSA and
0.271 kwh/lb for cryogenic.
Total energy for production of methanol which includes the energy of the
feedstock (natural gas) was estimated by IR&T to be 18,500 Btu/lb in 1971 and
17,890 Btu/lb in 1973. This averages to 18,200 Btu/lb or 1.73 kwh/lb. The
C/W/C report (2) estimates the energy for production of methanol as 0..7-1.4
kwh/lb.
The best estimates of energy consumption in the production of activated
carbon are given in a paper by Bernardin and Petura (16) of the Calgon
Corporation. They estimate energy for the manufacture of 51,200 Btu/lb of
activated carbon. This estimate includes the Btu content of the coal used
as feedstock. Energy for reactivation of spent carbon was estimated as
4260 Btu/lb or about 8.3% of the energy required for manufacture of the
carbon. Manufacturing energy is equivalent to 4.88 kwh/lb and reactivation
is equivalent to 0.41 kwh/lb.
18
-------
As mentioned earlier, the production of sulfuric acid involves the
burning of sulfur which produces about 2.6 million Btu or 1.1 tons of high
pressure steam per ton of sulfuric acid produced. The electrical energy
consumed is only about 9 kwh/ton of sulfuric acid produced. These data
are from the Sulfur Institute Technical Bulletin No. 8. It can be concluded
from this that the energy consumed in producing sulfuric acid is negligible.
If dewatering chemicals are estimated as 200 Ib of lime and 150 Ib of
ferric chloride per ton of dry sludge solids, the off-site energy used for
production of the chemicals will be about 91 kwh/mg. If the final effluent
stream is chlorinated at a concentration of 8 mg/1 and the cost of producing
chlorine is taken as one kwh/lb of chlorine, an additional expenditure of
67 kwh/mg will be incurred, making a total of 158 kwh/mg for off-site produc-
tion of chemicals.
19
-------
SECTION 6
HEAT ENERGY USED AT THE PLANT
Anaerobic Digester Heating:
Heat energy is used in the plant for holding the contents of the
anaerobic digester at 95°F, for heating the space used for housing equipment
and personnel, and for sludge incineration.
Heat energy is utilized in the anaerobic digester for raising the
temperature of the sludge stream up to the operating temperature of the
digester and also for overcoming the loss of heat through the digester walls,
If the difference between the digester operating temperature and the ambient
temperature of the sludge stream is called DT in F3 the sludge production
rate in Ib sludge solids produced mg of water treated is called W, and the
concentration of solids in the sludge feed stream is TSS in percent, the
amount of heat used to raise the sludge temperature can be computed as
follows:
Btu/mg - 100'WDT/TSS (7)
Heat loss through the digester walls can be computed using a rule-of-
thumb given in the WPCF Manual of Practice No. 8 (17) which states that in
the northern part of the U.S., daily heat losses can be estimated as 62,500
Btu/1000 cu. ft. of digester volume. This estimate can be adjusted for
climatic conditions by multiplying by a factor (F) of 0.5 for digesters in
middle U.S. and by a factor of 0.3 in southern U.S. Primary digesters are
normally heated while secondary digesters are not. If the hydraulic
retention time (HRT) for the primary digester is estimated at 15 days, the
heat loss through the walls in northern U.S. locations can be expressed as
follows:
Btu/mg = 100'WHRT'F/TSS (8)
HRT = hydraulic retention time, days
F = factor to account for
geographical location
These two relationships can be added to find the total heat energy used by
anaerobic digesters as follows:
20
-------
Btu/mg = (100'W/TSS)(DT + HRT'F) (9)
The temperature of the digester feed stream will also depend on the
climate at the plant. For example, the temperature of wastewater averages
58°F at Buffalo, NY, 60°F at Chicago, IL, 63°F at Richmond, IN, 62°F at
Minneapolis-St. Paul, and 51 F at Ely, Minnesota. Therefore, conservative
estimates for sludge temperature might be about 58 F for northern U.S.,
63 F for middle U.S., and about 68 F for southern U.S. Sludge production
in secondary plants averages about 2000 Ib solids/mg, and the concentration
of thickened combined sludge can be estimated at about 4.5%. Therefore,
if the hydraulic retention time of the primary digester is 15 days, the
total heat consumption for anaerobic digesters will be about 2.3 million
Btu/mg for northern U.S., 1.76 million Btu/mg for middle U.S., and 1.4
million Btu/mg for southern U.S. Using the conversion factor of 10,500
Btu/kwh, these estimates are equivalent to 219 kwh/mg, 168 kwh/mg, and 133
kwh/mg. Equation (9) shows that these estimates are directly proportional
to the sludge production and inversely proportional to the sludge concentra-
tion of the feed stream. For example, if the concentration of the sludge
was 3% instead of 4.5%, which might be the case without thickening, the
estimates would be increased by a factor of 1.5.
Space Heating:
Heating requirements for habitable space 'in wastewater treatment plants
is easily computed when a specific plant, for which all construction details
is known, is considered. Making estimates for plants in general is much
more difficult. Information of this kind was compiled by Voegtle (18) from
data for 10-12 plants. Voegtle1s estimates are shown by the horizontal
solid lines in Figure 2. Smith (1) estimated electrical energy consumption
in activated sludge plants to be about 900 kwh/mg. Voegtle1s estimates for
space heating can be seen to be about 12 billion Btu/yr at 10 mgd or about
250 billion Btu/yr at 100 mgd. Using the conversion of 10,500 Btu,
Voegtle's estimates correlate to 313 kwh/mg at 10 mgd and 752 kwh/mg
at 100 mgd. Thus, according to Voegtle, building heat requirements are a
significant fraction of the electrical power requirements. Because of the
apparent importance of space heating requirements, an independent study was
made based on information gathered from operating plants.
Estimates of heat consumed for space heating from ten plants are shown
by the data points in Figure 2. The squared point represents the Ely,
Minnesota plant which is atypical because the plant includes two-stage lime
clarification, tertiary filters, and vacuum filters for dewatering of all
sludges. Since all tertiary processes are housed, the space to be heated
at Ely is also atypical. It can be seen from Figure 2 that two of the
points fall within the range estimated by Voegtle, but the majority of the
points suggest that Voegtle's estimates are high. A tabulation of the data
gathered is shown in Table 7. Since the heat consumed for space heating
cleurly depends on the volume of the space heated, as well as the climate
at the plant site, an effort was made to relate heat consumed to these
variables.
21
-------
1000
o
o
c
o
= 100
IB
O
LU
X
O
Z
B 10
z
z
1
/
/
/
/
_L
Figure 2.
1 10 TOO 200
PLANT DESIGN CAPACITY, mgd
Space heating requirements for wasfrewaiar treatment
plants versus plant size. Solid lines represent estimates
by Voegtle, circled points are typical plant data,
squared point represents Ely, Minnesota tertiary plant,
dashed lines show estimates based on building volume
and 4 Btu/yr per cu ft per annual degree-days.
22
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TABLE 7. ESTIMATES OF HEATED VOLUME AND HEAT USED FOR SPACE HEATING
IN WASTEWATER TREATMENT PLANTS
(V)
CO
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
State
New York
Ohio
Ohio
Ohio
Virginia
Ohio
Ohio
Ohio
Wisconsin
Minnesota
Wisconsin
Plant Size
150 mgd
100 mgd
8 mgd
28 mgd
27 mgd
51
120 mgd
109 mgd
5.5 mgd
1 .5 mgd
54 mgd
Degree-days/yr
7218
6144
5522
5522
3818
5522
6525
5522
7536
Heated Volume, Annual Heat Load Btu/yr per cu ft
Million cu ft billion Btu/yr per degree-day
2.713 71.65 3.65
33.2
0.694 16.0 4.18
4.75
0.40 6.42 4.20
23.5
62.0
1.68 15.2 1.64
1.7
9.6
88.6
-------
In wastewater treatment plants, the principal loss of building heat
is associated with the ventilation requirements. For example, the Ten
State Standards (19) recommends one complete air change per minute for
chlorination housing, twelve complete air changes per hour for wet wells,
and six air changes per hour for tunnels and dry wells. Current practice
is to provide about six air changes per hour for enclosed sludge handling
facilities. The C/W/C report (2) estimated the ventilation and infiltration
requirements for the total composite building volume at 5.5 air changes per
hour. This estimate appears to represent reasonably well the observed heat
loss in operating plants. Heat loss through walls and ceilings is small
compared to the heat loss from ventilation; roughly 20% of the ventilation
heat loss. The ventilation heat loss can be readily calculated as follows:
Ventilation heat loss, Btu/yr = V x 0.075 x 5.5 x 24 x 0.24 x days/0.7.
In this equation, V is the building volume ventilated in cubic feet, 0.075
is the density of air (Ib/cu ft), 5.5 is the number of air changes per hour,
24 is the number of hours/day, 0.24 is the specific heat of air (Btu/lb/ F),
utilization efficiency is 0.7, and days is the number of degree-days per
year at the site of the building. Thus, with these assumptions, the heat
consumption for ventilation at 5.5 air changes/hr can be expressed as
3.394 Btu/yr per degree-day per cu. ft. of volume ventilated. This is shown
by the horizontal line in Figure 3.
Heat loss through the walls and ceiling of the building space is more
difficult to estimate. ASHRAE (American Society of Heating, Refrigerating,
and Air-Conditioning Engineers) Standard 90-75 (20) gives recommended
standards for heat loss coefficients for walls (U ) and ceilings (U ) in
terms of Btu per hour per sq. ft. per degree F. The value of U for non-
residential buildings is given by the following relationship:
Uw = 0.39 - 0.019(°days/1000) (10)
The corresponding relationship for heat loss through ceiling and roofs is
given by the following relationship:
U = 0.124 - 0.008(°days/1000) (11)
\*
If the buildings are assumed to be 20 ft high and 40 ft wide, the heat loss
through walls and roofs can be expressed as follows:
(Btu/yr)/cu ft/°day = 24(UW + Uc)/20 + (1600/V)UW 24 (12)
The second term approaches zero as the building volume becomes large.
The total heat loss is shown graphically by the two sloping lines in
Figure 3. The lower line is for very large (1,000,000 cu. ft.) buildings
and the upper line is for 30,000 cu. ft. which is characteristic of a 1 mgd
plant. The four points shown in Figure 3 are reported heat consumption
values.
The best way to estimate space heating requirements for wastewater
treatment plants appears to be in terms of (Btu/yr)/cu ft/( days/yr). An
average value for the three upper points in Figure 2 is about 4 Btu/yr per
24
-------
6
*• £. c
-------
cu ft per annual degree-days. The Federal Housing Administration set a
standard of 2 Btu/yr per cu ft per annual degree-days in 1965. Property
standards required by the HUD Operation Breakthrough reduced this figure
to 1.5 in 1970, and in 1972 FHA set their minimum standard for housing at
1 Btu/yr cu ft per annual degree-days. Thus, as might be expected, the
observed heating requirement for wastewater treatment plants is significantly
above conventional housing standards. This is undoubtedly due to the
requirement for adequate ventilation.
The volume of heated space in wastewater treatment plants can vary
widely, depending on the services provided at the plant. In Figure 4, the
three squared points are estimates made by George S. Russel of the consulting
firm of Russel and Axon in Daytona Beach, Florida, for Cornell University in
1960. The circled points depict recently gathered data from plants considered
to be typical. The triangular point represents the Ely, Minnesota tertiary
plant. The solid line in Figure 4 can be used as a rough estimate of the
amount of heated space to be found in secondary treatment plants.
Thus, if the heat requirement is taken as 4 Btu/yr per cu ft per annual
degree-days, an estimate can be found for building heat requirements. The
annual degree-days ranges from 3000-8000 for most of the United States.
These limits were used together with the estimate of 4 Btu/yr per cu ft per
annual degree-days and the estimate of heated volume shown in Figure 4 to
find the dashed lines shown in-Figure 2. It can be seen that the band
enclosed by the dashed lines is a fair representation of the data points
and can, therefore, be used as a reasonable estimate for building heat
requirements.
The conclusion that a major fraction of the heat used for space heating
is associated with heating ventilating air makes the use of energy wheels a
potentially attractive device for conservation of energy. The energy wheel,
as described by Pallio (21), is an air-to-air heat exchanger which picks up
heat from contaminated air being exhausted from the building and transfers'
it to the fresh outside air being drawn into the building for ventilation.
Pallio estimates the efficiency of the energy wheel at 80%.
Heat Pumps:
Heat pumps are being installed in some wastewater treatment plants (21")
to remove heat from the final effluent stream and deliver it at a highjsr
temperature (above 100 F) for heating building space. The performance of
heat pumps is expressed as the coefficient of performance (COP) which is
defined as the amount of heat removed from the wastewater stream divided by
the heat equivalent (1 kwh = 3413 Btu) of the electrical power consumed. The
COP normally ranges from 2 to 3, depending on the temperature of the waste-
water stream6 The ASHRAE standard (20) for COP of heat pumps with a water
source at 60 F is 2.2. The alternative to using heat pumps is to burn the
fuel at the plant site and produce hot water for heating building space. The
overall efficiency of a hot water boiler is about 60%. Therefore, if 10,500
Btu of fuel energy is supplied to the boiler, about 6300 Btu will be delivered
for heating. If this same fuel is burned in the electrical power generating
26
-------
0.01
100
PLANT SIZE, mgd
Figure 4. Heated space in wasfewater treatment plants versus
plant size. Circled points are typical secondary plants,
squared points are estimates by George S. Russell,
triangular point is Ely, Minnesota tertiary plant.
27
-------
station, one kwh of electrical energy will be recovered and this will produce
7500 Btu of heat for heating building space when the COP is 2.2. Thus, it
can be seen that the energy efficiency provided by heat pumps that use the
wastewater effluent stream as a source is not dramatic. The heat pump,
however, does have the added advantage of being able to cool as well as
heat the building space.
Auxiliary Fuel for Sludge Incineration:
The amount of auxiliary fuel used for sludge incineration can vary
widely depending on factors such as the water content of the sludge, the
flue gas temperature, and the thermal cycling requirements. The higher
heat value for volatile sludge solids varies from 7000-14,000 Btu/lb of
dry volatile solids (DVS) with about 10,000 Btu/lb DVS as the average.
Heat losses from the operating incinerator include radiation and convection
losses, enthalpy of the products of combustion and water content of the
sludge, cooling air loss, heat for conversion of dewatering chemicals, and
heat loss in the ash. When the sum of the losses is greater than the heat
value of the volatile sludge solids, auxiliary fuel is required. Thus, the
the amount of auxiliary fuel needed for operation can be calculated (22)
when the operating conditions are known.
Most of the heat lost from the incinerator is in the flue gas stream.
Flue gases are composed of the vaporized water content of the sludge and the
DVS products of combustion. If the fraction of dry solids in the sludge
is F and the fraction of dry solids which are volatile is F , the pounds of
water per pound of DVS (W ) can be calculated as (1-F )/(F F ). The
enthalpy of water vapor (above 60 F) can be represented wilhvlittle error
by the following relationship:
Btu/lb H20 = 1160 + 0.505(TS-300) (13)
T = flue gas set point temperature, °F
Thus, it can be seen that if the solids content of the sludge is 25% and
the solids are 70% volatile, about 4.3 Ib of water is associated with each
Ib of DVS, and the enthalpy loss in 800 F flue gases will be 6054 Btu/lb DVS
for evaporating the water content of the sludge.
If the chemical composition of the DVS is 56% carbon, 8% hydrogen, 30%
oxygen, 5^ nitrogen, and 1% sulfur, the enthalpy of the combustion products
(above 60 F) can be estimated by the following relationship:
Btu/lb DVS = (1300 + 450 Ev) + (2.55 + 2.09 EV)(T -300) (14)
X X S
E = excess air supplied/stoichiometric amount
A
The amount of excess air is adjusted to hold the flue gas temperature (T )
at some set point. Therefore, a value for E can only be found from a miss
balance around the incinerator. For example, if the minor losses for
radiation and convection, cooling air, conversion of chemicals, and ash are
28
-------
subtracted from the heat value of the DVS, a net heat available is found. If
the heat needed to evaporate the water content of the sludge is subtracted
from the net heat available, the heat available for combustion products (H )
can be found. This value can then be set equal to equation (14) and E
solved for. A minimum recommended value for E is 0.5. Therefore, if the
value found for E is less than 0.5, auxil,iaryxfuel must be added.
)\
If dewatering chemicals consist of 10% lime and 7.5% ferric chloride,
the heat used for conversion of the chemicals is about 240 Btu/lb DVS. When
30% of the cooling air is discharged to the atmosphere, about 190 Btu/lb DVS
is lost. About 46 Btu/lb DVS is lost in the ash. Estimates for radiation
and convection losses vary with the manufacturer, but an average value in
terms of Btu/hr per sq ft of hearth area is 8200 divided by the hearth area
raised to the 0.435 power. If the loading rate is taken as 2 Ib dry solids
per hour per sq ft of hearth area and the production rate is 2000 Ib/mg of
dry solids, the heat loss (Btu/lb DVS) can be estimated as 1160 divided by
the design capacity (mgd) raised to the 0.435 power. Thus, for a 10/mgd
plant, the radiation and convection loss will be about 426 Btu/lb DVS. The
minor losses from the incinerator are, therefore, about 900 Btu/lb DVS.
Heat available for combustion products (H ) can be estimated as 10,000
Btu/lb DVS minus 900 Btu/lb DVS minus thea6054 Btu/lb DVS needed to
evaporate the water content or 3046 Btu/lb DVS. Equating equation (14) to
3046 Btu/lb DVS yields a value of 31.5% for E , indicating that some auxiliary
fuel is required to hold the flue gas temperature at 800 F.
The higher heat value for fuel oil is about 19,000 Btu/lb or 140,000
Btu/gal. A fraction of this heat, however, is lost in the combustion
products of the fuel oil. The heat lost per pound of fuel oil burned with
no excess air is approximately (700 + 4.14T ) Btu/lb fuel oil. If the
amount of fuel oil used is expressed as gallons of fuel oil per ton of DVS,
(Rf) the heat contributed by the fuel oil per Ib DVS can be calculated as
lows:
Btu/lb DVS = Rf(67.42 - 0.0152 Tg) (15)
\ IN-C I
fOlll
Thus, since the heat deficit needed to hold the value of E at 0.5 with a
flue gas temperature of 800 F was 277 Btu/lb DVS, it can be seen that the
value for Rf is about 5 gal. of fuel oil per ton of DVS.
In general, when H and T are known, the value for E can be calculated
/- -i -i a S A
as follows:
E - (H - 535 - 2.55TJ/(2.09T - 177) (16)
x a ss
If the computed value for E is less than the recommended minimum of 0.5,
E is set at 0.5 and the fuel requirement can be calculated as follows:
A
R, = (3.6T + 446.5 - H )/(67.42 - 0.0152T ) (17)
T s a s
29
-------
CO
o
1400
1300
;v
111 > ii
//////
// ' ' i i
'
-5-4-3-2-101234567
HEAT AVAILABLE FOR COMBUSTION PRODUCTS, 1000's BTU/lb DVS
Figure 5. Auxiliary fuel (Rf = gal fuel oil/ton DVS) shown by dashed lines and excess air ratio
(Ex = air supplied for combustion/stoichiometric amount) shown by solid lines for holding
incinerator flue gas temperature at any set point as functions of heat available for combustion products.
-------
These relationships are shown in Figure 5 as functions of H for T for
easy reference. To find the conditions for which the combustion process
will be self sustaining, Rf can be set equal to zero in equation (17). For
example, the flue gas temperature achievable with no auxiliary fuel is
expressed as follows:
T = (H - 446.5)/3.6 (18)
b a
It is also of interest to find the sludge moisture content which can be
tolerated for self-sustaining combustion at any flue gas temperature. The
pounds of water per pound of DVS (W ) below which the process will require
no fuel can be calculated when the net heat content of the DVS is 9100
Btu/lb DVS as follows:
Ws = (9100 - 446.5 - 3.6TS)/(1008.5 + 0.505TS) (19)
Thus, it can be seen that the value for W is 4.09 at 800°F, 3.34 at 1000°F,
2.69 at 1200°F and 2.11 at 1400 F, If the sludge is 70% volatile, these
values will correlate to dry solids concentrations of 25.9%, 30.0%, 34.7%
and 40.4% at 800, 1000, 1200, and 1400 F, respectively. Thus, it can be
seen that the flue gas temperature achievable with no auxiliary fuel is very
dependent on the water content of the sludge.
When an afterburner is provided to raise the flue gas set point
temperature (T ) to a higher temperature (T.), usually 1400 F, to prevent
the escape of odors, the total amount of fuel used is calculated as follows
where the value of E is found from equation (17), and H is evaluated at
the higher temperature (T.).
R, = (535 - 177E + (2.55 + 2.09E )T. - H )/(67.42 - 0.0152T,) (20)
T X X 1 a 1
Fuel requirements for thermal cycling can also be computed with reason-
able accuracy using the methods given in ref. (22). However, the number and
size of the incinerators and the duty cycles must be known. An example
worked out in ref. (23) showed that a 10 mgd secondary plant with anaerobic
digestion can incinerate 9845 Ib dry solids per day using two 510 sq ft
hearth area incinerators which use 1475 Btu/lb DVS for thermal cycling.
Total fuel consumption in 8 separate plants for 16 operating periods
is reported in ref. (22). The heat content of the dry volatile solids
ranged from 9500-14,000 Btu/lb DVS, and the operating flue gas temperature
ranged from 685-1360 F. Percent solids in the sludge ranged from 13.5% to
48%, and the volatile fraction ranged from 0.43 to 0.65. Total auxiliary
fuel used averaged 3382 Btu/lb DVS and ranged from zero to 6178 Btu/lb DVS.
The fuel consumption reported is in reasonably good agreement with the
relationships presented here, but unknowns such as fuel used for thermal
cycling and approximate values for some of the variables make accurate
comparison impossible.
In order to assign a median value to fuel consumption for incineration,
assume that the DVS have a heat value of 10,000 Btu/lb, the miscellaneous
31
-------
incinerated will
3448 x 700 or
taken as
be estimated as
losses are 900 Btu/lb DVS, the solids content of the sludge is 25% of which
50% are volatile. The 50% value is more characteristic of anaerobically
digested sludge. If the flue gas operating temperature is 800 F, the value
for H will be 625 Btu/lb DVS and the fuel consumption will be 2698 Btu/lb
DVS o^ 49 gal. of fuel oil per ton of DVS. The 1475 Btu/lb DVS fuel
requirement for thermal cycling is characteristic of a 10 mgd plant which
is near the lower limit of plants practicing incineration. Thus, 750 Btu/lb
DVS might be a more appropriate estimate for thermal cycling. Adding this
to the operating fuel requirement gives a total of 3448 Btu/lb DVS which
is close to the average of 16 examples given in ref. (22) of 3382 Btu/lb DVS.
If an activated sludge plant produces 2000 Ib/mg of dry solids which are
70% volatile and anaerobically digests all sludge with a reduction of 50%
in the volatile solids, the amount of volatile solids to be
be 700 Ib/mg. The estimated fuel usage will, therefore, be
2.41 million Btu/mg. If the electrical power equivalent is
10,500 Btu/kwh, the energy consumption for incineration can
230 kwh/mg.
Trucking of Sludge To Land Disposal:
Trucking of dewatered sludge to a land disposal site will generally
require less energy than incineration. For example, if sludge is dewatered
to a density of 55 Ib/cu ft or 0.743 tons/cu yd and hauled in a 10 cu yd
dump truck with an average gasoline mileage of 4.5 miles/gallon gasoline,
usage can be expressed as 0.06 gal/ton-mile, based on the one-way distance.
Furthermore, if the energy content of gasoline is taken as 140,000 Btu/gal
and the electrical energy equivalent of heat energy as 10,500 Btu/kwh, the
energy cost of sludge hauling can be expressed as 0.8 kwh/ton-mile. If the
one-way hauling distance is 20 miles, and the activated sludge plant produces
2000 Ib/mg of undigested sludge or 1300 Ib/mg of digested sludge, the energy
cost for hauling dewatered sludge will be 20 x 0.8 or 16 kwh/mg without
digestion or 10.4 kwh/mg with digestion.
Trucking of liquid sludge to land disposal can use as much or more
energy than incineration of dewatered sludge, depending on the solids content
of the sludge and the distance hauled. Energy used for liquid sludge trucking
is directly proportional to the distance hauled and inversely proportional to
the solids content of the sludge. For example, if 4% solids content sludge
is hauled in a 2500 gallon tank truck having an average gasoline mileage of
4.5 miles/gallon, the energy consumed can be expressed as 1.07 gallons of
gasoline per ton per mile of one-way distance hauled. Again assuming*the
energy content of gasoline as 140,000 Btu/gal and the equivalent of one kwh
as 10,500 Btu, the energy used for hauling 4% liquid sludge is 14 kwh/ton-mile,
Thus, if an activated sludge plant produces 1300 Ib/mg of digested liquid
sludge and the sludge is hauled 20 miles one-way to land disposal, the energy
used is 14 x 20 x 1300/2000 = 182 kwh/mg. Since the energy cost for hauling
is inversely proportional to the solids content, it can be seen that if the
sludge hauled had been 3.2% solids, the energy used would have been equivalent
to the value estimated for incineration of 230 kwh/mg.
32
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SECTION 7
POTENTIAL FOR ENERGY RECOVERY
In activated sludge plants, sludge production is about 2000 Ib of dry
solids per million gallon treated. Since this sludge is usually 65-80%
volatile, the organic dry solids represent a resource of heat energy which
can be recovered to produce mechanical or electrical energy at the plant
site. Raw wastewater has a suspended solids concentration of about 225
mg/1, and about 60% of the suspended solids can be removed in the primary
settler. Production of primary sludge will, therefore, be 1125 Ib/mg and
if the sludge is 60% volatile, production of volatile solids will be about
675 Ib/mg. Volatile solids production in the activated sludge process is
about 0.64 Ib/lb BOD removed at an SRT of 3-5 days. Thus, volatile sludge
production in the activated sludge process is about 750 Ib VSS/mg or 933
Ib/mg of dry solids. Total sludge production in the activated sludge plant
is about 2060 Ib/mg of suspended solids or 1425 Ib/mg of volatile solids.
The most commonly used process for recovering energy from the organic
component of the sludge is anaerobic digestion of the sludge to produce
digester gas. Reduction of volatile solids during digestion is in the range
of 50-60% with about 15 scf of digester gas produced per Ib of volatile solids
destroyed. Thus, production of digester gas will be about 10,700 scf/mg
when the more conservative estimate of 50% reduction is used. The higher
heat value for digester gas is about 600 Btu/scf. When the gas is used as
fuel for internal combustion (1C) engines, the energy conversion rate is in
the range of 7000-8500 Btu/hp-hr. If an average conversion rate of 8000 Btu
per hp-hr is used, the energy recovered will be about 800 hp-hr/mg or 33 hp
per mgd of activated sludge plant capacity. If the mechanical energy is
used to generate electrical energy using an electric generator with 96%
efficiency, the electrical energy production will be about 573 kwh/mg. This
represents 50-70% of the total plant electrical requirements, depending on
plant size. Waste heat rejected by the diesel 1C engines can be recovered
from the engine jacket water to heat the digesters. Roughly 25% of the heat
in the digester gas supplied to the 1C engines can be recovered from the
jacket water. The heat recovered from the jacket water will be about 1.6
million Btu/mg which is near the center of the estimated heat requirement of
2.3-1.4 million Btu/mg. Thus, some auxiliary fuel may be needed for heating
the anaerobic digesters.
Another energy recovery scheme which has been proposed (24-25) for
large plants where ultimate disposal of the sludge is a difficult problem is
the concept of incinerating all of the sludge and recovering heat from the
incinerator flue gases by means of waste heat boilers often called econo-
33
-------
mizers. This scheme can also be applied to multiple hearth furnaces used
for lime recalcination or activated carbon regeneration. Although suppliers
of waste heat boilers feel that the exit gas temperature can be as low as
300-400 F before corrosion becomes limiting, a more conservative design
value which might be more appropriate for wastewater treatment plant instal-
lations in 500 F.
The total or gross amount of heat recovered in the waste heat boiler
equals the difference in enthalpy of the flue gases across the boiler. The
net amount of heat recovered from the dry volatile sludge solids alone can
be found by subtracting the sum of the miscellaneous losses and the enthalpy
of the gas stream at the boiler exit temperature (T ) from the hhv of the
DVS. Therefore, if the hhv of the DVS and the miscellaneous losses are
fixed, at say 9100 Btu/lb DVS, the net amount of heat recovered is a
function of T and E . Both T and E should be minimized to maximize the
net heat recovered. XE is minimized when the set point temperaturg (T ) is
as great as allowed by the incinerator materials. A value of 1100 F will
be used here as the highest practicable value. Where control of odors is
important, or where air pollution control standards demand it, an after-
burner may be required to raise the flue gas temperature from 1100 F at the
incinerator exit to 1400 F at the entrance of the waste heat boiler. The
value for E , however, will be determined by T , as shown by equation (16).
A O
Flue gases are composed of superheated steam resulting from the water
content of the sludge and the products of combustion for the DVS and the
auxiliary fuel, if any is used. From equation (13) it can be seen that the
heat recovered from the water content of the sludge will be 0.505(T. - T )W ,
where T. is the temperature at the inlet of the waste heat boiler and T °
is the temperature at the exit. °
Heat recovered in the waste heat boiler from the volatile solids com-
bustion products per Ib DVS is (2.55 + 2.09E )(T. - T ), as shown by equation
(14). The fraction of excess air (E ) is computed from equation (16) using
a value of 1100 F for T . E must b§ set equal to 0.5 if the computed value
is less than 0.5. b x
The amount of heat recovered from the auxiliary fuel products of
combustion is 0.0152(7. - T ) Rf, where Rf (gal. fuel oil/ton DVS) is the
amount of £uel required to Hold T. at liofi F when no afterburner is required
or at 1400 F when an afterburner is used. The amount of auxiliary fuel (Rf)
is computed from equation (20) by evaluating H at the temperature T..*
a i
The total amount of heat recovered in the waste heat boiler is the sum
of the three components quantified above and can be expressed as follows:
Btu/lb DVS = (0.505W + 2.55 + 2.09 Ev + 0.0152 R,)(T- - T ) (21)
** X T 1 0
The net heat recovered from the volatile sludge solids alone can be
found by subtracting the miscellaneous losses (900 Btu/lb DVS), the enthalpy
of the water content of the sludge given by equation (13) by substituting
To for Ts' and the entnalPy of tne products of combustion of the DVS given
34
-------
by equation (14) with T substituted for T from the hhv of the DVS (10,000
Btu/lb DVS). The value used for E in equation (14) is computed from
equation (16) except where the computed value for E is less than 0.5, in
which case, a value of 0.5 is used for E . Thus, i£ can be seen that the
net heat recovered is a function of only T , T , and the water content of the
sludge. The computed value for net heat recovered from the DVS is shown
in Figure 6 by the solid lines, assuming a value of 1100 F for T , 900 Btu
per Ib DVS for miscellaneous losses, and a hhv of 10,000 Btu/lb DVS for the
DVS. It can be seen from Figure 6 that the net heat recovered frorq the
DVS is zero when
when F is about
DVS is zero when F is 23.5% with a boiler exit temperature of 500 F and
2T% with a T value of 350 F.
The total amount of heat recovered from the DVS and the auxiliary
fuel is shown in Figure 6 by the dashed lines. Fuel needed to hold the
set point temperature at 1100 F is zero when F has a value above about
32.2%. The vertical distance between the 1100-500 F line and 1400-500 F
line is the heat recovered from the auxiliary fuel used in the afterburner.
Computed values for percent excess air and auxiliary fuel used in
making estimates of the amount of heat recovered are shown in Figure 7.
For a set point temperature of 1100 F, the combustion process is self-
sustaining when F is above 32.2%. Thus, E is greater than 0.5 and
auxiliary fuel without the afterburner is zero when F is greater than
32.2%. Auxiliary fuel used in the afterburner is the difference between
the two dashed lines.
Mechanical energy for direct driving of pumps and blowers can be pro-
duced at the plant by allowing steam from the waste heat boiler to expand
through a steam turbine. Maximum energy available from each pound of steam
can be found from steam tables by assuming that the steam expands isentrop-
ically from the inlet conditions to the pressure provided in the condenser.
Theoretical efficiency is usually expressed as pounds of steam used per kwh
produced defined as the isentropic enthalpy drop divided by 3413 Btu/kwh.
Theoretical steam rates for typical inlet conditions are shown in Table 8.
The amount of heat used to produce the steam is the enthalpy at the
turbine inlet minus the enthalpy of condenser water returned to the waste
heat boiler. Therefore, if the turbine is supplied with 600 psig steam at
750 F and the turbine is equipped with a condenser operating at 2" Hg, the
heat added to the steam is 1379 Btu/lb steam at the inlet minus 70 Btu/lb
steam in the condenser water or 1309 Btu/lb steam used in the turbine.
Multiplying this value by the steam rate of 7.08 Ib/kwh gives the amount of
heat used to generate one kwh: 9268 Btu/kwh. Thus, the theoretical thermal
efficiency is 3413/9268 or 36.8%.
To account for turbine inefficiency a factor must be applied to the
theoretical steam rate. In general, turbines with lower speeds (rpm) and
greater horsepower are more efficient. Turbines with horsepower ratings
in the 5000-15,000 hp range and speeds in the 7500-10,000 rpm range have
a quoted efficiency of about 76%. Therefore, the steam rate will be
7.08/0.76 or 9.32 instead of the theoretical value of 7.08. Industrial
35
-------
>
Q
CO
CTs
O
o
34
36
38
40
42
44
46
48
50
« SLUDGE SOLIDS FRACTION, percent
Figure 6. Amount of heat (BTU/lb.DVS) recoverable in the waste heat boiter. Solid lines show heat
recoverable from the DVS alone. Dashed lines show total recoverable heat from both DVS
and auxiliary fuel. Flue gas set point temperature is 1100°F. Afterburner used to raise flue
gas temperature to 1400°F. Boiler exit temperatures are 500°F and 350°F.
-------
V)
Q
CO
01
Q
UJ
CO
O
SLUDGE SOLIDS FRACTION, percent
Figure 7. Excess air ratio shown by solid line to hold incinerator flue gas temperature at 1100° F.
Auxiliary fuel required is shown by dashed lines for combustion of DVS at 1100° F and
for raising the gas temperature to 1400° F with an afterburner.
-------
TABLE 8. THEORETICAL STEAM RATES (Ibs/kwh)
CO
OD
2" HGA
4" HGA
0 PSIG
50 PSIG
TOO PSIG
200 PSIG
300 PSIG
400 PSIG
600 PSIG
250 PSIG 400 PSIG 600 PSIG
550°F 750°F 750°F
8.78 7.36 7.08
9.67 7.98 7.64
14.57 11.19 10.40
26.75 17.56 15.36
42.40 23.86 19.43
43.51 29.00
43.72
-
„ _ =
600 PSIG
850°F
6.66
7.17
9.64
13.98
17.64
26.33
39.70
-
_
900 PSIG
900°F
6.26
6.69
8.74
12.06
14.50
19.45
25.37
33.22
63.40
1500 PSIG
900°F
6.08
6.48
8.26
10.94
12.75
15.84
18.94
22.32
30.75
2000 PSIG
950°F
5.84
6.20
7.78
10.07
11.55
13.96
16.19
18.49
23.63
-------
synchronous electric generators in the range of 3000-5000 kwh have
efficiencies between 95-97%, depending on the percent load. Therefore, the
steam rate used to predict electrical power generation will be 9.32/0.96
or 9.7 Ib/kwh. With this steam rate the thermal efficiency of the turbine
and generator combined will be 34137(9.7 x 1309) or 26.9%. The heat used
to produce one kwh of electrical energy can be estimated as 9.7 Ib steam/kwh
times 1309 Btu/lb steam or 12,697 Btu/kwh. If the heat recovery rate in
the waste heat boiler (Btu/lb DVS) is divided by the heat used to produce
electrical energy (Btu/kwh), an overall conversion rate of kwh/lb DVS can
be found. For example, if the sludge incinerated is 40% solids, 70%
volatile, and the waste heat boiler operates over the 1400-350 F range, the
total amount of heat recovered will be about 6900 Btu/lb DVS, and this value
divided by 12,697 Btu/kwh gives 0.5434 kwh/lb DVS. Heat recovered from the
DVS alone is 4550 Btu/lb DVS which corresponds to a conversion rate of 0.358
kwh/lb DVS.
Since activated sludge plants produce about 1400 Ib/mg of DVS, these
values are equivalent to 761 kwh/mg and 501 kwh/mg for total recoverable
electrical energy and net recoverable energy from the DVS alone. These
values can be compared to the 573 kwh/mg found earlier for anaerobically
digesting the sludge and using the digester gas as fuel for 1C engines.
This procedure can be used for finding gross and net recoverable electrical
energy for any of the heat recovery estimates shown in Figure 6.
This analysis shows that energy recovery using a waste heat boiler to
salvage heat from incinerator flue gases is competitive with anaerobic
digestion only under the most favorable conditions. For example, heat losses
from the waste heat boiler, which might be as much as 10%, have been neglected
and the turbine efficiency used corresponds to turbines used only in very
large plants. Where sludge with 40% dry solids is incinerated, and the flue
gas temperature is raised to 1400 F using auxiliary fuel, power production
(assuming a 10% heat loss in the boiler) will be about 38 hp/mgd. Thus, a
5000 hp turbine would be installed in a plant treating about 130 mgd of
wastewater with the activated sludge process. The analysis has also shown
that the moisture content of the incinerated sludge is a critical parameter
if maximum power recovery is the goal.
The principal advantage of the incineration with heat recovery scheme
is that it minimizes the mass of residual for ultimate disposal. Another
advantage is that low pressure steam can be tapped off the turbine for
building heat or for dewatering liquid sludge. Also, when air pollution
standards demand that the flue gas temperature be raised to 1400 F with
auxiliary fuel, a part of the heat supplied by the fuel can be recovered
in the waste heat boiler.
39
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REFERENCES
1. Smith, R, "Electrical Power Consumption for Municipal Wastewater
Treatment". EPA-R2-73-281, U.S. Environmental Protection Agency,
Cincinnati, Ohio, 1973.
2. Wesner, G. M. et al, "Energy Conservation in Municipal Wastewater
Treatment". EPA-430/9-77-011, U.S. Environmental Protection Agency,
unpublished.
3. Leary, R. D., L. A. Ernest, and W. J. Katz, "Effect of Oyxgen-Transfer
Capabilities on Wastewater Treatment Plant Performance". Journal of
Water Pollution Control Federation, Vol. 40, p. 1298, 1968.
4. Leary, R. D., L. A. Ernest, and W. J. Katz, "Full-Scale Oxygen
Transfer Studies of Seven Diffuser Systems". Journal of Water Pollution
Control Federation, Vol. 41, p. 459, 1969.
5. Smith, R. and R. G. Eilers, "Control Schemes for the Activated-Sludge
Process". EPA-670/2-74-069, U.S. Environmental Protection Agency,
Cincinnati, Ohio, 1974.
6. Petersack, J. F. and R. G. Smith, "Advanced Automatic Control Strategies
for the Activated Sludge Treatment Process." EPA-670/2-75-039, U.S.
Environmental Protection Agency, Office of Research and Development, 1975,
7. Herendeen, R. A. and Clark W. Bullard, III, "Energy Cost of Goods
and Services", Center for Advanced Computation, University of Illinois
at Urbana-Champaign, Document No. 140, 1974.
8. "Sewer and Sewage Treatment Plant Construction Cost Index". FWPCA, U.S.
Dept. of Interior, 1967.
9. "Voluntary Industrial Energy Conservation". Progress Report 4, November
1976, U.S. Dept. of Commerce and Federal Energy Administration.
10. "Energy Consumption in Manufacturing", a Report to the Energy
Policy Project of the Ford Foundation, Ballinger Publishing Company,
Cambridge, Mass., 1974.
11. "Areawide Assessment Procedures Manual", John G. Myers, et al of
the Conference Board, EPA-600/9-76-014, U.S. Environmental Protection
Agency, Office of Research and Development, 1976.
40
-------
12. Zarnett, G. D., "Energy Requirements for Conventional and Advanced
Wastewater Treatment". Publication N. W47, Ministry of the Environment,
Toronto, Ontario, Canada, October 1975.
13. Shreve, R. N., "The Chemical Process Industries". McGraw-Hill Book
Company, 1956, New York.
14. Saxton, J. C. et al, "Industrial Energy Study of Industrial Chemicals
Group". International Research and Technology Corporation under
Contract 14-01-0001-1654 for U.S. Dept. of Commerce and the Federal
Energy Office.
15. Smith, R., W. F. McMichael, and R. G. Eilers, "Alternative Systems for
Supplying Dissolved Oxygen in Wastewater Treatment". Unpublished EPA
report.
16. Bernardin, F. E. Jr., and J. C. Petura, "Energy Considerations in
Adsorption as a Wastewater Renovation Technique". Proceedings of the
Second National Conference on Complete Water Reuse, Chicago,
May 4-8, 1975.
17. "Sewage Treatment Plant Design". Water Pollution Control Federation
Manual of Practice No. 8, 1967.
18. Voegtle, J. A., "Be Conservative About Energy". Water Pollution
Control Federation Deeds and Data, February, 1975.
19. "Recommended Standards for Sewage Works". Great Lakes-Upper Mississippi
River Board of State Sanitary Engineers, 1973.
20. "Energy Conservation in New Building Design". American Society of
Heating, Refrigerating, and Air-Conditioning Engineers (ASHRAE) Standard,
90-75, 1975.
21. Pallio, Frank S., "Energy Conservation and Heat Recovery in Wastewater
Treatment Plants". Water and Sewage Works, pg. 62-65, February 1977.
22. Unterberg, W., R. J. Sherwood, and G. R. Schneider, "Computerized
Design and Cost Estimation for Multiple-Hearth Sludge Incineration",
17070 EBP 07/71, U.S. Environmental Protection Agency, Office of
Research and Development, 1971.
23. Smith, R. and R. G. Eilers, "Computer Evaluation of Sludge Handling
and Disposal Costs". Proc. of 1975 National Conference on Municipal
Sludge Management and Disposal.
24. "A Plan for Sludge Management". Havens and Emerson, Ltd., Consulting
Engineers for The Commonwealth of Massachusetts Metropolitan District
Commission, 1973.
41
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25. "Phase I Report of Technical Alternatives to Ocean Disposal of Sludge in
the New York-New Jersey Metropolitan Area". Camp, Dresser & McKee and
Alexander Potter Associates, Environmental Engineers for Interstate
Sanitation Commission, June 1975.
42
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TECHNICAL REPORT DATA
(Please read Instructions on the reverse before completing)
REPORT NO.
EPA-600/2-78-149
i. RECIPIENT'S ACCESSION-NO.
TITLE ANDSUBTITLE
TOTAL ENERGY CONSUMPTION FOR MUNICIPAL
WASTEWATER TREATMENT
5. REPORT DATE
August 1978 (Issuing Date)
6. PERFORMING ORGANIZATION CODE
AUTHOR(S)
Robert Smith
8. PERFORMING ORGANIZATION REPORT NO.
PEFjFORMING ORGANIZATION NAME AND ADDRESS
Municipal Environmental Research Laboratory--Cin, OH
Office of Research and Development
U.S. Environmental Protection Agency
Cincinnati, Ohio 45268
10. PROGRAM ELEMENT NO.
1BC611
11. CONTRACT/GRANT NO.
12. SPONSORING AGENCY NAME AND ADDRESS
Same as above
13. TYPE OF RE PORT AND PERIOD COVERED
Inhouse (Jan-Dec 1Q77)
14. SPONSORING AGENCY CODE
EPA/600/14
15. SUPPLEMENTARY NOTES
Project Officer: Robert Smith 513/684-7624
16. ABSTRACT
Quantities of all forms of energy consumed for collection and treatment of
municipal wastewater are estimated. Heat energy is equated to electrical energy by
a conversion factor of 10,500 Btu/kwh. Total energy consumption, expressed as kwh/mg
of wastewater treated, ranges from 2300-3700 kwh/mg. Energy used for construction
of the treatment plant and the sewerage system represents 35-55% of the total energy
consumed. The remainder used for plant operation is predominately (65-75%) electrical1
energy. The use of high efficiency aeration devices combined with good maintenance
practices appears to offer the best opportunity for conservation of energy within
the plant. Recovery of energy from the sludge produced at the plant can be accom-
plished by anaerobically digesting the sludge and using the digester gas as fuel for
internal combustion engines. In large plants, when the sludge is sufficiently
dewatered, it is also possible to recover energy by incinerating the dewatered sludge
with production of steam in a waste heat boiler. The steam can then be used within
the plant or expanded through a steam turbine to produce mechanical or electrical
energy.
KEY WORDS AND DOCUMENT ANALYSIS
DESCRIPTORS
b. IDENTIFIERS/OPEN ENDED TERMS jc. COSATI Field/Group
*Sewage Treatment
*Sewage Treatment—Water Treatment
Sewers
*Electric Power Demand
Wastewater Treatment
Processes
Sewage Treatment Plants
Energy Consumption
Energy Recovery
13B
13. DISTRIBUTION STATEMENT
Release to public
19. SECURITY CLASS (This Report)
Unclassified
21. NO. OF PAGES
51
20. SECURITY CLASS (This page)
Unclassified
22. PRICE
EPA Form 2220-1 (9-73)
43
"A U.S. GOVERNMENT PRINTING OFFICE. 1978— 757-140/1426
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