EPA-600/2-77-214
November 1977
ENERGY REQUIREMENTS FOR
MUNICIPAL POLLUTION CONTROL FACILITIES
by
G. M. Wesner
Culp/Wesner/Culp
Clean Water Consultants
Santa Ana, California 92707
Contract No. 68-03-2186
Project Officer
Francis L. Evans, III
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 publication,
Approval does not signify that the contents necessarily reflect the views and
policies of the U.S. Environmental Protection Agency, nor does mention of
trade names or commercial products constitute endorsement or recommendation
for use.
11
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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.
In view of the worldwide energy situation, it is important that
designers of municipal pollution control facilities consider the energy
requirements for various control methods. This report presents information
on energy requirements for various treatment processes and requirements for
production of consumable materials commonly used in municipal pollution
control facilities.
Francis T. Mayo, Director
Municipal Environmental Research
Laboratory
111
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EXECUTIVE SUMMARY
This report presents information on energy requirements in municipal
pollution control facilities for several major areas of interest.
1. Pumping energy for filtration and granular carbon adsorption of secondary
effluent - Pumping requirements are developed for all elements of the
filtration process including: (a) main stream, (b) backwash, (c) surface
wash, (d) wash water return, and (e) chemical feed. The estimates show
that main stream pumping consumes by far the greatest part of the energy
used for filtration. Energy for gravity filtration at a rate of 5 gpm
per sq ft varies from about 0.028 kwh per 1000 gal in a 1 mgd plant to
0.013 kwh per 1000 gal in a 100 mgd plant. Energy for pressure filtration
at a rate of 5 gpm per sq ft varies from about 0.12 kwh per 1000 gal in a
1 mgd plant to 0.08 kwh per 1000 gal in a 100 mgd plant. A lower filter
rate of 2 gpm per sq ft requires more energy if it is assumed that the
wash rate (15 gpm per sq ft for estimates herein) remains constant.
2. Heat Requirements - Estimated heat requirements are developed for several
operations that may be used in municipal wastewater treatment plants.
(a) Building heat. Wastewater treatment plant heating requirements
are presented as a function of plant capacity for three cities:
Minneapolis, New York and Los Angeles.
Treatment Building Heating Requirements
Plant Capacity (million Btu/yr)
(mgd) Los Angeles New York Minneapolis
1 77 290 450
10 230 900 1,600
100 1,100 4,400 6,500
(b) Anaerobic digestion. Heat requirements for anaerobic digestion
at 95 F in standard and high rate digesters are given as a func-
tion of influent sludge temperature.
Digester Heat Required
Influent Sludge (million Btu/mgd)
Temperature North U.S. South U.S.
(°F) High Rate Standard Rate High Rate Standard Rate
40 4.2 5.85 3.0 3.55
70 2.35 3.95 1.8 2.3
(c) Heat treatment of sludges. Requirements are presented for both
heat conditioning prior to dewatering and for oxidation prior to
ultimate disposal. Fuel requirements are given as a function of
thermal treatment capacity. The effects on energy requirements
of treatment of waste liquors and odors produced in heat treatment
iv
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process are discussed. Fuel requirements vary from 2.8 to 5.0
billion Btu/yr in a 10 gpm thermal treatment capacity plant.
Fuel requirements vary directly with plant capacity.
(d) Lime recalcination. Fuel requirements are presented as a function
of hearth area for recalcination of six different sludges in
multiple hearth furnaces. Natural gas requirements vary from 14
to 120 million scf/yr for a furnace with a 1,000 sq ft hearth area
loaded at a rate of 7 psf/hr.
(e) Granular carbon regeneration. The maximum total energy required
for on-site regeneration of activated carbon is estimated to be
8,300 Btu per Ib. This total includes furnace fuel, steam, fuel
for an afterburner and a small amount of electrical energy.
3. Utilization of Anaerobic Digester Gas - It is estimated from a survey of
existing installations, and data in the literature, that about 6.5 million
Btu per million gallons of wastewater treated are available from gas pro-
duced by anaerobic digestion of sludge from primary and activated sludge
treatment. Cost estimates are presented for cleaning and storing digester
gas, and for use as fuel in internal combustion engines that are coupled to
pumps, blowers or electrical generators. On-site electricity generation
costs are estimated to be $0.028 per kwh in a 100 mgd plant and $0.047
per kwh in a 10 mgd plant.
4. Estimated energy requirements are presented for the off-site production of
the following consumable materials used in some wastewater treatment
processes:
Consumable Fuel Electricity
Material Million Btu/ton kwh/lb
Activated Carbon 102* 4.9
Alum 2* 0.1
Ammonium Hydroxide 41* 2.0
Carbon Dioxide 2 to 54 0.1 to 2.6*
Chlorine 42 2.0*
Ferric Chloride 10 0.5*
Lime (Calcium Oxide) 5.5* 0.3
Methanol 36* 1.7
Oxygen 5.3 0.3*
Salt (Sodium Chloride)
Evaporated 4* 0.2
Rock & Solar 0.5 <0.1*
Sodium Hydroxide (50% NaOH) 37 1.8*
Sulfur Dioxide 0.5 <0.1*
Sulfuric Acid 1.5* 0.1
*Indicates principal type of energy used in production.
This report was submitted in partial fulfillment of Contract 68-03-2186
by Gulp/Wesner/Gulp - Clean Water Consultants under the sponsorship of the
U.S. Environmental Protection Agency.
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CONTENTS
Foreword
Executive Summary iv
Figures i-x
Tables « x
List of Abbreviations and Symbols xi
1. Introduction 1
2. Pumping Energy for Filtration and Granular Carbon
Adsorption of Secondary Effluent 3
3. Heat Requirements 9
Building Heat 9
Anaerobic Digester Heat Requirements 11
Heat Treatment of Sludge 15
Thermal Conditioning 17
Wet Oxidation 17
Heating Requirements 17
Sidestreams 18
Heat Required For Lime Recovery by Recalcination .... 21
Heat Required For Granular Carbon Regeneration 23
4. Utilization of Anaerobic Digester Gas 27
Existing Treatment Facilities 27
Atlanta, Georgia 28
Bloom Township, Illinois 28
Buffalo, New York 28
Cincinnati, Ohio 28
Cleveland, Ohio 28
Fort Worth, Texas 28
Los Angeles, California 30
Los Angeles County Sanitation District 31
Orange County Sanitation District (California) ... 31
Philadelphia, Pennsylvania 33
San Jose, California 33
Tucson, Arizona 34
Gas Production 34
Gas Utilization 37
Off-Site Use 37
Use in Internal Combustion Engines 37
Cost Estimates - Digester Gas Utilization 43
Cleaning and Storing Digester Gas 43
On-Site Electricity Generation 45
Example Cost Estimate 49
5. Production of Consumable Materials 55
Activated Carbon 57
Alum 59
Ammonium Hydroxide 59
Carbon Dioxide 60
Chlorine 61
Ferric Chloride 62
Lime (Calcium Oxide) 63
vn
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Methanol 64
Oxygen 65
Sodium Chloride 55
Sodium Hydroxide 66
Sulfur Dioxide 67
Sulfuric Acid 68
References 70
Metric Unit Conversion Factors 72
Vlll
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FIGURES
Number Page
1. Hydraulic efficiency of centrifugal pumps ...... 4
2. Energy required for pressure and gravity filtration . 7
3. Heat required for 1,000 sq ft building ....... 12
4. Floor area required in wastewater treatment plants . 13
5. Building heating required in wastewater treatment
plants ....................... 14
6. Anaerobic digester heat required for primary plus
waste activated sludge ............... 16
7. Fuel required for heat treatment of sludge ..... 20
8. Natural gas required for lime recalcining,
see Table 5 for sludge characteristics ....... 25
9. Anaerobic digester gas utilization system ...... 38
10. Construction cost to clean and store digester gas . . 47
11. Operation and maintenance costs to clean and store
digester gas .................... 48
12. Construction and maintenance material costs for 600 rpm
1C engines with heat recovery and alternate fuel
systems ....................... 50
13. Alternate fuel and labor requirements for 600 rpm
1C engines with heat recovery and alternate fuel
systems ....................... 51
14. Construction and maintenance material costs for complete
electrical generation system shown in Figure 9 ... 52
15. Labor and energy requirements for complete electrical
generation system shown in Figure 9 ......... 53
IX
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TABLES
Number Page
"• *" ^ '""'"Q
1. Filtration energy requirements 6
2. Average monthly degree days (heating) for various
cities 10
3. Fuel requirements for thermal treatment with air
addition 19
4. Energy consumption for odor control systems 22
5. Feed characteristics for lime recalcining fuel
requirements 24
6. Electricity generated at Cincinnati Mill Creek
Treatment Works 29
7. Digester gas analyses
(Los Angeles County Sanitation District) 32
8. Summary of plant operations, Tucson, Arizona .... 35
9. Internal combustion engine efficiency operating on
digester gas 40
10. Typical heat recovery rates for dual fuel engines . . 41
11. Anaerobic digester gas production and use 42
12. Digester gas compression costs 44
13. Digester gas cleaning and storage costs 45
14. Estimated energy requirements for the production of
consumable materials 56
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LIST OF ABBREVIATIONS AND SYMBOLS
Baume Be
British thermal unit Btu
cubic foot (feet) cu ft
degree
degree Celsius C
degree Fahrenheit F
feet (foot) ft
gallon (s) gal
gallons per day per square foot gpd/sq ft
gallons per minute gpm
gallons per minute per square foot gpm/sq ft
horsepower hp
horsepower hour (s) hp-hr
hour (s) hr
internal combustion 1C
kilogram (s) kg
kilowatt kw
kilowatt-hour kwh
milligrams per liter mg/1
million mil
million gallons mil gal
million gallons per day mgd
minute (s) min
pound (s) lb
pounds per square inch psi
pounds per square inch absolute psia
pounds per square inch gage psig
square foot (feet) sq ft
total dynamic head TDK
volatile solids VS
year (s) yr
XI
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SECTION 1
INTRODUCTION
The total energy required to treat all wastewaters in the United States is
small compared to the total national energy use. One report estimates
that it would require about one percent of the current energy demand in the
United States to operate all of the following pollution control facilities:
(1) sulfur dioxide control for power plants, (2) municipal wastewater
treatment to the tertiary level, and (3) solid waste collection and disposal.
Nevertheless, even though energy requirements for wastewater treatment are
small compared to total national energy demands, designers and owners have
become increasingly concerned with energy and chemical costs.
Still more recently, a new facet has been added to the problems of process
selection in wastewater treatment plant design. The fuel shortage, which
began in the winter of 1973 has made it necessary to consider not only the
cost of electric power, fuel and chemicals, but also the availability of
these commodities.2 Since supplies of this nature ultimately depend heavily
on the availability of crude oil or possible alternate fuels, it is essential
that the treatment plant designer take into account the total energy require-
ments of the treatment facility under consideration so that the desired
effluent standards may be achieved with the minimum practical energy consump-
tion.
A unique tool for making preliminary estimates of capital costs and opera-
ting and maintenance costs for wastewater treatment systems exists in the
form of a digital computer program3 developed at MERL, Cincinnati. This pro-
gram is ideally suited for calculating estimated total energy requirements
for any selected treatment system. In order for these total energy require-
ments to be calculated, the energy requirements for individual treatment
processes must be developed and entered into the computer program.
One objective of this study is the procurement of organized information
pertaining to the total energy requirements of various wastewater treatment
processes. This includes both the energy consumed at the treatment plant
and the energy required, at the source, for production of the supplies and
chemicals consumed in the wastewater treatment processes. Although some work
has already been done by Smith1* in estimating electrical power consumption
in sewage treatment processes and in estimating fuel consumption for
incineration of sludges, additional information is required.
In addition to the estimation of energy consumed in individual processes
of a treatment system it is also of interest to estimate the potential for
producing energy from the by-products of treatment; for example, the use of
anaerobic digester gas. Digester gas, after being cleaned and stored, can be
used as fuel for internal combustion engines or for supplying heat to indi-
vidual processes or buildings. Internal combustion engines can be directly
coupled to water pumps or air blowers or used to generate electrical power
which can then be used for general in-plant use. Another alternative is to
use digester gas to generate steam which can then be piped around the plant
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to drive water pumps and air blowers, or for various types of building and
process heating. One of the major elements of this task is to estimate
the cost associated with the use of digester gas in order that the in-plant
use of the gas can be compared to the alternative of selling the gas to a
utility.
English units are used extensively in this report because of their
common use in the municipal pollution control literature. A list of
English-metric unit conversion factors is included following the references.
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SECTION 2
PUMPING ENERGY FOR
FILTRATION AND GRANULAR CARBON ADSORPTION OF SECONDARY EFFLUENT
Energy for filtration can be estimated from the power required for water
and chemical pumping:
TE = P + BW + SW + WWR + CF (1)
where
TE = total energy required
P = main stream pumping
BW = backwash pumping
SW = surface wash pumping
WWR = wash water return pumping
CF = chemical feed pumping
Power required for water pumping is given in the EPA Report by Smith ^ in
the following relationship:
Q x 0.17546 x TDK (2)
hp = £
where
hp = horsepower
Q = flow, mgd
TDH = total dynamic head, ft
E = hydraulic efficiency
Electricity required for pumping is determined from equation (2) by con-
verting hp to kw and including a time factor:
. U/J 0.746 x T x Q x 0.17546 x TDH
Electricity, kwh/day = ^
t,
= 0.131 x T x Q x TDH (3)
E
where T = hr per day pump is operated
and Q, TDH and E are the same as before
It is necessary to estimate TDH and E to determine P; and T, Q, TDH and
E to determine BW, SW and WWR.
The following criteria and estimates are used herein to calculate filtra-
tion energy:
0 Efficiencies (E) of electric motors and centrifugal pumps. Overall,
or wire to water, efficiencies were derived and are shown in Figure 1.
Figure 1 is used to determine all hydraulic efficiencies.
0 P, main stream pumping
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c
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TDH = 15 ft
., , , ,,, 0.131 x 24 x Q x 15
therefore, kwh/day = - - — - -
E
E per mgd
A U • A 2.63
and hp required = -
E per mgd
^ BW, backwash pumping
T = 20 min per day per filter
Q = 15 gpm per sq ft of filter area
TDH = 25 ft
0 SW, surface wash pumping
T = 10 min per day per filter
Q = 1 gpm per sq ft of filter area
TDH = 200 ft
0 WWR, wash water return pumping; pump to operate 24 hr per day and return
all backwash and surface wash water to plant influent
T = 24 hr per day
Q = BW + SW volume per day
TDH = 25 ft
0 CF, chemical feed pumping
2 feed pumps: 1 alum 1 polymer
Q = 50 gal per hr each pump
maximum dosage required: alum = 20 mg/1
polymer = 0.8 mg/1
The energy required for filtration using these criteria, at 5 gpm per
sq ft and a maximum filter area of 700 sq ft is shown in Figure 2. Energy
requirements for a filtration rate of 5 gpm per sq ft and 2 gpm per sq ft
are summarized in Table 1.
The energy estimates shown in Table 1 and Figure 2 are requirements using
motor sizes that are commonly available. For example, calculated main stream
pumping hp for a 5 mgd plant is about 18.8
2.63 _ 2.63 c 10 0 ,
~Y~ x 5 = oTTo x 5 = 18'8 hp
If a 20 hp motor is used, then 20 x 0.746 x 24 = 360 kwh required per day.
Whereas using the relationship 47.1 per mgd, gives 336 kwh required per day.
E
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TABLE 1
FILTRATION ENERGY REQUIREMENTS
(kwh per day)
Plant
Capacity
mgd
1
3
5
10
15
30
50
75
100
P
Main
Stream
Pump
90
270
360
720
1075
1800
3600
4500
6300
BW
Back
Wash
5
15
25
50
75
150
250
375
500
SW
Surface
Wash
2
6
8
15
23
45
75
113
150
WWR
Wash
Water
Return
9
27
36
72
90
179
270
405
540
CF
Chemical
Feed
12
12
12
18
18
36
72
108
144
TE
Total
Pressure Gravity
5 gpm/sq ft
118
330
441
875
1281
2210
4267
5501
7634
28
60
81
155
206
410
667
1001
1334
2 gpm/sq ft
1
5
10
50
100
90
360
720
3600
6300
13
63
125
625
1250
5
19
38
188
375
18
90
135
720
1350
12
12
18
72
144
138
544
1036
5205
9419
48
184
316
1605
3119
-------
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Figure 2. Energy required for pressure and gravity filtration.
100
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The estimates in Table 1 indicate that energy for gravity filtration at
a rate of 5 gpm per sq ft varies from about 0.028 kwh per 1000 gal in a 1 mgd
plant to 0.013 kwh per 1000 gal in a 100 mgd plant. Energy for pressure
filtration at a rate of 5 gpm per sq ft varies from about 0.12 kwh per 1000
gal in a 1 mgd plant to 0.08 kwh per 1000 gal in a 100 mgd plant. A lower
filter rate of 2 gpm per sq ft requires more energy if it is assumed that the
wash rate (15 gpm per sq ft for estimates in Table 1) remains constant.
These data show that main stream pumping consumes by far the greatest part
of the energy required for filtration. The energy required for pumping is
directly proportional to TDK; therefore, TDH in pressure filters (15 ft used
in Table 1 and Figure 2) greatly influences the total energy required. Also,
no energy for main stream pumping is assigned to gravity filtration in these
estimates; however, energy is used (usually about 8 to 10 ft head loss at
backwash) in gravity filters.
Equation (1) can also be used to calculate energy required for granular
carbon adsorption by eliminating the term for chemical feeding. Also, TDH for
carbon treatment in pressure contactors is higher than 15 ft in most installa-
tions because of the greater bed depths used.
Equation (1) may be used (in a computer program if desired) to calculate
energy requirements for any set of flow, head and time conditions in gravity
and pressure filters and carbon contactors. The program could also be written
to use commonly available pump sizes for energy calculations.
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SECTION 3
HEAT REQUIREMENTS
Estimates of heat requirements are presented for the following operations:
1. Building heat
2. Anaerobic digestion
3. Heat conditioning of sludge to improve dewatering
4. Wet oxidation of sludge
5. Lime recovery by recalcination
6. Granular carbon regeneration
BUILDING HEAT
Energy required for space heating in a wastewater treatment plant depends
upon several factors including: (1) building size, (2) location (climate),
and (3) type of construction. The degree-day (deg-day) system is one method
of estimating energy required for space heating.
The deg-day is defined as 65°F minus the mean temperature for the day.
If the mean temperature of the day is 65 F or greater, then the number of
deg-days for heating is zero. The deg-day method is based on the findings of
the American Gas Association that the quantity of energy required for heating
is proportional to the number of deg-day. For example, a building requires
twice as much heat on a day when the temperature is 45 F (20 deg-day) than
when the temperature is 55°F (10 deg-day). Table 2 shows the average number
of deg-day per month computed from about 30 years of record, for 25 cities
in the United States.
The general equation used for estimating energy required for space heat-
ing is:
_ _ 24 x H x D
E " U
E = energy consumption, Btu
U = utilization efficiency
H = hourly heat loss for building, Btu/hr/ F
D = deg-day, F day
The utilization efficiency is the ratio of the heat loss from the struc-
ture to the heat input and is a function of several factors including control
of heating equipment and type of construction. Values from 45 to 90 percent
have been reported. The hourly heat loss can be computed5 or can be measured
directly. It is expressed in Btu/hr/°F and includes the heat losses through
the walls, ceiling, floor, windows and infiltration air. This quantity is
highly variable from structure to structure depending on insulation, building
materials and ratio of floor area to volume. Some representative heat loss
values have been published for insulated and uninsulated walls and ceilings.
Based on these values, and neglecting air infiltration rate, H values were
determined for the following three cases:
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• Case A corresponds to an uninsulated building of 1000 sq ft with
H = 820 Btu/hr/°F.
• Case B is a 1000 sq ft building with 3.5 in. wall insulation, 6 in.
ceiling insulation and storm windows. The insulation and storm windows
give a reduction of about 45 percent in the heat loss rate and H =
450 Btu/hr/°F.
t Case C is the same as Case B, but includes double glazed windows and
floor insulation and gives H = 325 Btu/hr/°F.
These three cases are shown in Figure 3 as a function of the number of
deg-day and a U of 0.70. Infiltration air can substantially increase these
values. For example, an infiltration rate of 1.5 times the building volume
per hour will increase the values for Cases A, B and C by 13, 24 and 33
percent, respectively.
In wastewater treatment plants, 4 to 6 air changes per hour is a common
design standard7. This rate will increase the heating requirement and
should not be neglected. For example, assuming 4 air changes/hr, 70 percent
utilization factor, 5000 deg-day climate, and 1000 sq ft floor area with an
8 ft ceiling gives an additional heat requirement of about 99 million Btu/yr
Building heating requirements for wastewater treatment plants can be
estimated from the above information if the total floor area is known.
Typical floor areas as a function of treatment plant size are given in the
EPA report by Smith4 and are shown in Figure 4. The data in these tables
and figures can be used to estimate building heating requirements. As
an example, the curves shown in Figure 5 were derived from these data for
Los Angeles, New York and Minneapolis.
ANAEROBIC DIGESTION HEAT REQUIREMENTS
Heat is required in the anaerobic digestion process to (1) raise the
temperature of the influent sludge to the level of the digester, and (2)
compensate for heat losses from the digester through its walls, bottom
and cover. The optimum temperature for sludge digestion in the mesophilic
range is about 95°F. The heat required to raise the influent sludge
temperature can be calculated from the following relationship:
Q = WC (TD - Tg) (5)
where
Q = heat required, Btu
W = weight of influent sludge, Ib
C = specific heat of sludge, 1.0 Btu/lb/ F
for 1 to 10% solids sludge
T_ = temperature in digester, °F
T = temperature of influent sludge, °F
The WPCF Manual of Practice No. 8, gives the following criteria for
digester heating:8
11
-------
250
CASE A:
UNINSULATED
CASE B:
ADDED WALL a
CEILING INSULATION
WITH STORM WINDOWS
CASE C:
WALL 8 CEILING
INSULATION
DOUBLE GLAZED WINDOWS
8 FLOOR INSULATION
34567
THOUSAND, deg day/yr
Figure 3. Heat required for 1,000 sq ft building.
12
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Figure 4. Floor area required in wastewater treatment plants.
13
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DESIGN ASSUMPTIONS:
FOUR FRESH AIR CHANGES/hr
STORM WINDOWS a INSULATED WALLS S CEILINGS
70 PERCENT FUEL UTILIZATION FACTOR
Figure 5. Building heating required in wastewater treatment plants.
14
-------
Data accumulated from numerous digester installations have made it
convenient to use factors for estimation of heat losses from diges-
ters without considering separately the loss through each element of
the digester. For the normal installation it is assumed that a 1°F
drop in temperature occurs for the entire tank contents in 24 hrs. A
correction factor is applied for outside temperature, depending upon
location and special conditions, such as the presence of ground water.
For each 1,000 cu ft of contents, this amounts then to 1,000 x 62.5 x
1.0 = 62,500 Btu per day; or 62,500 = 2,600 Btu per hr. Correction
24
factors for geographical location by which the value of 2,600 Btu per
hr is multiplied are as follows:
Northern United States 1.0
Middle United States 0.5
Southern United States 0.3
The WPCF Manual of Practice No. II9 gives the following loadings for
anaerobic digesters:
Standard Rate
High Rate
Loading, Ib VS/day/cu ft
0.03 to 0.1
0.1 to 0.4
Digester heat requirements for this paper are based on loadings of 0.05
and 0.15 Ib VS/day/cu ft. These criteria give the following digester
capacities:
Sludge
Type
Solids
Content
(percent)
Total
Solids
(Ib/mil gal)
Volatile Total
Solids Sludge
(Ib/mil gal) (Ib/mil gal)
Digester Capacity
(cu ft/mil gal)
Loading
(Ib VS/day/cu ft)
0.05 0.15
Primary 5 1155
Primary 4.5 (thickened) 2100
Plus WAS
690
1446
23,100
46,600
13,800
28,900
4,600
9,600
The total heat required for digestion at 95°F at the two loadings is
shown in Figure 6 for primary plus waste activated sludge. These heat
requirements are based on the above criteria for sludge heating and digester
heat loss and a 75 percent heat transfer efficiency.
HEAT TREATMENT OF SLUDGE
Heat treatment comprises several related processes in which sludges
are heated for conditioning prior to dewatering or for stabilization prior
to disposal. All the processes involve heating sludge for relatively short
periods of time in pressurized reactors. The reactor's environment - temp-
erature, pressure, residence time and oxygen content - is selected based
on the desired degree of sludge conditioning or stabilization. As the
15
-------
DIGESTER LOADING
0.05 Ib. VS/day/cu ft
0. 15
DIGESTION TEMPERATURE : 95°F
Figure 6.
40 50 60 70
SLUDGE TEMPERATURE TO DIGESTER, °F
Anaerobic digester heat required for primary plus
waste activated sludge.
16
-------
temperature and amount of available oxygen are increased a greater amount of
stabilization or oxidation takes place. Heat treatment processes are
divided into two main categories depending on the desired results: thermal
conditioning and wet oxidation.
Thermal Conditioning
Thermal conditioning is used to condition sludge for subsequent dewater-
ing. Under heat and pressure in a reactor, bound water and intercellular
water are released from the sludge and much of the smaller and more
hydrated particulate matter is solubilized. The result is a mixture of
relatively innocuous, sterile particulate matter and a liquid. The two
phases are easily separated after discharge by decantation and mechanical
dewatering processes. The dewatered solids are inoffensive and can be
used as soil conditioner. The liquid phase is highly colored, often has
a very offensive odor and has a BOD ranging between 3,000 and 15,000 mg/1.
For thermal conditioning of most municipal sludges, reactor temperatures
and pressures range between 300 and 500°F and 200 and 400 psi. Residence
time in the reactor is usually about 30 to 45 minutes at design flow. A
primary purpose in pressurizing the reactor is to prevent the liquid contents
from flashing to steam at the high temperatures involved. Air may be added
to the system to assist with heat transfer and to partially oxidize the
sludge.
Wet Oxidation
This process oxidizes organic materials in the sludge to ash. Wet
oxidation is similar to thermal conditioning in that sludge is heated in a
pressurized reactor, but it's purpose is to stabilize the sludge rather
than condition it for dewatering. This requires an increase in reactor
temperatures to a range between 450 and 700°F and pressures to between
750 psi and 1800 psi. The reactor's environment is selected based on the
characteristics of the sludge and the degree of oxidation desired. Air
is added to the reactor to supply the oxygen needed by the chemical reactions
taking place. The degree of oxidation of the sludge can be controlled and
can range up to over 95 percent of the influent COD for some sludges.
This is equivalent to results attainable in dry incineration processes, but
in wet oxidation, temperatures are much lower, fly ash is not a problem
and the sludge need not be dewatered before being oxidized.
Heating Requirements
In order to operate any heat treatment process, the temperature of the
incoming sludge must be raised to the selected reactor temperature. To
heat one gallon of sludge from 50°F to a thermal conditioning temperature
of 350°F requires 2500 Btu and to raise the temperature to 700°F for
complete oxidation requires about 5500 Btu. Thus a 10 mgd treatment plant
producing 10 tons per day of sludge requires approximately 150 mil Btu/day
for thermal conditioning and 320 mil Btu/day for wet oxidation. These
values are net heats required by the sludge and must be increased to reflect
17
-------
the efficiency of the heat generating and transferring system and losses
from the overall system. The actual energy input is, therefore, almost
double the above figures.
Heat exchangers are incorporated into the processes to capture the
heat from the treated sludge in the reactor outlet. In this manner, incom-
ing sludge is heated to within 40 to 50°F of the reactor temperature with
a corresponding drop in required input energy. With an efficient heat
exchange system, about 420 Btu/gal is required to reach the reactor
temperature and, accounting for system inefficiencies, a total energy input
of about 900 Btu/gal is required. This heat is normally supplied by injec-
ting steam into the reactor.
Heat to generate the steam is usually produced in gas or oil-fired
boilers. However, when sludge incinerators follow thermal conditioning
plants, waste heat boilers deriving heat from the incinerator stack gases
have been used successfully to provide all the required heat.
Injection of air into the reactor allows heat-producing oxidation
reactions to occur. In those thermal conditioning systems where air is
supplied, oxidation of about 5 to 10 percent of the volatile solids takes
place. Assuming typical wastewater sludges and a heat value of 10,000
Btu/lb of volatile solids, the required heat input is reduced from 900
Btu/gal to between 500 and 700 Btu/gal. This reduction in required heat
is accompanied, however, by an increase in electrical energy needed to
compress the air. Table 3 shows the heat input required for thermal condi-
tioning of several sludges and Figure 7 shows the annual heat requirements
for various sludges.
By increasing the degree of oxidation, as is done in wet oxidation,
to 20 to 30 percent of the volatile solids content, enough heat is produced
in the reactor to offset the need for supplementary steam. Steam is then
needed only to initially heat the system to the reaction temperature.
Further increase in the degree of oxidation produces excess heat which may
be used to generate steam or hot water for other uses. Or, hot, pressurized
off-gases from the reactor can be expanded through a turbine to drive
process equipment or an electrical generator.
The recoverable energy from a wet oxidation system treating the primary
and waste activated sludge mixture described in Table 3 can yield almost
16 horsepower per gpm of capacity. Comparing this recoverable energy with
the energy required to operate the system shows that the output very nearly
equals input. Of course, the energy balance will change for different
sludges amd system conditions, but in all systems a large amount of the
input energy is recoverable.
Sidestreams
Besides the direct energy requirements of heat treatment, other related
areas of energy use must be considered. These are the treatment of the
high-strength liquors produced in the reactor and the treatment of odorous
gases emanating from air-water separators, storage tanks, and subsequent
18
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dewatering processes. Often, costs and energy requirements for these
operations are incorrectly excluded when making feasibility studies involv-
ing the processes. Their impacts on energy consumption can be subs! -,-mti n 1 .
Strong liquors from thermal conditioning processes which inc.hide
supernatant from decanting operations and filtrate or cent-rate from -U>wai er-
ing operations, must be treated before discharge. These liquors are usually
treated in one of three ways: (1) separate biological treatment (Aerobic r.T
anaerobic) perhaps followed by adsorption on activated carbon, (?) r e«-yrlod
directly back to the primary or secondary treatment plant, or (3) biological
pretreatment and then recycled back to the main treatment plant for .ndditinnal
treatment. Because of its high-strength (BOD of 3,000 to 15,000 mg/1. and
suspended solids of 10,000 to 20,000 mg/1) and even though the volume is
low (0.4 to 0.8 percent of the inflow to the treatment plant), the increased
load due to recycling or separately treating can be quite significant,
Recycling strong liquor directly to an activated sludge plant can increase
the air requirements, and consequently the energy requirement, by as much
as 30 percent.
Most of the various systems available to control concentrated process
odors also consume relatively large amounts of energy. The methods most
commonly used and most generally effective for controlling odors from thermal
treatment are high temperature incineration, adsorption on activated carbon,
and chemical scrubbing. Table 4 shows the requirements for the three methods
based on a typical 1,000 cfm odor control system. A concentrated gas stream
of 1,000 cfm corresponds to a thermal treatment plant size of 200 to ?'>()
gpm or a sewage treatment plant size of 50 to 60 mgd. The energy require-
ments developed for the three methods represent the needs of complete odor
control systems and include requirements for collection of gases; ducting;
fans; chemical feeding, mixing, and storage equipment; automatic control
systems; disposal of removed and waste materials; and discharge of treated
gases as well as for odor removal itself.
The incineration or afterburning process considered consists of pre-
treatment by water scrubbing using treated effluent in a packed bed and
direct-flame incineration of 1,500°F with recovery of 40 percent of the
input heat. The carbon adsorption process includes prescrubbing with
effluent, dual-bed adsorption on activated carbon, regeneration of carbon
with low pressure steam, condensation of vapors, and incineration of the
waste organic stream. The chemical scrubbing system utilizes three stages
of scrubbing in packed beds. The first two stages use secondary effluent
and a final stage uses a buffered, potassium permanganate solution.
HEAT REQUIRED FOR LIME RECOVERY BY RECALCINATION
Wastewater treated with lime produces sludge composed of varying
amounts of relatively inert and non-combustible material, such as calcium
carbonate, CaCOs, magnesium hydroxide, Mg(OH)2, phosphorus precipitates
and others. The sludges also contain combustible organic material with
with some heat value. Lime as calcium oxide, CaO, is recovered in rh«
recalcining process according to the following relationship:
21
-------
TABLE 4
ENERGY CONSUMPTION FOR ODOR CONTROL SYSTEMS
Electrical Energy
kwh/1000 cu ft
kwh/yr (1 mgd)2
kwh/yr (1 gpm)3
Incineration
122
1285
321
Carbon
Adsorption
146
1540
385
Chemical
Scrubbing
146
1540
385
Fuel
million Btu/1000 cu ft 36.8
million Btu/yr (1 mgd)2 387
million Btu/yr (1 gpm)3 97
1
11
2.7
on continuous operation
\
1 mgd indicates approximate sewage treatment
plant capacity
1 gpm represents approximate thermal treatment
plant capacity
22
-------
CaCOa + heat -> Ca02 + CO (gas)
Fuel requirements for several different types of lime sludges are
shown in Figure 8. The sludge characteristics are given in Table 5. These
fuel requirements are based on the experience of furnace manufacturers.
• Case A illustrates a typical sludge resulting from lime
addition to raw sewage where no lime recycle is practiced.
In this instance, the multiple hearth furnace is actually
being used for incineration and disposal rather than recalcining,.
• Case B is based on a system where raw sewage is lime coagulated
and the lime is recovered and recycled.
• Case C illustrates a tertiary lime coagulation system where
the sludge is not classified prior to recalcination.
• Cases D, E, and F illustrate tertiary systems where classification
is practiced with varying sludge moisture content entering the
furnace. The heating value of natural gas is taken as 1,000
Btu per cu ft.
HEAT REQUIRED FOR GRANULAR CARBON REGENERATION
Granular activated carbon is reactivated in multiple hearth furnaces
fueled by natural gas or other fossil fuels. Steam is also commonly used
in the reactivation process. Relatively small amounts of electricity are
required for furnace operation and for carbon transfer.
Operating data reported at South Lake Tahoe10 indicate the following
energy requirements for on-site regeneration of activated carbon:
Btu per Ib
Carbon Reactivated
Electricity
Natural Gas (furnace)
TOTAL
Energy to supply steam and to operate an afterburner is not included
in the total of 4,300 Btu per Ib.
A paper written by employees of a carbon manufacturer11 gives the
following requirements for reactivation of granular carbon:
23
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HEARTH AREA, sq ft
Figure 8. Natural gas required for lime recalcining
(See Table 5 for sludge characteristics).
25
-------
Btu per Ib
Carbon Reactivated
Reactivation 2,180
Afterburner 2,080
TOTAL 4,260
The maximum total energy required for on-site reactivation of granular
carbon is considered to be about 8,300 Btu per Ib including requirements
for air pollution control equipment.
Btu per Ib
Carbon Reactivated
Electricity 700
Furnace fuel 3,600
Steam 1,600
Afterburner 2,400
TOTAL 8,300
26
-------
SECTION 4
UTILIZATION OF ANAEROBIC DIGESTER GAS
The purposes of this section include the following:
1. Estimate the total cost for cleaning and storing digester gas as
a function of the amount of storage and the amount of gas processed.
2. Estimate the electrical energy available as a function of gas
consumption.
3. Estimate waste heat available from exhaust gas and water jackets of
internal combustion (1C) engines large enough to make a practical
system.
4. Estimate the cost of an installation for generating electrical
power from digester gas as a function of the kilowatt capacity
of the installation. Alternate fuel, either natural gas or fuel
oil must be provided when the supply of digester gas is inadequate.
Include waste heat recovery where size makes this practical.
5. Estimate the cost and horsepower of direct coupled 1C engines
for driving influent pumps or air blowers as a function of gas
consumption.
6. All costs to be broken down into construction cost, operating and
maintenance labor, materials and supplies and energy similar to
the Black and Veatch report.12
EXISTING TREATMENT FACILITIES
Information was obtained from the following agencies that utilize sludge
digester gas:
Atlanta, Georgia
Bloom Township, Illinois
Buffalo, New York
Cincinnati, Ohio
Cleveland, Ohio
Fort Worth, Texas
Los Angeles, California (City)
Los Angeles County Sanitation District
Orange County Sanitation District (California)
Philadelphia, Pennsylvania
San Jose, California
Tucson, Arizona
27
-------
Madison, Wisconsin and Racine, Wisconsin are also using some digester
gas but no data was obtained from these cities. Following is a summary of
the information that was obtained.
At_.lant_a1 Georgia
A 90 mgd treatment plant was recently completed and no data is avail-
able on the quantity or details of utilization of digester gas. The plant
is equipped with three dual fuel engines which are designed to drive
blowers.
Bloom Township , Illinois
Digester gas is not now used in internal combustion engines in this
plant because of high maintenance costs. From May 1973 through April 1974,
an average of about 58,000 cu ft/day of gas was produced and about 3.5
cu ft was produced per Ib of VS added to the digester.
J^'^l'^liN New Yojrk
Internal combustion engines are not used at this plant. Sludge digester
gas is used as fuel for: two boilers to heat digesters; an incinerator
which burns sludge cake; and building heat. There are no gas cleaning or
storage facilities. Accurate records of gas production are not available.
_C_inc i i ma t i ^ OMp
Digester gas is utilized at the Mill Creek Treatment Works in four 1910
hp turbo-charged dual fuel engines to drive four 1350 kw generators. Heat
recovery units are used to furnish steam for heating the digesters. Data
from 1973-75 are summarized in Table 6 and show that an average of 17.8
scf of digester gas was required to produce one kwh of electricity. Data
from other plants in Cincinnati indicate that digester gas produced ranged
from 10.9 to 13.4 cu ft per Ib of VS destroyed.
The sludge digester gas system will be removed from this plant in the
near future in connection with the expansion and installation of a different
solids handling system. Digester gas is not used for engine fuel but is
used to heat the digesters and as fuel for a sludge incinerator. Digester
gas is produced at the rate of about 500,000 cu ft per day and about 5 cu ft
per Ib of VS destroyed.
Fort Worth^ Texas
The following information is based on the period October 1, 1973
through September 30, 1974.
28
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Average flow treated 38.8 mgd
Average VS destroyed
in digesters 47 percent
Gas produced 4.2 scf/lb of VS destroyed
Average power generated 19.7 scf digester gas
required to generate
1 kwh electricity
Two 1620 hp White Superior dual fuel engine generator sets were instal-
led in June 1972. The generators are rated at 1180 kw each. One 1440 hp
gas engine is used to drive one blower. The engines are equipped with heat
recovery units which are used to heat the digesters. Gas is compressed and
stored at 35 to 45 psi in a 50 ft diam sphere (65,000 cu ft capacity).
An iron sponge type scrubbing system was installed with the engines but is
not used because the hydrogen sulfide concentration is less than 1,000 ppm.
The large White Superior engines are turbo-charged and gas must be supplied
at a minimum pressure of 35 psi.
Los Angeles, California
The Hyperion Plant treats an average flow of 340 mgd all of which
receives primary treatment and 100 mgd receives conventional activated
sludge treatment. Sludge treated in the digesters is about 92 percent
primary and 8 percent waste activated. There are 18 digesters, 15 operate
at 95°F and three at 122°F. Following is a summary of engine operation
and gas production data during the last three fiscal years:
Gas Production
million cu ft per day
Heat Value*, Btu/cu ft
cu ft gas produced/lb
VS destroyed
Engine Operation
Btu/hp-hr
1971-72
4.186
590
17.7
6,469
1972-73
3.843
590
13.4
6,428
1973-74
3.548
590
11.7
Electricity Generated
kwh/day
58,533 59,349
*Lower heating value from laboratory tests
7,675
56,847
Engineers at the Hyperion Treatment Plant believe that the reduction in
gas production indicated in the last two years is the result of poor metering
and does not represent a change in actual gas production. The gas is com-
pressed to 35 psi and stored. The hydrogen sulfide content is about 800 ppm
and scrubbing has never been used.
The digester gas is used primarily in 10 supercharged 8 cylinder
Worthington engines rated at 1688 hp. The engines are dual fuel and
30
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continuously utilize about 5 percent fuel oil. Five of the engines operate
generators each rated at 1190 kw. The other five engines are direct coupled
to blowers each rated at 40,000 cfm. The engines are each equipped with
heat recovery units which are used to heat the digesters. The data shows that
about 40 percent of the installed capacity of 5950 kw was utilized in gener-
ating electricity (about 58,000 kwh/day).
Los Angeles County Sanitation District
The primary treatment plant for the Sanitation District treats an
average of about 385 mgd and is equipped with 30 digesters. An average of
5.5 million cu ft of gas is produced and 16 cu ft per Ib of VS destroyed.
The digester gas is about 60 percent methane with a high heat value of
607 Btu. A summary of digester gas analyses from December 1973 through May
1975 is shown in Table 7. The lower heating value of the digester gas
would be about 577 Btu per cu ft. This data also shows that the average
hydrogen sulfide concentration was very low, about 28 ppm, with the highest
figure reported to be 147 ppm.
Gas is transferred directly from the digesters to 12 Ingersoll-Rand
engines without any treatment, compression or storage. The standby fuel is
propane and the engines are not equipped with heat recovery units. There is
an emergency waste gas burner on site, but normally any excess gas is taken
by a contractor at $0.15 per 1,000 cu ft. Five of the engines are direct
coupled to pumps rated at 97,000 gpm each; the other seven engines are
connected to generators as follows:
Rated Engine, bhp Rated Generation Capacity, kw
2 engines at 1180 each 835 each
1 engine at 1100 775
2 engines at 888 each 615 each
2 engines at 800 each 560 each
TOTAL 6836 4795
The engines operate at low rpm (330 to 360) and some have been operating
for 20 years with no significant down time.
Orange County Sanitation District (California)
During the 1972-73 fiscal year digester gas production in two plants
averaged 2,214,000 cu ft/day. The gas is used in (a) naturally aspirated
internal combustion engines coupled to influent and effluent pumps, (b)
boilers, and (c) rag incinerators. All engines are spark ignited with
natural gas for standby fuel. Heat recovery systems on the engines are
utilized to heat the digesters. The plant is also equipped with a gas
turbine generator set which is used for standby power. The gas turbine
is equipped with a heat recovery unit which furnishes steam to a turbine
and another generator. This heat recovery system has not performed satis-
factorily and has been removed from service.
31
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TABLE 7
DIGESTER GAS ANALYSES
Los Angeles County Sanitation District
Date
December 1973
March 1974
April 1974
May 1974
July 1974
August: 1974
September 1974
October 1974
November 1974
December 1974
January 1975
February 1975
March 1975
April 1975
May 1975
AVERAGE
No.
Days Sampled
17
18
22
21
21
22
18
22
16
19
22
18
21
20
21
Average
% CO?
36.9
37.1
37.0
37.0
36.4
36.2
36.0
36.5
37.2
36.7
36.8
36.9
37.2
37.7
37.2
36.9
Average
59.9
60.0
59.8
59.6
60.0
60.3
60.3
59.9
59.8
60.2
60.0
59.9
59.6
59.1
59.6
59.9
Average
Btu/cu ft*
607
608
606
604
608
611
611
607
606
610
608
607
604
600
604
607
Note: Data from March 1974 throimh Fphma™ 1Q?q •»«,!•,• „.,*.«„ 4-u^
the average H2S concentration is 28 ppm+17 ppm. The highest
figure reported for this period was 147 ppm or 0.015% by
weight of a cubic foot of digester gas.
Based on a higher heating value of 1013 Btu/cu ft
32
-------
A 45 mgd activated sludge plant is currently under construction and two
1500 hp Enterprise-Delaval engines will be installed to drive blowers rated
at 35,000 scfm at 7 psi discharge pressure. The engines will be spark
ignited and will operate at 350 rpm. Two White Superior 1200 hp engines
will be installed for effluent pumping and two 250 hp White Superior engines
will be installed for in-plant pumping. Natural gas will also be the
standby fuel for these new engines.
Gas withdrawn from each digester passes through a sediment trap and is
conveyed to gas compressors. The compressors normally compress the gas to
40 psi with a maximum capability of 50 psi. Compressed gas is stored at a
maximum pressure of 50 psi in two 32 ft diameter spheres (17,000 cu ft
capacity). Gas pressure is reduced from the storage pressure of 40 - 50
psi to 2 - 5 psi prior to use in the engines, boilers and incinerators. The
digester gas has a high hydrogen sulfide concentration of as much as 3,000
ppm, but scrubbers have never been used.
The District estimates that present work equivalent performed per day
using digester gas as fuel amounts to 74,300 hp-hr. This amounts to 58 per-
cent of the total energy required for collection and treatment based on
actual work performed. Other energy sources used in the two plants are
electrical, which accounts for 38 percent of the work and natural gas, which
accounts for 4 percent.
Philadelphia, Pennsylvania
Digester gas is used to heat buildings and digesters, but no internal
combustion engines are operated on digester gas. The gas is not cleaned,
compressed or stored before use. A yearly average of 6.4 cu ft of gas is
produced per Ib of VS destroyed.
San Jose, California
The 160 mgd plant has eight primary digesters heated to 95 F and three
unheated secondary digesters. The digesters reduce VS by 50 to 55 percent.
Primary digesters are heated with an external heat exchanger by hot water
from internal combustion engine heat recovery units.
Average heat value of the digester gas is 550 Btu/cu ft and is mixed
with natural gas to produce a blend with a heat value of 700 Btu/cu ft. No
cleaning or scrubbing, except water removal, is provided. Digester gas is
compressed to 60 psi before blending and no storage is provided before use
in engines. Generally 85 to 90 percent of digester gas is used and 10 to 15
percent is flared.
The blended gas is used as fuel for 11 internal combustion engines.
Five dual fuel Enterprise-Delaval engines drive electrical generators:
2 - 800 hp and 3 - 2500 hp. Six tri fuel spark ignited Cooper-Bessemer
engines drive blowers: 3 - 2400 hp and 3 - 1800 hp.
33
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Tucson, Arizona
Digester gas is used as fuel for 300 hp Waukesha internal combustion
engines which are direct coupled to blowers. Data from the last two fiscal
years was taken from the 1973-74 Annual Report and is summarized in Table 8.
There is no explanation for the high gas production reported.
GAS PRODUCTION
Perhaps the most important design criterion that must be selected is
the volume of gas produced per unit of organic material destroyed in the
digester. Virtually all operating data, as well as data in the literature,
is reported in cu ft of gas produced per Ib of VS destroyed. In some cases
the gas production is recorded in total Ib of VS supplied to the digester.
The EPA report by Smith4 discusses the volume of gas produced as follows:
"The volume of gas produced per Ib of VS destroyed is reported
as 17-18 scf/lb at the larger and better instrumented plants.
Smaller plants report lesser values, sometimes as low as 6 scf per Ib
VS destroyed, but these lower values are probably due to poor
measurement techniques."
The Water Pollution Control Federation's Manual of Practice on Anaerobic
T Q
Sludge Digestion ^ gives the following data on anaerobic conversions of the
chief types of organic matter in sewage sludge:
Type and Average
Concentration
Carbohydrate (C6H1005)n
Fat C5oH9006
Insoluble Soap
Protein 6C-2NH3-3H20
Gas Produced
(cu ft gas/lb organic matter digested)
14.2
24.6
22.3
9.4
These data were developed from extensive experimental work conducted
at the Los Angeles County Sanitation Districts.
The WPCF manual on sewage treatment plant design8 gives the following
gas production data:
"In terms of solids digested, the average yield adjusted to
standard temperature of 60°F is about 15 cu ft of gas per Ib
of VS destroyed. These gas volumes are for normal plant operating
pressures of 6 to 8 inches of water."
The EPA Process Design Manual for Sludge Treatment and Disposal gives
the following sludge and digester gas data. ^
"In general, treatment of 1 mgd of municipal wastewater will provide
1 ton of mixed primary and activated sludge solids which translates
to 0.2 to 0.3 Ib solids/capita/'day. An unheated digester will typically
34
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TABLE 8
SUMMARY OF PLANT* OPERATIONS
Tucson, Arizona
1972-73 1973-74
Population served 325,318 341,930
Average daily flow, mgd 33 32
Average influent suspended solids, mg/1 211 236
Average influent BOD , mg/1 227 235
Average suspended solids to digester, Ib/day 38,192 35,589
Average volatile solids
To digesters, percent of SS 72 79
To digesters, Ib/day 27,452 28,137
Destroyed, Ib/day 12,490 14,430
Reduction, percent 45.5 51.3
Average digester gas produced
Thousand cfd 341,970 367,668
cu ft/lb volatile solids to digester 12.5 13.1
cu ft/lb volatile solids destroyed 27.4 25.5
*Sewage is treated in three plants: two activated sludge and one trickling
filter.
35
-------
produce 0.32 to 0.56 cu ft of gas/capita while a heated digester w
produce from 0.56 to 0.74 cu ft of gas/capita. This is equivalent to
a maximum gas production of approximately 11 to 12 cu ft of gas/lb
of total solids digested. The heat value of sludge gas is approximately
566 Btu/cu ft."
A range of 14 to 19 cu ft of digester gas produced per Ib of VS des-
troyed was reported for Chicago.15
Data collected from operating plants during this study indicates that
17 to 18 scf/lb of VS destroyed is not routinely obtained even at some well
operated facilities and much lower values are reported in some presumably
well operated plants. Therefore, 15 scf/lb VS destroyed is recommended
for sizing typical digester gas utilization systems, unless data are avail-
able for a specific waste to be treated.
The amount of sludge produced in a wastewater treatment plant, the VS
content of the sludge, and the gas produced by anaerobic digestion varies
with influent suspended solids concentration, BOD and type and efficiency of
the biological treatment processes. A published review16 of sludge quantities
produced in municipal wastewater treatment plants concludes that 915 and
1,085 Ib/million gallons treated are typical quantities of sludge produced
by primary and secondary treatment respectively. The following sludge
quantities are based on a review of data from several sources and are con-
sidered representative of typical primary and activated sludge plants:
Sludge Solids
(Ib/million gallons)
Volatile
01 J m Total Volatile (percent of total)
Sludge Type — —
Primary 1,155 690 60
Waste Activated 945 756 80
TOTAL 2,100 1,446 —69~
A review of the literature, and data collected from operating plants
during this study, indicates that about 50 percent of the volatile solids are
destroyed by anaerobic digestion and that the gas produced has a heat value
of about 600 Btu/scf.
These criteria give the following estimates for gas and heat available
from anaerobic digestion:
Waste
Primary Activated
„ j Sludge Sludge TOTAL
Gas Produced, scf per million 5,175 5,670 10,845
gallons treated
Heat Available, Btu per million 3,105,000 3,402,000 6,507,000
gallons treated
36
-------
For planning purposes, and in the absence of more specific information,,
it may be assumed that about 6.5 million Btu per million gallons of waste-
water treated are available from gas produced by anaerobic digestion of
sludge produced by primary and conventional activated sludge treatment.
GAS UTILIZATION
Digester gas can be used for on-site generation of electricity and/or
for any in-plant purpose requiring fuel. Digester gas could also be used
off-site in a natural gas supply system.
Off-Site Use
Off-site use of digester gas will usually require treatment to remove
trace impurities such as hydrogen sulfide and moisture; in most cases the
heat value of the digester gas must be increased by removal of carbon dioxide
before it can be used in a natural gas system. Carbon dioxide removal is not
commonly practiced at wastewater treatment plants but information on systems
used in the chemical industry is available. 7 The estimated cost in 1974 to
treat digester gas, from a 125 mgd plant in Dallas, Texas, for use in a
natural gas system was $0.46 per 1,000 scf of methane.18 This cost included
a carbon dioxide removal system manufactured by Union Carbide that uses a
monoethanolamine absorbent. In-plant energy requirements for primary and
secondary treatment always exceed the energy available from digester gas;
therefore, the remainder of this section is devoted to on-site use as fuel
in internal combustion engines.
Use In Internal Combustion Engines
Diesel or gas internal combustion engines can be used to drive electric
generators, air blowers or pumps in a wastewater treatment plant. A typical
system illustrating these potential uses is shown in Figure 9.
Diesel engines operate on fuel oil that is ignited entirely by the heat
resulting from the compression of the air supplied for combustion. Gas-
Diesel engines operate on a combustible gas (anaerobic digester gas in this
case) as primary fuel; the ignition of the digester gas is accomplished by
the injection of a small amount of pilot fuel oil. Commonly 5 to 10 percent
fuel oil is required to operate a dual fuel engine. Dual fuel Diesel engines
are equipped to operate on fuel oil only or as a gas-Diesel. Fuel oil is
normally used in the alternate fuel system for dual fuel engines in a
wastewater treatment plant; however, it is possible to equip this type of
engine to also operate on natural gas or propane.
A gas internal combustion engine operates on a combustible gas fuel
(anaerobic digester gas in this case) that is ignited by an electric spark.
Natural gas or propane could be used as an alternate source of fuel in a
gas engine.
There are many variations in engine design, and auxiliary equipment
required, for these two basic engine types. The operating speed and
37
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turbocharging are basic differences between engines supplied by different
manufacturers. These variations in engine types result in equipment cost
and operation and maintenance cost variations.
The efficiency of engines varies depending on the basic engine design
and method of operation. In general, low speed, turbocharged or dual fuel
engines require less fuel per hp-hr than higher speed naturally aspirated
engines. However, capital costs are greater for the more efficient engines.
Average efficiencies obtained at the Hyperion Treatment Plant during three
years of operating 10 dual fuel engines are compared with other estimates
in Table 9.
The use of heat recovery equipment will increase the overall efficiency,
One manufacturer estimates energy supplied to internal combustion engines
is used as follows:
Energy Use
(percent)
Jacket water and lube oil 45
Exhaust 15
Radiation 10
Work 30
Heat recovery has been used successfully for many years particularly
with large slow speed engines. Typical heat recovery rates for dual fuel
engines manufactured by White Superior are shown in Table 10. This data
shows that recovered heat varies from 20 to 31 percent of fuel input.
Typical heat recovery rates in percent of fuel supplied to the engine are:
jacket water, 18 to 20 percent; exhaust, 10 to 13 percent; combination of
both jacket water and exhaust heat recovery, 20 to 33 percent. This
recovered heat added to the 30 to 37 percent efficiency of the engine
results in a total thermal efficiency ranging from 50 to 70 percent.
One generally used method of recovering jacket water heat is through
ebullient cooling, that is, raising the jacket water temperature to just
above the boiling point (215° to 220°F) and collecting the steam in an
external separator. The low pressure steam thus produced may be used for
digester heating, sludge drying, building heating or other purposes.
Exhaust heat is typically recovered by use of combination exhaust silencer
and heat recovery boilers. In some installations the jacket water and ex-
haust heat are recovered in a single combined unit. The cost of heat
recovery equipment varies considerably, but usually in proportion to the
size of the engine, with lower unit costs for larger engines.
Table 11 is a summary of gas, heat and power available for various
size treatment plants based on the following criteria:
1. Total dry solids to digester = 2,100 Ib/million gallons and VS =
1,446 Ib/million gallons from primary and conventional activated
sludge treatment.
39
-------
TABLE 9
INTERNAL COMBUSTION ENGINE EFFICIENCY
OPERATING ON DIGESTER GAS
Engine
Rating Efficiency
(Btu/hp-hr) (percent)
Hyperion Plant
1971-72 6469 qo /
-* 39-4
J 6428 39 6
7675 33.1
EPA Report" 7000 ^^
Engine Manufacturers
Caterpillar 8500 3Q Q
Delaval 6630 38*4
White Superior
Gas fuel, naturally aspirated,
spark ignited 8300 30.7
Gas fuel, turbo-charged,
spark ignited 7700 33 i
Dual fuel 7000 (or less) 36.'4
40
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2. Fifty percent of VS destroyed in digester.
3. Digester gas produced = 15 scf/lb VS destroyed
4. Heat available = 600 Btu/scf gas or 9,000 Btu/lb VS destroyed.
5. 1C engine efficiency =36.4 percent (7,000 Btu/hp-hr).
6. Engine-generator efficiency = 30 percent (11,400 Btu/hp-hr).
COST ESTIMATES - DIGESTER GAS UTILIZATION
Construction costs in this report include all elements of construction
cost a contract bidder would normally encounter in furnishing a complete
facility. Construction costs include materials, labor, equipment, electric-
al, normal excavation and contractor overhead and profit. Construction
costs do not include costs for land, engineering, legal, fiscal and
administrative services or interest during construction. Construction
costs include the same elements included in construction costs in the Black
and Veatch report.
Equipment costs were obtained through quotes from various suppliers arid
manufacturers. Construction costs include allowances for the following:
overhead and profit (25 percent), equipment installation (35 percent),
electrical (15 percent), piping and miscellaneous items (15 percent) and,
other site work and contingency (15 percent). Compounding these allowances
gives a construction cost of 2.6 times equipment cost. Operation and
maintenance is broken down into three categories: (1) operating and main-
tenance labor in hr/yr, (2) materials and supplies in $1,000 yr, and (3)
energy in kwh/yr or Btu/yr.
Cleaning and Storing Digester Gas
Hydrogen sulfide (H2S) can be removed from digester gas by treatment in
a chemical scrubbing system using sodium hypochlorite or other oxidizing
agents. The reaction with sodium hypochlorite requires 2.2 Ib of NaOCI to
remove one Ib of H2S:
H2S + 4NaOCl •* H2S04 + 4NaCl
It is possible to use activated carbon for H2S removal but the carbon
must be regenerated with steam. Chemical scrubbing systems are more econ-
omical and simpler to operate. It may be possible to use other chemicals,
or other sources of hypochlorite, to furnish less expensive scrubbing systems
than shown herein. Iron sponge scrubbers have been installed in some treat-
ment plants.
Estimated construction costs and operation and maintenance data for
compressors are shown in Table 12. Equipment costs are based on recent
quotes from manufacturers, operation and maintenance estimates are based
on records of the Orange County (California) Sanitation District.
43
-------
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The following construction costs of conventional size spheres to store
gas at 50 psi are based on a recent quote from a supplier in Southern
California.
Sphere Diameter Volume Construction Cost
(ft) (cu/ft) ($1,000)
32 17,000 65
36 24,000 90
46 50,000 185
60 113,000 400
Unit costs for diameters larger than 60 ft are higher because of struc-
tural features that must be incorporated.
Construction costs for scrubbing with NaOCl in a packed tower, include
on-site hypochlorite generation. Operating and maintenance costs for this
type of scrubbing system assume the removal of 1,000 ppm H2S from the
digester gas. The estimated construction costs to cLean and store digester
are summarized in Table 13. Construction costs are shown in Figure 10;
operation and maintenance data are shown in Figure 11. Construction costs
are greatly influenced by the storage capacity provided. The storage capa-
city used in these estimates is based on one sphere per plant, up to plant
sizes of about 100 mgd.
On-Site Electricity Generation
The primary components of a system to generate electricity with digester
gas, in addition to gas cleaning and storage facilities, are shown on the
anaerobic digester gas utilization system schematic (Figure 9) and include:
(1) 1C engine, (2) generator, (3) heat recovery unit, and (4) alternate fuel
system.
Cost estimates for electric power generation are based on the following
criteria:
1. Engine and engine-generator equipment costs are based on data
furnished by several major engine manufacturers: Ingersoll Rand,
Enterprise Delaval, White Superior, Fairbanks Colt and Waukesha.
2. Both dual fuel and gas engine costs are included for equipment
available in 600 rpm speeds.
3. Dual fuel engines are turbocharged and gas engines are naturally
aspirated.
4. Engine costs include all auxiliary equipment required for an
operating installation, such as: skid base, exhaust silencer, air
inlet filter, starting equipment , gas and dual fuel pumps, regula-
tors, safety devices, control equipment and main circuit breaker.
Heat recovery units are shown as a separate item.
45
-------
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46
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GAS STORED, 1000 cu ft
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100
Figure 10. Construction cost to clean and store digester gas
47
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a:
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DIGESTER GAS CLEANED 8 COMPRESSED, scfm
Figure 11. Operation and maintenance costs to clean and
store digester gas.
48
-------
5. Alternate fuel systems are fuel oil for dual fuel engines and
propane for gas engines.
Fuel oil: 142,500 Btu per gal
Propane : 91,500 Btu per gal
6. Heat recovery costs are based on ebullient systems and data furnish-
ed by Vaporphase Systems and the engine manufacturers.
7. Operation and maintenance estimates are based on a detailed analysis
of four years data from the Orange County Sanitation District for
six engines operating on digester gas.
Estimated costs for 600 rpm internal combustion engines equipped with
heat recovery and alternate fuel systems are shown in Figures 12 and 13.
These cost curves include data for both dual fuel and gas engines. Operation
and maintenance costs are greatly affected by the alternate fuel consumed.
Propane alternate fuel systems are more costly than fuel oil systems;
however, gas engines that would require propane are less costly than dual
fuel engines that require fuel oil. Dual fuel engines require about 10
percent fuel oil on an average annual basis. Gas engines could operate
without using any alternate fuel. However, for these estimates, it is
assumed that 10 percent would be consumed. Propane would have to be used
(or at least paid for) to obtain contracts for a firm supply.
Estimated costs for complete systems to generate electricity with diges-
ter gas are shown in Figures 14 and 15. These costs are for a system as
shown in Figure 11.
Example Cost Estimate
The cost curves may be used to estimate on-site electricity generation
costs as shown in the following example for a 100 mgd plant:
Construction cost (Figure 14) $2,500,000
Material (Figure 14) 55,000/yr
Labor (Figure 15) 5,800 hr/yr
Electricity (Figure 15) 1,500,000 kwh/yr
Fuel (Figure 15) 23 x 109Btu/yr
Annual costs:
$ Construction $319,000 per year
$2,500,000 plus 35 percent for engineering, administra-
tion, interest during construction and other costs =
$3,375,000 total. Amortize for 20 years at 7 percent
interest, ($3,375,000) (0.09439) = $319,000
$ Operation and Maintenance $220,000 per year
Labor 5,800 hr @ $10/hr $58,000
49
-------
10,000
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1 1
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-- 10
L
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7 8 910,000
Figure 12. Construction and maintenance
material costs for 600 rpm 1C
engines with heat recovery and
alternate fuel systems.
50
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tr
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1C ENGINE, hp
Figure 13. Alternate fuel and labor requirements
for 600 rpm 1C engines with heat
recovery and alternate fuel systems.
51
-------
O
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u
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TREATMENT PLANT CAPACITY, mgd
Figure 14. Construction and maintenance material
costs for complete electrical generation
system shown in Figure 9.
52
-------
100,000
8
7
6
5
CD
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Ul
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a:
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Material $55,000
Electricity 1,500,000
kwh @ $0.025/kwh 38,000
Fuel 23xl09 Btu/yr
@ $3/mil Btu 69,000
• Total Annual Cost $539,000 per year
Column (7) in Table 11 estimates that there are 2400 kw (21,000,000
kwh/yr) available from a 100 mgd plant. This gives a unit electricity
generation cost of $0.026 per kwh. If the generating facility operates
only 80 percent of the time, the unit cost increases to $0.032/kwh. These
costs do not take credit for recovered heat. Column (8), Table 11 estimates
that 162.5 mil Btu/day (59 x 109 Btu/yr) could be recovered in a 100 mgd
plant. Valuing this waste at $1.50/mil Btu reduces the unit costs to
$0.022/kwh and $0.028/kwh for 100 percent and 80 percent operating time
respectively. A similar calculation for 10 mgd plant, including credit
for recovered heat, gives $0.037 and $0.047 per kwh for 100 percent and
80 percent operating time respectively.
54
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SECTION 5
PRODUCTION OF CONSUMABLE MATERIALS
Estimated energy requirements are presented for off-site production
of the following consumable materials:
Activated Carbon Lime(Calcium Oxide)
Alum Methanol
Ammonium Hydroxide Oxygen
Carbon Dioxide Sodium Chloride
Chlorine Sodium Hydroxide
Ferric Chloride Sulfur Dioxide
Sulfuric Acid
Data on energy required to manufacture consumable materials was obtained
from several sources including: (1) contact during this study with manu-
facturing companies, (2) technical journals and books, and (3) calculations
based on descriptions of production processes contained in the technical
literature or furnished by manufacturers.
Specific energy requirements for some materials are somewhat difficult
to obtain for the following reasons:
1. Some companies consider this type of information proprietary and
will not release details of the manufacturing process or the energy
required. Other companies could not, or would not, furnish energy
data for a variety of reasons including the belief that it would
jeopardize their competitive position, and insufficient records.
2. Some manufacturing processes produce more than one product, e.g.,
chlorine and sodium hydroxide, or a primary product and a by-product
e.g., ammonia and carbon dioxide.
3. By-product or waste from one process used as feedstock in manufac-
turing process, e.g., ferric chloride and sulfuric acid.
4. Most chemicals are produced by more than one process, or with differ-
ent methods of obtaining feedstock, with different energy require-
ments, e.g., sulfuric acid, carbon dioxide and methanol.
The estimated energy requirements for production are summarized in
Table 14. These total energy estimates include the fuel required to generate
electricity required for production.
The following sections discuss the energy estimates for each consumable.
List prices for each chemical were used as a general guide to the reason-
ableness of the estimates. The following costs and other factors were used
in developing the energy estimates:
55
-------
TABLE 14
ESTIMATED ENERGY REQUIREMENTS FOR THE PRODUCTION
OF CONSUMABLE MATERIALS
Fuel Electricity
Material Million Btu/ton kwh/lb
Activated Carbon 102* 4 9
Alum 2* 0.1
Ammonium Hydroxide 41* 2 n
Carbon Dioxide 2 to 54 Q>1 tQ ^^
Chlorine 42 2>Q*
Ferric Chloride 10 0 5*
Lime (Calcium Oxide) 5.5* 0 3
Methanol 35* ^ 7
Oxygen 5.3 0.3*
Salt (Sodium Chloride)
Evaporated 4* Q ~
Rock & Solar 0.5 <0 i*
Sodium Hydroxide (50% NaOH) 37 l g*
Sulfur Dioxide 0.5 <0 i*
Sulfuric Acid 1.5* 0 1
Indicates principal type of energy used in product
ion .
56
-------
Electricity $0.028/kwh
Natural Gas $1.30/million Btu
Fuel
Natural gas 1,000 Btu/cu ft
Coal 25,000,000 Btu/ton
Diesel fuel 142,500 Btu/gal
Electricity generation 10,500 Btu/kwh (32.5% effi-
Steam generation (low pressure) 1,600 Btu/lb ciency)
ACTIVATED CARBON
The manufacture of activated carbon is a highly competitive industry
and the companies will not divulge specific details of their production
process nor will they furnish information on the energy required for
production. There are several books devoted to activated carbon,19' 20> 21
but none contain information on energy requirements for manufacturing.
Regarding the manufacture of granular carbon for water treatment
Hassler,20 notes:
"The production of granular carbons for liquid phase applications
was long delayed because of the additional activation necessary to
provide the required types of adsorptive capacity. The additional
activation oxidized the walls of pores and thereby weakened the structure.
As a result, the finished carbons lacked the mechanical strength to
withstand the abrasion incident to continual recycling required of
granular carbons. The difficulty was finally surmounted about the
time of World War II. Granular carbons with effective adsorptive
capacity combined with adequate mechanical strength have been
available for liquid systems for a number of years."
The granular activated carbon now used in most reactivation systems
throughout the world is made from bituminous coal.
Powdered carbon is made from granular activated carbon by grinding
the dry granular material. Hassler20 describes the preparation of powdered
carbon as follows:
"The preparation of powdered carbon should be accomplished by
the mildest possible pulverizing action. A powerful crushing
action as by heavy weights of balls in a ball mill can damage
the filterability. It can also impair the adsorptive power of
decolorizing types of carbon.
Carbons should preferably be very dry when pulverized because
the presence of moisture augments adverse effects of filterability."
The common requirements of all the processes for the production of
activated carbon are that the raw material is carbonized at temperatures
usually in the 500 to 800 C range and then activation is achieved either
by the addition of reagents to the raw material or by a subsequent activation
stage. In processes involving gaseous activation with steam or carbon
57
-------
dioxide, the activating reaction is endo thermic. If, however, the product
gases are burned to provide heat, the overall reaction is merely the
combustion of part of the carbon which is an exothermic reaction. Carbons
used in wastewater treatment are activated with heat and steam and thus
fuel (usually natural gas) is required for the activation process. Hassler20
describes the process as follows:
"In a typical process, coal is pulverized and mixed with sufficient
binder to form a plastic mass which is briguetted or extruded at pressures
variously described ranging from 100 to 2,000 Ibs per square inch
The pellets or spaghetti-like strings are carbonized slowly to avoid
rapid evolution of gas, after which the char is steam activated."
A report by one manufacturer11 states that 12.8 million Btu are required
to produce 250 Ib of new carbon, or about 51,000 Btu/lb (102 million
Btu/ton). The following are reported by Smisek and Cerny21 to be the normal
consumption of material and energy for the production of one ton of carbon
activated with steam when wood charcoal is used as the starting material:
Wood-tar 1,500 kg
Wood-Charcoal 3,000 kg
steam 10,000 kg
Electricity 2,000 kwh
17 nnnT "Ju*rements would give about 10,500 Btu/lb for electricity and
i/,UOO Btu/lb for steam for a total of about 27,500 Btu/lb (55 million
Btu/ton). This total does not include any energy required in the carbon-
ization process to produce the wood charcoal.
Garber, et al.,22 estimate the energy required to produce one ton of
granular activated carbon as follows:
Btu/ton Carbon
Mining and transporting
coal 140,000
On-site production 36,000,000
TOTAL 36,140,000
ah n/ionn/ waftewater treatment grade granular activated carbon is
about $0.45/lb ($900/ton). The cost of natural gas for granular activated
(at 51,000UBtu/lb) W°Uld ran§e ^^ $°'°23/lb (at 18'°°° Btu/lb) to $
The higher energy requirement (51,000 Btu/lb) and energy cost does not
appear ^unreasonable. Since the higher estimate is from a manufacturer and
there is no data to support a lower figure, 51,000 Btu/lb (102 million
Btu/ton) is shown in Table 14. There is no basis for differentiating
between energy required for powdered and granular carbon production.
58
-------
ALUM
Aluminum sulfate is produced by reacting bauxite ore (Al2C>3 • 2H20)
or clays which are rich in aluminum oxide with sulfuric acid. The reaction
is represented by the following formula:
A1203 • 2H20 + 3H2S(\ •> A2 (80^)3 + 3H20
The process used for aluminum sulfate production is well-established and
fairly standard among producers. It requires 670 Ib bauxite (55 percent
A1203) and 1,140 Ib sulfuric acid (60 Be) to produce one ton of aluminum
sulfate (17 percent A1203).23
Information furnished by three manufacturers ranged from 125,000 to
770,000 Btu required to produce one ton of alum. One manufacturer reported
a total requirement of 4.7 million Btu/ ton with 83 percent of the heat
requirement supplied as a by-product of sulfuric acid manufacture. The
report by Garber, et al.,22 estimates an energy requirement for alum
production of 9.2 million Btu/ ton. Adding the energy required for sulfuric
acid production (1.5 million Btu/ ton) to the high figure reported by a
manufacturer, gives a requirement of about 1.6 million Btu/ ton.
The chemical reaction is exothermic and it appears that relatively
small amounts of energy are required in subsequent processing operations;
therefore, an energy requirement of 2.0 million Btu/ ton appears adequate
for production, including energy required for feedstocks.
AMMONIUM HYDROXIDE
Ammonium hydroxide, or aqua ammonia as it is termed commercially, is
commonly produced in solutions varying from about 20 to 30 percent ammonia
through the reaction of ammonia gas with water:
NH + H20 $ NH^OH £ NH4+ + OH~
Ammonia is produced by the catalytic reaction of nitrogen and hydrogen
at high temperature and pressure. The nitrogen is derived from air by means
of liquefaction, the producer gas reaction, or by burning out the oxygen
in air with hydrogen. Hydrogen is obtained from many sources, including
water gas, coke-oven gas, natural gas, fuel oil, catalytic reformer gases, and
the electrolysis of water or brine. Since World War II, natural gas has
become the most important hydrogen source. Currently, petroleum or natural
gas-derived ammonia represents 90 percent of production and ammonia is the
number one petrochemical in terms of volume of production. Natural gas
curtailments have reduced ammonia production since 1972.
Faith23 gives the following requirements for producing one ton of
liquid ammonia:
Natural gas (92% CH4) 26,000 cu ft
Catalyst for shift reaction 0.3 Ib
59
-------
Synthesis catalyst 0.5 lb
Caustic soda (100%) 8* lb
Monoethanolamine 0.3 lb
Fuel gas (for driving
compressors) 22,000,000 Btu
Electricity IQQ
Water 6,000 gal
er «t S2?T °5 T^uUSe ^ the industrial chemicals industry by Saxton,
et al., found that the average energy required for the production of am-
monia in 1971 and 1973 was about 41 million Btu/ton. This total energy
requirement is divided between feedstock energy (about 55 percent) and pro-
cess energy fuel and electricity (about 45 percent).
The cost of energy supplied by natural gas would be about $62.40/ton
for an energy requirement of 48 million Btu/ton. The cost of process energy
and energy represented by natural gas feedstock is about 38 percent of the
list price for ammonia hydroxide.
CARBON DIOXIDE
Pure liquid or solid carbon dioxide (C02) is produced from various
sources of dilute C02. Primary sources of dilute C02 gas include:
(1) gases from the decomposition of carbonates, and (2) combustion of coke
'
« in n resultinS Sases> Banging in C02 content from
about 10 to 40 percent, are treated by absorption to remove C0? . After
the concentrated gas is purified, it is compressed and refrigerated to give
liquid or solid C02 . Coke, oil, and natural gas are burned carefully to
produce a gas containing 17 to 18 percent C02, and the heat obtained is
converted into energy for the compressors.
Because the thermal decomposition of limestone, dolomite, magnesite
marble and similar materials yields gases containing 32 to 42 percent COo
by-product recovery is often carried out on kiln gases at cement and lime
plants. When limestone or dolomite is used as a raw material, however
° C°ke °rdlnarily mixed with every ton of limestone burned
The following are material and utility requirements reported by Faith23
to produce one ton solid C02 from 18 percent flue gas:
Energy
Btu/ton Solid C02
Natural gas 22,000 cu ft 22,000,000
Sodium carbonate 25 lb
Water 20,000 gal
60
-------
Energy
Btu/ton Solid C02
Steam 20,000 Ib 32,000,000
Electricity 10 kwh 105,000
TOTAL 54,105,000
These material and utility requirements result in a total energy
requirement of about 54,000,000 Btu, at a cost of about $70, to produce
one ton C02•
Data was furnished by three C02 manufacturers. One manufacturer reported
that C02 gas is a by-product of ammonia production and is liquefied and sold.
This company reported an energy requirement of 200 kwh/ton to liquefy the C02-
Another large producer also reported that CO^ was a by-product of other chem-
ical manufacturing processes and they had no way to estimate the energy re-
quired for its production. The third manufacturer reported a requirement of
160 kwh/ton of C02 produced. A study for the Ford Foundation25 estimated 40
kwh/ton CO2 produced. Based on this data, a range of 2 to 54 million Btu per
ton is shown in Table 14. The high value represents the total energy requir-
ed to produce C02 from all new materials. Carbon dioxide used in wastewater
treatment will often be produced at the plant site with purchased C02 use
limited to standby and emergency purposes.
CHLORINE
Over 95 percent of the chlorine now manufactured in the United States is
produced from the electrolysis of brine by two different methods: (1)
diaphragm cells, and (2) mercury cells. The production of one ton of chlor-
ine, in addition to electricity, requires about 3,660 Ib sodium chloride,
steam and refrigeration. The process also produces about 2,285 Ib of sodium
hydroxide and 57 Ib of hydrogen gas per ton of chlorine produced. »°
The gas produced at the anode is about 97.5 percent chlorine. The
diaphragm cell produces an 11 to 12 percent solution of sodium hydroxide
while the concentration is about 50 percent in the mercury cell method.
Therefore, no evaporation is needed in a mercury cell to produce the usual
commercial strength of 50 percent caustic. Despite this advantage, the
use of mercury cells is being discontinued because of mercury in the waste
discharge.
Typical electrical power requirements for diaphragm and mercury cells
are reported by White:2°
kwh/lb
Chlorine Produced
Diaphragm cells 1.36 - 1.41
Mercury cells 1.47 - 1.57
Chlorine gas produced in a diaphragm cell must be cooled, dried, compressed,
scrubbed of impurities and liquefied by refrigeration for shipment. All of
61
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these operations consume energy, which is not included in the 1.36 to 1.41
kwh/lb shown above. However, the process also produces sodium hydroxide
and some energy use should be charged to this chemical.
The survey of energy use in industrial chemical production by Saxton24
gives a national average energy requirement to manufacture chlorine of 43.3
million Btu/ton in 1971 and 41.5 million Btu/ton in 1973. The Ford Foundation
study25 reports about 42 million Btu required to manufacture one ton dry
chlorine and 1.13 tons caustic soda in 50 percent solution by the diaphragm
cell process.
One manufacturer reported a requirement of 9,600 Btu/lb of chlorine
produced and two others reported about 21,000 Btu/lb. The higher values
also include steam for evaporation to produce a 50 percent solution of sodium
hydroxide in the diaphragm cell method.
Power and salt required for the on-site generation of sodium hypochlorite
are reported by several manufacturers as follows:
Electrical Energy Salt
kwh/lb Ib/lb
Manufacturer Chlorine Equivalent Chlorine Equivalent
Ionics 1.6-2.5 1.8-2.0
Englehard 1.7-2.8 3-4
Pacific Engineering 2.3-3.0 3.2
Diamond Shamrock 2.5 3.5
An average electrical energy requirement for chlorine production of 2.0
kwh/lb is shown in Table 4-1 (this converts to 42 million Btu/ton using
10,500 Btu required to generate one kwh). The 2.0 kwh/lb includes necessary
feedstocks, the electrolytic cell and other required processes and no credit
for the sodium hydroxide and hydrogen produced.
FERRIC CHLORIDE
Little information was obtained from manufacturers on the processes used
for ferric chloride production. One producer in Southern California manufac-
tures ferric chloride using waste pickling liquor from a nearby steel mill.
This particular mill uses hydrochloric acid for steel cleaning; the waste
acid and ferrous chloride is supplied to the chemical producer. The waste is
neutralized, concentrated by solar evaporation and reacted with chlorine
solution to form ferric chloride.
FeCl2 + Jg C12 -»• FeCl3
Sulfuric acid is used in many steel manufacturing pickling operations.
The reaction with waste pickle liquor from this type of operation is:
+ FeCl3
62
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The stoichiometric reaction with ferrous chloride requires 437 Ib
chlorine to produce one ton of ferric chloride. An energy requirement of
about 9.2 million Btu/ton for production of ferric chloride is given by
this reaction by using an energy requirement for chlorine production of 2.0
kwh/lb. The reaction with ferrous sulfate would require 1,312 Ib chlorine
to produce one ton of ferric chloride and the energy requirement would be
about 4,000 kwh/ton.
The 1976 price of ferric chloride was $80 to $100 ton with $90 per ton
quoted in California. The 1976 price of chlorine was quoted at $125 to
$150 per ton. However, some agencies in Southern California are paying
$220 per ton for chlorine delivered in one ton cylinders. A manufacturing
process to produce one ton ferric chloride would require chlorine with a
list price of $25 to $48; a process starting with ferrous sulfate would
require chlorine valued at $75 to $145.
The current price of ferric chloride appears consistent with a manu-
facturing process utilizing ferrous chloride as feedstock. Energy required
for production by this method would be about 0.5 kwh/lb including feedstocks
and processing energy
LIME (CALCIUM OXIDE)
Quick lime (CaO) is produced by burning various types of limestone
(CaC03) in shaft or rotary kilns as illustrated in the following reaction:
heat
CaC03 •> CaO + C02 t
Shaft kilns are directly fired by oil, natural gas or producer gas.
Rotary kilns are also fired with oil, natural gas or producer gas, but
the trend has been to firing with pulverized coal. The modern trend is to
large rotary kilns with capacities of at least 200 tons/day. Energy required
to manufacture quick lime depends upon: (1) raw material, (2) type of
furnace, (3) type of fuel, and (4) efficiency of equipment.
Shaft kilns are considered more efficient in terms of fuel economy than
are rotary kilns. The most modern shaft kilns may approach a fuel ratio of
5 tons of lime per ton of coal and for the larger rotary kilns this ratio
may average around 4.2. The national average for all quick lime production
is about 7 million Btu/ton of quick lime. This may drop into the 5 million
range as the larger kilns, both shaft and rotary, come on stream and the
smaller, less efficient, kilns are retired.
The following requirements to produce one ton of quick lime were
reported by Faith, et al.,27 and one manufacturer:
Limestone (pure) 3,750 Ib
Coal (bituminous) 650 Ib
Using a heat value for coal of 12,500 Btu/lb gives 8.1 million Btu
required to produce one ton of quick lime plus energy used in mining and
delivering limestone.
63
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The Flintkote Company, U.S. Lime Division, recently began operating a
coal fired, rotary kiln in Nelson, Arizona, rated at 800 tons pebble quick
lime per day. Initial operating results indicate that this new and
efficient plant will require about 4.5 million Btu to produce one ton of
quick lime. Another new plant is under construction in Kentucky which will
be equipped with three 1,000 ton/day kilns. Estimates are that this plant
will require from 4.5 to 5.2 million Btu to produce one ton of quick lime.
A report by the Stanford Research Institute28 gives 5.6 million Btu
required to produce one ton of lime. It appears that in modern plants
about 5.5 million Btu should be adequate to produce one ton of quick lime,
including limestone production.
METHANOL
Methanol is synthesized by the reaction of hydrogen and carbon monoxide
under high pressures:
CO + 2H2 + CH3OH
This reaction has an efficiency of about 60 percent without recycle.
The reactants are obtained by a variety of methods from different raw
materials. The most critical raw material for production in the United
States is natural gas. Natural gas which has been desulfurized by passage
over activated carbon is preheated and mixed with carbon dioxide and steam
at 30 psig. The mixture is passed into heated alloy-steel tubes in a
furnace. The tubes are normally packed with a promoted nickel catalyst.
The reaction which takes place at 800°C is essentially:
3CHU + ~C02 + 2H20 -* 4CO + 8H2
The resulting synthesis gas is cooled by passage through waste heat boilers,
various heat exchangers, and water coolers.
A few plants produce methanol by using carbon dioxide instead of carbon
monoxide:
C02 + 3H2 + CH3OH + H20
One manufacturer reported a total energy requirement of 30 million Btu/ton
for production of methanol including methane feedstock, steam and electri-
city. Another manufacturer reported 15 million Btu/ton including all steam
used in the process.
The survey of energy use in industrial chemical production by Saxton24
gives a national average energy requirement to manufacture methanol of 37.0
million Btu/ton in 1971 and 35.8 million Btu/ton in 1973. This total
energy requirement is divided between feedstock energy (about 72 percent)
and process energy (about 28 percent).
Because of the differences in manufacturing methods, the 1973 national
average of about 36 million Btu/ton is shown in Table 14 for methanol.
64
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OXYGEN
Pure oxygen is produced by the liquefaction and subsequent fractionation
of air. Several different variations of the basic process are used by manu-
facturers including variations in the methods for air compression, purifica-
tion, and refrigeration; and differences in the design of heat-exchange,
rectifying, evaporating, and condensing equipment. The pressures of the
various cycles range from about 60 to 3,000 psi.
Union Carbide Corporation is one of the largest producers and employs
two basic oxygen generator designs. The cryogenic process is used in large
installations and a pressure swing adsorption (PSA) system is used in smaller
plants (usually less than 50 tons per day). The average power requirement
for oxygen generation for each system is reported by Union Carbide29 as
follows:
PSA - 0.35 hp/lb 02 transferred/hr at 90% oxygen utilization
Cryogenic - 0.22 hp/lb Q^ transferred/hr at 90% oxygen utilization
At a constant load, cryogenic oxygen systems use the least power for
oxygen generation; however, the PSA unit turns down more linearly.
The Union Carbide system proposed for Amherst, Massachusetts would require
3720 hp-hr to generate six tons oxygen per day, or 620 hp-hr (470 kwh) per
ton. A 350 ton/day system proposed for the City of Los Angeles would require
345 kwh/ton. Another manufacturer reported 500 kwh/ton required to generate
oxygen gas and 800 kwh/ton for liquid oxygen.
Faith,27 gives a range of energy requirements for oxygen production from
about 290 kwh/ton in a 300 to 500 ton/day plant to 370 kwh/ton in a 25 ton/
day plant. Another review of chemical technology30 reports energy require-
ments for oxygen production of 500 kwh/ton for gas and 800 kwh/ton for liquid.
The Ford Foundation study25 estimates energy required for oxygen production
of 425 kwh/ton for gas and 780 kwh/ton for liquid.
All of these reported requirements are in the range of 290 to 800 kwh/ton.
It appears that 500 kwh/ton can be achieved in even small to medium size
plants and this value is shown in Table 14. Pure oxygen use in wastewater
treatment is similar to the use of carbon dioxide in that oxygen will most
often be produced at the plant site.
SODIUM CHLORIDE
Sodium chloride is produced commercially in the United States by essen-
tially three processes:
1. Multiple effect evaporation (evaporated salt) - 99.8 percent NaCl
2. Mining (rock salt) - 98.5 percent NaCl
3. Solar evaporation (solar salt) - 95 percent NaCl
Salt required for regeneration in selective ion exchange processes can be
supplied by any of these three manufacturing methods. One manufacturer report-
ed energy requirements for all three processes:
65
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kwh/ton
Salt Produced
Evaporated salt (vacuum pan method) 2,340
Rock salt 22
Solar salt 150
Another manufacturer reported a steam requirement of 2,000 Ib/ton.
Using 1600 Btu/lb steam gives an energy requirement of 3.2 million Btu/ton.
Faith gives the following requirements to produce one ton evaporated salt
(99.8 percent NaCl):
Saturated brine 7,600 Ib
Soda ash (58%) 7.5 Ib
Caustic soda (50%) 0.8 Ib
Steam (actual) 2,500 Ib (with triple
effect evaporation)
The energy requirement would be about 4 million Btu/ton for steam, plus
brine pumping. Estimated energy requirements are shown in Table 14 for all
three types of salt.
SODIUM HYDROXIDE
In the electrolytic process for the manufacture of sodium hydroxide, an
electric current is passed through a cell containing a sodium chloride
solution. The salt brine is decomposed by the current to form a 10 to 70
percent sodium hydroxide solution, with hydrogen gas forming at the cathode
and chlorine gas at the anode as co-products. Two types of cells, the mer-
cury cathode and diaphragm, are used in the United States. These are the
same units used to produce chlorine gas.
Mercury cells produce sodium hydroxide of 20 to 70 percent concentration
and diaphragm cells produce a 10 to 12 percent solution. The weak solutions
are concentrated in multi-effect evaporators to produce a 50 percent standard
grade solution.
Four manufacturers furnished data on energy required for production of
sodium hydroxide ranging from 0.9 to 2.1 kwh/lb. Data in the Chlorine
section indicates that about 42 million Btu are required to produce 1.13
tons of sodium hydroxide, in 50 percent solution, and one ton chlorine.
Sodium hydroxide is also produced commercially from lime and soda ash
according to the following reaction:
Na2C03 + Ca(OH)2 + 2NaOH + CaC03
In this process, a solution of sodium carbonate (soda ash) is treated
with calcium hydroxide (hydrated lime) to produce a precipitate of calcium
carbonate and an aqueous solution of sodium hydroxide. After removal of the
66
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insoluble carbonate, the solution is concentrated to give various grades of
caustic soda. Material and utility requirements to produce one ton sodium
hydroxide (11 percent solution) by this method are:
Sodium carbonate (58 percent) 3,000 Ib
Lime (make-up - 98 percent CaO) 165 Ib
Water 2,200 gal
Steam 2,700 Ib
Fuel (reburning) 13,000,000 Btu
Electricity 18 kwh
The manufacture of caustic soda is related both to the chlorine industry
and to the ammonia-soda industry in that it is produced as a profitable item
by both. In the first case, caustic soda is a joint product with chlorine;
in the second, production is secondary to soda ash. The percentage of total
production manufactured by the electrolytic process was 29 percent in 1925,
44 percent in 1935, and more than 85 percent in 1954. Demand for chlorine is
increasing faster than demand for caustic soda. There is a definite tendency
for chlorine consumers to build electrolytic plants to supply their own
chlorine needs and to market excess caustic.2
The energy requirement shown in Table 14 is on the same basis as that
shown for chlorine (1 ton chlorine and about 1.13 ton of sodium hydroxide
are co-products in the electrolytic process). The 37 million Btu is the
energy required to produce one ton of sodium hydroxide; this same 37 million
Btu would also produce 1750 Ib chlorine and 50 Ib hydrogen.
SULFUR DIOXIDE
Sulfur dioxide (862) is the basic raw material for the manufacture of
sulfuric acid and S02 for this purpose is derived from several sources
including:
1. Sulfur. Burning with the proper ratio of air yields a gas which
is 8 to 11 percent S02-
2. Metal sulfides. Heating ferrous sulfides (FeS2 or FeySs) releases
S02- The remaining iron oxides may be utilized as iron ore in some
cases. The sulfur must be driven off of the sulfides of copper,
lead, nickel and zinc that are mined for their metal content. Air
pollution control laws now require smelters to remove SC>2 from the
stack gas discharge.
3. Hydrogen sulfide. H2S recovered in the production of fuel gases can
be burned directly to S02• In one process the H2S is stripped from
the fuel gas with an ethanolamine solution and later liberated from
the solvent. The concentrated H2S is then converted to S02 by burn-
ing at. 1000°C in a pressurized boiler where 80 percent of the total
heat of reaction can be recovered by generating steam.
67
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Energy required for production of S02 varies depending upon the manufac-
turing process and the source of feedstock. Gases containing S02 are cooled,
purified and liquefied by compressing. Production by a process similar to
that used for C02 would require about 200 kwh/ton plus feedstock requirements
Sulfur mined by the Frasch process requires about 8 million Btu/ton for hot
water and pumping energy while several of the other feedstock sources are
waste gases from other processes.
It is noted in the Ford Foundation study25 that compound gas (acetylene
and carbon dioxide) require much less energy for production than elemental
gases (oxygen, nitrogen and hydrogen) . One manufacturer reported that more
energy is recovered by waste heat boilers in the production process than is
used in purification and liquefaction. Another manufacturer considers the
production energy requirement to be about 150 Btu/ton for liquefaction. An
energy requirement of 0.5 million Btu/ton is considered representative for
S02 production and is shown in Table 14.
SULFURIC ACID
Sulfuric acid is produced commercially in the United States by two basic
methods: (1) contact process, and (2) chamber process. Very few new chamber
plants have been constructed since the advent of the contact process.
Both the contact and chamber processes for producing sulfuric acid
utilize sulfur dioxide as the basic raw material. The primary difference in
the processes is the method of oxidizing the sulfur dioxide to sulfur triox-
ide. However, the chamber process can more easily use sulfur dioxide of low
purity and may, therefore, be better adapted to producing sulfuric acid from
pyrites and waste gases.
gives the following requirements to produce one ton sulfuric
acid (100 percent l^SO^) in plants with 50 ton/day capacity:
Contact Chamber
Process Process
Sulfur, Ib 688 677
Water, gal 4,000 2,500
Air, cu ft 250,000 275,000
Electricity, kwh 5 15
The electrical energy required to produce sulfuric acid from sulfur
dioxide ranges from 5 to 15 kwh/ton. The sulfur dioxide can be produced
from several sources as described in the previous section.
One manufacturer reported a production energy requirement of 1 million
Btu/ton. Another manufacturer reported 2.6 million Btu/ton. These figures
include sulfur dioxide production by burning sulfur but do not include
energy required for sulfur production. As noted in the previous section on
sulfur dioxide, heat is produced and recovered in the sulfur burning process
68
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The study of energy use in industrial chemicals production by Saxton24
gives about 160,000 Btu/ton for sulfuric acid manufacture. This does not
include the energy consumed in mining sulfur. This study also notes that a
great deal of excess steam is available from sulfuric acid plants because
of the heat released when sulfur is burned in air.
An energy requirement of 1.5 million Btu/ton is considered adequate
for sulfuric acid manufacture, including energy required for all raw
materials, with the exception of sulfur production by the Frasch process.
More total energy would be consumed by plants using native sulfur mined
by the Frasch process (not considering heat recovery) and less energy would
be required by plants using waste gases as a sulfur source.
69
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REFERENCES
1. Huffman, G. L., "Effects of Pollution Control on the Demand for Energy,"
paper presented at California Water Pollution Control Association Meeting,
San Jose, California, April 15, 1974.
2. Voegtle, J. A., "Be Conservative About Energy," JWPCF Deeds and Data,
February, 1975.
3. Eilers, R. G. and Smith, R., "Executive Digital Computer Program for
Preliminary Design of Wastewater Treatment Systems," Water Pollution
Control Research Series WP-20-14.
4. Smith, R., "Electrical Power Consumption for Municipal Wastewater Treat-
ment," EPA-R-2-73-281, July 1973.
5. ASHRAE Guide and Data Book, Fundamentals for 1965 and 1966, American
Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc.
N.Y.
6. Morgan, M. Granyer, "Energy and Man: Technical and Social Aspects of
Energy," IEEE Press, N.Y., 1975.
7. "Operation of Wastewater Treatment Plants," WPCF Manual of Practice No. 11,
1970.
8. "Sewage Treatment Plant Design," WPCF Manual of Practice No. 8, 1967.
9. "Operation of Wastewater Treatment Plants," WPCF Manual of Practice
No. 11, 1976.
10. Evans, D. R. and Wilson, J. C., "Capital and Operating Costs - AWT,"
Journal WPCF, Vol. 44, No. 1, pp. 1-13, January 1972.
11. Bernardin, F.E., Jr., and Petura, J. C., "Energy Considerations in
Adsorption as a Wastewater Renovation Technique," Second National
Conference on Water Reuse, Chicago, May 4-8, 1975.
12. Patterson, W. L. and Banker, R. F., "Estimating Costs and Manpower
Requirements for Conventional Wastewater Treatment Facilities," EPA
Water Pollution Control Research Series 17090 DAN, October 1971.
13. "Anaerobic Sludge Digestion," WPCF Manual of Practice No. 16, p. 7,
1968.
14. "Process Design Manual for Sludge Treatment and Disposal," EPA 625/
1-74-006, pp. 5-17. October 1974.
15. Graef, Steven P., "Anaerobic Digester Operation at the Metropolitan
Sanitary Districts of Greater Chicago," Proceeding of the National
Conference on Municipal Sludge Management, Pittsburgh, Pa., June 11-13, 1974
70
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16. Kormanik, Richard A., "Estimating Solids Production for Sludge Handling/'
Water and Sewage Works, pp. 72-74, December 1972.
17. Strelzoff, S., "Choosing the Optimum C02 Removal System", Chemical
Engineering, pp. 115-120, September 15, 1975.
18. "Digester Gas Reclamation, City of Dallas Central Wastewater Treatment
Plant", Black and Veatch, 1974.
19. Mantell, C. I., "Carbon and Graphite Handbook," Interscience Publishers,
New York, 1968.
20. Hassler, J. W., "Purification With Activated Carbon," Chemical Publishing
Company, New York, 1974.
21. Smisek, M. and Cerny, S., "Active Carbon, Manufacture, Properties and
Applications," Elsevier Publishing Co., New York, p. 41, 1970.
22. Garber, W. F., et al., "Energy-Wastewater Treatment and Solids Disposal,"
Journal of the Environmental Engineering Division, ASCE, Vol. 101, No. EE3,
Proc. Paper 11357, pp. 319-332, June 1975.
23. Lowenheim, F. A. and Moran, M. K., "Faith, Keyes and Clarks's Industrial
Chemicals," Fourth Edition, John Wiley and Sons, Inc., New York, 1975.
24. Saxton, J. C., et al., "Industrial Energy Study of the Industrial Chemicals
Group," International Research and Technology Corporation, NTIS PB -
236322, August, 1974.
25. "Energy Comsumption in Manufacturing," report to the Energy Policy
Project of the Ford Foundation, Ballinger Publ. Co., Cambridge, Mass., 1974
26. White, G. C., "Handbook of Chlorination," Van Nostrand Reinhold Co., 1972.
27. Faith, W. L., et al., "Industrial Chemicals," Second Edition, John Wiley
and Sons, Inc., New York, 1957.
28. Stanford Research Institute, "Patterns of Energy Consumption in the United
States," U. S. Government Printing Office, p. 152, January, 1972.
29. "Comparison of Fundamentals, Design Parameters, and Operational Character-
istics of Air and Oxygen Activated Sludge Systems," Union Carbide Corp.,
Linde Division, Environmental Systems Department (undated).
30. "Chemical Technology: An Encyclopedic Treatment," Volume I, Barnes and
Noble, New York, 1968.
71
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METRIC UNIT CONVERSION FACTORS
English Unit
Btu
Btu/lb
cu ft
cu yd
ft
gal
gpd/sq ft
gpm
gpm/sq ft
hp
hp-hr
in.
Ib (mass)
mil gal
mgd
ppm (by weight)
psi
sq ft
tons (short)
Multiplier
1.055
2.326
0.12832
0.765
0.555 (°F - 32)
0.3048
3.785
0.04074
0.06308
0.67902
0.7457
2.685
25.4
0.4536
3785
3785
essentially
6.895
0.0929
907.2
Metric Unit
kJ
kJ/Kg
1
m3
°C
m
1
m3/m
1/s
1/m2
kw
MJ
mm
m
3/d
mg/1
kN/
m
m
72
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TECHNICAL REPORT DATA
(Please read Instructions on the reverse before completing)
1 . REPORT NO.
EPA-600/2-77-214
2.
3. RECIPIENT'S ACCESSION-NO.
4. TITLE AND SUBTITLE
ENERGY REQUIREMENTS FOR MUNICIPAL
POLLUTION CONTROL FACILITIES
5. REPORT DATE
November 1977
(Issuing Date)
6. PERFORMING ORGANIZATION CODE
7. AUTHOR(S)
G. M. Wesner
8. PERFORMING ORGANIZATION REPORT NO.
9. PERFORMING ORGANIZATION NAME AND ADDRESS
Culp/Wesner/Culp
Clean Water Consultants
Santa Ana, California 92707
10. PROGRAM ELEMENT NO.
1BC611
11. CONTRACT/GRANT NO.
Cont. No. 68-03-2186
12. SPONSORING AGENCY NAME AND ADDRESS
Municipal Environmental Research Laboratory--Cin.,OH
Office of Research § Development
U.S. Environmental Protection Agency
Cincinnati, Ohio 45268
13. TYPE OF REPORT AND PERIOD COVERED
Final
14. SPONSORING AGENCY CODE
EPA/600/14
15. SUPPLEMENTARY NOTES
Project Officer: Francis L. Evans, III
EPA-MERL-WRD, Cincinnati, Ohio
45268, 513/684-7610
16. ABSTRACT
This report presents information on energy requirements in municipal pollution
control facilities for several major areas of interest.
1. Pumping energy for filtration and granular carbon adsorption of secondary
effluent - Pumping requirements are developed for all elements of the filtration
process including: (a) main stream, (b) backwash, (c) surface wash, (d) wash water
return, and (e) chemical feed.
2. Heat Requirements - Estimated heat requirements are developed for:
(a) Building heat. For three cities, heating requirements are presented as a
function of plant capacity.
(b) Anaerobic digestion,, Heat requirements for anaerobic digestion at 95 F in
standard and high rate digesters are given as a function of influent sludge
temperature.
(c) Heat treatment of sludges„ Fuel requirements as a function of thermal
treatment capacity are presented for both heat conditioning prior to dewatering and
for oxidation prior to ultimate disposal.
3. Utilization of Anaerobic Digester Gas - Cost estimates are presented for
cleaning and storing digester gas, and for use as fuel in internal combustion engines
that are coupled to pumps, blowers or electrical generators,,
4. Secondary Energy Requirements - Estimations are made for off-site production
of—soffle—e£~tire~consttfflaklc5 used in wastcwatcr treatment-processes„ —
17.
KEY WORDS AND DOCUMENT ANALYSIS
DESCRIPTORS
b.IDENTIFIERS/OPEN ENDED TERMS
c. cos AT I Field/Group
Waste treatment, Sewage filtration, Sludge
digestion, Activated carbon, Drying, Energy,
Power, Pumping, Heat, Heating load, Heat
recovery, Conversion, Cost analysis, Cost
estimates
Energy requirements,
Digester gas (cleaning,
storage, reuse), Pumping
energy--filtration,
Granular carbon adsorp-
tion, Secondary energy
requirements--consumables
13B
13. DISTRIBUTION STATEMENT
Release to public
19. SECURITY CLASS (This Report)
Unclassified
21. NO. OF PAGES
85
20. SECURITY CLASS (Thispage)
Unclassified
22. PRICE
EPA Form 2220-1 (9-73)
73
•e, U.S. GOVERNMENT PRINTING OFFICE: 1978— 757-140 /66 31
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