625377009C
Controlling Mlutfon
from the Manufacturing
and Coating
of Metal Roducts
Do not WEED. This document
should be retained in the EPA
Region 5 Library Collection.
EPA Technology Transfer Seminar Publication
Water Fbllution Control
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EPA-625/3-77-009
CONTROLLING POLLUTION
FROM THE MANUFACTURING
& COATING OF METAL PRODUCTS
Water Pollution Control
^eo S7;%
U.S. ENVIRONMENTAL PROTECTION AGENCY
Environmental Research Information Center • Technology Transfer
MAY 1977
U.S. Environmental Protection Afinqr
jwtaon Bpulcvard, 12th Ftof
lL 60604-3590 ^
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ACKNOWLEDGMENTS
This seminar publication contains materials prepared for the U.S. Envi-
ronmental Protection Agency Technology Transfer Program and has been pre-
sented at Technology Transfer design seminars throughout the United
States.
The technical information in this publication was prepared by Centec
Consultants of Reston, Virginia.
NOTICE
The mention of trade names or commercial products in this publication is
for illustration purposes, and does not constitute endorsement or recommenda-
tion for use by the U.S. Environmental Protection Agency.
- , , faction
U|S. Environments. —
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CONTENTS
Page
Chapter I — INTRODUCTION 1
Chapter II — REVIEW OF PL 92-500 2
Effluent Guidelines and New Source Performance Standards 2
The National Pollutant Discharge Elimination System 3
Self-Monitor ing 4
Pretreatment Requirements for Discharge into Municipal Systems .... 5
Cost-Recovery Charges 5
Toxic Substances 5
Ocean Disposal 6
Oil and Hazardous Materials Spills 6
Thermal Discharges 6
Water Quality Standards 6
Aids to Manufacturers 7
Research and Development 7
Steps for Compliance 7
General Wastewater Characteristics 8
Chapter III - IN-PLANT CONTROLS 20
Waste Survey 20
Flow Reduction 25
Flow Equalization 34
Water Reuse 38
iii
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Page
Recycling 38
Summary 42
Chapter IV — RELATIONSHIP WITH THE MUNICIPAL SYSTEM 43
Regulatory Requirements for Pretreatment 43
Industrial Cost Recovery 45
The Economics of Pretreatment 47
Chapter V - WASTEWATER TREATMENT 50
Primary Cleanup (Grit and Free-Oil Removal) 53
Wastewater Conditioning 57
Coagulation Equipment 81
The Sedimentation Process 83
Filtration , ^Q^
Further Solids Concentration 105
Chapter VI — RESIDUAL OIL AND GREASE 109
Oil Properties Affecting Removal 109
Mechanisms for Separation Ill
Separation Equipment 112
Equipment Costs -^20
Chapter VH - HANDLING WASTE-ACID STREAMS 124
Neutralization 124
Acid Recovery 129
Chapter VIE - INDUSTRIAL CASE HISTORY 134
REFERENCES 140
IV
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FIGURES
Figure Page
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
5-Day Waste Stream Variation
Short Circuiting
Counter Current Rinsing (Tanks)
Effect of Added Rinse Stages on Water Use
Counter Current Rinsing (Dip and Spray)
Two Spraying Methods
Flow Variations within a Plant
Flow Equalization
Examples of Recycle Systems
White Sidewall Grinding
Wastewater Components
Typical Wastewater Treatment System
Gravity Separator with Grit Removal
Floating Skimmer
API Separator
Common Skimming Devices
Solubility of Heavy Metals as a Function of pH
Jar Test Apparatus
Typical Settling Curves Obtained from Jar Tests
Zeta Meter Apparatus
Predicting Optimum Coagulant Dosage
.... 22
.... 28
. . . . 31
32
.... 34
.... 35
.... 36
.... 37
.... 39
.... 41
.... 50
.... 52
. . . . 54
55
. . . . 56
.... 58
. . . . 61
. . . 63
64
. . . . 66
.... 67
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Figure Page
22 Lime Slaking Equipment .................... 70
23 Lime Feed System ....................... 71
24 Liquid Handling System .................... 77
25 Typical Solids Feed Equipment ................. 78
26 Common Metering Pumps .................... 80
27 Typical Chemical Mixing Patterns ................ 82
28 Flocculation Equipment ..................... 84
29 Typical Wastewater Treatment System .............. 85
30 Sedimentation System Showing Various Zones of Settling ...... 87
31 Settling Velocities in an Ideal Sedimentation Basin ......... 88
32 Four Zones of a Sedimentation Basin ............... 90
33 Effective Settling Area for Circular and Rectangular Basins .... 91
34 Relation of Overflow Rates to Surface Area and Flow Rate
for Rectangular Basins ..................... 92
35 Center-Feed Circular Clarifier ................. 93
36 Circular Basin Flow Patterns .................. 94
37 Combination of Processes within a Clarifier ........... 95
38 Rectangular Clarifier Basin ................... 96
39 Clarifier with Enlarged Sludge Zone for Thickening ........ 97
40 Tube Settler .......................... 98
41 Plate Separator ........................ 99
42 Thickening Equipment on Plate Settlers ............. 100
43 Approximate Costs for Circular Basin Clarification Equipment . . . 102
44 Installed Costs for Tube or Plate Clarification Equipment ..... 103
VI
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Figure Page
45 Filters 104
46 Rotary Drum Vacuum Filter 106
47 Operating Zones of a Vacuum Filter 106
48 Centrifuge 108
49 Plate-Type Coalescing Oil-Water Separator 113
50 Packed Oil-Water Separators 114
51 Cartridge-Type Coalescing Oil-Water Separator 115
52 Carbon Adsorption 118
53 Ultrafiltration Process 119
54 Capital Cost vs. Flow Rate of Oil-Water Separators of
API Specifications 121
55 Capital Cost vs. Flow Rate for Various Oil-Water Separators . . . 122
56 Capital Costs for Ultrafiltration of Oil-Water Mixtures 123
57 pH Scale—Concentration of H+ at Various pH Values 125
58 Neutralization Systems 127
59 Diversion Pond to Control pH 128
60 Pickling System with Acid Recovery Unit 131
61 Atlantic Wire Water Flow System 135
62 Batch Sulfuric Acid Recovery Unit 136
63 Atlantic Wire Neutralization System 138
vn
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TABLES
Table Page
1-9
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
Raw Waste Characteristics (for various subcategories):
Casting and Molding
Mechanical Material Removal
Metal Forming (except plastics)
Physical Property Modifications
Assembly Operations
Chemical - Electro-Chemical Operations
Material Coating
Smelting and Refining
Molding and Forming (plastics)
Cost Comparisons for Treatment of Process and
Nonprocess Water
Economics of Staged Rinsing for One Set of Conditions
Summary of Recycle Study
Preliminary Analysis' of Pretreatment Economics
Polyelectrolytes
Cost Comparison of Common Commercial Alkaline Agents
Operating Cost Comparison of Pickle Liquor Waste
Treatment Methods
Discharge Composition
11
12
13
14
15
16
17
18
19
29
33
42
48
73
130
132
139
Vlll
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CHAPTER I
INTRODUCTION
This volume addresses, for the most part, managers, engineers, and other in-
dustry personnel responsible for resolving the water pollution problems of a manufac-
turing facility. At present, this means complying with the requirements of the Federal
Water Pollution Control Act Amendments of 1972 (PL 92-500). We assume here that,
although you want to meet the requirements of PL 92-500, your basic interest lies in
turning out a product at a profit. We also realize that you are not interested in starting
a new career in pollution control. Thus, the discussion will be limited to the practical
aspects of compliance, with emphasis on minimizing costs and avoiding disruption of
the manufacturing process.
A viable, cost-effective pollution control plan must be based on detailed familiarity
with both plant operations and pollution control technology. Obviously, you bring the
first of these requisites to your plant's pollution control effort. The object of this vol-
ume is to discuss the control techniques themselves—and the physical and chemical
principles underlying them—in enough detail that you can work knowledgeably with con-
sultants, suppliers, and other experts who may assist in planning for wastewater
compliance.
The volume has a general, five-part format: (1) a summary of current regulations
affecting the industry, discussion of those likely to be issued, and some pointers that
may enable your plant to avoid classification in the more restrictive categories; (2) a
presentation of techniques for resolving water pollution problems at the source, through
in~plant controls; (3) suggestions for establishing the most advantageous relationship
with the municipality; (4) a presentation of three methods for wastewater treatment;
and (5) a brief case history of one metal-products plant in achieving compliance and in
upgrading its controls to meet new requirements.
In summary, this volume, supplemented by references from the bibliography, will
serve as a practical guide to the regulatory requirements and the technology available
for meeting them. It has been prepared for the U.S. Environmental Protection Agency
Technology Transfer Seminar Series for manufacturers in the broad industrial category
of metal machining, fabricating, and coating. Readers are also referred to other pub-
lications in the Series, including specific volumes on emissions control for the metal
coating and metal cleaning industries and a volume on management perspectives.
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CHAPTER II
REVIEW OF PL 92-500
PL 92-500 is a complex law covering a wide range of regulations on water pollution
control. For the average manufacturer, however, the key points are those relating to
the discharge of industrial wastewater to waterways or public systems. A summary of
these points follow.
EFFLUENT GUIDELINES AND NEW SOURCE PERFORMANCE STANDARDS
For the manufacturing community, the most far-reaching feature of the law is the
requirement that all industries discharging waste into waterways must implement a
base level of pollution control by July 1, 1977. EPA was directed to establish the
amount of pollution reduction that could be obtained by application of the "best practi-
cable control technology currently available" (BPCTCA/BPT). This base level of ef-
fluent reduction is to be met by all point sources discharging into waterways.
The law also required that EPA establish the level of pollution reduction achievable
from a more advanced technology, called "best available technology economically
achievable" (BATEA/BAT). This will be required of all point sources by 1983. Fur-
ther, a new standard, also calling for advanced technology, is to be applied to any fa-
cility constructed after regulations for new sources are proposed. This will involve
"best available demonstrated control technology, processes, operating methods, and
other alternatives including, where practicable, a standard permitting no discharge of
pollutants" (New Source Requirements).
In effect, then, EPA is to set three levels of performance for each industrial
subcategory:*
• For existing plants to meet in 1977 ("BPT" or "BPCTCA")
• For existing plants to meet in 1983 ("BAT" or "BATEA")
• For new plants ("New Source Requirements")
Another significant point is that EPA is also to set pretreatment standards for plants
discharging to public systems.
*0ne of the controversial aspects of the law has been disagreement as to the degree of stringency indicated by var-
ious terms used to describe the expected level of pollution control. This publication does not address this aspect
of PL 92-500 and is strictly limited to providing a basic understanding of the law and the technology available for
pollution control.
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If the application of BPCTCA pollution control techniques by all sources discharg-
ing into a watercourse is inadequate to effect established water quality standards, both
EPA and the state involved are authorized to impose even more stringent pollution con-
trol requirements. In all cases, the states are authorized to impose requirements
more stringent than the federal guidelines. Plants located on "water quality limited
streams" may be required to install additional pollution control.
PL 92-500 directed EPA to set standards for manufacturers that were to be met
nationally, regardless of location. This was done to simplify government administra-
tion and to prevent the establishment of "pollution havens" in states anxious to attract
industry. In effect, all industries were told to apply a base level of pollution control
technology, regardless of the nature and use of a waterway.
THE NATIONAL POLLUTANT DISCHARGE ELIMINATION SYSTEM
The key to enforcing PL 92-500 is the National Pollutant Discharge Elimination
System (NPDES). This provision of the law establishes the activities of the EPA pro-
grams, which began with the 1899 Refuse Act. While the states retain primary respon-
sibility for combating water pollution, they must do so within a specific federal frame-
work. If the states cannot, or do not choose to, establish programs for meeting federal
requirements, the federal government assumes the responsibility.
The heart of the system is the NPDES permit. This is the mechanism by which the
federal government ensures that all of the requirements of PL 92-500 are being met.
Each point source discharging into a waterway must apply for a permit. Manufacturers
discharging to municipalities come under other provisions, but are also regulated.
Permit applications filed previously under the 1899 Refuse Act are considered valid
under PL 92-500.
Through either the state or the federal EPA offices (depending upon the status of
the state permit program), a manufacturer discharging into a waterway must obtain a
permit, which spells out the following conditions:
• The pollutants that may be legally discharged. (The average and maximum
daily amounts will be specified. These amounts may be derived from the efflu-
ent guidelines for the industry, from the requirements for toxic substances, or
from requirements derived from water quality standards.)
• A compliance schedule, with the specific steps the manufacturer must take to
achieve the best practicable control technology by July 1, 1977, in conformance
with the law.
• A listing of the monitoring and reporting requirements stipulated by both EPA
and the state.
• The period for which the permit is valid, not to exceed 5 years.
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Moreover, all permits are issued subject to the following conditions:
• The permit holder must report any new or increased discharges.
• Any discharge of a new pollutant, or excessive discharge of an authorized pol-
lutant, will be a violation of the permit.
• The permit may be modified, suspended, or revoked if its terms are violated
or if it was obtained by misrepresentation or failure to disclose all relevant
facts.
• The permit holder will allow EPA or state water pollution control officials to
enter and inspect the plant, the required records, and the monitoring equip-
ment. Samples of discharges can also be taken for EPA analysis.
• The permit holder will keep his pollution control systems in good working
order.
The federal government is given ample authority to ensure compliance with the
conditions of the permit. If violations of permits issued by the states are not followed
by appropriate enforcement action, the EPA can, after 30-days' notice, initiate its
own action. The law further states that any person who willfully or negligently vio-
lates any permit condition shall be punished by a fine of not more than $25,000, by
imprisonment for not more than 6 months, or both. In this case, the term "person"
means any responsible corporate officer.
SELF-MONITORING
In order to provide regulatory officials with information for establishing effluent
standards and for maintaining records on the amounts of pollutants being discharged,
a manufacturer may be required to monitor his own plant wastes in a manner to be
specified by EPA. The manufacturer must keep adequate records to be provided to
EPA on request or on an established schedule.
EPA also has the right, upon presenting proper credentials, to visit any manufac-
turing site and examine records, check sampling and monitoring equipment, and take
any samples required for checking the results submitted by the manufacturer. All this
information becomes public except where it is established that public access would re-
veal trade secrets or proprietary information. In this case, access will be limited to
regulatory officials.
This "self-monitoring" requirement plays a very important part in the implemen-
tation of controls on manufacturers. Penalties for refusing to comply, or for falsifying
information, are severe. EPA also can require a manufacturer to provide information
to assist it in developing guidelines. Requests from EPA for such information are
known as "308 letters," after Section 308 of the law, which authorizes them.
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Each state may develop its own procedures for inspection, monitoring, and entry.
After EPA approval, the state will be authorized to carry out its procedures in the
same manner as the EPA.
PRETREATMEIMT REQUIREMENTS FOR DISCHARGE INTO MUNICIPAL SYSTEMS
The law also firmly establishes federal control over the operation of municipal
treatment plants by requiring municipal systems to obtain discharge permits under
NPDES. The law specifically directs EPA to develop pretreatment standards for in-
dustrial discharges into publicly owned treatment works (POTWs). These standards
will prevent the introduction of pollutants determined to be nonsusceptible to treatment
in the municipal system or to hamper the operation of the municipal treatment works.
For new plants, the law also requires that EPA develop pretreatment standards simul-
taneously with effluent discharge standards. Compliance with these standards will be
required within 3 years after their promulgation.
COST-RECOVERY CHARGES
Under PL 92-500, the federal government pays 75 percent of the total construction
costs for municipal treatment systems. Since many states contribute an additional 15
percent of the cost of municipal systems, many cities will be required to contribute
only 10 percent of the cost under the new law. Before the federal share can be ap-
proved, however, the municipality must show that a plan has been developed to recover
a proportionate share of the total federal construction costs from any industrial users.
EPA is directed to develop guidelines enabling municipalities to arrive at cost alloca-
tion systems that meet requirements of the law. In the past, municipalities only re-
covered the local share of the cost from industry; now they must also recover the
federal costs. Thus, there will be a sharp rise in industrial charges for the use of
municipal systems.
TOXIC SUBSTANCES
PL 92-500 contains a special section for dealing with substances designated as
toxic. Effluent standards or total prohibitions of these pollutants are to be established
under criteria that take into account toxicity, persistence, degradability, likelihood of
affected organisms in the water, nature and extent of the substance on the organisms,
and the importance of affected organisms.
EPA was directed to prepare a list of substances to be designated as toxic and to
develop effluent standards for their control. This has proved to be a difficult task.
There is, however, a proposed list of toxics with allowable discharges.
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OCEAN DISPOSAL
The law directs EPA to establish guidelines for determining the degradation of the
oceans, territorial seas, and contiguous zones. Based on these guidelines, ocean dis-
charge permits will be issued.
OIL AND HAZARDOUS MATERIALS SPILLS
EPA is directed to designate as hazardous compounds, and to develop regulations
for, any substances which, when discharged in quantity to the navigable waters, will
"present an imminent and substantial danger to the public health and/or welfare in-
cluding, but not limited to, fish, shellfish, wildlife, shorelines, and beaches." EPA
also was to determine if such substances, once spilled, can actually be removed.
The proposed regulations for hazardous materials have only recently been issued.
For manufacturers who routinely handle or ship oil or hazardous materials, this pro-
vision of the law is extremely important.
THERMAL DISCHARGES
The law specifically addresses thermal discharges. States are required to provide
estimates of the maximum thermal load that could be tolerated by waters within their
jurisdictions without harming shellfish, fish, and wildlife.
Any manufacturing facility that can demonstrate to EPA that the prescribed use of
cooling towers or other technology to cool its discharges is in excess of that required
to protect fish, shellfish, and wildlife will not be required to employ that technology.
Where cooling and water intake structures are needed, the law specifies that they
shall be designed to minimize the environmental impact.
These provisions are important for large industries using large amounts of water,
such as the power industry. In general, medium and small manufacturers have not
been affected by the requirements for thermal pollution control.
WATER QUALITY STANDARDS
In addition to the level of pollution control that may be established as the result of
the "best practicable pollution control technology currently available," the law allows
even more severe restrictions when the use of this control technology by all dischargers
fails to raise the quality of the water to an acceptable standard. In these cases, the
permit will be based on a level of treatment that the regulatory agency determines is
required to meet the water quality criteria.
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PL 92-500 set up two outside organizations to provide an overview of EPA's activ-
ities. The first is the Effluent Standards Water Quality Information Advisory Commit-
tee (ESWQIAC), composed of a chairman and eight members who are appointed by the
Administrator of EPA. They are selected from the scientific community and are to be
"qualified by education, training and experience to provide, assess and evaluate scien-
tific and technical information on effluent standards and limitations."
The second outside organization is the National Commission on Water Quality, cre-
ated to conduct a "full and complete" investigation of all the technological, economic,
social, and environmental effects of achieving—or not achieving—the effluent discharge
goals for 1983 (Best Available Technology Economically Achievable).
This Commission consists of five members of the Senate Public Works Committee,
five members of the House Public Works Committee, and five from the public sector
appointed by the President. The heads of all departments and agencies of the executive
branch of the federal government were directed to cooperate with the Commission.
In brief, while the National Commission on Water Quality is to address itself to
long-term goals, ESWQIAC is responsible for an overview of the more immediate ef-
fluent guidelines requiring that the "best practicable" control technology be installed
by 1977; it advises the Administrator of EPA.
AIDS TO MANUFACTURERS
PL 92-500 amended the Small Business Act to allow the Small Business Adminis-
tration (SBA) to make loans assisting small business firms in meeting the requirements
of the law. The applicant must furnish the Small Business Administration with a written
statement from EPA that additions or alterations are necessary and will be adequate for
compliance. For this purpose, up to $800 million can be appropriated to SBA disaster
funds.
RESEARCH AND DEVELOPMENT
EPA was authorized to continue a research and development program, authorized
under previous legislation, for developing new techniques to control industrial wastes
and for studies to determine the actual effects of pollutants on the ecology of streams.
Grants of up to 70 percent of the cost can be given to industries or municipalities that
implement advanced concepts.
STEPS FOR COMPLIANCE
There are five basic rules for resolving the water pollution problems of a manu-
facturing facility. If they are kept in mind, the manager should be able to comply with
PL 92-500 in a manner that is responsible, yet cost effective:
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1. Make sure you understand exactly what is required. Although this may seem
basic, in actual practice it is often complex. Many plants have had unpleasant
surprises because of misunderstandings with the regulatory agency. The en-
vironmental manager or engineer must have a thorough knowledge of which
streams are to be monitored and of the required sampling and analytical pro-
cedures. As will be discussed later, the difference between "process" and
"nonprocess" wastes is often critical and can affect the entire program. It is
important to both EPA and the manufacturer that the technical basis for the
permit be mutually understood. If there are any misconceptions, it is far
cheaper to resolve them at this early stage.
2. Know the current situation at your plant. The discovery of a new waste source
or of unexpected wastewater variations after control equipment has been
ordered—or worse yet, installed—is embarrassing and expensive. Yet it hap-
pens frequently. To avoid it, make a firm commitment at the outset to thor-
oughly analyze the wastewater characteristics of your plant.
3. Formulate a balanced, realistic program of in-plant and end-of-pipe treatment
technology. There are many in-plant changes (to be discussed later) that can
reduce the pollutants requiring treatment and cut costs. End-of-pipe treat-
ment, on the other hand, requires less interference with the manufacturing
process. For a good pollution control program, evaluate realistically what
can be expected from in-plant programs and improved operator awareness,
and arrive at an economic balance between in-plant controls and discharge
treatment.
4. Keep the regulating agencies informed of your progress. State and federal
agencies want to see advances toward water pollution control. However, for
reasons beyond your control, you may not be able to meet all your deadlines.
Missing a date on a compliance schedule is less of a problem if the agencies
are kept aware of the reasons. Many of the confrontations between plants and
the regulatory agency are due to misunderstandings that can and should be
minimized.
5. Keep monitoring your pollution control systems. It is common for these sys-
tems to lose ground after initial success. Flows and pollutants change and the
operating conditions for which the equipment was designed are forgotten. This
is especially true for in-plant control techniques, where old habits may soon
creep back. The NPDES monthly report is a constant reminder not to let con-
ditions regress to the point where permit violations begin to occur. Unfortu-
nately, deterioration of a finely tuned pollution control program can often take
weeks or months to correct. To avoid this effort and expense, it is essential
that the entire program be maintained.
GENERAL WASTEWATER CHARACTERISTICS
The machinery and mechanical products industries make up the broadest category
of manufacturers for which EPA is developing effluent guidelines. In the EPA Develop-
ment Document,^ the diverse industries in this category are subclassified according to
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the major operations of metal machining, fabricating, and coating. Because of the
variety of products, raw materials, and plant sizes and locations, a single listing of
wastewater characteristics is impossible.
The EPA contractor who did a study of the industry to provide data for establishing
effluent guidelines divided it into 12 subcategories.2 Tables 1-9 at the end of this
chapter list the "typical" waste characteristics for the nine subcategories that are par-
ticularly relevant to metal machining, fabricating, and coating operations. The values
in these tables are important for the manufacturer as an indication of the difficulties he
may face in meeting any EPA limitation. The problems will be greater if the waste-
water flow is unusually high. Since most pollutant limitations are specified by both
EPA and the states in terms of total pounds (obtained by multiplying a model flow by a
concentration), the plants with large flows per unit of production will have to attain
lower concentrations to keep within the limits allowed.
Basically, the major pollutants targeted for control in 1977 in the Municipal Pro-
grams Category are suspended solids, oil and grease, heavy metals, cyanides, and
possibly some organic chemicals.* The technology for controlling these pollutants is
generally well-established, although obtaining the best performance under each plant's
specific conditions requires a separate evaluation.
There are several characteristics of plant-water discharges—not shown in these
tables—that are vital to the pollution control program and that should be understood by
anyone involved in a water pollution control effort. These are the distinction between
"process" and "nonprocess" wastewater and the plant's wastewater variations.
PROCESS AND NONPROCESS WASTEWATER
Process wastewater is water that comes in contact with pollutants as part of the
manufacturing process. Water used to rinse work being pickled in acid is an example.
This water is directly regulated by the effluent limitations.
Nonprocess wastewater is water that does not normally come in contact with pol-
lutants. Once-through cooling water used in heat exchangers is an example of non-
process water, which is not monitored or regulated with the same stringency as
process wastewater. For example, noncontact cooling water is usually only monitored
for temperature. There is, of course, an indeterminate area involving water that
sometimes becomes slightly contaminated.
The designation of water as process or nonprocess can have significant conse-
quences for the control program. The manager should strive to prevent the contami-
nation of any large stream that is basically clean so that it will have a nonprocess
classification. Means for accomplishing this will be discussed in the next chapter.
*Control of the pollutants introduced by metal finishing operations (such as cyanides and chromium) are covered in
a separate EPA Technology Transfer Publication for pollution control in metal finishing, which is not discussed here.
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The mixing of process and nonprocess waste streams is a major reason for the
wide range of waste flows shown in the tables for most categories.
SINGLE-PLANT WASTEWATER VARIATIONS
In addition to separation of process and nonprocess water, another factor of ex-
treme importance is variability of wastewater flows. This is the single most common
reason that pollution control systems fail to live up to expectations and performance.
Consideration of flow equalization methods to reduce the variability of the wastewater
must be made. The ranges shown in Tables 1-9 refer to variations between plants.
The manager or engineer must know how the waste characteristics vary within his own
plant. This will also be discussed in the next chapter.
10
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Table 1
Raw Waste Characteristics
Subcategory 1 — Casting & Molding
(variations among plants)
Parameter(a)
PH*
Turbidity (JTU)(b)
Temperature
Dissolved Oxygen
Sulfide
Cyanide
Total Solids
Total Suspended Solids*
Settleable Solids
Cadmium*
Chromium, total
Chromium, hexavalent
Copper*
Fluoride
Iron, total*
Iron, dissolved
Lead*
Oil, Grease*
COD*
Total Phosphates
Zinc*
Boron
Mercury
Nickel*
Silver*
Minimum
2.2
25
10°C
1.0
.010
.007
155
6
.20
.002
.005
.005
.014
.500
.200
.100
.010
1.75
54
16.7
.020
.100
.002
.004
.002
Maximum
10.0
1200
63°C
12.0
4.0
.007
28770
28390
40
.443
.049
.020
104.3
14
95
75.3
5.6
13510
20630
40000
146.4
21.3
.005
51.3
.21
Mean
—
576.4
23.2°C
5.9
.6
.007
3625 .4
11.5
.044
.025
.011
9.7
6
13.8
11.7
.61
1046.8
1746.7
2425 .7
10.85
2
.003
4.7
.019
Flow (for a production floor area of 200,000 ft2) — 317,000 GPD
* Under consideration for regulation by national effluent guidelines.
(a) All parameters measured in mg/liter except pH, turbidity, and temperature.
(b) Jackson Turbidity Units.
11
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Table 2
Raw Waste Characteristics
Subcategory 2 — Mechanical Material Removal
(variations among plants)
Parameter!0)
PH*
Turbidity (JTU)(b)
Temperature
Dissolved Oxygen
Sulfide
Cyanide
Total Solids
Total Suspended Solids*
Settleable Solids
Cadmium*
Chromium, total*
Chromium, hexavalent
Copper*
Fluoride*
Iron, total*
Iron, dissolved
Lead*
Oil, Grease*
COD*
Total Phosphates*
Zinc*
Boron
Mercury*
Nickel*
Silver
Minimum
2.1
.380
2°C
2.0
.01
.01
40.5
2.4
.200
.002
.005
.005
.016
.130
.103
.003
.006
.400
3.7
.200
.020
.030
.003
.004
.002
Maximum
12.3
3800
65°C
12
24
.425
28770
28390
40
61
400
.033
184.6
240
95
75.3
103
13510
40000
92
181
21.3
.003
93.5
.010
Mean
340
22.7°C
7.5
1.2
.072
2647
1223.6
5.4
2.4
19
.009
4.5
8.5
9
6.2
2.04
668.2
3087.4
10.2
7.2
2.3
.003
3.3
.004
Flow (for a production floor area of 200,000 ft2) - 45000 GPD
* Under consideration for regulation by national guidelines.
(a) All parameters measured in mg/liter except pH, turbidity, and temperature.
(b) Jackson Turbidity Units.
12
-------
Table 3
Raw Waste Characteristics
Subcategory 3 — Metal Forming (except plastics)
(variations among plants)
Parameter (a)
PH*
Turbidity (JTU)(b)
Temperature
Dissolved Oxygen
Sulfide
Cyanide
Total Solids
Total Suspended Solids*
Settleable Solids
Cadmium
Chromium, total
Chromium, hexavalent
Copper*
Fluoride
Iron, total*
Iron, dissolved
Lead*
Oil, Grease*
COD*
Total Phosphates*
Zinc*
Boron
Mercury
Nickel*
Silver*
Minimum
1.5
2.2
12°C
1.0
.010
.020
155
2.0
.20
.002
.005
.005
.016
.120
.110
.030
.010
.500
1.0
.20
.020
.030
.002
.004
.002
Maximum
12.0
3800
65° C
12.0
6.4
1.8
63090
11990
40
.43
.417
.030
145
1.8
600
250
103
8056
19170
45.4
146.4
16.3
.002
165.2
.044
Mean
—
472.4
24° C
5.6
.645
.523
5294
1034
8.5
.064
.085
.011
8.5
.73
29.4
14.3
2.7
600
2826.5
6.5
10.7
1.5
.002
5.8
.005
Flow (for a production floor area of 200,000 ft2) - 63,400 GPD
* Under consideration for regulation by national guidelines.
(a) All parameters measured in mg/liter except pH, turbidity, and temperature.
(b) Jackson Turbidity Units.
13
-------
Table 4
Raw Waste Characteristics
Subcategory 4 — Physical Property Modifications
(variations among plants)
Parameter!0)
pH*
Turbidity (JTU)(b)
Temperature
Dissolved Oxygen
Sulfide
Cyanide*
Total Solids
Total Suspended Solids*
Settleable Solids
Cadmium
Chromium, total
Chromium, hexavalent
Copper
Fluoride
Iron, total*
Iron, dissolved
Lead*
Oil, Grease*
COD*
Total Phosphates
Zinc
Boron
Mercury
Nickel*
Silver
Minimum
2.1
1.5
13°C
4.0
.010
.010
151
.400
.200
.003
.005
.005
.129
3.2
.077
.030
.012
.400
1.0
.600
.120
.030
.010
.004
.003
Maximum
12.0
3800
87°C
12.0
6.4
900
12280
11990
40
.012
.013
.005
.952
3.2
422.2
80
102.8
13510
18070
.600
1.7
16.3
.010
17.7
.003
Mean
—
349
24° C
6.9
.56
67.2
2016.1
715.8
6.3
.006
.008
.005
.434
3.2
14.5
5.9
2.7
681.4
2357
.600
1.2
1.5
.010
1.2
.003
Flow (for a production floor area of 200,000 ft2) - 190,000 GPD
* Under consideration for regulation by national guidelines.
(a) All parameters measured in mg/liter except pH, turbidity, and temperature.
(b) Jackson Turbidity Units.
14
-------
Table 5
Raw Waste Characteristics
Subcategory 5 — Assembly Operations
(variations among plants)
ParameterW
PH*
Turbidity (JTU)(b)
Temperature
Dissolved Oxygen
Sulfide
Cyanide
Total Solids
Total Suspended Solids*
Settleable Solids
Cadmium*
Chromium, total
Chromium, hexavalent
Copper*
Fluoride*
Iron, total*
Iron, dissolved
Lead*
Oil, Grease*
COD*
Total Phosphates*
Zinc*
Boron
Mercury*
Nickel*
Silver*
Minimum
1.8
2.2
2°C
1.0
.010
.048
165
6.3
.002
.002
.005
.005
.013
.110
.070
.030
.007
.400
11.6
.250
.020
.030
.002
.004
.002
Maximum
11.5
3800
63° C
12.0
4.8
.192
28770
28390
60.9
60.9
.026
.007
184.6
325
95.4
77.5
102.8
13510
40000
62.4
33.9
17
.055
93.5
.052
Mean
—
379
6.25
.345
.120
2578
1060.8
1.3
1.3
.015
.005
3.5
14.7
9.6
7.6
3.3
720
2435 .9
8
2.9
1.9
.012
2.3
.005
Flow (for a production floor area of 200,000 ft2) — 32,000 GPD
* Under consideration for regulation by national guidelines.
(a) All parameters measured in mg/liter except pH, turbidity, and temperature.
(b) Jackson Turbidity Units.
15
-------
Table 6
Raw Waste Characteristics
Subcategory 6 — Chemical - Electro-Chemical Operations
(variations among plants)
Parameter!0)
pH*
Turbidity (JTU)(b)
Temperature
Dissolved Oxygen
Sulfide
Cyanide
Total Solids
Total Suspended Solids*
Settleable Solids
Cadmium
Chromium, total*
Chromium, hexavalent*
Copper*
Fluoride*
Iron, total*
Iron, dissolved
Lead
Oil, Grease*
COD*
Total Phosphates
Zinc
Boron
Mercury
Nickel
Silver
Minimum
.9
1.2
10°C
4.0
.100
.019
151
1.6
.150
.002
.005
.005
.017
.150
.023
.138
.018
.40
1.0
.200
.153
.120
.008
.006
.002
Maximum
7.4
500
65° C
12.0
1.2
.20
54210
16560
18
.029
119.1
89.6
155.4
7.4
600
26.6
2.0
1730
6040
17.5
164.3
1.8
.008
84.5
.010
Mean
112
21°C
7.5
.414
.105
5531 .8
837.2
7.8
.011
11 .5
3.8
22.9
1.6
30.4
4.7
.241
97
419
6.2
29.8
.416
.008
7.2
.005
Flow (for a production floor area of 43,000 ft2) — 285,000 GPD
* Under consideration for regulation by national guidelines.
(a) All parameters measured in mg/liter except pH, turbidity, and temperature.
(b) Jackson Turbidity Units.
16
-------
Table 7
Raw Waste Characteristics
Subcategory 7 — Material Coating
(variations among plants)
Parameter!0/
pH*
Turbidity (JTU)(b)
Temperature
Dissolved Oxygen
Sulfide
Cyanide
Total Solids
Total Suspended Solids*
Settleable Solids
Cadmium*
Chromium, total*
Chromium, hexavalent*
Copper*
Fluoride*
Iron, total*
Iron, dissolved
Lead*
Oil, Grease*
COD*
Total Phosphates*
Zinc*
Boron
Mercury*
Nickel
Silver*
Minimum
1.5
.300
9°C
1.0
.010
.010
35
.200
.200
.002
.005
.005
.011
.130
.103
.003
.006
.500
3.7
.200
.020
.050
.002
.007
.002
Maximum
11.3
3800
63° C
12.0
24
1.6
63090
28390
40
60.9
400
36.4
1060
110
422.2
367.7
102.8
13510
40000
62.4
86.5
21 .3
.055
.950
.100
Mean
395.6
23.3°C
7.0
1.3
1 .03
2917.9
917.8
10.5
2.1
20.1
1.5
21 .1
6.8
21.6
17.9
1.7
545.2
1837
9.5
4.6
2.5
.012
.207
.007
Flow (for a production floor area of 107,000 ft2) — 108,000 GPD
* Under consideration for regulation by national guidelines.
(a) All parameters measured in mg/liter except pH, turbidity, and temperature.
(b) Jackson Turbidity Units.
17
-------
Table 8
Raw Waste Characteristics
Subcategory 8 — Smelting and Refining
(variations among plants)
Parameter'0)
PH*
Turbidity (JTU)(b)
Temperature
Dissolved Oxygen
Sulfide
Cyanide
Total Solids
Total Suspended Solids*
Settleable Solids
Cadmium*
Chromium, total
Chromium, hexavalent
Copper*
Fluoride
Iron, total*
Iron, dissolved
Lead*
Oil, Grease*
COD*
Total Phosphates
Zinc*
Boron
Mercury*
Nickel*
Silver*
Minimum
.39
14
11°C
10.0
.010
.010
958
9.3
.30
.008
.009
.005
.015
.200
.109
.100
.020
2.8
1 .00
1.3
.060
.100
.003
.028
.003
Maximum
11.3
1200
63° C
10.0
.70
.70
32670
15110
40
5.8
.955
.120
38.4
1.3
592.6
119.7
22.8
1335
13510
13.7
112.9
2.1
.089
94.5
.210
Mean
258.6
24.1°C
10.0
.176
.197
7297.6
2088.1
21.1
.8
.205
.020
7.1
.670
96.4
36.3
4.6
166.1
1650
4.9
23.4
.831
.032
16.5
.049
Flow (for a production floor area of 200,000 ft2) - 172,000 GPD
* Under consideration for regulation by national guidelines.
(a) All parameters measured in mg/liter except pH, turbidity, and temperature.
(b) Jackson Turbidity Units.
18
-------
Table 9
Raw Waste Characteristics
Subcategory 9 — Molding and Forming (plastics)
(variations among plants)
Parameter'0/
PH
Turbidity (JTU)(b)
Temperature
Dissolved Oxygen
Sulfide
Cyanide
Total Solids
Total Suspended Solids
SefHeable Solids
Cadmium
Chromium, total
Chromium, hexavalent
Copper
Fluoride
Iron, total
Iron, dissolved
Lead
Oil, Grease
COD
Total Phosphates
Zinc
Boron
Mercury
Nickel
Silver
Minimum
^^
25°C
7.0
.100
.100
359.7
4.6
—
.002
.007
.005
.016
—
.035
_
.020
3.2
24.0
7.0
.237
—
—
—
—
Maximum
7.1
25°C
7.0
.100
.100
359.7
4.6
—
.002
.007
.005
.016
—
.035
—
.020
3.2
24.0
7.0
.237
—
—
—
—
Mean
.^
25°C
7.0
.100
.100
359.7
4.6
—
.002
.007
.005
.016
—
.035
—
.020
3.2
24.0
7.0
.237
—
—
—
—
Flow(c)
* Under consideration for regulation by national guidelines. (Not applicable)
(a) All parameters measured in mg/liter except pH, turbidity, and temperature.
(b) Jackson Turbidity Units.
(c) No process wastewater.
19
-------
CHAPTER III
IN-PLANT CONTROLS
A common way of reducing the cost of water pollution control is the use of in-plant
practices to minimize the pollutants to be treated. There is no doubt that for some
plants this is an effective way to keep down control costs and to meet limitations that
would otherwise be impossible. It is usually an economic necessity to consider in-
plant control techniques before installing a treatment system.
In most plants, however, a point will eventually be reached where the cost of fur-
ther flow and pollutant reduction by in-plant control becomes too high. Treatment then
becomes the only remaining option. This point differs from plant to plant, and each
manufacturer must reach a balanced approach for his own facility.
This chapter offers plant managers a five-step approach to determining the eco-
nomic limits of in-plant controls: waste survey; flow reduction; flow equalization;
water reuse; and recycling.
WASTE SURVEY
The first step in any water pollution control project at a manufacturing facility is
the wastewater survey. The goal is to understand the waste flows within the plant so
that a minimum-cost pollution control program can be devised. The scope of the sur-
vey can range from simple monitoring of the outfall for a few days to a detailed analy-
sis of all internal waste flows. In most cases, funds for studying internal waste flows
are repaid by savings on the treatment system that result from a better understanding
of the sources of pollution.
An excellent starting point for a manager considering either an in-house waste
survey or hiring a consultant is the "Handbook for Monitoring Industrial Waste Water,"
distributed by the U.S. EPA Technology Transfer Program.4 It contains a good gen-
eral discussion, in simple language, of the factors to be considered in a waste survey.
The more complete the survey, the more it will cost. Getting all the information
you would like to have is almost always too expensive. The manager, therefore, must
use judgment in balancing the savings gained from a better understanding of his plant's
waste against the cost of the survey.
A general rule will be helpful: It is better to have a few accurate numbers than to
have a large volume of uncertain data. Since wastewater flows and characteristics
vary considerably over a day (and between days), the initial results of the survey can
be confusing. It is important that the sampling and analytical procedures do not add to
this confusion.
20
-------
A waste survey consists essentially of
• Locating and measuring flows;
• Obtaining samples of these flows;
• Protecting the samples until they can be analyzed;
• Analyzing the samples; and
• Analyzing the data obtained.*
Finally, there are certain essential points that the manager should know about sam-
pling, flow measurement, and data analysis, whether the survey is being done in-house
or by an outside consultant.
SAMPLING
If a representative specimen is to be obtained, sampling of wastewaters requires
much greater precision than the production supervisor normally strives for in his
quality-control work. Some of the parameters set for the metals products industry
are especially troublesome if not closely watched. A basic consideration is the use of
grab samples vs. composite samples.
Grab Samples
A grab sample is very easy to obtain. It is an instantaneous sample of the waste-
water and should represent its condition at that exact time. The problems with grab
samples occur later, in analyzing their significance. Figure 1 shows the variations of
a waste stream over a 5-day period; it is obvious that a grab sample can be far re-
moved from the average or even typical value of a parameter. It is essential, if you
are using grab samples, to take enough of them to establish the changes occurring over
time.
Composite Samples
A composite sample represents an attempt to measure the true average value of a
waste stream over a specific period. Twenty-four-hour composites are common in
wastewater control, and many permits will be written requiring this kind of sampling.
In these samples, we try to obtain the value we would get if all of the wastewater were
collected over a 24-hour period, mixed thoroughly, and then sampled. There are a
variety of samplers now on the market that attempt to approximate this value by vari-
ous abbreviated methods. To reach a composite average, the effect of each sample
added to the composite must be weighted according to the flow and composition at the
time of sampling. This is called "flow proportioning" and is handled differently by
different samplers.**
*References 3, 4, and 9 discuss these steps in detail.
**References 3 and 4 are recommended for excellent discussions of problems and factors involved in sampling.
21
-------
075)
400 |—
300
- 200
to
100
Daily noon
"grab" samples
miss major variations
4 8 12 4 8 12 4
12 4
12 4 8 12 4 8 12 4 8 12 4 8 12 4 8 12 4 8 12
Day 1
Day 2
Day 3
Day 4
Day 5
Figure 1. 5-Day Waste Stream Variation (4-hour samples)
-------
The 24-hour composite, being an average value, does not give the actual situation
to be faced by the waste treatment system. Reliance on 24-hour composites alone can
result in unrealistic and overly optimistic treatment designs. For a comprehensive
understanding of your waste stream, a combination of grab and composite samples will
be required. A series of grab samples taken over one day shows the hourly variation.
The daily composites give the average values and show what probably will be reported
to the regulatory agencies.
Plants in the machinery and mechanical products category are being controlled on
oil and grease, suspended solids, pH value, and certain metals.
Oil and grease are among the most difficult of all pollutants to sample accurately.
There are several reasons for this. First, oil-containing samples have a tendency to
deposit oil on the line leading to the sampler, giving low values. If a sample for oil
and grease is taken as a composite, the large container used for the 24-hour composite
can collect a significant portion of the oil on its walls. When a small portion of this
"composite" sample is removed for analysis, the oil remaining on the walls is not
counted and a false low value is obtained. For this reason, the EPA Methods Manual^
recommends that only grab samples be used for oil and grease and that the entire sam-
ple should be analyzed. For a true analysis of the waste, many grab samples must be
taken.
A more serious problem, causing even greater inaccuracies in oil and grease
sampling, is the tendency of oil to form a film on the top of the wastewater stream. A
sample taken below this oil film may show false low values of oil, while one from the
top of the flowing stream will show overly high values. For an accurate value, the
sample must be taken in an area of complete mixing, commonly the turbulent overflow
point from weirs.
The situation is further complicated by the tendency of oil from many processes
to enter the wastewater system in slugs. Thus, it is entirely possible to take a good
sample at one point and to find different values an hour later. In addition, oil accumu-
lating on pipes or on floors may be washed down by a stream of water in one large
mass. In sum, the only way to obtain accurate analyses of the oil characteristics of
a wastewater stream is by taking a fairly large number of grab samples at one well-
mixed point.
Suspended solids can also present sampling problems, although these are not nor-
mally as severe as those of oil and grease. Basically, this is due to the difficulty of
reaching a low suspended-solids level in the stabilized particles (to be discussed more
fully in Chapter V, Wastewater Treatment). These stabilized particles tend to remain
in suspension in the waste lines. If a sample is not taken at a well-mixed point, grossly
inaccurate values may be obtained and may lead to undersized equipment.
Another problem with suspended-solids analyses is the possibility of destabilization
in the sample bottle after long settling times. This problem, however, relates more to
getting a representative sample for a settling test than to obtaining analytical results.
The sampling methods discussed in Reference 4 should normally assure reasonable
values of suspended solids.
23
-------
The pH value is another parameter that is easy to analyze but that may show great
variation over an operating day. Since the pH can affect the nature of both suspended
solids and oil and grease, it is important that this value be recorded for all samples
taken.
PROTECTION OF SAMPLES
The EPA Methods Manual outlines the protective measures that must be applied
to wastewater samples if proper analytical results are to be obtained. The basic rule
is that the sample should be analyzed as soon as possible. When this is not feasible,
certain preservation methods must be applied. Samples to be analyzed for metals,
for example, should be acidified. Make sure that your contractor or your laboratory
staff follow these methods.
ANALYTICAL TECHNIQUES
Analytical techniques for wastewater are well-developed. The EPA Methods Man-
ual is the bible for these techniques. If your samples are being run in an independent
laboratory, verify that it employs EPA methods; if not, find out why not, and the
possible adverse effects. Although the values reported on your NPDES permit must
stringently follow the EPA methods, samples taken in-plant for planning your treat-
ment system are not subject to this restriction. In some cases, your laboratory may
be able to recommend cheaper analytical techniques that will provide the information
you need. Such substitutions, however, should be approached with great caution.
Oil and grease levels reported from the standard analytical methods require some
understanding. The materials reported as oil and grease are really materials extracted
from the water with freon and remaining as residue after the freon is distilled off. This
figure, then, is not specific to the "oil" you might see floating on the wastewater and
can include some soluble oils as well. Since hexane was previously used as the ex-
tracting chemical, these materials are sometimes still called "hexane-extractable,"
but now more commonly "freon-extractable."
EPA now allows a different (and more costly) method that uses both freon extrac-
tion and infrared spectrophotometry. It is more specific to mineral oils, the major
pollutant being regulated under oil and grease. If there is another component or soluble
organic material making it impossible to meet your oil and grease requirements, this
alternate analytical method should be considered.
FLOW MEASUREMENT
The techniques for measuring flow in a manufacturing facility are explained in the
EPA Monitoring Manual. ^ Flow rates are quite often difficult to obtain, and accurate
determinations may tax the ingenuity of the engineer. This is particularly true if
sewers are intermingled and the plant sewer maps are not up-to-date. Accurate
measurement of flow rates is essential for determining the sources of waste and ap-
plying effective in-plant controls.
24
-------
ANALYSIS OF WASTE SURVEY RESULTS
At the completion of a waste survey, you should know the conditions at your plant
outfall; this means that you should be aware of its variability. The NPDES permit will
be based on a daily maximum value and a 30-day average value. The day-to-day vari-
ations of the plant's wastes may be sufficient to place it in violation of the 24-hour max-
imum, even though the 30-day average is within the allowance. This can only be ascer-
tained by a statistical analysis of conditions at the outfall. The monitoring handbook'*
describes a simple statistical analysis technique for considering this variability as you
estimate your outfall conditions.
The outfall conditions must also be known, so that the proper waste treatment sys-
tem can be designed. Information on in-plant conditions will be helpful in minimizing
the cost of this system. It is usually economical to perform in-plant surveys in those
areas where major wasteloads seem to be occurring. If analysis shows that the sus-
pected processes do account for the bulk of the wasteload at the outfall, then confidence
can be placed in the overall accuracy of the waste survey.
The results of the waste survey will also be used in an economic evaluation of all
the in-plant controls, to be discussed later.
FLOW REDUCTION
After the waste survey, you should be able to minimize your pollution control
costs. One of the simplest and most effective methods for reducing the cost of a water
pollution control system is to cut down the amount of wastewater to be treated. Re-
ducing the wastewater flow enables use of smaller, less-expensive equipment and
usually reduces the chemicals required for treatment.
If you hire a consultant to design a water pollution control system, flow reduction
should be one of the first considerations. If you can reduce flows below 50, 000 gallons
per day, you may put your plant into a new category, less stringently regulated. If you
are discharging to a municipal system, you may be able to move out of the "significant
contributor" category. The beneficial effects that flow reduction can have on pollution
control costs, for plants discharging both to streams and municipal systems, include:
• Minimizing the size of pollution control equipment;
• Maximizing the allowable level of concentration;
• Enabling use of less sophisticated equipment;
• Reducing chemical, pumping, mixing, and maintenance costs;
• Reducing water costs;
• Reducing the plant's share of municipal treatment system costs; and
• Possible reclassification out of the major contributory industry category for
plants discharging to municipal systems.
25
-------
There are several approaches to flow reduction that have been generally success-
ful. Not all of them apply to every plant. Putting a good flow reduction program into
practice requires the help of someone with a detailed knowledge of the manufacturing
process. In this discussion, we will proceed from less- to more-expensive techniques.
Many of these practices you may already have applied, and some may be impractical
for your plant.
HOUSEKEEPING
Eliminating Excess Water Use
The first task in reducing wastewater flow does not require a large capital outlay,
but a critical examination of the production area to eliminate unnecessary use of water.
You can usually achieve significant reductions just by ensuring that water is used only
when needed. This program will involve some operator education, but even more im-
portant will be small modifications to equipment to make it easier for the operator to
save water. For example:
• Every hose in the plant should have a spring-loaded cutoff valve that shuts off
water when released. Open hoses without nozzles can be a major source of
water waste. One unattended half-inch hose, for example, can use 23,000 gal/
day. Three of these hoses can use up the total allowance of 50,000 gal/day and
change a user from a minor to a major contributor to a municipal system. (The
implications of moving into this major category will be discussed in Chapter IV.)
• High-pressure spray nozzles should be used instead of direct water streams for
washing. A survey in one poultry plant showed that total wastewater could be
reduced 60 percent just by screwing in new spray nozzles.
• Leaky shut-off valves should be repaired.
• Wherever possible, dry cleanup should be used instead of routine flooding with
water. Water washdown is certainly a necessity in most manufacturing facili-
ties, but water is often used as a substitute for a broom. Under current water
pollution control regulations, you will find a water stream a very expensive
broom.
• Water flow to each piece of equipment should be cut to the minimum compatible
with good operations, and steps should be taken to ensure that only this amount
of water is used.
In any washing or rinsing operation, the natural tendency is to increase the flow of
water to meet the most extreme requirements—and then to leave it at that level. Al-
though it certainly isn't advisable to reduce water flow to the point where product qual-
ity is marginal or a full-time operator is needed just to maintain minimum flows, there
is usually a point at which production can continue efficiently with reduced water usage.
Determining this point requires that your water valve be somewhat adjustable. You
simply reduce water flow over several days until there is some indication of inadequate
flow and then increase the flow 10-15 percent to arrive at the minimum level for
26
-------
efficient operation. You then reset the valve at this point whenever you process the
same type of work. Two points of caution: First, this method of setting valve position
is good only if the line water pressure is constant. Changes in pressure will affect
the flow at the same valve position; it may be necessary to install a water pressure
regulator. Second, if gate valves are used, they probably will not provide the throttling
accuracy required, and it may be necessary to install globe or throttling ball-valves.
Reducing Short Circuiting
After you have cut the flow to rinsing or washing equipment to the minimum, there
will probably be room for further improvement because of "short circuiting." This
effect is depicted for a rinse tank in Figure 2, where only a small portion of the water
used actually passes over the product. There are methods for scientifically measuring
the effective flow pattern in a tank by using tracers. However, you can detect short
circuiting simply by adding a pulse of dye or other observable material into your inlet
water stream and watching the patterns form. In many cases, you will see that a large
portion of the water flow is missing the workpiece and is thus used unnecessarily.
There are several ways that short circuiting can be minimized. Sometimes a relocation
of the inlet and outlet nozzles is sufficient. In other cases, baffles are needed. Figure
2 shows various types of short circuiting and their corrections. In most cases, the
method of correction becomes obvious once the flow patterns are observed.
Once these "housekeeping" tasks have been accomplished (and they can be done by
your own staff), you are approaching the point where more detailed attention from op-
erators is required. For the next round of water flow reductions you will need the re-
sults of at least a preliminary waste survey.
SEPARATING IMONPROCESS WATER FROM PROCESS WATER
In many manufacturing plants, a large portion of the wastewater is actually cooling
water that is not—or is only slightly—contaminated. This cooling water stream is usu-
ally very large. If the cooling water does not come into contact with any pollutants,
EPA classifies it as "noncontact" water. It isn't part of your process wastewater load
and you do not have to monitor and treat it with the same stringency as contact water.
In many plants, however, much of this cooling water finds its way into the final
process waste stream, either through leaks that allow cooling water to run across
areas where it picks up oil and dirt or through oil leaks that end up in cooling water
sumps. Often, merely plugging oil leaks and/or cooling water leaks will enable a large
portion of contaminated water to be reclassified as noncontact water.
If this separation and protection can be accomplished without great expense, real
savings in water pollution control costs can result. Table 10 shows two cost estimates
from four plants. These were large plants, and costs for treating the total water flow
were high. Keeping oil out of the cooling water, which enabled separation of the waste
stream into nonprocess and process water, substantially reduced each plant's invest-
ment. Control of oil leaks to prevent cross-contamination made reclassification of
many sewers possible. This greatly reduced the water to be treated and enabled these
plants to meet the guidelines on their NPDES permits.
27
-------
Water
Water.
Workplace
Poor Distribution
Portions of water
bypassing work
Workplace
Poor Distribution:
portions of
water not
contacting work
K>
oo
Water -
Water
Workplace
Water
distributed
along work
Workplace
Baffles direct water to work
Figure 2. Short Circuiting and Corrective Measures
-------
Table 10
Cost Comparisons for Treatment of Process and Nonprocess Water
Plant
Case
Waste
collection
investment
Waste
treatment
investment
Total
capital
costs
1
A*
$1,147,800
235,000
$1,382,800
Bt
$454,000
220,000
$674,000
2
A
$ 827,000
289,000
$1,116,000
B
$320,000
275,000
$595,000
3
A
In excess
of
$5,000,000
B
Approx.
$700,000
4
A
$ 994,200
200,000
$1,194,200
B
$298,100
138,000
$436,100
to
CD
*A = All flows designated as "process" water.
fB = Cleanup and designation as "nonprocess" for certain flows.
-------
Causes of contamination of noncontact cooling water include:
• Oil leaks into cooling water sumps;
• Direct contact condensers;
• Cooling water leaks into process water;
• Excessive seal flushes; and
• Mixing of process waste with cooling water because of inadequate sev/er lines.
If you do have a significant portion of once-through cooling water intermingling
with your waste stream, it will be advantageous to find ways to prevent this cross-
contamination.
THE COUNTER CURRENT CONCEPT
After the basic housekeeping and process and nonprocess waste-separation tasks
have been surveyed, a fairly large amount of water may remain that is integral to your
process and cannot, by any stretch of imagination, be changed to nonprocess water.
The next step to further reduce your wastewater flow requires the understanding of a
very important concept—counter current operation, especially applicable to plants with
extensive rinsing operations.
The basic concept of counter current operation is to use the water from previous
rinsings to contact the more-contaminated work article. The fresh water enters the
process at the final rinse stage and then moves counter to the work flow to serve as
rinse water in the preceding stages. Normally, the closer you can approach this con-
cept in practice, the less water you will use.
Figure 3 illustrates three rinsing operations, each designed to remove the residual
acid in the water on the surface of the workpiece to .0001 percent. In illustration 3a,
the piece is dipped into one tank with continuously flowing water. In this case, we are
essentially diluting the acid on the surface of the workpiece to the required level.
Assuming complete mixing, the amount of water theoretically required to remove
10 Ib acid/100 ft2 surface area with a film drag of 1/16 Ib/inch is about 1,200 gallons
of water.
In illustration 3b, we have taken the first step towards counter current operation
with the addition of a second tank. The workpiece is now moving in a direction opposite
to the rinse water. The piece is rinsed with fresh makeup water prior to moving down
the assembly line. However, the fresh water from this final rinse tank is directed to
a second tank, where it meets the incoming, more-contaminated workpiece. Fresh
makeup water is used to give a final rinse to the article before it moves out of the
rinsing section, but the slightly contaminated water is reused to clean the article just
coming into the rinsing section. In this case (making the assumption that the tanks
are well-mixed and that there is a limit of . 1 percent acid in the first-stage rinse),
the water required is only 120 gallons, a 90-percent saving. If we increase the number
of stages, as shown in illustration 3c, further water reduction can be achieved.
30
-------
SINGLE RINSE
OUTGOING WATER
ttw
WORK MOVEMENT
INCOMING WATER
DOUBLE COUNTERFLOW
RINSE
OUTGOING WATER
WORK
--^MOVEMENT
INCOMING WATER
TRIPLE COUNTERFLOW
RINSE
|pz
rl
OUTGOIN
] j~j i
rj — ~— ~-f-/ i t
t_*__J j *_____ j
O K
3 WATER
WORK MOVEMENT
J
" ] [ '
^P^T"
*-*— j
tu^t
^ INCOMING
^_ WATER
Figure 3. Counter Current Rinsing (Tanks)
31
-------
Theoretically, the amount of water required is the amount of acid being removed by
single-stage requirements divided by the highest tolerable concentration in the outgoing
rinsewater. This theoretical reduction of water by a counter current multistage oper-
ation is shown in the curve graph in Figure 4. The actual flow reduction obtained is a
function of the drag-out and the type of contact occurring in the tanks. Large amounts
of short circuiting, for example, completely invalidate these projected reductions. If
you have reasonably good contact, however, major reductions in water use are possible.
An important point, illustrated for the single case in Figure 4, is that the largest
reductions are made by adding the first few stages. This is the familiar law of dimin-
ishing returns, which governs most in-plant reductions. The additional rinsing stages,
of course, cost additional money. The actual number of stages you may wish to add
depends on the specific layout and operating conditions at your plant. Table 11 presents
a sample analysis for one plant that indicates three stages as the most economical
1000 I—
750
500
250
Rinse Stages
Figure 4. Effect of Added Rinse Stages on Water Use
32
-------
Table 11
Economics of Staged Rinsing
for One Set of Conditions
Number
of Stages
1
2
3
4
Water
Flow (GPD)
125,000
12,000
1,500
250
Total Water
Costs (a) ($/yr)
$35,000
$ 3,400
$ 420
$ 70
Incremental
Capital
—
$25,000(b)
$35,000
$45,000
Payoff on
Initial
Investment
—
1 year
3 years
29 years
(a) Total watci costs (purchase plus treatment) at $.85/1,000 gallons.
(b) Initial investment allows for modifications to hoisting system. Additional labor not included.
choice. With higher costs for water and waste treatment, more stages might be eco-
nomical. With very low water costs, fewer stages would be economical. The space
available for additional tanks is also important. Many other factors will affect the
economics of this method; an evaluation must be done for each individual plant.
In addition to direct staging, many other configurations can be used for the counter
current system. One of the more common is the dip and spray technique, shown in
Figure 5. In this method, the work piece is dipped into a rinse tank to remove the
bulk of the contaminants and then sprayed with clean water as it is removed. The
water dripping from the piece into the tank provides the dilution necessary to control
the amount of contaminants reaching the fresh water spray.
If you are only spray rinsing, it is possible to arrange your spraying sequence so
as to approach a counter current process. The goal is a situation in which the partially
contaminated water flows over the the workpiece to remove the bulk of the contamina-
tion before the piece is sprayed with fresh rinse water. Figure 6 shows two methods
of spraying an article, one of which utilizes the counter current concept and one which
does not. If your operator employs the "random flood" method of rinsing down with a
hose (Figure 6-a), he is approaching the single-stage rinse system. If, on the other
hand, the piece is held so the water draining down cascades over the lower portions of
the workpiece (Figure 6-b), you are, in effect, approaching the counter current system.
The advisability of multistage counter current rinsing may finally depend upon the
layout of your plant. If the flow from stage to stage can be effected by gravity, the
counter current system is usually quite economic. If, on the other hand, you are
forced to use pumps and level controls, then another method, such as spray rinsing,
may be more feasible.
33
-------
Overflow
Fresh water
spray
Work is first dipped
in rinse water, then
held in spray of fresh
water for final cleaning
Figure 5. Counter Current Rinsing (Dip and Spray)
Another factor to consider before installing multistage rinse systems is the need
for agitation. In cases where water is cascading in enormous quantities over a work-
piece, the high flow usually provides enough agitation. As you apply more staging to
reduce the amount of water, you will reach the point where the flow of the water itself
is not sufficient to provide agitation. This necessitates either careful baffling of the
tanks or additional mechanical agitation.
FLOW EQUALIZATION
Another valuable method for keeping down costs in a wastewater treatment system
is flow equalization. Even after the maximum in-plant controls have been implemented,
most plants have a wide variation in the flow of wastewater. Figure 7 shows how flows
varied for one plant. Flow variations will be extremely high if your manufacturing
process includes many batch-type operations. When one tank is dumped, a large flow
will occur for a few minutes. Continuous processes have fewer variations; yet even
here flow variation is a significant factor in the design of pollution control equipment.
34
-------
CO
en
Rinse
water
Workpiece
\
Rinse
water
Rinse
water
Rinse
water
-o—
Workpiece
. Rinse
water
a
Random Flooding
Systematic Spraying
Figure 6. Two Spraying Methods
-------
100
75
50
25
1 1 1
6 12 18
DAY 1
1 1 1
6 12 18
DAY 2
1 1 1
6 12 18
DAYS
1 1 1
6 12 18
DAY 4
Figure 7. Flow Variations within a Plant
-------
Pollution control equipment would be designed for the maximum rate. In the case
of the plant in Figure 7, the equipment would be sized to handle the flow of 80,000 gal/
day even though the average flow is only about 50,000 gal/day. The difference between
designing for average flow and peak flow would normally be about 30 percent of the total
system cost. The reductions that can be achieved in the size and costs of equipment by
designing closer to average flow are what make flow equalization well worth considering.
The flow equalization will also bring about at least some equalization of wastewater
characteristics and composition. Although continuous systems that supply complete
equalization of wastewater characteristics (as opposed to flow) are not practical, the
dampening effect upon discharges might make the difference between success and fail-
ure in operation of the treatment system.
Flow equalization is accomplished in several ways. A certain amount of equaliza-
tion and dampening occurs in the sewer lines. The pollution control equipment, having
a given surge capacity, will also dampen some of these variations.
Both suspended-solids removal equipment and oil-water separators and filters are
designed on the basis of flow rates plus pollutant concentrations. A surge of flow can
cause a clarifier to lose its sludge blanket, resulting in a catastrophic amount of sus-
pended solids in the overflow. Similarly, a surge can overload the oil-water separators
and cause oil spills. Figure 8 shows one type of flow equalization system. Here the
tank has a level that is allowed to rise during the day when the flow rates are normally
higher. Meanwhile, the pump maintains a fixed flow to the waste treatment system.
The flow equalization in this system is very good. With wastewater pollutants, how-
ever, large upsets may still occur if the concentration entering the flow equalization
system is very high.
Underestimating the variability of the waste is the single most important reason that
pollution control systems fail to meet expectations. The designer must know the peak
values and the rates of change of the flows to which his pollution control equipment will
be subject. Flow equalization will minimize the risk of overloading the system.
Total
plant
Mixing
discharges
Equalization
tank
To waste
treatment
Fixed
flow
pump
Figure 8. Flow Equalization
37
-------
WATER REUSE
In discussing counter current operations, we illustrated the benefits of reusing
the fresh water from final cleaning of a workpiece to clean dirtier pieces. Instead of
looking at each rinsing process by itself, the entire manufacturing sequence should be
examined to decide if water from one application is suitable for use in other areas re-
quiring less purity. This is the basic concept of water reuse.
To assess the amount of water that can be practicably reused in your plant, you
must know the water quality requirements for each water-use point. This information
is not usually known precisely, but engineers or operators familiar with a manufactur-
ing process can often present fairly good estimates. Careful experimentation may be
required. An inventory of water quality requirements is then matched against a similar
listing or diagram of the wastewater being generated.
In practice, rearrangement of water use in a plant is not so simple as it sounds.
First, the water to be reused has to match the needs of the particular area to which it
is available. In many plants, this will require the addition of surge capacity equip-
ment, since the production units may not operate in matched sequence. Second, there
can be some unpleasant product quality changes when contaminated water is used on a
product. A basic rule in incorporating water reuse practices into a plant is to slowly
test each application over a period of time. If changes are made simultaneously and a
problem arises, it is difficult to sort out the cause.
RECYCLING
In the context of our discussion, to recycle means to rechannel water for reuse in
a process in which it has already been used. If for a specific plant operation you take
the water that has been used and return it to the inlet, as shown in Figure 9-b, you have
closed the loop, or placed the system on zero water-discharge. If this is done as illus-
trated, and if new impurities are continually being added to the loop, the recirculating
water in the system will very quickly reach a high pollutant value. At this point, it will
no longer be able to remove further contaminants from the workpiece, since the con-
centration in the circulating water will be as high as that on the piece being processed.
Product quality will quickly deteriorate.
To transfer any pollutant to water, there must be a concentration difference be-
tween the source of the material and the water itself, maintained by removing pollutants
from the circulating system. One way is to drain off a portion of the circulating stream
and add an equal portion of fresh makeup water (Figure 9-c). The system shown here
works well. Material being transported out of the system equals that brought in by the
workpieces, and thus a state of equilibrium exists. The flow-out is low, but of high
concentration. A second method is to set up a side stream and treat it to remove the
contaminant.
The impurity to be removed may be either soluble or insoluble. Methods for keep-
ing both types of contaminants at acceptable levels in recycle streams are discussed
in the following paragraphs.
38
-------
Contaminated water
(with impurities)
to sewer
b)
Impurities
in
Contaminated
water
Contaminated
water
returns
Impurities
in
Excess
overflow is •*-
purge
Fresh
water
added
Figure 9. Examples of Recycle System?
-------
RECYCLE LOOPS WITH SOLUBLE CONTAMINANTS
To recycle water in your plant for a specific process, you must know the level of
impurities that can be tolerated within the recycle stream. The amount of impurities
tolerable in relation to impurities already present in the incoming water determines
the amount of recycle.
As an example, let us suppose we are removing a treatment solution from a prod-
uct and that the work brings 30 pounds of some type of salt into the process per hour.
If we were to completely close the loop, this salt would quickly build until the circu-
lating water would not remove any further salt from the work. If we begin to bleed a
portion of the recirculating stream, creating a "blow-down," a new equilibrium will be
established in which the salt removed by the blow-down is equal to the salt being added
by the pieces.
In closing the loop, you must determine how high you can allow the level of salt to
go in the circulating stream while still maintaining product quality; that is, the concen-
tration that will allow the process to continue satisfactorily.
The concept of balancing water reuse against product quality can be applied to any
system for which recycle is being considered. Basically, this involves starting with a
high purge rate, followed by a slow reduction of the flow of fresh water into the system.
The new flow rate is allowed to reach equilibrium, and its effect on the product is then
evaluated over a period of time. Each step may take days and each change should be
small; you should be sure of the results of one step before proceeding to the next.
Table 12 shows a result that might typically be obtained. In the case shown, it was
found that the first five reductions still provided ample "operating room." At the sev-
enth step change, however, operations began to be shaky. In this particular case, it
would be best to stabilize operations at level five, a level that showed ample water
savings over level one at a low cost.
RECYCLE LOOPS WITH INSOLUBLE CONTAMINANTS
In many cases, the critical contaminant in a recycle stream is insoluble.
An example occurs in the white sidewall grinding operation on rubber tires. To
bring out the white sidewall, each tire is ground with an abrasive material. Water
washing after this abrasive grinding operation picks up particles of rubber and of the
abrasive itself. One type of water flow for this washing is shown in Figure 10-a, in
which the water applied to the tire empties directly into the waste treatment system,
resulting in a high level of suspended solids in the sewer.
Since the material in this case is an insoluble solid, it can be removed easily and
the water quality requirements at the inlet of the process can be satisfied. In the sys-
tem shown in Figure 10-b, we have removed the entire stream as a plant waste source
and have in fact closed the loop. In this system, the flow from the grinding operation
goes to a settling device, where some oils float to the top and other solid materials
40
-------
a
Soap and solution
Water makeup to replace loss
Tire in •
Grinding
Operation
Tire.
in
Tire
out
Grinding
Operation
Highly contaminated
wastes
Tire out
Note: Circulating solution must be re-
placed. In This case, it was
hauled out by special contractor
Figure 10. White Sidewall Grinding
-------
Table 12
Summary of Recycle Study
Step
1
2
3
4
5
6
Flow
Over Work
GPM
500
500
500
500
500
500
Overflow
to Sewer
GPM
500
(once
through)
250
200
150
100
75
Fresh Water
Required
GPM
500
250
200
150
100
75
Product
Quality
Good
Good
Good
Good
Good
Good
System
Mechanical Operation
Good
Good
Good
Good
Good
Flow Nozzles Clogging
Change nozzles to larger size and proceed
7
8
500
500
75
50
75
50
Good
Slippage
Good
Good
Operate at 75-100 GPM.
sink to the bottom. The pump takes the contaminated but still-adequate water back to
the inlet of the process, where it is reused. This is a complete recycle system. The
purge and makeup water will only equal the small amount of the water carried out by
the workpiece.
In examining your own manufacturing processes for opportunities to close recycle
loops, look first for areas where the mechanical or cooling properties of water are be-
ing used rather than the solvent properties. For example, almost any process where
water is used to transport solids will be a prime target for recycle. Look also for
processes in which water quality requirements are not extremely high. In the case of
white sidewall grinding, water was being used to mechanically remove the grinding
debris and to provide cooling. With a very simple settling system, the water could be
reconditioned to perform the function again.
SUMMARY
This chapter has dealt with the more important principles involved in reducing
wastewater flows by in-plant controls. Although in-plant controls are an extremely
important part of any pollution control program, it is difficult to specify the most eco-
nomic measures without a detailed knowledge of a plant's operations and layout; spe-
cific decisions can only be based on an analysis at each individual facility.
42
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CHAPTER IV
RELATIONSHIP WITH THE
MUNICIPAL SYSTEM
Plants connected to a municipal treatment system face a somewhat different set of
rules than plants discharging directly to a stream. This section outlines some points
applicable especially to plants discharging into municipal or public systems.
Managers of plants discharging into municipal systems must bear in mind two im-
portant considerations of PL 92-500: (1) the cost recovery provisions for upgrading of
municipal systems; and (2) pretreatment requirements. Both of these points, unless
fully considered, can adversely affect the economics of pollution control.
Under the old grant system that existed before PL 92-500, municipalities received
35-55 percent of the cost of constructing municipal treatment systems from the federal
government. Under the new law, they receive 75 percent. In the past, it was common
for municipalities to recover only their share of the grant from the participating indus-
tries. Under PL 92-500, however, municipalities are required by the federal govern-
ment to recover the federal share of the grant also. This means that industry's share
of the municipal system will increase. Further, the cost of the municipal system will
be higher than in the past, since the municipality is required to meet higher discharge
standards.
The method that a municipality can use to recover the industrial costs can vary,
but is subject to some EPA guidelines. It can be based upon total flow or upon a more
elaborate formula relating to the actual amounts of materials discharged. In addition,
for new construction the industry and the municipality must sign a contract, which, in
effect, commits the industry to use a certain portion of the municipal treatment
capacity.
If you are now connected to a municipal system and are a "major contributing in-
dustry" (defined below), it is important that you become aware of your municipality's
progress in upgrading its treatment system, since this will guide you in your own plans.
For example, if you are now sending large amounts of essentially uncontaminated cool-
ing water to your municipal system and find that the municipality is devising a cost-
recovery system proportioned largely by flow, you should consider some process
changes in your plant.
REGULATORY REQUIREMENTS FOR PRETREATMENT
The pretreatment requirements for each industry subcategory dictate what must be
done before discharge into the municipal system. Under the general requirements for
43
-------
meeting national EPA guidelines, pH must be above 5 and all of the other requirements
met. Since no specific pretreatment regulations have been issued for metal products
industries at the time of this writing, the plant must be prepared only to satisfy the
municipality that nothing is being discharged that would harm the treatment system.
This means good spill control, perhaps with some grit and free-oil removal facilities;
these systems are discussed in Chapter V, Wastewater Treatment.
Keep in mind that regardless of what EPA may specify, the municipality can invoke
more stringent regulations. Although this generally is not a problem, a good spill-
control program is advisable for staying on the right side of municipal officials.
The general EPA pretreatment regulations will consist of, first, general guidance
for municipalities receiving industrial wastes, and second, the specific pretreatment
requirements for each industrial category.
Under the general regulations there is one important definition: what constitutes
a "major contributing industry." According to the regulations, a major contributing
industry is an industrial user of a publicly owned treatment works that:
• Contributes a flow of 50,000 gallons or more per average work day;
• Contributes a flow greater than 5 percent of the flow carried by the municipal
system receiving the waste;
• Has in its waste a toxic pollutant; and/or
• Is found by the permit issuance authority to have significant impact, either by
itself or in combination with other contributing industries, on the treatment
works.
Many of the regulations apply only to major contributing industries; that is, if you
can meet the conditions that keep you from being a major contributing industry, your
regulatory requirements will be easier to meet. If you are working around the clock,
the 50,000 gallons per day translates to about 34 gallons per minute. If your average
waste flow is less than 34 gallons a minute, you will not be a major contributor if the
other conditions are met. If you are on an 8-hour shift per day, you can discharge up
to 104 gallons per minute, on the average, without being considered a major contribu-
tor. If you are close to this quantity now and have a possibility of reducing your flows
by in-plant changes, the option may be a wise one.
You may also have a stake in learning what pollutants are actually defined as toxic
by PL 92-500 and what amounts are allowed. It will be to your advantage to eliminate
a toxic pollutant from your plant's wastes by pretreatment or by switching to a different
material, if this will gain the plant exclusion from the "major contributing industry"
category.
The general guidance for pretreatment standards basically states that there are
four "compatible" pollutants: biochemical oxygen demand (BOD), suspended solids,
pH in values or amounts that exceed the limits, and fecal coliform bacteria. Additional
44
-------
pollutants may be defined as compatible in the NPDES permit if the publicly owned
treatment works was designed to treat them and does, in fact, remove them to a sub-
stantial degree. This is of interest to the metal products industry because the regula-
tions give examples of such additional pollutants as chemical oxygen demand (COD) and
fats, oils, or grease of animal or vegetable origin.
The latitude in designating compatible pollutants means that your municipality can,
if you have the right relationship, help you by agreeing to remove portions of the pol-
lutants in your waste.
In the general regulations, certain wastes are prohibited from publicly owned
treatment works. These are:
• Wastes that create a fire or explosion hazard;
• Wastes that cause structural corrosion damage to treatment works, such as
acids (the regulations currently state that any waste with a pH lower than 5
cannot be discharged unless the works is designed to take such wastes);
• Solid or viscous wastes in amounts that would obstruct flow in the sewers; and
• Wastes entering the treatment facility at a flow rate and a pollutant discharge
rate that is excessive over relatively short periods, so that the treatment
process is upset.
For the compatible pollutants (in the metal products industry, pH and suspended
solids), pretreatment is not required by these regulations. However, the state or the
municipality can add regulations to require pretreatment if they desire. Bear in mind
also that even though you may not be required to treat the compatible pollutants your-
self, the municipality can (and probably will) set up its cost-recovery system so that
you are charged for treatment by the municipal system.
For "incompatible" pollutants, the guidelines state that the pretreatment standard
should require pretreatment to a level in which the final effluent, after passing through
the municipal system, will be equivalent to that obtained if the best practicable control
technology currently available were applied. The only exception is when the publicly
owned treatment works is committed in its NPDES permit to remove a specified per-
centage of a pollutant; in this case, the industrial discharger may have his allowance
adjusted upward to account for this municipal removal.
Therefore, for those plants discharging to a municipal system, the effluent guide-
lines pretreatment requirements are quite important, especially if you are a major
contributing industry. If you are not, none of these regulations apply and your require-
ments are strictly between you and the city.
INDUSTRIAL COST RECOVERY
Current EPA guidelines require that any municipality receiving a federal grant for
upgrading its treatment system or for building a new system must recover from industry
45
-------
the costs due to industrial usage of the system. These funds are then returned to the
federal government. Every manufacturer discharging to a municipal system should be
familiar with these regulations.* Before receiving any amount above 50 percent of its
grant, each municipality must furnish evidence that it has made progress in developing
a satisfactory industrial cost recovery system (ICR).
An important guideline for municipalities is that payment must be in proportion to
those industrial wastewater characteristics that influence the cost of construction.
Characteristics that may be included as affecting the cost are the strength, volume,
and flow rate of the waste stream.
EPA says the following points must be considered in the ICR system:
* If the maximum flow of a plant contributed to the cost of the construction works,
it shall be the basis for that user's payment. No credit is given to the industrial
user for the period when his plant is not operating and not discharging water
(which is one reason that flow equalization and reduction is an important in-
plant control).
• If an industrial user discharges cooling water into a municipal system, even
though it is uncontaminated, it is to be considered process waste and must be
included in the ICR computation. (If discharged to a stream, uncontaminated
cooling water is not called a process waste and can usually be discharged with-
out treatment.)
• The cost of unreserved excess capacity in the municipal treatment system for
expanded future use need not be recovered from existing users.
• If you reserve some of the treatment system's excess capacity for your plant's
future use, however, the cost of your reserved capacity must be reckoned.
ICR is not required for infiltration/inflow correction or correction of combined
storm overflow and collection or treatment of storm waters. Additionally ICR does
not apply to grants for projects which will not benefit industrial users.
"Major" contributing industries must be monitored on a regular basis. "Minor"
industries may be monitored on a random basis or, if monitoring is not feasible,
wastewater characteristics may be determined by using historical records and data
from similar industries.
There is, therefore, some leeway in how you can be included in the municipal
cost-recovery system. If a manufacturer has the option of either discharging to a
stream or to a municipal system, it is important to examine the economics of both
options in light of his plant's specific situation. In making this assessment, the man-
ufacturer must be aware of his municipality's future plans.
* A copy of Industrial Cost Recovery Systems can be obtained from the EPA Municipal Wastewater Treatment Works
Construction Grant Program, Washington, D.C. 20460.
46
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THE ECONOMICS OF PRETREATMENT
Pretreatment in a plant can be roughly divided into (1) pretreatment required to
meet the regulatory requirements, and (2) pretreatment installed because it pays for
itself in reduced municipal charges. The treatment necessary to meet municipal reg-
ulatory requirements has been discussed above. In this section we are concerned with
installing additional pretreatment systems as an investment yielding a favorable return
in reduced municipal charges.
The economics of installing a pretreatment system that exceeds the regulatory re-
quirements solely to reduce user or ICR charges are dependent on (1) the fee schedule
for the municipal system and (2) the waste characteristics of the plant. Using the sur-
charge rates for one major city, 2.4 cents per pound of suspended solids above those
in the first 10,000 gallons per day, a plant discharging 100,000 gallons per day of
wastewater with 1,000 ppm suspended solids could save about $5,000 per year in sur-
charges by reducing suspended solids to 100 ppm. If conventional treatment of the
entire stream with clarifiers were required to reach the 100 ppm level, it would not be
a particularly attractive investment, because the payback on the investment would prob-
ably exceed 10 years. On the other hand, if this reduction could be made by simple in-
plant modification—such as recycling one internal stream—an attractive saving might
be possible.
Similarly, if the plant were discharging 2, 000 ppm of suspended solids, then pre-
treatment to remove some of this load might be economical. Each plant must make
this calculation, based upon the formula used by the local municipality and the estimated
cost. Generally speaking, however, for plants of the size of most metal products man-
ufacturers, the most attractive route to reducing payments to the municipality is the
use of improved in-plant controls (Chapter III) rather than the installation of hardware.
This statement is based on the typical situation in which the manufacturer has a rela-
tively low flow. Obviously, there will be exceptions. Table 13, which follows, is a
rough analysis for a specific plant, using the surcharge costs in the previous example.
For the case illustrated, the conclusion is clear: It is cheaper to pay the surcharge
than to install pretreatment equipment.
Flow reductions save in both the water bill and the sewer charges as well as pos-
sibly bringing the plant below the "major contributing industry" level.
47
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Table 13
Preliminary Analysis of Pretreatment Economics
Example Surcharges for a Municipality
• Suspended solids surcharge of 2.4 cents/lb for all solids beyond that which
would be obtained in the first 10, 000 gal/day
• Sanitary allowance of 4 mills/day per employee for sanitary sewage
discharged
• 2.1 cents/1,000 gal of flow
Plant Conditions
190,000 gal/day @ 716 ppm of suspended solids (SS)
25 employees
330 days operation
Yearly operating costs associated with solids removal
(labor, maintenance) = $3,000 (estimated)
Objectives — To study installation of equipment to:
Reduce suspended solids to 100 ppm by clarification and to spread solids on
plant property. Determine total capital costs and payback period which can be
obtained by reduced municipal charges.
A. Present Surcharges for Plant
Total surcharge = flow surcharge plus suspended solids surcharge minus
sanitary allowance
Flow Surcharge
190,000 - 10,000 gal $.021 330 days
day 1,000 gal year
Suspended Solids Surcharge:
= $1,247
m , t, ..„„ 190,000 gal 330 days 8.34 Ib of water „„„ „..„,,
Total Ib of SS = '- — x — x 716 ppm x — = 374,410 Ib
day year gal of water
Pounds of suspended solids excluded from surcharge:
10,000 gal 330 day _.„ 8.34 Ib water -„„„,,,,
—^i — x x 716 ppm x = 19,706 Ib
day year gal water
48
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Pounds of suspended solids which will be chargeable:
374,410 - 19,706 = 354,704 Ib
$ 024
Suspended Solids Surcharge = total Ib of 354,704 x *' =8,513
$ 001
Surcharge Allowance; = —— x (25 employees) x (330 days) = $8
IT «/
Total Surcharge = $1,247 + $8,513 - $8 = $9,752
B. Adding Hardware for Reducing the Suspended Solids to 100 ppm;
Savings = Suspended Solids Surcharge reduction minus operating costs
SS surcharge reduction = ($8,512) f 716 " 10° \ = $7,323
Operating cost estimate - 3,000
Total Savings = $4,323
Cost of Equipment: $110,000 (Figure 45)
_, , . /110,000 \ nr
Payback = I 4 g23 1 = 25 years
Conclusion: Not Economic
49
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CHAPTER V
WASTEWATER TREATMENT
Once in-plant controls have been applied to their practical limit, the manufacturer
must look to wastewater treatment techniques for the final removal of pollutants to a
level that will satisfy the NPDES permit or the municipal system requirements.
Wastewater will contain both dissolved and suspended substances. The goal of
treatment is to reduce these materials to levels that will not pollute the receiving
stream.
Figure 11 shows the components of wastewater for most metal machining, fabri-
cating, and coating facilities. The water will include some coarse solid materials
picked up from the scaling or cleaning operations, along with free and perhaps dis-
solved oil, some dissolved metals and other solids, and colloidal material. Basically,
the treatment system will remove insoluble solids by gravity settling, free oil by float-
ing and skimming, and dissolved metals by chemical reactions that convert them to an
insoluble form so they, too, can be removed by gravity settling. The mechanical as-
pects of this treatment are not particularly complex. As will be seen, however, cer-
tain chemical and physical phenomena occur that make it necessary to perform interim
testing to ensure that low pollutant levels are being reached.
By visually examining a sample of a plant's wastewater, we can see that certain
pollutants can be readily removed; these are the materials that soon either settle to the
bottom or float to the top. These highly visible materials are often responsible for the
gross unsightliness that the public associates with pollution.
\
Dissolved
oils, metals, solids
Floating free oil
and grease
Jnsoluble
materials
Figure 11. Wastewater Components
50
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The solids that rapidly settle to the bottom of a beaker of raw wastewater are
known as "settleable solids."* These are the larger, coarser suspended solids. In
water pollution control technology, there is a specific measurement for their determi-
nation. The wastewater is allowed to settle for one hour in a special apparatus. The
solids that settle during that time are going to be relatively easy to remove from your
wastewater; in fact, some of them will settle out in the sewer lines at periods of low
flow.
After this quiet settling time, however, there will still be many solids present in
the sample. These are finely divided particles that remain suspended indefinitely un-
less some treatment is applied to make them "settleable."
An upper layer will also normally form on wastewater left quietly in a beaker for
a short time. This will be the free oil and grease that readily agglomerates to large
droplets and floats to the surface. If you give the wastewater a chance to reach quies-
cent conditions for a given period, this free oil will rise and can be easily skimmed off.
Assuming that the wastewater has now been passed through the proper equipment
(which will be discussed shortly) to remove the easily settleable solids and the free-
floating oil, the treated water will still have a very cloudy, turbid appearance and may
still contain significant amounts of oil and grease. There also may be significant quan-
tities of dissolved heavy metals, some of which may also be regulated on your NPDES
permit. Removal of these remaining pollutants requires chemical and/or mechanical
treatment to first precipitate the soluble pollutants and, second, to create conditions
where the suspended solids can join together into larger particles that can be removed
by gravity settling. The suspended or emulsified oils must also be treated so that
smaller particles join together and become "freed" for removal by mechanical
separation.
Figure 12 illustrates the normal sequence of treatment for wastewater containing
the pollutants just described. Basically, the philosophy is to first remove those mate-
rials that can be easily separated; this avoids later interference with the more critical
chemical and physical techniques used in final treatment. Thus, the wastewater is
first passed through a crude settling and skimming operation, followed by a precipita-
tion and clarification process, and finally, if required, through an emulsion-breaking,
coalescing operation where the fine oil particles are freed from the waste stream.
The sludge removed from the water might then be sent to a further "thickening" oper-
ation where the percentage of solids in the sludge is increased for easier transport to
disposal.
For discussion purposes, the treatment sequence has been divided into three gen-
eral operations: primary separation, the crude separation of easily settled solids and
bulk oil; wastewater conditioning, or chemically treating the remaining pollutants for
removal; and actual removal of these pollutants by sedimentation or filtration.
*Almost all the terminology used in wastewater treatment is derived from the sanitary engineering profession. Since
this terminology does not always coincide with common usage, we will define each key term as it appears.
51
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Industrial
Wastewater
Oil
Skimming
Condi-
tioning
Coagu-
lation
Floccu-
lation
Grit
removal
Suspended
solids
separation
Fiee oil
removal
Solids
removal
T
Water
return
Solids
Disposal
Solids
thickening
01
CO
Wastewater
effluent ~
Emulsion
Breaking
Fine oil
removal
Figure 12. Typical Wastewater Treatment System
-------
Waste treatment is simply the accomplishment of these three tasks. In this chap-
ter, we will discuss the basic means by which pollutants are removed; describe the
equipment; present data to help estimate costs; and provide guidance to the manager
for avoiding many pitfalls that can result in extensive violations or added costs.
PRIMARY CLEANUP (GRIT AND FREE-OIL REMOVAL)
The first step in the treatment of industrial wastewater is the removal of grit and
free oil, which are suspended in the waste stream by turbulent flow. Once the flow
rate is lowered to a nonturbulent level, the grit (heavier than water) settles to the bot-
tom of the container and the free oil (lighter than water) floats to the top.
GRIT REMOVAL
Components of the grit—dirt, metal shavings, and other heavy solid materials-
are generally classified as settleable solids and are removed from the waste stream
by installing grit chambers. A grit chamber is normally a rectangular vessel or tank,
sized to reduce the horizontal velocity of the waste stream to approximately 1 ft/second
and having a detention time sufficient to remove particles with a. settling velocity greater
than 2.5 ft/minute. Given the detention time (settling velocity), horizontal velocity, and
the waste stream flow rate, the overall dimensions of the chamber can be determined.
During nonturbulent flow conditions, the solids fall to the bottom of the grit cham-
ber into an accumulation space. The settled grit is removed periodically from the grit
chamber by conveyors (such as screw conveyors) or collection baskets, which are
raised out of the chamber with an overhead lift or monorail system. Collection baskets
are preferred when the grit is abrasive. These grit chambers can be combined into one
unit with the free oil-water separators, as in Figure 13. Grit chambers can usually
eliminate all particles with sizes greater than 100 mesh.
OIL REMOVAL
Oil-water separators and skimming equipment are used to separate and remove
free oil from a waste stream prior to further treatment. In some instances, flotation
equipment is also used to induce oil droplets to the water's surface for removal. These
classic oil removal techniques usually result in residual concentrations of approximately
50 ppm, depending on the nature of the oil. Under certain conditions, lower free-oil
values can be obtained.
Skimming Devices
Free oil and grease floating on or near the surface can be effectively removed by
skimming devices, once the flow rate is reduced in a basin. Some of these are units
that remove free oil by indiscriminately removing the entire surface layer of the waste
stream. Other skimming devices aie self-contained units, designed to rest on the
water surface and consisting basically of a floating platform. An adjustable weir allows
the different skimming depths. The weir overflow, containing the oil and grease, is
-------
CONCRETE SLAB
SEALED REMOVABLE LIDS
"T" PIPE OUTLET
TO SEWER
OPEN GRATE
(can be provided
for direct influent)
BACKFILL
(sand or gravel)
WEIR
V*OIL RETENTION
BAFFLE
ROTARY PIPE SKIMMER (STD)
(removes skimmed oil to slop tank)
SUSPENDED SOLIDS BAFFLE
SLUDGE CHAMBER WALL
SLUDGE ACCUMULATION
(to be dumped periodically)
SLUDGE REMOVAL BASKET
Figure 13. Gravity Separator with Grit Removal
-------
then pumped from the platform to a waste disposal area (Figure 14). While skimming
operations can remove essentially all of the floating oil (depending on the depth setting),
the units do not separate the components of the layer removed and the oil can contain
considerable water.
Gravity Separations
The most common gravity separator for oil removal is the American Petroleum
Institute (API) type. Proprietary versions of this type of oil removal equipment are
also available. The API manual cites specific minimum and maximum standards for
these units. ^ Figure 15 illustrates a common separator of the API type.
The API separator has two main sections: the inlet and the oil-water separator
channels. An adaptation of the API separator to include grit removal was shown in
Figure 13.
The inlet, or forebay, helps reduce the flow velocity of the influent while collecting
floating oil. This section is also used for grit removal, providing the necessary re-
tention time and removal mechanisms (conveyors or buckets). The influent then flows
into the separator channels through control gateways. Downstream from the gateways
is a velocity diffusion device that reduces turbulence and distributes the flow equally to
OIL WEIR OVERFLOW
INTERFACE FLOAT
CONTROL
MOTOR AND PUMP
ADJUSTABLE FLOATS
CLEAN WATER
DISCHARGE
N\^<%*S^
Figure 14. Floating Skimmer
55
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INLET SEWER
TRASH RACK
ftOATING Oil SUBMERGED
SKIMMER HOSE
COVERED PRE SEPARATOR FLUME
HOW —-
— TRANSITION SECTION- - — PRFSEPARATOR SECTION
NOTF 1 - SLUDGE REMOVAL FROM FOREBAV
BY CLAM SHELL OR VACUUM TRUCK
NOTE 2 - SEPARATOR PUMPS ARE ALLOCATED FOB
SFcCIFIC SERVICE AS FOLLOWS BUT ARE
MANIFOLDED TO PERMI* SPARING AND TO
PROVIDE FLEXIBHITY OF OPERATIONS
PUMP A - TRANSFER OF SETTLED WATER
FRDM COVERED SKIMMED OIL SUMP
TO INLET 0^ PRESEPARATOR FLUME
PUMP B - TRANSFER OF SEPARATED OIL FROM
COVERED SKIMMED Oil SUMP TO
SLOP HANDLING tftCluTIES
PUMP C- TRANSFER OF SLUDGE FROM
SEPARATOR 10 SLUDGE HANDLING
FACILITIES
WORKING
PLATFORM
DIL I
SEPARATOR PUMPS -
NOTE 2
,J
sR )
VN Y
KB -'TV
*m
-~m
-UL]
O-
5
O
§
o
a:
O
MEO —
I LINE
NT WEIR ,
WALL
NO
GATEVH
\ GATEWAYS
/ 0 \ 0
XTXIxKX
i i
FLIGHT SCRAPER
CHAIN SPROCKET
FLIGHT SCRAPER
" CHAIN '
/
WOOC FLIGHT
\
n — n
E
Y PIER
FIOW GATE SLOTS
t .- f
I U U
XlXlxXKtX-
H— — nr
FLOW
i
fi — - n
U U
-'^ EFFLUENT FUME
FIOW
DIFFUSION DEVICE
-(VERTICAL SLOT
BAFFLE!
SLUDGE-COLLECTING
HOPPERS
SIUDGE PUMP
SUCTION
_J
ROTATABLt OIL
— SKIMMING PIPE
-OIL RETENTION
BAFFLE
0 EFFLUENT
=» SEWER
Ihll FT ^fWFR !_. r.n.,rnrn nnrPrn.n*Tnnr,,i.»r
(' ' WATER LEVtl CANVAS CURTAIN^
\ X
(.) FLOW TRASH RACK /
"—^^C^rr (-
TRASH PAN
i— y
/ r-n
/ x1111
FLOATING SKIMMER
-SEPARATOR CHANNEL^)
— . , — n — ,
\
,11
V /
- TRANSITION SECTION-
- PRESEPARATOR SECT'ON
CHANNEL GATEWAYS
GATEWAY PIERS
SECTION A A
DIFFUSION OEVICL
(VERTICAL SLOT BAFFlE,
\
GATEWAY PIER
V
J
\
" ~ — 3V^
OIL RETENTION BAFFLE
FLIGHT SCRAPER. CHAIN SPROCKET FLIGHT SCRAPER ROTATABLE OH ~"^V/
/ WOOD FLIGHTS CHAIN WATER LEVEL SKIMMING PIPE /
P CN^/T
o FL°W o J
EFFLUENT WEIR AND WALL
SB
EFFLUENT
^EFFLUENT
FLUME
SLOT FOR CHANNEL GATE
SIUDGE COLLECTING HOPPER
SLUDGE COILECTING HOPPER
DISCHARGE WITH LEAD PLUG
SLUDGE PUMP SUCTION PIPE
SECTION B B
Figure 15. API Separator
56
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the channels. The separator channels are where oil-water separation takes place; each
one has a chain-type oil- and sludge-moving conveyor, with flights attached for half of
the length. The flights, traveling with the flow, move free oil toward the skimmer and
push the sludge into a collection area.
An oil skimmer and oil-retention baffle are also located in the effluent area of the
separator. Beyond this, the waste stream flows over the wall of the effluent weir into
the sewer.
Figure 16-a shows a slotted-pipe skimming arrangement. The pipe is installed in
a tank parallel to the water surface. The slot is set so that the depth of oil to be re-
moved flows into the pipe, where it travels by gravity to a collection area.
Figure 16-b illustrates another piping assembly. The skimming pan, or trough,
is set at the desired level and free oil on the surface flows into the pan and down a hose
to a collection area. The float on this particular skimmer constantly adjusts the pan
level.
Air Flotation
In this technique, air is induced into a pressurized waste stream tank to dissolve
the air in the water. The air-saturated, pressurized water becomes supersaturated
when released through a pressure-reducing valve. The excess air, in its super-
saturated state, forms small bubbles in the water that attach to oil droplets, which
then float to the surface for removal. Chemical coagulants can be added before air is
released into the tank to aid in adsorbing bubbles. Removal of emulsified oil from
large wastewater flows by combined chemical flocculation and air flotation is a com-
mon procedure.
WASTEWATER CONDITIONING
Grit and free oil quickly settle out of the waste stream when the flow rate is re-
duced to a nonturbulent state. The remaining pollutants settle at an unsatisfactory
rate or remain in suspension. These particles include suspended and dissolved solids
and emulsified oils that are stable in the water. This stability is due both to the small
size of the particles and to charges between the particles and water that overcome
density differences and gravitational effects. The fraction of these solids that remains
suspended is termed colloids.
To remove these pollutants, the water must be conditioned or pretreated by adding
chemicals to change the stability of the colloids by making them larger, enabling small
particles to join into masses (floes) with sufficient density to settle. Chemical pre-
treatment is also used to adjust the pH value of the stream or to cause a reaction with
the dissolved solids to yield an insoluble, settleable product.
The types of pretreatment for conditioning the wastewater for solids removal are:
(1) pH adjustment (to alter solubility of dissolved solids); and (2) destabilization treat-
ment (to promote coagulation of colloids).
57
-------
Figure 16. Common Skimming Devices
58
-------
The treatment for emulsified oil is slightly different and includes a combination of
chemicals and rapid agitation to free the oil from the emulsion. This step, emulsion
breaking, will be discussed in Chapter VI, Residual Oil and Grease.
pH ADJUSTMENT
To meet the regulations, the pH value of a waste stream must be in the range of
6-9 (water alone is 7). Normally, a waste stream will be acidic (pH less than 7), and
a base such as lime or caustic soda can be added to increase the pH. If the waste
stream has a pH greater than 9, an acid can be added. Dosages are determined by
standard laboratory experiments, explained in chemistry books and vendor literature.
A detailed discussion on neutralization chemistry and hardware will be presented in
Chapter VII, Handling Waste Acid Streams.
The importance of pH control by neutralization extends beyond meeting the regula-
tory figure, since it affects the subsequent reaction with dissolved solids (such as heavy
metals) and the efficiency of coagulants in forming settleable floes. For particularly
troublesome waste streams, the pH may be adjusted to a value beyond the regulatory
range for optimum removal of pollutants and then readjusted back before discharging
the water.
NEUTRALIZATION TREATMENT: PRECIPITATING HEAVY METALS
Heavy metals dissolved in waste streams are one of the most prevalent pollution
control problems.
The EPA is extending its regulation of heavy metals for 1977; iron, lead, chro-
mium, zinc, copper, arsenic, cadmium, selenium, tin, nickel, and mercury are all
being regulated in some industries.
Problems in reaching the required levels of metals in waste streams normally
occur when:
• The flow of wastewater exceeds that allowed in the guidelines;
• Mixtures of heavy metals are being removed; or
• The precipitated metal salt is difficult to separate from the wastewater by
gravity settling.
The majority of heavy metals will be removed from wastewater by combining the
pH adjustment step, which precipitates the metals, and gravity settling. Adjustment
of pH is the first step. Insoluble metal hydroxides are formed by the reaction of the
basic chemical with the metal at a pH range of 6-9. Not all metals precipitate (become
insoluble) at the same pH or to the same extent. Initial separation should therefore be
made at the pH value that gives the most complete precipitation or that reduces the
most toxic metals to the lowest degree.
59
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Existing effluent limitations for various metals are closely related to the sol-
ubility of the metal in water and are regulated according to the total pounds of the metal
per unit of production. Since the minimum concentration of a metal in wastewater is
set by its solubility, any increase in the flow of the waste stream results in a propor-
tional increase of the total pounds of metal at the same concentration. The manufac-
turer has no control over this. However, if the plant can reduce its flow below that
anticipated in EPA's effluent guidelines, it may be able to meet metals-concentration
requirements with less strain.
The problem is compounded for plants treating a mixture of heavy metals in their
waste streams, since each metal has different solubilities at different pH levels. Fig-
ure 17 shows the theoretical solubility of several heavy metals as a function of pH. If
each metal were the only component in the waste stream, then copper could theoreti-
cally be reduced to a solubility of .0001 mg/liter at a pH of 9 and nickel could be re-
duced to a solubility of .001 mg/liter at a pH of 10.5. At a pH of 8.5 the solubilities
are .0005 mg/liter for copper and 1 mg/liter for nickel. When the two metals are
mixed, however, the choice is either a pH optimum to one of them or a compromise
between the two. Since complex interactions can occur between the metals, the situa-
tion is often even more complex. Thus, it is important to determine the exact charac-
teristics of the waste stream through experimentation. If only nickel and copper were
present (and removals close to theoretical), the concentration of each metal would be
equal at a pH of 10, with a solubility of .0015 mg/liter.
Another problem in meeting heavy-metal guidelines is the difficulty of removing
the metal precipitate resulting from pH adjustment. Settling rates vary widely with the
final compound, requiring careful selection of the neutralizing reactant. Certain forms
of iron hydroxide, for example, are very difficult to settle. Poor settling rates for
metal salts resulting from neutralization will mean failure to meet limitations for both
heavy metals and total suspended solids. The manufacturer must make sure that the
neutralization reagent is compatible with the best overall removal efficiencies in the
final separation step.
DESTABILIZATION METHODS
Destabilization is also a function of pH. Dosages of chemicals for destabilization
are normally adjusted to achieve maximum floe formation at the pH value effecting
minimum solubility of the metal salts.
As discussed earlier, suspended solids contain particles that will not separate
from the wastewater in a reasonable time without additional chemical action to agglom-
erate them.
These very small colloidal particles are kept in suspension (stabilized) by electri-
cal surface charges. The goal of chemical treatment is to neutralize these charges,
causing the small particles to come together in sufficient size and density to settle
readily. The chemicals used for this are called coagulants or flocculants (see Refer-
ence 14 for a more detailed discussion).
60
-------
001 —
0001 —
Solution, pH
(From EPA-440/1-73-003)
Figure 17. Solubility of Heavy Metals as a Function of pH
61
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More specifically, destabilization and flocculation are usually effected by adsorp-
tion or enmeshment (entrainnient) of the colloids, which enables the formation of floes
(masses of colloids). Adsorption is accomplished by adding polymers or metal salts
to achieve physical or chemical bonding between particles. Enmeshment uses metal
salts or lime to create a chemical that produces a precipitate. As the precipitate
forms, the colloidal particles are trapped by it. The effectiveness of a coagulant is a
function of the pH, the colloidal composition, and the coagulant material and dosage.
Coagulation is the reaction that neutralizes the repellant charges of the colloidal par-
ticles and permits their agglomeration. Flocculation then promotes collision and in-
teraction of the coagulated particles to develop the clusters, or floes. Effective floc-
culation requires that the proper amount of energy be applied—just enough to bring the
particles together; too much mixing will break up the flocculated material.
The first step in removing stabilized solids is to determine the most promising
combination and amounts of chemicals and polymers. If excess chemicals are added,
it is possible to restabilize the solid particles, since they will again become charged
in the opposite direction.
The effects of adding coagulants will be charge-reduction of the ions in solution
and a change in the pH. Metal salts such as alum (aluminum sulfate) or ferric chloride
are common coagulants, used in combination with polymeric flocculants. Proper pH
control of the waste stream alone can sometimes produce coagulation by enabling pre-
cipitation of materials, heavy metals, for example, already present (see Reference 14
for detailed discussion).
LABORATORY TESTS
It is impossible to determine coagulant dosages from theoretical calculations.
Laboratory tests must be run on the effect of pH and various coagulants. Experiments
have been developed that establish optimum coagulant concentrations and dosages;
equipment vendors and chemical manufacturers can conduct the necessary tests. They
have also developed favored combinations of neutralizing chemicals and coagulants.
In these tests, the pH is held constant while various dosages of coagulants are
added. The pH is then changed in one direction by adding a suitable chemical, and the
dosages are checked. If an improvement is noted, the pH is increased or decreased
again until the optimum result is obtained. The most widely used laboratory tests are
the jar test and a test to measure the conductivity—the "zeta potential"—of the particle
charge.
Jar Test
This test is commonly used to determine the dosage and other parameters for op-
timizing solids settling rates and removal. It attempts to simulate the full-scale co-
agulation process, and since 1918 has remained the most common coagulant-control
test in the laboratory.
62
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The jar test uses a series of containers stirred by individual, mechanically oper-
ated stirrers (Figure 18). Wastewater samples are placed in the containers and treat-
ment chemicals are added as the contents are being stirred. A simulated range of
conditions is selected that represents optimum conditions. After a short period of
rapid stirring (1-5 minutes) to ensure complete dispersion of the coagulant, the stirring
rate is decreased and flocculation is allowed to continue for a variable period, 10-30
minutes or longer, depending on conditions simulated. The stirring is then stopped
and the floes are allowed to settle for a controlled time. The supernatant (layer of
liquid above settled solids) is then analyzed for critical parameters. The usual analy-
ses would be for turbidity, pH, and residual coagulant.
Figure 18. Jar Test Appratus
63
-------
Supernatant samples may be taken at intervals during the settling period for con-
structing a series of settling curves, which provide more information on the settling
characteristics of the floes than a single sample. A dynamic settling test may also be
used in which the paddles are operated at 2-5 rpm during the settling period, more
closely representing settling conditions in a large horizontal basin with continuous flow.
Several 6-position stirrers are commercially available for jar tests. An excellent
discussion of jar tests is included in the EPA Technology Transfer publication Process
Design Manual for Suspended Solids Removal. •*-
Figure 19 shows typical curves obtained from settling tests such as the jar test.
Curve A represents a coagulation that produced a uniformly fine floe, so small that at
the end of 1-2 minutes of settling the supernatant had a turbidity about equal to that of
the starting water. This degree of coagulation is unsatisfactory. Curve B represents
the most common settling picture, in which the settling rate during the first 5 minutes
is practically a straight line on a semilog plot; settling is rapid and clarification satis-
factory. Curve C shows a mixture of large, rapid-settling floes and small, slow-settling
50 -
20-
10-
5-
1 2 3
Settling time —minutes (from Reference 12)
Figure 19. Typical Settling Curves Obtained from Jar Tests
64
-------
particles. Settling was rapid for the first 2 minutes, but there was little further clari-
fication. High residual turbidity may also have resulted from incomplete coagulation.
Curve D shows excellent coagulation. The floe particles were so large and dense that
97 percent settled within 3 minutes. By then, sedimentation (solids settling) was essen-
tially complete, since only 0.5 percent additional floe settled in the next 27 minutes.
Final clarity of the supernatant was entirely satisfactory.
Measurement of turbidity is the quickest indicator of the degree of solids removal.
The recommended procedure for turbidity measurement by light scattering is given in
Standard Methods for Examination of Water and Wastewater;^ however, other methods
can be used, from simple visual evaluation of the level between the solids and the clear
water portion to measurement of light transmitted on a laboratory spectrophotometer.
Measurement of residual suspended solids is the only procedure that gives actual weight
concentration of solids remaining, but it is too slow for process control. Residuals-of
coagulant in the supernatant liquid are usually significant and may be measured either
manually or with automated equipment.
The experiments enable visual observations of the effects of coagulants and pH ad-
justment on sludge characteristics and volume and also of the effluent composition to
be expected from the plant. The data from these experiments can be the basis for the
plant's wastewater treatment design; in many instances, it provides the only available
basis for this design.
Zeta Potential
The principal stabilizing factor for colloidal suspensions is electrostatic force and
the repulsion between particles of similar charges. The "zeta potential" is the measure
of these forces between the clear solution and the solution held within the boundary of
the colloidal particles; it is the residual charge of the interface between the layer of
bound water and the clear water. The importance of this value lies less in understand-
ing these rather complex relationships than in their laboratory measurement for selec-
tion of coagulants.
The zeta potential is measured with a commercially available instrument, shown
in Figure 20. A special cell is placed in a sample of treated water containing the co-
agulant and the floe formed from colloidal suspension. The cell is then placed under a
microscope and a voltage is applied to electrodes at the ends of the cell. The charged
particles migrate through the solution to the electrode that has the opposite charge.
The velocity of the particle is proportional to the applied electrode voltage and the par-
ticle charge. The velocity is calculated by dividing the distance the particle travels by
the time of travel, the distance being measured by an ocular micrometer. The zeta
potential is then obtained from charts, provided with the instrument, that combine the
particle velocity with the instrumental parameters. The procedure is repeated, using
various dosages of coagulants to arrive at the optimum level.
A sample plot showing zeta potential and turbidity measurements vs. coagulant
dosage is given in Figure 21. In this case, the optimum dosage of the coagulant alum
was 150 mg/liter. This value corresponded to a zeta potential of -3 millivolts. The
65
-------
Figure 20. Zeta Meter Apparatus
control point will normally fall in the range of 0 to -10 millivolts for effective coagulant
dosages. The initial zeta potential without coagulant added to the sample was negative;
therefore, a cationic (positively charged) coagulant would be a logical choice. Higher
concentrations of the coagulant restabilized the colloidal suspension; the turbidity in-
creased from 4 units at the optimum dosage of 150 mg/liter to 8 units at a dosage of
300 mg/liter. The curve appears to level out at dosages above 350 mg/liter.
Zeta potential curves are often combined with the settling results from the jar test
in determining optimum coagulant types and dosages. Once the initial coagulant dosage
and type is determined, the zeta potential can be rechecked during actual operation as
an indicator of possible wastewater changes.
COAGULANT TYPES
The selection of coagulants is based on the laboratory tests, described above,
which indicate the settling rates at various coagulant dosages. Since the dosage must
be carefully controlled, coagulant feed rates are also important. In many'cases, sev-
eral coagulants will produce similar destabilization efficiencies. The final selection
then becomes one of economics and ease of handling. Commonly used coagulants are
discussed below, with an explanation of how each one works and how it should be used.
Aluminum Sulfate (Alum)
The most common aluminum salt used for coagulation is alum. It is particularly
effective in pH ranges from 5 to 7. The approximate chemical formula of alum is
A12(SO4)3 ' 14H2O. The solubility of dry alum in water is high and ranges between
6.03 Ib/gal at a temperature of 32°F to 8.45 Ib/gal at 85°F. It is white in color,
acidic, and a 1-percent solution has a pH below 4. It can be stored without any special
precautions other than moisture control.
66
-------
+ 10
S =>
o z
a -10
-20
20
15
10
1 1 1
1 1
100 200 300
Dosage {mg/liter)
Figure 21. Predicting Optimum Coagulant Dosage
400
500
67
-------
Many varieties of alum are marketed. The one most commonly used in water pol-
lution control is ordinary ground or rice alum. Alum is also marketed in solution
form, which does not require solids -handling and -dissolving facilities at the plant site.
The price for liquid alum on a 17-percent A12O3 basis, F.O.B. point of manufacture,
is approximately $62/ton. Generally, the decision to use liquid or dry alum is based
on the distance between the plant and the purchaser, since shipping costs of the liquid
include 50-percent carrying water. Liquid alum is normally shipped as a 45-50 per-
cent solution as A12(SO4)2 • 14H2O.
Although dry alum is not particularly corrosive, liquid alum is, and requires the
use of plastic, stainless steel, or rubber-lined tanks and piping systems. The price
for dr3^ alum is approximately $15/100 Ib.
Alum -handling precautions are not stringent. No particular industrial hazards are
encountered. Standard protection, such as gloves and eye protectors, should be used.
Alum is an irritant and should be promptly washed off if body contact occurs. Alum
dust can be abrasive to industrial machinery and should be controlled.
Ferric Chloride
Ferric chloride for coagulant use is usually purchased as a liquid. The solution is
a dark -brown, oily mixture, containing between 35-45 percent ferric chloride (FeCl3)
and having a density of 11.2-12.4 Ib/gal. A pH of 2 is obtained with a 1-percent solu-
tion of ferric chloride . A crystalline form of ferric chloride has a chemical formula
of FeCl3 • 6H2O.
Ferric chloride solution is marketed in tank-car volumes of 4,000-10,000 gal, in
truck volumes of 3,000-4,000 gal, and in 5-13 gal containers. The price, F.O.B.
point of manufacture, is approximately $4. 50-$7. 50/100 Ib as FeCl3, dependent upon
the solution quality; anhydrous technical grade FeCl? costs approximately $12.50/100
Ib.
Normally, rubber-lined steel or plastic is used for constructing storage tanks and
piping systems for ferric chloride. Ferric chloride should be used with the normal
precautions for handling chemicals.
Ferrous Sulfate
Ferrous sulfate, or copperas, is a by-product of pickling steel and is produced as
granules, crystals, and powder. The most common commercial form of ferrous sul-
fate is ferrous sulfate heptahydrate , FeSO4 ' 7H2O, with a molecular weight of 278,
and containing 55-58 percent FeSO4 and 20-21 percent Fe. The product has a bulk
density of 62-66 lb/ft3. When dissolved, ferrous sulfate is acidic. The composition
of ferrous sulfate is quite variable and should be established in each case by consulting
the supplier. It is readily dissolved in water to approximately 35-percent solution at
room temperature. It is most effective as a coagulant when stream pH is above 9.
The price of ferrous sulfate in bulk carload and truckload quantities is about $18/ton
(21 percent Fe). The bagged cost is $24/ton. Ferrous sulfate is sometimes available
at lower prices as a by-product of sulfuric acid recovery systems.
68
-------
Precautions for dust and handling are the same as for other acidic solutions. Mix-
ing quicklime and ferrous sulfate produces high temperatures and should be avoided
unless proper control is exercised.
The granular form of ferrous sulfate is easiest to feed; either gravimetric or vol-
umetric feeding equipment may be used. The optimum chemical-to-water ratio for
continuous dissolving is 0.5 Ib/gal to produce a 6-percent solution. The detention time
is 5 minutes in the dissolver. Lead, rubber, iron, plastics, and type 304 stainless
steel can be used for equipment for handling solutions of ferrous sulfate.
Ferrous sulfate reacts with the alkalinity of wastewater, or with added alkaline
materials such as lime, to precipitate ferrous hydroxide. The ferrous hydroxide is
oxidized to ferric hydroxide by oxygen dissolved in wastewater. Ferrous hydroxide is
soluble, and oxidation to the more insoluble ferric hydroxide is necessary if high iron
residuals in effluents are to be avoided. Flocculation with ferrous iron is improved by
the addition of lime or caustic soda at a rate of 1-2 mg/mg Fe as floe conditioning
agents. Polymers are also generally required to produce a clear effluent.
Lime Varieties for Wastewater Treatment
Ground limestone usually appears attractive in preliminary water treatment
studies, because it is readily available at a low price. It is shipped directly from the
mines and its properties vary widely with the source. It is not as reactive as lime and
is more easily fouled. For these reasons, limestone should be selected only after
pilot studies.
Lime is manufactured from limestone by heating to very high temperatures to
drive off CO2 . An excellent reference on the use of lime in treating wastewater is
available from the National Lime Association.-^
Lime has a dual use in wastewater treatment: as a coagulant and for pH adjustment
or control. It is generally available in two forms: quicklime or hydrated lime. Quick-
lime must be reacted with water (called slaking) to produce the hydrated compound
which is the reactant or coagulant. A saturated solution of lime and water has a pH of
12.4.
Quicklime (CaO) can be purchased in bulk in both car and truckload lots. It is also
shipped in 80- and 100-lb multiwall "moisture-proof" paper bags, costing approximately
$842/ton. The bulk density is approximately 55-75 lb/ft3 and the molecular weight is
56.1. Quicklime can contain significant amounts of impurities such as dirt and grit.
It is highly hydroscopic. Quicklime is an extremely strong irritant, affecting eyes,
mucous membranes, and upper respiratory tract. Hot lime suspensions can cause
severe burns and eye injuries; adequate personnel-protection measures are required.
Hydrated lime [Ca(OH)2] is usually a white powder (200 to 400 mesh); has a bulk
density of 20-50 lb/ft3; contains 82 98 percent Ca(OH)2; and costs approximately $55/
ton. It is slightly hydroscopic. Bagged hydrated lime is available, but costs $4-$16/
ton more than in bulk.
69
-------
Bagged lime should be stored indoors to prevent absorption of moisture. Cast
iron or mild steel is satisfactory construction material for piping and storage facilities
for both dry feeds and slurries. An efficient dust-collecting system is recommended
at lime-handling points.
Lime is normally fed as a water slurry containing 15-30 percent lime as CaO.
When large quantities are required, it is most economical to purchase quicklime and
to install a slaking unit. Slaking is the designation for the reaction of quicklime to hy-
drated lime, shown below:
CaO
Quicklime
H2O
water
Ca(OH)2
Slaked or Hydrated Lime
Heat is evolved as the reaction takes place.
Lime slaking equipment is of two general types, "paste-type" and "slurry-type."
Each system contains a mechanical quicklime feeder capable of flow variation and hav-
ing an accuracy rate within 1 percent; a slaking -water feed system and controls; slaking
compartments; and a section that removes the gnt and 'iirt from the quicklime. Slaking
reaction time is normally 5 minutes during continuous operation. Hydrated lime is con-
tinuously discharged. A lime slaker is shown in Figure -;>2, while the complete line feed
system is shown in Figure 23.
Polymers
Polymers used as coagulants in wastewater treatment can be either synthetic or
natural. They are composed of many small compounds (monomers), which can be
QUICKLIME^,
TORQUE CONTROLLED WATER VALVE
DILUTION CHAMBER
SLURRY DISCHARGE SECTION
GRIT DISCHARGE
PADDLES
SLAKING
COMPARTMENT
WATER FOR GRIT WASHING
CLASSIFIER
GRIT ELEVATOR
Figure 22. Lime Slaking Equipment
70
-------
,DUST COLLECTOR
/y*—FILL PIPE (PNEUMATIC)
SL
DUST COLLECTOR
WATER
SUPPLY
BULK STORAGE
BIN
DAY HOPPER
FOR DRY CHEMICAL
FROM BAGS OR DRUMS
FLEXIBLE
CONNECTION
ALTERNATE SUPPLIES DEPENDING
ON STORAGE
DUST AND VAPOR REMOVER
SCALE OR SAMPLE CHUTE
BAG FILL
SCREEN
WITH BREAKER
PRESSURE REDUCING
VALVE
GRAVITY TO
APPLICATION
WATER SUPPLY
HOLDING
TANK
c
_». PUMP
TO APPLICATION
Figure 23. Lime Feed System
identical to each other, or of different materials. Polymers are chains of monomers
whose number is varied by reaction to produce polymers with different molecular
weights; those used in wastewater treatment normally range from 100,000to 1,000,000.
Their prime function is to act as a "coagulant aid," a term applied to chemicals such
as lime that are added to wastewater, in addition to conventional coagulants, to im-
prove colloidal destabilization and flocculation.
The term "polyelectrolyte" is used to describe chemicals of charged groups in the
form of a polymer. The polyelectrolytes are further classified according to the type of
charge they will have in solution, as follows:
71
-------
Anionic Polyelectrolytes—negative charges
Cationic Polyelectrolytes— positive charges
Nonionic Polymers (Nonionic Polyelectrolytes)~neutral charge
The variety of monomers available as building materials for polymers is endless,
meaning that there is a large assortment of polyelectrolytes available for use in coag-
ulation. A partial listing, with physical properties, is provided in Table 14.
Polymers can be purchased dry or in liquid form. They are easily handled at the
plant site and are nonhazardous. The usual protection from dust is required, and the
storage facilities for the dry powders must be moisture-free. The materials of con-
struction for storage and handling equipment are normally stainless steel 316 and plas-
tics; selection is made on the basis of the polyelectrolyte chosen.
The coagulation mechanisms of polyeJectrolytes are associated with charge reduc-
tion of the colloids, which results in adsorption or enmeshment of the individual par-
ticles to form a settleable mass. Due to the size of the polymeric compound, the
polymers can attach themselves to the surfaces of the suspension at one or more sites.
The excess part of the polymer chain extends into the solution and adsorbs onto other
particles. The size of the floe being formed is generally restricted by the strength of
the attractive forces between the particles and those between the solids and the stream.
Polyelectrolytes are being xised extensively in coagulation processes because of
their versatility, range of properties, handling ease, and effect on coagulation rates.
Generally, cationic polyelectrolytes are used alone or as aids to other coagulants in
forming metallic salts. Anionic polymers are used as coagulant aids in colloidal de-
stabilization where positive charges are required to bridge the positively charged col-
loids. Nonionic polyelectrolytes are added to increase colloid concentration, thereby
aiding floe formation, and to increase floe size by attaching themselves to agglomerated
colloids.
Polyelectrolytes will:
• Increase the size and stability of the floes;
• Decrease dosages of conventional chemical coagulants, such as alum;
• Decrease floe formation times;
• Extend the effective range of coagulant dosage;
• Extend range of pH over which conventional coagulants are effective; and
• Increase suspended-solids removal efficiencies.
COAGULANT STORAGE AND HANDLING
Handling, storage, and feed-flow control facilities are as important as proper
selection of coagulants and determination of the coagulant dosage. As discussed
72
-------
Table 14
Dry Polyelectrolytes
Polyelecfrolyte
Aquafloc 409*
Aquafloc 41 1*
Aquafloc 414
Aquafloc 418
Aquarid 49-702
Calgon C-2256
Calgon C-2260
Calgon C-2270
Calgon C-2300
Calgon C-2325
Calgon C-2350
Calgon C-2400
Calgon C-2425
Calgon WT-2600
Calgon WT-2630
Calgon WT2660 (ST-260)
Calgon WT-2690
Calgon WT-2700
Calgon WT-2900
Calgon WT-3000
Hamaco 196*
Hercofloc 810
Hercofloc 812
Hercofloc 818
lonoc NA-710
Jaguar Plus
Magnifloc 530C
Magnifloc 820A
Magnifloc 835A
Magnifloc 836A
Magnifloc 837A
Magnifloc 865A
Magnifloc 870A
Magnifloc 875A
Magnifloc 880A
Magnifloc 900N
Magnifloc 901 N
Magnifloc 902N
Magnifloc 905N
Nalcolyte 610
Nalco 633-HD
Nalco 636-HD
Nalco 635
Nalco D-2339
Nalcolyre 675
Polymer F3
Polyfloc 1100
Polyfloc 1110
Polyfloc 1120
Polyfloc 1130
Polyfloc 1150
Polyfloc 1 160
Purifloc A-23
Superfloc 128
Tychem 8024*
Tychem 8013
Zero Floe C*
Zeta Floe O*
Zero Floe K (+KMnO2)*
Zeta Floe S
Zeta Floe WA*
Zero Flox WN
— '
Type
fable
3
AP
AP
NP
CP
CM
CP
CP
CP
NP
AP
AP
AP
AP
CP
CP
CP
NP
AP
AP
AP
S
CP
CP
AP
AP
CG
CP
AD
AD
AD
AD
AD
AD
AD
AD
ND
ND
ND
ND
P
CP
CP
AP
AP
AP
AG
AD
AD
AD
AD
AD
CD
CD
ND
ND
ND
BC1
BN1
BC1
BC1
BA1
BN1
Bulk Density
Ib/cu ft
Loose
25
42
48
33
30
24
10
10
11
10
16
23
29
27
9
10
8
16
20
22
21
30
38
31
34
30
27
28
40
32
27
26
42
45
38
39
35
32
34
36
36
35
33
34
42
28
33
48
52
48
54
54
54
Pack
54
53
59
44
43
35
16
16
19
16
28
34
42
39
16
18
13
29
25
31
31
40
47
40
40
42
35
36
42
40
35
40
68
53
50
53
50
40
40
42
42
48
45
40
53
33
43
68
74
68
78
78
78
Work
28
45
61
36
34
28
25
13
13
14
13
22
27
33
31
12
14
10
22
21
25
24
33
40
22
35
34
29
30
50
28
28
26
33
29
31
52
47
41
43
40
34
35
37
37
39
36
35
45
29
40
36
54
59
54
61
61
61
F low
table
4
CNKL
CNKL
CNKL
DPKL
E S
DLKP
DLKP
DIP
BIN
BIN
BIN
ALKM
BLKN
DKLP
DKLP
DKLP
ALM
DLP
AKLN
AKLN
DLP
EKLR
EKLR
DKLN
CNL
FLPR
CLP
DKP
CKP
DLP
CLN
CLN
CLN
DKLP
DJLN
DLN
DLN
DJLS
DLN
DLN
CKN
CKN
CKN
DKP
FJR
CJN
BKP
CLN
CLN
EKR
EJR
EKR
E S
E S
E S
Time to
disperse
into a
col 1 . solution
hour(s)
1-2
1-2
1-2
1-2
1-2
1/2
1/2
1/2
3/4-1
3/4-1
3/4-1
3/4-1
3/4-1
1/2
1/2
1/2
1/2
1/2
1/2
1/2
1/2-1
1-2
1-2
1-2
1/2-1
1-2
1-2
1/2-1
1/2-1
1/2-1
1 1/2-2
1/4
1/4
1-2
1-2
-2
1-2
1-2
1/2-1
1/4-1/2
1/4-1/2
3/4-1
1/2-1
1/2-3/4
-2
-2
-2
-2
-2
1/2-1
1/2-1
-2
-2
-2
-2
1/4-1/2
1/4-1/2
1/4-1/2
1/4-1/2
1/4-1/2
1/4-1/2
Solution— room temp.
Vise.
percent
1
2
2
2
1
1
1
0.5
0.5
0.25
0.25
0.25
1
1.5
0.5
0.5
0.25
0.25
0.25
1
0.5
0.5
0.5
0.2
1
1
0.1
0.1
0.5
1
0.5
0.5
0.5
1
1
1
1
0.5
2
1 .5
0.25
0.3
0.25
4
0.5
0.5
0.5
1
1
1
0.1
1
1.0
1 .0
cp
2350
480
660
23
75
40
35
24
275
425
740
800
38
20
20
23
80
160
250
<50
400
250
1000
ISO
800
160
450
360
620
130
38
38
20
150
300
200
500
700
190
33
2500
2000
2500
800
2000
2400
3500
3000
1000
1300
950
750
200
750
Sp gr
1
1
1
1
I
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
]
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
PH
4.5
4.5
7
2.4
7.9
6.4
6.7
7.3
7
7
7
7
7
4
4
4
7
7.5
7.5
7.5
<6-7
<6-7
<6-7
<8-9
5-6
8-9
4.2
6-7
6-7
5
4.1
7.5
7.5
5.5
4.7
4.7
4.7
4.7
5.1
3
8.5
7.0
8.5
9.5
8.5
7.5
8.8
8.8
6-7
5-6
10
6-7
5.5-6
6
8.2
8.2
8.5
Percent
max solution
concentration
recommended
1
2
2
2
1 .5
1 .5
1 .5
0.5
0.5
0.25
0.25
0.25
1 .5
1 .5
0.5
0.5
0.25
0.25
0.25
2
0.5
0.5
0.5
1
1
0.5
1
0.1
0.1
0.5
1
1
1
1
1
1
1
1
1
2
1 .5
0.25
0.5
0.25
4
0.5
0.5
0.5
0.5
1
1
0.5
1
1.0
1
1
1
1
1
1
1
'Approved by USPHS for potable water u<
^ Aluminum Silicate added.
(trom Reterence 121
73
-------
Table 14
Liquid Poly electrolytes
Pol/electrolyte
Aquafloc 403
Aquafloc 405
Aquafloc 407
Aquafloc 408*
Aquafloc 410
Aquafloc 412
Aquafloc 415
Aquarid 49-700
Aquarid 49-701
Aquarid 49-703
Cat-Floe (WT-2870)*
Magnifloc 521-C
Nalcolyre 603
Nalcolyte 607
Natron 86, 18%
Polyfloc 1170
Polyfloc 1175
Type
table
o
AH
CH
NH
AH
CH
CHi
AH
CM
CM
AH2
CH3
CH
CH
CH
CH
H
H
Solution
strength
percent
Full
Full
Full
Full
Full
Full
Full
Full
Full
Full
Full
Full
Full
Full
Full
Full
Full
Sp gr
room temp .
(about)
1 .12
1 .06
.00
.01
.03
.25
.03
.1
.1
.2
.025
.15
.6
.17
1 .06
1 .06
1 .08
Viscosity— cp
room temp .
100
1,000
10,000
1,500
1,000
50
1,000
100-500
25-150
200-500
2,000
225-325
80
50
300-500
50
700
PH
(about)
9.6
6.3
4.3
3.4
10.2
1 .0
11 .5
7.8
7.8
11 .5
4.2
4.5
8
7.5
3
9
9
Dilution
4.1
10.1
10.1
10.1
10.1
4.1
10.1
10.1
10.1
20.1
<10.1
<10.1
<10.1
<10.1
< 0.5
"Approved by USPHS for potable water use
Plus a primary coagulant
^A linear homopolymer of diallyldimethyl ammonium chloride
^Polyaromatic.
TYPES
A - Anionic
As - Slightly Anionic
B - Bentonite Clay or Clay, natural, col loidal-like type
b - plus Bentonite
C - Cationic
D - Polyacrylamide , Synthetic, HighM.W., Polyelectrolyte Polymer
E - Polyacrylonitrile, Synthetic Polyelectrolyte
F - Sulfonated Polymer
G - Guar Gum, Polysaccharide, Natural Polymer
H - High M.W., Organic Polymer
j - Alkyl Guanidineamine Complex
K - Sodium Alginate or Algin Derivative, Natural Polymer
L - Loguminous Seed Derivative, Natural Polymer
M - Polyamine, Synthetic, HighM.W., Polyelectrolyte Polymer
N - Nonionic
P - Synthetic High M .W ., Polyelectrolyte Polymer
R - Polyacrylamide and Carboxylic Group
S - Starch, derivative, modified, etc., Natural Polymer
T - Synthetic Polymer and Caustic Soda
U - Sodium Carboxymethylcellulose, Natural Polymer
X - Efhylene Oxide Polymer
Y - Carboxyl Polymer
2 - Biocolloid -*- Inorganic Coagulant + Caustic Soda
3 - Hydrophylic Colloid + Pregelatinized Starch in Alkalai
4 - Aluminum Hydroxide + Complex Organic Polymer
5 - Alumina + Polymer + Caustic Soda
6 - Polyacrylic Acid or Polyacrylate of Sodium or Ammonium
7 - Aluminum Hydrate + Caustic Soda
8 - Alkalai Concentrate + Metallic Ions
9 - Chemically Modified Natural Polymer
74
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Table 14
Liquid Polyelectrolytes (continued)
FLOW
A. Soft flakes, may hang up if packed excessively in a confining area, otherwise free flowing. Usually
will not need aid (vibration or agitation).
B. Powdered, soft flakes, hang up if packed excessively in a confining area, may or may not need aid
according to rate of feed, etc.
C. Soft granules, sometimes fibrous or flatfish, may hang up if packed excessively in a confining area,
otherwise free flowing. Usually will not need aid.
D. Powdered, soft granules, sometimes fibrous or flatfish, hang up if packed excessively, may or may
not need aid, according to other factors.
E. Granular, fluid powder, will arch if packed and can be fluidized or is floodable (to very floodable).
Needs aid and may need rotor, according to rate, etc.
F. Granules and powder, will arch and can be fluidized . Needs aid and could need rotor, etc .
G . Cohesive powder and granules, will arch, but will not flood . Needs aid .
H. Cake at room relative humidity.
J. Tendency to cake (or mass) at higher relative humidity.
K. Cake at higher relative humidity.
L. Moisture absorption, may lessen flowability .
M. Practically no dust.
N . Very little dust.
P . Some dust.
R. Dusty.
S . Very dusty .
TRADE NAMES
Aquarid - Reichhold Chemicals
Calgon C - Calgon Corp.
Calgon WT - Calgon Corp.
Cat-Floe - Calgon Corp.
Hamco - A. E. Staley Manufacturing Co.
Hercofloc - Hercules, Inc.
lonacNA-710 - lonac Chemical
Natron - National Starch and Chemical Corp.
Polyfloc - Betz Laboratories, Inc.
Polymer F3 - Stein-Hall
Tychem - Standard Brands Chem. Ind., Inc.
Zeto Floe - Norvon Mining and Chemical Co.
75
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earlier, the wrong coagulant dosage can adversely affect the settling and flocculation
rates of suspended solids by causing the colloidal suspension to restabilize. Low co-
agulant flow rates have similar effects.
A typical solids handling system identical to the line feed system is shown in Fig-
ure 23. In general, the solids are fed as a solution or slurry into the waste treatment
plant at chemical mixing sections. The system includes a storage bin for the solids,
a solids conveyor to transport the material to the dissolver, a dissolver tank equipped
with a mechanical agitator, water-feed system and control, and a hold tank. If the
chemical is fed dry, the coagulant enters the treatment system from the feeder. The
piping system, which includes pumps, instrumentation, and controls, is designed ac-
cording to process conditions and physical properties of the fluids. A typical liquid
handling system is shown in Figure 24.
Bins
Storage bins are available in numerous sizes, shapes, and materials. In general,
the bin should be large enough to hold 30-45 days of feed and have a closed top and pro-
visions for dust collection, air drying, and possibly fluidization or bin vibrators. The
slope on the bin outlet is normally 55°-60° to permit free flow into the feeder.
Solids Feeders
Characteristics of solids vary greatly. Some are granular; others are pellets or
powders. Some of them tend to break apart and upset the feed-rate control in volumet-
ric units. Thus, selection of a feeder must be considered carefully, particularly in
smaller facilities where a single feeder may be used for more than one chemical.
Generally, provisions should be made to keep all chemicals cool and dry; other-
wise, they may cause problems in feeding equipment. Dryness is especially important,
as water-absorbing (hydroscopic) chemicals may become lumpy, viscous, or rock-hard;
chemicals with lower affinity for water may become sticky from moisture on the par-
ticulate surfaces, causing increased arching in hoppers. In either case, this will affect
the density of the chemical and may result in inefficient coagulant flow into the treat-
ment plant, since the measured weight or volume is increased by the added water.
The simplest method for feeding solid chemicals is by hand. Chemicals may be
preweighed or simply shoveled or poured by the bagful into a dissolving tank. This
method is limited to very small operations, or to chemicals used in very weak solutions.
Volumetric feeding of solids is normally restricted to small plants, to chemicals
that are constant in composition and relatively unaffected by handling and storage con-
ditions, and to low feed-rate requirements. Within these restrictions, several types
of volumetric feeders are available, examples of which are shown in Figure 25-a and
b. Feeding accuracy is usually less than 97 percent.
The volumetric dry feeder in Figure 25-a uses a continuous belt of specific width
from under the hopper to the dissolving tank. A mechanical gate mechanism regulates
76
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TRUCK FILL LINE
DILUTION
WATER
SODIUM HYDROXIDE
STORAGE TANK
VENT, OVERFLOW
AND DRAIN
VENT, OVERFLOW
AND DRAIN
MIXER
SAMPLE TAP
SODIUM HYDROXIDE
FEEDER
POINT OF
APPLICATION
Figure 24. Liquid Handling System
the depth of the material on the belt. The feed rate is governed by the speed of the belt
and/or the height of the gate opening. The storage hopper is normally equipped with a
vibratory mechanism to reduce arching and to permit a uniform, continuous flow of
solids to the feeder. This type of feeder is not well-suited to easily fluidized materials.
The type of feeder shown in Figure 25-b employs a screw or helix running from the
bottom of the hopper through a tube opening slightly larger than the diameter of the
screw or helix. Rate of feed is governed by the speed of the screw or helix rotation.
Some screw-type designs are self-cleaning; those that are not are subject to clogging.
77
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MOTOR
GEAR REDUCER
FEED RATE REGISTER AND.
FEED ADJUSTING KNOB
SOLUTION CHAMBER
HOPPER
ROTATING AND
RECIPROCATING
FEED SCREW
JET MIXER
Figure 25. Typical Solids Feed Equipment
78
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Most remaining types of volumetric feeders fall into the positive-displacement
category. These designs, not shown, are normally used with powders or granular
(nondusting) chemicals. All designs are based on some form of moving cavity, which
can be either constant or variable in size. A unique design is the nonreciprocating
progressive cavity metering pump. In all positive-displacement feeders, the chemical
falls into the cavity, where it is more-or-less fully enclosed and separated from the
hopper's feedline. The flow rate of chemicals is governed by the size of the cavity and
the rate at which it moves and discharges. The positive-displacement control of the
chemical feed rate will have an irreducible lower limit. To assure accuracy and con-
sistency, these feeder units must be selected with the lowest and highest flow rates
within 20-80 percent of the total band. Positive-displacement feeders often use air in-
jection to enhance the flow of the material.
The basic drawback of volumetric feeders is their inability to compensate for
changes in density of materials. This problem is overcome by modifying the design to
include a gravimetric, or loss-in-weight, controller that allows for weighing of the
material as it is fed. Beam-balance weight controllers measure the actual mass. They
are considerably more accurate, over a long period, than the less-common, spring-
loaded gravimetric designs. Gravimetric feeders are used where feed accuracy of
about 99 percent is required for economy, as with very expensive chemicals or high
flow rates. This makes them applicable to large-scale operations or where materials
must be used in small, precise quantities to assure satisfactory coagulant or chemical-
pretreatment performance. It should be noted, however, that even gravimetric feeders
cannot compensate for weight added to the chemical by excess moisture.
Many volumetric feeders can be converted to loss-in-weight controllers by placing
the entire feeder on a platform scale adjusted to neutralize the weight of the feeder.
The volumetric flow is checked by calculating the weight loss over a specified period
and dividing that rate by the specific volume of the chemical.
Liquid Feeders
Liquid feeders are generally a type of metering pump which offers the simplest
control method, ?Uhough the most expensive. These pumps are of the positive-
displacement variety, either plunger or diaphragm type, with a variable-speed drive.
The choice of liquid feeder is dependent on the conditions of the process; on chemical
properties such as solubility; on suction and discharge heads; and on internal pressure-
relief requirements. Once the correct drive speed is selected, the flow rate will re-
main constant and is usually not affected by changes in downstream pressure. The
pumps can develop pressures exceeding several hundred pounds, making relief valves
necessary to prevent damage to equipment such as sight-flow indicators. A high-
pressure alarm with automatic shutoff is advisable where plugging of lines may occur.
If possible, valves should be eliminated from the discharge lines to prevent damage to
equipment if the valve is inadvertently closed. Although the pumps are calibrated, it
is necessary to recalibrate them over the adjustable ranges to check accuracy and re-
producibility of the flow rates. Examples of liquid feeders are shown in Figure 26-a
and b.
79
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Plunger coupling, self-aligning
Plunger, superfinished
y*>A>£
Throat and follower bushings
Bleed i/alve, pressure operated
Positive packless rod seal
Corrugated one-piece
/•' I "^V
TFE process diaphragm VX/ I il , v
Figure 26. Common Metering Pumps
80
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In some cases, control valves and rotameters with standard centrifugal pumps
may be all that is required for liquid feeding. The flow rate is maintained by throttling
back on the centrifugal pump discharge until the rotameter reading is satisfactory.
Normally the rotameter, which cannot be used with highly viscous fluids or fluids con-
taining solids, should be calibrated using the actual fluid. In other cases, such as
lime slurry feeding, centrifugal pumps with open impellers are used with a metering
device; this can be a scale tank with a positive displacement meter.
Solids Conveyors
In most cases, the solids feeder can act as the conveyor, as well as the flow con-
troller, for moving solids from the bulk storage tank to the dissolver. If the distance
between the two vessels is large, however, a conveyor will be needed, and a feeder,
used before the conveyor, will provide the correct flow rate for chemicals. The con-
veyor can be one of several types, all familiar to members of the metal products in-
dustry: the carrying type (using belts, vibrating pans, or buckets); the drag or push
type (using a screw or helix); or the elevator type (using buckets).
Dissolvers
Most chemicals added to waste streams for pretreatment must be mixed with water
for the desired strength. Thus, most feeders, regardless of type, discharge their ma-
terial to a small dissolving tank equipped with a nozzle system and/or mechanical agi-
tator, depending on the solubility of the chemical. Insoluble materials such as poly-
electrolytes may be spread out and subjected to a vortex spray or washdown jet of water
just before they enter the dissolver. It is essential that the surface of each particle be
thoroughly wet before the material enters the feed tank, to ensure dispersal and avoid
clumping, settling, or floating.
A dissolver for a dry chemical feeder cannot be adjusted to vary its speed or ca-
pacity and must be designed for the specific job; one suitable for a rate of 10 Ib/hr may
not be capable of dissolving at 100 Ib/hr. As a general rule, however, dissolvers may
have excess capacity, except those for commercial ferric sulfate or lime slakers,
which do not perform well if oversized.
COAGULATION EQUIPMENT
Once the laboratory tests for suspended solids removal have been conducted and
the best coagulants and coagulant feed systems selected, the next step is to select the
hardware for coagulation. The laboratory tests have confirmed that the solids can be
settled at a reasonable rate by:
• Adjusting the pH (neutralization)
• Adding coagulants (flash mixing)
• Permitting the treated solids to form a settleable mass (flocculation)
81
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The flash-mixing and flocculation steps constitute the coagulation process that
prepares for solids removal by gravity settling. This section describes the basic
equipment for the coagulation step.
CHEMICAL (FLASH) MIXING EQUIPMENT
Chemical mixing is the step in which the coagulant is added to the waste stream
and thoroughly dispersed by mechanical or baffled action. This mixing promotes suf-
ficient contact between the coagulant and the colloids and suspended solids so that the
coagulant can be adsorbed and a precipitate formed.
The mixing is normally by mechanical agitation. The intensity and duration of the
mixing must be controlled; overmixing or excessive agitation can break up colloidal
masses or settled solids already present in the stream, while overly slow agitation
will not adequately disperse the coagulant. Since the reaction rates are extremely
fast—a fraction of a second—the mixing equipment designed for minimum residence
time and for the maximum mixing speed at which there will be adequate dispersion
without fractionation of the solids. Detention times vary with the type of stream, but
normally range between 5-30 seconds. Typical chemical mixing patterns produced by
various mixing blade designs are shown in Figure 27.
r— Baffles
Side View
Baffles
s
Side View
Side View
Bottom View
Bottom View
Bottom View
Figure 27. Typical Chemical Mixing Patterns
82
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FLOCCULATION EQUIPMENT
After dispersion and destabilization of the coagulants in the mixing stage, floccu-
lation of the destabilized colloidal suspension is induced, increasing the natural col-
lision rates of particles. This is important, since the rate of floe formation is deter-
mined by the rate at which collisions occur, as is the binding strength between the
colloidal particles and the coagulant. Basically, the same parameters apply as for
chemical mixing, except that retention times are longer, mixing is less turbulent, and
the equipment is more exotic.
For new applications, pilot tests should be conducted on the plant wastewater to
determine the residence time and mixing speed. The flocculator is then designed to
provide the experimental residence time and velocity gradient as well as to include
design safety factors for conditions that can affect flocculation rates, floe mass, and
settling velocities.
Flocculation equipment varies, but is generally of two types: mechanically mixed
and baffled units (Figure 28). Figure 28-b illustrates typical baffled basins. These
units have low operating and maintenance costs, but high space requirements. The
mechanical flocculators (Figure 28-a) are similar to the chemical mixing tanks, but
are much larger and require larger agitators operating at lower speeds. They nor-
mally have variable-speed mixing drives.
Retention times in the flocculators vary from less than a minute to nearly an hour.
If the flocculation time is short, chemical mixing and flocculation can take place in the
same mixing tank.
THE SEDIMENTATION PROCESS
After the suspended solids have been destabilized by coagulation (chemical mixing
and flocculation), they are normally removed from the waste stream by sedimentation.
Sedimentation hardware is designed to create the quiescent conditions necessary for
settling. Because of their relative simplicity, sedimentation units are usually the
most economic method for solids removal from high-volume flows.
This section describes the physical mechanisms affecting the settling of suspended
solids, since these are the principles upon which the equipment is designed. The dif-
ferent types of sedimentation hardware, and their costs, will also be discussed. This
information should enable the reader to participate more fully with the consultant or
vendor in choosing the most effective equipment at the lowest operating and capital
costs.
BASIC OBJECTIVES
The basic objectives for the sedimentation process are clarification or thickening;
the hardware for the two functions are clarifiers and thickeners.
83
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CONTROL
VALVE
INFLUENT
1
1
1
1
1
1
1
^
1
1
1
1
1
^.
r
>
>
^
r
j
~\
v.
j
EFFLUENT
PLAN
|
r
!
r
s
^ 77777777
-\
\.
r
J
•\
^
f
y
>
<
—
s
—
~~
~~
—
~~~
D"-*l
DEPTH OF
FLOW-D
1
T
SECTION
b
Figure 28. Flocculation Equipment
Confusion over the terms is common. Figure 29, which illustrates the sequence
of clarification and thickening processes, will help to explain the difference. Clarifiers
are used to produce a clear liquid discharge or effluent. The feed to the clarifier is
very dilute and is normally received directly from the coagulation step. A clarifier
gives a clear overflow stream, but will not produce a bottoms or sludge with a solids
content much above 5 percent.
Since it is costly to dispose of a volume of water along with the solids, the bottoms
discharge from the clarifier is often fed to a thickener to increase the solids concen-
tration to a range of 5-10 percent. The liquid effluent from a thickener will not be as
low in suspended solids as the overflow from a clarifier, since the feed is concentrated
and the settling mechanisms are more susceptible to upsets. If a clear effluent is re-
quired, the dilute overflow from the thickener is recycled to the clarifier. Therefore,
84
-------
Wastestream
Chemical Mixing
Flocculation
Clarification
Clear
~^~ Effluent
Solids
Effluent
(Recycle)
Thickener
Concentrated
Solids
Figure 29. Typical Wastewater Treatment System
clarifiers and thickeners are often used together when both high solids content and a
clear effluent must be obtained. Since the thickener handles solids whose settling ve-
locities have already been maximized by coagulation, the coagulation step is rarely
used again prior to thickening.
TYPES OF SETTLING
There are four basic zones through which a particle normally passes to its final
disposition from a thickener. Basically, as the particles settle in the wastewater, the
solution concentrates and further settling is restricted by the mass density of the sus-
pended solids in solution. The requirements for each zone must be factored into the
85
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design of the hardware. The four basic settling zones, or classifications, have been
designated as:
Class 1: Free settling from a dilute suspension
Class 2: Hindered settling from a dilute suspension
Class 3: Zone settling
Class 4: Compression settling
It is common to have one or more of these settling actions taking place at once.
Some of the particles will reach each stage before the rest of the mass; this depends
mainly on the density of the particle. The design of sedimentation hardware is based
on observations of each stage; this data is critical to the sizing specifications for each
sedimentation process. Figure 30 shows a cross section of a typical settling system,
as observed in a cylinder. The different settling zones also indicate the degree of con-
centration from top to bottom. The diagram in the figure indicates the time it takes for
the particles to sink from one zone to the next. It is on this relationship between time
and concentration that the design of sedimentation equipment is based.
A general description follows of each settling classification and its relationship to
the design of sedimentation equipment.
Class 1: Free Settling
This process is similar to grit removal, discussed earlier. The particles settle
independently, without interacting with other particles in the dilute suspension and at a
rate directly related to their size, their density, and the density of the fluid.
Although free settling is the simplest of the settling classifications, equipment de-
sign for this process is complicated by the fact that the sedimentation tank must take
into account the flow rate through the treatment system. Consideration must also be
given to the overflow concentration to be achieved. If a very clear overflow is desired,
retention time must be increased. Figure 31 gives some indication of the effects of the
flow of the liquid through the system on the settling velocities of the particles. The
particle is never settled out of solution until it reaches the bottom of the tank. The
settler has an outlet zone where the turbulence is great enough that the settling rate of
the particle is overcome by the change in velocity and direction. When the particles
reach the settling zone, their settling velocity and direction are affected by the horizon-
tal velocity of the liquid passing through the settling zone. Therefore, they follow a
somewhat diagonal path. If the velocity of the particle is fast enough to permit it to
settle before the outlet zone, it is considered to be settled out of solution. If its set-
tling velocity is slow enough that it arrives at the outlet zone without reaching the bot-
tom of the settler, it will be entrained and carried into the overflow.
Good discussions on the use of settling velocities and laboratory experiments in de-
termining dimensions of a settling basin can be found in References 8 and 12. For our
purposes here, it is important to define two of the basic parameters in the design of
sedimentation processes:
86
-------
Class 1
Class 2
oo
Class 3
Class 4
Clear
water
Free
settling
Hindered
settling
Zone
settling
Compression
region
D)
'clJ
I
Clarification
Thickening
HO
h= height of zone
t=time of settling
Cc= Compression area
Cf|= Depth of settled particles
Time
Figure 30. Sedimentation System Showing Various Zones of Settling
-------
Inlet
zone
Vs= Vertical component of the particle velocity (settling velocity) I
Vn= horizontal component of the particle velocity I
Vj= terminal velocity J
Outlet
Settling tone
Figure 31. Settling Velocities in an Ideal Sedimentation Basin
• Terminal Velocity. As shown in Figure 31, terminal velocity (vt) is that veloc-
ity at which particles with settling velocities greater than vt settle out from
wastewater.
• Surface Hydraulic Loading. This term is related to both the total residence
time (expressed as the ratio of the flow rate in gallons/day to the horizontal
area of the sedimentation unit in square feet). Since a particle is not settled
out of solution until it reaches the bottom of the sedimentation unit, the depth
of the tank is important in determining the residence time for the particles.
Unfortunately for the designer, free settling without particle interaction is rare in
practice; some interaction normally occurs. Further, settling conditions are never
perfectly quiescent, and the design of the system must take into account fluid flow, di-
rectional changes, partial settling rates, and fluid turbulence. Since it is difficult to
relate these complicating factors to laboratory procedures, the designer normally be-
gins with the procedures and formulas discussed here for Class I settling and further
explained in Reference 8; he then applies various scale-up factors, along with data
from previous commercial installations, to allow for the difference between laboratory
tests and the normal in-plant efficiencies.
Class 2: Hindered Settling from a Dilute Suspension
As the particles settle under quiescent conditions, the density of their mass be-
comes greater, which will sometimes permit further flocculation during the settling.
Turbulence in the settler may have the same effect. Hindered settling then occurs as
larger, faster-settling particles overcome slower ones. The extent of this additional
88
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flocculation and interaction is dependent on the coagulation process and the depth of
the tank (which permits greater contact among particles). As was shown in Figure 30,
hindered settling causes a slight decrease in the settling rate.
Hindered settling is a more complicated consideration in the design of a sedimen-
tation process than free settling. To overcome the lack of mathematical models for
design of the hardware, laboratory settling tests are performed in columns of the same
height as those envisioned for the equipment. As the solids settle within the columns,
samples are extracted at intervals and the percent solids determined at each port loca-
tion along the column. When the desired concentration for the overflow is achieved,
the settling time and the depth of the sediment are noted; the sedimentation basin area,
residence time, and depth are then determined as for free settling. Detailed examples
of how hindered-settling data are used in designing sedimentation equipment are pre-
sented in References 8 and 14.
The first two settling classifications are normally used to design clarification sys-
tems. The last two classifications, discussed below, are used in designing thickeners
for the concentration of solids.
Class 3: Zone Settling
Zone settling involves the movement of a mass of particles that have coagulated
during the flocculation in the hindered zone. The density of the particles is much
greater than in the previous two zones. The solids settle as a consolidated mass, and
a sharp boundary exists between this mass and the clarified liquid. All particles set-
tle at the same rate. However, as shown in Figure 30, this rate sharply decreases as
the particles try to settle further in the solution.
The designer uses laboratory settling tests very similar to those used for free-
settling sedimentation equipment. However, with zone settling the procedures for cal-
culating the design parameters are more complicated because the settling rate is not
linear. In addition, the size is not based on a clear overflow, but on the desired final
concentration of the sludge. Once the residence time and concentration have been re-
lated to the settling curve, the designer again applies the basic principles of free set-
tling to determine the overall dimensions for the thickener. These laboratory testing
procedures are detailed in References 8 and 12.
Class 4: Compression Settling
The final settling mechanism involves compaction of the settled solids. This phe-
nomenon is dependent upon the retention time and on the compression depth of both the
solids layer and the liquid above it. The compression is resisted by each layer of
solids, which prevents the water held within the compression area from escaping. In
order to break up the different layers of solids in the compression zone, the designer
may install mechanisms for slowly stirring the settled mass. As was shown in Figure
30, the rate of compression is very slow as compared to the other sedimentation pro-
cesses. For this reason, compression of solids by gravity settling may become un-
economical, since the size of the sediment eventually becomes prohibitive. In most
89
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cases, compression will be rejected in favor of other methods for further concentrat-
ing the sludge.
FACTORS AFFECTING SEDIMENTATION EFFICIENCIES
In addition to determining the settling characteristics of the wastewater and using
various scale-up and design factors for applying laboratory data to plant practice, it is
also important to consider factors that alter the settling efficiency obtained under qui-
escent laboratory conditions. These considerations include short circuiting, turbu-
lence, distortion patterns, and scour velocity, discussed in detail in Reference 14.
SEDIMENTATION EQUIPMENT
The two basic functions of sedimentation hardware are clarification (obtaining a
clear overflow) and thickening (concentrating the solids underflow).
Sedimentation equipment contains four zones as shown in Figure 32: (1) the inlet
zone, which distributes the suspension over the cross sectional area of the basin;
(2) the effective-settling zone, where the majority of the settling occurs; (3) the solids
removal or sludge zone, where the solids are stored; and (4) the outlet zone, where
the clarified wastewater is collected and discharged. In each of these zones, fluid
patterns develop, and it is these flow effects that must be controlled for effective re-
moval of solids. Figure 33 shows theoretical settling areas for circular and rectangu-
lar basins.
Suspended solids gravity-settling equipment can assume a variety of shapes, sizes,
and designs. Again, the specific design of the equipment and the selection of hardware
Inlet
zone
Outlet
zone
T
Effective settling zone
* i M u t "*:
Solids removal zone
t
Figure 32. Four Zones of a Sedimentation Basin
90
-------
\ \ \
Effective settling area
\\ \ \
Sec A-A Side View
Sec A-A Side View
s
Inlet
zone
1
Effective settling area
Figure 33. Effective Settling Area for Circular and Rectangular Basins
must be based on laboratory tests of each plant's waste stream and the application of
design scale-up factors by experienced designers. Generally, sedimentation equip-
ment will include chemical mixers, flocculators, and a gravity settler. Class 1, or
free-particle settling, will normally include only the gravity settler, although free-
particle settling is usually not encountered except in grit removal.
This section will discuss the types of sedimentation equipment that include the
chemical-mixing and flocculation hardware as integral parts. Two general types will
be covered: units with circular and rectangular basins, and tubular and plate high-rate
settling devices.
91
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Sedimentation Basins
As noted, the sedimentation basin is divided into four zones: inlet, outlet, settling,
and sludge storage. The inlet zone must dissipate the velocity of the feed and provide
a uniform distribution of the flow into the settling zone without excessive turbulence.
The outlet zone is less critical to performance, but low-approach velocities to effluent
weirs must be maintained for maximum sedimentation efficiency.
The rectangular basin shown in Figure 31 illustrates a situation in which a perfect
inlet distribution of discrete particles occurs, with no particle interaction and no re-
suspension of particles from the sludge. In practice, uniform flow has proved unattain-
able in large tanks because of the complexities of each sedimentation phase and the
variance from ideal conditions.
The sludge zone serves the dual purpose of retaining the solids so that a minimum
of resuspension occurs and providing sufficient time for thickening.
The main factor in basin design is the overflow rate, expressed by dividing the
flow (gal/day) by the surface area (ft2) of the basin. The graph in Figure 34 relates
daily flow, surface area, and overflow rate for a rectangular sedimentation basin.
Although removal rates from basins will vary, there is generally a straight-line
inverse relationship between overflow rate and suspended solids removal. As the over-
flow increases, the solids removal decreases.
5,000 -
4.000
"Sr 3,000
2,000
1,000
15 gpm/ft2 overflow rate
3 gpm/ft2 overflow rate
5 gpm/f(2 overflow rate
I
I
I
1
8
0
2 46
Flow rate - millions of gallons per day
Figure 34. Relation of Overflow Rates to Surface Area and Flow Rate for Rectangular Basins
92
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Center-Feed Circular Clarifiers
In center-feed basin clarifiers (Figure 35), the influent flows up a central column
and enters a feed well where the inlet velocity head is dissipated and the flow equally
distributed. The flow out of the well is downward into the basin. Gravity separation
occurs in the settling zone due to differences in specific gravities and densities between
the wastes and the water. Additional flocculation of the pretreated wastestream occurs
in the feed inlet and settling zones due to flow patterns. Solids settle to the bottom of
the basin to form the sludge, while clear effluent flows over a weir around the circum-
ference of the tank. Figure 36-a illustrates flow patterns in the center-feed clarifier.
Continuous sludge removal is carried out by equipment supported on beams span-
ning the tank. The sludge is moved to the center of the basin by rotating arms, equipped
with scrapers and usually adjustable to any bottom slope. (Lightweight sludges may use
suction mechanisms on the arms for removal.)
Peripheral-Feed Circular Clarifiers
Figure 36-b illustrates the flow pattern of peripheral-feed clarifiers. Nozzles or
orifices are arranged around the inner wall of the basin to distribute the influent flow
evenly. Their operation and configuration is similar to the center-feed types.
Many clarifiers are capable of combining the processes of mixing, flocculation,
solids separation, and sludge removal in a single unit (Figure 37). Chambers added
-DRIVE
COLLECTOR ARM
•5
f U- INFLUENT
WELL
SKIMMER ^
f — ™iik
•fl
EFFLUENT
Figure 35. Center-Feed Circular Clarifier
93
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Inlet center
well
Sludge removal
Influent feed pipe
a) Center feed
Effluent
Sludge removal
b) Peripheral feed
Figure 36. Circular Basin Flow Patterns
to the basin increase reaction among the chemicals and suspended solids. The influent
enters a central draft tube, where it is mixed with treatment chemicals. Sludge and
raw water rise through the tube and are discharged to the flocculation compartment.
Most of the water and suspended solids enter the lower end of the draft to be recircu-
lated, while part of the water enters the clarified water area. Sludge is removed as
in other clarifier systems.
94
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DRIVE
EFFLUENT
• INFLUENT
Figure 37. Combination of Processes within a Clarifier
Rectangular Basins
Rectangular clarifiers depend on a straight flow pattern to increase flocculation
and lessen retention time. Most basins are designed with a standard length-to-width
ratio of 3:1 or greater, which provides a larger effective settling zone and more
closely resembles the idealized settling basin. Figure 38 illustrates a common rec-
tangular clarifier.
Sludge removal is accomplished in approximately the same way in both circular
and rectangular clarifiers; however, the rectangular basin employs a slowly moving
"bridge" spanning the basin, with submerged scrapers to push the sludge to a collec-
tion end. Reversing the direction of the bridge can cause reentrainment of the sludge;
to avoid this, some rectangular basins use an endless conveyor with cross pieces, or
flights, extending the width of the tank to move the sludge. Rectangular basins are
generally more expensive than circular basins.
95
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Influent
mixing chamber
Surface skimmer blade
Scum trough
Traveling bridge and traction drive
Effluent weir plate
7
Sludge drawoff pipe
Sludge well
Sludge scraper blade
Figure 38. Rectangular Clarif ier Basin
Effluent pipe
Thickeners
Sludge volumes generated in the clarifiers are usually less than 1 percent of the
total flow, and solids concentrations are often only 2-5 percent, depending on the waste
stream. Therefore, thickeners are commonly used to increase solids concentration.
The equipment is very similar to the clarifiers.
A continuous gravity thickener is normally circular, consisting of two truss-type
steel scraper arms mounted on a hollow shaft. A truss-type bridge is fastened to the
tank walls, or in some cases to steel columns that span the tank and support the entire
mechanism.
96
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Often, the thickener is incorporated into the clarifier by enlarging the sludge zone,
as in Figure 39. The settled solids are very gently agitated by a moving rake, which
dislodges gas bubbles, keeps the sludge moving toward the center well, and aids com-
paction. The thickened solids are removed at the center well.
Two of the most important design criteria for a thickener are the solids concen-
tration and the volume for the solids storage area.. This volume is calculated by de-
termining the retention time necessary for the desired buildup of solids. To calculate
the residence time, the solids-volume ratio is determined as the volume of the thick-
ened solids (dependent upon concentration and density) in the basin divided by the daily
volume of thickened solids pumped from the thickener. This relationship is expressed
in units of days and is used as a measure of the average retention time of thickened
solids.
In some cases, sludge thickening may simply involve pumping the sludge from the
clarifier to an appropriate holding tank where, after further detention and compaction,
residual water can be decanted and the sludge removed.
HIGH-RATE SETTLERS (CLARIFIERS)
A particle is settled out of solution when it touches the bottom of the settler. The
shorter the distance it must travel (excluding wind factors, scouring velocities, etc.),
the better.
However, the basin settlers need significant depth to eliminate or retard turbu-
lence effects and to reduce velocities that cause reentrainment. High-rate settlers
have recently been developed to reduce this distance, significantly cutting the time it
takes a particle to reach the settling surface. The types of high-rate settlers used in
Circular Clarifier
Sludge thickening
pickets
Scraper blades
Sludge drawoff
Sludge pit
Figure 39. Clarifier with Enlarged Sludge Zone for Thickening
97
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wastewater treatment are tube and plate settlers. They are noted not only for their
increased settling speed, but also for overall compactness. Their disadvantages are
related to their construction materials; solids may sometimes stick on the settling
surfaces. Therefore, high-rate settlers must be evaluated in terms of the solids
properties and the consistency of the stream.
Tube Settlers
In the tube-settler system, wastewater is clarified by sedimentation as it moves
upward from an entry point through modules of small, steeply inclined (45°-60°) tubes
and into a collection area. Solids settle on the bottom of the tubes, slide downward,
and exit at the end of the tubes (Figure 40).
Tube and plate settlers are highly adaptable to sedimentation processes because
they increase the solids-settling surface and reduce the settling distance from more
than 6 ft, normal in a conventional basin, to 1-2 inches.
A standard tube module may be constructed of 2 x 2 in. channels, 30 in. wide,
20 in. deep, and 10 ft long (actual dimensions must be determined by laborating settling
tests). Such a module provides 300 ft2 of settling area and creates a water surface
area of only 25 ft2.
INLET CONNECTIONI^
, TUBE CLARIFICATION
COLLECTOR
CLARIFIED EFFUENT
FLOCCULATOR-
TO SLUDGE
DISPOSAL
FLOCCULATOR TUBE CLARIFIES
Figure 40. Tube Settler
SLUDGE
SIPHON
98
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Plate Settlers
Utilizing a similar principle, plate separators, shown in Figure 41, have a series
of parallel, inclined plates. In this system, the effective settling area becomes the
area of each plate projected on a horizontal surface, approximately 10 ft2 of settling
area is available for each ft2 of floor space taken by a 1 ft2, 10-plate unit.
INFLUENT
EFFLUENT BOX
EFFLUENT
RETURN
TUBES
LAMELLA
PLATE PACK
EFFLUENT
SLUDGE
Figure 41. Plate Separator
99
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The waste stream is introduced into the feed box of the separator and distributed
evenly onto the plates through nozzles on the side. The flow proceeds upward and exits
at the top of the separator, while the solids settle onto the plate surfaces and slide down
into a sludge hopper for removal.
Unlike the tube-settler process, the solids do not have to settle through the feed
flow. Typical plate modules consist of perhaps 10-100 plates, 4 ft wide and 3 ft long,
spaced 6-12 in. apart and inclined at 25°-40° angles. (Precise dimensions are set, as
for other equipment, by laboratory settling tests on the waste stream.)
Several manufacturers have provided solids-thickening equipment with tube and
plate settlers. This is usually an additional chamber, or hopper, beneath the tube or
plate module. The chamber contains a rotating set of pickets (Figure 42) to gently stir
the solids, opening channels for water to escape in much the same manner as in a basin
thickener. Settlers lacking such equipment must be used with conventional thickeners
for maximum solids thickening.
Aside from weight, flow, size, and space-advantages, tube and plate settlers have
fewer moving parts, meaning further reductions in initial and maintenance costs. Any
SIDE VIEW
PLATE PACKS —. . THICKENER/SCRAPER DRIVE
—^ -*-
INFLUENT
EFFLUENT
FLOCCULATION
PICKET-FENCE THICKENER
SLUDGE SCRAPERS
UNDERFLOW
->-
SLUDGE
PLAN VIEW
INFLUENT
INFLUENT
D
D
EFFLUENT
Figure 42. Thickening Equipment on Plate Settlers
100
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systems external to a clarifier, such as chemical feed equipment, stirring mechanisms,
and solid pumps, add to overall costs and affect the reliability and maintenance of the
treatment process. Generally, it has been estimated that conventional settling systems
cost approximately $19/ft2 of effective settling area, while tube and plate settlers cost
about $ll/ft2. However, these settlers have difficulty handling corrosive liquids or
sticky solids, which may plug the unit.
COSTS
The wide variation in the waste streams of the metals machining, fabricating, and
coating industries makes it impossible to compile a universal cost list of sedimentation
equipment. Apart from material and labor expenses, the cost for equipment such as
clarifiers depends largely on individual wastewater composition. The interdependence
of such factors as settling velocities, overflow rates, coagulant dosage, and final ef-
fluent quality regulate the treatment system design.
While only laboratory testing will yield data on these parameters for a detailed eco-
nomic evaluation, we can cite approximate costs for sedimentation equipment based on
the size of the flow and desired overflow rate. Figure 43 shows approximate installed
costs for circular clarifier s; costs for rectangular clarifiers will be slightly higher.
The internal mechanisms referred to in the figure include skimmers, scrapers, sludge
collection mechanisms and draw-off s, and supports and walkways. Items such as land,
buildings, or chemical feed equipment are not included.
Figure 44 gives estimates on total installed costs for tube or plate settlers. It is
difficult to pinpoint functions by which the costs of this equipment vary, as variations
depend a great deal on the materials of construction. Also, the equipment is often
modular (which helps keep construction costs down), assembled to the purchaser's
requirements.
FILTRATION
Gravity settling, as the least expensive method for removing suspended solids,
should be the first one investigated. In some cases, however, gravity-settling tech-
niques will not remove suspended solids to the level required on an NPDES permit or
for reuse within the process. In this case, filtration must be considered.
There are two types of filters generally used in wastewater treatment: sand filters
and mixed-media filters. In both types, surface charges on the fine particles will have
the same adverse effect on performance as in gravity settlers. Highly colloidal matter
with surface charges will eventually charge the filter medium itself, setting up repel-
lant forces that prevent adsorption or adherence of later particles. Before investing
in the relatively high cost of a filter, therefore, make sure that the wastewater's de-
stabilization phenomenon has been fully investigated. A filter is not the solution to poor
solids/liquids separation resulting from particles that cannot be adequately destabilized
by coagulation.
101
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250
200
o
o
150
100
50
Based on 0.5 gpm/ft^ overflow rate
Estimated installed
cost
Equipment costs include all in-
ternal mechanisms, flocculator,
pumps, electrical system, etc,
1976.
Installation costs include
$300/yd 3 for cement, concrete,
excavation, reinforcement, etc.
Purchased costs
.4 .6
Flow rate - millions of gallons per day
10
Figure 43. Approximate Costs for Circular Basin Clarification Equipment
SAND FILTERS
Figure 45-a shows a conventional sand filter. The filtration basically occurs on
the surface, which eventually becomes saturated with particles. When this occurs, the
pressure drop across the filter increases and the filter must be backwashed to be made
operable again. Backwashing involves a sequence of reverse water flow combined with
what is called a surface scour, which essentially means that jets of water are directed
at the filter surface to break up the compacted material. The backwash cycle—that is,
the need to minimize it—is a major factor in determining the size (and initial cost) of
a filter. Therefore, filters should normally be used only for "polishing" operations,
with the bulk of the suspended solids removed prior to filtering.
102
-------
100
80
o
CD
O
60
40
20
Based on 0 5 gpm/ft^ overlow rate
Equipment costs include all
internal mechanisms, floccu-
lator, flash mixer, etc
Installation costs include
erection, piping, wiring, etc
.4
Flow rate — millions of gallons per day
.8
1 0
Figure 44. Installed Costs for Tube or Plate Clarification Equipment
MIXED-MEDIA FILTERS
Figure 45-b shows a single-media filter bed. In contrast, the mixed-media filter
bed in Figure 45-c, in which the size of the filtering media is varied so that bulk of the
filtration occurs deep in the bed, allows a far more effective use of surface area; it
also increases the time between backwashes. Even with the mixed-media filter, how-
ever, presettling is necessary to avoid overloading of solids.
The amount of solids in the effluent from a filter is a function of the influent solids,
the properties of the filter, and the degree of stabilization in the suspended solids.
References 8 and 12 provide general background on the use of wastewater filters. If
further treatment is needed to meet final limits for suspended solids at your plant, you
should carefully consider further flow reductions before adding a filtration step.
103
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Operating
table
Rate of flow and loss
of head gages
Filter bed wash-
water troughs
Operating
floor
Pipe gallery
floor
Filter dram
Filter to waste
Influent to filters
Perforated
laterals
Concrete filter
tank
Pressure lines to
hydraulic valves from
operating tables
Cast-iron
manifold
Effluent to
clear well
Drain
typical sand filter
UNDERDRAIN—r1
CHAMBER
INFLUENT
EFFLUENT
COARSE MEDIA—1/7
INTERMIX
ZONE
FINER MEDIA
FINEST MEDIA
UNDERDRAIN
CHAMBER
GARNET SAND
single-media
filter bed
Figure 45. Filters
mixed-media
filter bed
104
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FURTHER SOLIDS CONCENTRATION
The sludge from a conventional clarifier will normally contain only 1-3 percent
solids. A thickener can raise the solids content to 5-10 percent, based upon the solids-
settling and compaction properties. However, in most cases it is neither economical
nor feasible to use gravity-settling equipment with the high residence time needed to
achieve this thickness. To reach a higher solids concentration by other means, you
must either use extra force, such as vacuum or centrifugal, or allow a very long com-
paction time. Time-induced compaction is possible with sludge lagoons. In most
cases, the least expensive method for solids disposal is to find a nearby disposal site
(sludge pond), preferably on the plant grounds. The watery solids, if allowed to stand
for months, will eventually compact. The liquid overflow may be high enough in other
pollutants to require routing back to the treatment plant. Thus, it is essential to locate
the sludge pond in an area where excessive rainfall will not carry the solids into a
waterway.
One method of solids disposal is contract hauling. Since waste solids contain sig-
nificant amounts of water, and contract haulers charge by the pound, the costs for dis-
posal will be largely based on the weight of the water. If contract hauling is required
and there is no land for a pond, further concentrating of solids in the plant by dewater-
ing methods may become attractive. The two generally accepted mechanical approaches
are vacuum sludge filtration and centrifugation. The equipment for these methods is
expensive and is feasible for small plants only when no space for a pond is available.
As in the case of wastewater systems design, each option for solids concentration
must be separately evaluated if the best and most economical system is to be designed
for the plant. Factors that must be assessed for each dewatering process will include
physical properties of the solids, the current solids concentration, desired final con-
centration, construction materials, and filtering rates. It is common practice to con-
sult with equipment suppliers or engineering consultants who can predict the perfor-
mance of a solids concentration system and determine the costs of installation.
VACUUM FILTRATION
Many types of filters are available for dewatering solids. The systems can be op-
erated continuously or as batch operations. The selection of filter type depends pri-
marily on the filtering rate, which is basically the ease with which the water is removed
from the solids and the solids cake is formed. One of the most common filters for de-
watering is the rotary drum vacuum filter, shown in Figure 46. This filter has three
basic operating zones, shown in Figure 47: slurry pick-up, cake drying and formation,
and discharge. It is constructed as a cylinder, using common materials such as carbon
steel or stainless steel. A series of pipes is located beneath the cylinder to provide
suction and remove water from the cake. The drum can be covered with various types
of filter media, sized to pick up the solids slurry, to permit the best range of solids
removal and cake formation, and to reduce loss of solids into the filtrate.
105
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CLOTH CAULKING
STRIPS
DRUM
AUTOMATIC VALVE
SLURRY FEED
AIR BLOW-BACK LINE
-I O
Source: EPA Process Design Manual for Suspended Solids Removal.
Figure 46. Rotary Drum Vacuum Filter
Source: EPA Process Design Manual for Suspended Solids Removal.' *
Figure 47. Operating Zones of a Vacuum Filter
106
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Initially, the slurry is fed from the clarifier or thickener into the submergence
tank under the rotating drum, where it is mechanically agitated to keep the solids in
suspension. The drum is normally submerged to about 40 percent of its circumference
and rotated at a rate that permits the optimum slurry pick-up, cake thickness, and
water removal. The slurry is initially picked up on the filter medium by the vacuum
exerted through the filter medium and pipes, pulled with a standard-type vacuum pump.
As the drum rotates and exits the submergence tank into the drying zone, the water is
drawn through the pipes underneath the filter. At this point, the initial drying stage
begins and the cake is forming. The cake is further dried by air pulled through it in
the last section of the drying zone. The drum continues to rotate to the discharge area,
where a scraper and air ejection blow-out remove the solids from the filter medium.
The solids are then collected in a hopper and hauled to the disposal site.
Water and air drawn out of the cake into the discharge pipes are subsequently fed
into a filtrate receiver, a separating device that removes the air from the water. The
receiver is sized to reduce the velocity of the water and air entering the tank for effi-
cient removal of the vapor and liquid. The water is periodically discharged from this
vessel with a standard centrifugal pump and is usually recycled back to the clarifier or
thickener. The vacuum maintained at the pump is normally in the range of 18-22 in.
of mercury vacuum.
The performance of vacuum filters, measured by the pounds of dry solids per-
hour/ft2 of filter medium, varies with the application. However, a cake containing
20- to 40-percent solids, produced at a rate of three to six pounds of dry solids per-
hour/ft2, is common.
CENTRIFUGATION
When the volume of solids to be dewatered is very high, it is normally more eco-
nomical to use centrifuges than filters. Evaluating a centrifuge for dewatering use is
essentially the same as evaluating a filter. A centrifuge will generally be able to
achieve higher solids concentration while using less space in the plant. Its disadvan-
tage is greater maintenance due to its rotating bowl.
The force of rotation increases the settling forces on the solid particles to achieve
separation efficiencies higher than those obtained with filters. A typical centrifuge is
shown in Figure 48. Centrifuges, like filters, may be operated as either batch or con-
tinuous operations.
As the feed enters the centrifuge, it is accelerated to the rotational speed of the
device and distributed along the bowl. The heavier solids are directed toward the
sludge discharge by a screw-conveyor. The liquid flows through the filter screen of
the centrifuge wall and is collected on the outside rim, where it flows over a weir to
the liquid outlet. The liquid that has been separated from the solids is recycled back
to the clarifier or thickener. Solids are discharged into a hopper for disposal.
107
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o
00
i— CONVEYOR DRIVE
•BOWL DRIVE
CYCLOGEAR
SLUDGE
DISCHARGE
CONVEYOR
BOWL
REGULATING
RING
(N1.ET
IMPELLFR
Figure 48. Centrifuge
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CHAPTER VI
RESIDUAL OIL AND GREASE
Oil and grease that contaminate water in a separate, distinct phase can be removed
by a variety of techniques, selected on the basis of the quantity or physical character-
istics of the oil and the desired effluent concentration. Some of this "free" oil can usu-
ally be easily separated from water by flotation, skimming, or other gravity-motivated
processes (Chapter V). As the oil droplets get smaller or their density approaches
that of the wastewater, however, more complex separation technology is required.
Earlier sections have discussed removal of settleable and suspended solids. After
these steps, the wastewater may still contain some oil in an emulsified or very finely
divided state. This oil, kept in suspension by chemical and physical forces, will not
separate from the wastewater even after long periods of standing in a container; re-
moving it requires that the emulsion be broken and the fine oil droplets forced to co-
alesce into large drops of free oil. Techniques for this are similar to those applied
for agglomeration of fine suspended solids.
OIL PROPERTIES AFFECTING REMOVAL
Many factors affect the dispersion of oil droplets in wastewater. Pumping and the
presence of emulsifying chemicals are especially important. Pumps with high-shear
characteristics can cause formation of fine oil droplets; contaminants such as soaps,
detergents, or emulsifying agents tend to affect the complete and nearly permanent
dispersion of oil droplets in water.
Oil can be present in a waste stream in three forms:
• Free (insoluble) oil, composed of discrete particles, usually larger than 20
microns, that can rise through the water because of its lower specific gravity;
• Emulsified oil (partially dissolved), in droplets less than one micron that are
unable to rise to the surface regardless of detention time; and
• Dissolved oil, having no discrete particles and requiring advanced separation
techniques.
Characteristics of oil-water mixtures that should be carefully considered before
selecting a process for removing or separating out the oil are particle size, amount of
suspended matter, degree of emulsification, and the oil concentration, discussed below.
109
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PARTICLE SIZE
The size of oil particles dispersed in water has a significant bearing on the ease
of their removal. Oily waste should be handled in a way that prevents division or dis-
persion of oil droplets. The finer the oil droplets, the more difficult, and usually
more expensive, they are to remove. The size of droplets suitable for a given filtra-
tion technique cannot be specified, since droplets often change in both size and shape
during processing. The pumps supplied with, or recommended for use with, oil-water
separation equipment are frequently of the low-velocity, low-shear type. Low-speed
positive-displacement or centrifugal pumps are the most commonly used feed pumps to
oil-water separation equipment.
SUSPENDED OR PARTICULATE MATTER
The presence in water of solid particles other than oil may strongly affect oil re-
moval systems or equipment. This is particularly true with separators of the coalesc-
ing type (discussed later in this chapter), since solid particles have a tendency to plug
the fine pores and passages of coalescing media. The result is that the oil-water mix-
ture is forced through the remaining open pores at a much higher velocity and the ef-
fectiveness of the coalescing separation is diminished. In extreme cases, the plugging
of a high percentage of the passages results in the almost-complete blockage of oil-
water flow through coalescing cartridges.
DEGREE OF EMULSIFICATION
The extent to which oil in water is emulsified depends on particle or droplet size.
When oil particles are so small that their dispersion in water is essentially permanent
(with internal physical forces keeping them separate), they are said to be emulsified.
Oil-water mixtures in this state cannot be separated by the usual gravity, or even co-
alescing, types of equipment, but only by the most advanced adsorption or filtration
techniques.
The presence of soaps, detergents, and other chemical emulsifying agents in water
promotes the stable suspension of fine oil droplets. In some cases, these chemicals
are added intentionally to maintain emulsified oil, as in the case of water-soluble cut-
ting oils or coolants. In other cases, they are inadvertently added through interming-
ling of industrial wastes. The active portion of the emulsifying agent attaches itself to
the oil particles, giving them a physical-chemical nature that tends to keep them sepa-
rate from all other particles. The result is a very high-stability system, and no
amount of inducing oil droplets or particles to contact each other will result in their
agglomeration.
The removal of emulsified oils depends upon first destroying the forces holding the
fine oil particles apart and then providing conditions where they can physically combine.
This is analogous to the flocculation and agglomeration techniques used in suspended
solids removal (Chapter III). Truly emulsified mixtures of oil and water can be treated
by some of the adsorptive and advanced ultrafiltration processes. In many cases,
110
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emulsions can be broken by the application of heat, strong acids, or other chemicals
that destroy the soap or detergent causing the emulsion. In cases where the emulsion
is enhanced by high oil densities, addition of a salt to increase the density of the water
will aid separation.
INFLUENT OIL CONCENTRATION
The quantity of oil in the feed to coalescing and filtration oil-water separators can
be of significance for oil removal. Some devices have a capability for handling large
slugs of oil without difficulty or disruption. Other systems might suffer a serious up-
set in processing and discharging oil at the proper rate if a high percentage of oil, or
a nearly pure slug of it, should enter the equipment. Some equipment is outfitted with
automatic oil-unloading devices that discharge water-free oil in direct proportion to
the amount of oil entering the equipment.
All of these methods require special technical knowledge and the addition of other
materials to the wastewater. It is recommended that these techniques be implemented
with the cooperation of an experienced consultant or vendor.
MECHANISMS FOR SEPARATION
COALESCENCE
Coalescence is a process for promoting the agglomeration of small droplets. It
works by causing the oil-water mixture to flow in a manner that encourages contact
among fine droplets or particles. As might be expected, a normal straight-line flow
would have the effect of allowing oil droplets or particles to remain separate. A highly
turbulent flow will induce contact between the oil particles, but not in a way that pro-
motes their agglomeration into larger globules. Coalescence is the carefully designed
flow of an oil-water mixture through an area requiring many changes of path, forcing
the contact of multiple oil droplets; at the same time high turbulence and high velocity
are avoided. This promotes the formation of larger globules of oil as the oil-water
mixture leaves thp coalescing media.
Coalescing filters and similar equipment can normally produce effluents of a qual-
ity of 5-10 ppm or less, dependent on the nature of the oil-water mixture. Normally,
the gravity separation processes are better able to handle large slugs of oil. Coalescers
and filters, while not as well-suited to gross amounts of oil, generally produce a con-
sistently higher quality effluent with low residual oil.
FILTRATION AND ADSORPTION
The classical mechanism of filtration is retention of the unwanted material on one
side of a filtration medium. Perhaps the best-known example is bed-type filtration,
which uses granules of sand, gravel, anthracite coal, or carbon. In the chemical pro-
cess industries, filtration is also accomplished through the use of cloth, porous metal
plates, and, more recently, even micro-porous plastic sheets or membranes.
Ill
-------
In filtration, the passages for water flow are restricted in such a way that larger
particules or globules are retained. This mechanism is very consistent when the ma-
terial to be filtered is discrete particles of sand, clay, or other solids dispersed in
water. With oil mixtures in water, the mechanism is less effective but remains simi-
lar to that for solids removal: globules or particles of oil larger than the pores of the
filter are retained. This separation occurs on the gross-pore scale of standard types
of filters as well as on the micro-scale of membrane processes such as ultrafiltration.
In most cases, coalescence and adsorption will be occurring simultaneously with
true filtration. Thus, as an oil-water mixture is flowing through beds of particles.,
droplets are contacting droplets and some coalescence is taking place. Simultaneously,
a surf ace-sticking or adsorption of some of the oil might also occur, as is the case
with filter media such as activated carbon. In this instance, the major mechanism is
the adherence or adsorption of the oil on the large surface area of the activated carbon
granules.
SEPARATION EQUIPMENT
The equipment for oil-water separation by coalescence, filtration, and ultrafiltra-
tion processes is discussed below in some detail.
COALESCING SYSTEMS
Coalescing oil-water separators are generally of two major types—extended-
surface and cartridge coalescers.
Extended Surface Coalescers
This type of coalescer promotes a tortuous flow of water through the system,
forcing oil droplets into intimate contact. These units are typified by plate coalescers
and those using packing or other media. Plate or media-packed coalescers operate at
atmospheric pressure and often have open-tank construction.
Figure 49 shows a plate separator, a typical extended-surface coalescing separator.
The oil-water mixture flows in a turbulence-free manner between the plates. The ac-
tion of the plates promotes the agglomeration of oil droplets and directs them to a con-
venient oil off-take port. These separators can handle variable quantities of oil influ-
ent, including large slugs of oil. Since they have no moving parts, they generally
require low maintenance. Typical effluents contain approximately 10-25 ppm of free
oil, although under specific conditions the outlet oil concentration can be lowered to
about 5 ppm of free oil.
Another extended-surf ace coalescer design is based on plastic vertical tubes. The
internal mechanism is similar to the API separator mentioned earlier, except that a
matrix of plastic tubes is installed in the separation chamber. The tubes are perfo-
rated, and when they are stacked vertically, the grid pattern provides an interference
path for the oil and other solids. The plastic material attracts the oil globules and
112
-------
Adjustable
effluent
weir
Separated
oil
tank
Oil
skimmer
weir
Slotted
-oil
baffle
Oily
waste
Cleaned
water
Figure 49. Plate-Type Coalescing Oil-Water Separator
rejects the water; as suspended solids are accumulated on the plastic surface, they
agglomerate and drop to the bottom of the separator. The oil globules increase in size
and buoyancy until they break away from the plastic tubes and rise. The tubes allow
free passage of the oil and sludge in the desired direction.
A similar result is achieved when the surface is in the form of random packing,
as shown in Figure 50. The packing can consist of saddles, Pall rings, raschig rings,
113
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OIL/WATER
MIXTURE
INLET
BASKET
WATER
OVERFLOW
WEIR
OUTER
BASKET
OIL
OVERFLOW
WEIR
OILTRAP
MEDIA
BOTTOM SCREEN
Figure 50. Packed Oil-Water Separators
and other packing media used in the chemical process industries. The oil-water mix-
ture flows slowly through the packing, with intimate contacting of the oil droplets be-
cause of the high-surface area. Both plate and random-packing coalescers can be
outfitted with either manual or automatic oil-unloading equipment. Solenoid valves and
instruments sense the quantity of water in a particular area of the device. When the
quantity of oil is high, the appropriate valves open and discharge the water-free oil
from the unit. Manually adjusted unloading of the oil is sometimes carried out through
simple overflow weirs or skimming assemblies. When manual oil-unloading is used,
the equipment is less responsive to slugs of influent oil. Plate or media coalescers
are probably best suited to oily waste streams containing suspended solids or other
physical contaminants.
114
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Cartridge Separators
Cartridge coalescers are devices in which the oil-water mixture is passed through
fabricated paper or fibrous assemblies. The construction of the paper or coalescing
media is such that it promotes the tortuous type of flow described earlier without caus-
ing high shear between the oil and water. The cartridges are usually cylindrical and
are often constructed with pleated fabric, paper, or other porous media. Most car-
tridges have a discrete life expectancy and eventually lose their coalescence property.
This loss of function is often actually a plugging of the cartridge rather than a destruc-
tion of its inherent ability to coalesce oil droplets. In some cases, cartridges can be
rejuvenated by special techniques; in other cases it is more practical to replace them.
A typical cartridge-type coalescer is shown in Figure 51. In the coalescing section
of these devices, the fluid flow is normally from the inside of a cylindrical cartridge to
the outside. Both the oil and water flow through the medium, and as the liquids pass
through the twisted pathways of the paper, the oil droplets are joined. On the outside
of the paper cartridge, the oil is generally in the form of large globules that easily
oily mixture
inlet
oily water
outlet
clean water
outlet
Figure 51. Cartridge-Type Coalescing Oil-Water Separator
115
-------
float to the top of the tank or pressure vessel. As in other oil separation devices, the
quantity of oil accumulated at the top of the tank may be sensed and then discharged
either through an automatic solenoid valve or a simple manual valve.
In many cases, cartridge-type coalescers are preceded by cartridge filters de-
signed to remove any particles from the oily waste mixture that might plug the passages
of the coalescing cartridges. Cartridge prefilters are usually of about 10-micron po-
rosity. This degree of filtration removes particles of dirt, metal chips, and other
fibrous matter that might be detrimental to coalescing cartridges. Where prefiltration
cartridges are used, the life expectancy of coalescer cartridges is strongly dependent
on the percentage of particulates eliminated by the prefilteration. Coalescer cartridge
life is usually measured in months; in many devices, cartridges are replaced every
4-6 months. However, if careful attention is not given to the system, upset conditions
or improper feed can overload cartridges in a matter of days.
The advantage of cartridge coalescers over plate coalescers is better separation
and removal of oil droplets from the water. It is not uncommon for cartridge coales-
cers to produce an effluent quality in the range of 5-10 ppm of residual oil. However,
truly emulsified or soluble oil is not removed even by cartridge type coalescers. If
the oil is in droplets, or if it is chemically emulsified, it can usually find its way di-
rectly through the coalescing medium and continue to contaminate the product water.
Cartridge coalescers, although generally requiring more operating labor and attention,
offset this by producing higher quality effluent water. The choice must be made on the
basis of characteristics and variability of the influent together with the degree of oil-
water separation to be accomplished.
FILTRATION SYSTEMS
Oil removal filters generally fall into two broad categories. The first includes
those with a filter medium that adsorbs the oil phase in oil-water mixtures, typified by
the various carbon or charcoal bed or cartridge devices. The other is ultrafiltration,
which uses several kinds of micro-fine porous media.
Carbon Charcoal Bed Filters
In this equipment, two mechanisms are generally occurring simultaneously: inter-
mingling and coalescing of fine droplets of oil in the mixture and adhering of the oil to
the surface area of the filter medium.
Most equipment of this type has a fixed capacity for adsorption of oil; that is, a
point is reached when the medium can no longer retain oil from the influent water. At
this point, normal practice is to remove the filter medium from service and regenerate
it, stripping the oil by physical or chemical means. In some units, an oil solvent is
pumped through the filter bed to dissolve the oil; the solvent is recovered through a
separate oil-separation and solvent-distillation system. Other equipment may use
steam to strip the oil from the adsorber surface. In some cases, the activated medium
is simply used to saturation and then disposed of as a solid waste; the wisdom of this
approach depends on the quantity of oil to be removed and the replacement cost for the
116
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medium. When large quantities of activated carbon are used to remove only trace oils
from water, it may be practical to send the spent carbon to a central processing station
for thermal reactivation.
A typical activated carbon oil filter is shown in Figure 52. In many ways, this
equipment resembles classical carbon or bed-type filtration systems. The oil-water
mixture enters the device and then is thoroughly distributed through the bed of granules
or particles. The usual combination of coalescence and adsorption operates to separate
the oil from the water. In some equipment, provision is made for the separated oil to
rise to the top of the vessel.
When the carbon or other filter medium is loaded to capacity with oil, it is removed
from the vessel or simply regenerated in place, for instance, by steam-stripping or
solvent-washing of the activated bed. This removes virtually all the oil. Clean carbon
or other activated media can often remove oil to below-detectable limits. However,
when the carbon or medium becomes saturated, there is usually a total breakthrough of
oil. In extreme cases, the effluent from the device may contain the same amount of oil
as the influent water, or may even be higher in oil concentration for brief periods.
As mentioned earlier, the labor and replacement costs of filtration systems make
them more expensive than other oil-removal devices. However, when properly oper-
ated they probably give the highest quality of oil removal.
Ultrafiltration
The second type of oil-water filtration device is based on special porous media.
Examples of this technology are membrane ultrafilters or specially treated cloth media
in conventional plate and frame or drum-type filters.
A distinction should be made between these two types of media filters. In filters
using cloth, the entire liquid flow is normally pumped directly against the separating
sheet. Water passes freely through the sheet, while the oil is generally retained or
brought off as a separate side stream. In membrane ultrafilters, the bulk fluid flow is
parallel to the membrane face, with a pressure difference occurring across the mem-
brane. This pressure difference prompts the oil-free water to flow through the micro-
fine pores of the feed side and the oil to pass off in a more concentrated side stream.
In ultrafilters, as shown in Figure 53, plastic membrane sheets with extremely
fine porosity are used; the pores often range from .02 to .04 microns, smaller than
those in sophisticated membrane filtration equipment. Membrame filters with pores
from .2 to .4 microns are used for such specialized separation as the removal of bac-
terial, colloidal, and other very fine matter from water. Ultrafiltration membranes
with their much finer porosity permit only the passage of water molecules, while oil
droplets and even emulsified oil molecules are retained by the membrane. Ultrafiltra-
tion can be used to remove even totally soluble oils from water. The quality of water
produced by Ultrafiltration is extremely high, usually approaching no-detectable-oil and
with a substantially reduced chemical oxygen demand. Ultrafiltration equipment is nor-
mally more expensive, and consideration must be given to the life expectancy—and the
117
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•^-^^"vr:>'?^~r^—^v^- -^ '-T-~^'.-"^-
~- -^Aji----^^-^^-"*. - --^ •-•" •'•".'> _•"•'
'^•'T~-±--'-^-'.'2/J. ^f^-^^.^'—!?
Clean
Water
Outlet
Oily
Waste
Inlet
Regenerated
Carbon
Return
Figure 52. Carbon Adsorption
118
-------
' ' ," V .' . • : Membrane
Oil
concentrate
(a)
Clean
water
outlet
Oily
waste
inlet
t
Oil
Oil
concentrate
outlet
Water
I
\
Oil
Water
Membranes
(b)
Figure 53. Ultrafiltration Process
119
-------
replacement cost—of the active membranes. Another characteristic of ultrafiltration
that must be considered is that oil is normally removed as a concentrate, in a typical
"blow-down" fashion. This blow-down is seldom 100-percent oil and often contains
residual water. However, the blow-down usually responds well to typical gravity-type
separation, and if the concentrate is permitted to stand quietly for a short time, a
distinct oil layer is formed. The oil-free water separated from this concentrate can
usually be recycled to the equipment.
Ultrafilters may have many mechanical configurations. Some of those in common
use are tubular types, flat membrane "plate and frame" types, and sheet membranes
used in various cartridges (usually spiral-wound).
EQUIPMENT COSTS
The economics of oil-water separation is an imprecise subject because of the di-
versity of feedwater concentrations and differences in effectiveness of equipment. The
curves in Figures 54 through 56 give approximate capital costs for equipment, based
on gallons per day of feed for various types of control processes. The wide divergence
is obvious. In general, more sophisticated processes with the capability to totally re-
move oils from water are more expensive than simpler ones that accomplish only a
gross separation. Equipment costs can range from a fraction of a dollar per day per
gallon treated to as high as $10 per day for each gallon treated. The less expensive
devices are typically the plate or cartridge type coalescers, while the more expensive
ones are the highly specialized ultrafilters.
The cost of media replacement for oil-water separation equipment is difficult to
predict. Devices using extended surfaces, such as plate and packed coalescers, have
virtually no replacement costs for maintaining their oil-water separation capacities,
whereas the life expectancy of the cartridge coalescers may be uncertain and will de-
pend on the wastewater characteristics. The usual prices for replacement of cartridges
for either the prefiltration or coalescing sections of coalescing-type separators range
from approximately $10 to $30. It is not unusual for a single cartridge of this type to
have a capacity to treat 10, 000 gal/day. At the extreme end of the scale, the mem-
branes used in ultrafiltration may cost anywhere from a few dollars up to $30-$40/ft2.
However, a single ft2 of membrane may have the capacity to treat only about 20-30
gallons of influent water per day. Good ultrafiltration membranes, properly operated,
may have a life expectancy of several years.
Thus, the economic picture for separating-media is complex. It is necessary to
consider both the degree of oil removal required and the technical suitability of the
process providing the most practical, long-term operation at the lowest operating ex-
pense. As might be expected, the most practical answer is sometimes a combination
of processes. Thus, a pretreatment step using gravity separation, coalescers, or
other devices for bulk oil removal can accomplish gross separation at least cost. If
fairly complete oil removal or polishing is required for part or all of the stream, the
more sophisticated—and usually more expensive— processes can be employed. With
120
-------
25.000
20,000
15,000
10,000
5,000
I
I
I
2 4 .6 8
Flow rate - millions of gallons per day
Figure 54. Capital Cost vs. Flow Rate of Oil-Water Separators of API Specifications
1.0
pretreatment by earlier processes, it is reasonable to expect effective and quite ac-
ceptable economical performance from final polishing systems. Part of the economy
of combining oil separation processes is that each kind of equipment can be used only
in the operation mode most favorable to its life expectancy and the performance of its
medium.
Recovery and reuse of the oil from separation processes should also be considered.
As described earlier, some operations return clean, water-free oil. In others, the oil
and all other contaminants are accumulated as a "junk stream." Consideration should
be given to the economic and pollution-control implications of the removed-oil stream.
121
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30,000
25,000
20,000
15,000
10,000
5.000
Cartridge
Coalescing
Plate
1 .2 3
Flow rate - millions of gallons per day
Figure 55. Capital Cost vs. Flow Rate for Various Oil-Water Separators
122
-------
600
500
400
s 300
200
100
Based on hydraulic loading of 3 gpm/ft2
I
I
.02 04 06
Flow rate —millions of gallons per day
08
Figure 56. Capital Costs for Ultraf iltration of Oil-Water Mixtures
123
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CHAPTER VII
HANDLING WASTE-ACID STREAMS
A manufacturer with a waste-acid stream must either recover the acid for reuse
or must neutralize the stream, settle out the suspended solids, and discharge it. The
neutralization requirement will eventually apply even to plants discharging to municipal
systems because of the low pH prohibitions discussed in Chapter III. In this case, it
may be possible to discharge the neutralized stream into municipal systems.
Generally speaking, sulfuric acid recovery is the only proven recovery technology
available to the small manufacturer. Although there are commercially operating hy-
drochloric acid recovery systems, the economics are currently favorable only to the
manufacturer with large muriatic acid pickling facilities. There is no proven technol-
ogy for recovering mixtures of hydrochloric, sulfuric, nitric, or hydrofluoric acids.
Chromic acid recovery is possible through ion exchange or evaporation under cer-
tain conditions and is discussed in the EPA technology transfer metal finishing
publication. •>
In this chapter, we will discuss only neutralization and sulfuric acid recovery
from pickling operations.
NEUTRALIZATION
Neutralization, the oldest and most widely used treatment for acid solutions, in-
volves treating acids with alkalies to achieve a satisfactory effluent. The techniques
of suspended-solids removal, that is, chemical precipitation, sedimentation, and
clarification, are then applied to the neutralized stream. If neutralization is to be
effective, it must not only reduce the acidity or alkalinity of a solution, but must also
convert the metals into insoluble metal hydroxides that can be removed in downstream
separation equipment. Thus, neutralization can be viewed as a pretreatment method
in a total waste treatment operation.
CHEMISTRY
The chemical equation that describes neutralization is:
ACID + BASE »~ SALT + WATER
All water solutions of acids and bases are measured by their relative hydrogen ion
(H+) and hydroxyl ion (OH~) concentrations. In water, the equilibrium product of the
124
-------
H+ and OH concentration is a constant, 10~14 at 25°C. The scale used to measure
this relationship is referred to as pH, defined as:
Log (hydrogen ion concentration in g-moles/liter)
Pure water at 25°C, in which the H+ and OH" concentrations are equal, 10 7 , has a pH
of:
Log 10~7 =7
Figure 57 covers a range of acid and base solutions. Acid solutions increase in
strength as the pH value falls below 7, while base solutions increase in strength above
7. It should be noted that the pH scale is not linear with respect to acidity or alkalinity
14 K-
4 -
2 -
Strong Bases
Weak Bases
Weak Acids
Strong Acids
10-14 10-12 10-8 10-4
Figure 57. pH Scale-Concentration of H at Various pH Values
125
-------
and that a change in one unit of pH represents a tenfold change in the strength of the
acid or base. In designing a control system, this is an important concept.
SELECTING A pH CONTROL SYSTEM
Although neutralization is a very simple concept, the design and operation of a
neutralization system requires a surprising amount of care. This is especially true if
the effluent is to be monitored on a continuous, instantaneous basis rather than by a
24-hour composite and if the resulting suspended solids are difficult to settle.
There are two initial decisions to be made: (1) whether the system should be batch,
semi-continuous, or continuous; and (2) the type of neutralizing chemical to be used.
The first determination can be made entirely on the basis of the flow of the acidic
stream and the space available at the plant. Batch neutralization eliminates control
and compliance problems associated with continuous neutralization, since the pH of the
entire stream can be measured and brought within compliance before discharge; the
chance of a violation is thus very low. Batch neutralization, however, quickly becomes
unwieldy, and excessive mixing power is required to maintain uniformity. The econom-
ics of switching from batch to semi-continuous to continuous neutralization depend on
plant conditions. If the flow is 5,000 gal/day or less, batch operation (Figure 58-a) is
usually feasible. The waste stream can be collected in one basin, and the neutraliza-
tion agent (usually lime, limestone, or caustic) can be added until the desired pH is ob-
tained. Depending on the strength of the waste acid, considerable heat can be generated.
Settling time for solids is required after neutralization, and two basins or tanks may be
needed.
In semi-continuous operation, Figure 58-b, the neutralizing agent is added contin-
uously with the acid, but the effluent discharge is handled in batches after the final ad-
justment. This method is very common and allows a more even neutralization process.
However, it has the same disadvantage as the pure batch system in that as the flows
become large, the size of the neutralization system eventually becomes prohibitive.
The continuous process (Figure 58-c) allows larger flows to be handled with smaller
equipment or basin sizes. For plants with large flows, this is usually the only available
option. In designing a pH control system, it is important to know the amount of change
in pH the system is expected to effect. If there is a possibility of significant variation
in loads, then staging will be required. The first stage makes the major adjustment
and the following one or two stages are used to trim the pH to its required final value.
In the system shown in Figure 58-c, the first stage brings pH to about 3.0, the second
to 3.5, and the last stage brings it to the required value.
If the total water flow is large and only occasionally requires pH adjustment (as in
the case of an acid spill, for example), it is usually less expensive to install a diversion
pond that is used only when the pH is out of range. A system of this type is shown in
Figure 59. In this case, no pH adjustment is made until the stream is outside the de-
sired range, at which point the wastewater is directed to the diversion pond until the
126
-------
Acid
b) Semi-continuous
( pH
Acid is collected until pre-set level is
reached
Base is added with mixing until pH is within
range Then contents of tank are discharged.
Acid
1
1
1
1
1
psTC^i ^
^
^
o
Mixer
O
Neutralization Tank
Base is added along with acid
Effluent is retained then discharged,
batchwise, after pH is within range.
to
-3
Neutralized Stream
Base
Mixer
a
Mixer
a
c
\
0
#3
c>
o
#2
1
O
o
#1
•Acid
c) Continuous
Base is added along with acid in first and third stages
and effluent is continuously discharged.
Figure 58. Neutralization Systems
-------
0-
1
Wastestream •
to
oo
Base-
Effluent
Acid
1
Neutralized Stream
Holding Pond
Figure 59. Diversion Pond to Control pH
-------
pH imbalance is corrected. The water in the pond is then neutralized and discharged
over a convenient period. The pond must be sized to hold the maximum amount of
water expected to be discharged over the period required to correct the situation.
AUTOMATIC CONTROL SYSTEMS
In all the systems discussed, pH must be measured and an alkali added. For batch
and semi-continuous systems this need not be done automatically, since there is time to
measure pH and add further reagents. In continuous neutralization, however (unless the
holdup time is measured in days), automatical control should be used.
There are many suppliers of pH electrodes and complete-control systems. From
a titration curve of your waste stream, these manufacturers can design a system for
you. It is important, however, that they be fully aware of the variability likely to occur
in the effluent to the neutralization system.
CHOOSING A NEUTRALIZING AGENT
The choice of neutralizing agent is a trade-off of capital costs vs. operating costs
and convenience. Limestone (if available within a reasonable distance) or quicklime
(if freight is a factor) will usually cost less per unit of neutralizing capability. How-
ever, quicklime requires a slaking system and limestone can present problems in neu-
tralization consistency. Calcium compounds such as lime produce insoluble calcium
sulfate, which requires settling if the plant faces suspended-solids limitations. Sodium
hydroxide, on the other hand, is very convenient to use, since it can be obtained as a
solution and accurately and reliably fed with a metering pump or control valve. As
shown in Table 15, however, it is more expensive to buy.
Economics will generally favor sodium hydroxide for small installations and lime
for larger ones, with the breakeven point depending on location.
A possibility worth investigating is availability of a basic waste stream from a
nearby plant. Although caution is required, in that you may be bringing new pollutants
into your own waste stream, if a suitable stream can be found you may be able to reach
a mutually beneficial agreement with the source facility.
Once again, vendors of neutralization systems will make recommendations, based
upon your waste stream, of the most economic neutralizing agent. Such recommenda-
tions should not be accepted blindly; in most cases, for small or medium flows it is
wise to estimate the costs of both lime and caustic.
ACID RECOVERY
Some plants may have enough spent sulfuric acid liquor from pickling steel to jus-
tify recovery of the free acid. There are several commercially available sulfuric acid
recovery processes. Most are similar, in that ferrous sulfate (the compound formed
129
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Table 15
Cost Comparison of Common Commercial Alkaline Agents
Agent
Chemical
Formula
Price
($/ton)
Lbs/lb H2SO4
Neutralized
Relative
Cost
Sodium Hydroxide
(Caustic)
NaOH
178.00
.810
7.80
Sodium Carbonate
(Soda ash)
Na2CO3
74.00
1.100
4.30
High Calcium Hydrate
(Hydrated lime)
Ca(OH)
34.10
.756
1.40
High Calcium Quicklime
(Lime)
CaO
32.50
.571
1.00
Steps and Costs for Raw Materials to Neutralize 5,000 gal of Spent Pickle
Liquor/Day:
Caustic Storage
Neutralizer - $818. 80/day (Caustic)
Hydrated Lime Storage
Slurry Tank
Neutralizer -
$194.40/day
(Hydrated lime)
Quicklime Storage —>- Slaking —>- Slurry Tank —»- Neutralizer - $139. 80/day
when steel is pickled with sulfuric acid) is crystallized from the spent solution and the
remaining free acid is reused. If a plant's pickling operations are in the order of
6,000 tons of steel per year or greater, it should investigate acid recovery.
PROCESSES
One widely used recovery system batch-cools the spent pickle liquor to remove
ferrous sulfate heptahydrate crystals (Figure 60). Another continuously operated sys-
tem uses vacuum evaporation and cooling crystallization to remove these crystals. For
pickle liquor volumes of less than 25,000 gal/day, the batch system is the most widely
used in the United States.
In the batch system, spent liquor is pumped to the crystallizer, where the temper-
ature is slowly reduced (over 8-16 hours) to about 50°F. Ferrous sulfate heptahydrate
is crystallized from the solution during this cooling cycle, while sulfuric acid, required
to replace the acid consumed, is added to enhance crystal yield.
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CHEMICAL FUME AND VAPOR FILTER
•~-f-f--r ---. ^ t t- r - - —- ;
TUSH-PULL'
;/FUME EXHAUS
SYSTEM
Figure 60. Pickling System with Acid Recovery Unit
The FeSO, • 7H9O crystal slurry is fed to a crystal collection chamber that re-
T" * _ . . . t i i_ ml. *. .u n
tains the crystals and allows liquor to pass through to an acid recovery tank. The re-
tained FeSO • 7H2O crystals are then washed with small amounts of fresh water to
remove them for storage or sale. The recovered acid is preheated with steam and
returned as fresh makeup acid.
Heat exchange is accomplished in this system by circulating chilled water through
cooling coils. A refrigeration unit maintains the cooling water temperature entering
the crystallizer at approximately 40°F. Noncorrosive teflon heat-exchange coils are
commonly used for cooling.
COSTS
Table 16 presents the comparative economics of neutralization and acid-recovery
waste treatment systems for a pickling facility handling 100,000 tons/year. These
acid-recovery process costs were estimated for the zero-discharge case, where mod-
ifications such as staged rinsing were made to the pickling tanks, and for the controlled
discharge case. The operating costs for this plant, if contract hauling of the spent
pickle liquor and pH adjustment of the rinse water were used, would be $436,500. (The
hauling fee is $0.14/gal and the pH adjustment cost is $2.50/1,000 gal rinse water.)
Contract hauling requires a much lower investment. As the capacity decreases, how-
ever, the economics can shift to the point where either neutralization or contract haul-
ing would be favorable.
131
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Table 16
Operating Cost Comparison of Pickle Liquor Waste Treatment Methods
Basis: 100,000 tons steel pickled/year; 1% Fe loss/ton steel pickled; spent acid com-
position 8% dissolved iron, 8% H2SO4; all figures in thousands of dollars/year.1
Item
Investment
Salaries & Wages
Operators
Foremen
Utilities
Steam
Process Water
Electricity
Raw Materials
H2S04
CaO ,
Shipping Costs
Crystals
Sludge
Maintenance
General Plant Overhead
Waste Water Costs
Sewer Fees
pH Adjustment
Taxes and Insurance
Depreciation
By -Product Credit
Total Annual Costs
Basis
$12,500/man year
$15,000/man year
$2.50/1,000 Ib
$0.30/1,000 gal
$0.02/kWh
$50/ton savings
$33 /ton
$5/ton FeSO4-7H2O
$5 /ton
6% of investment
$1.25 (wages &
salaries + main-
tenance & labor
$0.40/1,000 gal
$2.50/1,000 gal
rinse water
0. 5% of investment
10% of investment
$10/ton FeS04-7H2O
Acid Recovery
Zero^
Discharge
630.0
12.5
1.5
35.4
(8.1)
19.0
(50.0)
0
28.8
0
37.8
35.0
0
0
3.2
63.0
(52.0)
126.1
Controlled3
Discharge
470.0
12.5
1.5
0
—
19.0
50.0
0
28.8
28.2
30.5
0
4.0
2.1
47.0
(52.0)
71.6
Neutralization
770.0
12.5
3.8
4.2
—
4.0
—
70.1
0
110.0
46.2
41.8
9.2
0
3.9
77.0
0
382.7
1. The economics are valid for these conditions only.
2. Includes investment for rinse system and air exhaust modifications.
3. Only includes rinse system modification. Some discharge of rinse
water occurs and pH adjustment is necessary.
Present cost for contract hauling of spent pickle liquor @ $0.14/gal and pH adjustment of rinse water
would be $436,500.
Rinse water included in process water for acid recovery plants at 25 gal/ton pickled steel.
132
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The merits of acid recovery will also depend on the market for the recovered
ferrous sulfate crystals, the iron level in the pickle liquor, and the amount of free
acid in the spent liquor. The amount of dilute rinse water to be included in the re-
covery system is also important.
A detailed discussion is beyond the scope of this publication. Based on economics
only, it now appears that plants pickling more than 6,000 tons/year of steel (assuming
a 1-percent iron loss) might have an incentive to use sulfuric acid recovery instead of
neutralization. If your plant's production falls above this capacity, you should investi-
gate this option.
The EPA will soon be publishing a manual containing comprehensive information
on acid-recovery methods and costs.
133
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CHAPTER VIII
INDUSTRIAL CASE HISTORY
The 70-year-old Atlantic Wire Company is located approximately 1 1/2 miles from
Long Island Sound on Connecticut's Branford River and produces basic and specialty
wire for many industries. Wastewater, generated continuously from their manufactur-
ing operations (Figure 61) and primarily from their pickling process, is handled by two
individual treatment processes.
The raw material used, hot-rolled wire rod, is initially pickled in sulfuric acid
(H2SO4) to remove rust and scale. The wire is then coated with hot lime or some other
preservative and placed in an oven to bake on the coating; in some instances, wire may
also be treated with a special liquor finish such as copper, tin, or zinc. The wire is
then cold-drawn through dies to desired diameters, after which it is subjected to an
annealing process before final storage and shipping.
Pickling takes place on two separate lines, one for rods or coarse wire and the
other for fine-wire products. The acidic wastewater from these two sources (desig-
nated Large Cleaning House and Small Cleaning House in Figure 61) combines with
smaller streams contaminated with heavy metals (zinc, nickel, etc.) and amounts to
an average flow of 180 gal/minute.
Atlantic Wire began seriously considering wastewater treatment in the 1960s to
meet newly established state and local regulations. A system was selected that could
provide regenerated H2SO4 from the spent pickle liquor and also neutralize rinse
wastewaters. To integrate the system into the plant, several modifications were nec-
essary, as follows:
• Heat exchangers were installed in the pickle tanks (previously, live steam was
injected directly into the tanks);
• The water drain system throughout the plant was modified; and
• Pollutant sources were identified.
The result was a design, installed in 1966, that directed spent pickle liquor to the
H2SO4 acid recovery unit, as shown on Figure 62. (In special instances, Atlantic Wire
uses muriatic acid for pickling, in which case the spent liquor is hauled away by an
outside contractor.) The acid recovery unit consists of three compartments: crystal -
lizer, crystal drainage tank, and recovered acid tank. The crystallizer receives the
hot spent pickle liquor. As the liquor gradually cools to 50°F, ferrous sulfate crystals
(ferrous sulfate heptahydrate) are formed. The slurry is then dumped into the crystal
drainage tank, where the liquors drain into the recovered acid tank and the crystals are
134
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co
Ol
Contaminated
Water to
••• Neutralization
System
(180 gpm)
City
Water
Contaminated Flow
Processed Flow
Figure 61. Atlantic Wire Water Flow System
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1-CRYSTALLIZER
2-AIR/LIQUIO SEPARATOR
3-AGITATOR
4-HEAT EXCHANGER (COOLING COIL)
5-CRYSTAL DRAINAGE BOX
6-RECOVERED ACID TANK
7-ACID PROTECTED CONCRETE BASE
8-FERROUS SULFATE DISPOSAL BIN
Figure 62. Batch Sulf uric Acid Recovery Unit
136
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retained. The crystals are washed with fresh water and conveyed to disposal bins.
The liquor (plus makeup sulfuric acid), now containing a low level of iron as ferrous
sulfate, is recycled to the pickling tanks. Sulfuric acid savings are approximately 30
percent. The ferrous sulfate is a byproduct that is currently being hauled to a disposal
site.
Acidic rinse water and the other waste streams, on the other hand, are directed
to a neutralization basin, where caustic is injected to adjust the pH for precipitation of
the heavy metals. The wastewater from the neutralization then flows to the top of a
circular clarifier (24 x 29 ft) for settling aided by a polymer added at the top of the
tank (Figure 63).
The installed costs in 1966 for the acid recovery and neutralization systems were
$121,000 and $67,000, respectively. The only addition to this initial treatment system
came in 1972, when near-rupture of a pickling tank prompted the design and installation
of a fail-safe secondary drain system that would—in event of a major spill of the pick-
ling tanks—diiect the liquor to the neutralization system. This system raised the total
installed costs of Atlantic's treatment system to approximately $256,000.
By 1976, state and local standards for heavy metals and total suspended solids be-
came more stringent; improved settling efficiency was needed in the clarifier.
In order to comply with the new regulations, Atlantic Wire installed a plate sepa-
rator and automated the pH control and polymer feed system. The plate separator was
placed atop the high clarifier. The more efficient polymer injection, a flocculator tank
with a mixer, and the increased settling efficiency of the plate separator combined to
bring the effluent within the regulations. The clarifier was also changed to a holding
or thickening tank to further concentrate the settled solids from the separator.
This modification is illustrated in Figure 63. As wastewater enters the neutrali-
zation basin, caustic (50 percent) is automatically injected as needed to maintain pH
control between 8.5-10, which will more efficiently precipitate heavy metal. The neu-
tralized wastewater is pumped to the flocculator, where a polymer (Betz #1100) is
added; mixing occurs before the wastewater enters the plate separator. The waste-
water from the separator discharges with about 1-percent solids directly to the pre-
existing clarifier, where the solids concentration is increased to about 3 percent. The
clear effluent flows from the top of the separator and clarifier and is discharged to the
river. The installed costs for adding the plate separator and for automation of the
caustic and polymer feed system was approximately $58,000.
The volume of sludge that accumulates in the clarifier is about 30,000 gal/week.
It is periodically hauled to the town lagoon by a local contractor, at an average cost of
$175/week. Additional operating costs for the neutralization system include chemicals
(caustic and polymer) at about $525/week. Costs of maintenance and supervision bring
the total operation to approximately $l,100/week.
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co
oo
Solid lines indicate components of
original system.
Dotted lines indicate components of
present system.
/-
""X
/ Polymer »
V Add J
•^.^^
t Mixet
L y r~~"l
I Flocculation / |
( Caustic A
I Add /
Acidic
Wastewaters
I
' \
~ Mixer 1
I
^**
|| pH control
,
1 1
T
Neutralization Basin
pp— '
M
| / _)
S *\ ~i Plate l~~ Clarified
( Polymer ^ / SeParator / W3ter
VAV / /
pump
T v- — --^
t
Plate separator solids
discharged to clarifier
i
T ^ To
^ River
Sludge
^ hauled to
-^ town
Figure 63. Atlantic Wire Neutralization System
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Table 17
Discharge Composition
Chemical
Chromium
Copper
Iron
Nickel
Tin
Zinc
Total
Suspended Solids
Pre 1975 Performance
1976 State Standards
Current Performance
Average Daily Cone.
10.0 mg/1
5.0 mg/1
150.0 mg/1
5.0 mg/1
5.0 mg/1
10.0 mg/1
200.0 mg/1
1.0 mg/1
1.0 mg/1
2.0 mg/1
1.0 mg/1
1.0 mg/1
2.0 mg/1
25.0 mg/1
0.11 mg/1
. 05 mg/1
1.5 mg/1
0.3 mg/1
0.5 mg/1
1.15 mg/1
16.0 mg/1
Occasionally, problems occur with pH control mechanisms because of erratic flow
within the plant that interrupts the continuous control. But the present system is oper-
ating well within the limits set by authorities, as is shown in Table 17.
Recently, on-site tests were performed at Atlantic Wire to determine the feasibil-
ity of further dewatering of the solids by a centrifuge. The results indicated that, al-
though the centrifuge would increase the solids concentrations, the present system is
sufficient to meet regulations and does not warrant the addition of the centrifuge at this
time; it remains an option for the future. Reuse of the treated water in plant operations
is also under consideration.
The Atlantic Wire Company has thus achieved successful control of its wastewater
problems through an acid recovery process to handle spent pickle liquor, a neutraliza-
tion system for acidic rinse waters, and removal of heavy metals from the manufac-
turing processes. Over the years, Atlantic's efforts to continually reassess its waste-
water situation have resulted in appropriate technology for meeting state and local
regulations and for maintaining costs at acceptable levels.
139
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REFERENCES
1. Gulp, Russell L., and Gordon, L., Advanced Wastewater Treatment, New York: Van Nostrand
ReinholdCo., 1971.
2. "Development Document for Effluent Limitations Guidelines and Standards of Performance
for the Machinery and Mechanical Products Manufacturing Point Source Category" (Draft).
U.S. Environmental Protection Agency, 1975.
3. Handbook for Analytical Quality Control. U.S. Environmental Protection Agency, 1972.
4. Handbook for Monitoring Industrial Wastewater. U.S. Environmental Protection Agency, 1974.
5. In-Process Pollution Abatement: Upgrading Metal Finishing Facilities to Reduce Pollution.
U.S. Environmental Protection Agency, 1973.
6. Lund, Herbert \-., ed., Industrial Pollution Control Handbook. New York: McGraw-Hill Book
Co., 1971.
7. Manual on the Disposal of Refinery Wastes: Volume on Liquid Wastes. Washington, D.C.:
American Petroleum Institute, 1969.
8. Metcalf, and Eddy. Inc., Wastewater Engineering: Collection, Treatment, Disposal New York:
McGraw-Hill Book Co., 1972.
9. Methods for Chemical Analysis of Water and Wastes. U.S. Environmental Protection Agency,
1974.
10. Parsons, Dr. William A., Chemical Treatment of Sewage and Industrial Wastes. Washington, D.C.
National Lime Association, 1965.
11. Process Design Manual for Sludge Treatment and Disposal. U.S. Environmental Protection
Agency, 1974.
12. Process Design Manual for Suspended Solids Removal. U.S. Environmental Protection Agency,
1975.
13. Standard Methods for the Examination of Water and Wastewater. American Public Health
Association, Water Pollution Control Federation, 1975.
14. Weber, Walter }.,Physicochemical Processes for Water Quality Control. New York: Wiley
Interscience, 1972.
U.S. Environmental ProttcUon Agtncy
Region 5. library (PL-12J)
7? West Jackson Boulevard. 12th Ftoar
Cfcicaf»,U. 60604-3590
140 AUS GOVERNMENT PRINTING OFFICE 1977-757-056/6564
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53SZ
U.S. ENVIRONMENTAL PROTECTION AGENCY
ENVIRONMENTAL RESEARCH INFORMATION CENTER • TECHNOLOGY TRANSFER
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