EPA-600/2-77-031
December 1977
Environmental Protection Technology Series
REMOVAL AND RECOVERY OF SUlFffiE
FROM TANNED WAS11WMIR
industrial Environmental Research Laboratory
Office ef Research and Development
U.S. Environmental Protection Agency
Cincinnati, Ohio 45268
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RESEARCH REPORTING SERIES
Research reports of the Office of Research and Development, U.S. Environmental
Protection Agency, have been grouped into nine series. These nine broad cate-
gories were established to facilitate further development and application of en-
vironmental technology. Elimination of traditional grouping was consciously
planned to foster technology transfer and a maximum interface in related fields.
The nine series are:
1. Environmental Health Effects Research
2. Environmental Protection Technology
3. Ecological Research
4. Environmental Monitoring
5. Socioeconomic Environmental Studies
6. Scientific and Technical Assessment Reports (STAR)
7. Interagency Energy-Environment Research and Development
8. "Special" Reports
9. Miscellaneous Reports
This report has been assigned to the ENVIRONMENTAL PROTECTION TECH-
NOLOGY series. This series describes research performed to develop and dem-
onstrate instrumentation, equipment, and methodology to repair or prevent en-
vironmental degradation from point and non-point sources of pollution. This work
provides the new or improved technology required for the control and treatment
of pollution sources to meet environmental quality standards.
This document is available to the public through the National Technical Informa-
tion Service, Springfield, Virginia 22161.
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EPA-600/2-77-031
December 1977
REMOVAL AND RECOVERY OF
SULFIDE FROM TANNERY WASTEWATER
by
Robert H. Sayers and Roger J. Langlais
The Blueside Company, Inc.
St. Joseph, Missouri 64502
Grant No. 12120 EPC
Project Officer
W. L. Banks
Permits Branch
EPA Region VII
Kansas City, Missouri 64108
INDUSTRIAL ENVIRONMENTAL RESEARCH LABORATORY
OFFICE OF RESEARCH AND DEVELOPMENT
U.S. ENVIRONMENTAL PROTECTION AGENCY
CINCINNATI, OHIO 45268
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DISCLAIMER
This report has been reviewed by the Industrial Environmental Research
Laboratory, U.S. Environmental Protection Agency, and approved for publica-
tion. Approval does not signify that the contents necessarily reflect the
views and policies of the U.S. Environmental Protection Agency, nor does
mention of trade names or commercial products constitute endorsement or recom-
mendation for use.
ii
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FOREWORD
When energy and material resources are extracted, processed, converted,
and used the related pollutional impacts on our environment and even on
our health often require that new and increasingly more efficient pollution
control methods be used. The Industrial Environmental Research Laboratory-
Cincinnati (lERL-Ci) assists in developing and demonstrating new and improved
methodologies that will meet these needs both efficiently and economically.
This report describes technology for in-plant process change to remove
a major pollutant from tanning wastewater and recovery and reuse of a chemical.
The purpose of the project was to demonstrate the technical and economical
feasibility of a physical-chemical process. Also described in the report
is the development of the equipment design.
The report will be of interest to all tanners who have a beamhouse,
to engineering consultants and to municipalities that receive wastewater
from beamhouse operations.
Further information on the subject can be obtained from the Food and Wood
Products Branch, Corvallis Field Station, Industrial Environmental Research
Laboratory, Office of Research and Development, U.S. Environmental Protection
Agency, Corvallis, Oregon.
David G. Stephan
Director
Industrial Environmental Research Laboratory
Cincinnati
iii
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PREFACE
One of the major components of the effluent from the
tanning industry is sulfides. The leather industry uses large
quantities of sulfides, 1-5% based on the hide weight. These
sulfides present a serious disposal problem. When discharged
into a river or stream, they cause a drastic reduction in dis-
solved oxygen, the formation of dark-colored precipitates with
iron and other minerals present in the water, and, in an acid
media; cause a disagreeable odor. Toxic hydrogen sulfide may be
formed when mixed with acidic wastes. ?>
•:' *.
Sulfide can be removed from tannery effluent slowly by
aeration or more rapidly by air oxidation through the use of a
manganese sulfate catalyst. Sulfides in a secondary treatment
system decrease the effectiveness of the aeration due to the
consumption of oxygen and may result in the release of hydrogen
sulfide to the atmosphere. Catalytic chemical oxidation of the
sulfide with air results in a quantitative removal of the
sulfide. The process, though effective, is time consuming and
expensive in chemicals and power.
The purpose of this demonstration grant was to determine
the practicality of a sulfide removal-recovery system. The
system is based on the removal of the sulfide as hydrogen sulfide
gas from the clarified acidified wastewater. The hydrogen
sulfide gas released is absorbed in sodium hydroxide, forming a
solution of sodium sulfide which is re-used in the tannery's
unhairing processes.
The study was practical in nature with data taken on a full
production scale unit capable of recovering all of the sulfides
from a 5,000 hides—per-day—tannery. The advantages of the large-
scale testing was the elimination of scale-up problems for
future units. The disadvantage of the approach is that the
system was part of the entire operational scheme of the
plant and variables were required to be kept to a few operating
limits.
The Blueside Company is the first tannery of its type in
this country. This tannery produces only leather in the "blue"
state. Cattlehides are put through the unhairing process and
chrome tanned as wet blue hides. The chrome tanned hides are
shipped to other tanneries for further wet and dry processing
into finished leathers.
IV
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The sulfide reclamation unit is also a "first" and was
constructed at the same time the tannery was built. The tannery
grew from a starting production of 5,000 hides per week to its
present production of over 25,000 hides per week. Concurrently
to this growth, the sulfide recovery unit was being operated,
modified and improved.
The project goal to demonstrate a practical plant-scale
method of removal and recovery of sulfides from a tannery waste
stream was accomplished.
v
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ABSTRACT
A full scale sulfide reclamation plant was constructed to demonstrate
the feasibility of removing and recovering for reuse the sulfides in the
wastewater of a large cattlehide tannery producing 25,000 hides per week.
The combined tannery effluent from the soaking, unhairing, bate, pickle,
and chrome tanning wastes is screened and clarified. The clarified effluent
is pumped on a continuous basis to a degasifier in which acidification to a
pH 5.0 - 5.5 using sulfuric acid is effected. The hydrogen sulfide liberated
from the wastewater is carried by air stream to an absorption tower where
is is absorbed in recirculating caustic soda until a desired sodium sulfide
concentration is achieved. The sodium sulfide is then reused in the tannery's
unhairing process.
Quantitative removal and recovery of the sulfides is accomplished. The
sulfide reclamation plant is operational seven days a week.
The sulfide reclamation economics indicate a savings in material and
freight costs of approximately $92,052 per year or $84.84 per 1,000 hides.
An additional benefit resulting from the acidification of the total
tannery wastewater is the coagulation of the solubilized proteins which could
be removed by secondary sedimentation. Their removal will result in a size-
able reduction of the pollution load and the related sewer surcharge.
This report is submitted in fulfillment of Grant No. 12120 EPC by
Blueside Company, Inc., under the partial sponsorship of the U.S. Environ-
mental Protection Agency. This report covers a period from March 1970 to
April 1976, and work was completed as of April 1976.
vi
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CONTENTS
Foreword ill
Preface iv
Abstract vi
Figures viii
Tables ix
Acknowledgment xi
1. Introduction 1
2. Conclusions 4
3. Recommendations 6
4. History of the Project . . . 9
5. The Manufacturing Process and Pollution Loading 18
6. Waste Stream Processing 26
Pre-treatment prior" to sulfide recovery . 26
Wastewater treatment through the sulfide
recovery system 35
7. Equipment Design Factors 38
8. Operational Characteristics of Sulfide
Reclamation System 44
9. Data Analysis 59
Initial observations 59
Test runs after modifications of the
wastewater treatment system 59
10. Protein Coagulation and Sedimentation 82
11. Economic Evaluation of the Sulfide Reclamation
System and Wastewater Treatment System 87
12. Proposed System Modifications 95
Sulfide reclamation system 95
Protein sediment removal proposal 99
Chrome recovery and reuse 99
References 101
Appendices
A. Sulfide Chemistry 103
B. Original Specifications 113
1. Sulfide stripping system 113
2. Degasifier and overflow towers 119
3. Sulfide absorption tower 122
C. Degasifier Design Calculations 125
D. Sulfide Recovery System Operation 128
vii
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FIGURES
Number Page
1. Wastewater Treatment System, Original Design 10
2. Wastewater Treatment System (as constructed, 1970). . 12
3. Sulfide Reclamation System 13
4. Tannery Process Flow Diagram. 19
5. Wet Well 27
6. Wastewater Treatment System (as modified,
1975 - 1976) 30
7. Degasifier Tower and Absorber 34
8. Absorber Sulfide Capture Progression. 52
9. Sulfide Flue Emission vs Caustic Soda Concentration . 54
10. Effect of pH on Sulfide Removal 63
11. Effect of pH on Total Settleable Solids 65
12. Effect of pH on Suspended Solids 66
13. Effect of pH on Total Solids 67
14. Absorber Analysis: Sulfide Absorbed, Caustic Soda
Consumed, Flue Emission vs Time 70
15. Absorber Analysis: Caustic Soda Consumed, Sulfide
Absorbed, pH of Recirculation Caustic Tank vs
Elapsed Time 79
16. Proposed Modification of Sulfide Reclamation System . 96
17. Alternate Design of Sulfide Reclamation System. ... 97
18. Schematic Protein Sludge Removal 98
viii
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TABLES
Number Page
1. Tannery Wastewater Effluent to Municipal Sewer . * . 2~T
2. Sulfide Present after each Process Cycle 23
3. Sludge Analyses 24
4. Wastewater Treatment System Evaluation, 1973 . . . . 28
5. Wastewater Treatment System Evaluation, 1976 .».. 32
6. Influent Sulfide Contents vs Air Flow Rates
Outside of Explosive Range 45
7. Absorber Contact Time as related to Air Flow Rates . 47
8. Theoretical Operating Conditions for 99% Sulfide
Recovery - . . , 48
9. Acid Pump Setting vs Influent Flow in Gallons per
Minute 50
10. Analysis of Caustic Soda Solution Tank as Sulfide
Absorption Proceeds 55
11. Caustic Soda Recirculation Rates in Relation to
Sulfide and Caustic Soda in Absorber at Various
Degasifier Influent Flows 56
12. Ratio of Caustic Soda to Hydrogen Sulfide at
Various Influent and Caustic Soda Recirculation
Rates . . 58
13. Summary of Operating Results, Sulfide Reclamation
Plant 1971 - 1972 60
14. Summary of Operating Results, Sulfide Reclamation
Plant, 1975 - 1976 61
15. Summary of Solids Data 64
16. Degasifier Influent and Effluent Data 68
17. Absorber Data 69
18. Degasifier Sulfide Rates . 71
19. Material Balance Degasifier - Influent 73
20. Material Balance - Degasifier Effluent 74
21. Material Balance Degasifier Influent and Effluent
Sulfide Rates 75
22. Material Balance Absorber Analyses 76
23. Material Balance Run Absorber Exhaust Flue Data . . 78
24. Material Balance of the Sulfide Reclamation System . 80
25. Protein Coagulation and Sedimentation: Effect on
Pollution Loading 83
26. Effect of pH on Sedimentation 84
27. Analysis of Protein Sediment 84
28. Economic Evaluation of Protein Sedimentation
on Municipal Sewer Surcharge 86
29. Sulfide Reclamation System Equipment Costs 89
ix
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Number Page
30. Annual Estimated Economic Evaluation of Sulfide
Reclamation System, Original (1970) vs Current
(1976) 88
31. Sanitary Landfill Charges and Quantities 90
32. Wastewater Treatment Costs 91
33. Hydrogen Peroxide Cost Estimate 92
34. Chlorine Cost Estimate 93
35. Physical Properties of Hydrogen Sulfide 104
36. lonization of Hydrogen Sulfide 107
x
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ACKNOWLEDGMENT
This project was conducted by a number of people over its
terms of activity. The original project was negotiated by
Lee R. Lyon of Blueside Real Estate, Inc., Dr. Robert Culver of
Camp, Dresser, and McKee Consulting Engineers did the preliminary
technical work on the project including the designs and specifi-
cations of the equipment. Dr. Culver was also the consultant
on the project during the initial phases of the work.
Mr. Ronald Collins, of Blueside Real Estate, Inc.,
conducted the early studies in the project. The project was
completed by Robert H. Sayers and Roger J. Langlais.
Dr. Thomas Thorstensen, of Thorstensen Laboratory, was
consultant in the final phase of the project.
Mrs. Bea Porter, an executive secretary at Blueside
Company, graciously performed the typing and associated tasks
required in the preparation of this report.
This project was supported by the U. S. Environmental
Protection Agency and its agency predecessors through Research
and Demonstration Grant No, 12120 EPC.
The support, encouragement, advice, and patience of the
Project Officer, W. L. Banks, is greatfully acknowledged.
xi
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SECTION 1
INTRODUCTION
The largest segment of the leather tanning industry in the
United States is the tannage of cattlehides. Cattlehides are
primarily produced in meat packing areas in the midwest. Hides
are salt cured and shipped to the leather tanning sites at east,
midwest, and west coast locations. There is a trend to locate
tanneries in the area near the source of the hides for freight
economies.
Traditionally, hides have been cured near the slaughter
house, shipped to tanneries at varied locations, tanned to the
blue, retanned, and finished into leather at the single location.
Costs to ship cured hides is more than double the cost of ship-
ping partially tanned hides; i.e., chrome tanned blue stock.
New tanneries located near the source of the hides can effect
these savings and move pollution loadings from the older tannery
sites.
The Blueside Company's tanning operation at St. Joseph,
Missouri,is one of the first of a new type of tannery. The
Blueside Company receives both fresh and cured hides from pack-
ing plants. These are then given treatments of soaking, flesh-
ing, unhairing, bating, pickling, and chrome tannage. Chrome
tanned leather is then wrung, palletized, and shipped as
"leather in the blue" to other factories for further processing
into finished leather.
The tanning of leather results in very high pollution load-
ing particularly from the beamhouse operation (soaking and
unhairing). In the study of the leather industry (1) made under
the Environmental Protection Agency for the purpose of determin-
ing the nature of tannery wastes, it was reported that there
were approximately two hundred tanneries in the United States
processing approximately nineteen million cattlehides per year.
Industry data of wastewater from these tanneries, prior to
treatment, indicated that the wastes contain approximately 8.5
pounds of sulfide as well as 95 pounds of BOD and 140 pounds of
suspended solids per 1,000 pounds of hides processed. The
quantity of sulfide discharged in the waste stream, on the basis
of nineteen million cattlehides per year processed through the
unhairing, would be approximately eight million pounds. This
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sulfide in the tannery effluents is a source for recovery.
(2, 3, 4) Sulfides are objectionable for direct discharge into
waterways because of their toxicity to wildlife and aquatic
organisms.
Sulfides consume oxygen in the stream. They can generate
undesireable odors which become a public nuisance. Hydrogen
sulfide gas generated in a sewer or in concentrations above
Y 1/500 mg/m^is a deadly poison. Hydrogen sulfide in sewers can
' cause corrosion of iron pipes, and decrease the efficiency of
secondary treatment.
The removal of sulfide from the waste, prior to discharge,
can be done by aeration in the secondary treatment. There is
objection to using the sulfide oxidation by aeration in the
secondary treatment since there is some loss of sulfide directly
into the atmosphere. The sulfide in the aerator will consume
some of the oxygen thus reduces the effectiveness of the
secondary treatment.
Sulfide can also be removed by the oxidation of the sulfide
to sulfate by air using a manganese sulfate catalyst. In this
system, the sulfide bearing wastes are placed in a tank,
manganese sulfate is added as the catalyst, and the wastes are
aerated for four to six hours. This system is conducted on a
batch basis. It is costly in terms of power and can not be
adapted to a continuous process as is the system that is used
at The Blueside company. (5, 6, 7, 8, 9)
In the Blueside system, the sulfide is removed by acidifica-
tion of the wastes to form hydrogen sulfide. Hydrogen sulfide
with its limited solubility at the low pH is then removed by an
air stream.
The air stream is then conveyed to a gas scrubber within
which sodium hydroxide is recirculated. The sodium hydroxide
reacts with the hydrogen sulfide to form a solution of sodium
sulfide in the sodium hydroxide which can be reused in the
tannery processes.
The objectives of this demonstration grant was to determine
the operating characteristics and economics of the wastewater
treatment system and the sulfide removal-recovery system.
A study was conducted on the full flow of wastes from the
tannery. Small scale laboratory studies on the recovery of
wastes are known not to provide a representation of plant
conditions. The engineering of the equipment and the effective-
ness of the present design is of prime importance.
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The chemistry of sulfide removal-recovery system is well
understood. A discussion is presented in the Appendix,
Outside the scope of this project, but initiated by the
interest created by the project, the Blueside Company undertook
two additional studies. The company investigated the feasibility
of secondary sedimentation of the coagulated protein resulting
from the acidification of their effluent during the sulfide
removal. Secondary sedimentation would further reduce sewer sur-
charges on BOD5 and solids discharged. The feasibility of a system
for chrome recovery and reuse was also investigated.
All data presented uses English units of measurements except
for laboratory data. This follows industry practice.
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SECTION 2
CONCLUSIONS
This project has shown that the sulfides from the sulfide
bearing wastewater of a large cattlehide tannery can be reclaimed
for reuse in the tannery ,'s unhairing process. The system
produces a commercially acceptable grade of sodium sulfide in
caustic soda at a 15% sodium sulfide concentration with a
residual 2% caustic soda.
The results of this investigation show that the sulfide is
completely removed from the wastewater by acidification to a
5.0-5.5 p H. The recovery rate from the wastewater is
approximately 98%.
With adequate acidification of the sulfide bearing waste-
water, complete removal of the sulfides is assured and expensive
chlorination or oxidation of residual sulfide is not necessary.
The design expectations of the sulfide reclamation have been
exceeded. The system has satisfactorily allowed the recovery
of sulfide from tannery wastewater containing 1,400 mg/1 of
sulfides.
The operational characteristics of the system as related
to liquid and vapor flows have been established to ensure a
safe operation.
Three design problems with the sulfide system remain to be
solved. First, the air diffusers in the degasifier trays become
clogged with proteins that precipitate during acidification
causing downtime for cleaning every fifteen days for a twelve
hour period. Second, the air blower should be changed to include
a variable drive^ thereby, allowing greater control of the air
flow which serves to dilute the hydrogen sulfide enroute to the
absorber. At lower influent flows, a lower air volume is desir-
able. At higher influent flows, higher air flow to a maximum
of 800 CFM is desirable. Control of air flow is needed to ensure
that the hydrogen sulfide concentration enroute to the absorber
remains below the lower explosion limit of 4% E^S in air. Third,
additional absorber capacity is required.
Economic evaluation shows that the sulfide recovery-reuse
system was profitable. Total annual costs for the system's
operation, maintenance, and depreciation of equipment at 1976
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prices was $305,000. The annual saving in cost for sulfide
chemicals was $397,000. The net saving amounted to $85 per
1,000 hides processed.
The sulfide recovery-reuse system reduced the discharge
of sulfide to meet the municipal ordinance and the BODc of
the wastewater. These reductions in pollution loading^decreased
the municipal surcharges which are based on BODc, flow, and
suspended solids discharged to the sewer.
Secondary sedimentation of the coagulated proteins, result-
ing from the acidification necessary for sulfide reclamation,
will result in approximately 80% reduction of the suspended
solids, and 60% of reduction of the BODs pollution loadings.
These reductions would effect an estimated economy of $87,000
in sewer surcharges. Chrome reclamation and reuse indicate an
estimated economy of $150,000. Once these systems are on stream,
their economies would reduce the total wastewater treatment cost
at Blueside Company to an annual operating cost of $120,271
based on 350 days per year. At 21,700 hides per week, the total
wastewater treatment cost will equate to $95/1,000 hides
processed.
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SECTION 3
RECOMMENDATIONS
In order to minimize solids carry over into the degasifier,
additional emphasis must be placed on the clarifier to ensure
that short-circuiting of clarifier flows for proper settling of
solids does not occur. The influent flow into the clarifier
center should be baffled below the outflow to spread out the
flow causing the inflow to the basin to be more quiescent and
better distributed for its downward flow into the settling zone.
(12) Continuous grease skimming and removal should be maintained
to keep the clarifier surface clean. Turbine speed should be
adjusted to a speed which will allow quiescent mixing of the
inflow. Sludge removal from the bottom of the clarifier should
be controlled so that sludge depth is maintained below 20 inches
at all times ensuring sufficient depth space above the sludge
level for settleability of solids. A float activating switch
should be installed in the rim to signal the sulfide reclamation
system operator that the upper and lower levels of wastewater
available for the sulfide system have been reached.
Operation of the sulfide reclamation system has been manual
rather than automated through most of the investigations required
in the project studies. Automation should be refurbished and
simplified. Sensing pH electrodes should be installed in the
top tray of the degasifier and in the overflow tower. A record-
ing controller would not only provide a record of the pH during
the operation but control the acid feed pump running time
maintaining acidification within the high/low of 5.5 - 5.0 pH.
The sensing electrode in the top tray would be a flow through
type to prevent clogging. The sensing electrode in the overflow
tower would be an immersion type and would record the pH of the
degasifier effluent to the city sewer line. The high-low limit
controlling relay for this electrode would sound an alarm if
the pH was out of range. While it is recognized that the
degasifier influent averages to pH 8.5, a change in the flow rate
in order to maintain continuous operation of the system currently
requires a manual change of the acid feed pump.
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A variable drive air blower having air displacement capa-
bility in the 100 to 800 cfm range should be installed as a
replacement of the 1,000 cfm blower in use. If the current
blower is retained, it should have a 3 inch valve outlet install-
ed at a point prior to entry into the plenum at the base of the
degasifier. Opening or closing of this valve would allow the
operator to adjust the rate of air flow into the degasifier
at the necessary level required for the sulfide content being
reclaimed.
In the system as studied the concentration of the caustic
soda was limited to 10% to prevent the formation of sodium
sulfide solution in excess of 15%. Facilities should be winter-
ized to allow the use of 25% caustic soda. The incorporation of
a second absorber of the present type installed in series with
the first would allow the formation of sodium sulfhydrate which
is not subjected to as low freezing points as sodium sulfide.
The method would involve the use of two absorber towers in
series. The exit gas stream from the first tower would be
passed to the bottom of the second tower. Initially, both towers
would be filled with caustic soda solution. Hydrogen sulfide
would be passed to the first tower, forming Na2S first and then
NaSH. As the production of NaSH nears completion, the H2S that
is not consumed would be passed to the second tower, forming
Na2S in that tower. Once the NaSH was formed in the first
recirculating tank, the H2S flow from the degasifier would be
passed from the degasifier to the second tower to complete the
formation of Na2S and subsequently NaSH. Meanwhile the NaSH in
the first tower would be replaced with fresh caustic soda
solution. The gas stream from the second tower would then be
passed to the first tower. This process of alternating absorber
towers for the production of NaSH would be repeated. A system
of this type would eliminate flue emission in the current sulfide
reclamation system.
Future systems should consider a lower liquid level in the
degasifier tray and possibly the use of valve type trays as
designed by Koch Engineering Company. The valve type trays
consist of perforated decks on which round movable caps are
mounted. The caps which operate like check valves are approxi-
mately 2 inches in diameter and have a limited lift which is
accomplished either by a hold-down cage or by integral guide
legs and lift stops. The valves are made in different metal
gauges and are normally installed in alternating rows of light
and heavy valves, parallel to the outlet weir to provide good
vapor distribution over a wide range of air flow rates. At
lower air flow rates, the lighter valves are lifted to an open
position. As the flow rate increases, the lighter weight and
then the heavier weight valves open progressively wider to their
full open position. Even at the lowest loadings, air would flow
upward through the slightest crevice thus preventing any leakage
and making tray gasketing unnecessary. Tray gasketing, however,
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is available. The possibility is that this valve tray type
design using low liquid levels in each tray would eliminate the
downtime for clean-up that is required in the present system.
Low liquid levels in the trays with air flow mixing would keep
solids in the wastewater in suspension during its flow through
the degasifier. (16, 17) The number of trays required would be
increased dependent on retention time required. The same type of
tower of proper dimension and necessary trays could be used as
an absorber. Fresh caustic soda would flow into the top tray of
this unit and flow countercurrent to the H2S/air vapor. The
emergent solution at the bottom of the unit would move to tank-
age as sodium sulfhydrate or sodium sulfide solution in caustic
soda dependent on the number of trays used.
For the present sulfide reclamation at Blueside Company, a
second full set of check valve type air diffusers should.be in
stock for use as replacements when the tower is opened for clean-
ing. The downtime for the system would be reduced to 6 hours.
The dirty diffusers taken from the tower would be cleaned during
the interim period betxveen tower clean-outs. The size of the
drain-out pipe from each degasifier tray should be increased from
1% inch I.D. to 3 inch I.D. The level of the drain pipe should
be such that full drain out of the tray is possible. The
present drain pipe outlet on each tray allows one inch residual
of liquid in the trays and makes wash-out of solids difficult.
With a large drain pipe and additional spray heads in the water
line at the top of each tray section, spray washing of the tower
would be safer for the operator.
Secondary sedimentation of the degasifier effluent is
recommended for the removal of the coagulated proteins resulting
from acidification of the tannery wastewater to 5.5 - 5.0 pH.
The removal of these solids will reduce pollution loadings by
80% for suspended solids and 60% for BOD5 resulting in
substantial economies in sewer surcharges.
The sulfide reclamation system is recommended for use in
the treatment of sulfide bearing wastewaters. Waste stream
segregation of the sulfide bearing wastes coupled with pre-
treatment to minimize solids is necessary. The sulfide system's
useage reflects cost savings.
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SECTION 4
HISTORY OF THE PROJECT
This project was designed by Camp, Dresser and McKee of
Boston, Massachusetts and was proposed by The Blueside Company of
St. Joseph, Missouri to the Federal Water Pollution Control
Administration in 1970, A grant .was awarded covering the design,
construction, and operation of a sxilfide recovery process to
handle tannery waste, The plant was to be full scale, handling
all waste from a major tannery.
Preliminary experiments in the laboratory demonstrated
I that sulfides were released from tannery waste when sulfuric
I acid was added. Air was passed through the mixture to drive the
I hydrogen sulfide out of the liquid media. The air H2S mixture
was then run into an absorber where the H2S was converted to a
sodium sulfide solution in a reaction with sodium hydroxide.
Laboratory trials using this principle of successive
chemical reactions resulted in complete removal of the sulfide
from the liquid and the absorption in sodium hydroxide solution.
The resulting sodium sulfide solution was suited for unhairing
reuse. Based on the laboratory tests a pilot plant was designed.
A pilot plant was first installed at the Prime Tanning
Company, Berwick, Maine. Tests were conducted over a two week
period during which the degree of acidification and various flow
rates were tried. It soon became apparent that although acidi-
fication was releasing sulfide gas, it also was causing a
precipitate to form. The precipitate was the result of lowering
the isoelectric point of the proteins as the pH was changed from
12.0 to 5.0. The proposed design used a packed column in the
aeration step it was anticipated that the proteins would soon
plug the column.
The next modification tried was to use a diffusion technique
by bubbling air through the acidified waste water. Through this
work, rates were determined for complete sulfide release and for
the appropriate air rate to scrub the sulfide out of the water.
Calculations were made for scaling-up the pilot plant to a full
production sized unit.
The degasifier section design was based on pilot scale work
and an absorber was selected from commercially available
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TANNERY WASTE WATER
•W EQUALIZATION
OVERFLOW
AIR
PRIMARY
SEDIMENTATION
SLUDGE
DEWATERIN6
CENTRIFUGE
DEWATERED
SLUDGE
TO LANDFILL
AIR
H2S04
•AIR & H2S
H2S \
REMOVAL/
H2S
ADSORPTION
•NaOH
SECONDARY
SEDIMENTATION
I
CHLORINATION
TO TANNERY
TO
MUNI SEWER
SLUDGE
Figure 1. Waste water treatment, original design.
10
-------
equipment. The acififying degasifier section was of an
original design and included diffuser caps on the trays to
insure adequate mixing.
The operation of this equipment was first planned to follow
the scheme illustrated by Figure No. 1. This involved a pre-
treatment step of equalization and settling before acidification.
This would reduce the amount of solids in the acidification/
degasifier section. The desire to remove all the available
sulfide would not permit separation at the first step in the
process.
The alternate sequence adopted was to first mix in a wet
well, equalize and clarify in the next step. The clarified
supernatant was then sent to the degasifier. The sludge was to
be sent to a landfill. The resulting design is illustrated in
Figures No. 2 and 3.
The first plant runs of the sulfide reclamation system
proved to be frought with many problems and difficulties.
Solids build-up occurred around the diffuser caps and at the
bottom of the overflow drain. Flooding, carry-over of liquid
into the absorber and general imbalance of the hydraulics in
the towers also occurred.
The original construction was lacking ancillary units by
which to measure air flow, sample the air or liquid during a
run and to adequately control the process.
It had been recognized that the solids could increase
during the processing and bubble caps were judged necessary to
insure mixing. Solids deposition occurred on each level of the
degasifier and at the drain from the outer shell during the
first series of runs.
The acid mixing zone was on the top tray and the gas exited
at the side. The rate of air/gas flow caused material to be
entrained from the top of the tower into the absorber. This
caused chrome to reach the circulating caustic along with other
waste water and reduced the effectiveness of the absorption.
The carryover defect was not immediately apparent because
of seemingly normal operation for few hours each day. The back
pressure in the blower would increase from 6.5 - 7.5 psig to a
flooding range of 9 - 10 psig. The progressive increase in
pressure would allow only a few hours of operation before there
was a need to clean out the unit. It was during a cleaning
session that the carryover was identified.
The correction of the poorly located gas outlet pipe was
attempted by adding a sock filter in the piping to the absorber.
Although the sock removed material in excess of 1 micron in
11
-------
AIR SOURCE
TAMN1RY FOR SULFIDE
WASTE DEGASIFIER
WATER
CLARIFIER
10' BAFFLE
/ CLARIFIER
P-2
PUMPING STATION
L
M_f _M_ I
SLUDGE
CENTRIFUGE FOR
DEWATERING
I
SLUDGE
TO
LANDFILL
RIM
BYPASS
TO
MUNI
T SEWER
TO SULFIDE
RECOVERY
Figure 2. Waste water treatment, as constructed (1970).
-------
CLARIFIER VSUPPLYJ
RIM \^/
/
CLARIFIER
/
r
i
i
1 FLOW |—
SENSORI L
\
\
niiMnf
ruMKi
f-
A
DIFFUSER
[
tCID PUMP
f_, HYDROGEN SULFIDE GAS
V \ (Z& DOWNCOMER
Vl r^^SrH OVERFLOW
II • i nna r. -f-^ f^ir ,
Y-w.'Wr/, (^
Jr ^ ^
y/~ '/• / / ^*~ ' '.-'
/,/}.!•///• ' '/
SJ~ -If- '-
^ ^///^ m. •',
1. ;
>^i/2l^=^ V;
BypASS DEGASIFIERT TRAP
TO
MUNI
SEWER
JL
^\
•jMM
y' ^
y
^;-
:/,
^
(BLOWER
DEGA:
TOWER
ilFIER
AH FROM "FLUENT
WET WELL JO MUNI
SEWER
I [
NaOH TANKS
t-1
Figure 3. Sulfide reclamation system.
-------
diameter, the installation was difficult to maintain requiring
frequent cleanout and replacement.
Data on air flow was non-existant in early experiments and
with certain assumptions made, was labeled as 700 cfm. Later
work was to determine that the flow was in excess of 1,000 cfm.
The installation of sight glasses and pressure gauges gave
further insight to what was occurring in the tower. With these
and other corrections it was then assumed possible to monitor the
process. Flow rates, filling times, liquid levels and pressures
could now be obtained and more significant runs made.
The unit was modified to correct entrainment by moving the
gas outlet from the side to the center of the roof of the
degasifier. A two foot diameter column, four feet high was
installed as a demister section. A grating of plastic modules
were included for gas diffusion. This was followed by a water
spray of about five gpm, eighteen inches above the diffusion
layer. A six inch thick demister pad was placed above the spray
section and just before the exit to the gas line.
An U-shaped pipeline connection between the bottom level
downcomer pipe and the overflow was installed to insure adequate
water pressure between the degasifier and the absorber at the
moment the air blower was started. Without this balancing, the
air stream would follow the path of least resistance and create
difficulties. Proper operation, based on experimentation,
called for turning on the air blower when the lower sight glass
indicated a depth of 46 inches of liquid in the vessel.
A gate valve was installed between the degasifier and the
overflow tower to allow stand-by operation with the blower on.
The degasifier would then be full of effluent with effluent pumps
off and acid addition stopped, This was the mode of operation
used when effluent supply was exhausted. The operation of the
blower kept the diffuser caps from clogging and saved filling
time when processing was resumed.
A different type of bubble cap was designed utilizing a
flat sheet of neoprene with cross slits and held by a half union.
This was to minimize clogging. Trials were made in the top tray,
and while it did not clog, the neoprene flaps were soon distorted
and allowed severe leakage.
The diffuser design has not been solved since the unit still
requires cleaning every ten to fifteen days. This maintenance
consumes about twelve man-hours. The location of diffusers
immediately below the downcomer pipe caused bypassing of the air.
This in turn restricted the effluent flow and contributed to
the overflow condition. Selected diffusers were plugged and this
problem was reduced.
14
-------
A vent consisting of a two inch opening was installed to
allow trapped air to escape. This also aided in the correcting
of the overflow condition.
Imbalance existed in the absorber and caused flooding to
a height of sixteen inches above the caustic return line. The
flooding caused heavy misting of the caustic solution into the
tower flue. Examination disclosed that the plastic packing was
in complete disarray and the lower support grating out of place.
The movement of the packing into the recirculation line caused a
partial blockage. The condition may have existed from the first
day of installation or could have been caused by severe flooding.
Repacking of the absorber solved the flooding problem. In
addition, the caustic spray manifold had been installed so as to
aim at the demister pad. When this was turned downward the
caustic flow into the flue ceased. At the same time the high air
flow rate, 1,000 cfm, had contributed to carrying large droplets
into the stack. The lower flow rates tried further reduced the
carryover.
The unit was installed out of doors with no winterizing.
This resulted in frozen pipes, broken fittings and the unheated
caustic/sulfide solutions solidified to cause blockage. Sludge
build-up in pumps whenever they were stopped without flushing
hampered the systems operation. Corrosion in the solenoid
valves, automatic console components and electrical short
circuits caused excessive maintenance and consumed time. As
each repair was made a solution was tried which eventually
reduced these bottlenecks to a reasonable level. Insulation,
heating of solutions and tracing of lines with steam solved
many of the defects.
The original system specification called for the use of 10%
caustic soda in the recirculating tanks in order to form a 15%
sodium sulfide solution suitable for re-cycling as an unhairing
liquor. The low concentrations were necessary because of
freezing characteristic of the solutions. An 8% to 12% sodium
sulfide solution will freeze at 15°F as will a 10% caustic soda
solution. Higher concentrations freeze at higher temperatures
and make pumping difficult. The addition of heated lines,- tanks
and lines would allow higher caustic concentrations and minimize
flue gas emissions.
When the Blueside plant first started the use of the sulfide
recovery system, the production ranged between 5,000 to 10,000
hides a week. The effluent contained between 200 to 400 ppm of
sulfide. The increase to 20,000 plus hides a week raised the
sulfide content of the effluent to over 1,300 ppm.
Corrosion was one of the major deterrents to progress in
the sulfide recovery system. The center column of the degasifier
15
-------
supporting the bubbler cap trays was found to experience
corrosion at the welds. Seams required re-welding and cementing
to prevent further corrosion. The black iron sulfuric acid line
required replacement on two separate occasions. A check valve
arrangement should eliminate clogging and syphoning during
intermittent running. If dilute waste acid becomes available
then a polypropylene acid resistant piping would be required.
The acid proportioning pump experienced a broken elastomeric
diaphram on numerous occasions. Replacement of the diaphragm
provided no relief and a pump replacement presented the same
problem.
A rubber expansion boot between the air blower and its
muffler hardened with age and broke apart. Condensation in the
muffler caused corrosion and air leaks. The sound level at 1,000
cfm measured 95 decibels and required ear protection, Lower air
velocities were accommodated without ear protection.
The first plans called for a centrifuge to be used to
concentrate 8% solids from the bottom of the clarifier. This
was a poor choice and was soon discarded. The 15% level could
not be obtained as this was the lowest acceptable in a landfill.
The clarifier in turn was found unable to cope with the
heavy solids load. No baffle was provided at the center which
would direct the solids in a downward direction. There was no
skimmer to remove grease and soap scum.
With all of the system difficulties, it became necessary to
utilize the clarifier as a settling basin in order to produce
the 15% solids concentration required by the landfill operator.
This meant that a sludge depth of 24 - 36 inches was necessary to
reach the 15% content. Companion to this difficulty, entrance
roads at the only available landfill became impassable during
winter months. The clarifier rakes were stopped and the sludge
depth increased at the rate of 0.75 inches per 20,000 pounds of
bluestock produced. The fact that no other contractor would
take the sludge due to hauling distances and without adequate
clarification, the evaluation of the sulfide reclamation came to
a standstill during the winters of 1973/74 and 1975/76.
The pollution loading, generated by the processing of
20,000 hides per week, is comparatively very high. In order to
stay in production, the clarifier was modified to pump the sludge
out of the bottom of the basin to the clarifier rim. Here it
was allowed to concentrate for manual removal. It was this
ingenious arrangement that allowed production to continue as well
as clarifier repair in 1974 and major plant modifications in
1975. The pretreatment and sulfide recovery system has been
modified and expanded to avoid all the problems related in this
16
-------
history of the project. Other sections of this report detail
the present system and explain how the problems of the past
are now avoided.
17
-------
SECTION 5
THE MANUFACTURING PROCESS AND POLLUTION LOADING
MANUFACTURING PROCESS
The Blueside Company in St. Joseph, Missouri, is a wholly
owned subsidiary of Prime Tanning Company of Berwick, Maine. The
Blueside Company is engaged in the manufacture of chrome tanned
leather. The leather is not processed into finished leather;
it is shipped to customers as chrome tanned hides (or sides) at
an approximate 58% moisture content.
The leather produced is best identified by the term "leather
in the wet blue state", or bluesides.
All hides are received pre-fleshed. The hides may be
conventionally salted, brine cured or fresh (without salt cure).
Fresh hides are processed immediately as received. Cured hides
are processed as needed for production scheduling.
Process sequence is shown in Figure 4 and proceeds in the
following order. All wet processing is conducted in hide
processors of the cement mixer type.
RECEIVING: Hides are unloaded from rail-cars and trucks
daily. Hide bundle ropes are removed and the hides counted as
they are placed in pre-weighed numbered plastic coated "hide
cans" affixed to pallets. When each hide can is full, it is
re-weighed. Batch weights are made up in a staging area for
loading into the soaking hide processors.
SOAKING: Hides are soaked in water containing a surfactant
and alkali to allow rehydration to that state existing when the
hides were first flayed from the animal. The soak xvaters,
pH: 9.0, are drained upon unloading to the "common drain pit"
into which all the tannery wastes flow within the plant.
FLESHING: As the hides are emptied from the soaker, they
are individually clamped to a cable conveyor for transfer to a
whole hide fleshing machine.
The fleshings are caught in a box and the water from the
fleshing operation drains into the "common drain pit". The
fleshings when drained are removed to landfill.
18
-------
PREFLESHED HIDES
Receiving
Count/Weighing
Flesh
Loading
Bate
Wash
Pickle
Tan
Unloading
Sort
Siding/Count
Palletize/Weigh
Shipping
Figure 4. Process flow diagram, Bluesicle Co,
19
-------
HAIRBURN: The unhairing of the hide is conducted using a
hair destruction system consisting of sulfide and lime. Drain
solution is at a pH of 12.3 - 12.5.
RELIME: The hides are conditioned in limewater. Solubiliz-
ing of unwanted proteins, scud, and pigmentation is conducted in
this cycle. Drain solution is at a pH of 12.3 - 12,4.
WASHES: Deliming and the elimination of waste products is
conducted by batch washing for the most economical use of water.
Drain solution is at a pH of 12.3 - 11.9.
i
BATE: Additional deliming salt solution, a pancreatic
enzyme, and a surfactant are added to further eliminate unwanted
protein and animal fats from the leather making collagen fibers.
Drain solution is at a pH of 8.0 - 9.3.
WASH: A final,water wash to eliminate the waste products
of the bating cycle. Drain solution is at a pH of 8.0 - 9.3.
PICKLE: Salt is added to provide an 8° Baume solution of
brine which will protect the hides from acid hydrolysis.
Sulfuric acid is then added to lower the pH to a range of 1,8 -
2.Q in preparation for chrome tanning.
CHROME TAN: Sodium formate and chrome tan is added to the
pickle solution and hide stock. The pH range's 2.8 - 3.2.
NEUTRALIZATION: An alkali salt is added slowly to increase
the pH of the tan liquor for chrome fixation. Drain solution
is at a pH of 3.8.
UNLOADING: As each of the hide processors are unloaded
in turn, the chrome tanned hides drop onto an open mesh
conveyor. The processed tan stock is transported to a large
collection tub where the hides are spread out with the hide
tail ends at the input side of a whole hide wringing machine.
WRINGING: The hides are wrung to a 58% moisture content.
As the hides pass through the wringing machine, they fall onto
a conveyor where they may be sorted as whole hides or allowed to
be cut into sides by a siding rotary knife blade.
The chrome liquor from the wringing is drained to the
"common drain pit".
PALLETIZING: Hides or sides are counted, folded and
palletized. The pallets are covered with a plastic sheet.
Pallets are weighed prior to shipping.
20
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TABLE 1. TANNERY WASTEWATER AFTER SEDIMENTATION TO MUNICIPAL SEWER
WEEKLY PRODUCTION: 21,700 HIDES
***
PARAMETER
Flow
BOD5
Total Suspended
Solids
COD
Oil & Grease
Chlorides
Sulfide
Sulfate
Total Nitrogen
Ammonia Nitrogen
Alkalinity
Total Solids
As CaCO3
Calcium
pH
ed
n
gen
CONCENTRATION
Parts Per
Million (p. p.m.)
-
4,800
5,020
13,160
1,500
7,970
1,395
4,802
1,350
600
1,110
26,783
340
350
8.9**
Lbs./1/OOO
Ibs . Hides
1500*
60.0
62.8
164.6
18.8
99.7
17.5
60.1
16.9
7.5
13.9
335
4.2
4.4
-
Lbs . /Day
300,000*
12,010
12,560
32,926
3,753
19,941
3,490
12,015
3,378
1,501
2,777
67,012
851
876
-
* Gallons
** Standard Units
*** 60% Salt Cured Hides; 40% Fresh Hides
21
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POLLUTION LOADING
The tannery operates 24 hours a day. Hide processors are
loaded as they are emptied. As a result, the effluent is more
steady in volume and composition than is usually found in
tanneries. Normal production is 200,000 pounds of hides per
day utilizing 300,000 gallons of water. The average discharge is
1,500 gallons per 1,000 pounds of hides at 208 gallons per
minute.
The water used in housekeeping measures, domestic sewage,
pollution processing, sewer flushing, and boiler blow-down is
included in the 1,500 gallons per 1,000 pounds of hides.
A composite made from grab samples taken every 30 minutes
over a 24 hour period was analyzed to indicate pollution loading
remaining in the tannery wastewater after the clarifier. .The
samples were taken at the manhole to the city sewer. Table No. 1
lists the pollution parameter tested and the results obtained.
The sulfide reclamation system was not in use during the
sampling period.
The municipal ordinance governing industrial wastewater
pollution limits at St. Joseph, Missouri allows a maximum
sulfide content of 10 parts per million to flow into the
municipal primary treatment plant.
Evaluation of the tannery's process cycles identifies the
distribution of sulfide bearing waste liquors in each batch of
hides to be as in Table No. 2.
22
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TABLE. NO. 2
SULFIDE PRESENT AFTER EACH PROCESS CYCLE
Production
Drain
Sequence
Hairburn
Chemical Re lime
Water Relime
Wash
Wash
Wash
Bate
Bate Wash
Brine - Pickle
Cycle Useage Sulfide
Waste Water Present
Volume-gallons S=, ppm
1,400
1,400
1,400
1,400
1,400
1,400
1,000
1,400
1,700
12,500
9,535
4,893
5,160
2,407
1,560
1,330
2,013
1,000
1,030
28,928
Sulfide
for
Recovery, %
33.0
16.9
17.8
8.3
5.4
4.6
6.9
3.5
3.6
100.0
The waste streams at Blueside Company are not segregated.
All of the process cycles drain into a common pit which flows
from the plant into a wet-well. This includes the pickle and
chrome tan liquors when the hide processors (cement mixer type)
are unloaded on completion of bluestock processing. The chrome
tan liquors from the wringing operation also flow into the
common drain pit. Soak liquors and housekeeping water require-
ments also flow to the same pit.
As this report is written, a chrome recovery and recycling
system is in its second month of trials. During the course of
studies for the sulfide reclamation system, all process cycle
drains were into the common pit. With chrome recovery and
recycle, the chrome tan liquors are handled separately.
The process cycles listed in Table No. 2 are all sulfide
bearing. A lead acetate drop test on a hide at the end of the
pickle cycle will indicate that the residual sulfide present at
the start has been removed. The acidification of the pickle
cycle converts the sulfide present to hydrogen sulfide. Each of
the hide processors are vented to the atmosphere by roof top fans
and the hydrogen sulfide is pulled from the mixer as it is formed.
23
-------
TABLE No. 3
SLUDGE ANALYSES
LIQUID PORTION
PARAMETER
Total Dissolved
Solids
Chemical Oxygen
Demand
Alkalinity as
CaC03
Chlorides
Bicarbonate
Carbonate
Cr203
Total Nitrogen
Nitrates
Calcium
Sulfates
Sodium
Sulfide
pH
Parts per
Million (ppm)
57,372
15,840
5,000
19,600
3,944
1,056
2.8
2,300
Less than
0.01
300
10,800
18,000
Less than
0.01
8.6*
SOLIDS PORTION
PARAMETER
Chemical Oxygen
Demand
Chromium
Total Nitrogen
Sulfide
Parts per
Million (ppm)
241,600
12,500
31,400
80
* Standard Unit
24
-------
The sulfide bearing wastes total to 12,500 gallons per
batch. Coupled with soaking liquors, boiler blowdown, chemical
mixing water, and general housekeeping water useage; the amount
of wastewater to be processed through the sulfide reclamation
system amounts to 300,000 gallons/day.
The sulfide reclamation system provides the means for
removing and recovering the sulfide from the total effluent on
a continuous basis for reuse in process. The clarifier into
which the plant effluent flows allows settleable solids to
produce a sludge that must be removed on a daily basis to a
sanitary landfill.
The settled solids form an 8% solids slurry in the bottom
of the basin which is pumped to flow equalizing tanks. It is
then pumped to filter presses for dewatering to a 50% solids
content for removal to landfill.
The quantity of sludge at 50% solids to be removed daily
amounts to approximately 43,000 pounds for 200,000 pounds of
hides processed. Table No. 3 shows the analyses conducted on
a composite sample of sludge representative of two days sluage
removal. The pollutants and quantities shown indicate the
effects of the settling in the clarifier. The sludge solids
and its liquid portion show a total of 257,440 mg/1 of COD
present in the sludge whereas Table No. 1 shows that the
clarifier effluent has 13,160 mg/1. The same beneficial
reduction of quantities for other parameters is affected by
sedimentation in the clarifier.
25
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SECTION 6
WASTE STREAM PROCESSING
PRE-TREATMENT PRIOR TO SULFIDE RECOVERY
The effluent treatment system as first conceived at the
initiation of this project is outlined in the flow chart,
Figure No. 1.
The initial design was modified because of the belief that
as Blueside Company's production increased during its formative
years, wastewater technology would also advance. The advances
in wastewater technology would, potentially, require the dis-
mantling of antiquated equipment, thereby, increasing the costs
of modifications. Essentially, it is cheaper and easier to add
to a basic facility than to correct what appears to be a complete
package. It was a wise decision based on the progressive changes
that have been and are occurring in the tannery's effluent
treatment system.
A basic treatment system, constructed in 1970, was in
accord with the schematic diagram in Figures No. 2 and 3, and
began operation in the mid-year of 1971.
The initial wastewater treatment starts out with the wet
well into which the tannery wastewater flows and will vary
chemically throughout the day. At some point in time, it will
be alkaline to a pH of 12.5 and at other times, it will be acid
to a 3.0 pH. The wet well is covered and is exhausted by being
the source of air for the air-blower. Sulfide gases forming in
the wet well are; therefore, drawn into the degasifier tower.
Figure No. 5 is a photograph of the wet well.
Two pumps remove the wastewater collected in the wet well
to the center of the clarifier. The pumps are electrically
controlled by probe levels. At a given level, one pump is
activated; at an increased level, when wastewater flow is higher,
two pumps are operating.
As the wastewater flows into the center of the clarifier,
it is caused to flow downward and toward the bottom of the
basin by a rotating turbine. This downward flow assists in the
settling of sludge solids. The downward flow is also outward
toward the basin wall then upward to a point of outflow into the
clarifier rim.
26
-------
Figure 5. Wet well.
27
-------
TABLE NO. 4
WASTEWATER TREATMENT SYSTEM EVALUATION,1973
WEEKLY PRODUCTION: 16,200 HIDES
Parameter
Unit: mg/1
pH*
BOD5
COD
Settleable Solids
Total Solids
Suspended Solids
Sulfide
Alkalinity
Oil & Grease
Cr203
NH3N
TKN
Total Phosphorus
Influent
Composite
9.7
9,133
19,867
10,600
48,9.00
9,067
250**
2,896
570
295
520
1,101
40
Effluent
Composite
9.8
5,123
13,167
300
25,400
2,767
264**
2,660
21
190
517
954
20
Percent
Reduction
43.9
33.7
97.2
48.0
69.5
-
8.1
96.3
35.6
-
13.4
50.0
* Standard Unit
**Sulfide Reclamation not operational
Pollution loading reductions of 43.9% in BOD5, 33.7% in
COD, 97% in Settleable Solids, 48% in Suspended Solids,
96.3%in Oii and Grease, 35.6% in Cr203, 13.4% in Kjeldahl
Nitrogen, and 50% in Total Phosphorus are reflected in the
test results.
28
-------
The clarifier is 57 feet in diameter and has a normal depth
of 16 feet. The capacity is approximately 340,000 gallons. The
sludge collecting in the bottom is continually raked toward the
center sump for removal to a dewatering system. A pumping
station houses the effluent pumps which draw from the wet well,
as well as the sludge pumps from the bottom sump to the de-
watering step.
The clarifier is an essential part of the sulfide recovery
system. It's purpose is to remove large solids which might
interfere with the operation of the degasifier tower and to act
as a reservoir to smooth out the fluctuations in flow and the
quality of wastewater from the tannery. A more or less constant
quality of wastewater can be pumped at a constant rate to the
degasifier.
The liquid surface level of the clarifier is designed to
fluctuate over a range of one foot six inches. The volume in
this range is about 29,000 gallons or about 2.1 hours at the
design rate of flow to the degasifier.
The clarifier rim weir separating the center from the rim
(in the initial design) had one-half inch diameter holes every
18 inches all around the rim. The holes were positioned
approximately 18 inches below the top of the rim weir. The
liquid level was held at about six inches above the holes. If
the level dropped to three inches above the holes, the pump (s)
from the clarifier rim to the degasifier were throttled slightly.
If the level rose more than twelve inches above the holes, the
flow rate of the pumps was increased sufficiently to compensate
for the higher liquid level. Adjustment of the flow rate was
not frequent in the sulfide recovery runs.
Adjustment of the flow rate from the clarifier rim to the
degasifier is kept to a minimum because each time the wastewater
flow rate is adjusted, it is necessary to adjust the acid feed
system to the degasifier.
Evaluation of the basic wastewater treatment system for
its efficiency of reducing pollution loading was conducted in
May, 1973. Composite samples of the influent to the clarifier
as well as to the municipal sewer were averaged to yield the
data shown in Table No. 4.
Grab samples of the tannery wastewater flowing into the
wet well were taken every 30 minutes through a 24 hour period
and composited. Grab samples were also taken of the clarifier
effluent at the same time interval and composited.
For this evaluation, the sulfide reclamation system was
non-operational.
29
-------
FILTRATE
CLARIFIER
4 TURBINE RIM WEIR
IBAFFLE
TUIEIT
WASTE
WATER
BYPASS
-M—MO
MUNI
^ r SEWER
TO SULFIDE
RECOVERY
FILTRATE
FILTER
PRESS
^ SLUDGE
I
J_
DEWATERFp SLUDGE
TO
.LANDFILL/
Figure 6. Waste water treatment, as modified (1975).
30
-------
Three defects in the system's operation became apparent
as the tannery's productivity increased; failure of the
centrifuge to de-water, clarifier not settling and rake arm
breakage and failure of internal baffling.
The centrifuge proved worthless as a means of dewatering
the sludge generated in the clarifier. The centrifuge's inner
cone wore out frequently requiring constant and costly repairs.
Under the best operating conditions, the sludge concentrate
produced was seldom dewatered to 15% solids and satisfactory for
removal to landfill. The centrate returning to the basin was
practically the same concentration as the sludge solids delivered
to the dumpster for removal. It was necessary to allow water
separation to occur in the dumpster and remove the water with a
sump pump before the contractor would accept the sludge for
disposal in the sanitary landfill.
The strength of the rake arm assembly in the basin proved
inadequate to move the sludge collecting in the bottom of the
basin to the center sump. The rakes became distorted resulting
in a prolonged break-down.
Within three years, a 10 foot deep poly-vinyl baffle,
initially installed peripherally within the clarifier, 10 feet
in from the rim, became embrittled and crumbled. The purpose of
this baffle was to insure a downward flow of the influent in its
path to the clarifier rim. The downward flow is heeded to
enhance sedimentation of the settleable solids.
This baffle was rebuilt using steel plate. Two years later
this baffle collapsed during a shut-down inspection. The
inspection team, while pumping from the clarifier to lower the
liquid level were not aware of the unequal pressures existing on
both sides of the steel baffle. The sludge build-up was
considerably higher on the outside of the baffle than in the
center of the basin. When the liquid was lowered below the
sludge level on the Outside of the baffle, the pressure of the
sludge against the bottom half of the baffle caused the baffle
to collapse inward toward the center. This indicated that a
great deal of sludge settling was occurring beyond the baffle.
Stress increased on the ends of the rake arms in the absence of
a proper sludge removal to landfill operation and contributed to
baffle and rake failure.
A major redesigning and expansion of the wastewater treat-
ment system was made in 1975. The schematic diagram, Figure
No. 6/illustrates the current process. Sufficient flexibility
in the piping and design lay-out of this system has been
incorporated to allow other planned improvements to be
incorporated.
31
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TABLE NO. 5
WASTEWATER TREATMENT SYSTEM EVALUATION, 1976
(Pollution Loading Reductions of the 1973
system are from Table No. 4 - Weekly
Production: 21,700 Hides)
Parameter
Unit: mg/1
PH*
BOD5
COD
Settleable Solids
Total Solids
Suspended Solids
Sulfide
Alkalinity
Oil & Grease
Cr203
NH3N
TKN
Total Phosphorus
Phenol
Chlorides
Influent
Composite
8.8
7,586
20,286
7,676
36,282
7,675
915
3,946
1,140
242
540
1,395
11,030
Effluent
Composite
8.7
4,590
8,485
3,250
27,644
3,256
930
3,840
985
150
532
850
15
13
8,950
Percent
Reduction
(1976) (1973)
39.5 43.9
58.2 33.7
57.7 97.2
23.8 48.0
57.6 69.5
-
2.7 8.1
13.6 96.3
38.0 35.6
-
39.1 13.4
50.0
-
18.8
*Standard Units
NOTE: Sampling accomplished when turbine not running, grease
skimmer not operational, sulfide system not operational
and hair screen operational.
32
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Innovations built into the clarifier included increasing
the slope at the bottom to provide easier flow of the sludge to
the center sump. The rakes were reinforced and rebuilt into a
tri-fork, i.e., three rake branches instead of two. Shear strips
were installed in the rake drive mechanism. These shear strips
are gauged to break under known torque, thereby, preventing
distortion of the rakes under major stress. A baffle was
installed immediately peripheral to the center turbine to allow
direct downward flow for the settleable solids and provide
strength to the baffle structure. A grease trap was installed
from the liquid surface through the clarifier wall. A grease
skimmer was installed on the rake arm ends to cause greases and
fats floating on the liquid surface to be pushed into the
grease trap.
Wastewater flow after the 1975 modifications as shown in
Figure No. 6, still flows into the wet well. A bar rake screen,
installed at the wet well, screens the wastewater for removal of
fleshings and large particles of foreign matter (metal, plastic,
etc.), these are collected in a dumpster for removal to a
sanitary landfill.
The wastewater is then pumped to two 17,000 gallons equal-
ization tanks in a new pollution treatment building. From these
tanks, the wastewater is pumped through nozzles to impact on a
hair screen (.020" gauge mesh) to remove the pulped hair residue.
The hair free wastewater is then pumped into the equalization
sedimentation clarifier where settleable solids form sludge in
the bottom.
The sludge from the bottom is pumped into two 17,000
gallons holding tanks in the pollution treatment building. The
sludge is then pumped into one of two filter presses which
de-water the sludge to a 50 ~ 60% solids concentration for
removal to sanitary landfill. The liquid removed from the sludge
is pumped to the clarifier.
The new system was evaluated for its efficiency in reducing
the tannery's pollution loading in September 1976. Composite
samples of the influent to the wet well and to the municipal
sewer were analyzed to yield the data presented in Table No. 5.
The composite of the influent consisted of mixing grab
samples of the tannery wastewater taken at the wet well every 30
minutes for a twenty-four hour period. The effluent composite
consisted of taking grab samples at the manhole to the municipal
sewer at the same time intervals as that of the sampling at the
wet well. At the time of sampling, the turbine in the clarifier,
the grease skimmer, and the sulfide reclamation system were
non-operational, the hair screen was in use.
33
-------
Fieure 7. Degasifier and absorber towers.
34
-------
Tannery process changes, occurring between May of 1973 and
the 1976 wastewater treatment system evaluation, involved
reduction in the use of lime in the hairburn and relime process
cycles plus a reduction in the use of ammonium sulfate in the
delime cycles and reductions in the use of pickle acid and
chrome tan. Tannery productivity during the same period in-
creased from 16,200 hides per week to 21,700 hides per week.
Water conservation measures affected during this period resulted
in total water useage of 300,000 gallons per day and remained
relatively stable.
Comparison of the influent composites for the two evaluation
periods revealed that the pollution loading in 1976 had 16.9%
less BOD5, 2.1% more COD, 27.5% less Settleable Solids, 25.8%
less Total Solids, 15."3% less Suspended Solids, 266% more
Sulfide, 100% more Oil and Grease, 17.9% less Cr20s, 3.8% more
Ammonia Nitrogen, and 26.7% more Kjeldahl Nitrogen, as related
to a 33.9% increase in tannery productivity.
The percent efficiency of reduction comparison, between the
1973 and 1976 wastewater treatment systems, shows that the 1976
system has 4.4% less reduction capability in 8005, 39.5% less
reduction in Settleable Solids, and 82.7% less reduction in Oil
and Grease. These lower efficiencies are related to the non-
operational turbine and grease skimmer.
It should be noted that the 1976 effluent composite
contained 10.4% less BOD5, 983% more Settleable Solids, 17.5%
more Suspended Solids, and 4590% more Oil and Grease than the
1973 sample. The 1976 wastewater, treatment system showed
improved efficiencies of reduction by removing 24.5% more COD,
and 25.7% more Kjeldahl Nitrogen. This benefit is related to
hair removal by screening.
WASTEWATER TREATMENT THROUGH THE SULFIDE RECOVERY SYSTEM
As with the initial wastewater treatment system, wastewater
from the clarifier rim is pumped to the sulfide reclamation
system; i.e., to the degasifier.
The pumps to the degasifier are located in the chemical
control area of the tannery adjacent to the automation console.
The sulfide recovery system can be controlled from this point.
The sulfide recovery system is schematically outlined in
Figure No. 3. The degasifier is shown on the left in Figure
No. 7 and the absorber on the right.
The degasifier tower has two functions, acidification and
air stripping of hydrogen sulfide gas released from the
acidified tannery wastewater. Sulfuric acid is added to the
wastewater until 5.0 and 5.5 pH is reached.
35
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The degasifier tower consists of four trays, each 6 feet
deep, and 10 feet in diameter. The trays are placed on top of
each other to form the tower. The trays are connected to each
other by 80 air diffusers in the bottom of each tray and a 12
inch diameter downcomer for the liquid. The liquid is pumped
into the top tray and flows downward and out of the bottom tray.
Air is blown into a two foot high plenum under the bottom tray
and bubbles up through each tray and exists at the top through a
scrubber and mist eliminator. The liquid depth in each tray is
maintained at 39 inches by the overflow level of the downcomer.
The liquid volume in each tray is normally 1,915 gallons, or
7,660 gallons total in the tower. At design capacity of 230 gpm,
the flow through time is about 33 minutes. The design air flow
is 700 cfm.
Air pressure is maintained in the tower by a liquid trap
formed by the overflow tower. The overflow tower consists of
an outer shell four feet in diameter and an inner overflow out-
let pipe 10 inches in diameter, with a seven inch diameter
adjustable overflow section. The adjustable overflow pipe can
be set from a maximum height of 15 feet 11 inches to a minimum
of 11 feet 11 inches above the overflow level of the lowest
downcomer in the degasifier tower. Liquid flow is from the top
tray where acidification occurs, downward through the tower and
countercurrent to air flow through each downcomer in the degasi-
fying tower into the outer shell of the overflow tower and then
down the inner outlet pipe to the sewer.
The gas exit from the degasifier tower is from the top
center of the tower through a three foot diameter scrubber and
demister. The scrubber section consists of a two foot deep bed
of one inch diameter plastic shapes (Koch rings). Four nozzles
located about 18 inches above the bed continuously spray the bed
with about 5 gpm of fresh water to wash the exit gas free from
entrained particles of solids or liquids. Located above the
spray nozzles is a four inch thick pad of polypropylene fibers
which serve to filter out any water droplets picked up from the
spray system.
The demisted gas then passes to the adsorption tower.
The adsorption tower is a standard fume scrubber, Model
731.5 produced by the Heil Process Equipment Corporation. This
scrubber is guaranteed to remove 99% of the H2S from the gas
stream if the air flow is limited to 700 cfm and the sodium
hydroxide solution maintained above pH 10. The recirculation
rate through the scrubber should be no greater than 20 gpm at
a pressure of 20 psig. As the air flow is increased to 1,000
cfm, the adsorption capability will drop significantly to a
range of 80 - 90%. Two solution tanks equipped with sensing
probes allow the preparation of a known concentration of sodium
36
-------
hydroxide solution for use in circulation. Each tank, in turn,
is allowed to circulate through the adsorption tower to a point
of sulfide saturation. The saturation point is sensed by a
continous sampling of the adsorption tower flue exhaust for the
presence of H2S gas. ~At the first emission of H2S, an alarm
sounds indicating that the solution tanks should be switched.
The first tank, after analysis, is sent to storage and a fresh
solution of caustic soda is routinely made up.
37
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SECTION 7
EQUIPMENT DESIGN FACTORS
Basic design data for the degasifier and absorber towers
requires knowledge of the liquid and vapor characteristics
that will be present therein. The data required includes the
following:
a. Flow rate per unit time
b. Density, lbs/ft3
c. Pressure, psig
d. Temperature, °F
e- Total wastewater volume to be treated daily
f. Corrosion resistance
g. pH conditions
The densities of liquid and vapor loadings are required at
actual inside tower conditions of temperature and pressure.
Design specifications for the sulfide reclamation system were
230 gpm or less of liquid flow into the top tray of a four tray
tower with each tray interconnected by a 12 inch diameter down-
comer and 80 check valve type air diffusers per tray. Air was
to be provided by a positive displacement blower at 700 cfm at
a maximum of 10 psig into a plenum at the base of the tower.
The liquid depth in each tray was specified at 39 inches. The
downcomers were to be immersed deep enough into the liquid of
the next tray so that air could not escape upward through them.
Air was to flow upward through the air diffusers countercurrent
to the liquid flow. The liquid was to leave the bottom tray
and flow into an overflow tower having an adjustable outlet pipe
for regulating liquid levels and pressures in the degasifier.
The liquid would leave the overflow tower and flow to the city
sewer. It was anticipated that the wastewater entering the
tower would range in temperature from 50°F to 80°F. The liquid
would have a maximum 1.2 specific gravity.
The mechanical requirements of the degasificr can be
calculated from the specifications. The internal liquid and
vapor loadings are required to ensure proper tray design.
The mechanical data requiring calculation include the
following:
38
-------
a. Wall and tray thicknesses
b. Tower cap
c. Manholes
d. Tray support rings
e. Downcomer
f. Vapor inlets and outlets
g. Liquid inlets and outlet
h. Holddown
i. Nuts and bolts
j. Number of trays
k. Vapor diffusion method
1. Corrosion allowance
The engineering calculations relating to the degasifier at
Blueside Company are shown in the Appendix. The current sulfide
reclamation was scaled up to a full production unit by Camp,
Dresser, McKee, Inc., of Boston, Massachusetts.
Degasifying or stripping is the release of a gas from a
solution by contacting the liquid with an inert gas. In
sulfide reclamation, acidification to 5.0 - 5.5 pH lowers the
solubility of the hydrogen sulfide in the wastewater causing its
release from solution. Air serves as the inert gas to strip the
gas and move it to the absorber.
Gas absorption is the opposite of stripping; i.e., in
absorption, the gas is caused to dissolve into a liquid or
react with the liquid.
The limiting amount of a gas dissolving in a liquid
at a given temperature and pressure is termed its solubility.
The method of determining the limit is to expose the liquid to
the gas for a sufficient length of time so that no more gas
dissolves at the given temperature or pressure. In order to
make the time as short as possible, vigorous mixing of the
solution is necessary as well as a relatively large area of
contact between the gas and the liquid.
In the solution of/or the reaction of a gas with a liquid,
the principle of countercurrent flow is used. The gas is passed
first through the almost saturated solution and consecutively
through less and less saturated solution and finally through the
pure liquid. The gas is commonly introduced at the bottom of
the tower, into the top of which the liquid is fed.
The tower may be packed with inert dispersion plastic rings
in random fashion or contain bubbling plates to furnish the
desired surface of contact and the necessary mixing.
A heat change always accompanies solution of a gas in a
liquid.
39
-------
The process of selective gas absorption is typified by the
sulfide reclamation system. The absorption of hydrogen sulfide
from a hydrogen sulfide/air mixture occurs in the absorption
tower using a sodium hydroxide solution. The air is the inert
gas and flows through the tower without reaction.
The absorption tower must be tall enough so that there is
sufficient contact time between the gas and the liquid and
large enough in cross sectional area so that the required
volume of flow can be accommodated. If too much liquid is run
down for the size of the tower, then the upward flow of gas
will hold up the liquid, causing the tower to flood.
The amount of gas that a liquid can dissolve at a given
temperature is determined by Henry's Law, which states that the
partial pressure of a gas in equilibrium with a solution is
equal to a constant times its concentration in the solution or
pa = H Xa
The constant, H, is different for each system and for each
temperature and it must be determined experimentally.
The concentration of the hydrogen sulfide in the flue
emission from the absorption tower must be less than the concen-
tration in the sodium hydroxide. The difference between the
actual concentration and the equilibrium concentration is
necessary in order that there be a driving force to cause
absorption to take place.
For each type of absorption tower and set of operating
conditions, there is a specific absorption coefficient. This
coefficient, K, depends on the type and composition of the gas
and solution involved, the type of packing, the temperature,
and the gas and liquid flow rates. The coefficient is defined
as the amount of material absorbed per unit time, per unit
contact area, per unit of driving force. The area of contact
through which the gas is being absorbed cannot be measured;
therefore, the unknown area is included with the coefficient, K,
and determined experimentally as coefficient times the area, Ka.
After the amount of material absorbed per unit time is determined
for the whole tower by direct measurement, it is divided by the
tower volume, and by the driving force giving the final form
Ka = Material Absorbed (Ibs)
Time (hr) x Tower volume (cu ft) X Ax
where Ax is the driving force. The Ax driving force is the
difference between the actual concentration of the liquid and
that which it would have if it were in equilibrium with the gas.
Since Ax may vary throughout the tower, an average may be used.
40
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For any given installation, the absorption coefficient
depends on the liquid flow rate, the gas flow rate, the temper-
ature and concentration of the liquid and gas. Because the
open cross-sectional area of the packed tower is not known, the
gas and liquid rates are usually given in superficial velocities.
This velocity is defined as the velocity the liquid would have if
it were flowing through and completely filling the tower when
empty of packing. (15, 17)
When the gas being absorbed is very soluble or reacts
completely, the liquid rate is not important and the coefficient
is affected most by the gas rate.
Absorption need not be performed in a packed tower. It
may be done in a bubble cap tower or in a tank where the gas is
bubbled through a liquid. If a tank is used, good dispersion
of the gas in the liquid is necessary and may be obtained by
violent agitation of the liquid with a stirrer or introduction of
the gas into the liquid through a porous plate.
One of the first steps in the design of a degasifier or an
absorber is to determine how many theoretical trays are required
to achieve full liberation of the H2S to the air flow for the
degasifier or full absorption of the H2S into the sodium
hydroxide for the absorber. On a theoretical tray, the concen-
tration of the H2S is in equilibrium with the H2S dissolved
in the liquid; Henry's Law, pa = HXa
Where: pa = partial pressure in the atmosphere of the
hydrogen sulfide
Xa = mole fraction of hydrogen sulfide in the
liquid
H = Henry's Law constant
Perry's Chemical Engineering Handbook lists data for H2S in H20
solution equilibrium at different temperatures. (10, 11)
Henry's Law constant for H2S in H20 at 20°C is given as
H = 4'.-8 2 x 104
Although the degasifier has a chemical phenomena associated
therewith; i.e.,
Effluent .Sulfides + H2S04 -^Effluent Sulfates +H2S
the reaction kinetics can be assumed instantaneous when
compared to the time to achieve vapor-liquid equilibrium.
The fifth data point in Table No. 21 shows that 2.21 pounds/minute
of H2§ were fed to the degasifier and 0.02
41
-------
pounds /minute flowed to the sewer in the degasified effluent
leaving 2.19 pounds/minute of E^S liberated from the wastewater
for movement to the absorber. At a molecular weight of 34,
the 2.19 pounds represents 6.44 x 10~2 moles/minute of H2S.
At an air flow rate of 320 cfm having a specific volume of
13.5 cu ft/lb, the air rate is 23.7 pounds/minute. Considering
an average molecular weight for air of 29, the air provides 0.817
moles/minute .
The mole fraction of H2S and air is 7.33 x 10~2 and 0.927
moles/minute respectively. Therefore, the total moles/minute
for the overhead gas is 0.872 moles/minute. Allowing 2 psig
for pressure drops through the demister pad, piping and absorber,
the internal pressure at the top of the degasifier is 1.15
atmosphere (absolute) . Thus, the partial pressure for the H2S
is 7.33 x 10~2 x 1.15 atmospheres or
pa = 8.43 x 10~2 atmospheres
Calculating for Henry's Law:
pa = H Xa
Xa = pa/H
Xa = 8.43 x 10~2/4.82 x 104
Xa = 1.74 x 10~6 mole fractions of H2S in the liquid
Determining the weight of H2S remaining in the solution on the
theoretical tray requires the calculation of the total moles on
the tray. At 240 gpm of influent flow at 8.5 Ibs/gallon, the
theoretical tray holds about 2,040 pounds/min of solution. Using
the molecular weight of the principal component, i.e., water at
18 Ibs/mole as a basis for calculation, there are 113.3 moles
on the tray. For 113.3 moles/min of water on the tray, there would
be:
113.3 moles/min x 1.74 x 10"6 mole fractions of H2S
or
1.98 x 10~4 moles of f^S in the liquid
At a molecular weight of 34 for H2S, 1.98 x 10~4 moles represents
6.72 x 10-3 or 0.00672 pounds/min of sulfide remaining in the tray's
solution.
Since the actual data shows 0.02 pounds/minute of H2S in
the degasifier effluent, no further tray calculations are
42
-------
necessary. The data shows that one theoretical equilibrium
tray is required in the degasifier.
To design for one theoretical tray with vigorous agitation
and expect equilibrium to be achieved momentarily is inconceiv-
able. The design philosophy provided one tray for acidification,
mixing, and distribution, another tray at the bottom for air
flow distribution, and two center trays of the same design at an
assumed efficiency of 50% to do "the work".
The absorber used in the present sulfide reclamation
system is a commercially available unit. However, the same
calculations using Henry's Law may be applied to determine
its adequacy. Data relating to the liquid-vapor system of the
absorber would be used. The calculations for the present
absorber indicates its adequacy at design specifications. If
flue emission is to be eliminated for higher sulfide input,
additional absorption capability is needed.
43
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SECTION 8
OPERATIONAL CHARACTERISTICS OF THE SULFIDE RECLAMATION SYSTEM
The sulfide reclamation system has the following process
variables that must be recognized for proper operation:
a. Sulfide content of the degasifier influent and
effluent, mg/1
b. Total wastewater volume to be processed daily
c. Influent flow rate, gpm
d. Air flow rate, cfm
e. Acid flow rate, ml/1 for acidification to 5.0 - 5.5 pH
f. Liquid levels existing within the degasifier trays
g. Overflow tower outlet setting for controlling liquid
levels
h. Absorber type and capacity
i. Caustic soda concentration
j. Caustic soda recirculation rates through the absorber
k. Basic knowledge of sulfide chemistry (covered in the
appendix section of this report)
Safety of operation in maintaining a mixture of hydrogen
sulfide gas in air below the lower explosive limit of 4% is a
governing factor in the operation of the sulfide reclamation
system.
The sulfide content in the clarified wastewater must be
known and checked periodically prior to and during operation of
the system. The sulfide content determines the influent flow
rate to be used in relation to the air flow rate. The influent
flow rate governs the amount of hydrogen sulfide gas that will
be liberated by acidification to a 5.0 - 5.5 pH.
The air flow rate should provide the amount of dilution air
to keep the concentration of hydrogen sulfide gas below the
lower explosion limit as it flows to the absorber. The relation-
ship of the influent flow rate and air flow rate is that the
influent flows governs the sulfide input into the degasifier
while the air flow serves to dilute the hydrogen sulfide gas
to a safe concentration for reclamation.
The total wastewater volume to be processed daily
establishes the sizing of the sulfide reclamation equipment and
the degasifier flow rate. Acidification to a 5.0 - 5.5 pH is a
44
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AIR
Table No. 6
INFLUENT SULFIDE CONTENTS
VS.
FLOW RATES OUTSIDE OF EXPLOSIVE RANGE
Influent Sulfide,mg/l
Influent Flow, gpm
Sulfide, Ibs./min.
H^S, Ibs./min. to
Absorber
Air Flow Rates
cfm Ibs./min.
100 7.41
200 14.82
300 22.23
400 29.64
500 37.05
600 44.46
700 51.87
800 59.28
Acceptable Air Flow Rates 1
Are Enclosed j
200
100
0.17
0.18
150
0.25
0.27
230
0.38
0.40
% H2S in Air
2.4
1.2
0.8
0.6
0.48
0.40
0.34
0.30
3.5
l.E"
1.2
0.9
0.72
0.6
0.52
0.45
5.1
2.6
1.8
1.3
1.1
0.89
0.76
0.67
400
100
0.33
0.35
150
0.50
0.53
230
800 1200
100
-T
0.77 0.67
0.82
% H9S in Air
4.5
2.3
1.6
1.2
0.9
0.78
0.67
0.58
6.7
3.4
2.3
1.8
1.4
1.2
1.0
0.89
9.9
5.2
3.6
2.7
2.2
1.8
1.6
1.4
0.71
150 I 230 100
1 • • • r
1.0 1.54 1.0
!
1.09 1.64 1.06
150
1.5
1.59
230
2.3
2.44
% H2S in Air % H~S in Air
8.7
4.6
3.1
2.3
1.9
1.6
1.4
1.2
12.5
6.7
4.6
3.4
2.8
2.3
2.0
1.8
18.1
9.9
6.9
5.2
4.2
3.6
3,1
2.7
12.5
6.7
4.6
3.4
2.R
2.3
2.0
1.8
17.7
,.7
6.7
5.1
4.1
3.5
3.0
2.6
24.8
12.9
9.9
7.6
6.2
5.2
4.5
3.9
1600
100
1.34
1.42
I
150
2.00
2.12
230
3.07
3.26
% H2S in Air
16.1
8.7
6.0
4.6
3.7
• 3.1
2.7
2.4
22.2
12.5
8.7
6.7
5.4
4.6
3.9
3.5
30.6
18.0
12.8
9.9
8.1
6.8
5.9
5.2
en
-------
function of the acid pump setting and running time. 0.8 ml/1
of acid per liter of influent for each 1.0 pH unit decrease is
required. The wastewater at Blueside Company has an average pH
of 8.5 and requires 2.7 ml/1 of 66° Baume sulfuric acid to lower
to 5.0 - 5.5 pH.
The sulfide content in the influent, the influent flow
rate, the air flow dilution rates and the concentration of
hydrogen sulfide in air enroute to the absorber is shown in
Table No. 6. This table defines sulfide reclamation ranges
of low to high sulfide bearing wastewater within varying influent
flow and/or the air flow for controlling the percent hydrogen
sulfide in air to a point below the lower explosive limit.
The design specifications for the sulfide reclamation
system set 700 cfm of air flow with a maximum sulfide capture
of 700 mg/1 from degasifier influent flow of 230 gpm or less.
The pounds of sulfide per minute for a degasifier influent of
230 gpm calculates to 1.34 pounds, or 1.42 pounds of hydrogen
sulfide gas when the effluent is acidified to 5.0 - 5.5 pH.
An air flow of 700 cfm at 13.5 cubic feet per pound of air
calculates to 51.85 pounds per minute of air. The percent of
H2S in air flowing to the absorber each minute equals :
1,42 lbs/H2S x 100 _
(51.85 Ibs of air + 1.42 Ibs H2-S)
The percent concentration of hydrogen sulfide in the air flow
is well below the lower explosive limit. Table No, 6 shows
that a high sulfide bearing influent having 1,600 mg/1 can be
processed through the sulfide reclamation system at 100 gpm
of influent flow and an air flow of 700 to 800 cfm of diluting
air allowing for proper reclamation below the lower explosive
limit ,
Lower sulfide content in the influent will allow the air
flow rate to be reduced. A lower air flow would increase the
contact time in the absorber. The effective absorbing zone in
the present absorber is 11.2 cubic feet. The calculated contact
time for the various air flow rates is shown in Table No. 7.
46
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TABLE NO. 7
ABSORBER CONTACT TIME AS RELATED TO AIR FLOW RATES
Air Flow, Contact Time,
cfm Seconds
100 6.72
200 3.36
300 2.24
400 1.68
500 1.34
600 1.12
700 0.96
800 0.84
900 0.75
1,000 0.67
The absorber in the present system was built by the Heil
Process Equipment Corporation and is listed as their Model 731.5.
The high sulfide reclamation of this unit is made possible by
its design. The air hydrogen sulfide stream enters the absorber
near the bottom and is contacted by a caustic soda solution
draining from the packing within the absorber. The air then
enters the packing and rises countercurrent to the flow of the
caustic soda solution. The route through the packing increases
the contact between the air stream and the caustic soda, for
maximum absorption of the hydrogen sulfide. The air stream is
given a final caustic soda wash as it passes through the sprays
used for distributing the caustic soda solution over the packing.
The clean air passes through a mist eliminator before leav-
ing the absorber which removes 99% of the entrained moisture.
The absorber has the capability to remove 99% of the hydrogen
sulfide providing the air flow is no greater than 700 cfm and
the recirculation of the caustic soda solution is 20 gpm at 20
psi with a minimum of 10.0 pH, The absorber has a maximum air
flow rate of 900 cfm.
The air blower installed in the sulfide reclamation
system can deliver 1,000 cfm of air. However, full use will
create an imbalance in the total system. A variable drive type
of air blower having a maximum air output of 800 cfm would be
preferred when degasifier influent and the air flow dilution
are considered. Proper and continuous operation of the system
requires the air flow to be reasonably unrestricted in its flow
through the degasifier to the absorber. The air flows from the
air blower through a six inch diameter pipe to the air plenum
at the bottom of the degasifier. The air flows through 80
air diffusers in each of the four trays in the degasifier. Each
diffuser has 12 orifice openings for air passage. The total area
of the orifice openings in the 80 diffusers on a tray is equal
47
-------
TABLE NO. 8
THEORETICAL OPERATING CONDITIONS FOR 99% SULFIDE RECOVERY
DEGAS IFIER TOWER
Air
Flow
CFM
700
Air
Pressure
psig
8
Pressure
Drop per
Level
1.8
1st Level
Press.
Gauge
psig
6.2
Sight
Glass
inches
2nd Level
Press .
Gauge
psig
4.4
Sight
Glass
inches
3rd Level
Press .
Gauge
psig
2.6
Sight
Glass
inches
4th Level
Press .
Gauge
psig
1.8-0
Sight
.Glass
inches
Overflow
„ .Tower
Sight
Glass
inches
Absorption
Tower
Caustic pH
Circulation
Min. pH 10
20 gpm @
20 psi
Effluent Flow; 200 qpm or be low; maximum ijs 2_30___gjpjn_ Sulf ide Absorption Capacity 700 ppm
Height downcomer level physical 39"; No. of bubblers is 80 with 12 orifice openings
ACTUAL OPERATING CONDITIONS
oo
!
Time
(mins)
0
30
60
90
120
150
180
210
240
270
1 '
Air
Flow
CFM
1080
Air
Temp.
OF
32
125
130
130
130
130
Console
psi
7.5
8
8
. °
8
8
8
1st Le
Press .
Gauge
psig
5.0
5.5
5 . 5
5.5
5.5
.
5.5
vel
Sight
Glass
inches
44
39
39
39
39
39
2nd L
Press.'
Gauge
psig
3.5 ^
4.0
4. 0
4.0
4.0
4.0
evel
Sight
Glass
inches
\
44.5
43
43,5
43.5
43.5
43.5
3rd ]
Press
Gauge
psig
2.2
...2.5
2.5
..2.5
2.5
2.5
Level
Sight
Glass
inches
44
49
49
4.9
49
49
4th I
Press .
Gauge
psig
0
_Q
.Q
0
0
0
jevel
Sight
Glass
inches
36
47
4-7
46
46
46
Overflow
Tower
Sight
Glass
inches
198
198
198
198
198
198
-------
to the cross-sectional area of the six inch diameter air input.
The orifice openings in the diffusers will begin to get
clogged with solids from the influent and a back-pressure created.
As more orifice openings plug up, the pressure builds to 12 psig
signalling the need for an internal cleaning of the degasifier.
The section of diffusers with longer service life has not been
achieved.
The operating step for starting the sulfide reclamation
system begins with the pumping of the clarified effluent to
fill the degasifier. The degasifier at start up will have 46
inches of effluent showing in the bottom tray's sight glass.
During continuous operation, the clarifier level change per unit
time is used to determine flow rate.
The air is provided at a constant rate and is limited by
the size of the air blower, the speed and the pressure under
which it operates. When the influent flow and air rates are
constant, the degasifier will stabilize with levels at each
tray in accordance with the pressure in each tray. Table No. 8
compares the theoretical conditions of operations as well as the
actual conditions existing during system operation. The
theoretical pressure drop from the bottom tray to the top was
calculated to be 1.8 psig per tray. In actual operation, the
pressure drop for the bottom tray was 2.5, for the second tray,
it was 1.5, for the third tray, it was 1.5, and the pressure
drop in the top tray was 2.5 psig or an average of 2 psig per
tray. The design pressure drop per tray is essentially accurate.
At the time these measurements were made, the air flow rate was
1,080 cfm, and three air diffusers were still located below the
downcomer in each tray.
The liquid levels as shown by the sight glass readings for
each tray indicate a variation in the downward flow of the
influent. Flooding is occurring with the top tray holding 46
inches of liquid (after 60 minutes of operation) the third
level 49 inches, the second level 43.5 inches and the bottom
level at 39 inches. The height of the downcomer in each tray
is 39 inches.
The lower tray reflected a condition where some air flow
was passing into the downcomer and flowing in part through the
overflow tower. The majority of the air flow is countercurrent
to the downward flow of the effluent.
The t'otal of the sight glass readings is 178.5 inches. The
pressure reading of the air blower was 7.5 psig and corresponds
to 207 inches of water pressure for the overflow tower setting.
The actual overflow tower setting was 198 inches. The
difference between the theoretical and the actual is within the
calibration of the pressure gauges. However, since flooding was
49
-------
TABLE NO. 9
ACID PUMP SETTING VERSUS INFLUENT FLOW IN GALLONS/MINUTE
TO REACH pH RANGE 5.0 - 5.5
m
o
Open Turns
on Influent
Pump Valve
0.7 - 0.75
1.0
2.5
Flow in
Gallons/
Minute
50
60
75
80
100
125
150
175
200
250
Liters of
Influent/
Minute
189.
227.
283.
302.
378.
473.
567.
662.
757.
946.
25
1
88
8
5
13
75
38
0
25
mis. of
Acid/Liter
Required
2
2
2
2
2
2
2
2
2
2
.7
.7
.7
.7
.7
.7
.7
.7
.7
.7
Total mis.
of Acid
Required
511
613.7
766
818
1022
1277
1533
1788
2044
2555
Acid Pump
Setting
72
82
109
116
146
175
205
238
280
351
-------
occurring, the overflow tower setting should have been increased
to compensate.
The levels of solution in the degasifier are controlled by
the overflow tower outlet pipe setting. In the overflow tower,
the outlet pipe is sleeved into a stand pipe allowing it to be
raised or lowered. This adjustment capability permits the
degasifier to be operated at varying pressures.
For the studies in this report, the level was established
at a steady flow and remained constant. When the diffusers in
the degasifier are clean, the height of the overflow tower
outlet is set for an operating pressure of 7 psig which converts
to 193.7 inches of height or, 27.67 inches of water per psig.
When clogging occurs this height is reset to a maximum of 12
psig.
The liquid effluent from the degasifier flows directly to
the city sewer system. Initially provision was made for
chlorination of unremoved sulfide. Hydrogen peroxide is also
suitable for the trace sulfide removal. Present costs indicate
that oxidation using hydrogen peroxide is more economical on a
standby basis. Under normal operation at a 5.0 - 5.5 pH, data
shows that the sulfide remaining is essentially zero. The need
for oxidation is, therefore, limited and the cost for oxidation
is negligible under proper operation. Under upset operation,
the quantity of oxidizing agent needed increases very signifi-
cantly. This will be discussed later in a separate analysis of
total costs.
The sulfide bearing influent entering the degasifier is
acidified with sulfuric acid. The sulfuric acid is fed at a
constant rate by pump under manual operation. The operator
measures the pH of the effluent and adjusts the pumping rate of
the acid. From the pH of the influent, the operator can make
desired setting on the acid pump as shown in Table No. 9. The
acid enters the top of the degasifier and mixes with the
incoming sulfide bearing wastes within the liquid zone. Complete
mixing is obtained by the turbulence in the solution caused by
the bubbles of air. The sulfide is converted to hydrogen sulfide
in accordance with the equation:
Effluent Sulfides + H2S04 -* Effluent Sulfates + H2S t
The ionization of hydrogen sulfide at pH 5.5 is less than
5% assuring 95% of the hydrogen sulfide as the molecule rather
than the ion. The hydrogen sulfide then mixes with the air
stream and is carried to the absorber. The degasifier has four
trays. Acidification takes place on the top tray. Two central
trays provide further diffusion of the II2S. The bottom tray is
used as a dis-engaging zone for the liquid and air stream.
51
-------
H2S - 2NaSH
NaOH + H2S - NaSH + H20
&
1-1 NaSH + NaOH - Na2S + H20
O1
0 15 30 45 60 75 90 105 120 135 150
TIME: MINUTES
Figure 8. Absorber sulfide capture progression.
52
-------
The degasifier influent flows downward through the
degasifier, countercurrent to the air. The four tray degasifier
tower assures a good removal of the hydrogen sulfide. A demister
above the top tray prevents the carry over of acid wastes to the
absorber .
The absorber, as previously described, has caustic soda
solution recirculating from a batch tank through the absorber.
The hydrogen sulfide is absorbed by the caustic soda in ac-
cordance with the following equations which occur simultaneously
in the presence of excess caustic soda:
HSH + NaOH -» NaSH + H20
and
Excess
NaSH + NaOH — » Na2S + 2 H20
When the caustic soda solution is depleted having absorbed
hydrogen sulfide to where the efficiency of the absorption
process is decreased, a discharge of hydrogen sulfide will occur
from the absorber flue. Control analyses were conducted during
system operation on the caustic recirculation tank at regular
intervals. When the analysis indicates 2% residual caustic
soda, a change is made to the other batch tank containing fresh
caustic soda. The saturated tank is analyzed for sodium sulfide
content and is sent to storage for tannery recycle.
If the tank at 2% caustic soda residual was allowed to
recircalate through the absorber for a prolonged period of time
in the presence of excess hydrogen sulfide (as indicated by H2S
emitting through the absorber flue) the reaction would proceed as
follows in the absence of caustic soda, resulting in the
formation of sodium sulfhydrate:
Na2S + HSH -> 2 NaSH
Table No. 10 shows the progressive change in chemical composition
of the caustic soda in the tank as it is circulated through the
absorber. The data is also presented in Figure No. 8.
Figure No. 8 illustrates the formation of sodium sulfide
proceeding gradually until approximately 2% caustic soda remains.
From this point on, hydrogen sulfide readily consumes the
residual caustic soda and proceeds to react with the sodium
sulfide to form sodium sulfhydrate. This reaction of
sulfhydrate formation requires an excess of hydrogen sulfide to
exist. This is not a desirable mode of operation as hydrogen
sulfide is discharged from the absorber flue. The flue emission
becomes excessive after approximately two hours of sulfide
reclamation for each fresh tank of caustic. Tanks of circulating
caustic are routinely switched each two hours of operation
53
-------
1306ppm
@ 3.86%
700-
600-
500-
400-
FLUE EMISSION
pptn S
1386
400
268
197
148
47.6
38.4
25.0
17.2
.355
% NaOH
3.86
6.73
9.15
11.7
14.2
19.6
23.2
26.2
29.8
39.0
" 300H
E
200-
100-
20 30
NaOH CONCENTRATION
Figure 9. Sulfide flue emission vs. caustic soda concentration.
54
-------
TABLE NO. 10
ANALYSIS OF CAUSTIC SODA SOLUTION TANK
AS SULFIDE ABSORPTION PROCEEDS
COMPONENT ANALYSIS, Percent
Elapsed Time Na2S NaHS NaOH
0
15
30
55
75
105
135
150
mins .
mins .
mins.
mins.
mins .
mins .
mins .
mins.
2
4
6
6
6
7
6
2
.35
.68
.13
.75
.91
.3
.56
.2
—
-
—
—
-
-
2.35
6.84
6.
4.
3.
2.
2.
2.
_
—
2
68
36
72
56
12
The flue emission analyses for sulfides from several trial
runs was also correlated with the analyses of the caustic soda
concentration. The data relationship which is shown in Figure
No. 9.
The effectiveness of the absorber is directly related to
the concentration of the caustic soda circulating through the
absorber. At higher caustic soda concentrations, the flue
emission decreases. As the caustic soda concentration decreases,
the flue emission of hydrogen sulfide gas increases.
There are several factors that contribute to the effective-
ness of the absorber. At high alkaline pH, the hydrogen sulfide
present above solutions can be calculated using Henry's Law.
An example of the calculation is made part of the Appendix of
this report. Specifically, if the caustic solution had a 12.24
pH, and contained 10% sodium, sulfide, there should only be 100
ppm of hydrogen sulfide existing above the surface of the
solution. In actual trial runs, this was not the case. A pH
of 13.2 allowed for 400 ppm of sulfide emission to the atmosphere
which would reflect the air flow influence on the absorption
capacity. The efficiency of the caustic spray to coat all of
the packing in the absorber is necessary to ensure intimate
contact with the available caustic soda solution. In the present
absorber, there is no spray head to coat the packing. The
caustic soda is caused to flow onto the packing through an 18
inch long pipe that has 0.25 inch holes every 2 inches. The
caustic soda flows through the holes to the packing. A
possibility that the full packing is not uniformily coated with
caustic soda exists. The recirculation rate of the caustic
soda through the absorber in relation to the hydrogen sulfide
flow is important. Design specifications called for a
recirculation rate of 20 gpm. During the course of the project,
the motor on the recirculation pump short-circuited and was
55
-------
TABLE NO. 11
CAUSTIC SODA RECIRCULATION RATES IN RELATION TO SULFIDE AND CAUSTIC SODA IN
ABSORBER AT VARIOUS DEGAS1FIER INFLUENT FLOWS
Influent Sulfide Data:
Recirculation Rate, gpm
NaOH, % Concentration
NaOH, Ibs. /gallon
NaOH , Ibs . /minute
Available in Absorber
H2S from Degasifier, Ibs./
Mintue into Absorber
NaOH, Ibs. /minute
Required to React
with Sulfide
Influent Sulfide Data:
H2S from Degasifier, Ibs./
Minute into Absorber
NaOH, Ibs. /minute Required
to React with Sulfide
Influent Sulfide Data:
H2S from Degasifier, Ibs./
Minute into Absorber
NaOH, Ibs. /Minute required
to React with Sulfide
r~ • - - — - - -
800 mg/1 at 230 gpm or 1.54 pounds of Sulfide per Minute
i
jl -1
5 ; 10
i .._. . . . i
. ...«-_ -,-. ..... n., ^ ......
10 J2
0.926 0.17
4.63 .85
1.64 1.64
1.93 1.93
' : i
10 2
0.926
9.26
1.64
1.93
0.17
1.7
1.64
1.93
"
15
....... . , . . .. . ,
10 2
0.926
13.80
1.64
1.93
0.17
2.55
1.64
1.93
20
10
0.926
18.52
1.64
1.93
2
0.17
3.4
1.64
1.93
1
i
...... |
800 mg/1 at 150 gpm or 1.0 pounds of Sulfide per Minute
i
1.06 1.06
*t
1.25 1.25
1.06
i
i
1.25
1.06
1.25
1.06
1.25
1.06
1.25
1.06
1.25
1.06
1.25
800 mg/1 at 100 gpm or 0.67 pounds of Sulfide per Minute
0.71 0.71
0.84 0.84
0.71
0.84
0.71
0.84
0.71
0.84
0.71
0.84
0.71
0.84
0.71
0.84
Oi
-------
changed. The available motor installed on the puinp had a
different speed. The recirculation rate dropped to 16 gpm.
Table No. 11 relates the recirculation rates to the sulfide
and caustic soda present in the absorber each minute at various
influent flow rates.
A degasifier influent sulfide content of 800 mg/1 was
considered at influent flows of 230, 150, and 100 gpm. Table
No. 11 lists the pounds of caustic soda that would be present if
a 10% or 2% concentration was recirculated through the absorber.
The recirculation rates considered are 5, 10, 15, and 20 gpm.
The pounds of hydrogen sulfide present in the absorber for the
various influent flows are shown.
To simplify recirculation rate analysis, Table No. 12
shows the ratio of caustic soda to H2S at the two caustic soda
concentrations and degasifier influent flows. As the recircula-
tion rate increases, the available caustic soda increases regard-
less of influent flows. As the influent flow decreases, the
sulfide input decreases and the ratio of caustic soda increases
regardless of recirculation rates.
A higher ratio of caustic soda to hydrogen sulfide beyond
the stoichiometric requirement ensures better absorption. A
prior series of experiments relating caustic concentrations to
H2S emission from the absorber flue showed that emission was
minimized by high caustic availability in the absorber.
The 2% caustic soda residual in sodium sulfide is considered
the end point for changeover to a fresh 10% concentration. At
the 5 gpm recirculation rate, insufficient caustic soda is
available at 230 and 150 gpm of influent flow. When the caustic
soda reaches 0.7%, H2S would be emitting freely at 100 gpm.
At 15 gpm, H2S would be emitting freely at 1.65, 1.05, and 0.75%
caustic soda concentrations at influent flows of 230, 150, and
100 gpm respectively. At 20 gpm, H2S would be emitting freely
at 1.65, 1.08, and 0.8% caustic soda concentrations at influent
flows of 230, 150, and 100 gpm respectively.
57
-------
TABLE NO. 12
RATIO OF CAUSTIC SODA TO HYDROGEN SULFIDE
AT VARIOUS INFLUENT AND CAUSTIC RECIRCULATION RATES
Caustic Recirculation
Rate into Absorber, gpm
NaOH , % Concentration
Influent Flow Rate,
gpm
230
150
100
5
10
2.82
4.36
6.52
2
0.52
0.80
1.2
10
10
5.65
8.73
13.0
2
1.03
1.6
2.4
15
10
8.47
13.1
19.6
2
1.55
2.4
3.5S
20
10
11.3
17.5
26.1
2
2.1
3.2
4.8
00
-------
SECTION 9
DATA ANALYSIS
INITIAL OBSERVATIONS
Initial evaluations during 1971 - 72 of the sulfide
reclamation system were conducted for several hours each day
subject to system problems existing at the time. Minimal data
was obtained during these early trials. Only one air flow rate
of 1,000 cfm was used. The data consisted of recording the
sulfide content of the degasifier influent and effluent once
per trial run. This data did not provide an adequate analysis
of the system.
Data relating to these early trials is typified by Table
No. 13 which relates to the end of 1971 and the beginning of
1972. The tannery's production during this time period was
5,000 to 6,000 hides per week.
The test method for sulfides used initially indicated
residual sulfide present in the degasifier effluent at levels
of 4 - 5.5 pH. This is not theoretically correct. In later
evaluations, the test method was changed because of interference
attributed to the iodine demand of the coagulated protein.
Test runs conducted in 1975 - 1976 were specifically
designed to obtain data that would provide system evaluation.
Samples were collected at 30 minute time intervals for the
degasifier influent and effluents and at the caustic soda
recirculation tanks to the absorber. Air flow and influent
flow measurements were made and the sulfide emission levels at
the absorber flue were evaluated.
TEST RUNS AFTER MODIFICATIONS OF THE WASTEWATER TREATMENT SYSTEM
Table No. 14 lists data collected from typical daily runs
in 1976.
The tannery's production during this time period averaged
21,700 hides per week or 3,617 per day.
The data in Table 14 shows that the effective removal of
sulfide is dependent upon the pH of the effluent from the
degasifier. Effective sulfide removal was attained at pH's of
5.9 and below. At pH above 6, the effectiveness of sulfide
59
-------
TABLE NO. 13
SUMMARY OF OPERATING RESULTS SULFIDE RECLAMATION PLANT (1971 - 1972)
DEGASIFYINO TOWER INFLUENT
DEGASIFYING TOWER EFFLUENT
nouns
OPEIIATEO
2.58
1.
1.58
2,58
2.42
2
2.08
2.0
1.83
2.25
2
"1/25
-
_
1.25
1.66
2.0
4
-
I
1
i
RATE
OF FLOW
«pm
170
150
200
180
180
180
i«n
180
. 180
180
180
lt)0
-
_
180
150
180
200
-
pll
9.8
9.5
9.0
9.2
9.2
9.4
9.7
9.6
9.65
8.9
9.4
9.4
9.6
9.2
9.4
'9.4
9.8
8.4
9.1
ALKALINITY
IACIOITYI
mu/l CoCO,
4375
2914
2400
2470
2791
2841
3S87
3591
3945
3360
•
-
3321
3704
3351
2937
4375
1800
3137
SETTLEAOLE
SOLIDS
ml/I
5
3.5
3
2.5
3
4.5
3
'2
4
4
9
3
8
3
3
3
5
10
3
TOTAL
SOLIDS
mg/l
26720
27480
27660
27530
2851 n
26890
7fiQ7n
30340
29990
32410
28450
27900
24140
29900
30080
50000
26720
27840
-
•
SUSPENDED
SOLIDS
mg/l
3280
1820
1130
1220
1 RHfl
2020
Ifififl
23fifi
2267
2925
3040
2680
2320
2000
2300
28fiO
3280
1403
1500
SULFIOE
mg/l S'
280
211
_
?1 f.
326
c.ni
?fin
_
220
-"
-
130
246
400
280
210
208
pll
.5.15
1.1
4.8
5.1
i n
5.5
5 0
1.4
6.2
fi.O
4.65
4.7
6.2
1.1
6.4
l.fi
4.65
5.1
5.1
ALKALINITY
(ACIDITYI
mg/l CoCO,
70
425
700
425
K4n
360
Ran
1474
1890
_
-
792
1955
_
187
fi20
-
425
-
SETTLEAOLE
SOLIDS
ml/1
245
250
30
10
?in
240
?in
Ton
260
110
200
150
200
300
150
20fi
200
250
110
TOTAL
SOL 103
mg/l
28228
25740
28000
27333
pfindfi
27590
•JBBfifl
29330
32770
31500
26545
25640
30460
32970
26400
25140
17730
25140
32510
SUSPENDED
SOLIOS
mg/l
2080
2660
1265
1340
i 7nn
2200
•)itn
3320
3166
1900
3220
2140
2860
2520
2780
1240
2870
2620
2540
SULFIOE
mj/l S'
33
38
4.0
87
58
17.6
11 n
.. 16.0 ,
16.8
32
28
14.4
11.0
24
52
42
28
38
24
-------
TABLE NO. 14
SUMMARY OF OPERATING RESULTS SULFIDE RECLAMATION PLANT - (1975-1976)
nouns
OPEIIATED
4.16
3.67
4.0
2,33
5.0
4.5
4Tb
9.25
4.25
2.08
3.75
3.0
4.75
1.08
2.92
RATE
OF FLOW
gpm
240
248
226
201
255
240
2HB
216
193
217
167
192
193
216
248
DEGASIFYINQ TOWER INFLUENT
pll
8.6
8.5
a. 6
8.6
8.6
8.6
U.7
8.8
8.9
8.6
8.8
9.1
8.5
8.5
8.8
ALKALINITY
IACIDITYI
mo/I CoCO,
3450
2880
3700
3460
3550
3400
1 3600
3550
3300.
3550
3700
3450
3600
3800
3850
SETTLEADLE
SOLIDS
ml/1
7
14
24
28
19
17
10
8.5
17
9
32
6.5
.11.2
4.5
15
TOTAL
60LIOS
"1(1/1
30180
23100 '
32900
26200
22350
30160 '
20650'
24350
28200
.29060
27650
20950
25250
30200
30550
SUSPENDED
SOLIDS
mg/l
3660
2000
1500
1750
1200
2100
3200
1850
3300
2750
3050
1550
2100
1750
3050
SULFIOE
mg/l S'
1000
1000
400
1140
1440
500
1080
1400
1400
1200
1120
760
600
•540
1240
OEGASIFYINQ TOWER EFFLUENT
PH
5.8
5.8
5.5
5.5
6.7
4.0
6.8
6.8
5.0
3.6
6.2
5.6
5.9
4.1
5.5
ALKALINITY
(ACIDITY)
mg/l CoCO,
0
10
20*
.25*
750
110*
700
725
20*
130*
600
25*
10
100*
15*
SETTLEADLE
SOLIDS
ml/I
325
195
210
388
180
317
467
562
275
380
462
588
210
400
250
TOTAL
SOLIDS
mg/i
32500 "
26600
26600
39050
26800
31850
25200
24400
28950
31800
25100
32050
29850
33300
26650
SUSPENDED
SOLIDS
mg/l
3800
3700
6566
6650
3850
2600
3150
2100
3200
1800
3500
5100
2250
6B50
2150
SULFlUL
mg/l 5'
0
0
0
0
40
0
360
320
0
0
120
0
0
0
0
-------
removal was not assured and at pH 6.8, the sulfide removal was
poor. The graph, Figure No. 10 is theoretical relationship of
the effect of acidification and the liberation of hydrogen
sulfide from sulfide solutions at various pH levels. Correspond-
ingly, the graph relating to sulfide remaining in the degasifier
effluent correlates reasonably well.
At pH of 6.0, 89% of the hydrogen sulfide should be
liberated from the sulfide effluent. The actual data shows that
91% has been removed. At pH 6.8, 55% should be liberated from
the solution, the actual data shows 76% sulfide has been
removed from the effluent.
-The acidification increased greatly the settleable solids,
suspended solids, and the total solids. This is due to the
precipitation of the solubilized protein. (Reference Table
No. 15, Figures 11, 12, 13). The flow rate of the degasifier
effluent, within the tested range, has no effect on the efficiency of sulfide
removal. Complete removal of the sulfide from the effluent can be
achieved by the system.
Specific trial runs were made for the analysis of the
overall efficiency of the component parts of the system; i.e.,
the degasifier, the absorber, the recirculating caustic soda
solution concentrations, the influent flow, the air flow, and
the absorber flue emission.
The test to demonstrate the effectiveness of the absorption
as the sodium hydroxide solution reached the limit of absorption
capacity is summarized in Table No. 16. The influent data shows
the variations in the sulfide during the day. The amount of
sulfide can be calculated as:
Flow (gpm) x 8.3 x time (minutes) x sulfide (mg/l)= Ibs sulfide
The sulfide in the effluent can be calculated by the same
formula. The difference between the sulfide in the influent
and the sulfide in the effluent is the amount of sulfide in
the air going to the absorber.
In the absorber, the reaction between hydrogen sulfide and
sodium hydroxide is as follows:
H2S + NaOH -» NaSH + H20
Therefore, the decrease in the sodium hydroxide is equal to
the sulfide absorbed on a mole to mole ratio. From the
analysis of the caustic soda solution the amount of sulfide
removed from the air is determined. The difference between
the amount of sulfide received by the absorber and that found
in the caustic soda solution is the amount lost to the
atmosphere from the absorption tower flue.
62
-------
FACTOR
2 1.00-
KEY
X-THEORETICAL
— ACTUAL DATA
10
Figure 10. Effect of pH on sulfide removal.
63
-------
TABLE NO. 15
SUMMARY OF SOLIDS DATA
pH
9.1
8.9
8.8
8.7
8.6
8.5
6.8
6.7
6.2
5.9
5.8
5.6
5.5
5.0
4.1
4.0
3.6
Total
Settleable
Solids, mg/1
—
17.5
18.5
10
17
9.9
515
180
462
210
260
588
283
275
400
317
380
Suspended
Solids, mg/1
1,550
3,300
2,650
3,200
2,160
1,950
2,625
3,850
3,500
2,250
3,750
5,100
5,122
3,200
6,850
2,600
1,800
Total
Solids, mg/1
20,950
28,200
27,517
20,650
28,475
26,183
25,200
25,600
25,100
29,850
29,550
32,050
30,750
28,950
33,300
31,850
31,800
64
-------
700
600
en
500
^ 400
TO pH 4.0
300
200
100
87654
pH UNITS
Figure 12. Effect of pH on suspended solids.
65
-------
5500-
5000-
4500-
TO pH 4.0
4000-
3500-
3000-
2500-
2000-
1500-
1000
87654
pH UNITS
Figure 12. Effect of pH on suspended solids.
66
-------
36000-
34000-
32000
~ 30000-
o
txi
2 28000
26000-
24000-
22000-
20000
8
7
6
5
4
pH UNITS
Figure 13. Effect of pH on total solids.
67
-------
TABLE NO. 16
DEGASIFIER INFLUENT AND EFFLUENT DATA
INFLUENT
Time (itiins.) 0 60 120 180 240
pH* 8.9 8.9 8.9 8.8 8.9
Suspended
Solids (mg/1) 2,500 2,700 2,600 2,500 3,300
Total Solids 30,300 30,800 30,700 30,700 31,200
(mg/1)
Settleable
Solids (mg/1) .5 .1 5 4 8
Sulf ide (mg/1) 1,280 1,240 1,040 960 1,040
EFFLUENT
pH* 4.5 4.9 4.5 3.75
Suspended
Solids (mg/1) 3,930 3,700 4,470 3,800
Total Solids (mg/1) 33,400 33,600 34,400 34,400
Settleable
Solids (mg/1) 390 409 660 300
Sulf ide (mg/1) 40 0 0 0
300
8.9
3,200
31,300
7
920
3.9
. 3,900
34,600
300
0
* Standard Unit
68
-------
TABLE NO. 17
ABSORBER DATA
Time, min 0 60
Specific
Gravity 1.120 1.16
NaOH,
Volume, gal 466 474
NaOH 4,355 4,414
Solution, Ibs
pH* 13.4 13.3
NaOH, % 13.2 9.5
NaOH, Ibs 574.8 419.6
NaOH, Ibs/hr 155.2
Sulfide, Ibs/hr 124.2
Sulfide, Ibs/hr
from Degasifier
Sulfide
Absorbed, %
Sulfide lost
to atmosphere, %
120
1.114
483
4,490
13.2
6.5
291.8
127.8
102.2
104.5
97.8
2.2
180
1.112
491
4,556
12.8
4.26
194.0
97.8
78.2
111.3
70.3
29.7
240
1.111
499
4,626
12.7
2.4
110.0
84.0 30
67.2 24
122.4 120
54.9 20
45.1 79
300
1.10
510
4,681
12.1
1.7
79.6
,._. s
V
.4
.3
-0
.2
,
.8
*Standard units
69
-------
90-
80-
70-
60-
50-
40-
30-
20-
10-
KEY
-% NaOH IN RECIRCULATION TANK
-% SULFIDE ABSORBED
LD-55 SULFIDE LOST TO ATMOSPHERE
40 80 120 160 200 240
ELAPSED TIME IN MINUTES
280
320
Figure 14. Absorber analysis: sulfide absorbed, caustic
soda consumed, flue emissions, VST.
(Ref. Table 17.)
70
-------
TABLE NO. 18
DEGASIPIER SULFIDE RATES
Time 0 60 120 180 240 300
Sulfide
Influent
Rate, Ib/min 2.37 2.30 1.93 1.78 2.30 1.70
Average 2.335 2.112 1.855 2.04 2.00
Sulfide
Effluent
Rate, Ib/min 2.37 0.74 0 0 0 0
Average 1.63 0.37 0 0 0
Sulfide
Influent, Ibs 126.7 111.3 122.4 120.0
Sulfide
Effluent, Ibs 22.2 0 0 0
Sulfide
to Absorber, Ibs 104.5 ill.3 122.4 120.0
71
-------
No method of analysis for the exhaust air was available
resulting in the need to determine flue emission on a sub-
tractive basis. The sulfide reclamation equipment as now in use
will remove essentially all hydrogen sulfide generated. As
the caustic becomes depleted the efficiency of the hydrogen
sulfide absorption will rapidly approach zero.
The amount of sulfide absorbed at any time can be determin-
ed from the strength of the caustic soda solution. During the
second hour, from Table No. 17, the amount of caustic soda
used was 127.8 Ibs, or 3.195 moles (Ib moles). The sulfide
absorbed was also 3.195 Ib moles or 102.2 Ibs of sulfide.
From the influent and effluent data, the amount of hydrogen
sulfide flowing from the degasifier is:
Sulfide rate = Average mg/1 sulfide x flow (gpm) x
8.345 Ibs/gal.
= 1,140 x 222 x 8.345 = 2.112 Ibs/min or
126.7 Ibs/hr.
Based on this method, the hourly rates for sulfide flow and
absorption were calculated. Table No. 18 and Figure No. 14
provide the results.
The data shows the system is working well during the first
two hours with very little loss to the atmosphere. As the
caustic soda is depleted there is less efficiency in the
take-up.
In the existing system, hydrogen sulfide gas will be
found in the absorption tower flue emission when the caustic
soda is approximately 7% concentration. This emphasizes the
need of a correction in the present design of the system. More
absorption capacity, or possibly a second absorber would correct
this problem.
The data for material balance (Tables 19, 20, 21) was obtained during a
test run where analyses of all flows were made at specific time
intervals of 30 minutes for a duration of four hours. Chemical
analyses were conducted on the influent and effluent for pH,
Alkalinity to pH 5.5, Suspended Solids, Total Solids, Settle-
able Solids, Sulfate, and Sulfide.
The influent flow rate was determined by the time needed
to fill the degasifier. The flow rate was found to be 247
gallons per minute.
Sulfide concentration of the influent and effluent as
related to the influent flow rate allow the pounds of sulfide
removed from the influent to be calculated. A slight error in
72
-------
TABLE NO. 19
MATERIAL BALANCE DEGASIFIER - INFLUENT*
Sample Identity
Time
Elapsed Time,
Minutes
pH
Alkalinity to
, pH 5.5
^^J i* T
1
12:00
0
8.9
4,278
2
12:30
30
8.9
3,945
3
13:00
60
8.9
4,000
4
13:30
90
9.0
3,806
5
14:00
120
8.9
3,778
6
14:30
150
8.9
3,695
7
15:00
180
8.8
3,834
8
15 : 30
210
9.0
1,182
9
16:00
240
8.8
1,099
mg/1 CaCo3
Total Suspended
Solids, mg/1 8,620 8,060 8,220 8,840 7,180 6,760 8,000 8,160 6,700
Settleable
Solids, mg/1 240 220 220 220 230 220 200 210 230
Total Solids,
mg/1 36,050 35,560 34,340 34,720 35,500 34,450 35,840 35,440 35,240
S04, mg/1 3,596 - - 5,728 - 5,586
Sulfide, mg/1 1,228 1,200 1,316 1,062 1,078 741 1,078 1,182 1,099
*Flow Rate 247 gpm
-------
TABLE NO. 20
MATERIAL BALANCE - DEGASIFIER EFFLUENT
Sample
Identity
Time
Elapsed Time
pH
1 2
12:00 12
0
6.9 6.
:30
30
7
3
13:00
60
7.3
4
13:30
90
6.3
5
14:00
120
5.4
6
14:
15
5.
30
0
3
7
15:00
180
5.2
8
15:
21
5.
30
2
3
9
16:00
240
5.1
Alkalinity
tooH 5.5 417 361 472 111 00000
mg/1 CaCO3
Total Suspended
Solids,mg/1 6,400 6,160 6,680 6,100 3,700 2,960 5,980 5,940 5,760
Settleable
Solids, mg/1 620 550 470 740 550 490 930 870 870
mq/11 Solids/ 37,020 36,190 38,930 32,770 35,340 34,050 36,350 37,980 38,320
S04, mg/1 9,333 - - 9,757 - - - 10,172
Sulfide, mg/1 87 50 98 14 10 0 0 0 0
-------
TABLE NO. 21
~* MATERIAL BALANCE - DEGASIFIER INFLUENT AND EFFLUENT'SULFIDE RATES
Number
Sulfide
Rate , Ib /min
INFLUENT
EFFLUENT
Average Sulfide
Rate/ Ib/min
INFLUENT
EFFLUENT
Sulfide/lbs.
INFLUENT
EFFLUENT
To Absorber
1 2
2.52 2.46
.18 .10
2.49 2.56
.14 .15
74.7 77.4
4.2 6.0
70.7 71.4 .
345 6 789
2.69 2.18 2.21 1.52 2.21 2.32 2.20
.20 .03 .02 0 0 0 0
2.43 2.19 1.86 1.86 2.27 2.26
.12 .02 .01 0 00
TOTAL
72.8 65.7 55.8 55.8 68.0 67.8 538.0
3.6 .6 .3 0 00 14.7
69.2 65.1 55.5 55.8 68.0 67.8 523.3
-------
TABLE NO. 22
MATERIAL BALANCE RUN - ABSORBER ANALYSIS*
Sample
Identity
Time
Temperature,
Volume, gal.
Density,
Specific gra
o\
NaOH Initial
Solution, Ibs
pH
NaOH , %
Sulfide, %
Sulfide, %
Absorbed
NaOH/ Ibs
1
12:00
°F 96
704
1.179
vity
6,926
-
13.7
14.2
2.74
73.6
90.4
2
12:30
100
710
1.1695
6, 929
13.5
12.89
3.80
65.6
71.6
3
13:00
102
720
1.1695
7,027
13.5
11.69
4.68
58.
77.
4
13:30
106
726
1.166
7,064
-
13.5
10.33
5.48
2
7
5
14:00
108
731
1.166
7,113
13.45
9.15
6.55
78.8 68
93.0 77
6
14:30
112
737
1.1555
7,106
13.3
8.07
7.52
.5 28.
.4 91.
7
15:00
114
742
1.1555
7,155
13.2
6.73
7.87
7 72
9 88
8 9
15:30 16:00
112 112
748 753
1.1510 1.1510
7,185 7,233
13.3 13.2
5.47 3.86
8.84 9.90
.0 81.0
.5 113.8
*Flow rate of NaOH recirculation = 16 qpm
-------
the calculated values is present and is related to a dwell time
of 0.75 hours for the influent passage through the degasifier.
Since the sulfide concentration of the influent is changing
slowly during the test period, the error attributed by the
dwell time should not be pronounced enough to affect the
material balance calculations.
The influent analysis remained constant in composition with regard
to pH, Settleable Solids, and Total Solids. During the test period, the
alkalinity of the solution decreased and the sulfide in the influent
increased. These changes were attributed to higher strength tanning
wastes coming from the clarifier than was present during the early stages
of the test. The feed rate of the sulfuric acid was constant during the
test run. As the alkalinity of the influent decreased, the pH of the ef-
fluent also decreased and a greater removal of sulfide was achieved.
The data on the suspended solids in the effluent is
inconsistent with that observed in other runs. A possible
explanation of this could be in the amount of lime carried with
the influent. Particulate lime upon reaction with the sulfuric
acid would become solubilized resulting in a net decrease in
suspended solids. Table No. 19 shows data for the degasifier
influent relating to suspended solids, settleable solids, total
solids, and sulfide content as measured every 30 minutes during
the material balance run. Table No. 20 shows data for the
same parameters relating to the degasifier effluent and taken
at the same time intervals.
The degasifier influent and effluent sulfide rates/minutes
were calculated for each 30 minute interval. Table No. 21
lists the sulfide rates of the influent, the effluent, and for
the sulfide enroute to the absorber by difference.
Samples from the caustic soda solution in the recirculation
tank and at the absorber flue were taken at the same time
intervals as from the degasifier influent and effluent.
Analyses were made for specific gravity, temperature, pH,
percent sodium hydroxide, and percent sulfide. The analytical
results are in Table No. 22. From this data, the pounds of
sodium hydroxide used and the pounds of sulfide absorbed were
determined.
At the start of the test run, there was some sulfide
present in the caustic soda solution. This is due to the
mechanical configuration of the tanks. Some caustic soda-
sodium sulfide solution remains in the tanks after emptying and
is present for the next run. Density, temperature and volume
measurements can be used in calculating the pounds of caustic
soda and sulfide in the system. The data on the sulfide
77
-------
CO
TABLE NO. 23
MATERIAL BALANCE RUN - ABSORBFR EXHAUST1 PLUF DATA
Sample , „
Identity
Time, 12:00 12:30
Temp. °F 82 96
Flow,cfm 490 450
Sulfide, as H25/
cu Meter by Zinc 150* 150*
Acetate Method
By Instrumentation - -
Sulfide., Ibs
Discharaed 0.13 0.14
3 4
13:00 13:30
100 106
360 330
197 186*
248
0.18 0.
5
14:00
108
320
268
337
21 0.
6 7
14:30 15:00
108 110
320 390
276* 400
450
23 0.27 0.35
8
15:30
110
380
400
450
0.70
9
16:00
112
430
1306
-
* Data by extrapolation
-------
15.0-
14.0-
13.0-
12.0-
11.0-
10.0-
: 9.0-
! 8.0-
7.0-
6.0-
5.0-
4.0-
3.0-
2.0-
1.0-
0
PH
0
PH OF NaOH IN RECIRCULATION TANK
0—% NaOH IN RECIRCULATION TANK
H—% SULFIDE ABSORBED
+ — pH
30
60
ISO
210
-13
-12
-11
-10
9
-8 ;
-7
-6
-5
0
4
-3
-2
-1
0
240
90 120 150
ELAPSED TIME IN MINUTES
Figure 15. Absorber analysis: caustic soda consumed, sulfide absorbed,
pH of circulation caustic tank vs. elapsed time.
79
-------
TABLE NO. 24
MATERIAL. BALANCE OF THE SULFIDE RECLAMATION SYSTEM
Sulfide in
Degasifier
Influent, Ibs
Sulfide in
Degasifier
Effluent, Ibs
Sulfide to
Absorber by
Difference, Ibs
Sulfide Absorbed
by Analysis, Ibs
Sulfide Flue
Emission, Ibs
Calculated
Using Average
Sulfide
548.77
14.23
534.54
526.4
8.14
(by difference)
Calculated
From Data
Tables
538.0
14.7
523.3
526.4
2.2 (by analyses)
Sulfide
Recovered by
Absorption, % 95.9
Sulfide in
Flue Emission, % 1.48
Sulfide in
Degasifier Effluent,% 2.59
Total % =
99.97
97.8
0.4
__2v7_
100.9
80
-------
take-up and the decrease in the caustic soda conforms to a
near stochiometric ratio.
The air discharge from the absorber was sampled and analyz-
ed using the same time schedule. Air flow and temperature
readings were also made. The analysis for sulfide in the air
discharge was obtained at several but not all data points. The
missing data points were estimated and shown along with other
data in Table No. 23. At the end of the four hour test the
concentration of the caustic soda is decreased to the extent
that the amount of hydrogen sulfide being discharged with the
air analyzes to be in excess of 1,000 mg/1.
Data on the hydrogen sulfide in the exhaust air was
measured by chemical analysis using the Zinc Acetate Absorption
Method and also using the Research Appliance Sulfide Analyzer.
The chemical data was consistently lower and was considered to
be more accurate. The Research Appliance apparatus is
satisfactory for operational control.
Figure No. 15 illustrates the decrease in sodium hydroxide,
the increase in sulfide absorbed, and the decrease in the pH
of the recirculation caustic soda used during the material
balance run.
The material balance data summarized in Table No. 24, shows
that the sulfide can be removed from the degasifier influent
when acidified to a 5.0 - 5.5 pH. Approximately 98% of the
sulfide in the tannery wastewater is recoverable for reuse
in the unhairing process. The small amount of effluent
reflected in the data is due to improper pH levels of 6.3 to 7.3
at the beginning of the run. Sulfates in the effluent show an
increase due to the addition of sulfuric acid for pH adjustment.
The loss of sulfide to the atmosphere and the effectiveness of
the caustic soda absorption could be improved by the use of
a larger absorption column, a larger caustic soda recirculation
tank, higher caustic concentration, or by use of two absorption
columns in series.
81
-------
SECTION 10
PROTEIN COAGULATION AND SEDIMENTATION:
EFFECT ON POLLUTION L021DING
Acidification of the degasifier influent to a 5.0 - 5.5
pH which is necessary for sulfide reclamation causes coagulation
of solubilized proteins. During the evaluations of the sulfide
system, comparative analyses were made of the pollution loading
benefits of removing the coagulated proteins by sedimentation
of the degasifier effluent. The initial design for the waste-
water treatment plant at Blueside Company had scheduled second-
ary sedimentation; however, it was not incorporated into the
plant as constructed.
Table No. 25 shows the reductions to be achieved over the
time periods involved as related to the tannery's weekly
production of hides. The 1976 data shows a 25% BOD^ reduction
by sulfide removal. Further protein removal effects a total 60%
6005 reduction in pollution loading. Composite samples of the
degasifiar influent and effluent were analyzed. The composite
samples consisted of grab samples taken every thirty minutes
during a four hour sulfide reclamation run. The degasifier
effluent sample was split into two samples. One was used for
analysis immediately, the second was allowed to settle for four
hours. The supernatant liquid was drawn off for analysis. The
residual protein solids were blotted with absorbent paper,
air dried and then analyzed.
The reductions in pollution loading by the removal of
coagulated proteins represents large savings to the company
as an indirect discharger. Correspondingly, for a tannery with
a similar sulfide system as a direct discharger, the decreases
in the pollution parameters would greatly simplify a secondary
treatment process.
The sale of the recovered protein may be doubtful, however,
the sludge produced by sedimentation could have value as a
fertilizer. As wastewater treatment system expands and
chromium recycle is a reality, the protein would perhaps have an
economic value. The protein sedimentation at the Blueside
Company represents removal of coagulated proteins from the
total tannery effluent.
82
-------
GO
U)
TABLE NO. 25
PROTEIN COAGULATION AND SEDIMENTATION: EFFECT ON POLLUTION LOADING
Parameter ,
mg/1
pH*
Sulfide
Alkalinity
to pH 5.5
Sulfates
Total
Solids
Suspended
Solids
Settleable
Solids,
BOD5
COD
Cr203
Oil and
Grease
1971 Operation
>,
4J 4J 4-> tn m c
C C C 4J C > (U
Oi M i|.j -H i_i m
H H H 3 Cn w H
9.5 5.3 5.3
1975 Operation
.p
c
M-l 4-> 0
M-l -H >-l
W S (^
5.3
21
—
8,950
22,560
315
1
2,435
4,155
26
— —
rH U
1976 Operation
Ifl C 4->
> 01 C
O -H (U
E 0 i 3
0) -H i -H
K M-r ! M-l
MH i C ,
* W : H
98
(87)**
26
94
""" ™~
49
68
93
— —
8.2
1,107
3,834
5,687
J5,820
6,520
150
4J
C
(U
3
rH
VW
m
w
5.5
13
0
a, 872
37,960
6,560
850
7,867 5,917
L5.625 12,660
555
512
320 1,032
4-1 01
C 4J C
0) 3 -H
3 O (U
rH X! 4->
M-l 4-) 0
M-l -t-l S-l
W- S fli
5.5
1Q
0
rH O
m c
O-H
E 0
pa
—
99
100
10,004 (76)**
22,100.
1,420
0
3,185
3,260
19
19
38
78
100
60
79
97
94
* Standard Units
** Increa&e due to CaSO4 Formation
-------
TABLE NO. 26
EFFECT OF pH ON PROTEIN SEDIMENTATION
AND
ANALYSIS OF PROTEIN SEDIMENT
Iinit: mq/1 unless otherwise stated
Effect of pH on Protein Sedimentation
Parameter
T, *
pH
Total
Settleable
Solids
Control
9.2
15
6.0 j 5.0 4.0 ; 3.0 i 2.0 ! 1.0 1
380 400 420 i 320
' i
I _. L
230 I 190
Supernatant Lictuor
Suspended
Solids
2,950
200
300
300 i 300
300
300
1 i
Total Solids J22.400 27,000 29,200 17,400 30,100 J33,100 70,300
PH"
Density,
°Baume, 72°F
Cr203
Cr
6 +
9.3
2.0
520
nil
6.3,
2.0
32 i
i
nil
5.2
2.5
I
38 i
4.3
2.5
82
3.3J
2.5
2.4
1.8
nil 1 nil
51
nil
3.0 5.0
44 25
nil ;nil
* standard units
TABLE NO. 27
ANALYSIS OF PROTEIN SEDIMENT
Parameter
Moisture (air dry basis)
Total Solids
95.2
4.8
_So_lj.ds Compos j.t ion (air dry basis)
Nitroaen Kieldah]
Protein Equivalent
Oil 6 Grease
Sulfates
Chlorides
Calcium
Cr+6
3.38
19.0
10.0
2.61
0.60
1.64
2.74
Nil
84
-------
Research conducted on protein recovery (19) showed that one
pound of protein could be recovered from each 7 to 9 gallons
of unhairing liquor sampled from a hide processor tannery
process. Considering 1,200 gallons of unhairing liquor from
each of 12 process batches per day at one pound of protein
yield for each eight gallons of liquor, the amount of protein
sediment attributable to the hairburn liquors would be 1,800
pounds per day.
Protein precipitation as reported began at 6.0 pH and
continued precipitation down to about 3.8 pH. Most proteins
precipitated between 4 and 5 pH. At pH 4.2, the protein
solution appeared to act as a buffer. Further acid addition
did not precipitate the proteins completely with small amounts
remaining in the supernatant solution.
In order to determine the effect of pH on protein
sedimentation, liter quantities of the clarifier effluent were
acidified using sulfuric acid to varying degrees ranging from
6.0 to 1.0 pH and allowed to settle in an Imhoff cone. The
settleable solids were recorded. The supernatant liquor was
decanted and analyzed for total solids, suspended solids, and
chrome content. The protein sediment from an acidification to
5.0 pH was blotted with absorbent paper and air dried. The
protein sediment was analyzed for Kjeldahl nitrogen and factored
to determine protein content. The air dried sediment was
analyzed for oil and grease, sulfates, chlorides, and chromium.
Tables 26 and 27 show the results of pH variation on sedimentation
and a protein sludge analyses, respectively. The data results
from taking a five gallon sample of the clarifier effluent
which was kept under agitation to allow representative liter
samples to be taken for the experiment. Following acidification
to the various pH levels, the samples were allowed to settle
for four hours.
As the pH decreases, the settleable solids will increase
until a 4.0 pH is achieved. Further acidification causes a
decrease in settleable solids. This may be due to hydrolysis
of the protein which occurs at 2.0 pH in the absence of suf-
ficient salts to prevent acid swelling from occurring. The
density of the supernatant liquor is 2° Baume and salinity
concentrations of 6° Baume are required to prevent hydrolysis of
proteins at 2.0 pH. The OBaume of the clarified effluent
increases from 2.0° at a 9.2 pH to 5.0 °Baume at a 1.0 pH. The
supernatant liquor becomes clearer up to a 4.0 pH and pro-
gressively milkier in appearance as 1.0 pH is approached. The
total solids in the supernatant liquor will increase to 5.0 pH,
decrease to a minimum at 4.0 pH, and increase to a maximum at
1.0 pH. This indicates that protein sedimentation would be
optimum at 4.0 pH and that hydrolysis of the protein is
occurring at 1.0 pH. The pH of the supernatant liquor was
85
-------
rechecked four hours after acidification and shows slight
reductions in acidity. The suspended solids and the chrome
content in the supernatant liquor are substantially reduced by
acidification and settling.
The coagulated protein is amber in color at 6.0 pH and
changes progressively to a whiter product as acidification to
1.0 pH proceeds. Analyses conducted on the protein sediment
show 19% protein, 10.0% Oil and Grease, 2.61% Sulfate, 0.60%
Chloride, 1.64% Calcium, and 2.74% Chromium, as
Sedimentation of the protein would increase the sludge
solids quantities for removal to landfill by approximately
2,150 pounds. Current daily sludge removal to landfill requires
43,000 pounds at 50% solids to be removed. The sludge is
dewatered in filter presses. The sludge results from the
processing of 200,000 pounds of hides/day. Protein sedimenta-
tion for removal would increase the sludge solids to 45,150
pounds. Since a dumpster of sludge for landfill contains
15,000 pounds, there should be no additional landfill costs.
Using the 1976 data from Table No. 25, the economies
shown in Table No. 28 could be effected by protein removal.
Credit is taken for sewer surcharges based on flow, BOD^, and
Suspended Solids.
TABLE NO. 28
ECONOMIC EVALUATION OF PROTEIN SEDIMENTATION
ON MUNICIPAL SEWER SURCHARGE
Surcharge with
Surcharge without Sulfide Reclamation
Parameter
Flow
BOD5
Suspended Solids
Monthly Surcharge
Sulfide Reclamation
$ 3,550
2,358
7,190
$ 13,098
And Protein Removal
$ 3,550
910
1,306
$ 5,766
ANNUAL SURCHARGE $157,176 $69,192
NET SAVINGS: $87,984
86
-------
SECTION 11
ECONOMIC EVALUATION OF THE SULFIDE RECLAMATION SYSTEM
AND WASTEWATER TREATMENT PROGRAM
The sulfide reclamation equipment cost estimated in 1968
is essentially accurate and listed in Table No. 29. Today's
costs would be substantially higher based on the inflation
trend of the last few years. Table No. 30 is a comparison of
the 1970 economies of the sulfide system with the estimated
economic evaluation for 1976.
The original labor cost estimates of 1970 were low. They
indicated labor requirements of 300 man-days for a 250 day
year operation with an annual cost estimate of $6,000 or 1.2
men per year.
Supervision and fringe benefits were not included in the
overall labor cost estimates, nor was the expense of heating
the system included. The estimated cost for electrical useage
was also too low. Depreciation of equipment was estimated at
$15,300 per year in the 1970 evaluation. The estimated chemical
costs and useage requirements were low, and in addition, the
expense for freight was not considered.
A net operating cost for the sulfide reclamation-reuse
system in 1970 was set at $37,700 without consideration of
freight costs. When recalculated with a 350 day year, the cost
is equivalent to-$25 per 1,000 hides processed.
The 1976 economic evaluation of the sulfide reclamation
system shows higher labor costs resulting from six years of
inflation and a change to an around-the-clock operation of the
system over a 350 day year, requiring 2.4 man-years for its
operation.
Costs for supervision, fringe benefits, more expensive
replacement parts, maintenance, heating, and greater electrical
requirements are included in the 1976 evaluation. An increase
in depreciation expense is also indicated and based on the
replacement of capital items during the past three years. The
chemical quantities and cost are shown in the 1976 evaluation
with and without freight expenses included.
87
-------
TABLE NO. 30
ANNUAL ESTIMATED ECONOMIC EVALUATION OF SULFIDE RECLAMATION SYSTEM
ORIGINAL (1970) vs CURRENT (1976)
00
00
ORIGINAL (1970)
Annual
Item Units Amount
DIRECT" coSfs
Labor man- 300
days
Supervision
Fringe
Maintenance Lump Sum
Gas - -
Electricity Kwh. 2.5 x 105
Total Direct Costs
INDIRECT COSTS
Depreciation
of Equipment 20%
Laboratory
Supplies
Total Indirect Costs
CHEMICAL- COSTS
Sulfuric Acid tons 750
Sodium
Hydroxide tons 500
Total Chemical Costs
Unit
Cost
$20
-
-
_
0.015
-
.
*40.
*80.
TOTAL ANNUAL OPERATING COSTS
Income from
Reclaimed tons SPO
Sulfide
NET GAIN (OR CDSSr
*69.
SuHf Annual
Total Costs
$6000
-
-
2000
—
3700
$11700
$15300
— •
15300
30000
40000
70000
' $97000
$59300
($37700)
Annual Unit
Amount Cost
4 men @ 60% of
CURRENT (197fi)
~3ub
total
$25208
Annual Costs
w/o Frc. v/Frt.
their time, year
round Lump s um
2250
5492
5349
588
3.441xl05 @$0. 02543 8750
Material: Cost
2750 38.00
740 140.00
$21400
250
w/Frt.
45.49
150.00
$47637 $47637
21650 21650
104500 125098
103600 111000
$277387 $305385
967 290. no
.
411.00
28430 397437
$3043 592052
Original""BasTsV 60~6(rhTdis7day, 230 operating " BasFs: Average" 21700"hTdes/week/350 days/year
days/year 18x8 hr. shifts/wk or 21700 £ .6 = 3617 hides/day
*Chemical Freight Costs not included
NOTE: Current Estimate has been calculated with and without Chemical Freight Costs to St. Joseph,MO
-------
TABLE NO. 29
SULFIDE RECLAMATION SYSTEM EQUIPMENT COSTS
EQUIPMENT AND MATERIALS AMOUNT
Feed pumps to degasifier (2) $ 7,000
Acid feed pump 2,000
Recirculation pump 1,500
Caustic soda feed pump 1,500
Degasifier Tower 25,000
Absorber Tower 5,000
Automation Control System 7,000
Chemical Storage Tanks 10,000
Air Blowers (2) 10,000
Dahl tube and flow indicator 3,000
TOTAL COSTS $72,000
An estimated net gain of $92,052 per year with the
operation of the sulfide reclamation system is the result of
savings in material costs, but primarily the result of a savings
in freight costs. This equates to a net gain of $84.84 per
1,000 hides with uninterrupted operation of the system.
Table No. 31 cites the cost of the sanitary landfill
operation for sludge removal.
A recapitulation of total wastewater treatment costs for
the Blueside Company is shown in Table No. 32. Costs included
are the sewer surcharge (for primary/secondary wastewater
treatment) by the municipality and sanitary landfill charges for
sludge disposal.
The estimated net gains expected from sulfide reclamation,
chrome recovery reuse, and protein sedimentation removal (not
yet operational) are considered in the 1976 economic evaluation.
The net operating cost of the wastewater treatment systems
89
-------
at Blueside Company is stated as $120,272r which amounts to
a net cost of $0,095 per hide processed.
The chemical treatment of sulfide by chlorination or
hydrogen peroxide is expensive and prohibitive, except when its
use is for the final removal of small quantities of sulfide.
With proper operation of the sulfide reclamation system at a
pH of 5,0 - 5.5, the use of chlorine or hydrogen peroxide to
negate residual sulfide in the wastewater to a maximum of 10 ppm,
as required by the municipal ordinance, would not be required.
TABLE NO. 31
SANITARY LANDFILL CHARGES AND QUANTITIES
Basis:Removal of 3 sludge dumpsters/day for six days/week "~
for a 50 week year or a total of 900 x 15,000 pounds
loads per year or 75 x 15,000 pounds loads per month.
MONTHLY CHARGE
Minimum Charge: 40 loads x $70 = $2,800.00
+ Dumpster Maintenance = 200.00
Sub-total 3,000,00
+ 35 loads x $55 = 1,925,00
Total Monthly Total $4,925.00
ANNUAL LANDFILL- CHARGE $59,100.00
ANNUAL LANDFILL QUANTITIES: 13,500,000 pounds
@ 50% solids
The tannery operates 18 eight-hour shifts out of 21
potential shifts per week. The downtime for cleaning of the
sulfide recovery system's degasifier is scheduled during the
21st and 1st shift periods at which time tannery wastewater
flow of sulfide-bearing waste is zero. Flow into Blueside
Company's wastewater treatment system and the municipal sewer
is virtually stopped.
Table No. 33 shows the estimated cost of hydrogen peroxide
to remove 40 ppm of residual sulfide and the average of 1,360
ppm sulfide present in the wastewater,
90
-------
TABLE NO. 32
WASTEWATER TREATMENT COSTS
Direct Costs Annual
Amount
Labor . 6 men x 2080 x $ 5.56 $ 69,389
Supervision 14,600
Fringe 16,798
Maintenance Lump Sum 21,000
Electricity 8.242 Kwh X105XO.02543 20^959_
Total Direct Costs $142,746
Indirect Costs
Depreciation of Equipment 86,000
Laboratory Services and Supplies 5,250
Total Indirect Costs 91,250
Waste B.andling Costs
Estimated sewer surcharge when municipal
secondary treatment is on stream 157,176
Sanitary Landfill charges for
sludge removal 59,100
Total Waste Handling Costs 216,276
TOTAL~ANNUAL OPERATING COSTS 450,272
Credit from Sulfide Reclamation System 92,000
Credit from Chrome Recovery System 150,000
Estimated credit from sewer surcharge
economies when coagulated proteins are
removed from degasified effluent 88,000
Total estimated income of pollution
control projects 330,000
Net Cost of, Wastewater Treatment at $120,272
21,700 hides/week for a 50 week year, the cost
would, be JOj!_015Zhide
91
-------
TABLE NO. 33
HYDROGEN PEROXIDE COST ESTIMATE
Condition
Average Sulfide
in Effluent, ppm
'SulrTde"
System
Operational
40
"SulfTde
System Not
Operational
1360
Daily Effluent
Volume, gallons
300,000
Sulfide needed to be
oxidized, allowing
10 ppm to city sewer, Ibs 75
pH Range
Cost/pound 50%
Hydrogen
Peroxide
5-7
$0.2125
300,000
3380
8-9
»
$0.2125
Stoichiometric
Ratio of 50%
Hydrogen Peroxide
to Sulfide
2 : 1
8 : 1
Pounds of Hydrogen
Peroxide (50%)
150
$42.50
27,040
» ^B ^H m^ ^^ ^M ^B ^M ^H ^^ m
$5,746.00
Cost Estimate per day
Cost per hide,
3617 hides per day
$0.0118
$1.59
*Proper operation of the sulfide reclamation system at 5.0
5.5 pH results in no sulfide in the wastewater flow to the
municipal sewer. The choice of 40 ppm is for calculation
comparisons of hydrogen peroxide to chlorine.
92
-------
TABLE NO. 34
CHLORINE COST ESTIMATE
Condition
Average Sulfide in
Effluent, ppm
Daily Effluent
Flow, in gallons
SuTflde"
System
Operational
40
300,000
Sulfide needed to be
oxidized, allowing only 75
10 ppm to city sewer, Ibs
Cost/Pound Chlorine
Stoichiometric Ratio
of Chlorine to
Sulfide in effluent
oxidation
$0.075
10:1 minimum
15:1 maximum
Not
Operational
1,360
300,000
3,380
$0.075
10:1 minimum
15:1 maximum
Pounds of Chlorine
Required
750 minimum
1,125 maximum
33,800 minimum
50,700 maximum
Cost Estimate
per day
$56.25 to $84.38
$2,535 to $3,802.50
Cost per hide based
on 3617 hides per
day
$0.0156 to $0.0233
$0.70 to $1.05
*Proper operation of the sulfide reclamation system at 5.0
pH results in no sulfide in the wastewater flow to the
municipal sewer. The choice of 40 ppm is for calculation
comparisons of chlorine to hydrogen peroxide.
- 5.5
93
-------
Table No. 34 shows the estimated cost of chlorine useage
at the same two levels of sulfide content as used in the
hydrogen peroxide cost estimate — with and without the system
operational.
Without the sulfide system operational, the cost of remov-
ing an average of 1,360 ppm to the 10 ppm required by ordinance
would be $1.59 per hide for hydrogen peroxide useage and $1.05
per hide for chlorine useage. With the sulfide system oper-
ational and presuming an average of 40 ppm residual in the
tannery wastewater, the cost comparison of removing the sulfide
would be $0.0118 per hide for hydrogen peroxide useage and
from $0.0156 to $0.0233 per hide for chlorine, dependent on
the stichiometric ratio required.
The lower cost of hydrogen peroxide with the sulfide
system operational is because the stoichiometric ratio require-
ment in wastewater at a pH range of 5 - 7 is 1:1. Proper
operation of the sulfide reclamation system at a 5.0 - 5.5
pll, coupled with scheduling of downtime for the system's
cleaning, will eliminate the need for expensive useage of
chemical elimination of sulfides using either hydrogen peroxide
or chlorine.
The chemistry of the hydrogen peroxide and chlorine
reactions with sulfide is covered in the appendix, "Sulfide
Chemistry".
94
-------
SECTION 12
PROPOSED SYSTEM MODIFICATIONS
SULFIDE RECLAMATION SYSTEM
Two plans for reducing the downtime in cleaning the
degasifier and overflow towers are under consideration and are
outlined in the following proposals and schematics.
The first proposal, Figure No. 16, shows the installation
of conical shaped trays installed in the existing degasifier.
The liquid depth in the tower would remain the same, but at
the apex of each conical tray a 6 inch diameter drain would be
added with the necessary valving.
Air diffusers, parallel to the side walls of the degasifier,
would still be used on each tray, but additional orifice open-
ings and caps would be provided on diffusers to reduce clean-out
frequency.
A full set of replacement check valve type air diffusers
would be kept in stock for installation while the tower is
open for cleaning. This would reduce the cleanout time, as the
dirty diffusers would be cleaned during the interim period
between tower clean-outs.
Additional spray heads, with piping capable of handling
a higher water volume, would be installed in the top of each
tray section to assist in clean-out.
In addition, a second absorber would be installed in series
with the first one to eliminate H2S flue emission.
A conical bottom and larger drain pipes at the base of the
overflow tower will assist in the cleanout and are depcited
in the schematic.
The second proposal, Figure No. 17, shows a degasifier
with low liquid level trays. The low liquid level trays could
be of the type designed by Koch Engineering Company. Flexitrays
are valve-type trays consisting of perforated decks upon which
round movable caps are mounted. The caps, which operate as
check valves, are approximately 2 inches in diameter and have a
limited lift capability accomplished by either a hold-down cage
or by integral guide legs with lift stops.
95
-------
Figure 16. Proposed modification of sulfide reclamation system.
96
-------
H2S GAS
ACID
INFLUENT'
DEGASIFIER WITH
LOW LIQUID LEVEL
VALVE TYPE TRAYS
FLUE LINE EXTENDED TO WET-WELL
DEMISTER
=
.IrrrrrJ.
VALVE-TYPE
TRAY ABSORBER
LOW-LIQUID LEVEL
CONICAL BOTTOM
ON OVERFLOW
6-in. I.D. TOWER
NaOH
TANK #1 _
Of TWO j=Q
WET WELL
AIR SOURCE
Figure 17. Alternate design of sulfide reclamation system.
-------
VO
OO
MODIFIED OVERFLOW
L>
s. — .
/
-
\ /
MODIFIED \
OVERFLOW
TOWER BASE
FOR
EASE
OF CLEANING
g
200 gpm
/
^_. j
-r^i
L_
*•
f
V~
DRAG-BOARD, CHAIN DRIVEN
i i i i i i i
o o o o 5
EQUALIZATION HEADBOX
«i
BAFFLE Q
S 20,000 GALLON
RECTANGULAR CLARIFIER TANK
o o o o o Q
-
-
V
^
SLUDGE SUM
CLARIFIER EFFLUENT
TO MUNICIPAL SEWER
CYCLONIC SEPARATOR
TO FILTER PRESSES
SLUDGE
COLLECTOR
TANK
OVERFLOW WEIR
BAFFLE
Figure 18. Schematic protein sludge removal.
-------
The valves, made of different metal gauges, would be
installed in alternating rows of light and heavy caps, parallel
to the outlet weir, and would provide good vapor distribution
over a wide range of air flow rates. At lower air flows, the
lighter valves would lift to an open position. But as the air
flow rate increased, the heavier weight valves would progressive-
ly open wider, eventually to their full open position. Even at
the lowest air flow volume, air would flow upward through the
crevices around the valves and would prevent leakage into the
air blower.
Tray gasketing would not be necessary, not only because
of the air flow upward through the crevices which would prevent
leakage, but because of the lower liquid level's reduced down-
ward force exerted on the valves. Tray gasketing, nevertheless,
is available.
The number of trays required would be increased dependent
upon the retention time necessary.
An additional low liquid level tower of proper dimension
equipped with identical valve-type trays .would be used as an
absorber. Fresh caustic soda would flow into the top tray of
this unit and flow countercurrent to the H2S/air vapor.
•y
The emergent solution at the bottoia of this unit would
move to tankage as sodium sulfhydrate or sodium sulfide solution
in caustic soda, dependent upon the number of trays used. The
flue from the absorber would be eliminated as all gas would
return to the wet well and be recycled;
PROTEIN SEDIMENT REMOVAL PROPOSAL
Figure No. 18 is a schematic for the collection of
degasifier effluent into a rectangular clarifier from a
modified overflow pipe in the overflow tower. Modification of
the overflow pipe would be required to have sufficient headroom
to allow gravity flow of degasified effluent into the reservoir.
The effluent would be pumped from the clarifier into a
separator (cyclonic or sludge blanket type) allowing further
protein sludge to be removed. The clarified effluent would
flow to the municipal sewer. The protein sludge would flow
into a sludge receiving tank from which it would be pumped to a
filter press for dewatering prior to disposal.
CHROME RECOVERY AND REUSE
Chrome recovery and reuse was not a part of this project.
Its adoption will further reduce pollution loadings. The
minimization of chrome in the tannery wastewater will lower the
COD and produce a less bulking sludge for sanitary landfill.
The elimiation of acidic components from the tannery effluent
will result in a higher pH existing in the equalizing tanks
99
-------
prior to the hair screen and in the clarifier. This will
decrease odor potential.
The chrome recovery and recycling at Blueside Company is
based on reuse of exhaust liquor and excess amounts of liquor
will accumulate which must be precipitated.
Indications are that a savings of $150,000 per year could
be realized.
100
-------
REFERENCES
1. Stanley Consultants, Development Document for Effluent
Limitation and Guidelines and Standards of Performance,
Leather Tanning and Finishing Industry, E.P.A., Contract
No. 68-01-9594, June, 1973.
2. Eye, J. D., and Graef, S. P., Literature Survey on Tannery
Effluents, Journal of American Leather Chemists Association,
62, 194, 167.
3. Wissman, D. J., Economic Analysis of Proposed Effluent
Guidelines Leather Tanning and Finishing, E.P.A.,
230/1-73-016, October, 1973.
4. Pretreatment of Pollutants into Publicly Owned Treatment
Works, U. S., E. P. A., Office of Water Operations,
October, 1973.
5. Berg, N., Miller, T. H., Pearce, A. P., Shuttleworth, S. Go,
and Williams Wyman, D. A., Studies on Elimination of
Sulfide from Tannery Beamhouse Effluents by Manganese
Catalyzed Oxidation, Journal of American Leather Chemists
Association., 62, 684, (1967).
6. Berg, E. L., Wastewater Treatment System at Caldwell Lace
Leather Company, Journal of American Leather Chemists
Association, 68, 73, 393.
7. Van Meer, A., JJ* , Some Aspects of Chemical Treatment of
Wastewater from the Beamhouse, Journal of American Leather
Chemists Association, 68, 73, 339.
8. Roulings, D. E., Woods, D. R., and Cooper, D. R. , and
Shuttelworth, S. G., The Effects of Manganese and
Neutralization on the Removal of Sulfide and Oxygen Demand
in Fellmongering Effluent, Journal of Society Leather
Technologists and Chemists, 59, 5, 129.
9. Roulings, D. E., Woods, D. R., and Cooper, D. R, , An
Experimental Study of Sulfide in the Aeration of Fellmonger-
ing Effluent, Journal of Society Leather Technologists
and Chemists., 59, 5, 143.
101
-------
10. Perry, R. H., Chilton* C. H., and Kirkpatrick, S. D-,
Perry's Chemical Engineering Handbook, Fourth Edition,
McGraw-Hill Book Company, New York, N. Y., 1963, pg. 14-6.
11. Smith, B. D., Design of Equilibrium Stage Processes, McGraw-
Hill Book Company, New York, N. Y., (1963), Page 256
12. Process Design Manual for Sludge Treatment and Disposal,
U. S. Environmental Protection Agency, Technology Transfer,
EPS 625/1-74-006, October, 1974.
13. Standard Methods for the Examination of Water and Waste-
Water, 13th Edition, American Public Health Association,
Washington, D. C., 1971.
14. Process1 Design Manual for Sulfide Control in Sanitary
Sewage Systems, U. S. Environmental Protection Agency,
Technology Transfer, October, 1974.
15. Little and Ives Complete Book of Science, Gas Absorption,
J. J. Little and Ives Company, Inc., Educational Publishers,
New York, N. Y. 1958, Page 842-843.
16. Koch Engineering Bulletin KT-5, Koch Engineering Company,
Inc., Houston, Texas, 1968, 8 pp
17. Considine, Douglas M., Chemical and Process Technology
Encyclopedia, McGraw Hill, Inc., New York, N. Y., pp 5-15,
602, 603, (1974)
18. Dreisbach, Robert H., M. D., P. H. D., Handbook of
Poisoning, 8th Edition, Lange Medical Publications, Los
Altos, California, pp 226, 227, (1974)
19. Happich, W. P., Happich, M. L., Cooper, J. E., Feairheller,
S. H., Taylor, M. M., Bailey, D. G. , Jones, H. W., Mellon,
E. F., and Naghski, J., Recovery of Proteins from Lime -
Sulfide Effluents from Unhairing Cattlehides, Journal
American Leather Chemists Association, pp 50 - 65, (1973)
20. Davis, M. H., and Scroggie, J. G., Investigation of
Commercial Chrome Tanning Systems, (Part V - Recycling
of Chrome Liquors in Commercial Practice, Journal of the
Society of Leather Technologists and Chemists, Volume
57, pp 173, (1973) .
102
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APPENDIX A
SULFIDE CHEMISTRY
Hydrogen sulfide at normal temperature and atmospheric
pressure is a colorless gas having the offensive odor of rotten
eggs. It is a flammable gas and may explode on ignition at a
temperature of 260°C (500°F) and a wide flammability range of
4.3% - 46% in air. The gas is highly toxic and at higher
concentrations it paralizes the olfactory nerves preventing
detection of the odor. It is a mild reducing agent and is
oxidized under suitable conditions using chlorine, oxygen,
sulfur dioxide, and sulfuric acid. It enters into reactions
with many organic compounds. With solutions of heavy metals,
(silver, lead, copper, manganese) , hydrogen sulfide form metal
sulfides.
Industrially, depending on the quantity and purity
required, hydrogen sulfide is prepared by one of the following
reactions:
Sulfur and hydrogen: S + H2 — * ^S
Sulfur and an alkali: 4 S + 2 NaOH + H20 -^ 2 H2S +
Sulfide an an acid: 2 NaHS + H2S04 -^ 2 H2S +
Hydrogen sulfide may be supplied by on-site generators or in
cylinder quantities in steel cylinders as a liquefied gas under
its own vapor pressure of 252 psig at 70°F.
A number of producers of sodium sulfhydrate (NaHS) will
purchase the hydrogen sulfide gas in large containers and allow
the gas to escape from the cylinder under its own pressure into
a packed column (or tray plate tower) where liquid caustic soda
is caused to flow. The reactions that take place are as
follows:
Reaction No. 1: NaOH + H2S -> NaSH + H20
Reaction No. 2: NaSH + NaOH -* Na2S + H20
In the presence of excess NaOH, reactions 1 and 2 take place
simultaneous. As additional H2S is fed into the Na2S, the
reaction goes to completion as follows:
103
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TABLE NO. 35
PHYSICAL PROPERTIES OF HYDROGEN SULFIDE (Ref. 17)
Molecular Weight 34 . 08
Boiling Point, °C -59.6
Melting Point, °C -82.9
Tripple Point at 0.23 atm, °C -85.5
Density
Gas at 21.1 °C, g/1 1.43
Liquid at boiling point, g/ml 0.993
Specific Gravity
Gas at 15°C (air = 1 ) 1.1895
Liquid, d6^ 0.96
Critical Temperature, °C 100.4
Critical Pressure, atm 88.9
Critical Density, G/cm^ 0.349
Expansion Ratio, liquid at Boiling Point to Gas
at 21.IOC 1:674
Solubility in Water of Gas at 26.7°C wt. % 0.32
Specific Heat of Gas at Constant Pressure at
21.1°C, cal/g mole (°C 8.2
Heat of Vaporization, cal/g mole 44.63
Heat of Fusion, cal/g mole 568
Viscosity of Gas at 0°C, cp 0.01166
Autoignition Temperature, °C 260
Flamable Limits in Air, volume % 4.3 - 46.0
104
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Reaction No. 3: Na2S + H2S —» 2 NaHS
The end product of the commercially absorbed H2S is NaHS, or
sodium sulfhydrate. Since the hydrogen sulfide is fed into the
absorber from a cylinder under its own pressure of 252 psig,
the system is airless.
The sulfide reclamation system at Blueside Company makes
use of a controlled air flow to carry the liberated hydrogen
sulfide from the degasifier to the absorber. The air flow
volume is controlled as related to the hydrogen sulfide
liberated from the degasifier influent to ensure that the H2S
concentration in air is below the 4% lower explosion limit." The
properties of hydrogen sulfide are listed in Table No. 35.
At Blueside Company, hydrogen sulfide is formed in the
degasifier by acidification of the sulfide bearing wastewater
influent to a 5.5 - 5.0 pH using sulfuric acid. The reaction
is as follows:
Effluent sulfides + H2S04 -> Effluent sulfates + H2S
The conversion of the hydrogen sulfide to sodium sulfide is
limited to reactions 1 and 2 above.
The removal of sulfides from product floxvs by selective
solvent absorption in packed towers or plate towers is common
place in the chemical and petro-chemical industries. It is
a safe operation as is the sulfide reclamation system at the
Blueside Company.
The solubility of hydrogen sulfide in water at any given
temperature is based on two phenomena; Henry's Lav; and the
ionization of hydrogen sulfide as a weak acid. Henry's Law,
simply stated, defines the distribution of a gas between a
liquid solvent and a gas phase as a constant proportion at a
given temperature. Henry's Law may be written as pa=H Xa where
pa is the mole fraction of the component in the gas above the
liquid, and Xa is the mole fraction of the component in the
liquid. A simplified approximate ratio may be stated H = pa/Xa
where pa = ppin H2S in the air and Xa = ppm in the liquid.
The hydrogen sulfide in the water in the relationship is
only the sulfide as hydrogen sulfide present. Since hydrogen
sulfide is a. weak dibasic acid, the degree of disassociation
is dependent on pH. At a low pH 5.0, the ionization of the
hydrogen sulfide is repressed and approximately 99% of hydrogen
sulfide is not in the ionized state. In Section 7 it was
stated that one theoretical tray was required to liberate the
hydrogen sulfide from the liquid. Henry's Law resulted in
0.00672 pounds of sulfide remaining in the liquid as opposed
to 0.02 pounds as reflected by the data. The hydrogen sulfide
105
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in the vapor phase above the liquid based on the data was
2.21 pounds - 0.02 pounds =2.19 pounds. Based on Henry's
Law, the amount above the liquid would be 2.21 - 0.00672
pounds = 2.2033 pounds, or 99.7% of the hydrogen sulfide
liberated from the liquid.
The ionization of hydrogen sulfide in water proceeds in
two steps in accord with the following equations:
H2S t; SH~ 4- H+
The ionization constant of this reaction is:
10~7 = (SH~) (H+)
H2S
The second ionization step is:
SH- ±; s= + H+
The ionization constant of this reaction is:
10~15 = (S=)
SH-
At any pH, the state of ionization of the sulfide can be
determined. In the influent to the sulfide reclamation plant
at pH 9.0, the calculations are as follows:
lO-7 = (SH~) 10~9
H2S
SH" = 10~7 = 102
H2S
The ratio of sulfhydrate to hydrogen sulfide is 100:1
10~15 = (S=) 10~9
S= = 10~15 = 10~6
SH- 10"9
The ratio of sulfide to sulfhydrate is therefore 1: 1000000
From these equations, the ionization of the sulfide at
the significant pH's for the sulfide reclamation can be
calculated. Table No. 36 lists the concentration of ions in
the solution at various pH levels.
106
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TABLE NO. 36
IONIZATION OF HYDROGEN SULFIDE
Source pH H2-S SH~ S=
Acidified
Influent
Acidified
Influent
Absorber
End point
of NaOH
Analysis
5
6
12
8
.0 IO2
.0 10
.0 10"5
.2 10"1'2
J. J. U ~°~
i io-9
1 10~3
1 lO"6'8
For the degasifier influent at pH 9.0, the sulfide is
present primarily as the sulfhydrate ion. Very little sulfide
is present. Approximately 1% is present as hydrogen sulfide.
Acidification to pH 5.0 results indicate that hydrogen sulfide
is not ionized and is the dominant sulfide present, sulfhydrate
is present at approximately 1% lex'el, and the sulfide S~
concentration is less than 1 ppm. At pH 12 in the absorber, the
sulfide is present predominantly as the sulfhydrate ion.
Hydrogen sulfide is present in the range under 10 ppm and about
0.1% of the sulfide is present as sulfide ion S~. At pH 8.2,
the sulfide is present primarily as the sulfhydrate ion.
Approximately 5% of the sulfide_is present as hydrogen sulfide
and the amount of the sulfide S~ ion is less than 1 ppm.
If commercially available sodium sulfide (Na2S) were placed
in solution at a concentration of 10% in water, the ionic
concentration of SH", S= and OH~ would be as follows:
For hydrolysis of S~
I S= + H20±; HS- + OH" Kh = IO"14 = 1Q
10"15
HS-$ H+ S= K2- = 1 x ID"15
H20 ^ H+ OH" Kw = 10"14
107
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H+ + S~ + H20 - HS~ 4- H+ + OH~
Kn = (HS~)(H+)(OH=)
(H*) (S=)
Kw = (H+) (OH~)
K2 = (H+) (S=)
(SH-)
Kh = Kw
K2
Equations I and II are the same equation.
Since Na?S and NaSH total to be 1.28 molar concentration, let
SH~ = X
Therefore (S=) = 1.28 -X and (HS~) = (OH~) from equation I
Kn = (HS~) (OH") or Kn = X2
™ 1,28 - X
10 = X2
1.28 - X
12.8 - 10X - X2 = 0
Using the quadratic equation: X = 10- (100 + 4 x 12.8) 2
-2
X = 1.148 Molar
SH~ = 1.148 Molar
S= = 1.28 - .1.148 = 0.1318 Molar
Therefore, a 10% sodium sulfide solution has the sulfhydrate
ion present in a 1.148 molar concentration and sulfide S= ion
present in a 0.1318 molar concentration, or a sulfhydrate to
sulfide ratio of 8.7 to 1.
Using Henry's Law, it is possible to determine what pH
level is necessary in the caustic soda recirculation tank to
the absorber to maintain the H2S escaping into the atmosphere
below 100 ppm from a 10% sodium sulfide solution. Henry's Law
constant for H2S at 90°F is calculated to be 12*14 atmosphere
molar -1. If the limit of H2S into the atmosphere is to be
100 ppm, then the partial pressure will be:
108
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100 10"14 atmospheres
1,000,000
Therefore, the solution concentration must be:
Km = V
where "m" is the molality, "K" is Henry's Law constant, and
"P" is the partial pressure of H2S. Calculating for m:
m = P ,/K
£
m = 10 q atmosphere
12 . 14 atmosphere molar
m = 8.234 x 10~6 molar
If 1*8 is passed through the packing tower containing
caustic soda solution to produce Na2S and NaSH, the pH calcula-
tions require determining the hydrolysis steps for preparation
of two equations which will then be solved using the quadratic
equation.
Hydrolysis steps
S= + H90 7L HS + OH KHS> = Kw
"
v
K2
h2
+ OH" Khl = 10~14
Assuming a concentration of 10% Na2S, the molarity will be
1.28 molar.
For hydrolysis, Let X = (HS~) and Y = (OH~)
Then: 1.28 - x =•- (S=)
For hydrolysis of S~
Kbo = (HS") (OH") = XY
n - ~
1.28-X
For hydrolysis of HS
KUT = (H2S) (OH~) = 8.234 x 10~6Y
n± gg= x
Therefore, the two equations for solving are as follows:
109
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Equation I 10 = XY
1.28 - X
Equation II 1.11 x 10~7 = 8.234 x 10~6Y
X
To find the OH~ concentration needed or Y, rearrange the
equations :
Equation II X = 8.234 x 1Q~6Y = 74. 1Y
1.11 x 10~7
Substituting Equation I 10 = 74.1 Y.Y or 12.8 - 74. 1Y - 74.1Y2=0
1.28 - 74. 1Y
Using the quadratic equation:
Y = + 741 - (7412 + 4 x 74.1)^
-2 x 74.1
Y - 0.0172 M or (OH~) = 0.0172M
pOH = - log 0.0172M
pOH =1.76
pH =14.0-1.76
pH = 12.24
A pH of 12.24 for a 10% solution of sodium sulfide will
assure no more than 100 ppm of H2S into the atmosphere.
Oxidation of Sulfides
Manganous Sulfate has been used as the catalyst in aeration
systems to convert sulfides to elemental sulfur and/or sulfates.
The reactions proceed as follows:
2 HS- + 02 + H+ Mn ++ ) 2 H20 + 2 S
or to sulfates
HS~ + 2 02 Mn++ ) S04 = + H+
Hydrogen peroxide may be used as a method of controlling
the sulfide content in an effluent. The hydrogen peroxide
reacts on a stoichrometric basis at a range of 6 - 7 pH.
H2S + H2°2 a-") S= + 2 H2°
110
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Under strongly alkaline conditions, the chemistry shifts
and four times as much hydrogen peroxide is needed:
4 H202 , s S04= + 9 H* + 8 Cl"
The addition of chlorine to an effluent depresses biological
activity, virtually stopping the consumption of oxygen, until
the chlorine residual has disappeared. During this period of
suppressed biological activity, the stream acquires an oxygen
reserve that will delay the reappearance of sulfide downstream.
(14)
On pure sulfide solutions, chlorine converts the sulfide
to sulfur requiring 2.2 parts chlorine to 1 part sulfide:
HS~ + C12 - } S + H+ +2 Cl~
The elimination of sulfides in an affluent reduces the
oxygen demand in the treatment plant and reduces the BOD^
pollution loading of the wastewater resulting in a lower
municipal sewer surcharge. Oxygenation of the sulfides in the
treatment plant would produce thio-compounds through to sulfates:
S= + 02 _ > S02, 803, S04
Toxicology of hydrogen sulfide gas dictates care in
personal safety and constant training for awareness of dangers.
Blueside Company has signs dictating the potential presence of
H2S in all areas where the possibility exists. Respirators,
gas masks, sensing devices, personal belt alarm units make for
a safe-guard approach in tannery areas where sulfides are used.
High ventilation rates of outside air through the potential
areas has ensured an H2S free plant.
The principal manifestation of H2S poisoning is irritation.
Ill
-------
a. Acute Poisoning
Hydrogen sulfide is detectable by odor at 0.5 ppm, and 10
ppm causes irritation and sensory loss. Concentrations above
50 ppm cause the following symptoms: painful conjunctivitis,
appearance of a halo around lights, headache, anosmia, nausea,
rawness in the throat, cough, dizziness, drowsiness, and
pulmonary edema. Concentrations above 500 ppm cause immediate
loss of consciousness, depressed respiration, and death in 30
to 60 minutes.
The threshold limit of exposure has been set at 10 ppm
for an 8 hour day.
b. Chronic Poisoning
Prolonged exposure causes persistent low blood pressure,
nausea, loss of appetite, weight loss, impaired gait and
balance, conjunctivities, and chronic cough.
c. Prognosis
In hydrogen sulfide poisoning, if the patient survives
for the first four hours, recovery is assured. (18)
112
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APPENDIX B .
ORIGINAL SPECIFICATIONS
1 SULFIDE STRIPPING SYSTEM
General
Furnish, install controls, instruments, and accessory
equipment necessary for the operation of a sulfide stripping
system, as specified herein.
The sulfide stripping system is to convert gaseous hydrogen
sulfide mixed with air to sodium sulfide for process use. This
is accomplished by blowing the hydrogen sulfide gas from the
degasifying tower through an absorption tower, in which sodium
hydroxide is being circulated. A chemical reaction will occur
and the sodium hydroxide will be converted to sodium sulfide.
A hydrogen sulfide monitor will take samples from the
exhaust air stream of the absorption tower and will indicate
when the sodium hydroxide is converted to sodium sulfide. At
that time, a new batch of sodium hydroxide will begin circulat-
ing. The sodium sulfide will be transferred to storage, and a
new batch of sodium hydroxide will be made up in the empty
tank.
The system will operate for eight hours per day initially,
and later will operate 24 hours per day, except for infrequent
shutdowns.
Data to be Furnished
Submit for approval six complete sets of shop drawings,
showing details of construction and erection, and four complete
sets of operating and maintenance instructions, including
wiring diagrams.
Design
Scope
One supplier shall assume the responsibility for the
sequence of operation of the sodium sulfide stripping system.
This shall be a complete and operating system, including at
least the following:
113
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(a) Control cabinet and necessary relays
(b) Hydrogen Sulfide detector
(c) Timers for control system
(d) Horn and lights
(e) Level sensors with relays for batch tanks
(f) Solenoid operated valves
Process piping shall be installed by others. Solenoid
and motor operated valves supplies under this section shall be
installed by others as a portion of the process pipework.
However, all responsibility for operation of these valves shall
remain under this section.
Operation
The sequence of operation of the sulfide stripping system
shall be as follows:
1. The system will be initiated by the "Start" push button,
on the control cabinet, for continuous operation of the
system until the "stop" push button is activated or by
interrupting power to the cabinet.
2. The caustic feed pump and valve to fill batch Tank
No. 1 shall be energized. Tank will fill to approxi-
mately 52.5 gallons wibh 25 percent caustic. Level
will be fixed by a level probe called the intermediate
level.
3. The intermediate level probe shall shut off the valve
to batch Tank No. 1 and stop the caustic feed pump.
The water valve to batch Tank No. 1 shall open to
complete the makeup of 10 percent caustic. An upper
level probe shall be fixed and close the water valve
to Tank No. 1.
4. Upper level probe shall energize a relay to start the
circulation pump. The circulation pump shall be locked
in until the system "Stop" push button is activated or
by interruption of power.
5. The upper level probe in batch Tank No. 1 shall open
the circulation valves to and from batch Tank No. 1.
6. A caustic solution shall be made up in batch Tank No. 2
through a similar sequence and shall be held ready
until called upon.
7. A hydrogen sulfide detector will monitor the exhaust
gas from the absorption tower and alarm when the
hydrogen sulfide content exceeds a preset limit. The
114
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The alarm shall indicate that the sodium hydroxide has
been converted to sodium sulfide.
8. An adjustable recycle timer shall be in the circuit
parallel to the hydrogen sulfide detector and will
start its cycle with each batch tank circulation system.
This is provided to replace the hydrogen sulfide
detector and will start its cycle with each batch tank
circulation system. This is provided to replace the
hydrogen sulfide detector alarm in case of failure of
the detector. The timer shall also be able to be
taken out of the circuit.
9. Upon indication of an alarm, a time delay relay shall
be activated to confirm that the preset excess of
hydrogen sulfide is escaping by taking another sample.
A relay will then be energized by either the alarm
or timer to close the circulation valve from batch
Tank No. 1. Another time delay relay shall be actuated
to drain the absorption tower before the circulation
valve to batch Tank No. 1 shall be closed (the tower
does not have to be completely drained). Immediately
thereafter the circulation valves with batch Tank No. 2
shall open, transferring circulation from batch Tank
No. 1 to No. 2.
10. If the transfer is not completed within an adjustable
time limit and an excess hydrogen sulfide is detected,
an audible and visual alarm shall go off. This will
notify the operator of trouble in the sequence of
operation.
11. The sodium sulfide drain valve from batch Tank No. 1
shall open and the sodium sulfide transfer pump shall
start. When batch Tank No. 1 is empty, as determined
by a low level probe, the sodium sulfide transfer
pump shall stop and the sodium sulfide drain valve
from batch Tank No. 1 shall close.
12. Batch Tank No. 1 shall then be refilled as in Steps
2 and 3 with fresh caustic solution and be held ready
until called upon.
13. VThen the sodium hydroxide solution in batch Tank No. 2
has been converted to sodium sulfide as indicated by
either alarm or time, the circulation shall be
transferred to batch Tank No. 1 as outlined in Step 9,
only switching from batch Tank No. 2 to No. 1. The
cycle will repeat as in Steps 10, 11, and 12 only with
batch Tank No. 2.
115
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14. The system shall be continuous in operation, alter-
nating between the batch tanks unless the sequence is
stopped manually or by power interruption, In that
case, the system shall be restarted manually, and
through the relays will resume operation where it was
stopped.
Control Cabinet
An electrical control cabinet shall be provided to house
the sequencing control for the hydrogen sulfide stripping
system. Cabinet shall conform to Joint Industry Standard with
hinged door and latch and be for wall mounting. The door shall
be gasketed to keep out dust and fumes. Cabinet exterior and
interior shall be cleaned, primed, and finished in an enamel
selected by the owner. Nameplates shall be black lamicoid
with white filled lettering 0.25 inch in height.
The relays, except for level sensing specified elsewhere,
shall be industrial duty equal to General Electric CR2790E or
CR120J with 120-volt coils for continuous duty. Push button
stations and selector switches shall be oiltight, heavy duty
construction equal to General Electric 2940 for flush panel
door mounting. An industrial type horn shall be provided. All
control wiring shall be of industrial duty equal to 600 volts,
Flaininol, installed in a workmanlike manner, with terminal
strips, as necessary, cabled where necessary, and securely
fastened to the cabinet interior by approved methods. All
wiring shall be coded for ease of identification in trouble
shooting.
An external fused disconnect shall be provided and mounted
adjacent to the cabinet.
Hydrogen Sulfide Sampler Detector
The sampler detector for hydrogen sulfide concentration
shall be an integral unit, complete with sampling pump, monitor,
timer, and alarm control circuitry. The monitor shall be
adjustable over the limits of 0.0025 to 25 ppm of hydrogen
sulfide. Unit shall have automatically adjusted transmission
through clean paper with built-in voltage regulator. Vacuum
air pump shall have a capacity of 0 to 30 SCFH free flow
continuous duty. A prefilter shall be provided for trapping
particular matter. Alarm contracts shall be rated at 120
volts, 5 amperes, noninductive load. Sensitive/tape shall only
advance when hydrogen sulfide exceeds a preset limit. This
shall be part of a tape-saving feature which includes a sampling
frequency adjustable from 1 to 60 minutes in intervals of one
minute. The tape shall advance automatically after an alarm
condition through an adjustable built-in timer. Analyzer shall
be provided with 30 rolls of 60 foot punched tape, sensitized
116
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for hydrogen sulfide. A humidity jar shall be attached to the
air inlet and the exhaust gases shall be passed through a soda
and lime tube to scrub out any hydrogen sulfide that may have
passed through the tape. The detector cabinet shall be for
wall mounting, next to the control cabinet,
The hydrogen sulfide detector shall be Research Appliance
Company, Allison Park, Pennsylvania, Catalog No, 2307-B,
Model F-2-A AISI, or approved equal.
Level Sensors
The level sensors shall be of the probe type. Each shall
consist of a probe holder with four No, 316 stainless steel
probes and differential relays having built-in transformers for
power supply isolation.
One probe on each holder shall be bare and extend to
within 0.50 inch of tank bottom to serve as a "ground" electrode.
A second probe shall extend to within one inch of tank bottom
and be the low level probe. A third probe shall terminate at
one-third tank capacity (about 50 gallons) and be the inter-
mediate level. High level will be the fourth probe and extend
within six inches of the top (150 gallons). The level probes
shall be PVC coated to within one inch of the tip, for service
in a 25 percent caustic solution.
The probe holder shall be supplied with a three inch
pipe threaded connection to fit on the batch makeup tank.
Probe relays shall be mounted in the control cabinet. The
relay types and contact arrangement shall be such that they
will operate in accordance with the automatic sulfide stripping
system.
The level sensing system shall be as manufactured by B/W
Controls, Birmingham, Michigan, or approved equal.
Timers
The timers shall be of the adjustable time delay type. The
time shall be adjustable from a knob on the front and the timers
shall be flush panel mounted on the control cabinet.
Timers shall be Bliss Eagle Signal Company, Davenport, Iowa,
Cycl-Flex, HP5 Series, or approved equal.
Solenoid Operated Valves
All process valves under the one inch pipe size shall be
two-way type solenoid valves selected for the specific service.
Valves handling caustic and alkaline sodium sulfide solutions
117
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shall have all-iron or stainless bodies with Teflon seats.
Diaphragm, when used, shall be resistant to twenty-five percent
caustic solution. Valves for cold water service shall have
bronze bodies with stainless steel trim.
Valves shall be equal to Automatic Switch Company, Florham
Park, New Jersey, ASCO, or approved equal.
Motor Operated Valves
Process valves one inch and over shall be ball valves
with reversing electric motor operators. Valves shall be
constructed of PVC, carbon steel or ductile iron, suitable for
the service intended * Valve seals shall be molded Teflon or of
other composition suitable for intermittent service. Motor
operators shall be supplied in weatherproof housings and for
operation on 120-volt, 60-cycle power, Valves shall be supplied
with 150 pound ASA flanges.
Valves shall be equal to Worcester Valve Company, Worcester,
Massachusetts, Econ-O-Miser with a Flow Mate operator, Hills-
McCanna Company, Carpentersville, Illinois, McCannaseal with
a Ramcon operator, or approved equal.
118
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2 DEGASIFYING AND OVERFLOW TOWERS
General Requirements
Furnish all materials and equipment, required for instal-
lation and satisfactory operation of the degas.ify.ing tower and
overflow tower,
Data to be Furnished
The selected manufacturer shall submit structural calcu-
lations for the tower to the Engineer for approval. All calcu-
lations shall be stamped by a Registered Professional Engineer.
The tox\7er shown on the drawings is illustrative and
indicates only the design features to be incorporated. Special
reinforcement, etc., is not shown. Piping orientation shall be
as indicated on the drawings.
Submit for approval six sets of complete shop drawings
showing details of construction and errection, and four complete
sets of operating and maintenance manuals.
Design.
The degasifying tower shall be provided for the removal
of sulfides from the settled plant effluent. The effluent will
be mixed with sulfuric acid and discharged into the top of the
tower. Air will be introduced into the bottom of the tower
and will flow upward through the tower thus removing the en-
trained hydrogen sulfide gas. The gas will be discharged out
the to~ of the tower to the adsorbtion tower. The treated
effuent will flow into an overflow tower where chlorination
will be provided. The effluent will flow from this tower to
the city sewer.
T-he four--stage degasifying tower shall be provided as
indie-ted on the drawings. The settled effluent will be pumped
at a rat« of approximately 220 gpm to the top stage of the
tower, at which point it will be mixed with concentrated
sulfuric acid. Air will be provided by a positxve displacement
blower at 'a rate of 700 cfm at a maximum of 10 psig into a plenum
at the base of the tower. The air will pass upward throughout
the tower and pass out through a discharge line in the top of
the tower. The tower shall be airtight.
119
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Each stage will be provided with check valve type air
diffusers. Each level will maintain a 3 foot 3 inch depth of
liquid. Downward flow shall be allowed only through the down-
comers. The waste will leave the tower through a 12 inch
effluent in the bottom stage.
The degasifying tower shall be constructed of polyester
resin and fiber glass or other suitable corrosion-resistant
materials approved by the Engineer. The tank shall be structur-
ally designed to withstand all possible load conditions
including wind loads.
Minimum standards for towers made by hand lay-up shall be
in compliance with Product Standard PS-122-C for Custom Contact-
Moulded Reinforced Polyester Chemical Resistant Process Equip-
ment issued by the U- S. Department of Commerce; and for towers
fabricated by filament winding, shall be in compliance with
the Proposed Product Standard for Filament Wound Reinforced
Polyester Tanks being developed by the Society of the Plastics
Industry.
The tower shall be provided with a six inch flanged inlet
in the top stage along with a one inch connection for concen-
trated sulfuric acid. The effluent connection shall be 12
inches in the bottom stage. All interior piping shall be fiber
glass or PVC. The air will enter through an eight inch
connection in the base section. A 12 inch air outlet connection
shall be provided in the top section. Each stage shall be
provided with a six inch viewing port with a window, a 24 inch
access manhole with a blank cover and a two inch connection to a
drain. All flanges shall be standard ANSI Class 150 pound
rated. All openings shall be reinforced for strength.
Each chamber shall be connected to the next one with a 12
inch diameter downcomer with a 24 inch funnel top section. They
shall be deep enough in the solution so air will not escape
upward through them. There shall be 80 air diffusers per
chamber with a ball check valve built-in. They shall be made
of a plastic that will not corrode and have an orifice adjust-
ment so the head loss and air flow may be adjusted. The
diffusers shall be Link-belt Adjust-air diffusers or approved
equal.
A spray system shall be provided in each chamber to prevent
possible foaming. The nozzles shall be of Everdur or approved
equal as manufactured by Schutte and Loerting. It shall be
possible to start and stop the system with an external valve.
All piping shall be internal and made of fiber glass, PVC or
other approved material that will resist corrosion. A minimum
of four (4) nozzles shall be provided in each chamber. The
piping shall be supported internally. The spray system shall
120
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not use more than 60 gpm. A strainer shall be used upstream
to prevent clogging.
The waste shall leave the degasifying tower and go to an
adjustable overflow tower. The overflow weir shall be adjust-
able over a range of 48 inches. This adjustment is required so
that the waste will flow out but gases will not escape. The
weir shall be a telescoping valve as manufactured by Rex
Chainbelt, Milwaukee, Wisconsin and made of stainless steel
parts within the tower. An electric motor operator shall be
furnished with a weatherproof push button station at the base
of the tower. The valve shall have a neoprene seal to prevent
leakage of gas or effluent between the body and telescoping
sections of the valve. After overflowing over the weir there
shall be provided a chlorine contact chamber in the base as
detailed on the drawings before going to the sewer. This adjust-
able weir tower shall be airtight and be provided with at least
two six inch viewing ports near the surface. A steel ladder
shall be attached to this so the viewing ports may be utilized.
Warranty^
The degasifying and overflow towers shall be warranted to
be of good quality and constructed with the best commercial
practice. The towers shall be air tight to prevent the escape
of hydrogen sulfide gas. Any material or workmanship that,
within two years after delivery to the job site, is found to
have been defective, shall be repaired or replaced by the
manufacturer.
Alternate
The manufacturer shall submit an alternate price for
providing tank insulation. It is anticipated that the waste-
water will enter the tower at approximately 50°F and could be
held in the tower a maximum of 64 hours with no inflow. Outside
temperature can be expected to drop to -10°F. Insulation
material should be sufficient to prevent freezing. Design
calculations and samples of the insulation shall be submitted
to the Engineer.
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3 ABSORPTION TOWER
Gen era1 Requi r emen t s
Furnish all materials and equipment required for instal-
lation and satisfactory operation of the absorption tower.
Data to be Furnished
If the selected manufacturer proposes to furnish specially
fabricated equipment, the Engineer shall require submittal of
structural calculations for the tower for approval. All
calculations shall be stamped by a Registered Professional
Engineer. If a standard tower is offered, calculations will
not be required.
Submit for approval six sets of complete shop drawings,
showing details of construction and erection, and four complete
sets of operating and maintenance instruction.
Design
An absorption tower shall be provided to convert hydrogen
sulfide gas from the degasifier to liquid sodium sulfide. A
solution of sodium hydroxide will be cycled through the tower
to convert the gas to sodium sulfide.
The absorption tower indicated on the drawings is strictly
diagrammatical and is intended only to indicate the required
design features. Standard manufacturers' absorption towers,
if they meet the requirements of this specification, will be
acceptable.
The tower shall be suitable for handling hydrogen sulfide
gas and 10 percent liquid sodium hydroxide solution. The tower
design shall incorporate but not be limited to the following
features:
1. Inlet - outlet connections
2. Positive seal on tank overflow
3. Sodium hydroxide spray system
4. Packing material
5. Tower drain
6. Diffuser plate
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It is the intent of these specifications that the equipment
operate as described herein.
The gas shall enter the side near the base of the tower.
The gas will pass through at a rate of approximately 700 cfm
at 3.5 psig minimum to 5.5 psig maximum. (This is the rate of
discharge of the positive displacement blower connected to the
degasifying tower.) The gas will pass through a contact bed
counter to the flow of sodium hydroxide. The contact bed shall
force intimate mixing between the gas and liquid. The sodium
hydroxide shall be uniformly distributed over the packing
through non-clogging spray nozzles. The nozzles shall be
designed so the entire system can handle 20 gpm flow at 20 psig.
The nozzles shall be stainless steel. An adequate number of
nozzles shall be provided so that the entire surface area is
in contact with the spray within a vertical distance of two feet
below the bottom of the nozzles. The nozzles shall be easily
removed for inspection and cleaning. An adjustable air seal
shall be provided in the base so all gases must pass through
the packing and not be short circuited to the batch tanks
located below.
The absorbtion tower shall be constructed of molded rein-
forced polyester or other suitable materials that will resist
corrosion and physically able to withstand the process involved.
Materials shall be subject to approval of the Engineer. The
tower shall be air tight.
Minimum standards for towers made by hand lay-up shall be
in compliance with Product Standard PS-122-C for Custom Contract-
Molded Reinforced Polyester Chemical Resistant Process Equipment
issued by the U. S. Department of Commerce; and for towers
fabricated by filament winding shall be in compliance with the
Proposed Product Standard for Filament Wound Reinforced
Polyester Tanks being developed by the Society of the Plastics
Industry.
The tower shall be provided with 12 inch inlet and exhaust
connection flanges. There shall also be provided a Ik inch
flange for the sodium hydroxide inlet, a four inch flanged
connection for the drain to the batch tanks and a two inch
connection with a valve for draining the tower. An additional
18 inch~flange shall be provided with a blank flange for access.
All openings shall be reinforced for strength and all flanges \
shall be standard ANSI Class 150 pound rated. The tower shall
be approximately four foot in diameter and 10 foot high and
shall be designed to operate when filled with sodium hydroxide.
The packing material shall be Koch Flex rings or approved
equal. This packing shall be supported on a diffuser plate
which will distribute the gas up through the packing and
allow the caustic solution to pass down through the batch tank
123
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for recycling. The diffuser plate shall be constructed of a
material that will withstand the process. The caustic solution
specified in another section will be fed to the tower at the
rate of 20 gpm.
The manufacturer shall supply the 12 inch exhaust pipe
with a tap for 0.25 inch sample line to the hydrogen sulfide
detector. Pipe shall be PVC.
Warranty
The absorption tower shall be warranted to be of good
quality and constructed in conformance with the best commercial
practice. Any material or workmanship that within two years
after delivery to the job site, is found to have been defective,
shall be repaired or replaced by the manufacturer.
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APPENDIX C
DEGASIFIER DESIGN CALCULATIONS
220 gpm
Air
-10
Typical
Liquid 3' 3"
Level
*-'
4.
-4 i--
700 cfm/10 psi
125
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Where P = Press (psi)
R = Radius (inches)
and t = Thickness (inches)
TOWER DESIGN
Girth Stress : SQ = PR
Design SG = 200 psi (Max.)
Wall thickness t = 13.5 (60) = 0.405"
2000
OR
V Nominal for maximum condition of flood
Axiel Stress
From Blower w/full tower
£ + 8 psi x 11,310 (in.)/:= 90,5001
£. v Vessel Dead Weight = 6,0001
Fluid - 7400 (8.33) 1.2 =74,000#
=80,000*
Net Axiel - 90,500 - 80,000 = 10,5001
or 0.93 psi on Bottom Tray
Therefore: Vessel should have 8 tie down lugs for
1315f. > each.
VESSEL
Size: 10' 0" I.D. x 26' 0" High
Wall: Circumference 31.4'
Bottom: Area 78.54 ft.2
Weight: @ h" nominal wall @ 4.45#/ft.
Wall = 26 x 31.4 x 4.45 = 3,640#
Bottom & Trays 78.5 x 5 x 4.45 = 1,750#
Tor 78.5 x 1-1 x 3.5 = 3001
Fittings & Inner Piping = 2001
Net vessel weight 5,890#
FLUID
Volume (585 gals./ft) @ 3 ft, 3 in. = 1,9,00 gal./tray
4 trays - 7,400 gallons normal operation
Total Tower - 585 x 26 - 15,200 gal.
Specific Gravity: Assume not to exqeed 1.2
Static Pressure: Per tray = 1.2 x .434 x 3.25 = 1.69 psig
: Flood Condition 1.2 x .434 x 26 = 13.5 psig
126
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AIR SIDE
To supply given 700 cfm and 10 psig of air, Ref: Link belt
644 x 15 adjust air loss and flow data.
a
a. At 80 diffusers/tray:
700 cfm 8.75 cfm At Design
80 Diffusers Diffuser
8.75 = 0.1 psi Drop @ 12 orifices open/diffuser
= 0.15 psi Drop @ 8 orifices open/diffuser
- 0.30 psi Drop @ 4 orifices open/diffuser
b. Pressure drop/tray
1.8 psi @ 12 Orifices/diffuser
1.9 psi § 8 Orifices/diffuser
2.0 psi @ 4 Orifices/diffuser
c. Four tray pressure drop
12 orifices/diffuser = 7.2 psi
8 orifices/diffuser = 7.6 psi
4 orifices/diffuser = 8.0 psi
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APPENDIX D
SULFIDE RECOVERY SYSTEM OPERATION
A. System Operating Procedure
1. Ensure the wastewater level in the sedimentation-
equalization basin is at about 12 inches above the
outlet orifice in the clarifier rim weir.
2. Be sure all drain valves are closed on degasifying
tower and in the overflow tower.
3. In the overflow tower set the adjustable overflow
outlet pipe at 16 feet - 4 inches above the lowest
tray's diffuser deck.
4. Start the wastewater pump and adjust the flow to
about 190 gpm. Fill the degasifying tower and the
overflow tower until the liquid level in the lowest
tray of the degasifying tower is at 46 inches. This
should require about 33 minutes. Observe the liquid
level in the sight glass. When the proper liquid level
has been reached, start the air compressor.
5. While waiting for the degasifying tower to fill, turn
on the spray water in the scrubber on top of the
degasifying tower. Adjust the flow to about 5 gpm.
Start the gas absorption tower. Check the recirculating
flow of NaOH. This should be between approximately
15 - 18 gallons per minute.
6. After starting the air compressor, start the acid
feeding system. Do not operate the acid feed system
when the air compressor is off because inadequate
mixing may take place with the result that strong acid
may damage the degasifying tower.
7. After starting the air compressor, observe the liquid
level in the degasifying tower sight glass. The
liquid level should drop to about the level of
downcomer. Allow about 10 minutes for the conditions
to stabilize. Adjust the height of the adjustable
overflow outlet until the liquid in the sight glass is
about 4 inches above the level of the downcomer.
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8. After the degasifying tower has been in operation for
about 10 minutes, a sample of wastewater should be
taken from the upper tray and checked for pH value.
If necessary, adjust the acid feed rate to obtain a
pH between 5.0 and 5.5 in the upper tray.
The pH should then be checked from time to time and
the acid feed rate adjusted as necessary. If the
wastewater pump rate is changed, the acid feed rate
will also have to be changed. Therefore, changes in
the wastewater pumping rate should be kept to a
minimum.
9. To shut down the system:
(a) Turn off the acid pump.
(b) Shut off the main pumps.
(c) Shut off the air compressor.
(d) Open the degasifying tower drains.
(e) Open the overflow tower bypass.
(f) Shut off the scrubber spray system.
(g) Shut off the H2S absorption system.
(h) After the degasifying tower is drained, flush it
out using the water spray system within each level.
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TECHNICAL REPORT DATA
(Please read Instructions on the reverse before completing)
1. REPORT NO.
EPA-600/2-77-031
3. RECIPIENT'S ACCESSION-NO.
4. TITLE AND SUBTITLE
REMOVAL AND RECOVERY OF SULFIDE FROM TANNERY
WASTEWATER
5. REPORT DATE
December 1977 issuing date
6. PERFORMING ORGANIZATION CODE
7. AUTHOR(S)
Robert H. Sayers
Roger J. Langlais
8. PERFORMING ORGANIZATION REPORT NO.
9. PERFORMING ORGANIZATION NAME AND ADDRESS
The Blueside Company, Inc.
Box 383
St. Joseph, Missouri 64502
10. PROGRAM ELEMENT NO.
1DB610
11. CONTRACT/GRANT NO.
12120 EPC
12. SPONSORING AGENCY NAME AND ADDRESS
13. TYPE OF REPORT A
Industrial Environmental Research Laboratory-Gin., OH
Office of Research & Development
U.S. Environmental Protection Agency
fh'nr-iTinati. OH 45268
RIOD COVERED
14. SPONSORING AGENCY CODE
EPA/600/12
15. SUPPLEMENTARY NOTES
Contact: Jack Witherow, FTS-420-4793
16. ABSTRACT
Recovery of sulfide from tannery waste was accomplished through acidification with
sulfuric acid in a closed system and removing hydrogen sulfide formed by blowing
with air. Sulfide was then absorbed in caustic solution to produce re-usable
sodium sulfide/sulfhydrate liquor for the tanning un-hairing process. Plant scale
equipment was used in demonstration. The recovered sulfide was reused at the
tannery and resulted in an annual savings of $397,437 for chemicals. The annual
cost of the recovery system was $305,385. Operational difficulties, design modifi-
cations, and cost effectiveness are discussed in detail. Protein recovery is
recognized as an adjunct to the acidification procedure and significant reduction
in BOD loading is achievable by subsequent clarification.
7.
KEY WORDS AND DOCUMENT ANALYSIS
DESCRIPTORS
b.lDENTIFIERS/OPEN ENDED TERMS
c. cos AT I Field/Group
Leather, Tanning Materials
Leather Industry, Waste
Abatement, Process
Modification, Engineer-
ing Design. Sulfide Re-
covery, Sulfide Destruc
tion, Waste Characteri-
zation and Loading
13B
3. DISTRIBUTION STATEMENT
Release to Public
19. SECURITY CLASS (ThisReport)
Unclassified
21. NO. OF PAGES
142
20. SECURITY CLASS (Thispage)
Unclassified
22. PRICE
EPA Form 2220-1 (9-73)
130
ft U.S. GOVERNMENT PRINTING OFFICE: 1978— 757-140/6671
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