EPA-660/2-73-021
DECEMBER 1974
Environmental Protection Technology Series
Waste Control and Abatement in the
Processing of Sweet Potatoes
National Environmental Research Center
Office of Research and Development
U.S. Environmental Protection Agency
Corvallis, Oregon 97330
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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
five series. These five broad categories were established to
facilitate further development and application of environmental
technology. Elimination of traditional grouping was consciously
planned to foster technology transfer and a maximum interface in
related fields. The five series are:
1. Environmental Health Effects Research
2. Environmental Protection Technology
3. Ecological Research
4. Environmental Monitoring
5. Socioeconomic Environmental Studies
This report has been assigned to the ENVIRONMENTAL PROTECTION
TECHNOLOGY STUDIES series. This series describes research
performed to develop and demonstrate instrumentation, equipment
and methodology to repair or prevent environmental 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 report has been reviewed by the Office of Research and
Development, EPA, and approved for publication. Approval does
not signify that the contents necessarily reflect the views and
policies of the Environmental Protection Agency, nor does mention
of trade names or commercial products constitute endorsement or
recommendation for use.
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EPA-660/2-73-021
December 1974
WASTE CONTROL AND ABATEMENT
IN THE
PROCESSING OF SWEET POTATOES
By
Charles Smallwood, Jr.
Robert S. Whitaker
Newton V. Colston
Project No. 12060 FRW
Pro-gram Element 1BB037
ROAP/TASK No. 21 BAB/031
Project Officer
Harold W. Thompson
Pacific Northwest Environmental Research Laboratory
National Environmental Research Center
Corvallis, Oregon 97330
NATIONAL ENVIRONMENTAL RESEARCH CENTER
OFFICE OF RESEARCH AND DEVELOPMENT
U. S. ENVIRONMENTAL PROTECTION AGENCY
CORVALLIS, OREGON 97330
For sale by the Superintendent of Documents, U.S. Government Printing Office
Washington, D.C. 20402 - Stock No. 5501-00975
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ABSTRACT
The conventional processing of sweet potatoes produces a
very strong caustic waste that is high in organic matter. Present
technology does not emphasize recirculation or other control of
water use.
Improved technology is available such as high presure low-
volume water sprays and a dry caustic peeling process that reduces
water use and converts the liquid caustic waste to a semi-solid
waste that can be disposed of in sanitary landfills, or sold as
cattle feed.
Developing technology offers the potential of lye recovery,
and improved steam peel or an infrared dry caustic peel that
increases yield.
In-plant control of waste through process modification and/
ox treatment is economical and may even provide a net return on
investment.
Biological treatment is effective.
The majority of the analytical data characterizing sweet
potato processing wastes presented in this report were obtained
from an in-depth study of one conventional sweet potato processing
plant during the 1971 processing season. It was originally
planned to follow this in depth study with a full scale demon-
stration of infrared dry peelingj water conservation through
water reuse and high pressure low-volume sprays; in-plant waste
separation and treatment; and end of pipe sequential screening,
but the plant burned down and the project was terminated.
This report was submitted in fulfillment of Contract No.
12060 FRW by North Carolina State University under the sponsor-
ship of the Environmental Protection Agency. Work was completed
as of July 26, 1974.
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CONTENTS
Abstract ii
List of Figures iv
List of Tables v
Acknowledgments vi
Sections
I Conclusions 1
II Recommendations 2
III Introduction 3
IV Current Processing Technology 5
Conventional Processing
Improved Processing
V Potential Processing Changes 22
Steam Peeling
Infrared Dry Caustic Process
Lye Recovery
By-products
VI In-plant Waste Control 27
Water Recirculation
Water Pressure Control
Grit Separation
Screening
VII Cost Effectiveness of Waste Control 30
Introduction
Assumptions
Water Recirculation
Water Conservation
Combined Recirculation and Conservation
Waste Screening
Waste Control by Process Modification
Summary
VIII Waste Treatment 40
Introduction
Caustic Peeling Waste
Nutrient Supplement
Land Irrigation
Anaerobic Lagoons
Aerobic Lagoons
Aerated Lagoons (Completely Mixed)
Aerated Lagoons (Incompletely Mixed)
Two-Stage Biological Treatment
IX References 47
iii
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FIGURES
No. Page
1 Water Use and Waste Sources for a Conventional 9
Sweet Potato Canning Operation
2 Locations of Water Meters at Tabor City Foods 11
3 Recirculation of Water and Separation of Waste 13
Streams in a Conventional Cannery
4 Portions of the Sweet Potato Lost During Processing 19
iv
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TABLES
No.
1 Gross Waste from Canning Sweet Potatoes 4
2 Mass Balance for a Sweet Potato Cannery 10
3 Gross Water Use in a Conventional Sweet Potato 12
Cannery Process
4 The Strength of Conventional Sweet Potato Wastes 15
After 20-Kesh Screening
5 Waste Loads from a Conventional Sweet Potato 17
Cannery After 20-Mesh Screening
6 Comparison of Conventional and Improved 21
Processing Technology
7 Suggested Screen Sizes for Sweet Potato Canneries 28
8 Comparison of Yield and Waste from Conventional 36
Wet Caustic Process and from Infrared Dry Caustic
Process
9 Some Comparative Aspects of Lagoon Treatment 46
Systems
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ACKNOWLEDGMENTS
1. Mr. Jimmy San-ell and Mr. Benny Prince of Tabor City Poods, Inc.
of Tabor City, North Carolina, provided the laboratory studies
and plant measurements of the waste.
2. Dr. W. S. Galler made the calculations of cost effectiveness,
and we are grateful for his advice and counsel.
3. Dr. M. W. Hoover, Mr. Norman Miller, and Mr. Ray Carawan of the
Food Science Department at North Carolina State University gave
freely of their knowledge and experience with the sweet potato
industry.
4. Mr. Roy Tew of H. P. Cannon and Sons was most helpful in showing
the utilization of dry peeling, and a spray irrigation disposal
of liquid wastes.
5. Professor C. Smallwood, Dr. H. V. Colston are on the faculty of
North Carolina State University in the Department of Civil
Engineering. Mr. R. S. Whitaker is a graduate student in Civil
Engineering (1975). '
vi
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SECTION I
CONCLUSIONS
1. The conventional wet caustic method of peeling- sweet potatoes
(l97l) utilizes excessive amounts of water.
2. Wet caustic peeling produces a very strong alkaline waste that
is expensive to treat and to dispose of.
3. The disposal of caustic waste by spreading on agricultural soils
may render the soils sticky, impermeable, and useless for
agriculture. Caustic wastes have been successfully treated in bio-
logical treatment systems without neutralization but careful con-
trol is necessary.
4. Process modifications in the fozn of high pressure low-volume
sprays and a dry caustic peeling process are in use and show
cost advantages over conventional processes.
5. A dry caustic peeling process is in use in the industry that
reduces water use by about 50$ and reduces the BOD in the
liquid waste stream by over 50&
6. Potential process changes such as infrared dry caustic peeling
and lye recovery have been shown to be feasible in laboratory
tests.
7. In-plant control of wastes by screening and grit removal are
simple processes that can reduce cost of waste treatment.
8. Sweet potato wastes are readily biodegradable but will probably
require supplemental nitrogen in biological treatment processes.
Phosphorus may be required.
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SECTION II
RECOMMENDATIONS
1. Sweet potato processors should adopt water conservation prac-
tices such as high pressure low-volume sprays.
2. Recirculated water should be adopted in all preliminary wash-
ing processes.
3. Processors should adopt a "dry" caustic peeling process in
order to obtain a waste that can be disposed of as a solid.
Solids are less mobile than air or water and thus solid wastes
are more economically attractive to handle than are liquid wastes.
Under adequate ultimate disposal practices solids are less detri-
mental to the environment.
4. "In-plant" control of wastes by screening and grit removal are
cost effective and should be employed in all plants.
5. Equipment manufacturers and processors should support a plant-
scale test of the infrared dry caustic peeling process as a
method of putting more product in the can and less in the waste
stream.
6. Equipment manufacturers and processors should study further the
steam peeling process to increase product yield and to reduce
production of caustic wastes.
7. The use of biological treatment processes provides satisfactory
treatment and the simplest method consistent with the size and
scope of the cannery should be provided.
8. Continued exploration and development of by-products and by-
product markets should be undertaken by the industry as an
alternative method of controlling wastes and increasing profits.
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SECTION III
INTRODUCTION
The sweet potato industry is located in the Southern, Middle
Atlantic States, and California. The production and consumption of
sweet potatoes reached a peak during the depression with 12.7 kg P®r
person being consumed annually and 2,177,280.0 kkg per year being
produced. The production and consumption have steadily fallen off
since the 1930's to the present level of 3.18 kg per person being
consumed compared to 68.0 kg per person for white potatoes. The
sweet potato canning has expanded from 1.2 million cases processed
in 1940 to 11 million cases processed in 1965. Since that time, how-
ever, the industry has remained steady at around 10 million cases
per year. 1> 2
The amount of sweet potatoes processed represents about 40$
of the total, crop grown, the remaining 60/£ being sold on the fresh
market. As a general rule, roots with diameters 4.45 cm and greater
are sold on the fresh market, while those less than 4.45 cm in di-
ameter are sold to the canneries for processing. 2
Processed sweet potatoes are sold principally as canned whole
and/or cuts. In 1971 other products such as puree, wafers, or sweet
potato flakes had a limited market value. These other products, how-
ever, represent an attempt to convert the potato into a saleable prod-
uct and keep it out of the waste stream.
Canning sweet potatoes is a seasonal operation restricted large-
ly to the fall months—from September through December. After De-
cember a few that have been stored are canned. This processing
represents a "fill-in1* operation and is primarily determined by
market demand and the availability of stored potatoes. Seasonal
operation is characteristic of most food processing plant. However,
potatoes may be processed in late summer, fall, winter and spring,
beans, peppers, and other vegetables in the summer and early fall,
so that large plants can operate most of the year.
The profit in canning sweet potatoes is small and.is dependent
on low capital investment, low cost labor, and low overhead. Most
plants use conventional wet causting peeling processes developed during
World War II that require manual handling, sorting, and final trim-
ming.
In a conventional plant, as much as 60^ of the potato (or as little
as 40$) 3 may be lost as waste. The gross pollution load from this
waste is high, particiilarly in suspended solids and BOD. Table 1
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gives the load from a conventional process after 20-mesh screen-
ing. 4
TABLE i
GROSS WASTE m)M CANNING SWEET POTATOES
Parameter Waste Load Prom Processing *
kg/kkg
BOD-5 12-39
COD 36-92
SS 4-24
Gross Water Consumption 10,000-12,000 1/kkg ** processed
pH 8.0 - 11.5
* Multiply kg/kkg by 2 to obtain Ibs/ton **Multiply 1/kkg by 0.24 to
obtain gal/ton
Until recently, sweet potato wastes were simply discharged to streams
and sewers, with only screening or sedimentation. The cost of waste con-
trol to a cannery was minimal and generated little concern.
Under present strict antipollution standards and regulations, how-
ever, the cost of waste control has become a serious concern. At an
assumed cost of $0.111 kg ($.05 pound) of BOD-5 removed and f0.111 kg
($.05 pound) of suspended solids removed, the total cost of treatment
for a cannery processing 100 kkg (220,000 Ib) per day could range be-
tween $176 and $693 per day of production.
A trend in recent years has been to consolidate the canneries into
large units with modern equipment and increasing mechanization in order
to improve profits. In-plant water and waste control is also being
utilized to reduce treatment costs.
New technology of waste abatement and control is directed toward
in-plant changes in equipment and processes, more solid waste disposal,
and pretreatment; all designed to reduce the amount of waste that must
eventually be treated in the wastewater stream.
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SECTION IV
CURRENT PROCESSING TECHNOLOGY
Traditionally the fresh sweet potato has been canned while the aged
potato is sold on the fresh market. The difference between fresh and
cured sweet potatoes becomes significant in processing. 5
1. The skin of the fresh potato is thinner and more easily removed
than that of the aged sweet potato. Accordingly, the sweet potato
is processed while it is still fresh, and the aged sweet potato is
canned only as a late season fill-in operation or as a way of meet-
ing a high demand for the product.
2. The fresh potato has a higher starch content than the aged potato.
Aging results in part of the starch being converted into sugar.
3. After canning, aged sweet potatoes tend to break down in the can
and become more mushy than do canned fresh potatoes.
Feeling is the main source of waste and utilizes a caustic process
to soften the sweet potato skin and outer layers to facilitate mechanical
removal. The peeler itself- is an abrasive device that wears away soften-
ed material of the potato, flushing it into the wastewater stream.
Current processing technology of sweet potatoes can be divided
into two types: the conventional process and the improved process.
The conventional process is characterized by high water usage and all
waste is discharged into the wastewater stream. The improved process
utilizes a dry peeling system characterized by lower water usage and
disposes of much of the waste as a solid. Since solids are less mo-
bile, they put less stress on the environment. It is also true that
solids disposal by landfill is generally much cheaper than liquid waste
treatment.
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CONVENTIONAL PROCESSING
The conventional processing of sweet potatoes was adopted during
World War II during a period of severe labor shortages. It is a wet
caustic process that dumps all waste substances into the municipal
wastewater plant for treatment or discharge. The process may be
used for both white potatoes and sweet potatoes, though there are
some distinctive differences between the two.
The basic steps of the conventional process are as follows:
Receiving and Unloading.* The sweet potatoes are brought to
the plant by truck and are commonly unloaded into a conveyer hopper
using manual labor or a front loader. The potatoes are then dry
cleaned on a vibrating screen where stones, some dirt, some of the
small potatoes are removed. If the potatoes arrive wet, they are
difficult to dry clean by screening.
Cleaning and Washing; After dry cleaning, the potatoes are
washed in a reel washer consisting of a rotating drum and a water
spray. As the potatoes go through the drum, they are rolled and
sprayed. Approximately 5^ of the gross weight of the potato
trucked in from the field is dirt that is removed during the re-
ceiving and cleaning operations.
The receiving and preliminary cleaning operations generate
a large amount of grit.
Pe_eling: The peeling process involves several steps.
1. Preheat: After the potatoes have been cleaned, they are
preheated in a hot water bath at 50-65° C. (l20° to 150° P) for 2
to 5 minutes. The preheating enchances peel removal and improves
the appearance of the finished product according to some.2>4
Hot water overflows the bath and whole potatoes may float
out of the preheater.
2. Lye bath: After preheating, the potatoes are immersed
in a lye bath of 5 to 12$ caustic, at 100-102° C (210° to 215° P) for
2 to 8 minutes.4 The caustic softens the skin and outer layers of
the potato and facilitates the peeling. The strength of the lye
bath, skin thickness, and the condition of the potatoes determines
the length of required exposure in the bath.
Normally fresh dug potatoes are canned. They have a thin skin.
Potatoes which are stored develope a tough skin and a thick corky
layer that is difficult to remove^3 and requires a harsher caustic
treatment.
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3. Peelers: After the lye bath, the potatoes are conveyed to
a peeler. The sides of the rotating drum peeler are coated with sand-
like abrasive. As the drum revolves, the peel is rubbed off along
with some of the potato softened by the lye. As much as 40$ of the
potato may be removed during this process. The conventional wet-and-
dry peeler then employs a high pressure water spray to remove the
abraded potato peel from the side of the drum.
The waste from this process is very caustic and high in organic
matter.
Snipping: The operation of snipping the ends of the sweet
potato may be placed either before or after the lye peeling oper-
ation. The snipper is a device that mechanically cuts off the ends
of the potatoes.
These ends then go into the clean-up stream. The mechanical
snipping operation requires further manual labor to finish trimming
the sweet potato.
Scrubbing; Scrubbing is a finishing and polishing process
following the peeler and involves two steps.
1. Abrasive peeler: The abrasive peeler is a rotating drum,
with a fine sandpaper surface to smooth the potato surface and re-
move the remaining skin and softened portion of the potato. Water
is used to clean the sides and carry the waste to the treatment units.
2. Brush washer: The brush washer employs soft bristles in a
rotating drum in combination with water sprays to polish the potato.
The unit removes the last traces of caustic and imparts a sheen that
is required for customer approval of the sweet potato.
The waste from the scrubbing operation is high in organic matter.
Trimming, Sizing, Slicing, and grading: These operations follow
scrubbing. Manual labor is used to inspect the potatoes and trim
and discard the parts not suitable for canning. A rotating drum with
different size slots separates the potatoes into the correct sizes for
canning. The larger potatoes move through a series of slicers to
reduce size before canning. Grading is a final inspection to insure
acceptable size and color and requires the bulk of the manual labor
used in the cannery.
The trimming, sizing, slicing, and grading operations generate a
large amount of solid waste.
Filling: The potato, after grading, moves onto a circular or
tumbler hand-pack filler with a series of can-size openings around
"•he perimeter. The potatoes are raked into cans passing below the
openings. Waste associated with this process is confined to spillage
and can be discarded as a solid waste.
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gyrupima;? The syrup used as a filler in the sweet potato can be
principally a sucrose sugar (40-50$). 2 The old style of filling the
can involved spillage. The syrup was continuously poured and the cans
moved under the stream. This process caused wastage as one can was
removed and another took its place. The waste from this operation
enters the wastewater stream. A new method of filling the cans with
syrup involves an automatic cut off on the syrup stream as the can
becomes filled and moved forward. There is little or no spillage
involved with this method. There is also a vacuum syruper in use
that minimizes spillage.
Exhausting: The filled cans are exhausted, using steam to drive
out any air and maintain proper closing temperature before the can is
sealed. There is no waste.
Retorting: The retorts are the cookers. The waste associated
with this operation comes from spilled syrup on the outside of the
can. The potatoes are cooked in the can using superheated water
under pressure. Water consumption during this operation accounts
for nearly 40$ of the total water used. 1
Cooling; The cooling bath is a large tank through which the
cans are moved after retorting and before packing for shipment.
Little waste load is associated with this operation.
The water from the exhausting, retorting and cooling opera-
tion is relatively free of BOD-5.
Figure 1 shows the sequence of operations in a conventional
sweet potato canning operation.
Arrow symbols in the left hand side of the diagram show the
quality of water commonly supplied to each operation. Arrow sym-
bols on the right hand side of the diagram show which wastes are
normally liquified and which are normally solid state, and also
the source of the caustic waste.
8
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FIGURE 1
WATER USE AND WASTE SOURCES
FOR A CONVENTIONAL SWEET POTATO CANNING OPERATION
R>
RAW POTATOES
RECEIVING-UNLOADING
CLEANING-WASHING
PREHEATER
LYE
BATH
PEEL REMOVAL
SNIPPING
ABRASIVE PEELER
BRUSH WASHER
TRIM-SLICE-SIZE-GRADE
FILLING
SYRUPING
EXHAUSTING-SEALING
RETORTING
COOLING
PACKAGING-SHIPPING
D>c
D>
O
R[> RECYCLED WATER
P^ POTABLE WATER
^ SOLID WASTE
[> LIQUID WASTE
C CAUSTIC
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For a conventional process, a mass balance of raw product is
shown in fable 2. *
TABLE 2 *
MASS BALANCE FOR A SWEET POTATO CANNERY
5$ of the raw product was field dirt
9.5$ of the raw product was unfi^ for canning but was sold or given
away for feed for swine or cattle
of the raw product was placed in cans and became a saleable
product
of the raw product was placed into the liquid waste stream
for the treatment plant
20.5$ of the raw product was organic snips and solids that are
partially removed by a 10-mesh screen and disposed of along with
field dirt to land fill
The water use and the character of the waste from each of the
processing operations has been studied by Colston and Smallwood ^
at Tabor City Foods which is operating a conventional cannery in
Tabor City, N. C.
The schematic layout of the plant is shown in Figure 2.
Water meters were installed at locations lettered A through L to
measure the water use in the major processing operations. A
six-inch Parshall flume was installed to measure the flow of cool-
ing and retort water that was discharged untreated to a small
creek. A nine-inch Parshall flume was installed to measure the
volume of strong organic waste stream from the processing opera-
tions that was discharged to the city sewer.
*A review of the data presented by the authors in reference 4 showed
that the effect of recirculation had been overlooked. Thus, only
25% of the raw material appears in the waste stream after screening
rather than the 33% originally reported.
10
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10
RECIRCULATION WATER
PUMP ROOM
RECEIVING a PRIMARY WASHING
PREHEATING
LYE PEELING
FIRST WASHING
SNIPPERS
SIZER
ABRASIVE PEELERS
BRUSH WASHING
HAND TRIMMING
SIZING
CUTTING
GRADING
FILLING 8 CLOSING
RETORTING
COOLING
PUREE MIXING
PUREE FILLING
PUREE CLOSING
SYRUP ROOM
WATER METERS
WELL WATER LINE
CITY WATER LINE
FIRST WASHER
BRUSH WASHER
HAND TRIMMING
ABRASIVE PEELERS
SNIPPERS
SYRUP ROOM
12-
13-
14-
15*-
16-
17-
18-
19-
20-
A-
B-
C-
D-
E-
F-
G-
H-
6
6
6
WELL WATER
o—
CITY WATER
I- RETORTING
J- PEELING CLEAN-UP
K- TRIMMING CLEANERS
L- FILLING AREA CLEAN-UP
FIGURE 2. LOCATIQNS OF WATER
METERS AT
TABOR CITY FOODS
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The gross water rise in the process operations is shown in
Table 3.4
TABLE 3
GROSS ¥ATBR USE IN A CONVENTIONAL
SWEET POTATO CANNERY PROCESS
Unit Process
Cleaning and Washing
Preheater
Peeling
1. Lye bath
2. Peel removal
Snipping
Scrubbing
1. Abrasive peeler
2. Brush washer
Trim, Slice, Size, Grade
Filling
Syruping (goes into can)
Exhausting, Sealing
Retorting
Cooling
Packaging - Shipment
Miscellaneous
1. Boiler water
2. Belt wetting
3. Cleanup
Rote: Multiply 1/kkg by 0.24
Divide I/case by 3.8
* Can size - #2-1/2
1/kkg Input 1/C,
ase
* % of Total
321.0
63.0
50.0
1168.0
10.6
2.0
1.7
37.5
Water Use
3.0
.5
.4
10.8
459.0
584.0
400.0
15.1
18.2
12.9
Included in Cleanup
150.0
5.0
10,900.0
to obtain gal/ton
to obtain gal/case
4.4
5.3
3.7
1.4
4170.0
259.0
0.0
1732.0
542.0
1002.0
132.5
3.8
0.0
56.8
17.8
32.6
38.3
1.1
0.0
16.4
5.2
9.5
100.0
12
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Almost 40$ of the gross use is recirculated from retort and
cooling operations to be used in the cleaning-washing, preheating
and lye bath operations.
There is also a consumptive use of about 8$ of the gross
water applied to the process, part of which is to product and part
to boiler evaporation.
Of the net water to waste a little over half goes to the
sewage treatment plant after 10-mesh screening and grit removal.
A little less than half of the net use is low BOD-5 cooling water
that is discharged to a drainage ditch untreated.
The pattern of water use is shown in Figure 3.
FIGURE 3. RECIRCULATION OF WATER S SEPARATION OF
WASTE STREAMS IN A CONVENTIONAL CANNERY
RECIRCULATED
NET
NEW
WATER
4,150 l/kkg
, rGROSS
6,750 l/kkg
IO,9OOI/kkg
PROCESS
5.850 l/kkg
10,0001/kkg
CONSUMPTIVE USE
900 l/kkg
WASTE TO
SEWAGE PLANT
3,000 l/kkg
WASTE TO
DITCH
(COOLING a RETORT)
2,850 l/kkg
NOTE: MULTIPLY l/kkg BY 0.24 TO OBTAIN gal/ton
13
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Tabor City Foods, Inc. personnel at the plant analyzed liquid
waste composited from four grab samples from the principal opera-
tions each day during the Fall of 1971 canning season. The efflu-
ent to the city sewer was also sampled. This waste received screen-
ing through a 10-mesh screen and then before analysis was passed
again through a 20-mesh screen.
Biochemical oxygen demand, total nitrogen, total phosphorus,
total solids, suspended solids, and settleable solids were deter-
mined in accordance with procedures prescribed by the Environmen-
tal Protection Agency.6 The chemical oxygen demand was determin-
ed in accordance with Standard Methods for the Analysis of Water
and Wastes.7 Nitrogen and Phosphorus in the waste were analyzed
by the laboratory of the North Carolina Office of Water and Air
Resources on single composited samples.
The concentrations of the wastes from the individual process
streams are shown in Table 4. An independent check of the waste
load was obtained from analysis performed on the effluent dis-
charged to the city sewer. Then using the flows from Table 3
and the waste concentrations from Table 4 the waste loads per
unit of product were calculated and are shown in Table 5.
The load from the individual processes adds up to 29.8 kg of
BOB-5 per kkg (59.6 Ib/ton) of sweet potatoes processed. An
independent calculation from the strength of the effluent to the
city sewer shown in Table 4 and the measured flows shown in Figure
3 yields a waste load of 27.7 kg of BOD-5 (55.4 Ib/ton) per kkg
of potatoes processed. The agreement is striking and perhaps
fortuitous in view of the variability of the collected data.
A separate laboratory study® of the raw sweet potato show-
ed that it averaged 16% water. When dried at 103°C (217°F)
1 kilogram of dried potato had a COD of 1.06 kg, a BOD-5 of 0.49
kg, a carbon content of 0.38 kg, a nitrogen content of 0.003 kg,
and a phosphorus content of 0.0016 kg. To obtain comparable
figures on a wet weight basis each of these figures should be
multiplied by 0.24. Thus 1 kg of whole sweet potato would have
0.24 kg of dry matter, 0.254 kg of COD, 0.118 kg of BOD-5, 0.72 g
of N and 0.4 g of P.
14
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TABLE 4
THE STRENGTH OF CONVENTIONAL SWEET POTATO WASTES
AFTER 20-MESH SCREENING 4
Total Nitrogen
Total Phosphorus
VJ1
Unit Process
Cleaning & Washing
Preheater
Peeling:
Lye bath
Feel Removal
Snipping
Scrubbing:
Abrasive Peeling
Brush Washer
Retort
Cleanup
Effluent to
City Sewer *
990
3,700
13,000
5,900
14,000
3,500
76
2,200
9,250
-5 (me/1)
s
680
1,020
4,600
1,800
3,750
1,120
24
2,100
3,150
n
8
5
13
13
11
69
7
7
8
COD (mg/ll
M 8
3,700 2,560
9,300 2,800
32,000 9,400
16,000 8,700
22,000 10,400
6,400 2,040
210 48
3,800 2,900
22,000 2,550
n
9
13
19
19
19
20
7
7
16
M
12
45
320
140
330
71
-
-
210
as N(mg/l)
s
7
-
210
53
175
64
-
-
43
n '
3
1
4
4
4
5
-
-
6
1
17
40
23
50
9
-
-
29
as P(mg/l)
8
1
-
19
11
12
8
-
-
12
n
2
1
4
3
4
4
-
-
6
* Excluding Retort Waters
M - mean or average value
s - standard deviation
n - number of samples analyzed
-------
TABLE 4 (continued)
Total Solids
Suspended Solids
Settleable Solids
Unit Process
Cleaning & Washing
Preheater
Peeling:
Lye Bath
Peel Removal
Snipping
Scrubbing:
Abrasive Feeling
Brush Washer
Retort
Cleanup
Effluent to
City Sewer *
M
2,100
8,400
35,000
13,000
23,000
4,300
300
2,700
26,000
(mg/1)
£
2,320
1,320
9,250
3,100
7,200
1,650
39
590
18,000
n
14
14
23
21
20
17
8
3
13
I
M
1,200
1,600
7,700
3,800
4,400
1,200
-
870
3,800
lmg/1)
s
475
860
2,300
2,200
1,800
1,300
-
210
2,600
n
6
5
14
9
9
n
«•
5
6
M
28
32
530
280
470
74
-
-
250
(ml/1)
S.
9
16
105
no
166
34
-
«H
130
a
20
18
24
20
29
30
-
-
25
* Excluding Retort Waters
M - mean or average value
s - standard deviation
n - number of samples analyzed
-------
TABLE 5
WASTE LOADS FROM A CONVENTIONAL SWEET POTATO
CANNERY* AFTER 20-MESH SCREENING
Unit Process
Cleaning & Washing
Preheater
Peeling
1. Lye bath
2. Peel removal
Snipping
Scrubbing
1. Abrasive peeler
2. Brush washer
BOD- 5
kg/kkg
.32
.23
Carried
15.80
2.71
8.05
.14
COD
kg/kkg
1.2
0.6
over to peel
39.0
7.4
12.9
2.5
Total
Solids
kg/kkg
0.7
0.6
removal
43.0
6.0
13.4
1.7
Suspended
Solids
kg/kkg
0.4
0.1
9.4
1.8
2.6
0.5
Trim, Slice, Size,
Grade
Filling
Syruping
Exhausting, sealing
Retorting
Cooling
Included in the cleanup operation
T! It
It II
.32
0.0
0.9
0.0
1.2
0.0
Packaging - Shipment Included in the cleanup operation
Mis cellaneous
1. Boiler 0.0
2. Belt wetting 0.0
3. Cleanup 2.2
0.0
0.0
3.8
0.0
0.0
2.7
Total Waste 29-8 68.3 69.3
Note: Multiply kg/kkg by 2 to obtain Ib/ton.
0.0
0.0
0.0
0.0
0.9
15.7
17
-------
Table 5 shows that about 30 kg of BOD-5/kkg of sweet potatoes
processed is discharged to the municipal treatment plant or that the
BOD-5 discharged will amount to about 30% by weight of the potatoes.
Since BOD-5 of whole s:\reet potatoes is about 12% by weight, it ap-
pears that at least 25% of the raw potato processed is delivered
to the waste treatment plant in this conventional wet lye process-
ing plant. Another 20% of the potato is removed by the 10-mesh
screens (refer to Table 2) and disposed of along with the field
dirt as a solid waste.
IMPROVED PROCESSING
In recent years the conventional wet caustic process has
been abandoned in some plants for a dry peeling system. The
system replaces the conventional wet peel removal and abrasive peeler
and brush washer with a low water use "dry peeler" and a low
water use brush washer. The low water content permits the waste
from the peeler and from the brush washer to be disposed of either
as a feed for cattle or hogs or as a semi-solid waste that can be
buried.
The chief concern in the peeling operation is removing the
eyes and skin of the sweet potato in order to produce a clean,
saleable product. It is also important to remove the cortex layer,
shown in Figure 4, that gives the canned potato a pasty look.
18
-------
FIGURE 4
PORTIONS OF THE SWEET POTATO LOST DURING PROCESSING
PORTION OF THE POTATO
LOST DURING SNIPPING
SNIPS
PORTION OF THE POTATO LOST DURING
/PEELING AND TRIMMING
PORTION OF THE POTATO CANNED
-------
The dry peeler equipments employs rubber studs rather than
the conventional sand abrasive on planetary rollers in a rotating
drum. In concept the rubber studs are flexible and facilitate a
more efficient removal of the potato eyes and the skin surrounding
irregularities. Abrasion by contrast is not flexible and must
remove all of the potato to the depth of the eye. Rubber studs
may be provided in different length, sizes, and stiffness, allow-
ing for interchange and combinations that provide the most effi-
cient peeling operation. Magnuson10estimates that 45-50$ pf the
raw potato will end in the can in contrast to only 40$ by the con-
ventional peel. The rapid rotation of the planetary rollers dis-
charges the peel waste to the interior wall of a containing drum
where it can be scraped off. Only a small quantity of water is
needed to lubricate the planetary rollers. The waste can be dis-
posed of as a semi-solid.
The washer is similar to the "dry-peeler" but employs nylon
bristles in place of fingers to polish the potato and prepare it
for the sorting, trimming, and slicing operation.
Inquiries were made of several canneries using the "dry-
peeling" system. Inquiries to five plants elicited only 2 re-
plies, In one case the canner reported that he had reduced his
water use by about 50/5. In another case the canner reported that
all of the waste from his peelers and brush washer was being
buried. However no data was provided so only estimates can be
made of the improvement. Reference to Tables 3 and 5 permits
the following estimates to be made of possible benefits from
employing a "dry-peeler" system;
Gross water use can be reduced from 10,900 1/kkg to 5450
1/ldcg (2620 gal/ton to 1310 gal/ton).
BOD-5 in the plant effluent can be reduced from about 30
kg/kkg (60 Ib/ton) to about 6 kg/kkg (12 Ib/ton) since
the peeling and scrubbing waste is removed from the effluent
stream and disposed of as a solid waste.
Table 6 tabulates the comparison of Conventional with Im-
proved Technology.
Thus a dry-peeler system offers a means of eliminating the
very caustic and very strong organic waste from the effluent.
The remaining wastes are readily treated by a system as simple
as spray irrigation or as complex as two stage activated sludge.
It would be desirable to have field data on waste reduction
by the dry peeling process.
-------
TABLE 6
COMPARISON OF CONVENTIONAL AND IMPROVED
PROCESSING TECHNOLOGY
Parameter
Product Yield %
Water Use 1/kkg
BOD-5 kg/kkg
COD kg/kkg
Suspended Solids kg/kkg
Total Solids kg/kkg
pH
Observed
Conventional
Wet Peeling
40
10,900
30
68
16
69
9.5-11.5
Estimated
Dry Peeling
45-50
5,000-8,000
6-8
13-15
4-6
10-15
7-8
21
-------
SECTION V
POTENTIAL PROCESSING CHANCES
The potential process changes have not been demonstrated
successfully in full scale plant operations, and their value to
the "state of the art" rests in their "potential." The follow-
ing processes have been investigated on an experimental basis or
have experienced some use in some part of the industry:
1. Steam peeling
2. Infrared dry caustic process
3. Lye recovery
4. By-product consumption
STEAM PEELING
Steam peeling has been reported to be successful on white
potatoes.11 However, personal conmunication from a major canner
reported that a three-year trial of the process on sweet potatoes
was unsuccessful.12 The failure resulted from poor sheen on the
potato as well as from erosion of valves by grit and gumming of
feed parts by sweet potato latex. Steam peeling works well for
other sweet potato products, such as flakes and puree, where
;-iheen is not important.
The continuous steam peeler consists of a pressure chamber
with an internal screw conveyor and feed and discharge valves.
In operation, sweet potatoes must first be throughly washed in
single or double washing units, depending on the amount of soil
on the surface of the product. After washing, the product is
conveyed upward on an inclined timing feed elevator and dis-
charged through the pressure feed valve to the pressure chamber.
The screw conveyor inside the pressure chamber carries sweet
potatoes the length of the chamber. The product is subjected to
steam under pressure to loosen the skin. At the discharge end,
the product is transferred through a second pressure valve to a
rotary washer, where loosened skins are washed off.
The batch peeler is equipped with a charging door and mount-
ed on hollow axles to permit rotation of the peeler during oper-
ation. After charging with the proper amount of sweet potatoes,
the door is closed and steam is admitted. After 30-45 second
exposure to 4.2 kg/sq cm (60 psi) pressure steam, the steam is
22
-------
released rapidly from the pressure chamber, the sweet potatoes
are discharged, and the softened tissue is removed by brushes
or water sprays.
The steam pressure and the exposure time may be varied to
regulate accurately the depth of heat penetration desired, de-
pending upon the -type and the condition of sweet potatoes.
Boyerl3 reported that superheated steam at atmospheric
pressure was very effective in peeling both new and old white
potatoes.
Bidt and MacArthur H compared the peeling losses on white
potatoes from different methods and concluded that on the aver-
age the losses from steam peeling are 18$, lye peeling 17$, and
25$ for abrasive peeling. Hammond 5 has reported that a peel and
trim loss of 5<$ is common and that 40$ is considered excellent
for sweet potatoes in a conventional caustic peeler.
The reported advantages of steam peeling over conventional
lye peeling are summarized:
1. Since steam attacks all surfaces of the product uniformly,
the shape of the potato has minor effect on peeling effi-
ciency.
2. Steam pressure and time of exposure can be easily adjusted
to meet specific peeling requirements of a potato variety
or condition.
3. No preheating step is required.
4. Elimination of caustic costs.
5. Elimination of caustic waste.
The disadvantages of steam peeling relative to conventional
lye peeling:
1. High cost of continuous peeling equipment.
2. High maintenance cost.
3. Steam peeling leaves a heat ring on the peeled surface which
is discernible in the finished product and usually reduces
consumer acceptance except in products such as potato chips
from white potatoes.
4. Steam peeling might result in discoloration, although,, ade-
quate cooling and post treatment can minimize this effect to
a great extent.
-------
5. The raw product must be absolutely free of grit to prevent
abrasion of steam parts.
6. The latex exuded from broken and bruised potatoes gums up
the steam parts.
INFRARED BEY CAUSTIC PROCESS
In 1967-68, Graham, Huxsoll, et al,!5 at the USDA's Western
Utilization Research and Development Division, Albany, California,
announced the development of a new method of peeling potatoes.
Their peeling method was based on the application of infrared
heat to light caustic treated potatoes followed by mechanical
peel removal.
The infrared dry caustic peeling process for potatoes in-
volves several steps. Wet-washed potatoes are immersed in a hot
dilute lye solution. The excess lye is drained and the potatoes
stand for about 5 minutes to allow the caustic to penetrate.
Following the holding period, the potatoes are subjected to in-
frared heat for 1 or 2 minutes. The infrared heat activates the
caustic and dries the surface layer of the potato. After con-
ditioning by the heat, the potatoes are placed in a rubber-
tipped mechanical peeler which removes the treated outer surface
of the potato. Finally the potatoes are brush washed to remove
a very small amount of soft sticky residue from their surfaces.
The gas-fired infrared heaters are of the type commonly used
for space heating. Combustion takes place just in front of a
ceramic mantle which radiates at 860° to 890° C (1550° to 1600° P),
A nichrome wire screen protects the mantle from contact with the
potatoes.
The peeling equipment consists of the dry peelers and scrub-
bers already described.
The advantages *5 of this process over the conventional wet
process are reported as follows:
1. lye consumption may be reduced 24$ to 56$.
2. Overall water consumption may be reduced 20$ by minimizing
water sprays in the peeling and brush washing operations.
3. Raw product canned may be increased by 12$ to 25$ because
of lower peel loss.
4. Waste control is enhanced by disposal of the peel as a solid
waste.
24
-------
The principal disadvantages of this process are:
1. The process has not been demonstrated on a plant scale on the
sweet potato, although it appears to work well on white po-
tatoes.
2. The process tends to leave a heat ring on the white potatoes
that is undesirable in the finished product if exposure to
heat is not carefully controlled.
3. Special conveyors are needed to rotate the potato as it passes
beneath the infrared burners to insure uniform and complete
exposure.
4. Additional capital expenditures for burners.
5. Unavailability of economical energy source for infrared units
at all locations.
Huxsoll has conducted bench scale experiments at the USDA
Western Utilization Research and Development Division, using
a one-potato process. Pilot plant studies, also sponsored by
USDA at a plant in Louisiana and at Tabor City Poods, N. C., l6
studied the application to sweet potatoes. The results were re-
ported favorable, but no full-scale experiment has been made.
LYE RECOVERY
A principal advantage of lye recovery is in removing it from
the wastewater stream and reusing it. Many wastewater treatment
problems are associated with caustic:
1. Sodium reacts with soil making the clays sticky and imperme-
able. Spray irrigation is not recommended1' as a treatment
process for a caustic waste water.
2. High caustic wastes may inhibit biological action unless com-
pletely mixed aeration systems are used.
3. High caustic concentrations tend to dissolve solid organic
matter in the wastewater and increases soluble BOD and COD
loadings.
4. Special facilities must be incorporated into the treatment
system to handle high caustic wastes.
25
-------
The use of a lye recovery system is not common to the potato
processing industry, though it has been useful in mercerizing
plants1-8 of the cotton textile industry. It seems probable that
the control of sodium discharge will sooner or later result in
lye recovery.
A laboratory study demonstrated that the use of a rotary drum
filter for reconditioning lye by removing solids and carbonate
offered a potential for cost savings by reducing new lye consump-
tion by 15^ to 50$. ^ The lye was simply evaporated to concen-
trate to 12$ total alkalinity after filtering. No difference in
the quality of peel was observed in the laboratory.
BY-PRODUCTS
The use of by-products from the sweet potato processing
operation offers the potential to partially recover costs asso-
ciated with product wastage. Saleable food products such as
puree, wafers, and instant sweet potato flakes have potential
markets. At this time, only puree has gained any consumer ac-
ceptance. All the other products noted are feasible.
The solid wastes have also been studied as potential feed
for cattle. White PQtato waste has rbeen used extensively as an
nd has*^» -»22 the nutritional value euivalent to
animal feed and has*» -» the nutritional value equivalent
corn when fed to cattle. Laboratory studies to determine the
potential of using sweet potato waste as cattle feed concluded
that:
1. Feeding dried sweet potato waste to cattle is feasible. ^5
2. Cost of drum drying the waste, however, is too high at
$ .044/1 24 of water removed for conventional waste in consid-
eration of food value obtained.
3. Direct feeding of the waste from the dry peelers is possible
if it is mixed with trim wastes and allowed to ferment for
24 hours to reduce pH to 9.0 *5 because of high pH neutrali-
zation required.
4. Cost of drying the sludges from sweet potatoes is higher
than that of white potatoes, so no direct comparison should
be made between the two.
5. No animal feeding studies have been conducted to test accept-
ability of the product to animals.
6. Market conditions may well bring by-product materials into favor
as feed sources.
26
-------
SECTION VI
IN-PLANT WASTE CONTROL
In-plant waste control and abatement is directed at reduc-
ing waste discharges and water consumption. Techniques (discussed
below) can be applied to almost every plant. Many are already
used in recently built plants.
WATER RECIRCULATION
An operation in which recirculated water is clearly useful
and beneficial is the reel washer. The water used in this process
does not need to be of a particularly high quality. The primary
source of grit in the wastewater comes from this washing of field
dirt from the potatoes.
Recirculation systems should include a method of settling
or hydrocloning to remove this grit to prevent excessive wear on
the equipment. A simple settling basin is satisfactory.
In a conventional cannery as shown in figures 1 and 3 as
much as 40$ of the gross water applied in the process is recir-
culated from retort and cooling operations and used in the clean-
ing, washing, preheating and lye operations. Even so, a larger
amount of relatively clean water is discharged without any re-
quired treatment to an open ditch. With chlorination this water
could be used for belt wetting or for cleanup. Assuming that all
of the clean water could be recirculated and reused 2850 1/kkg
(675 gal/ton) plus 4150 1/kkg (lOOO gal/ton) then new water coulA
be reduced to 3900 1/kkg (940 gal/ton) which represents a 35$
additional savings on water.
The application of counter flow principals would suggest
that cooling water could be chlorinated and reused in retorts,
peel removal, preheater and cleaning-washing operations. Then
the return of water from the retorts to preheater and cleaning-
washing operations would be efficient use of fuel.
WATER PRESSURE COHTROL
Conventional canneries allow water to flow without control
in all operations whether potatoes are moving or not. High
pressure low-volume water spray systems could be used in the
conventional process for water conservation in the reel washer,
the abrasive peeler, and snipper. The lowered use of water not
only would serve to reduce the amount and cost of the water but
also increases the concentrations of the waste and facilitates
27
-------
treatment. The total cost of installing high pressure low-
volume water sprays should be determined in light of equipment
costs, power consumption, water reduction, and wastewater treat-
ment costs. An example calculation of cost effectiveness is in-
cluded in the next section.
High pressure low-volume sprays could be used in all prelimi-
nary operations except the syruping and subsequent operations.
GRIT SEPARATION
Grit from the field dirt vibrated or washed from the potatoes
is a significant problem in sweet potato canneries. The grit fills
up basins, wears equipment, and clogs piping. Removal of this grit
before waste treatment or pumping represents a savings in mainte-
nance costs. Grit should be removed from any water recirculating
system. It can be removed in settling chambers or through hydro-
clones located after the washer.
i
SCREENING
Conventional canneries often employ either 10 or 20-mesh
screening of waste effluent from processing operations within the
cannery and remove a very substantial amount of suspended solids
from the total waste load. Screening of the combined plant efflu-
ent before discharge or treatment is also useful. Suggested screen
sizes for each operation are summarized in Table 7.
TABLE 7
SUGGESTED SCREEN SIZES FOR SWEET POTATO CANNERIES
Expected % Removal of
Operation Size Applied Suspended Solids
1. Receiving 6-10 mesh est. 80%
2. First Wash 20-40 mesh B 80%
3. Snippers 6-10 mesh " 80%
4. Brush Washer 40-50 mesh " 60%
5. Trimming, Slicing, Grading 20 mesh " 80%
6. Combined Plant Effluent (after 10-mesh screening)
a. Vibrating Screen 20 mesh 4 %
b. Rotary Screen 120x600 mesh 60 % -
c. Single Screen 20-50 mesh 20 % -
28
-------
Following is a check list of locations and processes where
screening or hydroclonea should be considered for waste control:
1. Receiving: Vibrating screens or dry reels can be used during
the receiving process to remove dirt and unusable potatoes. No
water should be used with this screen and the waste should be
disposed of as a solid waste.
2. First Washing: Mud that is not removed in receiving and un-
loading is the primary waste from the first wash. Other ma-
terials, however, may be present. Sticks, roots, leaves,
debris, small potatoes, etc., may be easily screened.
3. Snippers: The snipping operation involves cutting off the
ends of the potato. The snips are relatively large and easi-
ly screened.
4. Brush Polishers: The brush polishers create suspended solids
not easily removed by screening. The solids consists of bits
of peeling and a sticky outer portion of the potato. Screens
for this operation must be self -cleaning.
5. Trimming, Slicing, Sizing, Grading: The wastes from this
operation will be solids, consisting of discarded portions
of the potato.
6. Combined Plant Effluent: Screening of a combined plant ef-
fluent and plant cleanup water can remove many solids that
escape other processes. Laboratory studies by Swope2^ indi-
cated that a vibrating screen followed by a rotary screen,
could remove about 60$ of the suspended solids. The authors
are not aware that this combination is used in sweet potato
canneries. Screens used in this process should be self-
cleaning. Other studies by Hamza^ showed that 90$ of the
suspended solids could be removed when 2$ fly ash was added
to the waste.
The value of screening can be measured in terms of treat-
ment cost reduction. A waste pretreatment system consisting of
a vibrating screen followed by a rotating drum can remove approx-
imately 60$ of the suspended solids and BOD associated with these
solids. The cost saving is determined by the difference between
costs of in-plant waste collection and disposal and the charges
assessed by the city for treatment of these solids.
29
-------
SECTION VII
COST EFFECTIVENESS OF WASTE CONTROL
INTRODUCTION
In many cases, adoption of waste control and abatement measures
not only reduce pollution but provides savings (profit) to the can-
nery. Calculations made by Dr. W. S. Caller illustrate the point
and are presented based on data for one conventional cannery company.27
Each plant would have different figures but methodology would remain
the same.
ASSUMPTIONS
1. Production rate in 1972 was 13.61 kkg/hr (30,000 Ib/hr). The
1972 work day averaged 10 hours for six days per week for a six
week season.
2. The adoption of the infrared dry peeler process will reduce the
amount of lye required from 29 kg/kkg (58 Ib/ton) to 9.65 kg/kkg
(19 Ib/ton) and will reduce the amount of potatoes required to
maintain current production from 13.61 kkg/hr to 11.34 kkg/hr
in accordance with estimates from USDA1" and Magnuson Engineers,
Inc.15
3. The costs offered by the processor are correct:
Power @ $0.02/kwh
Water @ $0.000105/1 ($0.0004/gal)
Potatoes
-------
8. A unit weight of dissolved solids will have the same ultimate
BOD as a unit weight of suspended solids.
9. There is 0.49 kg of BOD per kg (0.49 Ib/lb) of dry sweet
potato solids.8 (Based on NCSU laboratory studies of cleaned
sweet potatoes dried at 103°C to constant weight.)
10. The cannery shown in Figure 2 discharges waste to the munic-
ipal treatment plant through a 10-mesh screen and a small
sedimentation basin. Thus, the data of the mass balance
shown in Table 2 shows that the system will prevent the dirt
and snips (20.5%) from entering the effluent leaving the plant,
but that 25% of the raw potatoes entering the plant will leave
the plant in the waste stream.
11. The water use in the plant (1972) is shown in Figure 3 and in
Table 3.
WATER RECIRCULATION
The data of Figure 3 shows that the plant reduces water con-
sumption by reusing water from retorting and cooling for washing
and peeling. The saving on the cost of water that is achieved by
recirculation over a once-through system is readily calculated.
Cost of Water
Once through
(10,900 x 136.1 x 36 x $0.000105) $5,608
With Recirculation and Reuse
(6,750 x 136.1 x 36 x $0.000105)
Net Saving on Water
However, if it is necessary to add a pump and piping to achieve
recirculation, the amortization of the investment must be considered.
In another context, it was proposed that a high pressure, low-volume
water spray system be added with costs as follows:
Pump $1,500
Pipe and Nozzles 1,200
Installation 1.687
Capital Cost $4,387
31
-------
However, a recirculating system would still require a pump but
only half as much pipe and none of the nozzles. Installation costs
are estimated to be half as much without the nozzles and ancillary
equipment, so that for a simple recirculation system similar to the
one in the plant in 1972 costs are estimated.
Pump
Pipe
Installation
Annual costs may be calculated.
Capital Cost x CRF (14 yr @ 10.5%)
Maintenance
Power (522 kwh @ $0.02/kwh)
Annual Cost
Water Cost with Recirculation
Total Annual Cost
Savings over a once-through system
($5,608 - 4,188) $1,420
Return on Investment
(1,420 x 100/2,943) 48.25%
Thus, it is clear that the recirculation system is a good in-
vestment.
WATER CONSERVATION
The high pressure, low-volume spray nozzle system already men-
tioned was estimated to be capable of reducing gross and net water
use by a third. The system would reduce water purchases from
6,750 1/kkg (1,620 gal/ton) to 4,498 1/kkg (1,080 gal/ton). The
capital cost as previously noted was $4,387, but the increase in
investment over that required to provide recirculation is only $1,444.
32
-------
Annual Cost Increment
Capital Cost x CRF (14 yr @ 10.5%) $201
Maintenance 144
Power, assume no change
Annual Cost $345~
Cost of Water
(4,498 x 136 x 36 x $0.000105) $2,312
Savings on Water ($3,473 - 2,312) $1,161
(low-volume spray over recirculation)
Net Annual Saving ($1,161 - 345) $816
The return on the investment in the high pressure, low-volume
feature is 816 x 100/1,444 = 56.5%
COMBINED RECIRCULATION AND CONSERVATION
The combined recirculation and high pressure, low-volume spray
system would also yield a satisfactory return on investment.
Capital Cost * $4,387
Annual Cost
Capital Cost x CRF (14 yr @ 10.5%) $ 612
Maintenance 439
Power ( 522 kwh @ $0.02/kwh) 10
$1,061
Cost of Water 2.312
Total Annual Cost $3,373
Annual Cost of Once-Through Water $5,608
Net Savings $2,235
Return on Investment
($2,235 x 100/4,387) 50.95%
During the first year only it would be possible to take an
additional 7% investment tax credit28 that would improve the return.
However, the savings could not be realized in future years and so is
neglected in these calculations.
33
-------
WASTE SCREENING
An existing 10-mesh screen and settling basin assures that
rejected potatoes, dirt, and snips and trim wastes do not enter the
liquid waste stream leaving the plant under conventional processing
(Table 2). Twenty-five percent of the raw product still leaves the
plant in the liquid waste stream. It was determined by Hamza-'-" from
laboratory studies that a two-stage screening system proposed by
Swope26 could remove a large fraction of this 25% leaving the plant.
Sixty percent of the suspended solids could be removed by a rotating
fine drum screen (600 x 120 mesh, 30 micron). The associated BOD
was also removed. If 2% fly ash was added as a filter aid, 90% of
the suspended solids and associated BOD could be removed. The BOD
load is determined by multiplying the dry suspended solids by 0.49.
The cost of a two-screen system installed was estimated at:
System Cost $43,216
The annual cost of the two-screen system was then estimated
to be:
System Cost x CRF (10 yr @ 10.5%) $ 7,185
Maintenance (at 10%) 4,322
Power (estimated) 300
Total Annual Cost $11,807
The total charge for treatment to the processor (assume 10-mesh
screening) by the municipal treatment plant was calculated.
Total Suspended Solids/Season Before Additional Screening
293,976 kg (647,000 Ib)
(136.1 x 36 x 25% x .24)
BOD-5 of Suspended Solids at 0.49 kg/kg
144,048 kg (317,000 Ib)
Estimated Annual Municipal Charge for Removal of Suspended
Solids and BOD-5 without Two-stage Screen at Plant:
438,024 kg @ 0.11/kg (964,000 Ib @ 0.05/lb) $48,183
34
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If the in-plant pretreatment removes 90% of the suspended solids
and associated BOD, the charge by the city for the remaining 10% would
be $4,818. (The fly ash is assumed to be generated at the plant and
thus free.)
The annual cost of pretreatment for 90% reduction would then be:
Screening System Annual Cost $11,807
City Treatment Charge for Remaining 10% 4,818
Total Cost $16,625
Charge without System $48,183
Charge with System 16,625
Net Savings $31,558
% Return on Investment 73%
If removal is only 60%, the total cost increases to $31,079 and
the net savings would be only $17,106 for a return on investment of
39.5%.
In either case the investment in additional screens is worthwhile.
WASTE CONTROL BY PROCESS MODIFICATION
The use of process modification to control waste and abate
pollution can be illustrated by the infrared dry caustic peeler
system. Assumptions are the same as mentioned at the beginning
of the section.
The estimated installed cost of an infrared dry caustic peeler
system was $83,960. It was estimated that this process would change
the yield of product1" and nature of waste (from Table 2) as shown
in Table 8.
35
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TABLE 8
COMPARISON OF YIELD AND WASTE FROM CONVENTIONAL WET
CAUSTIC PROCESS AND INFRARED DRY CAUSTIC PROCESS
Conventional Infrared
Process Process
% Raw Potato % Raw Potato
Yield 40 48
Dirt 5 5
Solid Waste 9.5 13.5
Snips 20.5 20.5
Waste to Treatment Plant
after 10-mesh Screening 25 13
The solid waste from the infrared process will be added to the
snips and screenings and removed from the liquid waste stream.
The two-stage screen of the previous paragraph without a filter
aid is capable of further reducing the waste in the liquid waste stream
leaving the plant from 13% of the potato (Table 8) to 5%.
The peel waste, which was formerly part of the liquid waste stream,
will be part of the solid waste stream for the infrared dry caustic
peeler system, thus increasing the solid waste stream from 9.5% to an
estimated 13.5% of the raw potato weight.
If the current daily output was maintained, the raw potato
requirements could decrease from 136.1 kkg per day (300,000 Ib/day) to
113.4 kkg per day (250,000 Ib/day). If raw sweet potatoes cost $40.85
per kkg ($37.00/ton), the net savings would be $33,351 per season. In
addition, if lye costs $0.154 per kg, then it is estimated that the
amount of lye neededlS could be reduced from 29 kg/kkg (58 Ib/ton) to
9.65 kg/kkg (19 Ib/ton) for a net savings on lye of $12,165 per season.
In addition, if the same through-put rate was maintained (13.61 kkg/hr),
the work day for the peeling process will be reduced 16.7% for the same
total output, but this is ignored in the calculation.
36
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The additional cost analysis is given below:
Cost of New Infrared Dry Caustic $83,960
Peeling System (13.61 kkg/hr)
Annual Cost
Cost x CRF (14 yr @ 10.5%) $11^710
Maintenance (at 10% of cost) 8,396
Power (estimated) 1,000
Total Annual Cost of the New Infrared
Dry Caustic Peeling System $21,106
Waste Treatment Cost of Infrared Dry Caustic Peeler
(based on 13% of the raw product ending up in the
liquid wastestream after 10-mesh screen only)
Suspended Solids 127,371 kg (280,216 Ib)
(113.4 x .13 x 36 x .24)
BOD of Suspended Solids 62,412 kg (137,306 Ib)
(127,371 x 0.49)
Treatment Cost at $.11 per kg (.05 Ib) $20,876
Waste Treatment Cost with Conventional Peel
(10-mesh screening) $48,183
(136.1 x 36 x 25% x 24% x (1+0.49) x 0.11)
Net Reduction in Waste Treatment Cost by
Adopting New System C$48,183 - 20,876) $27,307
37
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Comparison: Conventional Caustic Peeling System vs Infrared Dry
Caustic Peeler Including High Pressure Water System and Screens
but not Labor (annual costs based on six-week season)
1. Comparison of Peel Systems
Conventional New
Peel Equipment Amortized $ 11,710
Maintenance $ 5,000 (est.) 8,396
Power 750 (est.) 1,000
Sweet Potatoes 200,105 166,754
Lye (136.1 x 36 x 29 x .154) 21,878
(113.4 x 36 x 9.65 x .154) 6,066
Cost of Peel Systems $227,733 $193,926
Net Saving ($227,733 - 193,926) $33,807
Yield on Investment ($33,807 x 100/83,960) 40.27%
2. Comparison of Liquid Waste Treatment Charges
(after existing 10-mesh screen)
Conventional New
(136.1 x 36 x .25 x .24 x 1.49 x .11) $48,183
(113.4 x 36 x .13 x .24 x 1.49 x .11) $20,876
Net Savings in Treatment Charge 27,307
Yield on Investment (27,307 x 100/83,960) 32.52%
Combined Yield (61,114 x 100/83/960) 72.79%
38
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3. Effectiveness of Adding Two-Stage Screening to Infrared Peeling System
Conventional New
Annual Cost of Adding Two-Stage Screen $11,807 $11,807
Treatment Charge after Additional Screen
(136.1 x 36 x .25 x .24 x .4 x 1.49 x .11) 19,273
(113.4 x 36 x .13 x .24 x .4 x 1.49 x .11) 8,350
Cost of Two-Stage Screen 31,077 20,157
Costs of Conventional vs Infrared Dry Peel
plus Two-Stage Screening 258,810 214,083
Net Savings $44,727
Yield on Combined Investment
$44,727 x 100 (83,960 + 43,216) 35.17%
SUMMARY
These studies illustrate that pollution abatement and control through
in-plant conservation and pretreatment and through process modification
can be an effective way of reducing costs and increasing the return on
investment.
39
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SECTION VIII
WASTE TREATMENT
INTRODUCTION
Waste treatment costs can be minimized by careful attention to
pretreatment in the plant. The disposal of solids by feeding to
animals or by burial is economical and reduces the load on sub-
sequent biological treatment units and, thus, should receive high-
est priority.
Capital investment in sweet potato waste treatment, may be
limited by the seasonal operation of many plants. Accordingly,
discussion will proceed from the simplest to the most complex.
Only in very large plants should the two-stage biological treat-
ment be considered.
Work has been dene on the wastewater treatment of white po-
tato wastes,29,30,31 however, this work should not be directly re-
lated to the sweet potato because of the following differences in
the two potatoes.
1. The white potato, on a weight basis, is about 21$ dry mater-
ial, while the sweet potato is 24$ dry material. Consequently,
the sweet potato may produce more solids and BOD per pound of
peel loss than the white potato.4
2. A number-one peel (the requirement for a number-one peel is
that the product be clean, smooth, and require a minimum of
trimming) on white potatoes gives a weight loss of approximately
20$, while a number-one peel would waste approximately 25$ of sweet
potatoes* The increase in peel loss for sweet potatoes is caused
in part by the thickness of the stringy layer under the skin that
must be removed and in part by the tails that are removed.
3. Field data indicates a lye consumption of 13.36 to 26.72 kg
(30 to 60 pounds) of caustic per ton of white potatoes peeled
and a corresponding figure of 18.14 to 36.28 (40 to 80) for the
sweet potato.5
4. Sweet potato processing is a seasonal operation, while the
white potato industry is not. This means that any biological
treatment scheme must be able to handle the tremendous shock
load part of the year and yet survive during the off season.
5. Sweet potatoes are typically canned with syrup, whereas
white potatoes are not. The overflow of syrup from the cans may
contribute a substantial amount of BOD to a plant's waste stream.
6. Sweet potatoes when damaged exude a latex that is sticky
enough to cause clogging of steam processing equipment.
7. The skin of the sweet potato thickens more than the white
potato when stored.
40
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CAUSTIC PEELING WASTE
The caustic peeling waste in conventional wet canneries should
be separated from other wastes because of its high strength and
high pE. It has been pointed out that "improved technology" and
"potential technology" is directed at producing a low-water con-
tent peel waste that either can. be buried or can be neutralized
and sold for feed.
The high sodium content of the caustic peeling waste can
destroy the structure of clay soils so these wastes should be
applied directly to agricultural soils only with careful con-
sideration of future use of the soils.1?
The waste may be neutralized and/or held in a holding la-
goon to allow anaerobic fermentation and carbon dioxide from the
atmosphere to neutralize the excess caustic.
A neutralized waste may then be further treated in aerobic
lagoons or in aerated lagoons. Loadings of different types of
lagoons are noted in Table 9*
The high starch and other carbohydrate in the peel waste
will ferment rapidly when the caustic is reduced below a pH of 10.
The primary products of the fermentation are odorous volatile
fatty acids. These products will definitely attract the un-
favorable attention of neighbors. Accordingly, the waste should
be given additional treatment as rapidly as possible.
NUTRI13NT SUPPLEMENTS
High carbohydrate wastes are usually deficient in nitrogen
and phosphorus, and these elements must be added when treatment
is by biological processes. Sawyer^2 has proposed that nutrient
ratios should be no larger than:
BOD to N = 32 to 1
and BOD to P « 150 to 1.
Higher ratios will result in bulking.
Unpublished studies by Tyler8 show that for sweet potatoes
the
BOD to N s 107 to 1
and BOD to P - 194 to 1.
In the same studies, white potatoes showed:
BOD to N - 130 to 1
and BOD to P s 137 to 1.
-------
These figures suggest that for sweet potato, the waste is
deficient in both nitrogen and phosphorus, while for the white
potato, the waste is deficient in only nitrogen.
The amount of supplemental nitrogen would be 2.5 parts for
each part in the sweet potato waste.
The amount of supplemental phosphorus would be 0.3 part for
each part in the sweet potato waste.
Streebin, Reid, and Hu11 found that supplemental nutrients
were required at Stillwell Cannery Company to prevent bulking of
the suspended solids in the aeration basins. Their work called
for maintaining the ratio of total Kjeldahl-nitrogen to volatile
suspended solids at 5$.
LAND IRRIGATION
Irrigation is popular as a method of treatment where low-
cost land is available close to the cannery. Spray irrigation
is readily adaptable to the new technology associated with the
dry caustic peeler and in-plant waste control.
General concepts for a land irrigation system are as
follows:
1. Agriculture requirements are about 7m3/day/ha (750 gpd per
acre) for water to support crops in areas where rainfall is
around 100 cm/yr.
2. Cultivated agricultural soils can take 2Sm3/day/ha (3000
gpd per acre).
3. Grassed lands may take up to 240ra3/day/ha (25,000 gpd per
acre). The upper limit is governed by the perviousness of the soil.
4. The sodium concentration of the wastewater should be low
enough to prevent clogging of the soil. (Do not apply caustic wastes.)
5. One overland flow irrigation system operated at 67m3/ha/day
(7200 gpd per acre) and showed a 6 ppm BOD in the effluent of a
canned soup waste.33
6. The spray irrigation system used by a processor at Dunn, N. C.54
treats wastes from a sweet potato processing plant employing "dry
peeling." The treatment system has been in operation for two years
(1973) without any operational difficulty. This plant discharges
45m3/ha/day (200 gpm) to a field planted primarily in fescue. The
soil type is sandy clay loam. There is no runoff from the field and
to date no evidence of groundwater contamination has been found. The
plant showed investment of $250,000 in a 24.28 ha (60-acre) spray
irrigation system or about 810,300 per ha ($4000 per acre). A large
share of those costs was land acquisition. Operating cost (l97g)have
been estimated at 325,000 per year or about $1,030 per ha per year
($400 per acre) at a load of 45ra3/ha/day (4800 gpd per acre).
42
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ANAEROBIC LAGOONS
Anaerobic lagoons do not normally afford better than 50$
removal of BOD-5 from cannery wastes (Porges35). However, they
may be operated at very intensive loadings of 48 to 160 g/day/cu m
(150-500 kg/day/ha in a 3 m deep lagoon). Accordingly, anaero-
bic lagoons offer a high degree of organic removal per unit area,
but the effluents will require additional treatment to meet 1973
standards of performance.
Anaerobic lagoons are designed with small surface area and
depths of ten feet or more to minimize loss of heat and escape
of odorous volatile fatty acids that are produced. Dostal^
reports that detention periods of 30 days are required to obtain
the best treatment efficiency.
Deep ponds are subject to hydraulic short circuiting, that
is, large amounts of waste may flow quickly through the pond with-
out permitting time for decomposition. In addition, anaerobic
processes require considerable time for the buffering capacity to
build up, so that the fatty acids that are produced do not stop
the decomposition reaction. Bulking or floating of suspended
solids is another common difficulty that arises from the attach-
ment of fermentation gas bubbles to suspended particles when gas
production is very rapid.
Despite the low treatment efficiency and operating difficul-
ties, anaerobic lagoons may be used as an economical use of aur-
face area for preliminary treatment and as a treatment that can
improve the settling properties and the dewatering properties
of the colloidal solids from canneries.
AEROBIC LAGOONS
Aerobic lagoons with detention periods of as much as 37.5
days have afforded 98$ removal of BOD-5 from canneries at load-
ings as high as 7-4g/day/cu m (20 Ibs. per acre ft. per day)?5
(substantially lower than the reported loadings on anaerobic
lagoons).
Very shallow lagoons, less than 1.2? m (4 ft.), depend
upon natural surface aeration and algal production to supply
oxygen. The loadings upon such ponds in warm sunny latitudes
will range from 4g/day/cu m of BOD-5 in the winter to 12g/day/cu m
of BOD-5 in the summer. At higher loadings, 85$ removal is ex-
pected with detention periods of 30 days.
Deeper aerobic lagoons will be between 1 to 2 m deep (3 to 6
ft.). The bottom will probably be anaerobic, but enough oxygen
will be supplied through the surface and by algal production
that the dominant action will be aerobic. Loads as high as
36.8g/day/cu m of BOD-5 have been reported with removal effi-
ciencies of 90$. 24 The aerobic-anaerobic action is commonly
termed "facultative."
43
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The aerobic and facultative (aerobic-anaerobic) ponds are
subject to short circuiting and care must be taken to assure that
flow is distributed uniformly to all parts of the ponds.
The ponds may become "attractive nuisances" to children and
frequently must be fenced. Since the land requirements for po-
tato canneries are high, the combined cost of land and fencing
may force consideration of more intensive treatment.
AERATED) LAGOONS (Completely Mixed)
In the context of this report, aerated lagoons are defined
as simple earthen lagoons about 3 m (10 ft.) deep in which me-
chanical aerators are employed to transfer increased amounts of
oxygen to the liquid waste. No provision for the separation and
recirculation of biological suspended solids is included, since
in this discussion it is assumed that they are discharged with
the effluent.
The objective of an aerated lagoon is to provide a moderate-
ly high degree of treatment that requires less space and lower
detention periods, than simple aerobic lagoons, and which can
accept the fairly high unit loadings of plants such as canneries.
Dostal^6 reports 81$ removal of BOD-5 at loadings of
183g/day/cu m and a detention period of less than eight days.
The power input to achieve this degree of treatment was about
7.2 kwh/kg of BOD-5 removal. In this same instance the removal
of soluble BOD-5 was 93$ indicating that a more sophisticated
treatment system with a final settling tank and provisions for
solids disposal could improve the quality of effluent substantial-
ly.
The design and performance of completely mixed aerated la-
goons are described by mathematical equations developed by both
McKinney38 and Eckenfelder39.
ABHATED LAGOONS (incompletely Mixed)
In the "incompletely mixed" aerated lagoon an earthen lagoon
of about 3 m depth is employed as in the completely mixed system.37
However, a smaller mechanical aerator is employed so that complete
mixing is not achieved. Part of the BOD-5 in the influent and
synthesized suspended solids is removed by sedementation in the
bottom of the basin and is not discharged with the effluent. Ac-
cordingly one would expect results somewhere between those of a
simple aerobic lagoon and a completely mixed aerated lagoon.
An illustration may be taken from work by Dostal36 on white
potatoes. A small lagoon of 171 cu m capacity was loaded at
143g/day/cu m of BOD-5. A 3-7 kw aerator supplied 0.022 kw/cu m.
44
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The removal of BOD-5 was 84$ at a detention of 8.8 days. The
power requirement was 0.023 kwh/kg BOD-5 removed.
Benjes37 points out that performance of the incompletely
mixed aerated lagoon is not completely predictable by the
McKLnney58 or Eckenfelder39 mathematical relationship.
TWO-STAGE BIOLOGICAL TREATMENT
Streebin, et al,11 have reported a remarkable efficient two-
stage process of aeration in presence of solids, that is, capable
of removing 97$ of the COD at loadings that are of the order of
27,000 kg/day/ha of BOD-5. (See Table 9) The energy requirement
was of the order of 0.83 kw-hr per kg BOD-5 removed in comparison
to the 7.15 kw-hr/kg reported by Dostal on aerated lagoons. Such
a plant would be desirable in large canneries that operate on
a year-round basis.
Schematically the plant consists of a preliminary aeration
basin operated at 8,000 gms/cu m/day of BOD-5 for a detention
period of 4*8 hours. The high load unit is followed by parallel
extended aeration basins operated at 640 gms/cu m/day of BOD-5
for a detention period of 36 hours. A final clarifier designed
at an overflow rate of 32.64 cu m/day/sq m provides 1.89 hours
of settling at a weir loading of 124 cu m/day/m.
Solids are digested anaerobically in an open lagoon.
The system provides for recirculation of settled solids to
either one or both of the aeration basins.
In practice, the first aerator basin operates with a mixed
liquor suspended solids concentration of about 3500 og/1 at about
85$ volatile solids. The solids do have a tendency to bulk, which
is controlled when the total Kjeldahl nitrogen is raised 5% of
the volatile suspended solids.
The extended aeration unit also operates at a high mixed
liquor suspended solids concentration of over 3000 mg/1 at 83$
volatile solids.
A chemical feed unit provides 454 kg (1,000 Ibs.) per day
of supplemental nutrient to control bulking during the season
when potatoes (both sweet and white) are processed. The feeder
is operated until the total Kjeldahl nitrogen to volatile sus-
pended solids ratio in the primary aerator reaches 5$ (i.e.,
180 mg/1 per 3600 mg/l).
Dissolved oxygen in the primary aeration basin is maintain-
ed near 4 mg/1 with two 56 kw aerators. Dissolved oxygen in the
extended aeration unit is also maintained near 4 mg/1 with 6 sur-
face aerators (3 in each unit) of 30 kw each.
45
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TABLE 9
SOME COMPARATIVE ASPECTS OF LAGOON TREATMENT SYSTEMS
Approximate
Load Intensity
gms/day/cu m
kg /day /ha
% Removal
BOD-5
Spray
Irri-
gation
( 25,000)
234m3/ day/ha
22-44
90
Anaer-
obic
Lagoons
48-160
150-500
at 3m deep
50
Aerobic
Lagoons
4-12
5-15
at 1m deep
85.0
Incompletely
Mixed
Aerated
Lagoons
143
4,290
84
Completely
Mixed
Aerated
Lagoons
180
*
5,400
at 3m deep
90
Extended
Aeration
2-Stage*
900
27,000
at 2.5m
97
Detention Period
Days
Power
kwh/kg BOD-5
removed
30
30.0
88
4.3
7-8
7.2
0.83
*The design in the primary aeration basin is to transfer about 0.8 kg of oxygen per kg of BOD
removed, and it is estimated that about 40% of the BOD will be removed at these high loadings.
The design of the extended aeration unit aims at transferring about 1.2 kg of oxygen for each
kg of BOD-5 removed and about 60% of the load is removed in this unit.
-------
SECTION IX
REFERENCES
1. U. S. Department of Agriculture Statistical Report Service,
Potatoes, Sweet Potatoes, Statistical Bulletin No. 409,
Washington, D. C., (July 1967 and August 1969).
2. Sweet Potatoes: Production. Processing. Marketing. Edmond
and Ammenoan, AVI Publishing Company, Inc., 1971.
3. Woodroof, J. G., Dupree, W. E., and Cecil, S. R., "Canning
Sweet Potatoes," Agricultural Experiment Station Bulletin
N. S. 12. Georgia Experiment Station, 1956.
4. Colston, N. V., and Smallwood, C., Jr., "Waste Control in
the Processing of Sweet Potatoes," Proceedings. Third Na-
tional Symposium on Food Processing. Pacific Northwest
Water Laboratory, Environmental Protection Agency, Novem-
ber 1972, (EPA R-2-72-018).
5. Hammond, Leigh H./ and King, Richard A., "Planning Data for
the Sweet Potato Industry: 2: Costs and Returns for a Model
Canning Plant," Agricultural Information Series. No. 93,
Agricultural Economics Department, North Carolina State
University (1962).
6. FWPCA Methods for Chemical Analysis of Water and Wastes,
November 1969, Federal Water Pollution Control Administra-
tion, Division of Water Quality Research Analytical Quality
Control Laboratory, Cincinnati, Ohio (later publication be-
comes the Environmental Protection Agency Publication No.
16020 of Water Quality Office 2971).
7. Standard Methods for the Analysis of Water and Waste Water,
13th Edition (l97l), APHA, AWWA, WPCF (APHA - Washington,
D. C.)
8. Tyler, J., Internal Communication, North Carolina State
University, July 1972.
9. Willard, Miles in "Pilot Plant Study of the USDA-Magnuson
Infrared Peeling Process", 3067 Gustafson Circle, Idaho
Falls, Idaho, July 28, 1969.
10. Personal Communication to Mr. Norman Miller, North Carolina
State University Food Science Specialist from Robert M.
Magnuson, Magnuson Engineers, Inc., March 9, 1972.
47
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11. Streebin, L. E., Reid, G. W., and Hu, Alan, C. H., "Demonstra-
tion of a Full-Scale Waste Treatment System for a Cannery,"
Water Pollution Control Research Series 12060 DSB. Environ-
mental Protection Agency, September 1971*
12. Personal Communication to Mr. Norman Killer of N. C. State
University, Department of Food Science, from an engineer
of Green Giant Packing Co., 1969.
13. Boyer, J. L., "Effect of Temperature and Exposure on Peeling
of Fruits and Vegetables," Food Technology. Vol. 4, p. 206-
209, 1950.
14. Eidt, C. C., and MacArthur, M., "The Peeling of Fruits and
Vegetables for Processing," Food in Canada. Vol. 4» No. 7,
p. 31-35, 1944.
15. Graham, R. P., Huzsoll, C. C., Hart, M. R., Weaver, M. L.,
and Morgan, A. I., Jr., "Dry Caustic Peeling of Potatoes",
Food Technology. Vol. 23. February 1969.
16. Personal communication to Mr. Jimmy Garrell, Tabor City
Foods, Inc., from C. C. Huxsoll, Agricultural Engineer,
U. S. D. A. Agricultural Research Service, Western Util-
ization Research and Development Division, Albany, Col.,
December 12, 1969.
17. Thomas, R. P. and Law, J. P., Jr., "Soil Response to
Sewage Effluent Irrigation" presented at symposiums on
Use of Sewage Effluent for Irrigation at Louisiana Poly-
technic Institution, Ruston, Louisiana, July 30, 1968.
18. Steele, W. R., in "Application of Flue Gas to Disposal of
Caustic Textile Wastes" Proceedings—Third Southern Muni-
cipal and Industrial Waste Conference, March 18-19, 1954—
North Carolina State University, Raleigh, N. C.
19. Hamza, A., "Internal Report on Filtration of Sweet Potato
Wastes," NCSU, October 1971.
20. Dickey, H. C., Brugman, H. H., Highlands, M. E., and Plum-
mer, B. E., "The Use of By-products from Potato Starch and
Potato Processing," Proceedings. International Symposium
Utilization and Disposal of Potato Wastes. New Brunswick
Research and Productivity Council, New Brunswick, Canada,
1965.
48
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21. Morrison, P. D. Feeds and Feeding. Morrison Publishing Company,
Ithaca, New York, 1950.
22. Bell, T. D., "Livestock Peed Utilization of By-products of
Potato Processing Plants Using Dry Peel Method," University of
Idaho, 1970.
23. Personal communication from R. E. Perrin, Associate Professor
of Economics to Dr. N. V. Colston, Assistant Professor of Civil
Engineering, both of N. C. State University, September 25, 1969.
24. Raines, Brian, "Sweet Potato Processing Waste, A Potential
Source of Animal Peed," Internal Report. Department of Food
Science. North Carolina State University, December 1971.
25. Grames, Lloyd M. and Kueneman, R. W., "Primary Treatment of
Potato Processing Wastes," Journal. Water Pollution Control
Federation. 41 1358-1366 (1969).
26. Personal Communication to Dr. N. V. Colston, North Carolina
State University, from H. G. Swope, Consulting Chemist, Mad-
ison, Wisconsin, March 1972.
27. Internal Report to Tabor City Poods, Inc., North Carolina
State University, July 1972.
28. Personal Communication from Mr. Bill Hamilton, Environmental
Protection Agency, Cost Analysis Section to Professor C.
Smallwood, North Carolina State University, Raleigh, February
14, 1974.
29. Carlson, Dale A., and Guttormsen, Kristian, "Current Practice
in Potato Processing Waste Treatment," DAST-14 Water Pollu-
tion Control Research Series. (U. S. Department of Interior),
October 1969.
30. Vivian, Robert W., Dostal, Kenneth A., "Potato Processing
Wastes: Progress Report on Pilot Plant Studies of Second-
ary Treatment: Report No. PR-4," Pacific Northwest Water
Laboratory. Environmental Protection Agency, January 1968.
31. Atkins, Peter P., and Sproul, Otis J., "Feasibility of Bio-
logical Treatment of Potato Processing Wastes," Journal
Water Pollution Control Federation. Vol. 38, p. 1287, August
1966.
49
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32. Sawyer, C. N., "Bacterial Nutrition and Synthesis," Biologi-
cal Treatment of Sewage and Industrial Wastes. Vol. 1, Reinhold
Publishing Company, New York, 1956.
33. Gilde, L. C., "Experiences of Cannery and Poultry Waste Treat-
ment Operations," Proceedings of the 22nd Industrial Waste
Conference. Purdue University, Engineering Extension Series,
No. 129, p. 675 (1967).
34. Hoy Tew of H. P. Cannon & Sons, Dunn, N. C., Personal conver-
sation with Professor C. Smallwood, North Carolina State Uni-
versity, July 1973.
35. Porges, R., "Waste Treatment Lagoons," Journal. Water Pollu-
tion Control Federation. Vol. 35. No. 4, p. 456, 1963.
36. Dostal, K., "Secondary Treatment of Potato Processing Wastes,"
Report FR7, Federal Water Pollution Control Administration,
Northwest Region, Corvallis, Oregon 97330, July 1969.
37. Benjes, Henry, Jr., "Theory of Aerated Lagoons," in Second
International Symposium for Waste Treatment Lagoons. June 23-
27 (1970), Kansas City, Mo., published by the University of
Kansas, Lawrence, Kansas.
38. McKinney, R., E., "Mathematics of Complete Mining Activated
Sludge," J.A.S.C.E. S.E.E., Vol. 88, SA3, May 1962, p. 87-
113.
39. Eckenfelder, W. W., "Aerated Lagoons Manual of Treatment
Process,w> Vol. 1, Environmental Science Services Corpora-
tion, 1968.
50
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SELECTED WATER
RESOURCES ABSTRACTS
INPUT TRANSACTION FORM
ccfs.K/on No,
w
4, Title
WASTE CONTROL AND ABATEMENT IN THE PROCESSING OF
SWEET POTATOES
$,. R-epvrt-Daf*
1
S,
r. Authors) smallwood, Charles, Jr.; Whitaker, Robert S., and
Colston, Newton V.
9. Organization
Civil Engineering Department
North Carolina State University
Raleigh, N. C. 27607
. &wirot»eatai
10. Project No.
11. Contract/Grant f'o
03J1P00835
IS. 'fj:'il S&poti mi'
1 5, Supplemental otes
16. Abstract
The conventional processing of sweet potatoes produces a very strong caustic waste that
is high in organic matter. Present technology does not emphasize recirculation or other
control of water use.
Improved technology is available such as high pressure low-volume water sprays and a
dry caustic peeling process that reduce water use and convert the liquid caustic waste to
a semi-solid waste that can be disposed of in sanitary landfills or sold as cattle feed.
Developing technology offers the potential of lye recovery, an improved steam peel or
an infrared dry caustic peel that increases yield.
In-plant control of waste through process modification and/or treatment is economical
and may even provide a net return on investment.
Biological treatment is effective.
This report was prepared to make available the data collected under the first phase of
the Environmental Protection Agency's Grant Number 12060 FRW. The majority of the ana-
lytical data characterizing sweet potato processing wastes presented in this report were
obtained from an in-depth study of one conventional sweet potato processing plant during
the 1971 processing season. Grant 12060 FRW was terminated prior to initiating the sec-
ond and final phase of the grant. The second phase was to be a full scale demonstration
of infrared dry peeling; water conservation through water reuse and high pressure low-
yolume sprays; in-plant waste separation and treatment; and end of pipe sequential screen
17a. Descriptors
Sweet potatoes, In-plant control, Waste treatment, Pollution costs, Chemical
recovery.
17b. Identifiers
17V. COWRR Field dl Group
IS. Availability
m
20. Security Class,
27,
22, Price
Send To:
WATER RESOURCES SCIENTIFIC INFORMATION CENTER
U.S. DEPARTMENT OF THE INTERIOR
WASHINGTON, D. C. 2O24O
North Carolir^
JT DB'INT'ING OFFICE- 1Q7*-697-652_t62 REGION 10
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