20 'Aedia and Reagent for Micro"CO Laboratories, Inc., Detroit. and OSBORNE, M. F. 1950. t3-1836. ON, W. O., and HENDEL, C. S. Pat. 2,705,679. WRRI, "puffs" potatoes. Food

,R. M. 1951. Quick cooking

.E, C.W. 1955. Maintenance

P 05765

Treatment and Disposal of Potato Wastes R . E . Pailthorp J. W.Filbert G . A. Richter

isms. Appl. Microbial. 3, 361-

new snack item-French fried the technology, production and ypt. Agr. Misc. Publ. 695. WILLIAMS, J. H. 1945. U.S.

'ON, W. 0. 1955. Process of 'Pat. 2,707,684. ato snack item. Bakers Weekly

3.,and HEISLER, E. G . Potato 18-73-15. ilip confections. Can. Food Inds. The chemicals we get from pop ~ 190-194. . I, C. F., HEISLER, E. G., and uation of potato chip bars. Food

Pollution control is a pressing problem for existing processing plants and is a major consideration when comparing locations for new processing plants. Reduction of processing losses must be considered during the manufacture of the product to obtain the greatest economic returns and to ensure the lowest amount of polluting emuents. Processing plants with low product losses will have a low amount of waste. There is a demand for more and better finished products and also a demand for maintaining and improving the quality of U. S. public waters. The demand for increased water quality has been instrumental in the formation of a U. S. national policy that demands substantial treatment of processing wastes before they can be discharged to U. S. public waters. The potato-processing industry has developed methods for providing effective removal of settleable and dissolved solids from potato-processing wastes. The types, sizes, layouts, and details of treatment systems must be carefully considered and planned to obtain maximum benefits from the investment. In some cases this portion of a processing plant will be as important as the proper selection of the product process line. Reduction of the total quantity of waste through selection and operation of efficient peeling systems and processing lines and reduction of water flow through conservation and water reuse systems should be the first step in a pollution control program. 747

748 R. E. Pailthorp, J. W. Filbert, and G . A. Richter

POLLUTION Treatment of industrial wastes is necessary if effluent is discharged to public waters in the United States. Treatment may also be required prior to irrigation or other forms of land disposal. Pollution can be defined very broadly as anything that causes nuisance conditions in or adjacent to a receiving stream or anything that interferes with another beneficial use of a stream or groundwater. Pollution is not a quantitative term.However, many laymen consider a stream polluted only when apparent nuisance conditions exist. The following are some things that can cause pollution: (1) organic wastes (domestic sewage, food-processing wastes, cattle feed-lot drainage, etc.); (2)bacteria; (3) toxic compounds (chlorine, lead, hexavalent chrome, mercury, etc.); (4) nutrients (phosphates, nitrates); (5) floating material: (grease, oils, foam, etc.); (6) settleable materials: (ashes, soil, organic materials); (7) cloudiness (turbidity); (8)soluble ions in high concentrations (sodium, chloride, sulfate, nitrate); (9) acids and alkaline wastes; (10) heat; and (11)color. The most common pollution problem is associated with organic wastes, which undergo decomposition in water. The decomposition occurs when bacterial and other biological forms use the compounds as a food source. Oxygen is required for biological decomposition to take place without causing nuisance conditions in a stream. The oxygen for this process is taken from the stream. Only 9-10 mg/liter of oxygen will dissolve in water. Many forms of aquatic life require 4-5 mg/liter of oxygen to survive. When all of the oxygen is used from a stream it becomes unattractive; fish die, odorous gasses evolve, and decomposing solids float on the surface. As a consequence of this type of pollution, recreation is destroyed, and many other beneficial uses are impaired or destroyed. Treatment of industrial effluents to remove organic materials often alters many other objectionable waste characteristics.

Terminology The pollution control and effluent treatment field has its own terminology. Some of the most common terms must be learned to understand literature on pollution control and to evaluate statements made by representatives of control agencies and other people in the field of pollution control. The following is a glossary of the most common terms. Many of these will be used in this chapter.

20. Treatment and Disposal of Potato Wastes 749

Domestic Sewage. Wastewater from residential dwellings, apartment houses, and other living accommodations. Commercial Sewage. Wastewater from commercial establishments containing domestic sewage, along with possible other wastewaters such as those originating from laundries, bottling plants, ice plants, and restaurants. Industrial Wastes. Wastewater from industries using large volumes of water for processing industrial products, such as food-processing plants, paper mill refineries, and textile mills. BOD.-Biochemical oxygen demand is a measure of the oxygen necessary to satisfy the requirements for the aerobic decomposition of the waste. This provides an accurate measure or indication of the organic \ content or pollution strength of the waste. BOD,.-Measurement of oxygen required in a 5-day laboratory test. BOD &.-Measure of ultimate BOD; usually measured using a 20day test. COD.-Chemical oxygen demand is a measure of the amount of oxygen that will react chemically with a waste. This value varies with the type of oxidant used, with the testing method used, and with the type of waste. The result is not a direct measure of BOD, but can usually be correlated with BOD by a series of parallel tests. Suspended Solids.-Solids which can be mechanically filtered from the wastewater. Settleable Solids.-Suspended solids that will settle in sedimentation tanks in normal detention periods. Total Sol ids.-Both suspended and dissolved solids. Parts Per Million.-The pounds of material in one million pounds of flow (abbreviated as ppm). Milligrams Per Liter.-The milligrams of material in one liter of flow (abbreviated as mglliter). Primary Treatment.-The removal of suspended and settleable solids by screening, flotation, or sedimentation. Secondary Treatment.-The removal of organic matter by biological decomposition (usually preceded by primary treatment). Advanced Waste Treatment.-Treatment beyond secondary treatment. Aerobic Treatment.-Biological activity in the presence of dissolved oxygen (normally does not cause odors). Anaerobic Treatment.-Biological activity in the absence of dissolved oxygen (normally does cause odors). Toxicity.-Usually measured by placing fish or other aquatic life

750 R. E. Pailthorp, J. W. Filbert, and C. A. Richter

20. Treatment and Disposal of Potato Wastes 751

forms in the waste or diluted waste for several hours and noting fatalities.

Testing The waste characteristics must be known to properly size effluent treatment units and to evaluate the effectiveness of treatment units after they are installed. In some cases, these characteristics may be estimated from production records and losses, but sampling and testing of the effluent discharged from the plant is usually necessary. The most common measurements are those of various types of solids and of chemical or biological oxygen demand. The test procedures for solids determinations and for BOD can be found in “Standard Methods for the Examination of Water and Wastewater” (Amer. Public Health Assoc. 1985). Figure 20.1 represents the relationship of the various classifications of solids in liquid waste. The settleable solids portion represents the amount of waste that can be removed by sedimentation. The COD test is easier and quicker to run than the BOD test. A BOD test requires 5 days before the results are known; a COD test can be done in a few hours.

1

I

TOTAL

SOLIDS

C

SUSPENDED SOLIDS SUSPENDED VOLAT I L E I

/

I/

I

DISSOLVED SOLIDS DISSOLVED VOLATILE

1

N0N.S E T T L E A B L E

I

SUSPENDED FIXED SETTLEABLE DISSOLVED FIXED

Fig. 20.1. Classification of solids in wastewater

Regulations Regulations contralling surface waters and groundwater pollution are established by county, state, and federal agencies. The regulations usually establieh a n upper limit on several pollutants in the final

effluent. In some cases receiving water standards have been established when discharge limits are not appropriate to protect the environmen t . Common law rulings are another facet of pollution that should always be considered by an industry that discharges wastes. Even though a discharge meets the requirements set forth by a regulatory agency, a judgment can be obtained by court action against the discharger. Many industrial plants conduct extensive stream surveys above and below their discharge points in an effort to protect against common law actions.

History The history of waste treatment for the potato-processingindustry parallels that of many other industries. In the United States, there are three geographical areas of major potato-processing activity: (1)Idaho, eastern Oregon, and eastern Washington; (2) North Dakota and Minnesota; and (3) Maine. Most plants are located in sparsely populated areas where the waste load from the plants is extremely large compared to the domestic sewage load. Because of this, the waste was traditionally disposed of in streams. Potato chip and prepeeled potato plants in contrast, while expanding in number and size, are largely located near metropolitan areas, where the waste effluent is more easily handled by municipal facilities. In general, these plants are much smaller than French fry or dehydrated potato plants, and the processes do not contribute as great a unit waste load as the other types. A typical example of the recognition of a problem and the approach to solution was found in Idaho along the Snake River. During the 1950s, the potato-processing industry in this area experienced great growth and the stream was subjected to increased demands for irrigation and recreation. Fish kills and other nuisance conditions resulted. Therefore, in early 1961, the State Department of Health required the potato processors to provide for removal of all settleable solids and up to 90% reduction of BOD in the future. The processors of the region formed a joint committee (Potato Processors of Idaho) and undertook pilot-plant studies to determine the best way to provide the required treatment. By the processing season of 1964, most of the plants had installed primary treatment for removal of settleable solids. Stream conditions were materially improved by the primary treatment systems. This same committee fi-

762 R. E. Pailthorp, J. W. Filbert, and G. A. Richter

nanced a pilot-plant study in 1964 and 19 to evaluate secondary treatment methods to meet the requirements for 90% BOD reduction. The results of the study were used to design a full-scale aerobic secondary treatment facility at the R. T. French Company in Shelley, Idaho, as a federal demonstration project. Since the installation of primary treatment, the Potato Processors of Idaho have sponsored many research projects to explore various types of aerobic and anaerobic treatment, waste biological solids treatment, conditioning, disposal, and spray irrigation. By the beginning of the 1971-1972 processing season, most Idaho potato-processing plants had initiated either secondary treatment or spray irrigation systems. Most of the information on potato-processing waste treatment has been published since 1950. There were very few treatment systems for potato wastes in the United States until after 1960.

CHARACTERISTICS OF PROCESSING PLANT EFFLUENTS Components of Potato-Processing Waste The composition of a waste stream from a potato-processing plant is largely determined by the processes used. Most potato processing can be separated into the following general steps: washing the raw potatoes; peeling, which includes washing to remove softened tissue; trimming to remove defective portions; shaping, washing, and separation; heat treatment (optional); final processing or preservation; and packaging. The analysis of waste stream from potato-processing operations relates closely to the composition of the potato. Components foreign to the potato that also may be present include dirt, caustic, fat, cleaning and preserving chemicals, and other food ingredients in small quantities. A typical proximate analysis of potato waste solids from a plant employing steam or abrasive peeling is shown in Table 20.1. Normally, most waste streams in the plant are combined before discharge from the plant. Dirt. Dirt or silt adhering to the surface of the potato is removed either in an initial washing step or in the normal peeling waste stream. It contributes to the suspended, fixed solids and normally is treated separately from the other process water. For example, this water can be

20. Treatment and Disposal of Potato Wastes 753

Table 20.1. Percentage Composition of Potato Waste Solids Component Total organic nitrogen as N Carbon a s C Total phosphorus as P Total sulfur as S Volatile Solids

Amount

(%)

1.002

42.200 0.083 0.082

95.2

settled in a shallow pond or clarifier. The water from these systems contains soluble and suspended solids and must be treated for discharge. In-plant treatment and recycle of wash water has been'practiced. Treatment units available include screens, clarifiers and high-pressure liquid cyclone units. Some wash water is usually bled from these recycle systems and must be treated prior to discharge. Raw Pieces. Raw pieces that are not suitable for processing range in size from whole potatoes to small fragments. Since these materials are normally firm, they present little problem in removal by screening or settling. These are commonly used for cattle feed.

Raw Pulp. Raw potato that has been finely subdivided is usually designated as raw pulp. Sources of this include abrasion peeler discharge, cutting waste, and pulp from starch separation. Equipment handling raw potatoes will contribute finely divided raw potato solids when the equipment is cleaned. Because of the large amount of water normally in contact with the pulp, much of the soluble solids are leached out. The raw pulp may be removed from the waste stream by fine screening or settling. Raw starch settles from such streams so rapidly that it sometimes causes plugging of lines and cleaning problems. Pulp is commonly used for cattle feed. Cooked Pulp. The softening action of heat during peeling or processing steps weakens the intercellular bonds of the potato tuber and results in separation of large quantities of potato cells and agglomerates of cells during washing and handling steps. These rapidly disperse in the wastewater. Many such agglomerates are removed in screening, but the greatest portion passes through the normal 20-mesh screen opening. These solids settle rapidly in a properly designed clarifier and represent a major portion of the settleable solids removed in

764

R. E. Pailthorp, J. W. Filbert, and G. A. Richter

u?

OOONO

o mNu 4 N 3 w c4 n

primary treatment of potato-processing waste streams. Separated solids are used for cattle feed.

?

Dissolved Solids. Constituents of the potato that are readily water soluble appear as dissolved solids in the final waste stream. These include solubilized starch, proteins, amino acids, and sugars. This organic portion of the waste stream can be removed only by secondary treatment, namely some form of biological oxidation or land disposal. Starch plant waste characteristics are given in Table 20.2.

OOON u? O

O"CD2 "4

I I I I I

I I I I I

I I I I I

I I I I I

I I I I I

I I I I I

I I I I I

I I I I I

Effect of Process Variations in process methods make it virtually impossible to make generalizations concerning the quantities of waste produced by specific operations. Many references can be found for studies made in the major types of processing plants. These studies show wide variations in water usage, peeling losses, and methods of reporting the waste flow. In many cases, the data do not define whether the waste was screened before analysis. Several studies on the composition of wastes resulting from various types of potato processes are summarized in Table 20.6. Processes involving several heat treatment steps, such as blanching, cooking, caustic and steam peeling, will obviously produce an effluent containing gelatinized starch and coagulated proteins. In contrast, starch processing and potato chip processing produce emuents in which the components have not been heated. The disposal of starch protein water has been the subject of much research.

Design of Effluent Treatment Facilities For an existing plant, it is necessary to measure the flow of all waste streams and determine the quantity and character of the solids found in these flows. Procedures for accomplishing this are well known. Of major importance is the reduction of waste discharge into the final plant emuent and the reduction of water flow throughout the plant. For a proposed new plant for which the waste facilities must be designed, informption may possibly be found in the literature for a similar installation. In most cases, however, a reasonable estimate of the waste flow may be determined from the estimated capacity of the plant, the recovery of product expected, and the type of screening and clarification equipment to be installed. Accurate estimates of water

-2a

T??ZZ

-'u??u?cq

-+"mm

l-lnlnroln

ONAWQ,

I I I I I

I I I I I

I I I I I

I I I I I

20. Treatment and Disposal of Potato Wastes 757

756 R. E. Pailthorp, J. W. Filbert, and G . A. Richter

usage and methods of re-use to be employed are, of course, necessary. For preliminary estimates, it can be assumed that 1 lb of dry potato solids exerts a BOD of 0.65 lb and a COD of 1.1lb.

msmrvatlon

%r..nlnp

L r w a o of ..tar Procam. r*wlalonn Procam control MW product8

Unit Processes

WASTE TREATMENT PROCESSES The conventional waste treatment process is usually considered to occur in three phases: primary treatment, secondary treatment, and advanced waste treatment (AWT). Primary treatment involves the removal of suspended and settleable solids by screening, flotation, and sedimentation. Secondary treatment involves the biological decomposition of the organic matter, largely dissolved, that remains in the flow stream after treatment by primary treatment processes. Biological treatment can be accomplished by mechanical processes or by land disposal. The primary treatment process is frequently sufficient to safeguard public health and to prevent development of nuisance conditions in instances where large volumes of dilution water are available in receiving streams. Where the flow in the receiving stream is low or where pollution loads are high, secondary treatment must generally be provided. In 1974, the Environmental Protection Agency proposed nationwide minimum discharge limits for the potato-processing industry, which resulted in land disposal or secondary treatment (EPA 1973). Advanced waste treatment involves removal of pollutants that are not removed by conventional secondary treatment. Advanced treatment can include removal of nutrients, suspended solids, and organic and inorganic materials. In mechanical secondary treatment processes, the organic material remaining in the effluent from primary treatment processes receives further treatment by passing the flow through units in which biological oxidation of this organic matter takes place. Biological oxidation is a result of biological organisms using the organic matter as a food source. The flow from the biological units is then passed through sedimentation units so that the biological solids formed in the oxidation unit may be removed prior to the final discharge of the treated effluent to a stream. When irrigation is used as the secondary treatment system, bacteria in the topsoil stabilize the organic compounds. In addition, the soil may accomplish removal of some ions by adsorption or ion exchange. Ion exchange in some soils may cause system failure. In all cases, careful consideration must be given first to the steps that might be taken within the plant to reduce the waste load.

Secondary Treetment

Primary Treatment

Treatment Category

Sadlnantbtlon

Trlckllnp

S r t t l l n p pond1

& t Iwatmd

Advanced Treatment ChrlcaL trsaWOt

Illtar8

Cnrbon t r n m w n t

ILUdpD

Stmbllliatlon ponds Anmwoblc

F1Ltratlon

Ion mchanpa

fILtmr8

An..I-ObtC contact Anaerobic pond8 &?.rad pond.

hr.,..

0..0.l.

A l r itrlpplnp

I Irrlpatton

Unit Sequence

Fig. 20.2.

%BOD Removal

Generalized unit process sequence for waste disposal.

-

Stream

6 t o 10X

86 t o 8 5 1

4 0 t o 60s

I

I Llquld

-4 1 1 I I I

-C o n o e n t r n t l o n

Treatment Solids Disposal

Cmttl. Fmmd

Fig. 20.3.

C.ttl.

Fg*d

I

C a t s l a Faad Lnndfl L L Inolnar.tlon

I

Typical treatment sequence for potato-processing effluent.

758 R. E. Pailthorp, J.

W.Filbert, and G.A. Richter

20. Treatment and Disposal of Potato Wastes 759

SIL t Sepnratfon

S n t t l l n g Pond

Secondary

Trnntment

r l t h P a e l l n g ond

Water Strenm

Procasslng Westes

CLorlffer

I I I I -Yrf I lor

Llquld

Sollds

L V O C U U DF I

Lter Ornvy Thtokener

SoLldo DlopornL

Fig. 20.4.

Procedure for silt water treatment.

The sequence of steps involved in treating potato-processing emuent are outlined in Figs. 20.2-20.4 and described in detail in the following sections.

In-Plant Treatment Two main objectives are immediately evident in reducing waste disposal problems within the plant. The first is to minimize the solids disposed of into the waste stream by improved control of the processing and handling equipment and methods. The second is the use of minimum quantities of water in processing. In many plants, the reduced losses, improved product recovery, and reduced water usage resulting from attention directed to these areas to reduce treatment costs has more than offset the cost of treatment facilities. Steps that have been taken to reduce the loss of solids to plant waste streams include the following: Improvements in peeling facilities to obtain a cleaner potato with less loss of solids. Reduction of floor spillage by redesign of equipment. Collection of floor waste in receptacles that can be dumped instead of washing waste into floor drains. Removal of potato solids from waste carriage water as soon as possible after their introduction to minimize solubilization of the solids. Replacing operational steps that create large losses where possible. Avoiding conveying.

The peeling process contributes by far the largest quantity of waste to the plant emuent. Therefore, improvements in peeling efficiency can have a major effect on the waste treatment cost. Examples of steps that can be taken include better control of caustic concentration in caustic peeling, use of higher-pressure water sprays in washing facilities, circulation of caustic through peeling equipment to obtain better heat transfer, use of preheat facilities to reduce losses in both steam and caustic peeling, and use of continuous abrasion peelers rather than batch units. Reducing the water flow in the plant has two advantages. The size of treatment facilities is influenced by total water flow from the plant. Second, the efficiency of a primary settling system, or the quantity of waste that will be removed by a given piece of equipment, increases with the concentration of the waste. Water usage within the plant can be significantly reduced by the reuse of process water containing only minimum quantities of dissolved or suspended solids where fresh water is not necessary. For example, overflow from water cooking, water blanching, water cooling, and surge tanks can be used to remove peel after lye or steam treatment of potatoes. To avoid plugging nozzles in the washer, the process water should be screened first. Water used in the exit end of peel-removal washers following lye or steam peeling can be screened and pumped into the spray system at the entrance end of the washer. Process water can be used to furnish flume water for conveying raw potatoes from storage areas into the plant. Water used to defrost refrigeration coils can be used to replace fresh water in the plant. To reduce the possibility of bacterial contamination of the product, fresh chlorinated water should be used in the final steps of processing such as the final washing of blanched potatoes before dehydration. Inplant cleaning practices may require improvement to reduce bacterial buildup in lines carrying reclaimed process water. This water is frequently warmed, allowing bacterial growth to flourish if care is not taken. The advantages of reduced waste treatment and water usage will greatly outweigh the additional sanitation problems if they occur. Excessive foam production may be difficult to control in water reuse systems if sufficient fresh water is not introduced. Other steps that have been used to reduce the volume of emuent to be treated include (1)use of some method other than water fluming for conveying potatoes, (2) use of improved cleaning facilities for equipment and floors, such as shut-off nozzles for hoses and high-pressure, low-volume spray units, and (3) collecting clean waste streams and discharging to natural drainage or storm water systems.

I

760 R. E. Pailthorp, J. W. Filbert, and G . A. Richter

20. Treatment and Disposal of Potato Wastes 761 r

Screening (Pretreatment)

!

Screening is the first step in most treatment systems and is the most economical method of removing large solids. Screening protects other treatment units from plugging or damage and reduces the size of other solids-handling units. The solids removed are relatively dry and can be disposed of with comparative ease. Screening is often used to remove larger pieces so that the water can be reused within the processing plant. Three types of screens are commonly used: vibratory screens, rotary screens, and stationary gravity screens. Many screening devices have been custom-built for individual plants, but in most cases, standard manufactured units are more satisfactory. These units are similar to

i

I

Ij I I

j ,

!

i

Fig. 20.6.

i

! I I

iI I, I

iI

I

Rotary screen used in primary treatment.

(Courtesy CH2M Hill.)

screens used in dewatering products during processing. Mesh size normally is 20 to 40 per inch (Figs. 20.5 and 20.6). The question of the elevation and location of the waste screen is of considerable importance. In one design, plant wastewaters are collected in a sump pit below the floor level of the plant, from which they are pumped to the screen. The screen is elevated so that the solid wastes may fall by gravity into a suitable hopper. From here, the water flows by gravity into the primary treatment equipment or to the sewer. Another method is to locate the screens below the level of the plant drains if the elevations permit. After screening, the solid waste can be conveyed up to the waste hopper and the water pumped into the clarifier, or other disposal system.

Primary Treatment Fig. 20.5. DSM screen for dewatering plant effluent-45 Oliver, Inc.)

unit.

(Courtesy Door-

In the past, the removal of floatable and settleable solids from foodprocessing wastes was frequently done in a batch process. Two tanks

762

R. E.Pailthorp, J. W. Filbert, and G. A. Richter

20. Treatment and Disposal of Potato Wastes 763

would be provided which could be operated on a fill and draw schedule. Modern treatment systems, however, use continuous flow-through tanks, called clarifiers, of either rectangular or circular construction (Fig. 20.7). The required geometry, inlet conditions, and outlet conditions for successful operation of such units are known. Clarifiers are fitted with mechanical scraper mechanisms, which collect the solids that settle to the bottom or float to the top so that they can be removed from the tank easily and continuously for further processing. Figure 20.8 shows a cross section of a typical circular clarifier. Construction materials and methods will vary because of local conditions, preferences, and costs. In the primary treatment of potato wastes, the clarifier typically is designed for an overflow rate of 800-1000 gal/ft2/day and a depth of 10-12 ft. Most of the settleable solids are removed from the emuent in the clarifier. Generally, this primary treatment results in a decrease of 40-70% of the COD. To reduce the volume of the settled plant waste, which normally contains about 6% solids, some form of concentration is employed.

P L A N T EFFLUENT -*

CLARIFIER 800 G A L L O N S , PER S Q U A R E - F n n l PER D A Y

. --.

/SCREENS

. a \

T R E A T E D F L O W TO S T R E A M OR SECONDARY TREATMENT

F L O W MEASURMENT

C E N T R A T E OR

V A C U U M FILTER OR CENTRIFUGE SOLIDS 15. TO 20% J

Fig. 20.7.

Primary treatment schematic diagram.

\

\

WEIR

_.

SOLIDS WITHDRAWAL

Fig. 20.8.

WCRAPE HOPPER

MOVESOLIDS TO HOPPER

Primary clarifier.

Belt-type vacuum filters are used for this purpose. Additional dewatering of the underflow by pressing has not been successful. The withdrawal of the underflow from the bottom of the clarifier is accomplished by pumping. The solids that result from caustic peeling will, of course, have a high pH. The optimum pH level for best vacuum filtration of solids has been found to vary considerably from plant to plant. However, when the underflow withdrawal is adjusted to hold the solids in the clarifier for several hours, biological decomposition will start and the pH of the solids will be lowered substantially. A t a pH of between 5 and 7, these solids will dewater on a vacuum filter without the addition of coagulating chemicals. It was originally thought that both lime and a ferric salt would be necessary to condition the solids from caustic peel plants before they could be dewatered successfully on vacuum filters, but they did not prove necessary. The solids produced as a result of steam or abrasive peeling operations also undergo biological degradation in a few hours. If this decomposition proceeds too far, the solids will be difficult to dewater. A very low pH in the clarifier as a result of biological activity will cause damage to the structures and equipment. Agitation and internal mixing within the clarifier caused by excess fermentation and accompanying gas evolution greatly decrease the efficiency of separation. Control of the treatment process, especially the sludge withdrawal rate, must be maintained to prevent these problems. Flotation is another method of solids removal. This process uses fine

764 R. E. Pailthorp, J. W. Filbert, and G. A. Richter

gas bubbles, which are caused to form and attach to suspended solids. The gas bubbles float the solids to the surface of the tank where they are skimmed off by mechanical collectors. Several commercial units of this type are available.

Secondary Treatment Several biological systems are used to provide secondgry treatment (Fig. 20.9). In all cases, the secondary treatment units must provide an environment suitable for the growth of biological organisms, which do the actual work of waste treatment. Some of these treatment units depend on a sufficient supply of oxygen to support aerobic decomposition of the organic matter. Aerobic biological decomposition is practically odorless and is capable of very high removal of the organic matter contained in wastes. In some cases, anaerobic systems may be used for secondary treatment.

Fig. 20.9.

Secondary waste treatment facility. (Courtesy R.T. French Co.)

20. Treatment and Disposal of Potato Wastes 765

Most of the full-scale potato waste secondary treatment systems have been constructed since 1968 although considerable research of a pilot-plant scale had been conducted prior to that time. The R. T. French Company took one of the first steps with a full-scale federally supported project to demonstrate activated sludge treatment of their potato division waste in Shelley, Idaho. Since the R. T. French project, many other potato processors have installed biological treatment systems or land disposal irrigation systems. The unit processes described in the following sections differ in the method of providing the environment necessary for the biological action that occurs during secondary treatment. These various processes are the principal means of achieving secondary treatment. Data on a number of pilot-scale and full-scale secondary treatment designs are presented in Tables 20.3 and 20.4. Activated Sludge Process. Waste is discharged into large aeration basins into which atmospheric oxygen is diffused by releasing compressed air into the waste or by mechanical surface aerators (Fig. 20.10). The environment thus created is favorable to the growth of a heavy concentration of bacteria because of the presence of abundant organic food supply and oxygen. The organic content of the waste is removed by the life processes of the bacteria and stored within the bacterial mass as protoplasm. The bacterial mass, termed uctiuuted sludge, is then removed in sedimentation basins, thus providing highly treated eflluent. A diagram of a n activated sludge system is shown in Fig. 20.11. There are many variations of activated sludge processes; however, all operate in basically the same way. The variations are the result of unit arrangement and methods of introducing air and waste into the aeration basin. Biological Filters. Waste is distributed over filter beds constructed of rocks 3-4 in. in size, plastic media, and wood media. Atmospheric oxygen moves naturally through the void spaces in the filter material. In the environment thus created, biological slimes, consisting mainly of bacteria, flourish and colonize on the rock surfaces. As the waste trickles over the surface of the biological slime growths, removal of organic matter is accomplished. As the slime growths become more and more concentrated, their attachment to the media surface is weakened and the biological growth is washed from the filter. The pro-

Table 20.3. Characteristics of Various Pilot-Scale Secondary Treatment Designs Treatment process and process modification Complete mixing activated sludge

Type of process water

-

Hydraulic Organic loading removal (detention time) (8) Remarks 4 days 50-60 COD No nutrient added

Daily organic loading

2-4 days Complete mixing activated sludge

Potato starch

Complete mixing activated sludge Complete mixing activated slud e Contact stabhzation activated sludge

Lye peel

Oxidation ditch activated sludge Hi h rate trickling iGrs Super-rate biofilter

Lye peel

Anaerobic lagoons heated to 50°C and completely mixed Biological filter

15 h r

1000 Ib ~ . . .. MLSSH/hro 191-358 lb BOD/ 1000 ft3 200-400 lb BOD/ 1000 ft3

98 BOD

8 hr min

No nutrients added; Atkins and Sproul no pH adjustment (1964) Sproul (1966A)

1-1.5 hr contact and 6-8 h r reaeration 3 days

80 COD

pH adjustment

-

-

Potato starch

70 lb BODllOOO ft3 158 gal/ft*/day

Corn process water Simulated secondary potato process water Lye peel ,

72-123 lb BOD/acre 25 lb COD/1000

ft3

-

20 COD

Odor problems

Anderson (1961)

-

60 COD

2H2M Hill (1969)

70 BOD >40 COD

Problems maintaining solids in system Not fully acclimated Avg. liquid depth = 5ft Steam peel oper.; little odor; liquid depth = 3 ft Steam peel oper.; offensive odors; liquid depth = 3

Lye peel Lye peel

58 lb BODllOOO ft3 29 Ib BODllOOO ft3

-

B1-level & anerobic pond B1-level pond with aerator

Potato waste

111 BOD5

105 days

pond

Porges (1963)

:H2M Hill (1966)

Anaerobic filter Anaerobic pond

B1-level

Odor problems

Nutrients added

Lye peel

112 lb BOD/1000

-

91 BOD

1 day 5 days 15 days

Complete mixing activated sludge Anaerobic contact

Lye peel

ft3

Potato process + 140 BOD5b domestic waste Potato process + 300 BOD5b domestic waste

NO nutrients added; Sproul (1966A) filter clog ed Buzzell et al. (1964) Nutrients acfded Hatfield et al. (1956)

20 hr

-

ft3

Pasveer (1966)

-

-

BOD/1000 ft3 140 lb BOD/1000

90+ BOD

Atkins and Sproul (1964)

Nitrogen added; pH ndcIntosh and McGeorge 65+ in (1964) winter adjusted .. - -. .-. Ilay and Furgason pH adjusted 50 COD (1965) 70 COD 85-90 COD :H2M Hill(1966) Nutrients added 75 BOD

40 Ib BODllOOO ft3

Less than 400 lb

95 BOD

Recycle 5:l and 94 BOD 1O:lc

ft3

Buzzell et al. (1964)

-

-

159 ib BODllOOO

49-82 COD pH adjustment and nutrients added 95+ BOD No nutrients added

Anderson (1961)

6 hr min

Lye peel

Corn process water Lagoons-treatment Potato with domestic sewage Anaerobic lagoons Potato

Anaerobic lagoons

Less than 80 lb/

Reference

-

-

ft Source: Prepared by CHZM Hill, Corvallis, Oregon. a MLSS = mixed liquor suspended solids in aeration basis. b Lb/acre/day c Filter recirculation ration = number of times flow passed through filter.

2H2M (1970) 2H2M Hill (1970) Porges (1963) 3lson et al. (1964) IOlson

et al. (1964)

Table 20.4. Characteristics of Various Full-Scale Secondary Treatment Designs Treatment process and roCess modiication

Type of process

water

Daily organic loading

Hydraulic loading (detention time)

Organic removal (%)

Remarks

Reference

~~

Complete mixing activated sludge Complete mixing activated sludge Complete mixing activated slud e Multiple a e r a t e f lagoons Anaerobic pond and lye peel activated sludge Trickling filters (2 stages) Activated sludge and lye peel aerated lagoons

Dry caustic peel 32-39 IbllOOO ft3

2 days

73 BOD

During sludge bulking

Lye peel

28-84 lb/1000

1-2 days

70-90 BOD

Lye peel

60-180 IbllOOO

14 h r

87 BOD

Lye peel

3-6 lbl1000 ft3 in aerated lagoons

Removal varies with sludge bulking Slud e bulking will refuce removal Algal blooms will reduce removal

R3 ft3

25-80 IbllOOO R3 to activated sludge Dry caustic peel First stage-2060 lb/1000 ft3 60-150 IbllOOO ft3 in aeration basin 55 lblac in aerated & 8.%?2 in aerobic lagoon

16-20 days in ae98 BOD rated lagoons 105 days in aerobic lagoons 1 day 95 BOD

14 h r in aerated basin 52 days in aerated lagoons 60 days in aerobic lagoon

Slud e bulking will r e h c e removal

EPA (1973)

EPA (1973)

EPA (1973)

85 BOD Cold temperature will Landine and (both stages) reduce removal Dean (1973) 99 BOD Slud e bulking and al- EPA (1973) blooms will reuce removal

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