traitement patates

8/8/2019 traitement patates

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CHARACTERIZATION AND DEVELOPMENT OFTREATMENT ALTERNATIVES FORPOTATO PROCESSING WASTEWATERDon Richard, ScDChairmanDepartment of Civil EngineeringNorth Dakota State UniversityFargo, North Dakota 58105

Dennis Mathiason, PhDChairmanDepartment of ChemistryMoorhead State UniversityMoorhead, Minnesota 56560

ABSTRACTWastewater generated in the processing of potatoes forfrench fries creates several interesting treatment problems.The processor has a wide choice in treatment techniques.These range from simply treating all waste flows generatedto a more practical approach attempting to separate flowsand reclaim or reuse byproducts.The case study reported herein has a long history ofwastewater treatment problems. Generation of odors,incomplete BOD5 reduction and high nitrogen content in theeffluent have all caused problems for the potato processor

due to undersized and inefficient treatment processes.This paper presents some of the problems encountered intreating the wastewater, describes an in-house wastewaterflow monitoring study, discusses a bench scale treatabilitystudy and finally presents several options for possibletreatment.From pilot treatability studies, heated and mixed

andanaerobic digestion was found to effectively reduceif followed by aeration and storage, the wastewater cou d beland applied to allow nitrogen incorporation and uptakethrough land application.


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PROBLEM DEFINITIONThe potato processing plant makes frozen french friesfrom locally grown potatoes. Wastewater flows are generatedin all potato production operations and are generallycombined to flow to the wastewater treatment system. Thewastewater treatment process has never operated

satisfactorily and has generated very obnoxious odors whilenot adequately reducing organic loading and ammonia. Thepotato processor has worked very hard over the last severalyears to improve the wastewater operation althoughunsuccessfully. Wastewater flows have been greater thananticipated due to increased potato production and this hasfurther added to the treatment problems as the lagoontreatment system has not been expanded.IN-HOUSE STUDY

An in-house monitoring study was conducted to develop abetter understanding of the waste streams being generated byeach of the potato processing operations. Wastewater flowsand characteristics were determined for the following plantoperations:

FlumingPeelingCutting and slicingBelt and equipment spraysBlanchingFloor and equipment washdownRefrigeration condensation and defrostBoiler blowdownThe study was carried out by sampling each waste streamand simultaneously measuring the corresponding wastedischarge rate. Samples collected were analyzed forChemical Oxygen Demand (COD), Total Suspended Solids (TSS)Total Kjeldahl Nitrogen (TKN) Phosphate (PO4-P) I pH,Chlorides and temperature. Grab samples taken three timesduring the daytime operation were composited to allowanalysis.Figure 1 shows a layout of the potato processingoperation. The diagram indicates the flow path of the

potatoes as they are sluiced from the warehouse to thebarrel washer and then on to the processing operation.


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P A I T S C L I CI cR OOM

P O T A T O P R O C ES S IN G P L A N T L A YO U TFIGURE I

For the monitoring study, samples were taken at thenumbered locations shown in Figures 2 and 3. The diagramsare schematic and show the waste streams from the severalprocess operations being discharged into the floor flumeswhich direct the wastewater to the sump at the end of theoperating floor.8 R U S U T R I MSC R U EIER T I O L ES T E A M ( 6 1 1101PEEL ER WASMER

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Figures 4 and 5 reflect the results superimposed onthe operational schematic. The flumes allow water torecirculate for continuous cleaning operation. The diagramsprovide an understanding of the impact of the variousoperations. Table 1shows typical analytical results fromthe monitoring study.


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TABLE 1P O T A T O PLANT S N D YANALYTICAL RESULTS

P- 1P- 2P- 3P-4P- 5P-6P-7P- 8P-9P-10P-11e-12P-13P-14P-15P-16P-17P-18P-19P-20P-21P-22P-23P-24P-25sP-25UP-265P-26UP-27P-28P-29P-30sP-30U

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3 4 05 2 0 043803 4 8 0

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9 4 0 78 3 2 98 1 1 36 9 4 78 3 1 7

1 9 5 7 3203441 3 8 5 01 8 3 3 91 5 2 7 6

9 4 3 1356948247 9 6 99 2 5 9

1 5 1 1 62 1 2 4 01 9 9 4 41 1 1 7 6

3 8 8 61 2 0 1 99 3 3 53 8 8 660666 5 9 15 1 1 24884380229574 9 8 09 3 7 9

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AVEIUGE TMPEIUTUFES AT PLANTP U N T 7 6 FGRIT " 4 B E R 55 FANAEROBIC LAGOON 6 7 FAEROBIC U t O O N 4 6 F

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3 . 0 6 . 6 5 24 6 . 0 6 . 7 1 0 6

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The in-house study allowed the company an understandingof the types and quantities of wastes being generated duringnormal operation. The major waste streams were inconjunction with the steam peeler, the scrubber brushes andthe washers. The wastes generated in these operations werenot only high in COD but used a large quantity of water totransport the wastes. From this monitoring study it wasfound that about 4 2% of the waste load (COD) was beinggenerated at only three operations.Other wastes generated throughout the overall operationand their normal disposal process are defined following:Raw Storase. Solid materials, consisting primarily ofdirt, rocks and vines, are manually or mechanicallyremoved by eliminator chains while piling potatoes intothe storage warehouse. This material is hauled back tothe fields to fill low spots.


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Silt Removal (Grit Chambers). Solids, primarily sandand dirt which settle to the bottom of the pumping pitin the receiving area, are pumped into large settlingpits to allow the dirt to settle and the water toreturn to the pit. When the pit is full of dirt andthe water has drained, a payloader is used to load thedirt into dump trucks to haul to low spots in thefields.Peel Waste. This material consists primarily of cookedpotato tissue and is about 15-20% solids. The greaterportion is captured for sale as cattle feed. Thescrubbing equipment that removes the peel dischargesinto a pump which transports the slurry into overheadbins. These bins discharge into trucks which haul thepotato waste to a cattle feeding operation.Trim/InsDection Waste. This material includes anyportions of potato that are discarded as unacceptablefor the process. They include primarily: vines, stems,bruised portions, rot, green, and diseased parts orwhole potatoes. They are disposed of by depositing infloor gutters. Theses gutters carry material inrecycled water to the pump room where they pass over a4 0 mesh screen. The solids drop into one of theoverhead holding bins in the pump room. This bin alsodischarges into the same truck as the peel slurry. Itshould be noted that all materials entering the floorgutters are screened in this fashion.Starch Removal. The cutting operation frees somepotato starch which circulates with the recycled water.This starch is currently removed by a cyclone systemwhich deposits the starch in overhead totes in the pumproom. The water drains back into the floor pit and thestarch is periodically dumped into the same cattle feedtrucks with the trim waste and peel waste.

WASTEWATER TREATMENTThe wastewater treatment system flow diagram is shownin Figure 6. The 5 million gallon, lined, unheatedanaerobic pond is Styrofoam and straw covered. The covereffectively reduces heat loss, even in the winter months.

An approximate loo F loss is observed across the first pond.This pond overflows to a second, plastic-lined pond whichcontained 10-25Hp floating aerators. The second 5 milliongallon pond was initially intended to be aerobic, but due tothe heavy organic loading the aerators were inadequate foreither aeration or mixing. Water which cannot be directlyland applied is to be held in a large storage pond ( 2 0acres).


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P q O C E S S W A S T E U A T E R - 1IS L U I C E W A T E R G R ~ T IR EM O VA L

T R E A T E D W A T E R TOL AN D A m C A T l O N S T O R A G EA

W A S T E W A T E R T R E A T M E N T FL O W D I A G R A MF I G URE 6

At the time of the study, the wastewater coming fromthe potato processing had a BOD5 averaging about 5000 mg/lat a flow of over 250,000 gallons per day. This 10,425pounds of BOD per day would be equivalent to a population ofaround 61,000 persons. This high level of organic loadingrequires a significant treatment effort. High nitrogencontent in the water requires careful land applicationtechniques and close monitoring in order not to affect theground water. Light soil conditions in the wastewaterapplication areas permit rapid water movement vertically toa rather shallow groundwater table.

Water for land application is withdrawn from thestorage pond. Winter application was not originallyanticipated, but the potato processing exceeded proposedoperation and much excess water was generated. Monitoringof porewater and groundwater was necessary to insure thatnitrate limits as established by the regulatory agency werenot violated.

The wastewater treatment system never functioned asinitially designed due to heavy loading, faulty equipment,cold temperatures, and many other problems. Severalvariations for operation were tried to get the plant to meetthe demands placed upon it, but to date all have beenunsuccessful. Figure 7shows typical BOD removal acrossthe existing anaerobic/aerobic system. The anaerobic pondappears to be only operating as a large settling pond.BOD analysis of lysimeter water taken from the sprayfields yield a value of less than 10 mg/l at the five footsoil depth. Thus, ultimately, a very successful BOD removaloperation.


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I 1 A V E R A G E 1'""1J 1

P L A N T A N A E R O B I C A E R W I C S T O R A GEEFr LFF. E F F P O N O

8005 REMOVAL ACROSS TR EATMEN T SYSTEMFIGURE 7

TREATABILITY STUDIESInitial characterization showed the wastewater to behigh in COD, suspended solids and TKN while normal inchlorides. The water going to the anaerobic pond was foundto have a pH of about 5 to 5.5. Concerns for treatabilityof this type of wastewater led to laboratory bench scaletesting for COD/BOD reduction by anaerobic digestion.Several reactors holding about 50 liters each were setup in the laboratory to simulate the anaerobic pond. Thefbllowing****

conditions-were established:detention time 10 days (same as in field)reactors at room temperaturepH not adjusted in reactor onepH adjusted in reactor two

The data shown in Figure 8 indicates a low level ofefficiency for the reduction of COD. The pH buffered wasteshowed a slightly better COD reduction than did the non-buffered waste stream. However, in neither case was theremoval as high as had been anticipated or hoped for.

Figure 8 does show a slight improvement in CODreduction and further analysis indicated a similar reductionin the volatile acid-alkalinity ratio. It was suspectedthat this improved treatment efficiency was due to anuncontrolled increase in the laboratory room temperature.The wastewater temperature increased from about 72OF toabout 86OF.


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L A B R E A C T O R RESULTSFIGURE 8

To study the effect of temperature and mixing on theanaerobic digestion of the potato wastewater, a reactor wasset up in a constant temperature chamber at about 95OF. Thereactor, which had a volume of about 20 liters, waspartially mixed by recycling the reactor materials at a rateof about 100 ml per minute.The reactor was fed daily with two ( 2 ) liters of rawunbuffered wastewater. By removing the two ( 2 ) liters perday, a hydraulic detention time of about 10 days wasachieved.The COD reduction data due to heating and mixing thewastewater are shown in Figure 9. After only three weeksthis reactor was showing about an 80% reduction in COD. Infive weeks the reduction in COD exceeded 90%. The VA/ALKratio showed similar change.Through these simple laboratory tests the waste wasfound to be ameanable to controlled anaerobic treatmentwhich could be followed by aerobic treatment for polishingbefore land application.


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I300HEATED AND MIXED LA B REACTOR

FIGURE 9

TREATMENT OPTIONSOnce it has been determined just exactly what is to beaccomplished several treatment alternatives are available.The company has the responsibility to dispose of thewastewater and other byproducts without creating odors andwithout causing detrimental effect to the surface orgroundwater of the area.The simplest alternative would be to land apply thewastewater directly as it comes from the processing plant.

BOD5 and other nutrients could be taken up by the soil andplant growth if the wastewater is applied at an agronomicrate. Winter time application could be used, but sincelittle BOD5 reduction would take place and ammonificationwould be limited, the potential for groundwatercontamination and springtime odors would probably restrictthis alternative.Therefore, if storage for four or five months is

required, treatment for BOD5 reduction to control odorsbecomes essential. Aerobic or anaerobic treatment processesbecome options for consideration.Aerobic treatment of the raw effluent would requireintensive energy input especially during cold weather.Anaerobic treatment, properly controlled, was found to bemost effective in reducing BOD5 to a level that would allow


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aerobic treatment to be cost effective for polishing beforestorage.Figures 10 and 11 show two possible treatment processflow diagrams. A unique feature of the anaerobic contacttreatment scheme is the control of temperature in theanaerobic digester through the use of waste steam to heat

the below ground digesters. This would be a beneficial useof steam which presently is released to the atmosphere.Mixing or recycling of the flow would help to reduce shortcircuiting within the digester. To further reduce the BOD5levels, controlled aeration could be utilized either acrosstrickling filters, aerated ponds or oxidation ditches.Innovative BOD reduction could also convert ammonia tonitrate for bet er land application results.CONCLUSIONS

It can be concluded that potato processing wastewatersare strong and difficult to treat. Before selecting atreatment process, as much information as possibleconcerning treatability should be understood. The potatoprocessing operation itself should be studied in an effortto reduce the pollutant load. In this case study, about 4 2percent of the BOD5 generated came from the peeling andscrubbing operation. If these wastes can be isolated andremoved from the wastewater stream the resulting effect isdramatic in relation to wastewater treatment.

The wastewaters generated in this study were found tobe effectively treatable anaerobically when maintained atthe proper pH, temperature and mixed. A simple hole in theground, covered and called an anaerobic digester just is notsufficient.Treatment of this or any wastewater requires anunderstanding of the character of the waste product if weare going to design, construct and operate effectivewastewater treatment systems.


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