PROCESS DESIGN FOR TREATMENT OF CORN WET MILLING WASTES

H. 0. Bensing* and D. R. Brown**

INTRODUCTION

Starch has been manufactured from corn commercially f o r more t h a n 100 years. recovered; a l l the other components of the kernel were discarded.

In the early days of the industry, only starch was

Toward the end of the 1 9 t h century the corn millers began t o realize t h a t t e non-starch fraction of corn had value as an animal feed.(l! Later, a process f o r separating germ and recovering the corn oi l was developed. Improved methods f o r steeping the corn permitted evaporation of the s teepwater, and recovery of the solubles as part of the animal feed. By the beginning of the 20th century, practically a l l of the corn kernel was being Recovered, inc luding a large fraction of the solubles.

There remained, however, one large source of l i q u i d waste. used fo r washing solubles from the starch was sewered. solubles, amounting t o about 2% of the corn, must be removed to obtain the best quality starch and syrup products.

Water These

CLOSING THE LOOP

For many years this wash water stream was considered too d i lu te to recover. plant of Corn Products Refining Company (now CPC International, Inc.), i n cooperation w i t h the Chicago Sanitary District, which resulted i n a proc ss i n which the wash waters were used fo r steeping the corn.72, 3) The resulting "bottled up" wet milling process (Figure 1 ) i s now standard throughout the industry.

However, a study was s ta r ted i n 1920 a t the Argo

Corn i s steeped by soaking i n process waters. water is evaporated t o concentrate the solubles, and can be used as an additive t o the animal feed products, or so ld separately as a l iquid c a t t l e feed o r a nutrient f o r ant ibiot ic fermentations.

The resulting

*CPC International, Inc., Industrial Division, Pekin, I l l i no i s . **CPC International, Inc., Industrial Division, Moffett Technical

Center, Argo, I l l i no i s

277

Corn 4

>I Steeping . t--d Evaporation 1 i- ---

S L teepwater

I L----

Fiber V Corn Gluten Feed

> Washing

r--R9b I I L----, Starch-Gluten Hi gh Protein

Separation Gluten Meal > de-. 0.I-

Fresh Water 1

Starch Drying

Modi f i ed Starch

Corn Sugar and Syrups

Figure 1. Corn Wet Milling Process

278

After steeping, the corn is mildly milled, t o break open the kernel, and release the germ w i t h as l i t t l e damage as possible. i s separated from the other components of the kernel w i t h hydroclones, and then dried and processed t o extract the corn o i l . germ is added to the animal feed.

The germ

The o i l f ree

After separating the germ, the remainder of the starch and gluten is released from the hulls. from the hulls by countercurrent washing and screening. are dried, and combined w i t h the concentrated steepwater and the o i l free germ from previous steps to make corn gluten feed.

The mixture of starch and protein t h a t were separated from the hulls are processed t h r o u g h centrifuges and a ser ies of small diameter hydroclones. as well as removal of solubles from the starch.

The free starch and g lu ten a re separated The hulls

T h i s resul ts i n separation of the starch and protein,

The protein fract ion, referred t o as corn gluten, is concentrated, f i l t e r ed , and dried. protein additive t o feeds f o r broilers.

Fresh water is added t o the hydroclone s ta t ion , a t the rate of about 12 t o . 15 gallons per bushel of corn processed. A s ignif icant portion of this water consists of condensates recovered dur'ing evaporation of finished products.

The product is principally used as a h i g h

The finished starch product, s t i l l i n slurry form, may be d r i e d direct ly as an unmodified starch, or treated by various chemicals t o make modified starch. Part of the starch stream is hydrolyzed w i t h acid, enzymes, or a combination of acid and enzymes t o produce corn syrup o r dextrose.

WASTE SOURCES

The process shown i n F i g u r e 1 is usually referred t o as the wet starch process. The only waterborne waste from the wet starch process is the condensate result ing from the evaporation of steepwater. The condensate contains volatiles which are formed during the steeping process, and are vaporized dur tng evaporation.

The sources of other l i q u i d wastes vary w i t h i n the wet m i l l i n g industry, depending on the products made and the processes used. Typically, i n add i t ion t o the volatiles, the waste stream might contain f i l t r a t e s from the preparation of modified starches, w i t h dissolved chemicals used f o r modification, and Some soluble carbohydrate formed during the process. Another source of waste is the impurities removed dur ing the refining o f corn syrups and dextrose.

279

These volati les and carbohydrates exert an oxygen demand when discharged t o a stream or t o a municipal treatment plant. has been recognized for many years, and various treatment m thods

This

have been developed by members of the wet milling industry. f 4, 5 , 6)

WASTE CONTROL

CPC International operates four wet milling plants i n the United States. treatment plants. direct ly to waterways. As awareness of the envi ronmental effects of d i rec t discharge increased, i t became evident that t h i s practice could not continue.

Two of these pay fo r treatment o f t he i r wastes i n municipal In the past , the other two discharged waste

A consulting engineering company was retained to make a waste survey a t one of our plants, i n order to determine the requirements t o meet existing effluent standards. preliminary biological treatment design, based on the i r experience with industrial wastes, and the results of the survey.

The consultants prepared a

CPC International management was concerned about the r isk involved i n ins ta l l ing a treatment process w i t h no experimental da t a t o

I t was f e l t t h a t maximum treatment efficiency would resu l t i f specif ic treatment data were obtained f o r the waste stream.

. support the design.

The waste summary also indicated tha t manufacturing process improve- ments might significantly reduce the waste load t o be treated. Therefore, i t was decided t o study the t rea tab i l i ty of the waste stream, and a t the same time to conduct a vigorous waste load reduction program. The waste load reduction was an important and integral part of the over-all pollution abatement program. BOD o f the waste stream was reduced t o about one-half of i ts original value by isolating sanitary wastes and sending them t o the municipal treatment plant ; by instal la t ion of new process control instruments in cri t ical areas; by operator and supervisor training regarding process 1 osses supported by an extensi ve waste stream monitoring system; and by abandoning an intermittently operated process w h i c h generated a s ignif icant waste load.

TREATABILITY STUDIES

I t was expected that the carbohydrates and volati les in the waste stream would be readily biodegradable. program was limited to study o f the biological treatment process. The biological process u t i l i zes a mixed bacterial culture in contact w i t h the waste stream, t o convert soluble organics to insoluble bacterial ce l l s .

Therefore, the experimental

2 80

lVumerous va r ia t i ons o f the b i o l o g i c a l process have been devel oped. Our consul tant recommended t h a t the experimental program be 1 i m i t e d t o eva lua t i on of aerobic processes.

The experimental work was conducted under the guidance f i r s t o f A. W. Busch (then a t Rice Un ive rs i t y ) , and l a t e r W. B. Davis ( fo rmer ly a t Texas A & M Un ive rs i t y ) .

It was decided t o study two v a r i a t i o n s o f the aerobic b i o l o g i c a l process. i s simply a lagoon where oxygen i s supp l ied t o the mix tu re o f waste and cu l tu re , fo l lowed by a quiescent pond where the biomass i s al lowed t o s e t t l e . No biomass i s recyc led t o the aera t ion tank. company, and was described by McIntosh and McGeorge.(6) This process requi res very l i t t l e operator a t ten t i on , b u t has the disadvantage t h a t there are no operat ing con t ro l s t o compensate f o r va r ia t i ons i n waste c h a r a c t e r i s t i c s o r concentrat ion. A l a rge land area i s a lso requi red.

One was an aerated lagoon and s e t t l i n g process, which

This method o f treatment has been used by one wet m i l l i n g

The o the r system s tud ied was the completely mixed ac t i va ted sludge process.

.by a g r a v i t y c l a r i f i e r . tank can be c o n t r o l l e d by r e c y c l i n g c l a r i f i e r underflow. advantage o f t h i s method o f treatment i s t h a t opera t ing condi t ions can be c o n t r o l l e d by ad jus t i ng the biomass recyc le ra te .

This consis ts o f an aera t ion tank, usua l l y fo l lowed

The Biomass concentrat ion i n the aera t ion

EXPERIMENTAL RESULTS - AERATED LAGOON

The aerated lagoon process was s tud ied on a p i l o t p l a n t scale. The aera t ion tank volume was 35,000 gal lons, equipped w i t h a 5-hp sur face aerator . The s e t t l i n g pond was about the same s ize .

I t was found t h a t a s a t i s f a c t o r y e f f l u e n t could be obtained a t r e t e n t i o n times as low as 4 days. was obta ined i n the s e t t l i n g pond, although occas iona l l y i n warm weather clumps of sludge f l o a t e d t o the surface.

Pro jec t ions o f t he s i z e o f t he f u l l sca le treatment process showed t h a t s u f f i c i e n t l and was n o t a v a i l a b l e a t the p l a n t s i t e . no land was ava i l ab le adjacent t o the p l a n t s i t e , an expensive pumping and p i p i n g system would have been required. i t was decided t h a t the ac t i va ted sludge process would be the most economical f o r t h i s loca t ion .

S a t i s f a c t o r y biomass separat ion

Since

As a r e s u l t ,

281

EXPERIMENTAL RESULTS - ACTIVATED SLUDGE

Activated sludge treatment was evaluated o using techniques described by A. W . Busch.?7! The equipment i s diagrammed i n Figure 2. designed t o develop a culture acclimated to the waste under continuous flow conditions. After several weeks of continuous operation a t the selected conditions, the capacity of the culture for removal of soluble COD from the waste was determined by a batch t e s t as described by Busch. 7)

laboratory scale

The laboratory reactor studies were

The batch t e s t consists of aeratlng a mixture of waste and acclimated culture. t e s t , and analyzed for soluble COD and suspended solids. results are plotted as shown i n Figure 3. depletion curve, and corresponding values of suspended so l ids , are used t o prepare a u n i t ra te o f removal curve. of a u n i t rate of removal curve is shown i n F l g u r e 4. curve shows the rate of COD removal that can be obtained fo r any eff luent COD. aeration tank volume for the desired eff luent COD can be calculated. A ser ies of these curves was obtained d u r i n g the experimental

‘program, and the most conservative values were used f o r the final design.

Samples are taken frequently dur ing the The

An example

The slope of the COD

This

By selecting an MLSS concentration, the

Biomass growth ra te was estimated by material balance calculations from operation of the continuous laboratory reactors. The growth rate was correlated w i t h food to mass r a t io (F/M) as shown i n Figure 5.

Oxygen uptake rates were also determined from the continuous reactors. The a i r supply was s h u t o f f , the oxygen depletion measured, and correlated with F/M. Figure 6.

The batch test resul ts , together w i t h the biomass growth rate and oxygen uptake curves, were used t o design the aeration tank fo r the treatment plant.

These results are shown in

TREATMENT FACILITIES

The flow diagram of the treatment plant t h a t was designed from the laboratory data i s shown i n Figure 7. Some of the waste streams are discharged batchwise, so an equalization tank i s used to blend out variations i n flow, concentration, and pH. The equalization tank also provides some surge capacity fo r accumulation of the waste i n case o f equipment fa i lure , and as a source of supply t o maintain the culture d u r i n g weekends, when l i t t l e waste i s discharged from the manufacturing area.

282

To A s D i ra tor

Solenoid Substrate Valve

1 ) Reservoir

t 1 7

6

Pe

1

2 Liter Percolator

Li ter

rcolat

A i r Diffuse

Effluent Storage

x

rs

A Rotameter

rimer - / \

- P unlp I Waste Mi xed Liquor

Figure 2. Schematic Drawing of Laboratory Activated Sludge Reactor

,283

c \ P) E n

S 0

C, a L C,

.I-

S 0 u S 0 0

1200

1000

800

600

400

200

0 1 2 3 4 5

Time, Hours

Figure 3 . Typical Batch Test Data

284

100 200 300 400 500

Residual Biodegradable COD, mg/l

Figure 4. Typical Unit Rate o f Removal Curve

285

1 .o

0.8

0.6

0.4

0.2

1 2 3

l b Soluble COD Supplied/lb MLSS/Day

Figure 5. Biomass Growth Rate

286

60

40

20

0

Continuous Reactor Data

1 2 3

lb Soluble COD Supplied/lb MLSS/Day

Figure 6. Oxygen Utilization

287

h, 03 03

Treated P Disso lved A i r F l o t a t i o n E f f l u e n t

G r a v i t y [ I C l a r i f i e r

t Excess Biomass

F igu re 7. Treatment P l a n t Flow Diagram

Flow from the equa l i za t i on tank goes t o two aera t ion tanks i n p a r a l l e l . The raw waste temperature can get as h igh as 140°F, so some o r a l l o f the aera t ion tank supply can be d i rec ted over a coo l i ng tower, t o mainta in the design temperature o f 75°F i n the aera t ion tanks. F ixed mount sur face aerators are used t o prov ide the oxygen.

E f f l u e n t from the aera t ion tanks i s pumped t o a c i r c u l a r g r a v i t y c l a r i f i e r . o f the c l a r i f i e r .

Recycle and excess biomass are pumped from the bottom

Overflow from the c l a r i f i e r goes t o a d isso lved a i r f l o t a t i o n c e l l , be fore discharge t o the stream.

The treatment p l a n t f l ow sheet i s q u i t e conventional. The d i f f e rence between t h i s and many o the r i n s t a l l a t i o n s i s t h a t the process was designed f o r a s p e c i f i c waste, using r e l a t i v e l y new labo ra to ry methods f o r ob ta in ing the design data f o r a completely mixed ac t i va ted sludge process. I n recogn i t ion o f t h i s , the U.S. Environmental P ro tec t i on agency awarded CPC I n t e r n a t i o n a l a cons t ruc t ion and demonstrati on grant . Cer ta in e l i g i b l e demonstration costs were funded t o a maximum o f 70%, together w i t h a f i x e d amount f o r const ruct ion, based on 10% o f the estimated cons t ruc t ion cost. The maximum award was equ iva len t t o about 27% o f the t o t a l e l i g i b l e est imated cons t ruc t ion and demonstration costs. f a c i l i t i e s were n o t p a r t o f e l i g i b l e costs. TREATMENT PLANT OPERATING RESULTS

The t reatment p l a n t has been i n f u l l operat ion f o r about 6 months. The r e s u l t s have shown t h a t the r a t e o f so lub le COD removal c l o s e l y matches t h a t p red ic ted by the labora tory tes ts . Oxygen uptake a l so was c lose ly predic ted. P o s i t i v e d isso lved oxygen concentrat ions are maintained up t o the maximum design load ing o f the treatment p lan t .

I n p l a n t sewer changes, development work and biomass disposal

Excess biomass product ion i s a lso w i t h i n the range predic ted.

A major d i f f e rence Dctveen labo ra to ry and f u l l sca le performance has been i n the phys ica l c h a r a c t e r i s t i c s o f the biomass. During operat ion o f t he labora tory reactors , growth o f f i lamentous organisms was a f requent problem. c e l l was inc luded i n the treatment. I t had been found dur ing the 1 aboratory s tud ies t h a t the f i 1 amentous growth could be separated by d isso lved a i r f l o t a t i o n .

I t was because o f t h i s t h a t the f l o t a t i o n

I n the f u l l sca le p lan t , f i lamentous growth has n o t occurred. However, a cont inu ing problem has been a dispersed growth, which r e s u l t s i n excessive suspended s o l i d s i n the e f f l u e n t . While most o f the biomass forms a f l o c w i t h f a i r l y good s e t t l i n g cha rac te r i s t i cs , a f r a c t i o n o f the growth remains suspended as i n d i v i d u a l bac ter ia . i n the d isso lved a i r f l o t a t i o n c e l l w i thou t the use o f a chemical f l o c c u l a t i n g agent.

The p a r t i c l e s are too small t o be removed

289

The h igh suspended s o l i d s l e v e l i n the t r e a t e d e f f l u e n t n o t on ly r e s u l t s i n f a i l u r e t o meet the suspended s o l i d s c r i t e r i a , b u t a lso causes a h igh BOD due t o the oxygen demand o f suspended bac ter ia .

There has been a cont inu ing program t o determine the cause o f the dispersed growth. Variables t h a t have been o r are being s tud ied inc lude s t a b i l i t y o f waste load and pH, e f f e c t o f F/M r a t i o , f ac to rs a f f e c t i n g the growth of protozoa, which have genera l l y been lack ing i n t h i s system, and the e f f e c t o f ammonia n i t rogen concentrat ion. The r e s u l t s o f t h i s work w i l l be inc luded i n the p r o j e c t r e p o r t t h a t w i l l be prepared upon completion o f the demonstration gran t program.

SUMMARY

During many years o f development o f the corn wet m i 1 l i n g process , product losses t o l i q u i d e f f l u e n t streams were g r e a t l y reduced by r e c y c l i n g process waters, and recover ing so lub les as a by- product.

As the need f o r add i t i ona l improvement i n e f f l u e n t q u a l i t y became known , product 1 osses were f u r t h e r reduced by process mod i f i ca t ions . Laboratory t r e a t a b i 1 i ty s tud ies were used t o determi ne the requi re - ments f o r treatment of the remaining waste by the completely mixed a c t i v a t e d sludge process. labora tory and f u l l sca le operat ion w i t h respect t o r a t e o f so lub le COD removal, oxygen requirements, and biomass growth ra te . Physical charac ter is t i c s o f the biomass d i f f e r e d between the 1 aboratory and f u l l sca le processes. the labora tory , wh i l e dispersed i n d i v i d u a l b a c t e r i a has been a cont inu ing problem i n the f u l l sca le p lan t . This has r e s u l t e d i n f a i l u r e t o meet the design e f f l u e n t suspended s o l i d s and BOD concentrat ions. A program t o determine the cause o f t h i s problem i s s t i l l i n progress.

Good agreement was obta ined between

Filamentous growth was a major problem i n

290

REFERENCES

(1) Je f f r i es , F. L., Corn Grinding as I have Seen It. Corn Products Company.

(2 ) Mohlman, F. W. and A. J. Beck, I n d u s t r i a l and Engineering Chemistry - 21:205 (1929).

(3) Pul f rey, A. L., R. W . Kerr and H. R. Reint jes, I n d u s t r i a l Engi neer i ng Chemistry 32: 1483-1 487 (1940).

(4) Van Patten, E. M. and McIntosh, G. H., I n d u s t r i a l and Engineering Chemistry - 44:483-487 (1952).

(5) Ha t f i e ld , W. D., I n d u s t r i a l Wastes, Chapter 6, Corn Starch Processes ( W . Rudolfs, Ed i to r ) , Rheinhold Publ ishing Company, 1953.

(6) McIntosh, G. H. and G. G. McGeorge, Food Processing 25, No. 1, pp 82-86, January, 1964.

(7 ) Busch, A. W . , Chemical Engineering - 72, 71-76, 83-86 (1965).

291