EFFECT OF DISSOLVED OXYGEN CONCENTRATION

ON THE AEROBIC STABILIZATION OF SWINE WASTE

N.R. Bulley1 and J.T.R. Husdon2

1Department of Bio-Resource Engineering, University of British Columbia, 2075 Wesbrook Mall, Vancouver, B.C. V6T1W5and 2Canadian Bio Resources Consultants Ltd., 5691 - 176th St., Cloverdale, B.C.

Received 2 July 1976

Bulley, N.R. and J.T.R. Husdon. 1978. Effect of dissolved oxygen concentration on the aerobic stabilization of swine waste. Can.Agric. Eng. 20: 60-63.

Most aerobic waste treatment system designs recommend that the dissolved oxygen concentration be maintained at 0.5 - 2.0mg//. This design parameter may not be applicable to high strength wastes. Swine wastes (initial chemical oxygen demand(COD), 30,000 mg//) werebatch-digested at three levels ofdissolved oxygen (0.5-2.0 mg//; 5-8mg//; and 15-20 mg//) over 14-daytreatment periods. The results indicate that dissolved oxygen concentrations greater than 2 mg/ / produce a significant increase inthe rate of aerobicstabilizationof the waste. For this waste, the bacteriawereableto removethe organiccompoundsresponsiblefor the soluble COD fromsolutionwithin2 daysat all oxygen levels buttheywere ableto oxidizethesechemicals morerapidlyatthe higher oxygen concentrations. This increase in the rate of stabilization with oxygen concentration could have a direct effect onthe design of waste treatment systems for other high strength animal and food processing wastes.

INTRODUCTION

Intensified livestock production hasresulted in large volumes of animal manuresbeing produced on small land areas. In manycases there is insufficient land available to

use the nutrients in these manures optimallythrough a normal crop production program.This surplus has stimulated research todevelop treatment methods that will enableproducers to find alternate reuse or disposalsystems. Many of these systems involve theaerobic treatment of manures for conversion

into a form suitable for use as an animal feed

or to reduce their oxygen demand beforefinal disposal. Much of the work to date inaerobic animal waste treatment systems hasbeen based on applying principles that wereoriginally developed for the treatment ofmunicipal sewage. Because of the differencesin the characteristics of animal wastes and

municipal sewage, such as solidsconcentration, biodegradability,settleability and bacterial population, it isquite possible that many of these principlesare not directly applicable.

One of the areas where the differences

encountered could have a significant effecton systems design is in the concentration ofdissolved oxygen required for satisfactoryaerobic oxidation. Most systems that aredesigned for the aerobic treatment ofmunicipal wastes recommend the dissolvedoxygen concentration be maintained in therange of 0.5 - 2 mg//and this is supported bythe work of Thabaraj and Gaudy (1971).Oxygen concentrations greater than thishave been reported to give no significantimprovement in biochemical oxygendemand (BOD) removal efficiency foranimal wastes (Loehr 1971).

Recent studies on the use of pure oxygenin waste treatment systems have indicatedthat oxygen concentrations from 1 to 14mg// resulted in no change in substrateremoval kinetics (Ball and Humenick 1973).But, in activated sludge floes with a diametergreater than 500/i, calculations showedthat oxygen concentrations greater than 2.5

60

mg// would be required to maintaincompletely aerobic conditions (Englandeand Eckenfelder 1973). Other research usingoxygen concentrations of 8-10 mg// in anactivated sludge system indicated that adissolved oxygen concentration of 1 ppm inthe control experiment might be leading toanaerobic conditions inside the bacterial

floes (United States Department of theInterior 1970).

Irgens and Day (1966) in their studies onaerobic stabilization of swine wastemaintained a dissolved oxygen level of 1mg//during treatments. They recommendedthat a dissolved oxygen level of 0.5 - 2 mg//be used in the design of oxidation ditchesand this is supported by work of others(Jones et al. 1970).

The purpose of this study was todetermine whether the rate of stabilization

of high strength swine wastes is affected bydissolved oxygen concentration.

MATERIALS AND METHODS

A series of batch tests were conducted to

evaluate the effect of dissolved oxygenconcentration on the aerobic stabilization ofswine wastes. The batch tests wereconducted over a 15-day period and changesin the chemical oxygen demand (COD), andthe total organic carbon (TOC) weremonitored daily. Three 5-/capacity digesterswere used and were held at the followingdissolved oxygen concentrations: high 02level (15-20 mg//); medium 02 level (5-8mg//); and low 02 level (0.5-2 mg//).

The wastes used throughout the studywere collected from an anaerobic holdingtank in a commerical hog-finishingoperation. Prior to entering the holdingtank, the waste had undergone limitedanaerobic decomposition within the barnsover a 6-wk period and then passed throughan overflow system to the tank. The wasteswere collected and transported to alaboratory where they were stored at 4°Cuntil they were used in the experiments. Thewastes were aerated at room temperature

(20-23° C) using three specially constructed5-/ polyethylene digesters. Inverted T-barslocated close to the bottom of each digesterand connected to variable speed motorswere used to stir the contents of the

digesters. Aeration was provided by a smallair pump or pressurized cylinders of pureoxygen. The gases were passed throughsubmerged porous stones to maintain thedissolved oxygen concentration in thedigesters at preset levels.

Analytical Methods

The chemical oxygen demand (COD)was performed as outlined in StandardMethods (American Public Health Association (APHA) 1971) for the 20-m/ samplesize. A 20-m/ sample was taken from thedigester and a 5:1 dilution prepared using avolumetric flask. For soluble COD

determinations, the 5:1 diluted sample wasfiltered using No. 1 Whatman filter paperand 20 m/ of the filtrate used in the analysis.For total COD determination, the 5:1sample was diluted to 10:1 and a 20-m/portion was used in the analysis.

Values for total residue and total volatile

residue were obtained as outlined inStandard Methods (APHA 1971) with thefollowing modification. An exact volume ofsample was not used. A sample of thedigester contents was transferred to agraduated cylinder and the volumerecorded. This sample was then transferredto an evaporating dish and the analysis donein the normal manner. This modificationavoided the problems of settling which arecommon with high particulate wastes whenexact volume measures are taken.

The oxygen concentration of thedigesters was monitored using an oxygenmeter and probe (Yellow Springs Instrument Co., Model 54). The system wasperiodically calibrated against a knowndissolved oxygen concentration asdetermined by the Azide Modification of theWinkler Method (APHA 1971).

CANADIAN AGRICULTURAL ENGINEERING, VOL. 20 NO.l, JUNE 1978

Digester Test TrialAn initial test trial was carried out on the

three digesters to determine whether thedigesters acting under the same conditionswould produce the same biodegradation ofthe animal wastes.

Each of three digesters was filled with 5 /of anaerobic swine waste (COD, 24,000mg// wet weight basis). The waste wasoxidized over a 15-dayperiod and the rate ofbreakdown of the material was evaluatedusing daily COD determinations. Theoxygen concentration in each of thedigesters was maintained at a level of 6 - 8mg//.

For each of the subsequent trials the low,medium and high oxygen level in eachdigesterwaschosen randomly. The results ofthe experiments have been reported in thesame sequence for ease of reading.

Batch Tests A and B:Oxygen ConcentrationEffects

At the start of each test, each digester wasfilled with 5 / of anaerobic swine waste.Throughout the test, one digester wasmaintained at a low dissolved oxygenconcentration; .5-2 mg//. A seconddigester was maintained at a mediumdissolved oxygenconcentration; 5-8 mg//;and the third digester was maintained at ahighdissolved oxygen concentration; 15- 20mg//. The desired oxygen concentration wasmaintained by manually adjusting theproportions of air and pure oxygen beingsupplied by the digesters with the total flowrate being kept constant. Samples weretaken daily from the digesters throughoutthe 15-day experiment and were analyzedfor COD, total residue, and total volatileresidue.

Batch Test C — Effect of OxygenConcentration on Waste Stabilization afterSeeding with Aerated Swine Waste

This test was carried out to determine ifseeding the digesterswith previouslyaeratedswine waste would alter the effect thatdifferent levels of dissolved oxygen had onthe rate of stabilization of the waste. Aninitial seed with an active aerobic bacterialpopulation was established in a sample ofswine waste by maintaining the dissolvedoxygen at 5 ppm for 3 days. The threedigesters were each filled with 4 / ofanaerobic swine waste, aerated for 4 h toremove the dissolved gases; 1 / of thepreviously aerated waste was added to eachof the digesters, and the waste then aeratedfor a 15-day period.

The dissolved oxygen concentration wasmonitored for each of the three digesters asbefore and maintained at the low, mediumand high concentrations as in tests A and B.Oxygen uptake rates were determinedperiodically for the three digesters.

To determine oxygen uptake rates, theoxygen probe was inserted into the digester,the rotor speed was increased to providegood agitation and the oxygen supply wasshut off. The level of dissolved oxygen was

recorded at various time intervals and plotswere prepared using this data. The oxygenuptake rates in mg/min were determinedfrom these plots. In the digesters wherepureoxygen was being used, the digester contentswere purged with air so that the threedigesters all had the same initial oxygenconcentration at the beginning of the ratedetermination.

Daily COD determinations were madeon samples from each of the digesters beforeand after filtering using a No. 1 Whatmanfilter paper.

Viable organism concentrations weredetermined at various times using thestandard plate count with serial dilution andspot plate inoculation techniques (APHA1971).

RESULTS AND DISCUSSION

Digester Test TrialThe initial test trial was run to determine

if there were any differences among thedigesters when they were operated underessentially identical conditions. The p\\ inthe digesters started at 7.5 and rose to 8.9and then remained constant throughout theexperiment. The temperature in thedigesters was allowed to fluctuate with thelaboratory temperature (24.5 + 3°C).

The rates of COD reduction in the threedigesters were similar to each other. TheCOD vs. time curve for the medium oxygenconcentration in batch test A (Fig. 1) wastypical of all three digesters in this trial run.

A least squares linear regression on thechange in COD with time was calculatedfrom the data from each digester. Acovariance analysis was made on this CODdata and the regression lines for the threedigesters compared for differences in levelsand slopes (Snedecor 1956). There was nodifference at the 1% significance levelbetween digesters II and III. However,digester I had a lower rate of COD reductionthan digesters II and III. An examination ofthe daily COD analysis showed that therewas very little change in the COD fordigester I from day 3 to day 5.

This lag phase did not show up indigesters II or III. The records showed thatthe dissolved oxygen level in digester I haddropped to less than 0.1 mg//sometime after1600 h on day 2 until 1000 h on day 3. Thislow level of oxygen appears to have causedthe lag in COD reduction. An analysis ofthe data indicated that a 24-h lag phasecould account for the difference in slopedown to the 1% significance level betweendigester I and the other two digesters. Thisindicated that the aerobic treatment ofwastes in the digesters should operate in thesame manner under similar conditions but

that low oxygen concentrations couldadversely affect treatment rates.

Batch Tests A and B

Batch tests A and B were run todetermine what effect the level of dissolvedoxygen would have on COD removal. Thethree digesters were maintained at different

CANADIAN AGRICULTURAL ENGINEERING, VOL. 20 NO.l, JUNE 1978

TIME (days)

Figure 1. Changes in chemical oxygen demandduring aerobic stabilization of swinewaste as affected by dissolved oxygenconcentration.

levels of dissolved oxygen throughout theoxidation period. The TOC analyses carriedout on the same samples as the daily CODtests showed a positive correlation andprovided a check on the COD analyses. Therelationship between COD and TOC forboth filtered and unfiltered samples has beenreported previously (Bulley and Husdon1974). The COD values for the two batchtests are summarized in Table I.

There was a definite effect of oxygenconcentration on the COD reduction with48.7% reduction after 7 days at the highoxygen and a 15.7% reduction at the lowoxygen. After 14 days the percentagereductions had increased to 57.8% at thehigh oxygen and 38.9% at the low oxygen.The changes in COD with time for the threeoxygen treatments for batch test A areshown in Fig. 1. The slow COD reductionsat the low and medium 02 concentrationsappear to be due to a lag phase whichoccurred for the first 7 days at the lowoxygen and for about 3 days at the mediumoxygen concentration.

It can be seen in Fig. 1 that for the highoxygen treatment, the COD vs. timerelationship could be approximated by twodifferent linear functions, one for the first 6days and the second for the last 8 days. It wasnot possible to describe them by the samelinear function throughout. Since the maindifference in COD between oxygen levelsoccurred during the first 7 days (see Table I),the data was adjusted to consider this in thebest straight line to approximate the data.This was done by including only CODvalues over 16,000 mg//, since it was belowthis level that the rate of COD reductiondropped off sharply for the high oxygentreatment. With this adjustment, CODvalues were included for the first 6 days forthe high oxygen treatment, for the first 12days for the medium oxygen treatment, and

61

TABLE I. EFFECT OF OXYGEN CONCENTRATION, ON THE CHANGE IN COD (mg//) OFSWINE WASTE DURING DURING AEROBIC STABILIZATION

Batch test A Batch test B Batch test C

High oxygen (15-20 mg/l)

Initial 30,400 30,313 36,000

After 7 days 15,880 15,256 20,140

Percentage reduction 47.8 49.6 48.7

After 14 days 13,014 12,616 19,090t

Percentage reduction 57.2 58.4 47.0

Medium oxygen (5-8 mg/l)

Initial 31,776 30,993 32,120

After 7 days 21,629 18,972 24,000

Percentage reduction 32.0 38.8 25.3

After 14 days 15,886 15,061 20,000t

Percentage reduction

Low

50.0

oxygen (0.5-2 mg/l)

51.4 37.7

Initial 32,180 30,896 38,600

After 7 days 28,568 24,645 30,800

Percentage reduction 11.2 20.2 20.2

After 14 days 17,800 20,730 26,600t

Percentage reduction 44.7 32.9 31.1

115 days

TABLE II. PERCENTAGE REDUCTION TOTAL VOLATILE RESIDUE

Batch test A Batch test B

High oxygenMedium oxygenLow oxygen

19.1

11.8

17.4

32.6

23.1

21.1

for the entire oxidation period for the lowoxygen treatment. With these limitations, alinear regression analysis was made for thereduction of COD with time, combining theresults of batch tests A and B.

A statistical analysis of the data showedthat the slopes of the daily CODdeterminations were found to be different at

the 1% level of significance. These plots areshown in Fig. 2.

The reduction of total volatile residue

after 14 days, in batch tests both A and B,was greater for the high dissolved oxygentreatment than for the medium and low

levels. The reduction of total volatile residue

at the medium and low level was not

consistent. The low level reduced a greaterpercentage than the medium level in batchtest A but less than the medium level in batch

test B. These reductions are summarized in

Table II.

Batch Test C

It was felt that the lag in the removal ofCOD that was evident in the low and

medium levels of dissolved oxygen waspossibly due to the lack of a suitablebiological culture. Batch test C was run todetermine the effect of oxygenconcentration on the rate of biological

breakdown of swine wastes that had been

seeded with aerated swine waste. The COD

of the contents of the digesters was analyzeddaily (Fig. 3) and the overall reductions inCOD are summarized in Table I. The dailyCOD values for the filtrate from the filtered

samples are also shown (Fig. 3). The lines onFig. 3 have been hand drawn and areincluded to show trends during the digestionof the waste.

Seeding of digesters did not affect therate or degree of treatment significantly. Therate of COD removal at each of the oxygenconcentrations was similar to the rates

observed in experiments A and B. A leastsquares linear regression was calculated forthe pooled data of experiments A, B and C,and a covariance analysis made on this data.The slopes of the lines were significantlydifferent at the 1% confidence level.

A study of the filtered COD vs. time data,as compared to the unfiltered COD vs. timedata (Fig. 3), may shed more light on thecause of the reduced oxidation rate at the

lower oxygen concentration.During day 2 there was a rapid decrease

in soluble COD followed by a slow declineover the next 12 days. The high removal ofsoluble COD on day 2 was accompanied bya corresponding decline in total COD at the

32

28

1 24 \low 02'O

\ high 02medium 02 ^\

16

0 2 4 6 8 10 12 1

TIME (days)

Figure 2. A least squares linear regression ofchemical oxygen demand on time foraerobically digesting swine waste(batch tests A and B).

§24

0 4 8 12 16

TIME (day)

Figure 3. Changes in chemical oxygen demandduring aerobic stabilization of swinewaste as affected by dissolved oxygenconcentration for unfiltered and for the

filtrate from filtered digester samples,(low 02 •—•; medium O, O—O;high 02 "A ).

high and medium oxygen concentrationsbut not at the low oxygen concentration.

This would indicate that even thoughabout 40% of the soluble COD was rapidlyremoved in all three digesters, it was onlybeing stored within the micro-organisms atthe low 02 concentration. This wouldaccount for the fact that there was no

corresponding decrease in total COD. Thisinterpretation is supported by the work ofRobarts and Kempton (1971) who studiedthe removal of C,4 glucose by activatedsludge. Their studies showed that largeamounts of the substrate could be removed

and stored within the cell and that this

storage was separate from the oxidationprocess. This initial decrease in soluble CODcan also be, in part, attributed to the "initial

62 CANADIAN AGRICULTURAL ENGINEERING, VOL. 20 NO.l, JUNE 1978

TABLE III. BATCH TEST C, OXYGEN UPTAKE RATES AND BACTERIA COUNTS

(L( )\v02) (Med mm 02) (High 02)Day 02 uptake

(mg///min)Bacteria

count

(colonies/ml)

02 uptake(mg///min)

Bacteria

count

(colonies/ml)

02 uptake(mg///min)

Bacteria

count

(colonies/ ml)

1

5

15

.8

3.0

2.5

1,380X103

900X103

1,000X103

1.3

2.4

.4

1,440X103

1,620X103

1,200X103

3.2

1.0

.4

1,380X103

2,100X103

1,500X103

TIME (day)

Figure 4. Oxygen uptake rates during aerobicstabilization of swine waste at three

levels of dissolved oxygen,(low 02 •—•; medium 020—O;

high 02 •—A ).

removal phase" referred to by Eckenfelder(1970). This initial removal phase is the basisof the contact stabilization process in sewagetreatment.

The oxygen uptake rates and bacterialcounts are shown, for the three levels ofdissolved oxygen, at various times duringthe oxidation period (Fig. 4, Table III). Thebacterial populations for three levels ofdissolved oxygen were unexpectedly quiteuniform throughout the study. The minorchanges observed did not correspond to thechanges in oxygen uptake rates or CODremoval rates. At the high dissolved oxygenlevel, the oxygen uptake rates were thehighest during the first 3 days of the batchrun and then gradually decreased. Thebacteria count was higher after 5 days andhad declined again by day 15.

The medium level of dissolved oxygendid not reach its highest rate of oxygenuptake until the 3rd day even though itstarted with essentially the same size ofbacteria population as the highest oxygentreatment. At the low level of dissolved

oxygen the oxygen uptake rate did not reachits highest value until the 5th day and thenremained constant throughout the batchrun. The bacteria level had actually declinedby day 5 and was about the same level againon day 15.

The results indicate that the oxygenuptake rate is not solely dependent on thesize of the biological population but isinfluenced by the activity of the population.At the high oxygen concentration, thebiological population appears to be able tooxidize the substrate much faster than at the

medium and low oxygen concentrations.

This high COD removal rate wassustained for only a relatively short timebefore a much slower oxidation processtook over. Whether a continuous digestercould be operated at the high oxygen levelsand remove 45% of the COD in 7 days cannot be predicted from these results. Theseeding of the digesters did not help theCOD removal rates at the medium andespecially at the low oxygen concentrationsas might have been expected. This wouldindicate that the level of dissolved oxygen inthe digesters is very significant indetermining the rate of oxidation for theseconcentrated wastes.

Although this work was carried out usingswine wastes, the oxidation of other animaland food processing wastes with a highsolids content may also be restricted by themaintained level of dissolved oxygen.

With the increase in interest in refeedingof wastes to livestock, the partial orcomplete oxidation of wastes into a formsuitable as a livestock feed source should be

investigated.The capital costs of waste treatment

systems using pure oxygen are high but arepartially offset by the reduction in treatmenttime, equipment size, and process control.

Based on this research, further studiesshould be carried out using both batch andcontinuous treatment systems to determinethe feasibility and practicability ofincreasing the dissolved oxygenconcentrations in aerobic treatment systemsfor concentrated organic wastes.

CONCLUSIONS

1. During the batch aerobic stabilization ofswine waste, the oxygen concentrationdoes affect the rate of COD reduction.

2. For the unseeded batch treatment, therate for COD reduction during the first 7days of treatment was highest at high 02(15-20 mg/l), removing 49% of the COD.At the medium oxygen concentration (5-8 mg//), 35% of the COD was removedafter 7 days. The low oxygen digester(0.5-1.5 mg//) had a definite lag phaseduring the first 7 days, reducing the CODby only 15%.

3. For seeded treatment, the rate of CODreduction was highest at high 02 (15-20mg//).

4. There is a rapid uptake of organic

CANADIAN AGRICULTURAL ENGINEERING, VOL. 20 NO.l, JUNE 1978

compounds by the bacteria soon aftertreatment begins, resulting in a reductionin soluble COD. At high (15-20 mg//)and medium (5-8 mg//) 02, this uptake

of soluble COD is accompanied by arapid oxidation with accompanyingreduction in total COD. At low 02 (0.5-2 mg//), the rapid uptake of organiccompounds was followed by a slowsteady oxidation process.

5. After about 40% of the original COD hasbeen removed, the rate of COD removalappears to be less dependent on theoxygen concentration.

ACKNOWLEDGMENT

This research was supported by a grant fromAgriculture Canada.

AMERICAN PUBLIC HEALTH ASSOCIA

TION. 1971. Standard methods for the

examination of water and wastewater. 13th

ed. New York, N.Y.BALL, J.E. and M.J. HUMENICK. 1973.

Comparison of air and oxygen activatedsludge kinetics and settleability. In: R.E.Speece and J.F. Malina, Jr., eds. Applicationsof commercial oxygen to water and wastewater systems, Center for Research in WaterResources, University of Texas at Austin.

BULLEY, N.R. and J.T.R. HUSDON. 1974. Useof total organic carbon analysis as a measureof carbon reduction during the aerobicdigestion of swine waste. Can. Agric. Eng.16(1): 2-5.

ECKENFELDER, W.W. 1970. Water QualityEngineering. Barnes and Noble, Inc., NewYork, N.Y.

ENGLANDE, ANDREW, Jr. and WW.ECKENFELDER, Jr. 1973. Oxygen concentrations and turbulence as parameters ofactivated sludge scale-up. In: R.E. Speece andJ.F. Malina, Jr., eds. Applications ofcommercial oxygen to water and wastewatersystems, Center for Research in WaterResources, University of Texas at Austin.

IRGENS, R.L. and D.L. DAY. 1966. Laboratorystudies of aerobic stabilization of swine

wastes. J. Agric. Eng. Res. 11(1): 1-10.JONES, D.D., D.L. DAY, and A.C. DALE.

1970. Aerobic treatment of livestock wastes.

Bull. 737, University of Illinois, Urbana-Champaign, 111.

LOEHR, R.C. 1971. Liquid waste treatment. II.Oxidation ponds and aerated lagoons. In:Agricultural wastes: Principles and guidelinesfor practical solutions. Cornell UniversityConference on Agricultural Waste Management.

ROBARTS, R.D. and A.G. KEMPTON. 1971.Use of C14-glucose to study substrate removalby activated sludge. J. Water Pollut. Contr.Fed. 43(7): 1499-1506.

SNEDECOR, G.W. 1956. Statistical methods.Iowa University Press, Ames. Iowa.

THABARAJ, G.F. and A.F. GAUDY, Jr. 1971.Effect of dissolved oxygen concentrations onthe metabolic response of completely mixedactivated sludge. J. Water Pollut. Contr. Fed.41(8): R322-R335.

UNITED STATES DEPARTMENT OF THE

INTERIOR. 1970. Investigations of the use ofhigh purity oxygen aeration in the conventional activated sludge process. Water Pollut.Control. Res. Ser. No. 17050VNW05-70.

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