J. agric. Engng Res. (2001) 80 (3), 307}310doi:10.1006/jaer.2001.0736, available online at http://www.idealibrary.com onAP*Animal Production Technology

RESEARCH NOTE

Some Intermittent Aeration Kinetics from a Laboratory Study with Pig Slurry

Jun Zhu

University of Minnesota Southern Research and Outreach Centre, 35838-120th Street, Waseca, MN 56093, USA; e-mail: zhuxx034@umn.edu

(Received 31 August 2000; accepted in revised form 9 April 2001; published online 11 September 2001)

A 0)75 kW aeration system with a venturi air injector was used in this study to examine the properties of theoxygen uptake rate and overall oxygen transfer coe$cient in pig slurry. Data showed that both parametersvaried with the intermittent aeration process. The oxygen uptake rate increased from 0)46 mg l~1min~1 toabout 1)8 mg l~1min~1, while the overall oxygen transfer coe$cient decreased from 0)5 min~1 to about0)3 min~1 over a total number of 31 tests. The system had a tendency to run continuously since the o!-run timedecreased from 10 min at the beginning of the test to 3 min at the end of the test. This may imply that anintermittent aeration system, if controlled by dissolved oxygen concentration, might not be able to runintermittently due to increased consumption of oxygen by the manure. The aeration system tested achieved anaeration e$ciency of 3)20 kg [O

2] kWh~1 in this study.

( 2001 Silsoe Research Institute

1. Introduction

The objective of using intermittent aeration is to saveenergy and, at the same time, provide necessary oxygento the aerobes to decompose organic compounds. Thecommon practice in using this method is to automati-cally control the dissolved oxygen concentration inliquid. When the concentration falls outside the presetupper and lower limits, the aeration system will be eitherstopped or started (Johnston, 1984). However, this simplelogic may not work as expected in a real world since theperformance of any aeration systems is largely environ-ment speci"c. For instance, in manure environment, theoxygen uptake rate by the aerobes may play a role indetermining the performance of an intermittent aerationsystem (Saxon, 1972; Cumby, 1987). As the availablesoluble substrate is oxidized, signi"cant variations in theoxygen uptake rate may be observed at di!erent aerationstages. Unfortunately, there is not much informationavailable about such variations in an intermittent aer-ation course in swine manure. Without this information,it is di$cult to design an optimal intermittent aerationscheme to save energy and, at the same time, accomplishaerobic treatment. Therefore, this research note presentsdata that reveal the changes in the oxygen uptake rate

0021-8634/01/110307#04 $35.00/0 30

R and the overall oxygen transfer coe$cient kLa

of a ven-turi air injector system during an intermittent aerationprocess and how those changes a!ect the performance ofsuch an aeration system.

2. Materials and methods

2.1. Experimental apparatus

The test arrangement is illustrated in Fig. 1. An 1890 ltank was used to contain manure for the test. Freshmanure of 1514 l at 0)5% total solids level was containedin the tank. Prior to loading the tank, the manure wasscreened using a sieve with 1 mm pore size to removecoarse materials to avoid possible clogging for the ven-turi air injector. There was no addition or reduction ofmanure amount in the tank during the test period. Theliquid manure used was collected from a commercialswine "nishing building with an initial chemical oxygendemand of 2980 mg l~1.

A 0)75 kW centrifugal water pump was used to circu-late liquid manure through the venturi air injector whichwas capable of entraining air into liquid at a rate of4)72 l~1 s~1. The liquid #ow rate was 151)4 l~1min~1.

7 ( 2001 Silsoe Research Institute

J . ZHU308

Notation

C oxygen concentration in liquid, mg l~1

Cs

oxygen saturation concentration, mg l~1

Ct0,

Ctt

oxygen concentration when t"0 and t,mg l~1

kLa

overall oxygen transfer coe$cient, 1 min~1

R oxygen uptake rate by the aerobes in ma-nure, mg l~1min~1

r2 coe$cient of determination

To maximize air retention time in the liquid, two perfor-ated polyvinyl chloride (PVC) pipes of 2)545 cm in dia-meter, joined perpendicular to each other, were placedhorizontally at the bottom of tank so that the oxygenatedliquid was always dispersed into the tank from bottomup, with the deoxygenated liquid at the top being suckedinto the outside system for air entrapment. The pumpand air injector were provided by Environmental Systems,Technology and Research, Inc. (Brussels, WI, USA).

An on-site computer was used to automatically recordmeasurements from the dissolved oxygen probe (PointFour Systems Inc., British Columbia, Canada) at set timeintervals. The probe was equipped with a battery pow-ered propeller in its front end to cause liquid movementso recording could e!ectively continue when aerationstopped. The computer also controlled the pump switchoperated by the dissolved oxygen (DO) levels in the tank.The ambient temperature was not controlled and variedapproximately between 18 and 233C over the test period.

2.2. Measurement design

To study intermittent aeration and overall oxygentransfer e$ciency, the aeration process was programmed

Fig. 1. Schematic of th

in such a way that it would start at the beginning of every3 h and stop when the DO in liquid reached certainconcentrations. The liquid DO concentrations, whichwere recorded by the computer at a 5 s interval onceaeration was initiated, determined the length of eachaeration run. When the di!erence between the means oftwo consecutive sets of 12 DO readings each (1 min)picked up by the computer was less than 0)05 mg l~1, theconcentration of DO in liquid was considered as havingachieved a steady state and the pump was then switchedo! by the computer. This recording time frame wasdetermined by observations of several trials prior to theformal test. It best re#ected the oxygenation kinetics ofthe test manure. A total of 31 runs were conducted so theentire experiment lasted 93 h.

After the aeration process stopped, the computer keptrecording DO concentrations in the liquid at an intervalof 1 min until it approached zero. Data collected in thisstage were used to calculate the oxygen uptake rate of theliquid manure (mixed liquor) for each aeration run byplotting out DO concentrations versus time. The slope ofthe linear regression for each data set was designated asthe oxygen uptake rate.

To determine the aeration system e$ciency, chemicaloxygen demand (COD) was monitored during the test.Liquid samples were taken from the tank at varyingintervals and were analysed using the standard methodsrecommended by the American Public Health Associ-ation (APHA, 1998).

2.3. Determination of overall oxygen transfercoe.cient

Under non-steady state conditions in which thedissolved oxygen is allowed to approach zero bymicrobial respiration, the oxygen transfer rate into

e experimental design

Fig. 3. Overall oxygen transfer coezcient measurement duringthe test period

309AERATION KINETICS FROM A STUDY WITH PIG SLURRY

mixed liquor can be described by (Casey & Karmo,1974).

dC

dt"k

La(C

s!C)!R (1)

where: kLa

is the overall oxygen transfer coe$cient; Csis

the oxygen saturation concentration in mg l~1; C is theoxygen concentration in mg l~1 at time t; and R is theoxygen uptake rate by the mixed liquor in mg l~1 s~1.

Assuming the oxygen uptake rate is constant for eachspeci"c run, the integral of Eqn (1) yields:

kLa"

1

tln A

kLa

(Cs!C

t0)!R

kLa

(Cs!C

tt)!R B (2)

where: Ct0

and Ctt

are the oxygen concentrations at t"0and t.

With Eqn (2), the overall oxygen transfer coe$cientcan be calculated by the repetitive substitution method.Since temperature was not monitored in this study, allcalculations were based upon the assumption of 203C.The saturation concentration of dissolved oxygen inwater at 203C, 760 mm Hg, which was 9)1 mg l~1 (couldbe between 8)6 and 9)4 mg l~1), was used in calculations.

3. Results and discussion

3.1. Oxygen uptake rate and overall oxygen transfercoe.cient

The oxygen uptake rate R and overall oxygen transfercoe$cient k

Laof the test manure sampled at each 3-h

interval are presented in Figs 2 and 3. It can be seen thatboth parameters are not constant. The former increaseswith the increase of aeration time while the latter behavesin an opposite direction. This phenomenon may be dueto the biological activities of aerobic bacteria in manure(Saxon, 1972). Continuing, rapid growth of aerobes,

Fig. 2. Oxygen uptake rate measurement during the test period

driven by aeration, consumes more oxygen within a shortperiod of time, resulting in an increase in oxygen uptakerate. On the other hand, a strong and growing demandon oxygen exceeds the oxygen transfer capability of theaeration system, resulting in a decrease in overall oxygentransfer coe$cient.

3.2. Intermittent aeration

Figure 4 shows the time to reach steady state of dis-solved oxygen concentration in the test liquid for allaeration runs. Apparently, the time needed for achievingthe steady state increases from around 15 s to about 270 sover a total number of 31 tests. Although the increase intime is characteristic of linearity with a coe$cient ofcorrelation of 0)9567, the curve tends to bend upwards asindicated by the last few points, sugesting that apossibly substantial increase in time to reach steady DO

Fig. 4. Time to reach stable state of dissolved oxygen concentra-tion in liquid for each test; r2, coezcient of determination for

linear regression; , linear regression line

Fig. 5. Oxygen depletion time after reaching stable dissolvedoxygen concentration for each test; r2, coezcient of determinationfor linear regression; DO, dissolved oxygen; , linear regres-

sion line

J . ZHU310

concentrations in liquid may be needed thereafter. Thistime increase could be mainly due to the increasinggrowth and metabolic activity of aerobic bacteria, whichaccelerates the consumption of oxygen in liquid. How-ever, this hypothesis cannot be veri"ed in this studybecause of a lack of monitoring aerobic bacterial countsin the test liquid.

The increased bacterial activity can also be found inFig. 5. It can be seen that when aeration stops, the oxygenin liquid is depleted faster in later runs than in early runs,indicating a growing demand of oxygen. These observa-tions may potentially challenge the concept of usingintermittent aeration to treat swine manure. Accordingto this study, the pump-o! time declines from 10 min toabout 3 min in the 93 h period and has a tendency fordisappearing (Fig. 5). This implies that the aeration timehas to be increased run by run and, at some point, willbecome continuous to meet the preset oxygen concentra-tion if an automatic controller is used. Therefore, it canbe inferred that intermittent aeration systems, if solelycontrolled by DO concentrations in liquid, may event-ually end up in a continuous operating mode, withoutsaving energy at all.

The e$ciency of the aeration system in this study isestimated by the reduction of COD as opposed to the

total running time. At the end of test, the level of COD is960 mg l~1 so that the di!erence is 2020 mg l~1 (a reduc-tion of 68%), while the total pump running time adds upto 1)28 h. For a 0)75 kW pump, the system aeratione$ciency is around 3)20 kg [O

2] kWh~1.

4. Conclusion

According to this study, the oxygen uptake rate andthe overall oxygen transfer coe$cient of pig slurry arenot constant due to the increased biological activitiesduring the aeration process. The former increases from0)46 mg l~1 min~1 to about 1)8 mg l~1min~1 and thelatter decreases from around 0)5 min~1 to about0)3 min~1 over a total number of 31 tests. Therefore,simply controlling dissolved oxygen in liquid for inter-mittent aeration may not be able to achieve an on-and-o! operation due possibly to the increased demand onoxygen by rapidly growing aerobic bacteria. Furthere!orts should be speci"cally directed to understandingthe fundamentals in terms of the changing biologicalcharacteristics of the liquid manure during an aerationprocess. Speci"c to this study, the aeration system canreduce total COD by 68% in a 93 h test period and theaeration e$ciency is thus around 3)20 kg [O

2] kWh~1.

Reference

APHA (1998). Standard Methods for the Examination of Waterand Waste (18th Edn). American Public Health Association,Washington DC, USA

Casey T J; Karmo O T (1974). The in#uence of suspended solidson oxygen transfer in aeration systems. Water Research, 8(4),805}811

Cumby T R (1987). A review of slurry aeration 3. Performanceof aerators. Journal of Agricultural Engineering Research,36(2), 175}206

Johnston D W M (1984). Oxygen requirements, energy con-sumption and sludge production in extended-aeration plants.Water Pollution Control, 83(1), 100}115

Saxon J R (1972). Bacteria associated with the treatment anddisposal of piggery waste. MS Thesis, Aberdeen University,Scotland