- Email: [email protected]
AP—Animal Production Technology
J. agric. Engng Res. (2001) 80 (3), 307}310 doi:10.1006/jaer.2001.0736, available online at http://www.idealibrary.com on AP*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: [email protected] (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 the oxygen uptake rate and overall oxygen transfer coe$cient in pig slurry. Data showed that both parameters varied with the intermittent aeration process. The oxygen uptake rate increased from 0)46 mg l\ min\ to about 1)8 mg l\ min\, while the overall oxygen transfer coe$cient decreased from 0)5 min\ to about 0)3 min\ over a total number of 31 tests. The system had a tendency to run continuously since the o!-run time decreased from 10 min at the beginning of the test to 3 min at the end of the test. This may imply that an intermittent aeration system, if controlled by dissolved oxygen concentration, might not be able to run intermittently due to increased consumption of oxygen by the manure. The aeration system tested achieved an aeration e$ciency of 3)20 kg [O ] kWh\ in this study. 2001 Silsoe Research Institute
1. Introduction The objective of using intermittent aeration is to save energy and, at the same time, provide necessary oxygen to the aerobes to decompose organic compounds. The common practice in using this method is to automatically control the dissolved oxygen concentration in liquid. When the concentration falls outside the preset upper and lower limits, the aeration system will be either stopped or started (Johnston, 1984). However, this simple logic may not work as expected in a real world since the performance of any aeration systems is largely environment speci"c. For instance, in manure environment, the oxygen uptake rate by the aerobes may play a role in determining the performance of an intermittent aeration system (Saxon, 1972; Cumby, 1987). As the available soluble substrate is oxidized, signi"cant variations in the oxygen uptake rate may be observed at di!erent aeration stages. Unfortunately, there is not much information available about such variations in an intermittent aeration course in swine manure. Without this information, it is di$cult to design an optimal intermittent aeration scheme to save energy and, at the same time, accomplish aerobic treatment. Therefore, this research note presents data that reveal the changes in the oxygen uptake rate 0021-8634/01/110307#04 $35.00/0
R and the overall oxygen transfer coe$cient k of a ven*? turi air injector system during an intermittent aeration process and how those changes a!ect the performance of such an aeration system.
2. Materials and methods 2.1. Experimental apparatus The test arrangement is illustrated in Fig. 1. An 1890 l tank was used to contain manure for the test. Fresh manure of 1514 l at 0)5% total solids level was contained in the tank. Prior to loading the tank, the manure was screened using a sieve with 1 mm pore size to remove coarse materials to avoid possible clogging for the venturi air injector. There was no addition or reduction of manure amount in the tank during the test period. The liquid manure used was collected from a commercial swine "nishing building with an initial chemical oxygen demand of 2980 mg l\. A 0)75 kW centrifugal water pump was used to circulate liquid manure through the venturi air injector which was capable of entraining air into liquid at a rate of 4)72 l\ s\. The liquid #ow rate was 151)4 l\ min\.
307
2001 Silsoe Research Institute
308
J. ZHU
Notation C oxygen concentration in liquid, mg l\ C oxygen saturation concentration, mg l\ Q C C oxygen concentration when t"0 and t, R RR mg l\ k overall oxygen transfer coe$cient, 1 min\ *? R oxygen uptake rate by the aerobes in manure, mg l\ min\ r coe$cient of determination
To maximize air retention time in the liquid, two perforated polyvinyl chloride (PVC) pipes of 2)545 cm in diameter, joined perpendicular to each other, were placed horizontally at the bottom of tank so that the oxygenated liquid was always dispersed into the tank from bottom up, with the deoxygenated liquid at the top being sucked into the outside system for air entrapment. The pump and air injector were provided by Environmental Systems, Technology and Research, Inc. (Brussels, WI, USA). An on-site computer was used to automatically record measurements from the dissolved oxygen probe (Point Four Systems Inc., British Columbia, Canada) at set time intervals. The probe was equipped with a battery powered propeller in its front end to cause liquid movement so recording could e!ectively continue when aeration stopped. The computer also controlled the pump switch operated by the dissolved oxygen (DO) levels in the tank. The ambient temperature was not controlled and varied approximately between 18 and 233C over the test period.
in such a way that it would start at the beginning of every 3 h and stop when the DO in liquid reached certain concentrations. The liquid DO concentrations, which were recorded by the computer at a 5 s interval once aeration was initiated, determined the length of each aeration run. When the di!erence between the means of two consecutive sets of 12 DO readings each (1 min) picked up by the computer was less than 0)05 mg l\, the concentration of DO in liquid was considered as having achieved a steady state and the pump was then switched o! by the computer. This recording time frame was determined by observations of several trials prior to the formal test. It best re#ected the oxygenation kinetics of the test manure. A total of 31 runs were conducted so the entire experiment lasted 93 h. After the aeration process stopped, the computer kept recording DO concentrations in the liquid at an interval of 1 min until it approached zero. Data collected in this stage were used to calculate the oxygen uptake rate of the liquid manure (mixed liquor) for each aeration run by plotting out DO concentrations versus time. The slope of the linear regression for each data set was designated as the oxygen uptake rate. To determine the aeration system e$ciency, chemical oxygen demand (COD) was monitored during the test. Liquid samples were taken from the tank at varying intervals and were analysed using the standard methods recommended by the American Public Health Association (APHA, 1998).
2.3. Determination of overall oxygen transfer coe.cient 2.2. Measurement design To study intermittent aeration and overall oxygen transfer e$ciency, the aeration process was programmed
Under non-steady state conditions in which the dissolved oxygen is allowed to approach zero by microbial respiration, the oxygen transfer rate into
Fig. 1. Schematic of the experimental design
A E RA TIO N K IN E TI C S F R OM A S T UD Y W I TH PI G S LU R RY
309
mixed liquor can be described by (Casey & Karmo, 1974). dC "k (C !C)!R *? Q dt
(1)
where: k is the overall oxygen transfer coe$cient; C is *? Q the oxygen saturation concentration in mg l\; C is the oxygen concentration in mg l\ at time t; and R is the oxygen uptake rate by the mixed liquor in mg l\ s\. Assuming the oxygen uptake rate is constant for each speci"c run, the integral of Eqn (1) yields:
1 k (C !C )!R R k " ln *? Q (2) *? t k (C !C )!R *? Q RR where: C and C are the oxygen concentrations at t"0 R RR and t. With Eqn (2), the overall oxygen transfer coe$cient can be calculated by the repetitive substitution method. Since temperature was not monitored in this study, all calculations were based upon the assumption of 203C. The saturation concentration of dissolved oxygen in water at 203C, 760 mm Hg, which was 9)1 mg l\ (could be between 8)6 and 9)4 mg l\), was used in calculations.
Fig. 3. Overall oxygen transfer coezcient measurement during the test period
driven by aeration, consumes more oxygen within a short period of time, resulting in an increase in oxygen uptake rate. On the other hand, a strong and growing demand on oxygen exceeds the oxygen transfer capability of the aeration system, resulting in a decrease in overall oxygen transfer coe$cient.
3.2. Intermittent aeration 3. Results and discussion 3.1. Oxygen uptake rate and overall oxygen transfer coe.cient The oxygen uptake rate R and overall oxygen transfer coe$cient k of the test manure sampled at each 3-h *? interval are presented in Figs 2 and 3. It can be seen that both parameters are not constant. The former increases with the increase of aeration time while the latter behaves in an opposite direction. This phenomenon may be due to 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
Figure 4 shows the time to reach steady state of dissolved oxygen concentration in the test liquid for all aeration runs. Apparently, the time needed for achieving the steady state increases from around 15 s to about 270 s over a total number of 31 tests. Although the increase in time is characteristic of linearity with a coe$cient of correlation of 0)9567, the curve tends to bend upwards as indicated by the last few points, sugesting that a possibly substantial increase in time to reach steady DO
Fig. 4. Time to reach stable state of dissolved oxygen concentration in liquid for each test; r 2, coezcient of determination for linear regression; , linear regression line
310
J. ZHU
total running time. At the end of test, the level of COD is 960 mg l\ so that the di!erence is 2020 mg l\ (a reduction of 68%), while the total pump running time adds up to 1)28 h. For a 0)75 kW pump, the system aeration e$ciency is around 3)20 kg [O ] kWh\. 4. Conclusion
Fig. 5. Oxygen depletion time after reaching stable dissolved oxygen concentration for each test; r 2, coezcient of determination for linear regression; DO, dissolved oxygen; , linear regression line
concentrations in liquid may be needed thereafter. This time increase could be mainly due to the increasing growth and metabolic activity of aerobic bacteria, which accelerates the consumption of oxygen in liquid. However, this hypothesis cannot be veri"ed in this study because of a lack of monitoring aerobic bacterial counts in the test liquid. The increased bacterial activity can also be found in Fig. 5. It can be seen that when aeration stops, the oxygen in liquid is depleted faster in later runs than in early runs, indicating a growing demand of oxygen. These observations may potentially challenge the concept of using intermittent aeration to treat swine manure. According to this study, the pump-o! time declines from 10 min to about 3 min in the 93 h period and has a tendency for disappearing (Fig. 5). This implies that the aeration time has to be increased run by run and, at some point, will become continuous to meet the preset oxygen concentration if an automatic controller is used. Therefore, it can be inferred that intermittent aeration systems, if solely controlled by DO concentrations in liquid, may eventually end up in a continuous operating mode, without saving energy at all. The e$ciency of the aeration system in this study is estimated by the reduction of COD as opposed to the
According to this study, the oxygen uptake rate and the overall oxygen transfer coe$cient of pig slurry are not constant due to the increased biological activities during the aeration process. The former increases from 0)46 mg l\ min\ to about 1)8 mg l\ min\ and the latter decreases from around 0)5 min\ to about 0)3 min\ over a total number of 31 tests. Therefore, simply controlling dissolved oxygen in liquid for intermittent aeration may not be able to achieve an on-ando! operation due possibly to the increased demand on oxygen by rapidly growing aerobic bacteria. Further e!orts should be speci"cally directed to understanding the fundamentals in terms of the changing biological characteristics of the liquid manure during an aeration process. Speci"c to this study, the aeration system can reduce total COD by 68% in a 93 h test period and the aeration e$ciency is thus around 3)20 kg [O ] kWh\. Reference APHA (1998). Standard Methods for the Examination of Water and Waste (18th Edn). American Public Health Association, Washington DC, USA Casey T J; Karmo O T (1974). The in#uence of suspended solids on oxygen transfer in aeration systems. Water Research, 8(4), 805}811 Cumby T R (1987). A review of slurry aeration 3. Performance of aerators. Journal of Agricultural Engineering Research, 36(2), 175}206 Johnston D W M (1984). Oxygen requirements, energy consumption and sludge production in extended-aeration plants. Water Pollution Control, 83(1), 100}115 Saxon J R (1972). Bacteria associated with the treatment and disposal of piggery waste. MS Thesis, Aberdeen University, Scotland
RESEARCH NOTE
Some Intermittent Aeration Kinetics from a Laboratory Study with Pig Slurry Jun Zhu University of Minnesota Southern Research and Outreach Centre, 35838-120th Street, Waseca, MN 56093, USA; e-mail: [email protected] (Received 31 August 2000; accepted in revised form 9 April 2001; published online 11 September 2001)
A 0)75 kW aeration system with a venturi air injector was used in this study to examine the properties of the oxygen uptake rate and overall oxygen transfer coe$cient in pig slurry. Data showed that both parameters varied with the intermittent aeration process. The oxygen uptake rate increased from 0)46 mg l\ min\ to about 1)8 mg l\ min\, while the overall oxygen transfer coe$cient decreased from 0)5 min\ to about 0)3 min\ over a total number of 31 tests. The system had a tendency to run continuously since the o!-run time decreased from 10 min at the beginning of the test to 3 min at the end of the test. This may imply that an intermittent aeration system, if controlled by dissolved oxygen concentration, might not be able to run intermittently due to increased consumption of oxygen by the manure. The aeration system tested achieved an aeration e$ciency of 3)20 kg [O ] kWh\ in this study. 2001 Silsoe Research Institute
1. Introduction The objective of using intermittent aeration is to save energy and, at the same time, provide necessary oxygen to the aerobes to decompose organic compounds. The common practice in using this method is to automatically control the dissolved oxygen concentration in liquid. When the concentration falls outside the preset upper and lower limits, the aeration system will be either stopped or started (Johnston, 1984). However, this simple logic may not work as expected in a real world since the performance of any aeration systems is largely environment speci"c. For instance, in manure environment, the oxygen uptake rate by the aerobes may play a role in determining the performance of an intermittent aeration system (Saxon, 1972; Cumby, 1987). As the available soluble substrate is oxidized, signi"cant variations in the oxygen uptake rate may be observed at di!erent aeration stages. Unfortunately, there is not much information available about such variations in an intermittent aeration course in swine manure. Without this information, it is di$cult to design an optimal intermittent aeration scheme to save energy and, at the same time, accomplish aerobic treatment. Therefore, this research note presents data that reveal the changes in the oxygen uptake rate 0021-8634/01/110307#04 $35.00/0
R and the overall oxygen transfer coe$cient k of a ven*? turi air injector system during an intermittent aeration process and how those changes a!ect the performance of such an aeration system.
2. Materials and methods 2.1. Experimental apparatus The test arrangement is illustrated in Fig. 1. An 1890 l tank was used to contain manure for the test. Fresh manure of 1514 l at 0)5% total solids level was contained in the tank. Prior to loading the tank, the manure was screened using a sieve with 1 mm pore size to remove coarse materials to avoid possible clogging for the venturi air injector. There was no addition or reduction of manure amount in the tank during the test period. The liquid manure used was collected from a commercial swine "nishing building with an initial chemical oxygen demand of 2980 mg l\. A 0)75 kW centrifugal water pump was used to circulate liquid manure through the venturi air injector which was capable of entraining air into liquid at a rate of 4)72 l\ s\. The liquid #ow rate was 151)4 l\ min\.
307
2001 Silsoe Research Institute
308
J. ZHU
Notation C oxygen concentration in liquid, mg l\ C oxygen saturation concentration, mg l\ Q C C oxygen concentration when t"0 and t, R RR mg l\ k overall oxygen transfer coe$cient, 1 min\ *? R oxygen uptake rate by the aerobes in manure, mg l\ min\ r coe$cient of determination
To maximize air retention time in the liquid, two perforated polyvinyl chloride (PVC) pipes of 2)545 cm in diameter, joined perpendicular to each other, were placed horizontally at the bottom of tank so that the oxygenated liquid was always dispersed into the tank from bottom up, with the deoxygenated liquid at the top being sucked into the outside system for air entrapment. The pump and air injector were provided by Environmental Systems, Technology and Research, Inc. (Brussels, WI, USA). An on-site computer was used to automatically record measurements from the dissolved oxygen probe (Point Four Systems Inc., British Columbia, Canada) at set time intervals. The probe was equipped with a battery powered propeller in its front end to cause liquid movement so recording could e!ectively continue when aeration stopped. The computer also controlled the pump switch operated by the dissolved oxygen (DO) levels in the tank. The ambient temperature was not controlled and varied approximately between 18 and 233C over the test period.
in such a way that it would start at the beginning of every 3 h and stop when the DO in liquid reached certain concentrations. The liquid DO concentrations, which were recorded by the computer at a 5 s interval once aeration was initiated, determined the length of each aeration run. When the di!erence between the means of two consecutive sets of 12 DO readings each (1 min) picked up by the computer was less than 0)05 mg l\, the concentration of DO in liquid was considered as having achieved a steady state and the pump was then switched o! by the computer. This recording time frame was determined by observations of several trials prior to the formal test. It best re#ected the oxygenation kinetics of the test manure. A total of 31 runs were conducted so the entire experiment lasted 93 h. After the aeration process stopped, the computer kept recording DO concentrations in the liquid at an interval of 1 min until it approached zero. Data collected in this stage were used to calculate the oxygen uptake rate of the liquid manure (mixed liquor) for each aeration run by plotting out DO concentrations versus time. The slope of the linear regression for each data set was designated as the oxygen uptake rate. To determine the aeration system e$ciency, chemical oxygen demand (COD) was monitored during the test. Liquid samples were taken from the tank at varying intervals and were analysed using the standard methods recommended by the American Public Health Association (APHA, 1998).
2.3. Determination of overall oxygen transfer coe.cient 2.2. Measurement design To study intermittent aeration and overall oxygen transfer e$ciency, the aeration process was programmed
Under non-steady state conditions in which the dissolved oxygen is allowed to approach zero by microbial respiration, the oxygen transfer rate into
Fig. 1. Schematic of the experimental design
A E RA TIO N K IN E TI C S F R OM A S T UD Y W I TH PI G S LU R RY
309
mixed liquor can be described by (Casey & Karmo, 1974). dC "k (C !C)!R *? Q dt
(1)
where: k is the overall oxygen transfer coe$cient; C is *? Q the oxygen saturation concentration in mg l\; C is the oxygen concentration in mg l\ at time t; and R is the oxygen uptake rate by the mixed liquor in mg l\ s\. Assuming the oxygen uptake rate is constant for each speci"c run, the integral of Eqn (1) yields:
1 k (C !C )!R R k " ln *? Q (2) *? t k (C !C )!R *? Q RR where: C and C are the oxygen concentrations at t"0 R RR and t. With Eqn (2), the overall oxygen transfer coe$cient can be calculated by the repetitive substitution method. Since temperature was not monitored in this study, all calculations were based upon the assumption of 203C. The saturation concentration of dissolved oxygen in water at 203C, 760 mm Hg, which was 9)1 mg l\ (could be between 8)6 and 9)4 mg l\), was used in calculations.
Fig. 3. Overall oxygen transfer coezcient measurement during the test period
driven by aeration, consumes more oxygen within a short period of time, resulting in an increase in oxygen uptake rate. On the other hand, a strong and growing demand on oxygen exceeds the oxygen transfer capability of the aeration system, resulting in a decrease in overall oxygen transfer coe$cient.
3.2. Intermittent aeration 3. Results and discussion 3.1. Oxygen uptake rate and overall oxygen transfer coe.cient The oxygen uptake rate R and overall oxygen transfer coe$cient k of the test manure sampled at each 3-h *? interval are presented in Figs 2 and 3. It can be seen that both parameters are not constant. The former increases with the increase of aeration time while the latter behaves in an opposite direction. This phenomenon may be due to 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
Figure 4 shows the time to reach steady state of dissolved oxygen concentration in the test liquid for all aeration runs. Apparently, the time needed for achieving the steady state increases from around 15 s to about 270 s over a total number of 31 tests. Although the increase in time is characteristic of linearity with a coe$cient of correlation of 0)9567, the curve tends to bend upwards as indicated by the last few points, sugesting that a possibly substantial increase in time to reach steady DO
Fig. 4. Time to reach stable state of dissolved oxygen concentration in liquid for each test; r 2, coezcient of determination for linear regression; , linear regression line
310
J. ZHU
total running time. At the end of test, the level of COD is 960 mg l\ so that the di!erence is 2020 mg l\ (a reduction of 68%), while the total pump running time adds up to 1)28 h. For a 0)75 kW pump, the system aeration e$ciency is around 3)20 kg [O ] kWh\. 4. Conclusion
Fig. 5. Oxygen depletion time after reaching stable dissolved oxygen concentration for each test; r 2, coezcient of determination for linear regression; DO, dissolved oxygen; , linear regression line
concentrations in liquid may be needed thereafter. This time increase could be mainly due to the increasing growth and metabolic activity of aerobic bacteria, which accelerates the consumption of oxygen in liquid. However, this hypothesis cannot be veri"ed in this study because of a lack of monitoring aerobic bacterial counts in the test liquid. The increased bacterial activity can also be found in Fig. 5. It can be seen that when aeration stops, the oxygen in liquid is depleted faster in later runs than in early runs, indicating a growing demand of oxygen. These observations may potentially challenge the concept of using intermittent aeration to treat swine manure. According to this study, the pump-o! time declines from 10 min to about 3 min in the 93 h period and has a tendency for disappearing (Fig. 5). This implies that the aeration time has to be increased run by run and, at some point, will become continuous to meet the preset oxygen concentration if an automatic controller is used. Therefore, it can be inferred that intermittent aeration systems, if solely controlled by DO concentrations in liquid, may eventually end up in a continuous operating mode, without saving energy at all. The e$ciency of the aeration system in this study is estimated by the reduction of COD as opposed to the
According to this study, the oxygen uptake rate and the overall oxygen transfer coe$cient of pig slurry are not constant due to the increased biological activities during the aeration process. The former increases from 0)46 mg l\ min\ to about 1)8 mg l\ min\ and the latter decreases from around 0)5 min\ to about 0)3 min\ over a total number of 31 tests. Therefore, simply controlling dissolved oxygen in liquid for intermittent aeration may not be able to achieve an on-ando! operation due possibly to the increased demand on oxygen by rapidly growing aerobic bacteria. Further e!orts should be speci"cally directed to understanding the fundamentals in terms of the changing biological characteristics of the liquid manure during an aeration process. Speci"c to this study, the aeration system can reduce total COD by 68% in a 93 h test period and the aeration e$ciency is thus around 3)20 kg [O ] kWh\. Reference APHA (1998). Standard Methods for the Examination of Water and Waste (18th Edn). American Public Health Association, Washington DC, USA Casey T J; Karmo O T (1974). The in#uence of suspended solids on oxygen transfer in aeration systems. Water Research, 8(4), 805}811 Cumby T R (1987). A review of slurry aeration 3. Performance of aerators. Journal of Agricultural Engineering Research, 36(2), 175}206 Johnston D W M (1984). Oxygen requirements, energy consumption and sludge production in extended-aeration plants. Water Pollution Control, 83(1), 100}115 Saxon J R (1972). Bacteria associated with the treatment and disposal of piggery waste. MS Thesis, Aberdeen University, Scotland