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Aeration of anaerobically treated potato-processing wastewater
Agricultural Wastes 12 (1985) 1-11
Aeration of Anaerobically Treated Potato-processing Wastewater K. C. Lin Department of Civil Engineering, Universityof New Brunswick, Fredericton, N.B., Canada
A BS TRA C T The performance of an aerated lagoon, .[ollowing the anaerobic lagoon-filter units, in treating unsettled potato-processing wastewater at low temperatures between 4 and 20°C has been evaluated from a laboratoo" scale model. COD, SS, p H and reaction rates data have been assessed. Such a combination of reactors provided a very stable and satisfactoo' effluent even at temperatures as low as 4°C.
INTRODUCTION Potato processing is one of the major industries in the Province of New Brunswick, Canada. In 1971,635 000 000 kg of potatoes were produced in the province. Of this amount, 144 100 000 kg were processed by McCain Foods Ltd--the major processor in New Brunswick. In 1981, of the approximately 545 000 000 kg produced, 236 400 000 kg were processed. An average daily production (1981) of 710000kg resulted in a biochemical oxygen demand (BOD) load in the waste of 22 000 kg d a y - 1 Five years ago, a study was initiated at the University of New Brunswick to investigate the efficacy of employing an anaerobic lagoon-filter system to treat the potato-processing wastewater (Lin & Brown, 1980). The results were very successful and encouraged further studies (Lin et al., 1982). It was found that the system provided high removals of BOD and SS (Suspended Solids) under different loading and temperature conditions. 1
Agricultural Wastes 0141-4607/85/$03"30 © ElsevierApplied Science Publishers Ltd, England, 1985. Printed in Great
Britain
2
K . C . Lin
It is often reported that reduced efficiency of anaerobic treatment occurs outside the mesophilic and thermophilic temperature ranges (Heukelekian, 1933; McCarty, 1964). O'Rourke (I 968) observed that, for optimum growth of mesophilic organisms, the lowest temperature limit appeared to be 20 °C. However, the anaerobic lagoon-filter system used by L i n e t al. (1982) performed well between 20°C and 10°C, although reduced efficiency was observed at lower temperatures. When the temperature was decreased to 2°C, the system approached failure, indicated by a pronounced increase in chemical oxygen demand (COD) and Total Volatile Acids of lagoon and filter effluents. The results are more in agreement with those of Lettinga et al. (1979) who found their Upflow Anaerobic Sludge Blanket (UASB) process effective at suboptimal temperatures when using lower organic loading rates. It was thought that the quality of the effluent of the laboratory scale anaerobic system used by Lin et al. (1982) could be improved by aeration. Such an additional treatment may reduce the organic content of the effluent and possibly satisfy some of the nitrogenous oxygen demand. The arrangement of the laboratory scale system was the same as that used in the previous studies (Lin & Brown, 1980; L i n e t al., 1982) with the exception of an attachment of an aerated lagoon unit at the end. The results of the study are reported below.
METHODS The aerated lagoon was connected in series with the anaerobic units. The anaerobic units consisted of two cylindrical Plexiglas columns. The anaerobic lagoon, with an inside diameter of 191 mm and a liquid height of 1.15 m, had a total volume of 32.61itres. The anaerobic filter that followed had an inside diameter of 152 m m and a liquid height of 0.91 m for a total volume of 16.21itres. The stone media in the filter were 25-40 m m in size, submerged 60-80 m m below the liquid surface. The liquid volume of the filter was 6.8 litres. Both the anaerobic lagoon and the anaerobic filter were sealed at the top to facilitate gas collection and measurement. The anaerobic filter effluent entered the aerated lagoon. The aerated lagoon was another Plexiglas column of 155 m m inside diameter and 760mm liquid height for a volume of 14.31itres. Compressed air was introduced via a perforated tube across the bottom of the aerobic lagoon.
Aeration o[anaerohically treated potato-processing wastewater
3
Continuous feed to the anaerobic-aerobic system consisted of unsettled primary clarifier influent (PC1) obtained from the potatoprocessing plant of McCain Foods Ltd in Fiorenceville, New Brunswick, Canada. The C O D of the feed averaged 2700 mglitre-1. The average hydraulic and organic loading rates were 4"21itres day i and 0.35 kg C O D per cubic metre per day, respectively, based on the anaerobic lagoon volume. The detention times for the anaerobic lagoon, the anaerobic filter and the aerated lagoon were 7, 1-5 and 3.5 days, respectively. No sludge was wasted from the system. As the initial pH of the PCI was low (approximately 4.5), lime was added to the feed tank and filter effluent was recycled back to the anaerobic lagoon at 100 % of the feed rate to obtain a suitable pH ( ~ 7.0) and adequate alkalinity ( - 800 mg litre 1 as CaCO3). Complete mixing was maintained in the aerated lagoon with a dissolved oxygen level of 8.0 mg litre- 1. The whole experimental system was temperature controlled in a coldstream environmental chamber. The temperature, initially at 20 °C, was dropped, in 2 °C decrements, down to 2 °C. The system was operated to reach steady state at each temperature except for 2 °C, at which the anaerobic stage approached failure after running for 56 days. At this time, the temperature was increased to 6 °C, then to 8 °C and, finally, to 20°C, for system recovery. Samples were taken from the influent and effluent of each reactor for analyses. Effluent quality was monitored by COD, BOD and SS tests. Other data like pH, alkalinity, Volatile Acids, gas production and solids accumulation were also collected. The performance of the anaerobic units was reported elsewhere (Lin & Brown, 1980; L i n e t al., 1982). The performance of the aerated lagoon in polishing the anaerobically treated wastewater is discussed below.
RESULTS A N D DISCUSSION Figure 1 shows the input of C O D and SS to the whole system during the course of the study. Care was taken to provide as near a constant input to the system as possible. However, due to the variable nature of the PC1 samples collected from the potato-processing plant, the influent concentrations of organics and solids varied somewhat. The average concentrations of C O D and SS in the feed (anaerobic lagoon influent)
K. C. Lin Temperature (°C) 18
I
16
I
i
14
12
I
I0
I
8
1
6
I
4
i
2
I
I
6000 C00
4000
(mg/O
2000 0
3000 SS
2000
(mgll) IO00
I0
20
30
40
50
70
60
Time (weeks)
Fig. 1.
Input to the anaerobic-aerobic system.
Temperature le
t
16
t
14
I
IZ
10
i
(°C) t
8
i
6
4
t
2
t
I000
/\ Anaerobic filter
COD (rag/I)
i /
800 effluent
600
i'
/ ,/" ,/
..
400
i
200 I
I0
I,
20
I
30 Time
Fig, 2.
I
1
40
50
!
60
(weeks)
COD concentrations in the influent and effluent of the aerated lagoon.
TO
Aeration of anaerobically treated potato-processing wastewater TemperQture 18 300
I
t6
14
I
i
12 I
5
(°C)
IO i
8
6
!
J
4 |
2 i
J
200 SS (rag/l)
fO0
Aaaera*,o ,.,.. ;
.,,..j\ •
,.,
/ ......
~
t'; .:'
,,,
elf ueat/ , . , ~ , / , ,., . ! ~ !~!
"-.'
",.-" , .,. J
Aerated 0
I I0
I Z0
I 5O
I 40
~ " . ...,.\ ,
.ii 'i"-:.' ~
lagoon effluent I 50
! 6O
70
Time (weeke]
Fig. 3. SS concentrations in the influent and effluent of the aerated lagoon. were 2700 and 1380 mg litre- 1, respectively; the corresponding loading rates were 0.35 kg per cubic metre per day and 0-18 kg per cubic metre per day. Figures 2 and 3 show the anaerobic effluent concentrations of C O D and SS, which acted as input to the aerated lagoon, as well as their final effluent concentrations after aeration. The C O D plot (Fig. 2) indicates that the aerated lagoon was able to reduce the C O D concentration in the anaerobic effluent by almost 50 % at all temperatures. The SS plot (Fig. 3) is similar to the C O D plot except at 2 °C, at which the SS concentration in the aerated lagoon effluent was even higher than that in the anaerobic effluent. In fact, both Figures show that the results at 2°C were very unstable. As temperature was decreased from 4 to 2 °C, C O D and SS in the aerated lagoon effluent increased substantially as a result of their high concentrations in the anaerobic effluent. After 56 days of operation at 2 °C, alkalinity of the anaerobic effluent dropped from an average of 990 mg litre-1 to a low level of 285 mg litre-1 while the Total Volatile Acids increased from less than 100 mg litre-1 to 390 mg litre-1. These, along with the substantial increase in C O D and SS concentrations in the anaerobic and aerobic effluents, aroused concern about system failure. Therefore, the 2 °C operation was halted and the system was allowed to recover by increasing the temperature. Per cent removals of C O D and SS by the anaerobic-aerobic system are shown in Figs 4 and 5. These are steady-state time-averages except for 2 °C. C O D removal by the aerated lagoon ranged from 20 % at 18 °C to
6
K . C . Lin
100
r ~
~=.~ s
I
80
60
COD Removal (%)
~.
40
~. - ~ - .~ - - - ~
-'~'-'~" :
20
~ ....
=
~
~
T o %'01 A n a e r obtc Aerated
units lagoon
Tamparat~e
Fig. 4.
(*C)
Removals of COD.
68 ~o at 8 °C. Below 8 °C the removal of C O D decreased to 60 ~ at 6 °C, 59 ~o at 4 °C and 50 ~o after 56 days of operation at 2 °C. The majority of C O D was removed in the anaerobic process, averaging 93 ~o within the 20 °C-4°C temperature range; the aerated lagoon added another 2-9 ~o, resulting in an average C O D removal of more than 96 ~o by the whole system between 20 °C and 4 °C. At 4 °C, a total of 94 ~o removal of COD was still attainable by the system. A pilot-plant study was reported by Dostal (1970) on the treatment of potato-processing wastes employing an aerated lagoon or IO0"
f" --~. •. -~-- • -~ ~
80
~u-.-~-.-~-.-~•
60
-~
--
Tota I Anaerobic -" A e r a t e d
units lagoon
SS Removal (%)
40
•
/i 20
z" negative i~f
i 4
% $ S removal i 6
at 2 ° C , 8
Temperoture
I10
i 12
(°C)
Fig. 5. Removals of SS.
I 14
, 16
i 18
I 20
Aeration of anaerobically treated potato-processing wastewater
7
anaerobic-aerobic lagoons in series after primary sedimentation. An overall BOD reduction of more than 90 ~o was observed (overall COD removal appeared to be lower). However, the pilot-plant study cannot be compared with this laboratory scale study for two main reasons. (1) Caustic peel waste was used in the pilot plant (pH = 10.6-12.1) but steampeeled waste was used in this study (initial pH = 4.5 adjusted to 7"0 by lime addition). (2) The aerated lagoons in the pilot plant were the main biological reactors and the one used in this study only served to polish the anaerobic effluent. SS removal in this study mainly occurred in the anaerobic process-more than 93 ~o between 20 and 4 °C. The highest SS removal noted was about 98 ~ at 14 °C. The high removal of SS by the anaerobic process was mainly due to the effects of sedimentation and filtering of solids through the bottom sludge layer in the anaerobic lagoon. The aerated lagoon removed an additional 0.6-3.5 ~o of SS between 20 °C and 4 °C, resulting in an average SS removal of more than 97 ~o by the whole system within this range of temperature. At 4°C, the system was still able to remove 96 ~ of SS. However, at 2 °C, the SS concentration in the effluent of the aerated lagoon was even higher than that in its influent (Fig. 3). The efficiency of biological waste treatment usually decreases at lower temperatures within the normal range of operation. The results of SS removal shown in Fig. 5 are in agreement with such an observation. When Fig. 4, on COD removal, is examined, some apparently different results are found. Between 18 °C and 8 °C, the aerated lagoon was able to remove more COD at lower temperatures. In fact, the 68 ~o COD removal at 8 °C was the highest among all the temperatures studied. Although the COD removal at 20 °C was higher than that at 18 °C, there was an overall trend of increasing COD removal at lower temperatures down to 8 °C. Below 8 °C, low temperature played a definite r61e in reducing the efficiency of the aerated lagoon in COD removal. The reversed pattern of COD removal by the aerated lagoon between 18 °C and 8 °C may be explained as follows. At higher temperatures, most of the easily biodegradable material was removed in the anaerobic process. The low organic loading to the aerated lagoon was composed of organics which were more difficult to biodegrade, resulting in a lower organics removal in the aerated lagoon. As temperature was decreased, the performance of the anaerobic lagoon and filter, especially the latter, deteriorated. More of the easily biodegradable material became available to the microorganisms in the aerated lagoon. This is supported by the
K. C. Lin
TABLE 1 COD and BOD Data for the Aerated Lagoon lnfluent
COD(mglitre-1) BOD (mglitre-1) BOD/COD
20
18
16
14
190 49 0-26
183 31 0.17
142 67 0-47
176 ---
Temperature ( ° C) 12 I0 8
118 62 0.53
130 67 0.52
6
303 154 156 167 0 . 5 1 (1.1)
4
2
290 186 0.64
700* 429* 0.61"
* Average values within 56 days of unsteady operation.
higher B O D / C O D ratios of the aerated lagoon influent at lower temperatures, as shown in Table 1. As a result, more of these easy-tobiodegrade organics were removed. This paradox of higher C O D removal at lower temperatures in the aerated lagoon disappeared at temperatures below 8 °C. Based on the mass-balance principle of mass in, mass out, and firstorder reaction in a steady-state, completely mixed reactor without recycle, the reaction rates were evaluated for the aerated lagoon at different temperatures according to: K-
E t(1 - E)
(1)
in which K = first-order reaction rate (per day); E = efficiency of reaction and t = hydraulic detention time (days). Considering the effects of sludge solids, eqn. (1) can be modified to: K' -
E xj(!
- E)
(2)
in which K' = reaction rate (litres m g - ~d a y - 1) and x, = average MLSS concentration (mg litre- 1). Values of K and K' based on BOD results at different temperatures for the aerated lagoon are shown in Table 2. It is seen that K varies between 0.186 and 1.32 per day and that K' varies between 0-0067 and 0.0549 litres m g - 1 d a y - 1 within the temperature range of 20 °C~, °C. The reported values in the literature are K = 0.3 to over 1.0 per day (Clark et al., 1977) and K' = 0.00006-0.0028 litres rag- ~h - ~, corresponding to 0.001 44-0.0672 litres m g - ~d a y - ~ (Eckenfelder, 1967). Therefore, the reaction rates, K and K', observed in this study are within the normal
Aeration o f anaerobically treated potato-processing wastewater
TABLE 2 Reaction Rates
K' (litres m g - t d a y -1) K (per day)
20
18
16
14
0"031 I 1.021
0.0067 0.186
-0.740
---
Temperature (°C) 12 10
8
0.0549 0.0379 0.0392 0 - 7 0 2 0.617 1.32
6
4
2
0.0307 0.0383 0.0035* 0.899 1.29 0.65*
* Average values within 56 days of unsteady operation.
ranges except for K = 0.186 per day at 18 °C. It is noticed that the lowest K' value of 0.0067 litres rag- ~day- ~measured in this study also occurred at 18 °C. An attempt was made to find the effect of temperature (T) on the reaction rates (K or K') according to the equation: (3)
K2 = KIOT,- rl
However, a statistical analysis of the data showed that there was no significant correlation between log K and T. Therefore, a meaningful value of the temperature coetficient, 0, could not be found. Usually, the lower the temperature, the lower the reaction rate. In this case, the effect of temperature is probably masked by the increase in easily biodegradable material supplied to the aerated lagoon at lower temperatures. The aerated lagoon not only helped to polish the effluent of the anaerobic process, but also increased the pH of the treated wastewater. Table 3 shows the variation in pH of the influent and effluent of the aerated lagoon; their ranges are 6.1-6-6 and 7.3-8.0, respectively. As the pH of the PCI entering the anaerobic stage was 6-0-6.3, it appears that increased alkalinity in the anaerobic process helped to raise the pH in the anaerobic units and that the destruction of some organic acids, together TABLE 3 p H D a t a for t h e A e r a t e d L a g o o n
Influent Etlluent
20
18
16
14
6"6 8.0
6.6 7.5
6.5 7.4
6.4 7-3
Temperature ( ° C ) 12 10 8 6.5 7.4
6.5 7.6
• A v e r a g e v a l u e s w i t h i n 56 d a y s o f u n s t e a d y o p e r a t i o n .
6-6 7.6
6
4
2
6-4 7.5
6'2 7.4
6.1 * 7-4*
I0
K. C. Lin
with the stripping of excess CO2 in the aerated lagoon, boosted the final pH to over 7.3. The advantage of adding an aerated lagoon after the anaerobic lagoon filter combination is quite obvious. Under the conditions studied, the capacity of the aerated lagoon was not fully utilized. The whole anaerobic-aerobic system was found to be stable and efficient in C O D and SS removals at all temperatures above 4°C. Any excesss sludge produced in aeration should not pose a problem, since it can be recycled back to the influent of the anaerobic units.
ACKN O W L E D G E M ENTS This research was financially supported by the Natural Sciences and Engineering Research Council of Canada under operating grant No. A9314. Experimental work was performed by Mr Stephen Bliss, a former graduate student in the Department of Civil Engineering at the University of New Brunswick. Wastewater samples were provided by McCain Foods Ltd, Florenceville, New Brunswick, Canada.
REFERENCES Clark, J. W., Viessman, Jr., W. & Hammer, M. J. (1977). Water supply and pollution control. (3rd edn.). Harper & Row Publishers, 857 pp. Dostal, K. A. (1970). Aerated lagoons for potato processing wastes. Second International Symposium for Waste Treatment Lagoons, Kansas City, Missouri, 243-9. Eckenfelder, Jr., W. W. (1967). Comparative biological waste treatment design. Journal of the Sanitary Engineering Division, Proceedings of the American Society of Civil Engineers, 93 (SA6), 157-70. Heukelekian, H. (1973). Digestion of solids between the thermophilic and nonthermophilic range. Sewage Works, 5(5), 757-62. Lettinga, G., van Velsen, A. F. M., deZeeuw, W. & Hobma, S. W. (1979). Feasibility of the upflow anaerobic sludge blanket (UASB) process. Proceedings of 1979 National Conference on Environmental Engineering, American Society of Civil Engineers, San Francisco, 35 45. Lin, K. C. & Brown, G. J. (1980). Treatment of potato-processing waste water by an anaerobic lagoon-filter system. Canadian Journal of Civil Engineering, 7(2), 373 83.
Aeration ~] anaerobically treated potato-processing wastewater
11
Lin, K. C., Landine, R. C. & Bliss, S. G. (1982). Temperature effect on the anaerobic treatment of potato-processing wastewater. Canadian Journal of Civil Engineering, 9(3), 549-57. McCarty, P. L. (1964). Anaerobic waste treatment fundamentals, Parts 1 and 2. Public' Works, Sept., 107-11; Oct., 123-6. O'Rourke, J. T. (1968). Kinetics o./ anaerobic waste treatment at reduced temperatures. PhD Thesis, Stanford University, California.
Aeration of Anaerobically Treated Potato-processing Wastewater K. C. Lin Department of Civil Engineering, Universityof New Brunswick, Fredericton, N.B., Canada
A BS TRA C T The performance of an aerated lagoon, .[ollowing the anaerobic lagoon-filter units, in treating unsettled potato-processing wastewater at low temperatures between 4 and 20°C has been evaluated from a laboratoo" scale model. COD, SS, p H and reaction rates data have been assessed. Such a combination of reactors provided a very stable and satisfactoo' effluent even at temperatures as low as 4°C.
INTRODUCTION Potato processing is one of the major industries in the Province of New Brunswick, Canada. In 1971,635 000 000 kg of potatoes were produced in the province. Of this amount, 144 100 000 kg were processed by McCain Foods Ltd--the major processor in New Brunswick. In 1981, of the approximately 545 000 000 kg produced, 236 400 000 kg were processed. An average daily production (1981) of 710000kg resulted in a biochemical oxygen demand (BOD) load in the waste of 22 000 kg d a y - 1 Five years ago, a study was initiated at the University of New Brunswick to investigate the efficacy of employing an anaerobic lagoon-filter system to treat the potato-processing wastewater (Lin & Brown, 1980). The results were very successful and encouraged further studies (Lin et al., 1982). It was found that the system provided high removals of BOD and SS (Suspended Solids) under different loading and temperature conditions. 1
Agricultural Wastes 0141-4607/85/$03"30 © ElsevierApplied Science Publishers Ltd, England, 1985. Printed in Great
Britain
2
K . C . Lin
It is often reported that reduced efficiency of anaerobic treatment occurs outside the mesophilic and thermophilic temperature ranges (Heukelekian, 1933; McCarty, 1964). O'Rourke (I 968) observed that, for optimum growth of mesophilic organisms, the lowest temperature limit appeared to be 20 °C. However, the anaerobic lagoon-filter system used by L i n e t al. (1982) performed well between 20°C and 10°C, although reduced efficiency was observed at lower temperatures. When the temperature was decreased to 2°C, the system approached failure, indicated by a pronounced increase in chemical oxygen demand (COD) and Total Volatile Acids of lagoon and filter effluents. The results are more in agreement with those of Lettinga et al. (1979) who found their Upflow Anaerobic Sludge Blanket (UASB) process effective at suboptimal temperatures when using lower organic loading rates. It was thought that the quality of the effluent of the laboratory scale anaerobic system used by Lin et al. (1982) could be improved by aeration. Such an additional treatment may reduce the organic content of the effluent and possibly satisfy some of the nitrogenous oxygen demand. The arrangement of the laboratory scale system was the same as that used in the previous studies (Lin & Brown, 1980; L i n e t al., 1982) with the exception of an attachment of an aerated lagoon unit at the end. The results of the study are reported below.
METHODS The aerated lagoon was connected in series with the anaerobic units. The anaerobic units consisted of two cylindrical Plexiglas columns. The anaerobic lagoon, with an inside diameter of 191 mm and a liquid height of 1.15 m, had a total volume of 32.61itres. The anaerobic filter that followed had an inside diameter of 152 m m and a liquid height of 0.91 m for a total volume of 16.21itres. The stone media in the filter were 25-40 m m in size, submerged 60-80 m m below the liquid surface. The liquid volume of the filter was 6.8 litres. Both the anaerobic lagoon and the anaerobic filter were sealed at the top to facilitate gas collection and measurement. The anaerobic filter effluent entered the aerated lagoon. The aerated lagoon was another Plexiglas column of 155 m m inside diameter and 760mm liquid height for a volume of 14.31itres. Compressed air was introduced via a perforated tube across the bottom of the aerobic lagoon.
Aeration o[anaerohically treated potato-processing wastewater
3
Continuous feed to the anaerobic-aerobic system consisted of unsettled primary clarifier influent (PC1) obtained from the potatoprocessing plant of McCain Foods Ltd in Fiorenceville, New Brunswick, Canada. The C O D of the feed averaged 2700 mglitre-1. The average hydraulic and organic loading rates were 4"21itres day i and 0.35 kg C O D per cubic metre per day, respectively, based on the anaerobic lagoon volume. The detention times for the anaerobic lagoon, the anaerobic filter and the aerated lagoon were 7, 1-5 and 3.5 days, respectively. No sludge was wasted from the system. As the initial pH of the PCI was low (approximately 4.5), lime was added to the feed tank and filter effluent was recycled back to the anaerobic lagoon at 100 % of the feed rate to obtain a suitable pH ( ~ 7.0) and adequate alkalinity ( - 800 mg litre 1 as CaCO3). Complete mixing was maintained in the aerated lagoon with a dissolved oxygen level of 8.0 mg litre- 1. The whole experimental system was temperature controlled in a coldstream environmental chamber. The temperature, initially at 20 °C, was dropped, in 2 °C decrements, down to 2 °C. The system was operated to reach steady state at each temperature except for 2 °C, at which the anaerobic stage approached failure after running for 56 days. At this time, the temperature was increased to 6 °C, then to 8 °C and, finally, to 20°C, for system recovery. Samples were taken from the influent and effluent of each reactor for analyses. Effluent quality was monitored by COD, BOD and SS tests. Other data like pH, alkalinity, Volatile Acids, gas production and solids accumulation were also collected. The performance of the anaerobic units was reported elsewhere (Lin & Brown, 1980; L i n e t al., 1982). The performance of the aerated lagoon in polishing the anaerobically treated wastewater is discussed below.
RESULTS A N D DISCUSSION Figure 1 shows the input of C O D and SS to the whole system during the course of the study. Care was taken to provide as near a constant input to the system as possible. However, due to the variable nature of the PC1 samples collected from the potato-processing plant, the influent concentrations of organics and solids varied somewhat. The average concentrations of C O D and SS in the feed (anaerobic lagoon influent)
K. C. Lin Temperature (°C) 18
I
16
I
i
14
12
I
I0
I
8
1
6
I
4
i
2
I
I
6000 C00
4000
(mg/O
2000 0
3000 SS
2000
(mgll) IO00
I0
20
30
40
50
70
60
Time (weeks)
Fig. 1.
Input to the anaerobic-aerobic system.
Temperature le
t
16
t
14
I
IZ
10
i
(°C) t
8
i
6
4
t
2
t
I000
/\ Anaerobic filter
COD (rag/I)
i /
800 effluent
600
i'
/ ,/" ,/
..
400
i
200 I
I0
I,
20
I
30 Time
Fig, 2.
I
1
40
50
!
60
(weeks)
COD concentrations in the influent and effluent of the aerated lagoon.
TO
Aeration of anaerobically treated potato-processing wastewater TemperQture 18 300
I
t6
14
I
i
12 I
5
(°C)
IO i
8
6
!
J
4 |
2 i
J
200 SS (rag/l)
fO0
Aaaera*,o ,.,.. ;
.,,..j\ •
,.,
/ ......
~
t'; .:'
,,,
elf ueat/ , . , ~ , / , ,., . ! ~ !~!
"-.'
",.-" , .,. J
Aerated 0
I I0
I Z0
I 5O
I 40
~ " . ...,.\ ,
.ii 'i"-:.' ~
lagoon effluent I 50
! 6O
70
Time (weeke]
Fig. 3. SS concentrations in the influent and effluent of the aerated lagoon. were 2700 and 1380 mg litre- 1, respectively; the corresponding loading rates were 0.35 kg per cubic metre per day and 0-18 kg per cubic metre per day. Figures 2 and 3 show the anaerobic effluent concentrations of C O D and SS, which acted as input to the aerated lagoon, as well as their final effluent concentrations after aeration. The C O D plot (Fig. 2) indicates that the aerated lagoon was able to reduce the C O D concentration in the anaerobic effluent by almost 50 % at all temperatures. The SS plot (Fig. 3) is similar to the C O D plot except at 2 °C, at which the SS concentration in the aerated lagoon effluent was even higher than that in the anaerobic effluent. In fact, both Figures show that the results at 2°C were very unstable. As temperature was decreased from 4 to 2 °C, C O D and SS in the aerated lagoon effluent increased substantially as a result of their high concentrations in the anaerobic effluent. After 56 days of operation at 2 °C, alkalinity of the anaerobic effluent dropped from an average of 990 mg litre-1 to a low level of 285 mg litre-1 while the Total Volatile Acids increased from less than 100 mg litre-1 to 390 mg litre-1. These, along with the substantial increase in C O D and SS concentrations in the anaerobic and aerobic effluents, aroused concern about system failure. Therefore, the 2 °C operation was halted and the system was allowed to recover by increasing the temperature. Per cent removals of C O D and SS by the anaerobic-aerobic system are shown in Figs 4 and 5. These are steady-state time-averages except for 2 °C. C O D removal by the aerated lagoon ranged from 20 % at 18 °C to
6
K . C . Lin
100
r ~
~=.~ s
I
80
60
COD Removal (%)
~.
40
~. - ~ - .~ - - - ~
-'~'-'~" :
20
~ ....
=
~
~
T o %'01 A n a e r obtc Aerated
units lagoon
Tamparat~e
Fig. 4.
(*C)
Removals of COD.
68 ~o at 8 °C. Below 8 °C the removal of C O D decreased to 60 ~ at 6 °C, 59 ~o at 4 °C and 50 ~o after 56 days of operation at 2 °C. The majority of C O D was removed in the anaerobic process, averaging 93 ~o within the 20 °C-4°C temperature range; the aerated lagoon added another 2-9 ~o, resulting in an average C O D removal of more than 96 ~o by the whole system between 20 °C and 4 °C. At 4 °C, a total of 94 ~o removal of COD was still attainable by the system. A pilot-plant study was reported by Dostal (1970) on the treatment of potato-processing wastes employing an aerated lagoon or IO0"
f" --~. •. -~-- • -~ ~
80
~u-.-~-.-~-.-~•
60
-~
--
Tota I Anaerobic -" A e r a t e d
units lagoon
SS Removal (%)
40
•
/i 20
z" negative i~f
i 4
% $ S removal i 6
at 2 ° C , 8
Temperoture
I10
i 12
(°C)
Fig. 5. Removals of SS.
I 14
, 16
i 18
I 20
Aeration of anaerobically treated potato-processing wastewater
7
anaerobic-aerobic lagoons in series after primary sedimentation. An overall BOD reduction of more than 90 ~o was observed (overall COD removal appeared to be lower). However, the pilot-plant study cannot be compared with this laboratory scale study for two main reasons. (1) Caustic peel waste was used in the pilot plant (pH = 10.6-12.1) but steampeeled waste was used in this study (initial pH = 4.5 adjusted to 7"0 by lime addition). (2) The aerated lagoons in the pilot plant were the main biological reactors and the one used in this study only served to polish the anaerobic effluent. SS removal in this study mainly occurred in the anaerobic process-more than 93 ~o between 20 and 4 °C. The highest SS removal noted was about 98 ~ at 14 °C. The high removal of SS by the anaerobic process was mainly due to the effects of sedimentation and filtering of solids through the bottom sludge layer in the anaerobic lagoon. The aerated lagoon removed an additional 0.6-3.5 ~o of SS between 20 °C and 4 °C, resulting in an average SS removal of more than 97 ~o by the whole system within this range of temperature. At 4°C, the system was still able to remove 96 ~ of SS. However, at 2 °C, the SS concentration in the effluent of the aerated lagoon was even higher than that in its influent (Fig. 3). The efficiency of biological waste treatment usually decreases at lower temperatures within the normal range of operation. The results of SS removal shown in Fig. 5 are in agreement with such an observation. When Fig. 4, on COD removal, is examined, some apparently different results are found. Between 18 °C and 8 °C, the aerated lagoon was able to remove more COD at lower temperatures. In fact, the 68 ~o COD removal at 8 °C was the highest among all the temperatures studied. Although the COD removal at 20 °C was higher than that at 18 °C, there was an overall trend of increasing COD removal at lower temperatures down to 8 °C. Below 8 °C, low temperature played a definite r61e in reducing the efficiency of the aerated lagoon in COD removal. The reversed pattern of COD removal by the aerated lagoon between 18 °C and 8 °C may be explained as follows. At higher temperatures, most of the easily biodegradable material was removed in the anaerobic process. The low organic loading to the aerated lagoon was composed of organics which were more difficult to biodegrade, resulting in a lower organics removal in the aerated lagoon. As temperature was decreased, the performance of the anaerobic lagoon and filter, especially the latter, deteriorated. More of the easily biodegradable material became available to the microorganisms in the aerated lagoon. This is supported by the
K. C. Lin
TABLE 1 COD and BOD Data for the Aerated Lagoon lnfluent
COD(mglitre-1) BOD (mglitre-1) BOD/COD
20
18
16
14
190 49 0-26
183 31 0.17
142 67 0-47
176 ---
Temperature ( ° C) 12 I0 8
118 62 0.53
130 67 0.52
6
303 154 156 167 0 . 5 1 (1.1)
4
2
290 186 0.64
700* 429* 0.61"
* Average values within 56 days of unsteady operation.
higher B O D / C O D ratios of the aerated lagoon influent at lower temperatures, as shown in Table 1. As a result, more of these easy-tobiodegrade organics were removed. This paradox of higher C O D removal at lower temperatures in the aerated lagoon disappeared at temperatures below 8 °C. Based on the mass-balance principle of mass in, mass out, and firstorder reaction in a steady-state, completely mixed reactor without recycle, the reaction rates were evaluated for the aerated lagoon at different temperatures according to: K-
E t(1 - E)
(1)
in which K = first-order reaction rate (per day); E = efficiency of reaction and t = hydraulic detention time (days). Considering the effects of sludge solids, eqn. (1) can be modified to: K' -
E xj(!
- E)
(2)
in which K' = reaction rate (litres m g - ~d a y - 1) and x, = average MLSS concentration (mg litre- 1). Values of K and K' based on BOD results at different temperatures for the aerated lagoon are shown in Table 2. It is seen that K varies between 0.186 and 1.32 per day and that K' varies between 0-0067 and 0.0549 litres m g - 1 d a y - 1 within the temperature range of 20 °C~, °C. The reported values in the literature are K = 0.3 to over 1.0 per day (Clark et al., 1977) and K' = 0.00006-0.0028 litres rag- ~h - ~, corresponding to 0.001 44-0.0672 litres m g - ~d a y - ~ (Eckenfelder, 1967). Therefore, the reaction rates, K and K', observed in this study are within the normal
Aeration o f anaerobically treated potato-processing wastewater
TABLE 2 Reaction Rates
K' (litres m g - t d a y -1) K (per day)
20
18
16
14
0"031 I 1.021
0.0067 0.186
-0.740
---
Temperature (°C) 12 10
8
0.0549 0.0379 0.0392 0 - 7 0 2 0.617 1.32
6
4
2
0.0307 0.0383 0.0035* 0.899 1.29 0.65*
* Average values within 56 days of unsteady operation.
ranges except for K = 0.186 per day at 18 °C. It is noticed that the lowest K' value of 0.0067 litres rag- ~day- ~measured in this study also occurred at 18 °C. An attempt was made to find the effect of temperature (T) on the reaction rates (K or K') according to the equation: (3)
K2 = KIOT,- rl
However, a statistical analysis of the data showed that there was no significant correlation between log K and T. Therefore, a meaningful value of the temperature coetficient, 0, could not be found. Usually, the lower the temperature, the lower the reaction rate. In this case, the effect of temperature is probably masked by the increase in easily biodegradable material supplied to the aerated lagoon at lower temperatures. The aerated lagoon not only helped to polish the effluent of the anaerobic process, but also increased the pH of the treated wastewater. Table 3 shows the variation in pH of the influent and effluent of the aerated lagoon; their ranges are 6.1-6-6 and 7.3-8.0, respectively. As the pH of the PCI entering the anaerobic stage was 6-0-6.3, it appears that increased alkalinity in the anaerobic process helped to raise the pH in the anaerobic units and that the destruction of some organic acids, together TABLE 3 p H D a t a for t h e A e r a t e d L a g o o n
Influent Etlluent
20
18
16
14
6"6 8.0
6.6 7.5
6.5 7.4
6.4 7-3
Temperature ( ° C ) 12 10 8 6.5 7.4
6.5 7.6
• A v e r a g e v a l u e s w i t h i n 56 d a y s o f u n s t e a d y o p e r a t i o n .
6-6 7.6
6
4
2
6-4 7.5
6'2 7.4
6.1 * 7-4*
I0
K. C. Lin
with the stripping of excess CO2 in the aerated lagoon, boosted the final pH to over 7.3. The advantage of adding an aerated lagoon after the anaerobic lagoon filter combination is quite obvious. Under the conditions studied, the capacity of the aerated lagoon was not fully utilized. The whole anaerobic-aerobic system was found to be stable and efficient in C O D and SS removals at all temperatures above 4°C. Any excesss sludge produced in aeration should not pose a problem, since it can be recycled back to the influent of the anaerobic units.
ACKN O W L E D G E M ENTS This research was financially supported by the Natural Sciences and Engineering Research Council of Canada under operating grant No. A9314. Experimental work was performed by Mr Stephen Bliss, a former graduate student in the Department of Civil Engineering at the University of New Brunswick. Wastewater samples were provided by McCain Foods Ltd, Florenceville, New Brunswick, Canada.
REFERENCES Clark, J. W., Viessman, Jr., W. & Hammer, M. J. (1977). Water supply and pollution control. (3rd edn.). Harper & Row Publishers, 857 pp. Dostal, K. A. (1970). Aerated lagoons for potato processing wastes. Second International Symposium for Waste Treatment Lagoons, Kansas City, Missouri, 243-9. Eckenfelder, Jr., W. W. (1967). Comparative biological waste treatment design. Journal of the Sanitary Engineering Division, Proceedings of the American Society of Civil Engineers, 93 (SA6), 157-70. Heukelekian, H. (1973). Digestion of solids between the thermophilic and nonthermophilic range. Sewage Works, 5(5), 757-62. Lettinga, G., van Velsen, A. F. M., deZeeuw, W. & Hobma, S. W. (1979). Feasibility of the upflow anaerobic sludge blanket (UASB) process. Proceedings of 1979 National Conference on Environmental Engineering, American Society of Civil Engineers, San Francisco, 35 45. Lin, K. C. & Brown, G. J. (1980). Treatment of potato-processing waste water by an anaerobic lagoon-filter system. Canadian Journal of Civil Engineering, 7(2), 373 83.
Aeration ~] anaerobically treated potato-processing wastewater
11
Lin, K. C., Landine, R. C. & Bliss, S. G. (1982). Temperature effect on the anaerobic treatment of potato-processing wastewater. Canadian Journal of Civil Engineering, 9(3), 549-57. McCarty, P. L. (1964). Anaerobic waste treatment fundamentals, Parts 1 and 2. Public' Works, Sept., 107-11; Oct., 123-6. O'Rourke, J. T. (1968). Kinetics o./ anaerobic waste treatment at reduced temperatures. PhD Thesis, Stanford University, California.