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Removal efficiency of the constructed wetland wastewater treatment system at Bainikeng, Shenzhen
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Wal. Sci. Tech. Vol. 32, No.3, pp. 31-40.1995. Copyright © 1995 IAWQ Printed in Great Britain. All rights reserved. 0273-1223/95 $9·50 + 0'00
Pergamon 0273-1223(95)00602-8
REMOVAL EFFICIENCY OF THE CONSTRUCTED WETLAND WASTEWATER TREATMENT SYSTEM AT BAINIKENG, SHENZHEN Yang Yang*, Xu Zhencheng*, Hu Kangping*, Wang Junsan* and Wang Guizhi** * South China Institute/or Environmental Sciences, NEPA, 7 West Street, Yuancun, Guangzhou. China ** East River Water Supply Management Agency, Shenzhen, China
ABSTRACT In this paper. three years study on a constructed wetland wastewater treatment system at Bainikeng, Shenzhen, is reviewed and summarized. The wetland system under study occupies an area of 840Om2, with a design flow of 3100 m 3 per day. The study was conducted to understand removal efficiencies of constructed wetland systems for municipal wastewaters from small or medium scale towns in the sub-tropics, Such parameters as biological oxygen demand. chemical oxygen demand, suspended solids. total nitrogen. and total phosphorus in the influent and effluent of the wetland system are examined. and their removal rates are determined, It is shown that the system is very effective in removing organic pollutants and suspended solids and its removal efficiency is much similar to those of the constructed wetlands at Tennessee Valley Authority (TVA) (Choate et al.• 1990) while better than those of conventional secondary biochemical treatments.
KEYWORDS Constructed wetland; vegetated gravel bed; removal efficiency. INTRODUCTION A constructed wetland is a man-made, engineered. marsh-like area designed and constructed to treat wastewater. Generally constructed wetlands may be classified as several kinds. while the wetland at Bainikeng belongs to one of the vegetated subsurface flow beds, namely the vegetated gravel beds. As the wastewater flows horizontally through the vegetated gravel bed, pollutants are removed by physical, chemical, and biological treatment mechanisms, and thus expected effluent quality is attained. In this paper, the removal efficiency of the wetland system constructed at Bainikeng, Shenzhen, is determined based on monitoring results for the trial operation in 1990 and the regular operation from 1991 through 1993. It is revealed that the wetland is very effective in wastewater treatment. For the convenience of presentation, systematic analysis is focused on the monitoring data for the period from 1991 to 1993, and at the same time comparative study with other wetland systems of the-same-kind is conducted.
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Removal efficiency of constructed wetland wastewater treatment
33
MONITORING STRATEGY Overview of the constructed wetland system Bainikeng village is located at Pinghu, Longgang, Shenzhen Special Economic Zone. A village on the Pearl River Delta - one of the fastest developing areas in the world, Bainikeng has been urbanized almost completely. The wetland system, occupying an area of 8400 m 2 with a design flow of 3100 m 3/d, was put into operation in July 1990. Its conceptional design was based on the experience of TVA's three small-scale experimental constructed wetland systems. The constructed wetland is a subsurface flow type, with seriesparallel oxygenation ponds attached. The wetland project is selected to purify municipal wastewater which is characterized by high BODs, COD, SS, nitrogen and phosphorus, and it is anticipated that a removal rate equal to or even higher than that of conventional secondary treatment could be achieved. The constructed wetland is composed of 8 subsurface flow marshes and 3 oxygenation ponds, as shown in Fig. I. Primary gravel bed. This consists of three parallel gravel-Phragmites communis cells; the design hydraulic loading rate is 95.4 cm per day and gravel bed is 80 ern deep, resulting in an area requirement of 1512 m 2. Wastewater flow is directed to the inlet area of the wetlands by pipe, Secondary gravel bed. This is composed of two parallel cells of equal size with gravel Phragmites communis and gravel Cyperus malaccensis, occupying an area of 1739 m2 and containing 100 em of gravel, to which the effluent of the primary gravel bed is conveyed. Oxygenation pond group. This is composed of three parallel iso-surface ponds of Nelumbo nutifera, Eichhornia crassipes, and algal-bacteria symbiotic system, occupying an area of 1710 m 2 with water depth of 150 cm, to which the outflow from the secondary gravel bed is conveyed. Fourth gravel bed. This is composed of two gravel Cyperus malaccensis cells and one gravel Lepironia articata cell in parallel; the design hydraulic loading rate tS 100.7 cm per day, covering an area of 2850 m 2 and containing 100 ern of gravel, to which the outflow of the oxygenation ponds is conveyed and the final effluent of the constructed wetland is also collected and discharged by pipe.
Sampling Weekly influent and effluent sampling began for the trial operation period 1990. Sampling was reduced to a monthly basis (sampling 72 hours continuously) during the regular operation period from 1991 through 1993. Influent samples were collected at the wetland inlet area, while effluent samples were collected at outlets of the four sub-systems, as shown in Fig. 1. RESULTS AND DISCUSSION Removal efficiency for primal:)' pollutants Table I lists the averaged values over the period from 1991 through 1993 for each sampling site. Monitoring results reveal that the constructed wetland is very effective in ensuring that the final system discharge is within the effluent standard limits of conventional secondary treatment (BODs, SS< 30 mg/L). Results for each parameter are presented below.
BODS' COD. It can be seen from Table 1 and Fig. 2 that the average effluent concentrations decreased from one sub-system to the next and that BODS was removed by a higher percentage than COD in every subsystem. The final effluent BODS and COD decreased to 6.9 ppm and 38.3 ppm respectively, resulting in removal rates of 90.5% and 73.5%. This reveals that the constructed wetland is very efficient in removing biochemically degradable organic pollutants, but less efficient for less biochemically degradable ones ,
Y. YANGetal.
34
Table I. Bainikeng constructed wetland data
Sample-site
#1
#2
#3
#4
#5
BODs(ppm) Re1l10val(%)
92.8
32.2 55.8
17.2 76.4
14.9 79.6
6.9 90.5
COD(pp1l1) Re1l10val(%)
144.7
83.3 42.4
60.0 58.5
56.2 61.2
38.3 73.5
SS(pp1l1) Rernovalf %)
140.9
34.6 76.1
26.7 81.1
23.9 83.1
10.9 92.6
TN(pp1l1) Removal/ %)
23.7
22.8 3.8
21.9 7.5
20.2 14.8
18.2 23.2
TP(pp1l1) Removalj %)
2.30
2.20 4.30
1.95 15.20
1.75 23.90
1.59 30.60
NH 4-N(pP1l1) Rell1oval(%)
20.70
19.70 4.80
19.10 7.70
18.90 8.60
18.50 10.60
N0 3'(pp1l1) Rell1oval(%)
0.62
0.42 32.30
0.35 43.50
0.36 41.90
0.38 38.70
R a II
0 ~
a
1
%
iC0 90 80 70 60 50 40 30 20 10 0
BOllS
COD
l'ZZ2ZJCell 1 ffi\SJCell 2 mCell 3 QCell 4 Figure 2. Removal of BODS and COD.
It is reported that the ratio of BODs to COD is an indicator of biochemical degradability. When the ratio is greater than 0.3. wastewater is said to be biochemically degradable. Table 1 shows that wastewater of Bainikeng village is biochemically degradable. The ratio decreased from one sub-system to the next and the
Removal efficiency of constructed wetland wastewater treatment
35
final system discharge has a ratio as low as 0.18, indicating that the wetland system is very efficient in removing such pollutants as BOD 5. 55. The primary removal mechanism of suspended solids is believed to be filtration. The residue of SS after being oxidized by microorganisms will become one part of the wetland media. Table 1 shows that the average influent SS was 140.9 mg/l and the final effluent 10.9 mg/I, resulting in a removal rate of 92.6%. Figure 3 illustrates that removal of SS mainly occurred in the primary gravel bed, thus resulting in a serious sludge choke problem in the first sub-system.
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S"I'\)lling Si1e Figure 3. Removal efficiency of suspended solids.
Nitrogen. The primary removal mechanisms of nitrogen are believed to be nitrification and denitrification by wetland microorganisms, a process far more complicated than plant uptake. Table I shows that influent TN and NH4-N were quite high but N03" was comparatively lower, and final effluent TN and NH 4+ decreased while N0 3" increased owing to the function of nitrifying bacteria. Table I also indicates that the wetland is not so efficient in removing nitrogen. Total nitrogen in the influent to the wetland averaged 23.7 mg/l and the effluent averaged 18.2 mg/l, resulting in a removal of 23.2%; the influent NH 4-N was 20.7 mg/l and the final effluent was 18.5 mg/l, only resulting in a removal of 10.6%. It is believed that radical variations of hydraulic and pollutant loading rates have great impacts on the growth of wetland nitrifying microorganisms, thus reducing the removal efficiency of nitrogen. This is also evident from the behaviour of TN and N0 3" in the system that the removal of TN at the oxygenation ponds was increased by 50% compared to that of the secondary gravel bed while N0 3" concentration increased rather than decreased (see Fig. 4). The phenomenon could be explained because the oxygenation ponds are relatively abundant in dissolved oxygen which is necessary for the growth of nitrifying bacteria. The actual hydraulic loading rate to Bainikeng wetland, with higher concentrations, has exceeded the design value by 40%, resulting in a vehement reduction condition which is unfavourable to nitrogen removal. TP. There are several processes for phosphorus removal from wastewaters, e.g. bacterial and algae assimilation, sediment absorption, aquatic plant uptake, and gravel absorption and ion exchange. The removal efficiency of TP is also affected by detention time or hydraulic and pollutant loading rates, as in the case of nitrogen removal. Table I indicates that total phosphorus in the influent to the wetland averaged 2.3 mg/l and the final effluent averaged 1.59 mg/l, resulting in a removal of 30.6%.
Y. YANGetal.
36
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313
"1 20
:I.
10 0
ND3
TN
E2ZZZJ Ce II 1 &SS::sJ Ce II 2 f:~:::;Jce II 3
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Figure 4. Removal rates of TN and N03-.
Conditional removal efficiency
Removal efficiency of different ecological structures. The wetland consists of three kinds of vegetated gravel beds, namely, Phragmites communis bed, Cyperus malaccensis bed, and Lepironia articata bed. Variance analysis (Zhou, 1991) was applied to the effluent BODs of the three different ecological structures. The results are listed in Table 2 and Table 3. Table 2. Removal efficiency of different plant systems Sample size
Group number
2-1 2-2 4-1 4-2
12 12 12 12 12
4-3
Average value 17.10 18.24 5.34 7.43 7.78
Variance deviation
Standard error
Standard coefficient
Variation
2.9957 9.5428 2.4298 1.7447 1.6984
1.7308 3.0891 1.5588 1.3209 1.3612
0.4996 0.8917 0.4499 0.3813 0.3929
0.1010 0.1690 0.2920 0.1778 0.1747
Table 3. Results of variance analysis Sampling site
Deviation source
Quadratic sum
Freedom degree
Mean-square deviation
Variance ratio
Secondary gravel bed
A* B C
442.19 43.31 398.88
23 I 22
43.31 18.18
2.38
P>0.05
Fuurth gravel bed
A B C
98.54 41.77 56.77
2 29
10.89 1.98
5.50
P
* A = total variation; B = variation between groups; C = variation among groups.
Removal efficiency of constructed wetland wastewater treatment
37
The following are the results of multiple comparison. Looking up significance level from Q-distribution table with group number = 2 and freedom degree = 29, it can be found that the difference is quite significant: QO.OI = 3.89, Qo.os = 2.89. [(2-1)-(2-2)] = 2.12, [(4-1)-(4-3)] = 14.78, and [(4-1)-(4-2)] = 12.67. The variance analysis indicates that different ecological structures make great differences in removing efficiency, especially between Cyperus malaccensis and Lepironia articata. Effluent BODs of the three ecological structures are: 17.1 mg/l for Cyperus malaccensis, 18.2 rng/l for Phragmites communis (sampling at secondary gravel bed); 5.3 rng/l for Cyperus malaccensis, and 7.78 rng/l for Lepironia articata (sampling at the fourth gravel bed). Although we should not jump to conclusions from these average values, it is still clear that among the three plants Cyperus malaccensis is the most efficient one at removing BODS while Lepironia articata is the least.
Comparison between oxygenation ponds and the whole wetland system. The wetland is roughly composed of two parts; one is vegetated gravel bed, and the other is oxygenation pond (see Fig. 1). Data at site #4 could represent the effluent quality of oxygenation pond and data at #5 imply that of the whole wetland system. Figure 5 is the comparison of their removal efficiencies. It can be seen that the removal rates of oxygenation ponds ranged from 2% to 8.7% while the whole wetland system had removal rates equal to or even higher than 75%. Figure 5 also indicates that vegetated gravel beds play the leading role in removing most of the pollutants while oxygenation ponds are quite important for the removal of nitrogen and phosphorus. l~~
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COD
f
SS
TP
Figure 5. Comparison of wetland system with oxygenation ponds .
Tolerance to influent concentrations Although the influent concentrations have varied greatly since the constructed wetland went into operation in 1990, the final effluent quality has been quite stable, indicating that the constructed wetland has great tolerance to the influent loading rate. The conclusion could be easily confirmed by F-test or variance ratio test results listed in Table 4. It can be seen from Table 4 that the influent variance is by far greater than the effluent variance for such constituents as BOD 5, COD, SS and TP, indicating the influent concentrations varied greatly while the effluent concentrations kept quite stable (because the values of variances reflect the behaviour of data dispersion, the larger the variances are. the more the concentrations vary). In other words, Bainikeng Wetland has great tolerance to influent concentrations.
38
Y. YANG et al.
Table 4. Results of F-test
BOD, SS COD TP TN NH~+
NO J '
,
S\'
S'
14.13 104.16 11.29 0.76 8.37 8.35 0.13
1.38 3.30 4.27 0.36 5.03 6.79 0.12
Parameter
SI2/ S, 2 104.84 996.26 6.99 4.46 3.03 1.58 1.17
F(n 1-I,n,-I)
Significance-level
(11,11) (11,11) (11,11) (11,11) (11,11) (11,11) (11, II)
very significant very significant very significant very significant significant insignificant insignificant
* SI =intluent variance at 1#; S,=efnuent variance at 5#. 1M . . , - - - - - - - - - - - - - - - - . . . ,
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TN
Pel lutants
=ViZ=.~ 1991 CSSSSJ 1992 01993
L.--
Figure 6. Removal efficiency in different years.
Table 5. Removal efficiency in different seasons and under different loading rates Parameter
Spring (8-25"C)
Summer (25-3Y'C)
Autumn (18-30°C)
Winter (2-15°C)
Loading-rate (kg/ha/d)
BOD, COD SS
325.2 498.2 523,6
331.5 535.7 543.0
306.4 485.8 501.5
406.0 590.5 576.2
Effluent (mg/L)
BOD; COD SS
6,7 35,2 9.8
6,1 29,2 9.2
6.5 33.1 9.6
9.2 52.1 13.2
Removal-rate
BOD, COD SS
89.8 73.4 91.8
91.4 78.1 93.2
90.8 74.0 92,5
89.2 67.8 90.5
(%)
39
Removal efficiency of constructed wetland wastewater treatment
Stability of removal efficiency Figure 6 shows the removal efficiency on a yearly basis. It can be seen that most of the pollutants, excluding TN and TP, were removed by a percent as high as or higher than 80%, among which BODs and SS were removed by 90% and 91 % respectively, and even COD was removed by 74%. Three years operation has proved that the wetland system is functioning in a more and more stable way and its removal efficiency for BODs, SS, and COD is no longer affected by such factors as climate or influent loading rate, as shown in Table 5. Table 6. Comparison with other treatment projects Parameter
Wetland
(kg/ha/d)
BOD 5
ss
TI'
TN
NH.-N
NOj-N
Inlet loading (mg/L)
Inlet (mg/L)
Outlet (kg/ha/d)
Removal loading(%)
Removal rate
A' B C D
342.0 32.5 32.5 22.0
92.8 67.0 55.0 26.0
6.9 9.0 9.5 9.0
316.7 28.1 26.9 14.0
92.6 86.6 82.7 65.4
A B C D
528.0 89.6 73.9 49.0
140.9 92.0 125.0 57.0
10.9 8.0 17.7 9.0
487.3 81.8 63.4 41.3
92.3 91.3 85.8 84.2
A B C D
8.5 5.8 2.8 3.9
2.3 6.0 4.8 4.5
1.6 3.2 2.0 4.1
2.6 2.7 1.6 0.3
30.9 46.7 58.3 8.9
A B C D
87.5 31.6 11.9 12.6
23.7 32.5 20.2 14.6
18.2 6.0 I/.6 1/.2
20.3 25.7 5.1 2.9
23.2 81.4 42.6 23.0
A B C D
76.4 71.5 5.6 4.1
20.7 73.5 9.4 4.8
18.5 3.7 9.9 8.3
8.0 51.9 -0.3 -3.3
10.5 72.6 -5.3 -81.4
0.9 0.2 0.2 0.1
38.7 25.0 45.7 21.4
A B C D
2.3 0.45 0.41 0.24
0.62 0.67 0.70 0.28
0.38 0.57 0.38 0.22
* A=Bainikeng; B=Pembroke; C=Hardin; D=Benton. Comparison with other wetlands A wetland system depends on natural ecosystem to improve water quality. Therefore, the removed amount of pollutants in unit time and area is a valid measure of removal efficiency. Table 6 lists the data of four typical wetlands for the purpose of comparison. It can be seen that Bainikeng Wetland had the greatest loading rate at a unit area and its removal rate of organic pollutants and SS was as much as 300 kg/he/d. greater than those of TVA CW-WTS. As far as TP,TN and NH 4-N were concerned, Pembroke Wetland and Bainikeng Wetland took respectively the first and second place in removal efficiency while Hardin Wetland
Y. YANG et at.
40
and Benton Wetland came after them. The conclusions are almost the same if the effluent concentrations and removal rates are used as comparison variables. It is also clear from Table 6 that Bainikeng Wetland is very efficient in removing organic pollutants and SS but its removal of nutrient!'; needs further study.
CONCLUSIONS The constructed wetland at Bainikeng has been very effective in removing organic pollutants from municipal wastewater. It has the advantages of low requirement for area, high tolerance to loading rates, stable removal efficiency, and easy operation. Its removal efficiency of primary pollutants such as BODS and SS is close to that of TV A constructed wetlands and higher than that of conventional secondary treatment. The technology can be made available to other small scale towns to tackle problems similar to that of Bainikeng Village. REFERENCES Choate, K. D.. Watson, J. T. and Steiner, G. R. (1990). Demonstration of Constructed Wetland for Treatment of Municipal Wastewaters. Monitoring Report, TVA, USA. Zhou, Z. (1991). Statistics. Jinan University Press, Guangzhou, China.
Wal. Sci. Tech. Vol. 32, No.3, pp. 31-40.1995. Copyright © 1995 IAWQ Printed in Great Britain. All rights reserved. 0273-1223/95 $9·50 + 0'00
Pergamon 0273-1223(95)00602-8
REMOVAL EFFICIENCY OF THE CONSTRUCTED WETLAND WASTEWATER TREATMENT SYSTEM AT BAINIKENG, SHENZHEN Yang Yang*, Xu Zhencheng*, Hu Kangping*, Wang Junsan* and Wang Guizhi** * South China Institute/or Environmental Sciences, NEPA, 7 West Street, Yuancun, Guangzhou. China ** East River Water Supply Management Agency, Shenzhen, China
ABSTRACT In this paper. three years study on a constructed wetland wastewater treatment system at Bainikeng, Shenzhen, is reviewed and summarized. The wetland system under study occupies an area of 840Om2, with a design flow of 3100 m 3 per day. The study was conducted to understand removal efficiencies of constructed wetland systems for municipal wastewaters from small or medium scale towns in the sub-tropics, Such parameters as biological oxygen demand. chemical oxygen demand, suspended solids. total nitrogen. and total phosphorus in the influent and effluent of the wetland system are examined. and their removal rates are determined, It is shown that the system is very effective in removing organic pollutants and suspended solids and its removal efficiency is much similar to those of the constructed wetlands at Tennessee Valley Authority (TVA) (Choate et al.• 1990) while better than those of conventional secondary biochemical treatments.
KEYWORDS Constructed wetland; vegetated gravel bed; removal efficiency. INTRODUCTION A constructed wetland is a man-made, engineered. marsh-like area designed and constructed to treat wastewater. Generally constructed wetlands may be classified as several kinds. while the wetland at Bainikeng belongs to one of the vegetated subsurface flow beds, namely the vegetated gravel beds. As the wastewater flows horizontally through the vegetated gravel bed, pollutants are removed by physical, chemical, and biological treatment mechanisms, and thus expected effluent quality is attained. In this paper, the removal efficiency of the wetland system constructed at Bainikeng, Shenzhen, is determined based on monitoring results for the trial operation in 1990 and the regular operation from 1991 through 1993. It is revealed that the wetland is very effective in wastewater treatment. For the convenience of presentation, systematic analysis is focused on the monitoring data for the period from 1991 to 1993, and at the same time comparative study with other wetland systems of the-same-kind is conducted.
31 .Mf 32-3-0
w
tv
Cell#l-l Cell#2-1: Cell#3-1: Cell#4-3
Cell#1-2 Cell#1-3: Phragmites communis bed CypenlS malaccensis bed Cell#2-2: Phragmites communis bed Cell#3-2 Cell#3-3: Oxygenation pond Cell#4-2: Lepironta articata bed Cel1#4-1 Cyperus malaccensis Floodway ~
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><:
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~
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o
00
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545,000
2,000 I- ~.
30,000
~ 2,000 1111.
50,000
2.000
''',1
Figure I. Plan view and sampling sites of the wetland.
45,000
1"6.000
Removal efficiency of constructed wetland wastewater treatment
33
MONITORING STRATEGY Overview of the constructed wetland system Bainikeng village is located at Pinghu, Longgang, Shenzhen Special Economic Zone. A village on the Pearl River Delta - one of the fastest developing areas in the world, Bainikeng has been urbanized almost completely. The wetland system, occupying an area of 8400 m 2 with a design flow of 3100 m 3/d, was put into operation in July 1990. Its conceptional design was based on the experience of TVA's three small-scale experimental constructed wetland systems. The constructed wetland is a subsurface flow type, with seriesparallel oxygenation ponds attached. The wetland project is selected to purify municipal wastewater which is characterized by high BODs, COD, SS, nitrogen and phosphorus, and it is anticipated that a removal rate equal to or even higher than that of conventional secondary treatment could be achieved. The constructed wetland is composed of 8 subsurface flow marshes and 3 oxygenation ponds, as shown in Fig. I. Primary gravel bed. This consists of three parallel gravel-Phragmites communis cells; the design hydraulic loading rate is 95.4 cm per day and gravel bed is 80 ern deep, resulting in an area requirement of 1512 m 2. Wastewater flow is directed to the inlet area of the wetlands by pipe, Secondary gravel bed. This is composed of two parallel cells of equal size with gravel Phragmites communis and gravel Cyperus malaccensis, occupying an area of 1739 m2 and containing 100 em of gravel, to which the effluent of the primary gravel bed is conveyed. Oxygenation pond group. This is composed of three parallel iso-surface ponds of Nelumbo nutifera, Eichhornia crassipes, and algal-bacteria symbiotic system, occupying an area of 1710 m 2 with water depth of 150 cm, to which the outflow from the secondary gravel bed is conveyed. Fourth gravel bed. This is composed of two gravel Cyperus malaccensis cells and one gravel Lepironia articata cell in parallel; the design hydraulic loading rate tS 100.7 cm per day, covering an area of 2850 m 2 and containing 100 ern of gravel, to which the outflow of the oxygenation ponds is conveyed and the final effluent of the constructed wetland is also collected and discharged by pipe.
Sampling Weekly influent and effluent sampling began for the trial operation period 1990. Sampling was reduced to a monthly basis (sampling 72 hours continuously) during the regular operation period from 1991 through 1993. Influent samples were collected at the wetland inlet area, while effluent samples were collected at outlets of the four sub-systems, as shown in Fig. 1. RESULTS AND DISCUSSION Removal efficiency for primal:)' pollutants Table I lists the averaged values over the period from 1991 through 1993 for each sampling site. Monitoring results reveal that the constructed wetland is very effective in ensuring that the final system discharge is within the effluent standard limits of conventional secondary treatment (BODs, SS< 30 mg/L). Results for each parameter are presented below.
BODS' COD. It can be seen from Table 1 and Fig. 2 that the average effluent concentrations decreased from one sub-system to the next and that BODS was removed by a higher percentage than COD in every subsystem. The final effluent BODS and COD decreased to 6.9 ppm and 38.3 ppm respectively, resulting in removal rates of 90.5% and 73.5%. This reveals that the constructed wetland is very efficient in removing biochemically degradable organic pollutants, but less efficient for less biochemically degradable ones ,
Y. YANGetal.
34
Table I. Bainikeng constructed wetland data
Sample-site
#1
#2
#3
#4
#5
BODs(ppm) Re1l10val(%)
92.8
32.2 55.8
17.2 76.4
14.9 79.6
6.9 90.5
COD(pp1l1) Re1l10val(%)
144.7
83.3 42.4
60.0 58.5
56.2 61.2
38.3 73.5
SS(pp1l1) Rernovalf %)
140.9
34.6 76.1
26.7 81.1
23.9 83.1
10.9 92.6
TN(pp1l1) Removal/ %)
23.7
22.8 3.8
21.9 7.5
20.2 14.8
18.2 23.2
TP(pp1l1) Removalj %)
2.30
2.20 4.30
1.95 15.20
1.75 23.90
1.59 30.60
NH 4-N(pP1l1) Rell1oval(%)
20.70
19.70 4.80
19.10 7.70
18.90 8.60
18.50 10.60
N0 3'(pp1l1) Rell1oval(%)
0.62
0.42 32.30
0.35 43.50
0.36 41.90
0.38 38.70
R a II
0 ~
a
1
%
iC0 90 80 70 60 50 40 30 20 10 0
BOllS
COD
l'ZZ2ZJCell 1 ffi\SJCell 2 mCell 3 QCell 4 Figure 2. Removal of BODS and COD.
It is reported that the ratio of BODs to COD is an indicator of biochemical degradability. When the ratio is greater than 0.3. wastewater is said to be biochemically degradable. Table 1 shows that wastewater of Bainikeng village is biochemically degradable. The ratio decreased from one sub-system to the next and the
Removal efficiency of constructed wetland wastewater treatment
35
final system discharge has a ratio as low as 0.18, indicating that the wetland system is very efficient in removing such pollutants as BOD 5. 55. The primary removal mechanism of suspended solids is believed to be filtration. The residue of SS after being oxidized by microorganisms will become one part of the wetland media. Table 1 shows that the average influent SS was 140.9 mg/l and the final effluent 10.9 mg/I, resulting in a removal rate of 92.6%. Figure 3 illustrates that removal of SS mainly occurred in the primary gravel bed, thus resulting in a serious sludge choke problem in the first sub-system.
100 - r - - - - - - - - - - - - - - - - - ,
90 80 R 70 ~
II
6e
V
f
50 40
;(
30
o
20 1e
o
.J....!LLi..'-..'-'i-'-':":"':'-:'.L. ""..i..i....i..-'+'-~:..L.-l'..LLL.f..4LLLL'-'--iW..!.."""+:.t...:.,;'-"J..J
#4
S"I'\)lling Si1e Figure 3. Removal efficiency of suspended solids.
Nitrogen. The primary removal mechanisms of nitrogen are believed to be nitrification and denitrification by wetland microorganisms, a process far more complicated than plant uptake. Table I shows that influent TN and NH4-N were quite high but N03" was comparatively lower, and final effluent TN and NH 4+ decreased while N0 3" increased owing to the function of nitrifying bacteria. Table I also indicates that the wetland is not so efficient in removing nitrogen. Total nitrogen in the influent to the wetland averaged 23.7 mg/l and the effluent averaged 18.2 mg/l, resulting in a removal of 23.2%; the influent NH 4-N was 20.7 mg/l and the final effluent was 18.5 mg/l, only resulting in a removal of 10.6%. It is believed that radical variations of hydraulic and pollutant loading rates have great impacts on the growth of wetland nitrifying microorganisms, thus reducing the removal efficiency of nitrogen. This is also evident from the behaviour of TN and N0 3" in the system that the removal of TN at the oxygenation ponds was increased by 50% compared to that of the secondary gravel bed while N0 3" concentration increased rather than decreased (see Fig. 4). The phenomenon could be explained because the oxygenation ponds are relatively abundant in dissolved oxygen which is necessary for the growth of nitrifying bacteria. The actual hydraulic loading rate to Bainikeng wetland, with higher concentrations, has exceeded the design value by 40%, resulting in a vehement reduction condition which is unfavourable to nitrogen removal. TP. There are several processes for phosphorus removal from wastewaters, e.g. bacterial and algae assimilation, sediment absorption, aquatic plant uptake, and gravel absorption and ion exchange. The removal efficiency of TP is also affected by detention time or hydraulic and pollutant loading rates, as in the case of nitrogen removal. Table I indicates that total phosphorus in the influent to the wetland averaged 2.3 mg/l and the final effluent averaged 1.59 mg/l, resulting in a removal of 30.6%.
Y. YANGetal.
36
613
50 R
e 40 tl 0
Y
313
"1 20
:I.
10 0
ND3
TN
E2ZZZJ Ce II 1 &SS::sJ Ce II 2 f:~:::;Jce II 3
t=:;::·;;ke II 4
Figure 4. Removal rates of TN and N03-.
Conditional removal efficiency
Removal efficiency of different ecological structures. The wetland consists of three kinds of vegetated gravel beds, namely, Phragmites communis bed, Cyperus malaccensis bed, and Lepironia articata bed. Variance analysis (Zhou, 1991) was applied to the effluent BODs of the three different ecological structures. The results are listed in Table 2 and Table 3. Table 2. Removal efficiency of different plant systems Sample size
Group number
2-1 2-2 4-1 4-2
12 12 12 12 12
4-3
Average value 17.10 18.24 5.34 7.43 7.78
Variance deviation
Standard error
Standard coefficient
Variation
2.9957 9.5428 2.4298 1.7447 1.6984
1.7308 3.0891 1.5588 1.3209 1.3612
0.4996 0.8917 0.4499 0.3813 0.3929
0.1010 0.1690 0.2920 0.1778 0.1747
Table 3. Results of variance analysis Sampling site
Deviation source
Quadratic sum
Freedom degree
Mean-square deviation
Variance ratio
Secondary gravel bed
A* B C
442.19 43.31 398.88
23 I 22
43.31 18.18
2.38
P>0.05
Fuurth gravel bed
A B C
98.54 41.77 56.77
2 29
10.89 1.98
5.50
P
* A = total variation; B = variation between groups; C = variation among groups.
Removal efficiency of constructed wetland wastewater treatment
37
The following are the results of multiple comparison. Looking up significance level from Q-distribution table with group number = 2 and freedom degree = 29, it can be found that the difference is quite significant: QO.OI = 3.89, Qo.os = 2.89. [(2-1)-(2-2)] = 2.12, [(4-1)-(4-3)] = 14.78, and [(4-1)-(4-2)] = 12.67. The variance analysis indicates that different ecological structures make great differences in removing efficiency, especially between Cyperus malaccensis and Lepironia articata. Effluent BODs of the three ecological structures are: 17.1 mg/l for Cyperus malaccensis, 18.2 rng/l for Phragmites communis (sampling at secondary gravel bed); 5.3 rng/l for Cyperus malaccensis, and 7.78 rng/l for Lepironia articata (sampling at the fourth gravel bed). Although we should not jump to conclusions from these average values, it is still clear that among the three plants Cyperus malaccensis is the most efficient one at removing BODS while Lepironia articata is the least.
Comparison between oxygenation ponds and the whole wetland system. The wetland is roughly composed of two parts; one is vegetated gravel bed, and the other is oxygenation pond (see Fig. 1). Data at site #4 could represent the effluent quality of oxygenation pond and data at #5 imply that of the whole wetland system. Figure 5 is the comparison of their removal efficiencies. It can be seen that the removal rates of oxygenation ponds ranged from 2% to 8.7% while the whole wetland system had removal rates equal to or even higher than 75%. Figure 5 also indicates that vegetated gravel beds play the leading role in removing most of the pollutants while oxygenation ponds are quite important for the removal of nitrogen and phosphorus. l~~
90
~iil'
S0
R e 70
il!'
M
I
0
V <\
60 5B
r
~:1;'
!/
I
I
4~
J.
3a
'III
20
:Ir,l; /iii
i
'1/ 1/;
irr
10 0
.r
''j' r
'///1 //
BODS
COD
f
SS
TP
Figure 5. Comparison of wetland system with oxygenation ponds .
Tolerance to influent concentrations Although the influent concentrations have varied greatly since the constructed wetland went into operation in 1990, the final effluent quality has been quite stable, indicating that the constructed wetland has great tolerance to the influent loading rate. The conclusion could be easily confirmed by F-test or variance ratio test results listed in Table 4. It can be seen from Table 4 that the influent variance is by far greater than the effluent variance for such constituents as BOD 5, COD, SS and TP, indicating the influent concentrations varied greatly while the effluent concentrations kept quite stable (because the values of variances reflect the behaviour of data dispersion, the larger the variances are. the more the concentrations vary). In other words, Bainikeng Wetland has great tolerance to influent concentrations.
38
Y. YANG et al.
Table 4. Results of F-test
BOD, SS COD TP TN NH~+
NO J '
,
S\'
S'
14.13 104.16 11.29 0.76 8.37 8.35 0.13
1.38 3.30 4.27 0.36 5.03 6.79 0.12
Parameter
SI2/ S, 2 104.84 996.26 6.99 4.46 3.03 1.58 1.17
F(n 1-I,n,-I)
Significance-level
(11,11) (11,11) (11,11) (11,11) (11,11) (11,11) (11, II)
very significant very significant very significant very significant significant insignificant insignificant
* SI =intluent variance at 1#; S,=efnuent variance at 5#. 1M . . , - - - - - - - - - - - - - - - - . . . ,
98 88 R 78 e M
68
V
58
r
40
o
% 38
28 18
e
-'-'-'..:."'-I.~-"-'
,,",+"--'--'-'-'-"--'+':""':""--"--'-"-'-r'-"-=-,,"-,-'"¥-'-::;.J../
IP
TN
Pel lutants
=ViZ=.~ 1991 CSSSSJ 1992 01993
L.--
Figure 6. Removal efficiency in different years.
Table 5. Removal efficiency in different seasons and under different loading rates Parameter
Spring (8-25"C)
Summer (25-3Y'C)
Autumn (18-30°C)
Winter (2-15°C)
Loading-rate (kg/ha/d)
BOD, COD SS
325.2 498.2 523,6
331.5 535.7 543.0
306.4 485.8 501.5
406.0 590.5 576.2
Effluent (mg/L)
BOD; COD SS
6,7 35,2 9.8
6,1 29,2 9.2
6.5 33.1 9.6
9.2 52.1 13.2
Removal-rate
BOD, COD SS
89.8 73.4 91.8
91.4 78.1 93.2
90.8 74.0 92,5
89.2 67.8 90.5
(%)
39
Removal efficiency of constructed wetland wastewater treatment
Stability of removal efficiency Figure 6 shows the removal efficiency on a yearly basis. It can be seen that most of the pollutants, excluding TN and TP, were removed by a percent as high as or higher than 80%, among which BODs and SS were removed by 90% and 91 % respectively, and even COD was removed by 74%. Three years operation has proved that the wetland system is functioning in a more and more stable way and its removal efficiency for BODs, SS, and COD is no longer affected by such factors as climate or influent loading rate, as shown in Table 5. Table 6. Comparison with other treatment projects Parameter
Wetland
(kg/ha/d)
BOD 5
ss
TI'
TN
NH.-N
NOj-N
Inlet loading (mg/L)
Inlet (mg/L)
Outlet (kg/ha/d)
Removal loading(%)
Removal rate
A' B C D
342.0 32.5 32.5 22.0
92.8 67.0 55.0 26.0
6.9 9.0 9.5 9.0
316.7 28.1 26.9 14.0
92.6 86.6 82.7 65.4
A B C D
528.0 89.6 73.9 49.0
140.9 92.0 125.0 57.0
10.9 8.0 17.7 9.0
487.3 81.8 63.4 41.3
92.3 91.3 85.8 84.2
A B C D
8.5 5.8 2.8 3.9
2.3 6.0 4.8 4.5
1.6 3.2 2.0 4.1
2.6 2.7 1.6 0.3
30.9 46.7 58.3 8.9
A B C D
87.5 31.6 11.9 12.6
23.7 32.5 20.2 14.6
18.2 6.0 I/.6 1/.2
20.3 25.7 5.1 2.9
23.2 81.4 42.6 23.0
A B C D
76.4 71.5 5.6 4.1
20.7 73.5 9.4 4.8
18.5 3.7 9.9 8.3
8.0 51.9 -0.3 -3.3
10.5 72.6 -5.3 -81.4
0.9 0.2 0.2 0.1
38.7 25.0 45.7 21.4
A B C D
2.3 0.45 0.41 0.24
0.62 0.67 0.70 0.28
0.38 0.57 0.38 0.22
* A=Bainikeng; B=Pembroke; C=Hardin; D=Benton. Comparison with other wetlands A wetland system depends on natural ecosystem to improve water quality. Therefore, the removed amount of pollutants in unit time and area is a valid measure of removal efficiency. Table 6 lists the data of four typical wetlands for the purpose of comparison. It can be seen that Bainikeng Wetland had the greatest loading rate at a unit area and its removal rate of organic pollutants and SS was as much as 300 kg/he/d. greater than those of TVA CW-WTS. As far as TP,TN and NH 4-N were concerned, Pembroke Wetland and Bainikeng Wetland took respectively the first and second place in removal efficiency while Hardin Wetland
Y. YANG et at.
40
and Benton Wetland came after them. The conclusions are almost the same if the effluent concentrations and removal rates are used as comparison variables. It is also clear from Table 6 that Bainikeng Wetland is very efficient in removing organic pollutants and SS but its removal of nutrient!'; needs further study.
CONCLUSIONS The constructed wetland at Bainikeng has been very effective in removing organic pollutants from municipal wastewater. It has the advantages of low requirement for area, high tolerance to loading rates, stable removal efficiency, and easy operation. Its removal efficiency of primary pollutants such as BODS and SS is close to that of TV A constructed wetlands and higher than that of conventional secondary treatment. The technology can be made available to other small scale towns to tackle problems similar to that of Bainikeng Village. REFERENCES Choate, K. D.. Watson, J. T. and Steiner, G. R. (1990). Demonstration of Constructed Wetland for Treatment of Municipal Wastewaters. Monitoring Report, TVA, USA. Zhou, Z. (1991). Statistics. Jinan University Press, Guangzhou, China.