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CONSTRUCTED WETLANDS DESIGN AND OPERATION TO MAXIMIZE NUTRIENT REMOVAL CAPABILITIES
CONSTRUCTED WETLANDS DESIGN A N D OPERATION TO MAXIMIZE NUTRIENT R E M O V A L CAPABILITIES C. E . Swindell and J. A . Jackson Post, Buckley, Schuh & Jernigan, Inc., 800 North Magnolia Suite 600, Orlando, FL 32803, USA
Avenue,
ABSTRACT A 494 ha constructed wetland operating since mid-1987 in Orlando, F l o r i d a , U.S.A. produced an effluent in 1989 with average concentrations of 0.92 mg/1 total nitrogen and 0.08 mg/1 total phosphorus. Influent flows were approximately 0.58 m3/s. Most o f the nitrogen and phosphorus was removed in the f i r s t 11 percent of the system. Nutrient removal e f f i c i e n c y peaked by a detention time of 5 days. During 1989, the average nitrogen uptake rate across the f i r s t 54 ha of the wetland was 2.31 kg/ha/day. Phosphorus uptake during 1989 across the same area was 0.41 kg/ha/day. D e n i t r i f i c a t i o n appeared to be the primary removal mechanism for nitrogen. The f i x a t i o n of phosphorus in bacteria and algae c e l l s and eventual storage of these c e l l s in the sediments was thought to be the primary phosphorus removal mechanism. Because of a design that allows for more control over internal flow rates than most other constructed wetlands, the system was able to provide an effluent with consistently low nutrient concentrations. KEY WORDS Wastewater; constructed wetlands; nitrogen; phosphorus; design; operation. INTRODUCTION A 494 ha constructed wetland in Orlando, F l o r i d a , U . S . A . , was designed to provide additional nutrient removal to 0.88 m3/s of treated municipal wastewater from the C i t y ' s Iron Bridge Regional Water Pollution Control F a c i l i t y (WPCF). Because o f the s e n s i t i v i t y o f r i v e r s and lakes in Florida to even small increases in nutrient concentrations, l e v e l s of treatment beyond advanced were required before discharge. The wetland is being used to provide this additional level of advanced treatment prior to discharge to the St. Johns R i v e r . The wetland was constructed on low-lying land that was heavily ditched and drained to pro vide c a t t l e pasture. Over 25 km of earthern berms were constructed on the s i t e to provide a segmented design that would maximize operational control (Figure 1 ) . Water passes between c e l l s through control structures consisting of pipes through berms with box culverts and flashboards on the upstream end of each pipe. The wetland design incorporated the use of four communities to enhance w i l d l i f e u t i l i z a t i o n : a deep marsh ( c a l l e d wet p r a i r i e ) , a shallow mixed marsh, a hardwood swamp, and a l a k e . The wetland is a surface flow system that has depths ranging up to 1 m in the wet p r a i r i e , to 0.6 m in the shallow mixed marsh, to 0.3 m in the hardwood swamp, and to 9 m in the l a k e . The substrate of the system consists of mineral s o i l with 2 to 15 percent organic matter, primarily resulting from former c a t t l e ranching. No s i t e grading was conducted. There is a
107
C. Ε . S W I N D E L L and J. A . J A C K S O N
108
natural grade o f approximately 0.2 percent from the influent to effluent discharge struc t u r e . The wet p r a i r i e was planted with Typha spp. and Scirpus s p p . . the mixed marsh with approximately 30 indigenous herbaceous s p e c i e s , and the hardwood swamp with nearly 160,000 seedlings o f a v a r i e t y o f tree s p e c i e s , but primarily Taxodium distichum.
LEGEND
^LAKE
SHORELINE
^BERM —kWATER
•
F i g . 1.
Orlando wetland s i t e
FLOW
MONITORING
STATIONS
plan.
The wetland began receiving flow from the Iron Bridge WPCF in July 1987. I n i t i a l flow was 0.35 m^/s but increased to 0.58 m^/s in August 1988. I t is anticipated that flows w i l l be increased to the full 0.88 m^/s design capacity by 1991. At design flows, the detention time through the wetland is estimated to be 30 days. Water leaving the constructed wetland sheet flows across an adjacent natural marsh to the S t . Johns R i v e r . MONITORING RESULTS AND DISCUSSION Introduction Monitoring data for the start-up operational phase of the Orlando wetland was presented in Jackson (1989). The f i r s t two years o f operational data, from January 1988 to December 1989, are presented herein. As i l l u s t r a t e d on Figure 1, the wetland water q u a l i t y was monitored at ten sample s t a t i o n s . The sample stations represent the wetland influent and f i v e strata o f the system as f o l l o w s : 0 0 0 0 0 0
Wetland Stratum Stratum Stratum Stratum Stratum
Influent - monitoring station WPl; 1 - Wet P r a i r i e - monitoring stations WP2 and 2 - Wet p r a i r i e - monitoring stations WP4 and 3 - Wet P r a i r i e - monitoring station WP6; 4 - Mixed Marsh - monitoring stations MM7 and 5 - Hardwood Swamp/Lake - e f f l u e n t monitoring
WP3; WP5; MM8; and station HSIO.
Maximization of nutrient removal capabilities
109
Water q u a l i t y data was obtained monthly from three consecutive d a i l y grab samples taken from each monitoring s t a t i o n , with the exception o f WPl and HSIO, which were sampled d a i l y . Annual average t o t a l nitrogen and t o t a l phosphorus concentrations for 1988 and 1989 are sum marized in Table 1. Maximum e f f l u e n t concentrations permitted by State and Federal regula tory agencies are 2.31 mg/1 t o t a l nitrogen and 0.20 mg/l t o t a l phosphorus. TABLE 1
Monitoring Station
Nitrogen* l9Óé 1989
WPl WP3 WP4,5 WP6 MM8 HSIO
4.18 1.53 1.51 1.27 0.96 0.84
WETLANDS NUTRIENT CONCENTRATIONS
Phosphorus* 1^88 1989 0.72 0.08 0.15 0.07 0.05 0.076
0.572 0.103 0.102 0.106 0.091 0.095
5.52 1.92 1.74 1.59 1.22 0.92
Percent Area
Detention Time*** 1989
0 11 16 32 67 100
0 6 9 18 31 57
0 5 7 14 24 45
**
Percent upstream area equals percent o f the wetland area upstream o f the sample s t a t i o n . *** Detention time in days approximated based on volume.
The monitoring stations described p r e v i o u s l y .
presented
in
Table
1 are
representative
of
each
of
the
strata
Nitrogen Most of the reduction in nitrogen concentration occured in Stratum 1. In both 1988 and 1989, over 78 percent o f the nitrogen removal in the wetland occurred in t h i s f i r s t 11 percent (54 ha) o f the system. Otherwise, no s i g n i f i c a n t decrease in nitrogen concentration occurred in any other stratum. The percent reduction in nitrogen concentration was very similar in 1988 and 1989, even though the influent nitrogen concentration increased by 32 percent in 1989. The o v e r a l l percent removal o f nitrogen through the wetland system was 80 percent in 1988 and 83 percent in 1989. There was an increased dominance in the organic nitrogen concentration as water passed through the wetland (Table 2 ) . This may have caused the l a t t e r portions o f the wetland to become nitrogen l i m i t e d , reducing the phosphorus removal p o t e n t i a l . TABLE 2
Monitoring Station WPl WP6 MM HSIO
PERCENT TOTAL ORGANIC ΝITROGEN
1988* TON %
1989* TON %
17 56 92 85
13 37 67 73
C. Ε . S W I N D E L L and J. A . J A C K S O N
110
Nitrogen reduction in the wetland appeared to be highly correlated to loading r a t e s , provid ing indirect evidence that microbial a c t i v i t y ( n i t r i f i c a t i o n / d e n i t r i f i c a t i o n ) was a more important nitrogen sink than macrophytic plant uptake. The lack of s i g n i f i c a n t seasonal variation in the data set (Figure 2) also reinforces the theory that d e n i t r i f i c a t i o n , rather than macrophytic plant uptake, was the primary nitrogen removal mechanism. Nitrogen concen trations were similar to average annual concentrations during the winter months (December, January, and February with approximate average temperatures of 1 6 ° C . ) .
i
\
Λ
\ \
\\ \\ \ WP3
WPl
WP4
WP5
WP6
ΜΜ8
HSIO
MONITORING STATION
............... Fig. 2.
AVERAGE CONCENTRATION 1988 AVERAGE CONCENTRATION 1989 AVERAGE CONCENTRATION DURING WINTER MONTHS 1987-1989
Nitrogen retention through the Orlando wetland.
While concentration measurements are convenient for general water quality assessment, load ing and removal rates are more essential parameters for constructed wetland design and analysis (Table 3 ) . Loading to the wetland increased from 147 kg/day in 1988 to 279 kg/day in 1989. Corresponding to this increased nitrogen loading was an increased nitrogen removal across the f i r s t two strata o f the wetland. TABLE 3
Monitoring Station
AVERAGE ANNUAL NITROGEN LOADING AND REMOVAL RATES
Nitrogen* Loading
1988 Nitrogen** Removal
Nitrogen* Loading
1989 Nitrogen** Removal
WPl
147
-
279
_
WP3
21
1.046
32
2.304
WP4, 5
10
0.155
14
0.458
9
0.224
13
0.149
MM8
12
0.105
21
0.105
HSIO
31
0.026
48
0.009
WP6
riyurca
lur
juauiny
are
in u n i
υτ
K.g/ady.
** Figures for removal are in units o f kg/ha/day.
Maximization of nutrient removal capabilities
HI
Nitrogen removal ranged from 0.009 to 2.304 kg/ha/day with o v e r a l l system removals o f 0.27 kg/ha/day and 0.47 kg/ha/day for 1988 and 1989, r e s p e c t i v e l y . A predicted removal rate o f 1.5 kg/ha/day was used for design o f the system, based on an influent loading o f 454 kg/day (Jackson, 1989). Based on the high removal rates in the f i r s t strata associated with i n f l u ent nitrogen loadings approximately 30 and 60 percent lower than design l o a d i n g s , the design basis may have been c o n s e r v a t i v e . Two parallel c e l l s were set up in the wetland to compare the nutrient removal a b i l i t i e s o f Typha spp. and Scirpus spp. The c e l l represented by monitoring station WP5 was planted with Typha spp. and WP4 planted with Scirpus spp. There was no s i g n i f i c a n t d i f f e r e n c e In the c a p a b i f i t i e s o f Scirpus spp. and Typha spp. to reduce nitrogen concentrations, although Scirpus spp. reduced nitrogen concentration to s l i g h t l y lower l e v e l s . This i s somewhat sur prising since Typha spp. appear to form a denser l i t t e r mat, which should Increase the sub strate surface area a v a i l a b l e to d e n i t r i f y i n g b a c t e r i a . Phosphorus When compared to the nitrogen data, a more s i g n i f i c a n t portion o f the phosphorus was removed in the f i r s t stratum. In 1988 and 1989, over 98 percent o f the phosphorus removal in the wetland occurred in this f i r s t 11 percent o f the system. In the remaining s t r a t a , phos phorus removal was less s i g n i f i c a n t , and in some strata there was an Increase in phosphorus concentration. The only other stratum with s i g n i f i c a n t phosphorus removal was Stratum 3 in 1989, where 12 percent o f the total phosphorus removal occurred. The o v e r a l l percent removal of phosphorus through the wetland system was 83 percent in 1988 and 89 percent in 1989. As presented in the discussion o f nitrogen removal, nitrogen may have become a l i m i t i n g nutrient in the system. Influencing the phosphorus reduction in the mixed marsh and hardwood swamp c e l l s . Increased inorganic nitrogen loadings to these c e l l s in 1989 appear to c o r r e spond to decreases in total phosphorus concentrations. A way to Improve phosphorus removal potential may be to increase inorganic nitrogen l o a d i n g s . As with nitrogen, when 1988 and 1989 phosphorus data are compared, i t can be seen that the response o f the f i r s t stratum to the increased phosphorus loading i s an increase in phos phorus removal (Table 4 ) . The phosphorus removal rate varied between 0.0 to 0.408 kg/ha/day. Overall phosphorus removal for 1989 was 0.07 kg/ha/day. The design o f the wet land was based on a predicted removal rate o f 0.15 kg/ha/day with an influent phosphorus loading of 56 kg/day (Jackson, 1989). Based on the high removal rates In the f i r s t strata associated with influent loadings 40 and 65 percent lower than design l o a d i n g s , the design basis may have been c o n s e r v a t i v e .
TABLE 4
AVERAGE ANNUAL PHOSPHORUS LOADING AND REMOVAL RATES
Phosphorus* Loading
1988 Phosphorus** Removal
WPl
22.7
-
WP3
1.4
0.235
1.4
0.408
WP4, 5
0.7
0.000
1.2
-0.122
WP6
0.7
-0.008
0.6
0.104
MM8
1.2
0.006
0.9
0.004
4.1
-0.009
Monitoring Station
1989 Phosphorus* Phosphorus** Loading Removal
HSIO 3.6 -0.001 * Figures for loading are 1n units o f kg/day. ** Figures for removal are in units o f kg/day.
36.3
-
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C. Ε . S W I N D E L L and J. A . J A C K S O N
A comparison o f the Orlando wetland o v e r a l l phosphorus removal rates with other wetland systems is summarized in Table 5. The Orlando wetland is in the lower end o f the range, further indicating the observed results represent conservative estimates o f phosphorus assimilation for this system. TABLE 5
COMPARISON OF PHOSPHORUS LOADING AND UPTAKE RATES*
Loading** Rate
Uptake** Rate
0.9
0.73
Orlando (1989)
2.7
2.38
Yonika and Lowry (1979)
7.1
3.34
15.2
4.86
17.2
7.00
Reference Boyt et al
(1977)
Spangler et al
(1977)
Ewel and Odum (1979)
from Davis (1989). ** Figures for loading and uptake are in units o f g P/m^/yr.
During the design o f the Orlando wetland, long-term phosphorus removal c a p a b i l i t y was a con cern. D e n i t r i f i c a t i o n i s a much studied process and can provide a long-term nitrogen sink. Unfortunately, no such process e x i s t s for phosphorus. Since e a r l y 1989, the South Florida Water Management D i s t r i c t in West Palm Beach, F l o r i d a , has been conducting a series o f work shops to discuss mechanisms o f phosphorus removal and c a p a c i t i e s o f phosphorus storage in wetlands. During these workshops, three basic mechanisms were presented as potential path ways for phosphorus removal: s o i l sorption; macrophytic plant uptake; and bacteria and algae f i x a t i o n . All three mechanisms influence phosphorus removal in the Orlando wetland to some extent. I f soil sorption was the primary phosphorus removal mechanism, then a consis tent drop in phosphorus concentration across the s i t e would be expected. The gradient between Stratum 1 and the remainder o f the wetland (Table 4) is inconsistent with expected s o i l sorption patterns. I f macrophytic plant uptake was a primary removal mechanism, the same pattern o f a gradual decrease in phosphorus concentration would be expected. The f i x a tion of phosphorus in bacteria and algae c e l l s and the eventual storage o f these c e l l s in the sediments appears to be the most s i g n i f i c a n t removal mechanism in the Orlando wetland since the microbial community can be responsive to changes in nutrient concentrations. Further reinforcing the theory that the microbial community is responsible for a s i g n i f i c a n t portion o f the phosphorus removal is the lack o f seasonal v a r i a t i o n in concentration (Figure 3). OPERATION AND DESIGN CONSIDERATIONS The a b i l i t y to control and manage flows through the Orlando wetland system may be a primary factor in the system's a b i l i t y to provide consistent l e v e l s o f treatment. Adverse e f f e c t s from extreme flushing events and short-term winter conditions have been prevented by storing water in the various wetland c e l l s followed by a gradual drawdown. The Orlando wetland was designed with 1 m o f freeboard (storage c a p a c i t y ) above the normal high-water l i n e , allowing for the storage o f 4.4 χ 10^ m3 o f water in addition to normal flows ( i . e . , an additional 58 days o f storage capacity at design f l o w s ) . The a b i l i t y to control water depths from several centimetres (sheet flow) to design high water l e v e l s allows the system to be operated in a mode that maximizes plant growth. Water depths are adjusted to mimic natural hydrologic cycles and in response to c l i m a t i c events. Water depths are increased during cold months to moderate local temperature and are decreased when tropical storms or other high-intensity r a i n f a l l events are f o r e c a s t . Another beneficial operational feature Is the a b i l i t y to i s o l a t e individual c e l l s from flow. This control mechanism allows for Individual c e l l s to be drained to o x i d i z e and compact
Maximization of nutrient removal capabilities
113
sediments. The c e l l can then be reflooded and allowed to s t a b i l i z e before i t is brought back o n - l i n e . The a b i l i t y to i s o l a t e c e l l s has also been used to control unexpected nutrient inputs. During breeding seasons, bird rookeries have become established in par t i c u l a r c e l l s , contributing high nutrient loadings. Discharge from these c e l l s has been discontinued until the birds migrated and nutrient concentrations s t a b i l i z e d .
0.8i
k
\
V
V\\ V \ \
\ \\
δ 0.2-
WPl
WP3
' WP4
WP5
HSK)
WP6
MONITORING STATION
AVERAGE CONCENTRATION 1988 AVERAGE CONCENTRATION 1989 AVERAGE CONCENTRATION DURING WINTER MONTHS 1987-1989 F i g . 3.
Phosphorus retention through the Orlando wetlands.
In November 1989, a new process came on-line at the Iron Bridge WPCF t h a t , during start-up, resulted in increased nutrient concentrations d e l i v e r e d to the wetland for several weeks (up to 12.6 mg/1 total nitrogen and 3.3 mg/1 total phosphorus at W P l ) . T h i s , combined with record cold a i r temperatures (as low as -13'*C) in mid-December 1989, resulted in l i t t l e overall impact on the performance of the wetland system. Effluent concentrations remained consistently lower than permitted l i m i t s following these events. This a b i l i t y o f the system to respond to adverse conditions has been attributed to the operational control features described above. CONCLUSIONS AND RECOMMENDATIONS A review of the two years of operating data from the Orlando wetland provides evidence that nutrient concentrations lower than those normally attainable by conventional advanced waste treatment processes can be achieved. The system was well within compliance of permitted effluent l i m i t s o f 2.31 mg/1 total nitrogen and 0.20 mg/1 total phosphorus even though these are among the most stringent l i m i t a t i o n s imposed in F l o r i d a . Through review of monitoring data and operating experience, theories were developed regarding nutrient removal pathways. The primary removal mechanisms appeared to be d e n i t r i f i c a t i o n for nitrogen removal and f i x a tion by bacteria and algae for phosphorus removal. Additional research i s being conducted to support these t h e o r i e s , including plant biomass and d e n i t r i f i c a t i o n studies. Design features of the wetland allowing for control of water depths and i s o l a t i o n o f por tions of the system were important factors in the a b i l i t y to provide consistent and r e l i a b l e nutrient removal. This same operational f l e x i b i l i t y is recommended for other similar sys tems. From the data presented, i t appears the system was underloaded. on the overall system performance, a less conservative approach
Rather than basing designs is recommended. For exam-
114
C. Ε . S W I N D E L L and J. A . J A C K S O N
p i e , the overall system nitrogen and phosphorus removal rates of 0.49 and 0.07 kg/ha/day, r e s p e c t i v e l y , are too conservative to serve as basis o f design for similar p r o j e c t s . For nitrogen removal, a recommended rate for design o f similar projects is 1.25 to 2.30 kg/ha/day, based on the f i r s t three and f i r s t strata of the wetlands, r e s p e c t i v e l y . Until more d e f i n i t i v e studies are complete, a somewhat conservative approach should s t i l l be taken in designing systems for phosphorus removal. Design phosphorus removal rates in the range of 0.15 to 0.20 kg/ha/day are recommended for similar p r o j e c t s , based on removal in the f i r s t three strata o f the Orlando wetland in 1988 and 1989, r e s p e c t i v e l y . In areas with colder climates or for systems with s i g n i f i c a n t l y greater nutrient loadings, a more conser v a t i v e approach is recommended. ACKNOWLEDGEMENT The authors and Post, Buckley, Schuh & Jernigan, Inc. would l i k e to express t h e i r apprecia tion to Mr. Thomas L. Lothrop and Mr. William Allman o f the City o f Orlando. Their commit ment to wetland operations may be unprecedented for constructed wetland treatment systems. REFERENCES Davis, Steve (1989). Personal communication. Management D i s t r i c t , West Palm Beach, F l o r i d a .
Wetlands
workshop.
South
Florida
Water
Jackson, JoAnn (1989). Man-made wetlands for wastewater treatment: two case s t u d i e s . In: Constructed Wetlands for Wastewater Treatment: Municipal. I n d u s t r i a l , and A g r i c u l t u r a l . D.A. Hammer ( E d . ) . Lewis Publishers, I n c . , Chelsea, Michigan, pp 574-580. M707/J
Avenue,
ABSTRACT A 494 ha constructed wetland operating since mid-1987 in Orlando, F l o r i d a , U.S.A. produced an effluent in 1989 with average concentrations of 0.92 mg/1 total nitrogen and 0.08 mg/1 total phosphorus. Influent flows were approximately 0.58 m3/s. Most o f the nitrogen and phosphorus was removed in the f i r s t 11 percent of the system. Nutrient removal e f f i c i e n c y peaked by a detention time of 5 days. During 1989, the average nitrogen uptake rate across the f i r s t 54 ha of the wetland was 2.31 kg/ha/day. Phosphorus uptake during 1989 across the same area was 0.41 kg/ha/day. D e n i t r i f i c a t i o n appeared to be the primary removal mechanism for nitrogen. The f i x a t i o n of phosphorus in bacteria and algae c e l l s and eventual storage of these c e l l s in the sediments was thought to be the primary phosphorus removal mechanism. Because of a design that allows for more control over internal flow rates than most other constructed wetlands, the system was able to provide an effluent with consistently low nutrient concentrations. KEY WORDS Wastewater; constructed wetlands; nitrogen; phosphorus; design; operation. INTRODUCTION A 494 ha constructed wetland in Orlando, F l o r i d a , U . S . A . , was designed to provide additional nutrient removal to 0.88 m3/s of treated municipal wastewater from the C i t y ' s Iron Bridge Regional Water Pollution Control F a c i l i t y (WPCF). Because o f the s e n s i t i v i t y o f r i v e r s and lakes in Florida to even small increases in nutrient concentrations, l e v e l s of treatment beyond advanced were required before discharge. The wetland is being used to provide this additional level of advanced treatment prior to discharge to the St. Johns R i v e r . The wetland was constructed on low-lying land that was heavily ditched and drained to pro vide c a t t l e pasture. Over 25 km of earthern berms were constructed on the s i t e to provide a segmented design that would maximize operational control (Figure 1 ) . Water passes between c e l l s through control structures consisting of pipes through berms with box culverts and flashboards on the upstream end of each pipe. The wetland design incorporated the use of four communities to enhance w i l d l i f e u t i l i z a t i o n : a deep marsh ( c a l l e d wet p r a i r i e ) , a shallow mixed marsh, a hardwood swamp, and a l a k e . The wetland is a surface flow system that has depths ranging up to 1 m in the wet p r a i r i e , to 0.6 m in the shallow mixed marsh, to 0.3 m in the hardwood swamp, and to 9 m in the l a k e . The substrate of the system consists of mineral s o i l with 2 to 15 percent organic matter, primarily resulting from former c a t t l e ranching. No s i t e grading was conducted. There is a
107
C. Ε . S W I N D E L L and J. A . J A C K S O N
108
natural grade o f approximately 0.2 percent from the influent to effluent discharge struc t u r e . The wet p r a i r i e was planted with Typha spp. and Scirpus s p p . . the mixed marsh with approximately 30 indigenous herbaceous s p e c i e s , and the hardwood swamp with nearly 160,000 seedlings o f a v a r i e t y o f tree s p e c i e s , but primarily Taxodium distichum.
LEGEND
^LAKE
SHORELINE
^BERM —kWATER
•
F i g . 1.
Orlando wetland s i t e
FLOW
MONITORING
STATIONS
plan.
The wetland began receiving flow from the Iron Bridge WPCF in July 1987. I n i t i a l flow was 0.35 m^/s but increased to 0.58 m^/s in August 1988. I t is anticipated that flows w i l l be increased to the full 0.88 m^/s design capacity by 1991. At design flows, the detention time through the wetland is estimated to be 30 days. Water leaving the constructed wetland sheet flows across an adjacent natural marsh to the S t . Johns R i v e r . MONITORING RESULTS AND DISCUSSION Introduction Monitoring data for the start-up operational phase of the Orlando wetland was presented in Jackson (1989). The f i r s t two years o f operational data, from January 1988 to December 1989, are presented herein. As i l l u s t r a t e d on Figure 1, the wetland water q u a l i t y was monitored at ten sample s t a t i o n s . The sample stations represent the wetland influent and f i v e strata o f the system as f o l l o w s : 0 0 0 0 0 0
Wetland Stratum Stratum Stratum Stratum Stratum
Influent - monitoring station WPl; 1 - Wet P r a i r i e - monitoring stations WP2 and 2 - Wet p r a i r i e - monitoring stations WP4 and 3 - Wet P r a i r i e - monitoring station WP6; 4 - Mixed Marsh - monitoring stations MM7 and 5 - Hardwood Swamp/Lake - e f f l u e n t monitoring
WP3; WP5; MM8; and station HSIO.
Maximization of nutrient removal capabilities
109
Water q u a l i t y data was obtained monthly from three consecutive d a i l y grab samples taken from each monitoring s t a t i o n , with the exception o f WPl and HSIO, which were sampled d a i l y . Annual average t o t a l nitrogen and t o t a l phosphorus concentrations for 1988 and 1989 are sum marized in Table 1. Maximum e f f l u e n t concentrations permitted by State and Federal regula tory agencies are 2.31 mg/1 t o t a l nitrogen and 0.20 mg/l t o t a l phosphorus. TABLE 1
Monitoring Station
Nitrogen* l9Óé 1989
WPl WP3 WP4,5 WP6 MM8 HSIO
4.18 1.53 1.51 1.27 0.96 0.84
WETLANDS NUTRIENT CONCENTRATIONS
Phosphorus* 1^88 1989 0.72 0.08 0.15 0.07 0.05 0.076
0.572 0.103 0.102 0.106 0.091 0.095
5.52 1.92 1.74 1.59 1.22 0.92
Percent Area
Detention Time*** 1989
0 11 16 32 67 100
0 6 9 18 31 57
0 5 7 14 24 45
**
Percent upstream area equals percent o f the wetland area upstream o f the sample s t a t i o n . *** Detention time in days approximated based on volume.
The monitoring stations described p r e v i o u s l y .
presented
in
Table
1 are
representative
of
each
of
the
strata
Nitrogen Most of the reduction in nitrogen concentration occured in Stratum 1. In both 1988 and 1989, over 78 percent o f the nitrogen removal in the wetland occurred in t h i s f i r s t 11 percent (54 ha) o f the system. Otherwise, no s i g n i f i c a n t decrease in nitrogen concentration occurred in any other stratum. The percent reduction in nitrogen concentration was very similar in 1988 and 1989, even though the influent nitrogen concentration increased by 32 percent in 1989. The o v e r a l l percent removal o f nitrogen through the wetland system was 80 percent in 1988 and 83 percent in 1989. There was an increased dominance in the organic nitrogen concentration as water passed through the wetland (Table 2 ) . This may have caused the l a t t e r portions o f the wetland to become nitrogen l i m i t e d , reducing the phosphorus removal p o t e n t i a l . TABLE 2
Monitoring Station WPl WP6 MM HSIO
PERCENT TOTAL ORGANIC ΝITROGEN
1988* TON %
1989* TON %
17 56 92 85
13 37 67 73
C. Ε . S W I N D E L L and J. A . J A C K S O N
110
Nitrogen reduction in the wetland appeared to be highly correlated to loading r a t e s , provid ing indirect evidence that microbial a c t i v i t y ( n i t r i f i c a t i o n / d e n i t r i f i c a t i o n ) was a more important nitrogen sink than macrophytic plant uptake. The lack of s i g n i f i c a n t seasonal variation in the data set (Figure 2) also reinforces the theory that d e n i t r i f i c a t i o n , rather than macrophytic plant uptake, was the primary nitrogen removal mechanism. Nitrogen concen trations were similar to average annual concentrations during the winter months (December, January, and February with approximate average temperatures of 1 6 ° C . ) .
i
\
Λ
\ \
\\ \\ \ WP3
WPl
WP4
WP5
WP6
ΜΜ8
HSIO
MONITORING STATION
............... Fig. 2.
AVERAGE CONCENTRATION 1988 AVERAGE CONCENTRATION 1989 AVERAGE CONCENTRATION DURING WINTER MONTHS 1987-1989
Nitrogen retention through the Orlando wetland.
While concentration measurements are convenient for general water quality assessment, load ing and removal rates are more essential parameters for constructed wetland design and analysis (Table 3 ) . Loading to the wetland increased from 147 kg/day in 1988 to 279 kg/day in 1989. Corresponding to this increased nitrogen loading was an increased nitrogen removal across the f i r s t two strata o f the wetland. TABLE 3
Monitoring Station
AVERAGE ANNUAL NITROGEN LOADING AND REMOVAL RATES
Nitrogen* Loading
1988 Nitrogen** Removal
Nitrogen* Loading
1989 Nitrogen** Removal
WPl
147
-
279
_
WP3
21
1.046
32
2.304
WP4, 5
10
0.155
14
0.458
9
0.224
13
0.149
MM8
12
0.105
21
0.105
HSIO
31
0.026
48
0.009
WP6
riyurca
lur
juauiny
are
in u n i
υτ
K.g/ady.
** Figures for removal are in units o f kg/ha/day.
Maximization of nutrient removal capabilities
HI
Nitrogen removal ranged from 0.009 to 2.304 kg/ha/day with o v e r a l l system removals o f 0.27 kg/ha/day and 0.47 kg/ha/day for 1988 and 1989, r e s p e c t i v e l y . A predicted removal rate o f 1.5 kg/ha/day was used for design o f the system, based on an influent loading o f 454 kg/day (Jackson, 1989). Based on the high removal rates in the f i r s t strata associated with i n f l u ent nitrogen loadings approximately 30 and 60 percent lower than design l o a d i n g s , the design basis may have been c o n s e r v a t i v e . Two parallel c e l l s were set up in the wetland to compare the nutrient removal a b i l i t i e s o f Typha spp. and Scirpus spp. The c e l l represented by monitoring station WP5 was planted with Typha spp. and WP4 planted with Scirpus spp. There was no s i g n i f i c a n t d i f f e r e n c e In the c a p a b i f i t i e s o f Scirpus spp. and Typha spp. to reduce nitrogen concentrations, although Scirpus spp. reduced nitrogen concentration to s l i g h t l y lower l e v e l s . This i s somewhat sur prising since Typha spp. appear to form a denser l i t t e r mat, which should Increase the sub strate surface area a v a i l a b l e to d e n i t r i f y i n g b a c t e r i a . Phosphorus When compared to the nitrogen data, a more s i g n i f i c a n t portion o f the phosphorus was removed in the f i r s t stratum. In 1988 and 1989, over 98 percent o f the phosphorus removal in the wetland occurred in this f i r s t 11 percent o f the system. In the remaining s t r a t a , phos phorus removal was less s i g n i f i c a n t , and in some strata there was an Increase in phosphorus concentration. The only other stratum with s i g n i f i c a n t phosphorus removal was Stratum 3 in 1989, where 12 percent o f the total phosphorus removal occurred. The o v e r a l l percent removal of phosphorus through the wetland system was 83 percent in 1988 and 89 percent in 1989. As presented in the discussion o f nitrogen removal, nitrogen may have become a l i m i t i n g nutrient in the system. Influencing the phosphorus reduction in the mixed marsh and hardwood swamp c e l l s . Increased inorganic nitrogen loadings to these c e l l s in 1989 appear to c o r r e spond to decreases in total phosphorus concentrations. A way to Improve phosphorus removal potential may be to increase inorganic nitrogen l o a d i n g s . As with nitrogen, when 1988 and 1989 phosphorus data are compared, i t can be seen that the response o f the f i r s t stratum to the increased phosphorus loading i s an increase in phos phorus removal (Table 4 ) . The phosphorus removal rate varied between 0.0 to 0.408 kg/ha/day. Overall phosphorus removal for 1989 was 0.07 kg/ha/day. The design o f the wet land was based on a predicted removal rate o f 0.15 kg/ha/day with an influent phosphorus loading of 56 kg/day (Jackson, 1989). Based on the high removal rates In the f i r s t strata associated with influent loadings 40 and 65 percent lower than design l o a d i n g s , the design basis may have been c o n s e r v a t i v e .
TABLE 4
AVERAGE ANNUAL PHOSPHORUS LOADING AND REMOVAL RATES
Phosphorus* Loading
1988 Phosphorus** Removal
WPl
22.7
-
WP3
1.4
0.235
1.4
0.408
WP4, 5
0.7
0.000
1.2
-0.122
WP6
0.7
-0.008
0.6
0.104
MM8
1.2
0.006
0.9
0.004
4.1
-0.009
Monitoring Station
1989 Phosphorus* Phosphorus** Loading Removal
HSIO 3.6 -0.001 * Figures for loading are 1n units o f kg/day. ** Figures for removal are in units o f kg/day.
36.3
-
112
C. Ε . S W I N D E L L and J. A . J A C K S O N
A comparison o f the Orlando wetland o v e r a l l phosphorus removal rates with other wetland systems is summarized in Table 5. The Orlando wetland is in the lower end o f the range, further indicating the observed results represent conservative estimates o f phosphorus assimilation for this system. TABLE 5
COMPARISON OF PHOSPHORUS LOADING AND UPTAKE RATES*
Loading** Rate
Uptake** Rate
0.9
0.73
Orlando (1989)
2.7
2.38
Yonika and Lowry (1979)
7.1
3.34
15.2
4.86
17.2
7.00
Reference Boyt et al
(1977)
Spangler et al
(1977)
Ewel and Odum (1979)
from Davis (1989). ** Figures for loading and uptake are in units o f g P/m^/yr.
During the design o f the Orlando wetland, long-term phosphorus removal c a p a b i l i t y was a con cern. D e n i t r i f i c a t i o n i s a much studied process and can provide a long-term nitrogen sink. Unfortunately, no such process e x i s t s for phosphorus. Since e a r l y 1989, the South Florida Water Management D i s t r i c t in West Palm Beach, F l o r i d a , has been conducting a series o f work shops to discuss mechanisms o f phosphorus removal and c a p a c i t i e s o f phosphorus storage in wetlands. During these workshops, three basic mechanisms were presented as potential path ways for phosphorus removal: s o i l sorption; macrophytic plant uptake; and bacteria and algae f i x a t i o n . All three mechanisms influence phosphorus removal in the Orlando wetland to some extent. I f soil sorption was the primary phosphorus removal mechanism, then a consis tent drop in phosphorus concentration across the s i t e would be expected. The gradient between Stratum 1 and the remainder o f the wetland (Table 4) is inconsistent with expected s o i l sorption patterns. I f macrophytic plant uptake was a primary removal mechanism, the same pattern o f a gradual decrease in phosphorus concentration would be expected. The f i x a tion of phosphorus in bacteria and algae c e l l s and the eventual storage o f these c e l l s in the sediments appears to be the most s i g n i f i c a n t removal mechanism in the Orlando wetland since the microbial community can be responsive to changes in nutrient concentrations. Further reinforcing the theory that the microbial community is responsible for a s i g n i f i c a n t portion o f the phosphorus removal is the lack o f seasonal v a r i a t i o n in concentration (Figure 3). OPERATION AND DESIGN CONSIDERATIONS The a b i l i t y to control and manage flows through the Orlando wetland system may be a primary factor in the system's a b i l i t y to provide consistent l e v e l s o f treatment. Adverse e f f e c t s from extreme flushing events and short-term winter conditions have been prevented by storing water in the various wetland c e l l s followed by a gradual drawdown. The Orlando wetland was designed with 1 m o f freeboard (storage c a p a c i t y ) above the normal high-water l i n e , allowing for the storage o f 4.4 χ 10^ m3 o f water in addition to normal flows ( i . e . , an additional 58 days o f storage capacity at design f l o w s ) . The a b i l i t y to control water depths from several centimetres (sheet flow) to design high water l e v e l s allows the system to be operated in a mode that maximizes plant growth. Water depths are adjusted to mimic natural hydrologic cycles and in response to c l i m a t i c events. Water depths are increased during cold months to moderate local temperature and are decreased when tropical storms or other high-intensity r a i n f a l l events are f o r e c a s t . Another beneficial operational feature Is the a b i l i t y to i s o l a t e individual c e l l s from flow. This control mechanism allows for Individual c e l l s to be drained to o x i d i z e and compact
Maximization of nutrient removal capabilities
113
sediments. The c e l l can then be reflooded and allowed to s t a b i l i z e before i t is brought back o n - l i n e . The a b i l i t y to i s o l a t e c e l l s has also been used to control unexpected nutrient inputs. During breeding seasons, bird rookeries have become established in par t i c u l a r c e l l s , contributing high nutrient loadings. Discharge from these c e l l s has been discontinued until the birds migrated and nutrient concentrations s t a b i l i z e d .
0.8i
k
\
V
V\\ V \ \
\ \\
δ 0.2-
WPl
WP3
' WP4
WP5
HSK)
WP6
MONITORING STATION
AVERAGE CONCENTRATION 1988 AVERAGE CONCENTRATION 1989 AVERAGE CONCENTRATION DURING WINTER MONTHS 1987-1989 F i g . 3.
Phosphorus retention through the Orlando wetlands.
In November 1989, a new process came on-line at the Iron Bridge WPCF t h a t , during start-up, resulted in increased nutrient concentrations d e l i v e r e d to the wetland for several weeks (up to 12.6 mg/1 total nitrogen and 3.3 mg/1 total phosphorus at W P l ) . T h i s , combined with record cold a i r temperatures (as low as -13'*C) in mid-December 1989, resulted in l i t t l e overall impact on the performance of the wetland system. Effluent concentrations remained consistently lower than permitted l i m i t s following these events. This a b i l i t y o f the system to respond to adverse conditions has been attributed to the operational control features described above. CONCLUSIONS AND RECOMMENDATIONS A review of the two years of operating data from the Orlando wetland provides evidence that nutrient concentrations lower than those normally attainable by conventional advanced waste treatment processes can be achieved. The system was well within compliance of permitted effluent l i m i t s o f 2.31 mg/1 total nitrogen and 0.20 mg/1 total phosphorus even though these are among the most stringent l i m i t a t i o n s imposed in F l o r i d a . Through review of monitoring data and operating experience, theories were developed regarding nutrient removal pathways. The primary removal mechanisms appeared to be d e n i t r i f i c a t i o n for nitrogen removal and f i x a tion by bacteria and algae for phosphorus removal. Additional research i s being conducted to support these t h e o r i e s , including plant biomass and d e n i t r i f i c a t i o n studies. Design features of the wetland allowing for control of water depths and i s o l a t i o n o f por tions of the system were important factors in the a b i l i t y to provide consistent and r e l i a b l e nutrient removal. This same operational f l e x i b i l i t y is recommended for other similar sys tems. From the data presented, i t appears the system was underloaded. on the overall system performance, a less conservative approach
Rather than basing designs is recommended. For exam-
114
C. Ε . S W I N D E L L and J. A . J A C K S O N
p i e , the overall system nitrogen and phosphorus removal rates of 0.49 and 0.07 kg/ha/day, r e s p e c t i v e l y , are too conservative to serve as basis o f design for similar p r o j e c t s . For nitrogen removal, a recommended rate for design o f similar projects is 1.25 to 2.30 kg/ha/day, based on the f i r s t three and f i r s t strata of the wetlands, r e s p e c t i v e l y . Until more d e f i n i t i v e studies are complete, a somewhat conservative approach should s t i l l be taken in designing systems for phosphorus removal. Design phosphorus removal rates in the range of 0.15 to 0.20 kg/ha/day are recommended for similar p r o j e c t s , based on removal in the f i r s t three strata o f the Orlando wetland in 1988 and 1989, r e s p e c t i v e l y . In areas with colder climates or for systems with s i g n i f i c a n t l y greater nutrient loadings, a more conser v a t i v e approach is recommended. ACKNOWLEDGEMENT The authors and Post, Buckley, Schuh & Jernigan, Inc. would l i k e to express t h e i r apprecia tion to Mr. Thomas L. Lothrop and Mr. William Allman o f the City o f Orlando. Their commit ment to wetland operations may be unprecedented for constructed wetland treatment systems. REFERENCES Davis, Steve (1989). Personal communication. Management D i s t r i c t , West Palm Beach, F l o r i d a .
Wetlands
workshop.
South
Florida
Water
Jackson, JoAnn (1989). Man-made wetlands for wastewater treatment: two case s t u d i e s . In: Constructed Wetlands for Wastewater Treatment: Municipal. I n d u s t r i a l , and A g r i c u l t u r a l . D.A. Hammer ( E d . ) . Lewis Publishers, I n c . , Chelsea, Michigan, pp 574-580. M707/J