nutrient removal from aquaculture wastewater using a constructed wetlands system

8/11/2019 Nutrient Removal From Aquaculture Wastewater Using a Constructed Wetlands System

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Nutrient removal from aquaculture wastewater

using a constructed wetlands system

Ying-Feng Lin a,*, Shuh-Ren Jing a, Der-Yuan Lee a,Tze-Wen Wang b

aDepartment of Environmental Engineering and Health, Chia-Nan University of Pharmacy and Science,

Tainan 717, TaiwanbDepartment of Pharmacy, Chia-Nan University of Pharmacy and Science, Tainan 717, Taiwan

Received 20 October 2000; received in revised form 15 August 2001; accepted 3 September 2001

Abstract

Nutrient removal is essential for aquaculture wastewater treatment to protect receiving waters

from eutrophication and for potential reuse of the treated water. A pilot-scale wastewater treatmentsystem consisting of a free water surface (FWS) and a subsurface flow (SSF) constructed wetlands

arranged in series was operated for around 8 months. The study was conducted to examine system

start-up phenomena and to evaluate system performance in removing inorganic nitrogen and

phosphate from aquaculture wastewater under various hydraulic loading rates (1.8 to 13.5 cm

day1). The wetlands system showed rapid start-up behaviors in which process stabilities were

achieved in the following sequence: phosphate removal in the SSF without an adaptation period,

nitrogen removal in the SSF after 1 month, nitrogen removal in the FWS after 2 to 3 months,

phosphate removal in the FWS after 3 months, and vegetation cover in both wetlands after 7 months

of operation. Nitrogen removals were excellent, with efficiencies of 86% to 98% for ammonium

nitrogen (NH4 N) and 95% to 98% for total inorganic nitrogen (TIN). Removal efficiencies were

affected little by the hydraulic loading trials. Phosphate removal of 32% to 71% occurred, with the

efficiencies being inversely related to hydraulic loading. The FWS wetland removed most inorganic

nitrogen, whereas the SSF wetland removed phosphate at a rate equal to or even greater than the

FWS. Removal of ammonium and nitrite (effluent concentrations < 0.3 mg NH4 N l1 and 0.01 mg

NO2 N l1) were sufficient for recycle in the aquaculture system without danger of harming the

fish. D 2002 Elsevier Science B.V. All rights reserved.

Keywords:Constructed wetlands; Aquaculture; Wastewater treatment; Nitrogen; Phosphate

0044-8486/02/$ - see front matterD 2002 Elsevier Science B.V. All rights reserved.

PII: S 0 0 4 4 - 8 4 8 6 ( 0 1 ) 0 0 8 0 1 - 8

* Corresponding author. Tel.: +886-6-266-4911-304-16; fax: +886-6-266-7323.

E-mail address:[email protected] (Y.-F. Lin).

www.elsevier.com/locate/aqua-online

Aquaculture 209 (2002) 169184


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1. Introduction

Aquaculture is an important industry in Taiwan and one which requires large

quantities of water. Because existing surface water sources are widely polluted, ground-water is the main fresh water source for aquaculture. Consequently, several areas have

faced ground subsidence as a result of over-withdrawal of groundwater. In addition, the

accumulation of feed residue and fish excreta during cultivation often causes water

quality deterioration in fishponds, resulting in toxic effects to the fish. Aquaculture farm

discharges contain considerable quantities of organic matter, nitrogen, and phosphorus

and can further degrade the water quality in receiving waters. Therefore, it is apparent

that an appropriate wastewater treatment process is helpful for sustaining aquaculture

development in Taiwan.

A number of physical, chemical, and biological methods used in conventional

wastewater treatment have been applied in aquaculture systems. Solids removal is

accomplished by sedimentation, sand filtration, or mechanical filtration. Biological

processes such as submerged biofilters, trickling filters, rotating biological contactors,

and fluidized bed reactors are employed for oxidation of organic matter, nitrification, or

denitrification (Van Rijn, 1996). These above methods do help with phosphorus removal;

however, little work has been focused on aquaculture wastewater. Conventional treatment

systems have the disadvantages of sludge production, high-energy demand, and frequent

maintenance requirements.

Natural treatment systems, including constructed wetlands, have grown in popularity

for wastewater treatment since the early 1980s (Reed et al., 1995). Constructed wetlandshave been used to treat acid mine drainage, storm water runoff, municipal wastewater,

industrial wastewater, and agricultural effluent from livestock operations. Researchers

have demonstrated that treatment wetland systems can remove significant amounts of

suspended solids, organic matter, nitrogen, phosphorus, trace elements, and microorgan-

isms contained in wastewater (Kadlec and Knight, 1996). Constructed wetland systems are

characterized by the advantages of moderate capital costs, low energy consumption and

maintenance requirements, and benefits of increased wildlife habitat (International Water

Association, 2000).

Use of constructed wetlands for aquaculture wastewater treatment is increasing

(Zachritz and Jacquez, 1993; Schwartz and Boyd, 1995; Panella et al., 1999). Variousbiotic and abiotic processes regulate pollutants removal in wetlands (Kadlec and Knight,

1996; Reddy and DAngelo, 1997). Microbial mineralization and transformation (e.g.,

nitrification denitrification) and uptake by vegetation are the major biotic processes.

Abiotic processes include precipitation, sedimentation, and substrate adsorption and may

be particularly important for phosphorus removal. In wetland systems, some removal

processes require only brief periods (e.g., the abiotic processes) in which to become

fully operative, while others can require months (e.g., microbial community establish-

ment) or years (e.g., plant litter development) to reach stability (International Water

Association, 2000). These adaptation periods may depend on antecedent soil or gravel

properties, and the initial hydrological and vegetation conditions. Much research hasbeen targeted at the performance results of constructed wetlands for long-term operation,

whereas little work has been conducted on start-up phenomena. For these reasons, we

Y.-F. Lin et al. / Aquaculture 209 (2002) 169184170


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set up a two-stage pilot-scale system comprised of a free water surface (FWS) and a

subsurface flow (SSF) constructed wetlands (Fig. 1). System performance was moni-

tored with respect to removal of ammonium, nitrite, nitrate, and phosphate fromaquaculture wastewater under various hydraulic loading rates ranging from 1.8 to

13.5 cm day1.

2. Materials and methods

2.1. Constructed wetlands system

The pilot-scale constructed wetland system was built adjacent to a 0.2-ha earthen

pond for production of milkfish (Chanos chanos). The milkfish pond is located inTainan County, Taiwan. The pilot-scale system consisted of a free water surface (FWS)

and a subsurface flow (SSF) constructed wetlands arranged in series (Fig. 1). The

wetlands were cast-iron vessels, each measuring 5 m by 1 m by 0.8 m (length, width,

height), and lined with impermeable plastic liners. The FWS wetland contained a 30-cm

layer of local soil (Jender silt loam, 25j37VN, 166j55VE) at bottom and 40 cm of free

water surface above the soil layer. The SSF wetland included 60 cm of river gravel

(nominal diameter 10 to 20 mm), providing a porosity of 45%, and 40 cm of subsurface

water flow within the gravel layer. The elevation level of the FWS wetland was 30 cm

higher than the SSF wetland. The water level remained constant for the duration of the

study.Because the groundwater source used in the aquaculture farm contains about 0.5%

salinity, the plants for the constructed wetland system had to be salt tolerant. The FWS

Fig. 1. Layout of the pilot-scale FWS SSF series constructed wetland system for treating fishpond water. (A)

Sampling location for the influent; (B) sampling location for the FWS effluent; (C) sampling location for the SSF

effluent.

Y.-F. Lin et al. / Aquaculture 209 (2002) 169184 171


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wetland was planted with water spinach (Ipomoea aquatica) in the front half and with a

native weed (Paspalum vaginatum) in the second half. The SSF wetland was planted

with common reed (Phragmites australis). During initial plant establishment (January

1999), the wetland water was kept static, with a water depth of 5 cm for the FWS and 40cm for the SSF. The planting densities were 12% of wetland cover for the FWS and four

plants m2 for the SSF. These three kinds of aquatic plants grew rapidly to colonize the

wetlands since influent was continuously added. Plants were not harvested during this

study.

2.2. System operation

Outdoor cultivation of fish in Taiwan normally begins in spring with harvest in the late

fall to avoid losing fish to the low water temperatures of the winter. The constructed

wetlands system in this study was operated from March 24, 1999 through November 20,

1999. This period was also the optimal season for vegetation growth and pollutant

processing in the wetlands.

The aquaculture wastewater was continuously fed into the FWS wetland directly from

the fishpond by a gear pump and then passed through the SSF wetland via gravity flow

(Fig. 1). A lateral perforated distribution pipe for inflow and a lateral trough-shaped

collector for drainage were installed at the inlet end and the distal end of the FWS tank,

respectively. Another lateral perforated pipe served as a collection drain and was installed

at the bottom of distal end of the SSF wetland. Sampling ports were set up at the inlet and

outlet of the FWS wetland, and at the outlet of the SSF wetland (Fig. 1). Five hydraulicloading rates and associated retention times were monitored; each operating for 1 to 2

months (Table 1).

Table 1

Hydraulic conditions for operating the constructed wetlands system in various stages trials

Trial

nos.

Operating date Q

(m3 day1)

q

(cm day1)

tfor overall

system (day)

tfor FWS

wetland (day)

tfor SSF

wetland (day)

Stage 1 24 Mar. 199931 May 1999

0.18 1.8 12.8 8.4 4.4

Stage 2 1 June 1999

9 Aug. 1999

0.23 2.3 10.0 6.5 3.5

Stage 3 10 Aug. 1999

6 Oct. 1999

0.34 3.4 6.8 4.4 2.4

Stage 4 7 Oct. 1999

25 Oct. 1999

0.68 6.8 3.4 2.2 1.2

Stage 5 26 Oct. 1999

20 Nov. 1999

1.35 13.5 1.7 1.1 0.6

Q = average rate of inflow and outflow.

q = hydraulic loading rate, which is the average flow rate ( Q) divided by surface area of wetland(s).

t= nominal hydraulic retention time, which can be computed as surface area times water depth times porosity of

wetland(s) divided by average flow rate. The porosity or fraction of the space available for water to flow through

the wetland was assumed to be 0.75 in FWS wetland and 0.4 in SSF wetland in this study.



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2.3. Performance evaluation

Pollutant loading rate (g m2 day1) was calculated by multiplying the hydraulic

loading rate (m day1) by the influent pollutant concentration (mg l1). Pollutant removalrate (g m2 day1) was defined as hydraulic loading rate times the difference in

concentration between the influent and the effluent. The first-order plug flow kinetic

model was applied accordingly:

C0

Ci expKt 1

Where Ci = influent pollutant concentration, mg l1; Co = effluent pollutant concen-

tration, mg l1; t= nominal hydraulic retention time, day; K= first-order removal rate

constant, day1.

Because average temperatures of the influent and effluent of the constructed wetlands

system in each experimental stage ranged from 24.3 to 29.6 jC (Table 2), we omitted the

temperature effect on K and considered the K determined as an apparent reaction rate

constant.

Table 2

Water qualities (meanF standard deviation) at sampling locations for various stages trials

Sampling

location

Temperature

(jC)

NH4 N

(mg l1)

NO2 N

(mg l1)

NO3 N

(mg l1)

TIN

(mg l1)

PO4 P

(mg l1)

Stage 1

Influent 27.0F 2.2 0.16F 0.18 0.030F 0.085 0.26F 0.027 0.45F 0.53 2.39F 0.91

FWS effluent 27.3F 2.5 0.59F 0.47 0.041F 0.091 0.30F 0.16 0.90F 0.56 3.47F 2.35

SSF effluent 27.0F 3.2 0.40F 0.51 0.005F 0.004 0.19F 0.18 0.60F 0.70 0.73F 0.28

Stage 2

Influent 29.6F 1.6 3.31F 3.65 0.432F 0.450 0.74F 1.38 4.48F 5.48 7.44F 2.56



Stage 3

Influent 29.6F 1.2 1.30F 0.98 0.647F 0.450 0.94F 0.40 2.88F 1.83 10.45F 3.49FWS effluent 29.0F 1.2 0.13F 0.11 0.006F 0.003 0.02F 0.01 0.16F 0.13 8.52F 3.83


Stage 4

Influent 27.7F 1.8 1.46F 0.34 0.474F 0.339 2.26F 1.33 4.19F 2.01 8.57F 2.8

FWS effluent 27.1F1.4 0.12F 0.10 0.003F 0.002 0.02F 0.001 0.14F 0.10 6.48F 1.87


Stage 5

Influent 25.3F 1.7 0.80F 0.56 0.423F 0.134 2.66F 0.63 3.88F 1.32 5.19F 1.64



NH4 N = ammonium nitrogen; NO2 N = nitrite nitrogen; NO3 N = nitrate nitrogen; TIN = total inorganic

nitrogen; PO4 P = ortho-phosphate phosphorous.



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2.4. Sampling and analysis

Water samples were taken twice each week for the duration of the study from the

influent of the system, effluent of the FWS wetland, and effluent of the SSF wetland. Suchsampling was usually carried out at around 10 A.M. in each sampling date. These samples

were analyzed for temperature, ammonium nitrogen (NH4N), nitrite nitrogen (NO2N),

nitrate nitrogen (NO3 N), and ortho-phosphate phosphorus (PO4 P) according to the

Standard Methods (American Public Health Association, 1985). Total inorganic nitrogen

(TIN) was calculated as the sum of NH4N, NO2 N, and NO3 N.

3. Results

3.1. Start-up behaviors

Continuous-flow operation of the constructed wetlands system was initiated with a low

hydraulic loading rate of 1.8 cm day1 in Stage 1. Wetland plants grew actively in this

stage, but the vegetation cover was still sparse (Fig. 2). During Stage 1 (Fig. 3), reductions

of inorganic nitrogen (ammonium, nitrite, and nitrate) and phosphate through the FWS

wetland were negligible, although the influent nutrient concentrations and hydraulic

loading rate were low. Nutrient concentrations in the effluent of the FWS wetland were

higher than influent concentrations, resulting in negative removal efficiencies (Table 2).

Significant removal of inorganic nitrogen and phosphate in the FWS wetland did not occuruntil around day 70 (nitrogen) and day 90 (phosphate) of operation (Fig. 3). Thereafter,

effluent concentrations of inorganic nitrogen in the FWS wetland were consistently low,

and effluent phosphate concentrations followed the trend of the influent concentrations

under various hydraulic loading rates.

Fig. 2. Time course of vegetation growth in FWS and SSF wetlands.



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Conversely, significant reductions in concentrations of inorganic nitrogen through the

SSF wetland were achieved after approximately 30 days of operation (Fig. 3). Sub-

sequently, the SSF wetland showed consistently low nitrogen concentrations in its effluent.

Furthermore, phosphate concentrations in the SSF effluent were stable and consistentlylower than in the FWS effluent during Stage 1, and it followed the trend of the FWS

effluent and gradually increased because of an increase in the phosphate loading rate.

Fig. 3. Performance transitions during start-up period and the stationary state influent effluent concentration data

of nitrogen and phosphate for the pilot-scale FWSSSF series constructed wetlands system.



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The wetlands system required approximately 7 months to attain a vegetative cover of near

80% (Fig. 2). Because of the unstable behavior, treatment results in Stage 1 were not

included for evaluation of system performance.

3.2. Nitrogen removal

Nutrient concentrations in the fishpond increased as feed residue and fish excreta

accumulated. In Stages 2 through 5, the influent concentrations in the constructed

wetlands system ranged from 0.12 to 14.7 mg NH4 N l1, 0.02 to 1.5 mg NO2 N l

1,

Fig. 4. Relationships between effluent nitrogen concentrations (Co) and loading rate (LR) for NH4 N and TIN in

the constructed wetlands system.



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0.01 to 5.3 mg NO3 N l1, and 3.1 to 17.7 mg PO4 P l

1. As shown in Fig. 3, the

wetlands system effluents in Stages 2 through 5 were consistently much lower in nitrogen

concentrations than the influents. Based on data in Table 2, the average removal

efficiencies of the wetlands system were 86% to 98% for NH4N, > 99% for NO2 N,82% to 99% for NO3 N, and 95% to 98% for TIN. These efficiencies were extremely high

and were only slightly affected by hydraulic loading rate. The observed decreases in

inorganic nitrogen in the FWS wetland were significantly greater (P< 0.01) than those in

the SSF wetland (Table 2). Effluent concentrations of NH4 N and TIN in the constructed

wetlands (Fig. 4) were positively correlated with loading rates (r= 0.462 for NH4 N and

0.114 for TIN). However, effluent concentrations were usually < 0.3 mg NH4 N l1, 0.01

mg NO2 N l1 and 0.38 mg NO3N l

1 in Stages 2 through 5. Even at high hydraulic

loading rates (Stage 5), effluent values remained low (0.11F 0.13 mg NH4N l1,

0.003F 0.002 mg NO2 N l1, and 0.07F 0.06 mg NO3 N l

1).

Removal rates for TIN showed a linear relationship to the loading rate (Fig. 5), with a

maximum removal rate of 0.55 g N m2 day1 occurring in Stage 5 when the loading rate

was increased to 0.59 g N m2 day1. The first-order removal rate constant was

determined by linear regression analysis of the Co/Ci(calculated by mean values in Table

Fig. 5. Comparison of TIN removal rates and loading rates observed in the present study with those demonstrated

from two comparable studies in the literature.



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2) vs. t in Stages 2 through 5. Ammonium reduction followed first-order kinetics, with

removal rate constants of 0.434 day1(r= 0.849) for the overall system, 0.573 day1 (or

17.2 cm day1, r= 0.874) for the FWS wetland, and 0.17 day1 (or 5.1 cm day1,

r= 0.812) for the SSF wetland. TIN data were unsatisfactory to fit the design modelbecause of low correlation coefficients in linear regression; but, a high rate constant could

still be obtained by computation directly using Eq. (1), with an average value of 1.175

day1 for the overall system, 1.452 day1 (43.6 cm day1) for the FWS wetland, and

0.665 day1 (10.6 cm day1) for the SSF wetland.

3.3. Phosphate removal

The SSF wetland reduced phosphate levels at a rate equal to or even greater than the

FWS wetland in Stages 2 through 5 (P= 0.92). Average overall removal efficiencies for

phosphate decreased markedly from 71.2% to 31.9% as the hydraulic loading rate

increased from 2.3 to 13.5 cm day1 (Table 2). As a result, phosphate concentrations in

the effluent were exponentially correlated (r= 0.82) with the phosphate loading rates (Fig.

6). Average phosphate removal rates rose with the increase in phosphate loading, gradually

Fig. 6. Relationship between effluent phosphate concentrations (Co) and phosphate loading rate (LR) in the

constructed wetlands system.



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reaching a plateau of about 0.23 g P m2 day1 (Fig. 7). A maximum removal rate of 0.46

g P m2 day1 was recorded in Stage 5 as the loading rate was increased to 0.9 g P m2

day1. Phosphate removal data followed the first-order removal model, with rate constants

of 0.117 day1 (r= 0.924) for the overall system, 0.063 day1 (or 1.89 cm day1,r= 0.74)

for the FWS wetland, and 0.215 day1

(or 3.44 cm day1

,r= 0.945) for the SSF wetland.

4. Discussion

4.1. System start-up

The wetlands system required 2 to 3 months to reach a consistent nitrogen removal

performance level for the FWS wetland and 1 month for the SSF wetland. This is a much

shorter period than that reported in a study of the Tres Rios treatment wetland, Arizona,

USA, in which total nitrogen removal became efficient only after 1 year of operation(International Water Association, 2000). The SSF wetland appeared to achieve stable low

nitrogen levels in the effluent more quickly than in the FWS wetland, probably because the

Fig. 7. Comparison of PO4 P removal rates and loading rates observed in the present study with those

demonstrated from two comparable studies in the literature.



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presence of gravel in the SSF wetland provided more specific surface area for biofilm

growth.

The FWS wetland required about 3 months to achieve significant phosphate removal.

In contrast, the SSF wetland showed efficient phosphate removal as soon as the wetlandsbegan to receive flow, even when vegetation was sparse. This implies that gravel

adsorption determined initial efficient phosphate removal. However, the ability of

adsorption may decrease with time as sorption sites on the gravel become saturated.

The brief start-up period for nutrient removal in this study can be attributed to factors such

as uniform hydrological conditions in a pilot-scale system and high water temperature

during the initial phase (27.6 jC averaged at that time). During the start-up period, a net

increase in nitrogen levels in both the FWS and SSF wetlands and in phosphate levels in

the FWS wetland occurred because the nutrient removal processes had not yet completely

developed and could not balance the nutrient release resulting from decomposition of

existing plant litter, leaching by the antecedent soil, or incomplete nitrification deni-

trification.

One growing season (from March to October) was required to reach about 80%

vegetation coverage in our study. International Water Association (2000) documented that

a period from 3 to 4 months up to 2 years is required for complete plant cover in treatment

wetlands. In this study, nutrient removal became stable 4 to 5 months before full

vegetation coverage was reached, suggesting that stable performance for pollutant removal

may be achieved without complete vegetation being established.

4.2. System performance

Removal efficiency for inorganic nitrogen was rather high after Stage 1 (86% to 98%

for NH4N, 95% to 98% for TIN), which might have resulted from the low nitrogen

concentrations generally found in aquaculture wastewater. The FWS wetland removed

most of the ammonium, nitrite, and nitrate once its performance became stable. This result

suggests that sufficient nitrification and denitrification proceeded concurrently in the FWS

wetland. Soil in the FWS wetland can provide ideal environments for microbial processes

including nutrient mineralization, nitrification and denitrification due to a variety of

microbial population and wide range of oxidation states present in soil (International Water

Association, 2000). The SSF wetland did not remove nitrogen as efficiently as the designobjective (although it did display quick start-up behavior) because the SSF wetland

received consistently lower load of nitrogen, thereby kinetically limiting nitrogen removal.

Therefore, it is not suitable to compare the performance data between the FWS and SSF

wetlands in such series combination system in which the loading rate to each wetland were

considerably different.

The removal rate of TIN ( < 0.55 g N m2 day1) was relatively low as compared to

other study (Tanner et al., 1995; Fig. 5). Nitrogen removal rate is dependent on nitrogen

loading rate or hydraulic loading rate (Fig. 5). Although a high mean hydraulic loading

rate up to 13.5 cm day1 was examined in this study, the maximum loading rate of

inorganic nitrogen was only 0.59 g N m

2 day

1 due to the characteristic of low strengthaquaculture wastewater. Therefore, the low removal rates in this study were apparently

limited by the low nitrogen loading rates. Both nitrogen removal efficiency and rate in this



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study were higher than those demonstrated in a study of a constructed wetlands system

receiving channel catfish pond effluents (Schwartz and Boyd, 1995; Fig. 5), probably due

to the smaller wetland scale used and warmer climate in our work.

Phosphate removal was substantial and was about equal in the FWS and the SSFwetland, indicating that phosphate was not a kinetically limited substrate in either wetland.

Phosphorus removal rates in relation to loading were similar to those documented for two

systems for treating fish farm wastewater (Schwartz and Boyd, 1995) and dairy farm

wastewater (Tanner et al., 1995) (Fig. 7). We believe that vegetation uptake and deposition

in soil and gravel are the two main mechanisms for phosphate removal in the study.

Sustainable phosphorus removal processes involve accretion of new wetlands sediments

and soils (International Water Association, 2000).

By combining different types of constructed wetlands in series, improved pollutant

removal can be achieved through a greater variety of treatment mechanisms. Similar to this

study, Schwartz and Boyd (1995) observed most of the nitrogen oxides, but not

ammonium, being removed in the first wetland of a series FWS FWS system from

low-concentration catfish pond effluents. However, in another study, Kadlec et al. (1997)

found that most nitrification occurred in the third SSF wetlands and most denitrification in

the final FWS wetland, whereas reduction of suspended solids and COD and ammoni-

fication took place in the first two FWS wetlands when treating high-concentration

wastewater with a four-stage FWSFWSSSFFWS system. These previous findings

and this study suggest that wastewater strength can affect the site (or retention time) of

pollutant removal in a series-constructed wetlands system. High concentrations of COD

and organic nitrogen (as in potato-processing wastewater) require longer retention times toremove ammonium and nitrate, whereas wastewater with lower concentrations (as in

aquaculture wastewater) requires a shorter retention time.

Many researchers have applied the first-order reaction model for removal of ammo-

nium, nitrate, total nitrogen, and phosphate in FWS and SSF wetlands (Reed et al., 1995;

Kadlec and Knight, 1996). Ammonium removal rate constants for the FWS wetland (0.573

day1 or 17.2 cm day1) and the overall system (0.434 day1) in this study are both higher

than the 0.218 day1 reported by Reed et al. (1995) and the 4.932 cm day1by Kadlec and

Knight (1996), both for the FWS wetland. This result might be caused by the high

ammonium removal efficiency in our system, even under a high hydraulic loading rate.

Phosphate removal rate constants for the SSF wetland (0.215 day1

or 3.44 cm day1

) andthe overall system (0.117 day1) are both similar in magnitude to the 2.73 cm day1

reported by Reed et al. (1995) and the 0.14 day1 by Tanner et al. (1995), both for SSF

wetlands.

The present constructed wetlands system also performed well with respect to the

removal of chemical oxygen demand (25 55%), suspended solids (47 86%) and

chlorophyll a (76 95%) from the fishpond effluent for the duration of this study

(unpublished data). Nutrient mineralization cannot be evidenced directly from the

influenteffluent results because organic nitrogen and phosphorous were not measured

in this study. However, the consistent removal of pollutants (COD, SS, algae, inorganic

nitrogen and phosphate) implies that a considerable amount of particulate organic nitrogenand phosphorous in fishpond effluent (generated from uneaten feed residue, fish excreta



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and phytoplankton biomass) was retained in the constructed wetlands and subsequently

mineralized and removed from the fishpond effluent by various biotic and abiotic processes.

4.3. Area requirement of constructed wetland

The main disadvantage of constructed wetland is the requirement of large area. The

total area of a FWSSSF wetlands system (Atotal) required for treating a given flow rate of

fishpond effluent can be calculated by rearranging Eq. (1) and using the following

equations:

tlnCo=Ci

K 1

tAehFWS AehSSF

Q 2

AFWS ASSF 3

Atotal AFWS ASSF 4

WhereQ = flow rate of fishpond effluent, m3 day1; e = porosity of wetland, assuming

0.75 in FWS wetland and 0.4 in SSF wetland in this study; h = water depth of wetlands,being 0.4 m for both FWS and SSF wetland; AFWS = area required for FWS wetland, m2;

ASSF = area required for SSF wetland, m2.

Assuming fishponds have a daily regular effluent averaging 5% of the pond volume,

the depth of fishpond is 1.5 m, ammonium removal rate constant for the overall system is

0.434 day1, and the annual production of milkfish is normally 20 ton ha1 year1, then 1-

ha fishpond would require 1.2 ha of wetlands or 1 ton of annual production of milkfish

would require 0.06 ha of wetlands to remove 80% of ammonium from fishpond effluent.

Schwartz and Boyd (1995) also estimated a similar wetland area of 0.72.7 times pond

area for treating catfish farm effluent.

Constructed wetland also can be potentially used for treating the recycle water withoperating at higher hydraulic loading rate and consequently with lower removal efficiency

in a recirculating intensive aquaculture system (Zachritz and Jacquez, 1993; Panella et al.,

1999). However, additional works on higher hydraulic loading rates and their effects on

water quality and fish growth in the recirculating aquaculture system are needed.

5. Conclusion

By observing the performance transitions and growth of wetland plants, we demon-

strated that FWS and SSF constructed wetlands showed quick start-up behaviors fortreating aquaculture wastewater, with the SSF wetland achieving stable performance

more rapidly than the FWS wetland. The start-up period around 3 months for nutrient



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removal in this study seemed to be shorter than those that occurred in other systems cited

from the literature. The reason for this might be that a pilot-scale constructed wetland

system can develop stable removal processes more quickly than a large-scale constructed

system or a natural wetland. Because the aquaculture wastewater has low nitrogenconcentrations, removal of inorganic nitrogen was extremely efficient under various

hydraulic loading trials (2.3 to 13.5 cm day1), and the removal rate constants for

ammonium and nitrogen oxides were typically high. Phosphate concentrations in the

aquaculture wastewater were moderately high, and removal efficiencies for phosphate

decreased significantly as hydraulic loading rate increased. However, hydraulic loading

rates and operating time were confounded because we cannot evaluate the influence of

time vs. the changes in loading rate. The subtropical climate prevailing in Taiwan

provides for suitable conditions for the constructed wetlands ecosystem, which also

might help to explain the rapid start-up phenomena and efficient nutrient removal in this

wetlands system. Levels of ammonium and nitrite in wetlands discharge ( < 0.3 mg

NH4N l1 and 0.01 mg NO2N l

1) were obtained that rendered the water harmless to

fish, so that the water potentially could be reused and recycled in the aquaculture system.

Acknowledgements

We would like to express our appreciation to the National Science Council of the

Republic of China (Project Number: NSC-88-2621-Z-041-001) for funding support for

this project, and to Dr. James P. Kaetz, Associate Professor, Department of English,Auburn University and Editor ofNational Forum, for help in editing this paper.

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nutrient removal from aquaculture wastewater using a constructed wetlands system

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