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Marine Pollution Bulletin 57 (2008) 727–734

Effects of salinity on treatment of municipal wastewaterby constructed mangrove wetland microcosms

Yan Wu a, N.F.Y. Tam a,*, M.H. Wong b

a Department of Biology and Chemistry, City University of Hong Kong, Kowloon Tong, Hong Kong, Chinab Croucher Institute of Environmental Sciences, Hong Kong Baptist University, Kowloon Tong, Hong Kong, China

Abstract

The effects of salinity on the removal of dissolved organic carbon and nutrients from municipal wastewater by constructed mangrovemicrocosms planted with Aegiceras corniculatum were investigated. During the four-month wastewater treatment, the treatment effi-ciency was reduced by high salinity, and the removal percentages of dissolved organic carbon, ammonia-N and inorganic N droppedfrom 91% to 71%, from 98% to 83% and from 78% to 56%, respectively, with salinity increasing from 0 to 30 parts per thousands(ppt). In spite of such inhibition at high salinity, 100% of the effluents discharge from the constructed mangrove microcosms still com-plied with the discharge standards set by the Hong Kong Government for Coastal Water Control Zones. These results suggested thatconstructed mangrove wetland treatment systems were promising to effectively treat municipal wastewater, even those with high salinity.In addition, the denitrification potential in soil was found to be retarded by the high salinity while mangrove plants grew best at 15 pptsalinity condition.� 2008 Elsevier Ltd. All rights reserved.

Keywords: Aegiceras corniculatum; Mangroves; Salinity; Dissolved organic carbon (DOC); Nutrients

1. Introduction

Many tropical marine ecosystems are increasinglydegraded due to the input of wastewater-borne pollutantsincluding suspended solid, nutrients and toxic compounds.Excessive discharge of nitrogen and phosphorus intonearby aquatic ecosystems will cause algal blooms, whichthen use up the dissolved oxygen in water leading tohypoxia and decreases in diversity and productivity (Bell,1992; Hauxwell et al., 2001). In the past decades, variouswetland treatment systems have been built widely aroundthe world to treat municipal wastewater (Gumbricht,1992; Cameron et al., 2003; Kaseva, 2004; Mbuligwe,2004; Greenway, 2005). However, most of the commonlyuse wetland plants are freshwater species which have poorsalinity tolerance, for instance, Typha latifolia could die off

0025-326X/$ - see front matter � 2008 Elsevier Ltd. All rights reserved.

doi:10.1016/j.marpolbul.2008.02.026

* Corresponding author. Tel.: +852 27887793; fax: +852 27887406.E-mail address: bhntam@cityu.edu.hk (N.F.Y. Tam).

rapidly under chronic salt stress (Kargi and Dincer, 1996;Hootsmans, 1998; Klomjek, 2005). The growth of baldcy-press, water tupelo, Chinese tallow and green ash wasinhibited at 10 ppt salinity (Conner et al., 1997). The con-structed wetlands using freshwater species are thereforenot suitable for treating wastewater containing salts or sal-ine wastewater.

In recent years, saline wastewater has been generated inmany areas because seawater is used for toilet flushing tosave freshwater resources. For example, about three quar-ters of the population in Hong Kong are supplied with saltwater for toilet flushing, the used toilet flushing water ismixed with wastewater from other sources and the mixedwastewater is discharged into the sewage treatment plant.The salt concentration of the mixed wastewater is between5000 and 6000 mg L�1, about one-fifth of the salt concen-tration in seawater in Hong Kong SAR (personal commu-nication). There is a need to develop constructed wetlandsfor treating saline wastewater and the plants must be ableto resist salinity.

728 Y. Wu et al. / Marine Pollution Bulletin 57 (2008) 727–734

Mangrove plants colonized the inter-tidal habitat arewell known for their ability to withstand various environ-mental stresses, in particular, high salinity, waterlogging,alternating aerobic and anaerobic conditions and unstablesubstratum (Clough et al., 1983). One of the most strikingfeatures of mangroves is their ability to tolerate NaCl to alevel of 30 ppt or higher (Takemura et al., 2000). Ye et al.(2001) found that under saline conditions, the removal ofammonium-N by Kandelia candel and Bruguiera gymnorrh-

iza were 93% and 98%, respectively, and the respectiveinorganic P removal percentages were 88% and 98%. Theseresearch works imply that constructed wetlands using man-grove plants may be applicable for treating saline munici-pal wastewater.

Aegiceras corniculatum, belonging to the Myrsinaceaefamily, is a salt-secreting species, has salt glands in itsleaves and its root initiation was not significantly retardedby salinity (Ye et al., 2005). The protein synthesis and pro-teolytic activity of A. corniculatum were not inhibited evenwhen it was planted in a hydroponic culture at 18 ppt salin-ity condition (Parida et al., 2004). These studies demon-strated that A. corniculatum was able to tolerate salinity.

The present study aims to explore the effect of salinityon the performance of constructed wetland microcosmsplanted with A. corniculatum, a true mangrove specieswidely distributed in the outer (seaward) fringe in HongKong mangrove swamps, in treating artificial municipalwastewater.

2. Materials and methods

2.1. Experimental set-up

The constructed mangrove wetland microcosms con-sisted of nine PVC tanks, each had a dimension of 0.67 m(L) � 0.54 m (W) � 0.38 m (D). Each tank was divided into

Inlet Zone

Wastewater

Gravel

Treatment Zone

Fig. 1. Schematic diagram of the constructed mangrove s

three zones; inlet (0.12 m � 0.54 m � 0.15 m), treatment(0.45 m � 0.54 m � 0.15 m) and outlet (0.10 m � 0.54 m �0.15 m), and were filled with different media (Fig. 1). In inletand outlet zones, the media was comprised of gravel(around 2.27–2.91 cm in diameter), while the treatmentzone was filled with soil collected from Sai Keng, a typicalmangrove swamp in Sai Kung, Hong Kong SAR. Six seed-lings of one-year old A. corniculatum were transplanted intothe treatment zone of each microcosm. The seedlings weregerminated from the hypocotyls collected from the sameswamp and grown in the greenhouse one year before thestart of the experiment. Mangrove microcosms were irri-gated with tap water every day for one month to ensure thatmangrove plants were acclimated in the set-ups prior towastewater discharge. Tap water was used for irrigationduring the acclimation period, mainly because our previousexperience showed that growth of mangrove plants in tapwater was comparable to that in deionized water with lowsalinity (0–5 ppt) and tap water is more economic and sim-pler in operation.

At the start of the experiment, artificial wastewater atdifferent salinities (0, 15 and 30 ppt) was prepared by spik-ing the wastewater with different amounts of instant oceansea salt that is free of nitrate and phosphorus. The concen-trations of dissolved organic matter and nutrients inartificial wastewater were the same as those in primary set-tled municipal sewage collected from a local sewage treat-ment plant, that is, 60 mg L�1 total organic carbon(TOC), 25 mg L�1 ammonia nitrogen (NH3-N), 0.46 mgL�1 nitrate nitrogen (NO�3 -N), 45 mg L�1 total Kjeldahlnitrogen (TKN), and 5.00 mg L�1 inorganic phosphate(PO3�

4 -P) by adding appropriate amounts of glucose,ammonium chloride, sodium nitrate, urea and potassiumdihydrogen phosphate, respectively. The pH of the artifi-cial wastewater was 7.4, same as the municipal sewage inHong Kong. The artificial wastewater with different salini-

Effluent

Sandy Soil

PVC Box

Plastic Septa

Outlet Zone

ystem showing the inlet, treatment and outlet zones.

Y. Wu et al. / Marine Pollution Bulletin 57 (2008) 727–734 729

ties was discharged to each microcosm everyday at ahydraulic loading of 1.3 L day�1, equivalent to 5-dayhydraulic retention time. This experiment started in March2006 and ended in July 2006.

2.2. Collection and analyses of water samples

About 100 mL of effluent samples were collected fromthe outlet zone of each tank at 10-day intervals. The col-lected effluent samples were filtered through WhatmanNo. 42 filter paper and stored at 4 �C in the dark. The anal-yses of NH3-N and PO3�

4 -P were completed within 24 h,and DOC and NO�3 -N were done within 28 days accordingto standard storage methods described by Eation et al.(1995). The concentrations of NH3-N, NO�3 -N andPO3�

4 -P in water samples were determined by Flow Injec-tion Analysis (FIA, Lachat QuikChem� 8000, LachatInstruments, USA). The DOC concentration was measuredusing the Autosampler ASI-5000A Total Organic CarbonAnalyzer TOC-5000A (Shimadzu, USA). Total Kjeldahlnitrogen (TKN) concentration was analysed once a monthby Flow Injection Analysis (FIA, Lachat QuikChem�

8000, Lachat Instruments, USA) after acid digestion(McGill and Figueiredo, 1993).

2.3. Analyses of soil samples

At the end of the four-month treatment, four mangroveplants in each tank were pulled out and shaken gently toremove the bulk soil. The soil still adhering to the rootsof the mangrove plants, around 1 mm thick, was definedas the rhizosphere soil. The rhizosphere soil of the four har-vested plants was scrubbed down using a small plasticspoon, collected and pooled for further analyses. Soil sam-ples were subdivided into two portions. For one portion,the soil sample was air dried and stored in plastic bottlesfor further analyses. The remaining portion was kept at4 �C in the dark and analyses were done within 1 month.A sample weighing about 5 g of sieved air-dried (through2 mm mesh) soil was extracted with 50 mL KCl (2M) foran hour. The concentrations of NH3- and NO�3 -N in the fil-tered extracted soil solution were analysed by FIA (LachatQuikChem� 8000, Lachat Instruments, USA). A sampleweighing about 2 g of sieved air-dried (through 2 mmmesh) soil was extracted with 40 mL NaHCO3 (0.5 M)for half an hour. The concentration of inorganic phospho-rus in the filtered extracted soil solution was analysed byFIA. The activity of dehydrogenase was analysed accord-ing to Alef (1995a). One gram of the soil sample storedat 4 �C was mixed with 1.5 mL of Tris buffer and 2 mLof 2-p-iodophenyl-3-(p-nitrophenyl)-5-phenyl tetrazoliumchloride (INT) solution in a test tube. After incubated at40 �C in the dark for two hours and extracted by 10 mLmixture of 100 mL N,N-dimethylformamide in 100 mL eth-anol, filtered samples were measured with a UV–VIS spec-trophotometer (UV-1206, SHIMADZU, Japan) at 464 nmagainst blank (extraction solution). For the control, soil

was autoclaved (13.5 psi/121 �C/17 min) first, and thentreated exactly the same as the samples. Determination ofdenitrification potential was done according to Knowles(1982) and Alef (1995b). Ten grams of the soil samplestored at 4 �C was collected and placed into a 100 mL con-ical flask. Filtered-sterilized medium (300 mL) consisted of1.433 g potassium nitrate, 40 mg glucose, 100 mg chlora-phenicol and 1 g peptone per litre was poured into theflask. The soil slurry was incubated in the presence of acet-ylene to assess N2O production by gas chromatography(GC Hewlett 5890 Packard Series II) with electron capturedetector.

2.4. Measurement of mangrove plant growth

At the beginning of the experiment, three replicates ofseedlings, each with four plants pooled together as a com-posite sample, having similar sizes as those planted in thetanks were collected. At the end of the experiment, fourmangrove plants in each tank were randomly harvestedand pooled together as a composite sample. The oven-driedbiomass of root, stem and leaf (70 �C for 48 h) was deter-mined. The stem height and leaf number of each plant atthe beginning and the end of every month during the exper-iment were measured. For analyses of total N and P con-centrations in mangrove plants, 0.2 g of air-dried, groundand homogenized sample of each plant part was added intoa 50 mL digestion tube, digested with concentrated sulfuricacid, and the concentrations of total N and P were deter-mined by FIA.

2.5. Statistical analyses

Mean and standard deviation values of three replicatesfor each treatment were calculated. The one-way analysisof variance (ANOVA) with the Tukey test was used tocompare the removal percentages of nutrients or dissolvedorganic matter at the three different salinities at the level ofp 6 0.05. The effects of salinities on growth of A. cornicul-atum, the change of concentrations of nutrients and totalorganic matter, denitrification potential and dehydroge-nase activity in soil were also examined by the same statis-tical test. The difference in soil nutrient concentrationsbefore and after the experiment under each salinity treat-ment was evaluated by the independent student t-test. Allstatistical analyses were performed by a computing pack-age called Statistical Package for Social Science (SPSS11.0 for Windows, SPSS Inc., Illinois, USA).

3. Results

3.1. Treatment efficiencies of wastewater at different

salinities

The average removal percentages of DOC during thefour-month wastewater treatment decreased significantlywith increasing salinities, from 91% at 0 ppt to 71% at 30

Table 1Effects of salinity on the removal efficiencies of artificial municipalwastewater by constructed mangrove wetland systems

Parameters 0 ppt 15 ppt 30 ppt

DOC 90.8a 76.4b 71.2c

Ammonia-N 98.5a 88.2b 83.6b

Inorganic N 78.2a 55.5b 55.8b

TKN 97.1a 96.6ab 96.2b

Ortho-phosphorus 98.7a 99.1a 98.8a

The mean removal percentages of 36 samples during four-month treat-ment period are shown; means followed by different letters at the super-script position within the same row indicated significant difference at thelevel of p 6 0.05 according to one-way ANOVA.

730 Y. Wu et al. / Marine Pollution Bulletin 57 (2008) 727–734

ppt (Table 1). Similar trend was also found in the removalof NH3-N and inorganic N, and the removal at 0 ppt salin-ity was significantly higher than that at the other two salin-ities (p < 0.05). The effluent NH3-N concentrations were allbelow 1 mg L�1in systems treating wastewater at 0 pptwhile the values at the other two salinities ranged from0.36 to 10.24 mg L�1 (Table 2). The concentration ofNO�3 -N in the effluent in systems at 0 ppt was also signifi-cantly lower than that at 15 ppt (p < 0.001) or 30 ppt(p = 0.016). The removal percentages of TKN in systemstreating wastewater at 15 ppt salinity were comparable tothat at 0 (p = 0.061) and 30 (p = 0.305) ppt salinities, buta significant difference was detected between systems at 0and 30 ppt salinities (p < 0.001) (Table 1). Despite theinhibitory effect of salinity on wastewater treatment, efflu-ent leaving all treatment systems could meet the total Ndischarge standard for Coastal Water Control Zones set

Table 2Effects of salinity on effluent quality and their percentages of compliance witGovernment (EPD, 1990)

Parameter Salinity(ppt)

Mean effluent concentration andrange (in bracket) in mg L�1

Percen

Inland

Group

NH3 0 0.39 (0.01–0.95) 10015 2.96 (0.81–9.58) 8.330 4.09 (0.36–10.24) 5.6

NO�3 0 5.02 (1.80–14.18) 10015 8.26 (3.05–17.20) 97.230 7.50 (1.15–17.22) 91.7

PO3�4 0 0.06 (0–0.30) 100

15 0.05 (0.01–0.24) 10030 0.06 (0.01–0.14) 100

Total N 0 6.37 (3.42–14.05) NA15 9.13 (3.78–15.29) NA30 10.65 (6.53–19.36) NA

Total P 0 0.06 (0–0.30) NA15 0.05 (0.01–0.24) NA30 0.06 (0.01–0.14) NA

NA: not applicable as no discharge standard is set by the government.a Group A: inland waters include all waters in water gathering grounds and w

standard for NH3, NO�3 and PO3�4 are 1, 15 and 1 mg L�1, respectively; Grou

Territories and standard for NH3, NO�3 and total P are 5, 30 and 10 mg L�1,Control Zones; Zone 2: Other Water Control Zones in Hong Kong with more oof total N and total P and the respective values are 20 and 8 mg L�1 for Zon

by HKEPD (Table 2). For ammonia-N, effluent from treat-ments of wastewater at 0 ppt completely fulfilled the dis-charge standards for water gathering ground (Group A)and the agricultural areas in the New Territories in HongKong SAR (Group B). If saline wastewater was treated,the compliance percentages were reduced to 88% and72% for wastewater at 15 and 30 ppt, respectively. On thecontrary, the removal of inorganic P was not affected bythe salinity of the wastewater and more than 98% of inor-ganic P was removed at all salinities (Table 1). The effluentconcentrations were below 0.06 mg L�1 and completelycompiled with the discharge standards set by the HongKong Government (Table 2).

3.2. Plant growth and nutrient uptake

At the beginning of the experiment, A. corniculatum hadaround seven leaves, an initial stem height of 21 cm, andthe initial dried biomass of root, stem and leaf was 1.07,1.54 and 0.65 g per plant, respectively (Table 3). Most ofthe dried biomass of A. corniculatum was kept in its rootsand stems. After the four-month wastewater treatment,plants in the system at 15 ppt salinity grew best and pro-duced more biomass than the other two salinities (Table3). The stem height of A. corniculatum at 15 ppt salinityincreased from an initial 21.1–39.0 cm at the end (Fig. 2).The leaf number also showed the largest increases at15 ppt salinity while 30 ppt had the least number (Fig. 2).

The concentration of N in the leaves and stems of A.corniculatum increased significantly during the wastewatertreatment, especially at 15 ppt salinity, while the increase

h discharge standards at different water bodies in Hong Kong set by the

tages (%) of effluents complied with the discharge standarda

water areas Coastal water controls

A Group B Zone 1 Zone 2

100 NA NA88 NA NA72 NA NA100 NA NA100 NA NA100 NA NANA NA NANA NA NANA NA NANA 100 100NA 100 100NA 100 100100 100 100100 100 100100 100 100

ithin the boundaries of country parks and is the most stringent dischargep B: inland waters are mainly those draining agricultural areas in the Newrespectively; Coastal Water Control Zone 1: Tolo and Port Shelter Waterpen water. In these control zones, discharge standards are set only in termse 1, 100 and 10 mg L�1 for Zone 2.

Table 3Biomass and nutrient status of A. corniculatum before and after treating wastewater at different salinities

Properties Initial After

0 ppt 15 ppt 30 ppt

Biomass (g dry weight per plant)

Leaf 0.65 ± 0.22a 2.47 ± 0.48bc 3.73 ± 0.20c 1.96 ± 1.00ab

Stem 1.54 ± 0.36a 2.12 ± 0.43a 3.44 ± 0.60b 1.97 ± 0.30a

Root 1.07 ± 0.16a 1.81 ± 0.43bc 2.43 ± 0.69c 1.40 ± 0.26ab

Total 3.26 ± 0.71a 6.40 ± 0.52b 9.60 ± 0.44c 5.33 ± 0.70ab

Nitrogen concentration (mg N g�1dry weight)

Leaf 8.25 ± 1.81a 15.31 ± 3.33b 18.66 ± 2.16b 17.65 ± 2.24b

Stem 3.98 ± 1.89a 10.63 ± 0.96b 9.91 ± 0.37b 9.76 ± 2.25b

Root 4.60 ± 1.01 5.95 ± 0.93 5.98 ± 0.45 8.03 ± 5.12

Nitrogen uptake (mg N per plant)

Leaf 5.47 ± 2.38a 37.39 ± 4.85b 69.94 ± 12.06c 35.73 ± 22.05b

Stem 6.09 ± 2.83a 22.44 ± 1.58b 33.95 ± 5.05c 22.86 ± 8.04bc

Root 4.68 ± 1.31a 10.71 ± 2.19b 14.50 ± 3.93b 10.93 ± 4.63b

Total 16.24 ± 4.31a 70.54 ± 7.52b 118.4 ± 19.29c 69.54 ± 17.93b

Phosphorus concentration (mg P g�1dry weight)

Leaf 0.98 ± 0.25 1.03 ± 0.23 1.33 ± 0.15 1.28 ± 0.28Stem 0.81 ± 0.19 0.90 ± 0.24 1.67 ± 0.38 1.16 ± 0.27Root 1.06 ± 0.25 0.62 ± 0.10 0.93 ± 0.41 0.81 ± 0.32

Phosphorus uptake (mg P per plant)

Leaf 0.61 ± 0.25a 2.51 ± 0.35b 4.98 ± 0.69c 2.59 ± 1.58b

Stem 1.47 ± 0.25a 1.91 ± 0.33a 5.74 ± 1.14b 2.28 ± 0.09a

Root 1.15 ± 0.24 1.23 ± 0.31 1.93 ± 0.99 1.12 ± 0.34Total 3.24 ± 0.96a 5.65 ± 0.78a 12.64 ± 2.36b 6.60 ± 1.78a

Mean and standard deviation values of six replicates are shown; Means followed by different letters at the superscript position within the same rowindicated significant difference at the level of p 6 0.05 according to one-way ANOVA.

Y. Wu et al. / Marine Pollution Bulletin 57 (2008) 727–734 731

in root was insignificant (Table 3). Mangrove plants grownin systems treating wastewater at 15 ppt salinity took upmore N (more than 100 mg N per plant) while the totaluptake was around 70 mg N at 0 and 30 ppt salinities. Atthe end of the experiment, most of the N was kept in theleaves and the least was in roots, very different fromthe equal distribution among three plant components atthe beginning. As for total P, the concentrations in roots,stems and leaves did not show any significant differencebefore and after the wastewater treatment (Table 3). Therewas no significant accumulation of P in the root tissues, buta higher accumulation occurred in leaves and stems in allsystems, probably due to more plant growth. The totalamount of P accumulated in plants receiving wastewaterat 15 ppt salinity was also higher than those at 0 and30 ppt salinities (Table 3).

3.3. Nutrients and microbial activities of mangrove soil aftertreatment of wastewater at different salinities

At the end of the four-month treatment, the concentra-tion of ammonium in mangrove soil receiving wastewaterwas comparable to that at the beginning for all salinities(Fig. 3 and Table 4). With respect to soil nitrate, the con-centration significantly declined at the end of experimentfor treatments at 15 ppt salinity while no significantchanges at 0 and 30 ppt salinities (Fig. 3 and Table 4). Sim-ilarly, the average concentrations of inorganic P in soil

were comparable before and after the experiment excepta significant decrease at 0 ppt salinity, from an initial7.71 to 4.68 lg g�1 (Fig. 4 and Table 4). These results sug-gested that even though wastewater was rich in NH3-N andPO3�

4 -P, these pollutants were not accumulated in man-grove soil and some might even leave the soil componentat certain salinity.

At the end of the experiment, the denitrification poten-tial in mangrove soil receiving wastewater at 0 ppt salinitywas significantly higher than the initial value while thepotential at 15 and 30 ppt salinities decreased with waste-water treatment (Table 5). These results indicated thatnitrate produced from oxidation of ammonium in waste-water at 0 ppt salinity would be lost from the system viadenitrification but saline wastewater would inhibit suchreaction. Different from denitrification potential, signifi-cant increases in dehydrogenase activity were found in allsystems and more increases were found at higher salinities(Table 5). The average values of dehydrogenase activityvaried from 10.5 to 37.7 g g�1 dwt 2 h�1, which was at least100-fold higher than that at the beginning.

4. Discussion

The negative effects of salinity on wastewater treatmentefficiency of the constructed wetland system have beenreported. Brown et al. (1999) found that the removal oftotal and inorganic N by the plant–soil system was

Days of measurement (days)

0 20 40 60 80 100 120 140

Days of measurement (days)

0 20 40 60 80 100 120 140

Ste

m h

eigh

t (cm

)

15

20

25

30

35

40

45

0 ppt 15 ppt 30 ppt

a

bb

Leaf

num

ber

(per

pla

nt)

0

5

10

15

20

25

30

35

0 ppt15 ppt30 ppt

a

ab

b

Fig. 2. Changes in stem height and leaf number of A. corniculatum duringthe four-month treatment of wastewater at different salinities (mean andstandard deviations of 18 replicates are shown; different letters indicatedsignificant difference at the level of p 6 0.05 according to one-wayANOVA).

MonthBegin (March) End (July)

Am

mon

ium

con

cent

atio

n (μ

g g-1

)

0

2

4

6

8

10

12

14

16 0 ppt 15 ppt 30 ppt

0 ppt 15 ppt 30 ppt

MonthBegin (March) End (July)

Nitr

ate

conc

enta

ion

(μg

g-1)

0

10

20

30

40

Fig. 3. Concentrations of ammonium-N and nitrate-N in mangrove soilbefore and after the four-month treatment of wastewater at differentsalinities (mean and standard deviations of triplicates are shown).

Table 4p-values of the independent t-tests showing the differences in theconcentrations of ammonium (NHþ4 -N), nitrate (NO3-N) and inorganicP in soil in mangrove microcosms before and after 4-month wastewatertreatment at each salinity treatment

Salinity (ppt) Ammonium Nitrate Inorganic P

0 0.52 0.06 0.0315 0.42 <0.001 0.1530 0.07 0.05 0.32

732 Y. Wu et al. / Marine Pollution Bulletin 57 (2008) 727–734

significantly inhibited when salinity increased from 0 to35 ppt. The present study also showed that the removalof dissolved organic carbon decreased as salinity of thewastewater increased, and the removal percentagesdeclined in the order of 0 ppt > 15 ppt > 30 ppt. The Nremoval was less sensitive to the salinity gradient thanthe DOC removal. The salinity effect on N removal wasnot significant between 0 and 15 ppt but there was signifi-cant difference between systems at 0 and 30 ppt salinities.Despite the inhibitory effect of salinity on wastewater treat-ment, effluent leaving all treatment systems could meet thetotal N and P discharge standard for Coastal Water Con-trol Zones set by HKEPD (Table 2). The current researchdemonstrated that the mangrove plants grew well evenwithout tidal flushing, implying that the constructed man-grove wetland treatment system could be employed ininland rural areas such as the New Territories in HongKong if the effluent could meet the discharge standard setfor inland water. The present results revealed that the efflu-

ent from the treatments irrigated with fresh wastewater(0 ppt) could completely fulfill the discharge standardsfor ammonia-N for water gathering ground (Group A)and the agricultural areas in the New Territories in HongKong SAR (Group B).

The negative effect of salinity on treatment efficienciesmay be due to its inhibition on microbial activities in soil,as reflected by decreases in soil denitrification potential athigh salinity (Table 5). Denitrification potential that con-verts nitrate to nitrogen gas under anaerobic conditions

Month

Begin (March) End (July)

Inor

gani

c P

con

cent

ratio

n (μ

g g-1

)

0

2

4

6

8

10

12 0 ppt 15 ppt 30 ppt

Fig. 4. Concentrations of inorganic phosphorus in mangrove soil beforeand after the four-month treatment of wastewater at different salinities(mean and standard deviation values of triplicates are shown).

Table 5Effects of salinity on denitrification potential and dehydrogenase activityin mangrove soil at the end of the four-month wastewater treatment

Salinity Denitrification potential Dehydrogenase activity

Values (lg N2O-Ng�1 dwt h�1)

Foldchanges

Values(g g�1 dwt 2 h�1)

Foldchanges

0 ppt 0.57 ± 0.47 1.51a 10.5 ± 5.5 115a

15 ppt 0.13 ± 0.05 �0.78b 12.1 ± 2.2 1250b

30 ppt 0.10 ± 0.04 �0.98c 37.7 ± 17.8 1506c

Mean and standard deviation values of triplicates and mean percentageschanges are shown (fold changes = [initial value � final value]/initialvalue, negative value means a decrease at the end of the experiment); foldchanges followed by different letters at the superscript position within thesame column indicated significant difference at the level of p 6 0.05according to one-way ANOVA.

Y. Wu et al. / Marine Pollution Bulletin 57 (2008) 727–734 733

was inhibited at 15 and 30 ppt salinities, on the other hand,discharge of wastewater at 0 ppt stimulated this potentialleading to lower effluent nitrate level. However, Magalhaeset al. (2005) reported that salinity did not retard the deni-trification rate in inter-tidal sandy sediment, probablybecause the denitrifier communities were dominated byhalotolerant bacteria. Antheunisse et al. (2007) also foundthat the denitrification rate in semi-natural and agriculturalgrassland soils was not affected by salinity. It is possiblethat the sensitivity of denitrifying bacteria to salinity mayvary from system to system. The low N removal efficiencyin the treatments receiving wastewater at 30 ppt with morenitrogen retained in mangrove soil and more ammonia ineffluent in this study might also be explained by the adverseeffect of salinity on microbial activities involved in nitrogencycling. It has been reported that ammonification (McC-lung and Frankernberger, 1987), nitrification (Azam andIfzal, 2006; Rasul et al., 2006), and degradation of organicnitrogen (Pathak and Rao, 1998; Wichern et al., 2006) were

retarded by salinity. The effects of salinity on microbial activ-ities in the constructed wetland wastewater treatment systemdeserved more in-depth research.

Although some previous studies found that salinity hada significant inhibitory effect on growth and biomass pro-duction of wetland plants, and their assimilation of nitro-gen and phosphorus (Conner et al., 1997; Brown et al.,1999), this phenomenon was not that obvious in the pres-ent study. Actually, growth as well as total nitrogen andphosphorus uptake increased with salinities from 0 to15 ppt, and 15 ppt was the optimum salinity for A. corni-

culatum. Parida and Das (2003) found that nitrate reduc-tase in leaves of a mangrove species, Bruguiera parviflora,increased in 6 ppt saline hydroponic culture but itdecreased gradually when salinities changed from 12 to24 ppt, and the decrease in nitrate reductase was followedby a decrease in the total amount of nitrogen accumulatedin the leaves. Their study also pointed out that a decreasein the total amount of nitrogen in mangrove plants maybe attributed to lower biomass production in high salinity.This is in accordance with the present findings that A. corn-

iculatum grown in saline wastewater (15 ppt) producedmore biomass and accumulated more N and P than thosegrown in wastewater at the other two salinities (0 and30 ppt). The lower amount of biomass produced at highsalinity may be attributed to the depression of carbonassimilation and photosynthetic capacity as reported byprevious researchers (Ball and Farquhar, 1984; Naidooand Wilert, 2004; Parida et al., 2004).

5. Conclusions

The present study demonstrated that salinity of thewastewater had an adverse effect on treatment efficienciesof DOC and N as these removal processes were mainlydue to microbial activities. As for DOC, removal efficien-cies declined from 91% to 71% when the salinity of waste-water increased from 0 to 30 ppt, and the respectivedecreases in ammonia-N removal were from 99 to 84%.In spite of the salinity effect, 100% of the effluents dis-charged from the constructed mangrove wetland systemscomplied with the discharge standards set by the HongKong Government for Coastal Water Control Zones.Salinity had less inhibition on the growth of mangroveplants and there was no significant difference in growthand nutrient uptake between 0 and 30 ppt. The optimalsalinity for growth and nutrient uptake by A. corniculatum

was 15 ppt. These results showed that constructed man-grove wetland treatment systems, without tidal flushing,could be used to treat municipal wastewater with salinityas high as 30 ppt.

Acknowledgement

The work described in this paper was supported by theAreas of Excellence Scheme established under the Univer-

734 Y. Wu et al. / Marine Pollution Bulletin 57 (2008) 727–734

sity Grants Committee of the Hong Kong SAR, China(Project No. AoE/P-04/2004).

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