Constructed mangrove wetland as secondary treatment system for municipal wastewater

Constructed mangrove wetland as secondary treatment system for municipal wastewater

e c o l o g i c a l e n g i n e e r i n g 3 4 ( 2 0 0 8 ) 137–146 available at www.sciencedirect.com journal homepage: www.elsevier.com/locate/ecole...

1007KB Sizes 1 Downloads 205 Views

e c o l o g i c a l e n g i n e e r i n g 3 4 ( 2 0 0 8 ) 137–146

available at www.sciencedirect.com

journal homepage: www.elsevier.com/locate/ecoleng

Constructed mangrove wetland as secondary treatment system for municipal wastewater Y. Wu a , A. Chung b , N.F.Y. Tam a,∗ , N. Pi a , M.H. Wong b a b

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

a r t i c l e

i n f o

a b s t r a c t

Article history:

Intermittent subsurface flow mangrove microcosms were constructed to investigate their

Received 15 November 2007

capabilities in treating primary settled municipal wastewater collected from a local sewage

Received in revised form 9 July 2008

treatment work in Hong Kong SAR and the effect of hydraulic retention time (HRT). The

Accepted 11 July 2008

study was carried out in a greenhouse and without any tidal flushing or tidal cycle, with half of the tanks planted with Kandelia candel and half without any plants. The removal percentages of dissolved organic carbon (DOC), ammonia-N, inorganic-N and total Kjel-

Keywords:

dahl nitrogen in the planted systems were 70.43–76.38%, 76.16–91.83%, 47.89–63.37% and

Mangrove microcosm

75.15–79.06%, respectively, significantly higher than in the unplanted system during the

Municipal wastewater

6-month treatment period. More than 97% ortho-phosphate and 86.65–91.83% total phos-

Hydraulic retention time

phorus were removed by the planted microcosms. The HRT of 10 days had better removal

Nutrients

than that of 5 days, and the best performance was obtained in the planted microcosms

Dissolved organic carbon (DOC)

with 10 days of retention time. During the 6-month experimental period, the concentrations of all forms of nitrogen in the treated effluent were within the standards for effluents discharged into Group B inland waters and coastal zone with open waters. In terms of phosphorus, the effluents met the standards for effluents discharged into Group A inland waters. These results suggest that it is feasible to use the constructed mangrove wetland without tidal flushing as a secondary treatment process for municipal wastewater. © 2008 Elsevier B.V. All rights reserved.

1.

Introduction

Wetland treatment systems have been considered as an alternative to conventional treatment methods, especially for small communities (Soukup et al., 1994; Kivaisi, 2001; Solano et al., 2003), because of their low treatment cost and low maintenance, especially in suburban or rural areas without any large-scale sewage treatment facilities. In the past few decades, constructed wetlands have been applied in treating municipal (Gumbricht, 1992; Kaseva, 2004; Chung et al., 2008), industrial (Mays, 2001; Jacob, 2004; Gottschall et



Corresponding author. Tel.: +852 27887793; fax: +852 27887406. E-mail address: [email protected] (N.F.Y. Tam). 0925-8574/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.ecoleng.2008.07.010

al., 2007), agricultural and livestock wastewater (Comín et al., 1997; Knight et al., 2000). The feasibility of employing constructed wetlands as a secondary treatment system is attractive because it saves land areas; however, the subject still needs more research. Most of the existing constructed wetlands use perennial plants such as common reeds, cattails and bulrush, which are strongly recommended to be harvested periodically (Hammer, 1989; Cronk, 1995; Gopal, 1999). Frequent harvesting would lead to poor plant growth and increase the cost of manpower (Cronk, 1995; Gopal, 1999). Hammer (1989) therefore suggested that other species

138

e c o l o g i c a l e n g i n e e r i n g 3 4 ( 2 0 0 8 ) 137–146

of perennial plants should be used to avoid annual plantings. Mangrove swamps commonly found along tropical and subtropical coastlines, usually between 35◦ N and 35◦ S latitude, are some of the most productive wetlands in the world (Tam and Wong, 2000; Al-Sayed et al., 2005). In previous decades, mangroves in many countries have been seriously damaged by human activities, and ecological restoration of mangroves through mangrove planting has been commonly used to reverse such losses in the last decade worldwide (Lewis, 2005). Because of their high productivity, mangrove plants have a large demand for nutrients and many mangrove habitats struggle with a nutrient deficiency problem (Li, 1997; Alongi et al., 2005). The beneficial effects of nutrient addition on growth of mangroves have been reported (Clough et al., 1983). Mangrove plants that have colonized the inter-tidal habitat are also well known for their ability to withstand various environmental stresses, including high salinity, waterlogging, alternating aerobic and anaerobic conditions and unstable substratum (Clough et al., 1983). Chiu and Chou (1993) reported that Kandelia candel could transfer oxygen from its aerial parts to its roots, and a portion of oxygen would be leaked from the roots into the adjacent soil, producing a thin aerobic zone surrounding around the plant roots. The very extensive root systems of mangrove plants thus create a significant aerobic zone in the rhizosphere for oxidation (Chen et al., 1995). In addition, the mangrove soil and roots were found to harbor diverse groups of microorganisms which played essential roles in nutrient transformation (Al-Sayed et al., 2005). These unique functions of mangrove ecosystems create a suitable environment for removing and transforming pollutants in wastewater. Some previous studies have demonstrated that mangrove wetland systems made a significant contribution to the removal of nutrients and organic matter from wastewater (Sansanayuth et al., 1996; Wong et al., 1997; Chu et al., 1998; Tam et al., in press). However, there are problems with using mangrove wetlands for nutrient removal. One of them is the need of frequent tidal flushing, which makes the use of mangroves less practical. It will be more cost-effective if the mangrove wetland treatment system can be operated without regular tidal cycles. Our recent research has demonstrated that mangrove plants can grow well in fresh water without tidal flushing (Tam, 2004).

Fig. 1 – Schematic diagram of constructed subsurface-flow mangrove microcosms showing the inlet, treatment and outlet zones.

Table 1 – Background properties of Sai Keng soils Properties

Mangrove soil

Porosity pH Salinity (‰) Conductivity (mS cm−1 ) Total organic matter (%) NH3 –N (␮g g−1 ) NO3 − –N (␮g g−1 ) Ortho-P (␮g g−1 ) Total N (%) Total P (%)

0.34 8.62 2.1 0.84 0.21 10.65 7.87 8.29 0.027 0.014

± ± ± ± ± ± ± ± ± ±

0.04 0.18 0.9 0.50 0.06 4.33 4.65 1.44 0.008 0.003

Number of replicates 3 24 24 24 24 72 72 72 72 72

The present study aims to determine the performance and efficiency of constructed subsurface-flow mangrove wetlands, without tidal flushing, as a secondary treatment system. The role of mangrove plants K. candel in treating primary settled municipal wastewater under two hydraulic retention times (5- and 10-day HRTs) will also be investigated. Among eight true mangrove plant species found in Hong Kong, K. candel is the most widely distributed and common species. It can be found in all mangrove swamps, while for the two genera in the same family as Kandelia, Bruguiera is present in 28 out of 44 mangrove swamps and Rhizophora is absent in Hong Kong (Tam and Wong, 2000).

2.

Materials and methods

2.1.

Experimental set-up

The mangrove wetland treatment microcosms consisted of 12 PVC tanks. Each microcosm had the dimensions of 0.67 m (L) × 0.54 m (W) × 0.38 m (D). Individual microcosms were divided into 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 filled with different media (Fig. 1). In inlet and outlet zones, the media consisted of gravel (around 2.27–2.91 cm in diameter), while the treatment zone was filled with soil collected from Sai Keng, a typical mangrove swamp in Hong Kong SAR. The background properties of the soil are summarized in Table 1. Six seedlings of 1-year-old K. candel, germinated and grown from propagules in the greenhouse, were transplanted to all of the treatment zones, except for the control tanks, which did not have any plants. Both planted and unplanted microcosms were irrigated with tap water every day for 1 month to ensure that mangrove plants were acclimatized in the set-ups prior to wastewater addition. After acclimatization, primary settled municipal wastewater collected from Shek Wu Hui Sewage Treatment Work in Hong Kong SAR was discharged to the surface of the inlet zone every day in the amounts of 1.3 and 2.6 L for microcosms under 5- and 10-day HRTs, respectively. Effluent samples were collected from the outlet of each microcosm every 10 days for 6 months. Each treatment procedure was performed in triplicate, and the tanks were randomly arranged on two benches in greenhouse.

139

e c o l o g i c a l e n g i n e e r i n g 3 4 ( 2 0 0 8 ) 137–146

Table 2 – Effects of plant and hydraulic retention time on removal efficiency (in % except nitrate that is in average concentration, mg L−1 ) during 6-month treatment of primary settled municipal sewage Parameters

5-day Planted

DOC Ammonia-N Nitrate-Na Inorganic N TKN Ortho-phosphorus TP a

10-day Unplanted

Planted

65.66 65.87 7.36 31.90 65.00 91.13 83.64

76.38 91.83 4.75 69.63 79.06 97.52 91.83

70.43 76.16 6.11 47.89 75.15 97.63 86.65

Unplanted 71.49 89.62 8.08 51.72 67.62 96.72 87.70

% removal was not calculated for nitrate as influent had concentrations lower than that in effluent nitrate because nitrate was produced by nitrification during wastewater.

2.2.

Collection and analyses of water samples

About 100 mL of effluent samples were collected from the outlet zone of each microcosm at 10-day intervals. The collected effluent samples were filtered through Whatman No. 42 filter paper and stored at 4 ◦ C in the dark. The analyses of NH3 –N and PO4 3− –P were completed within 24 h, and dissolved organic carbon (DOC) and NO3 − –N were done within 28 days according to standard storage methods described by Eation et al. (1995). The concentrations of NH3 –N, NO3 − –N and PO4 3− –P in water samples were determined by Flow Injection Analysis (FIA, Lachat QuikChem® 8000, Lachat Instruments, U.S.A.). The DOC concentration was measured using the Autosampler ASI-5000A Total Organic Carbon Analyzer TOC-5000A (Shimadzu, U.S.A.). Total Kjeldahl nitrogen (TKN) and total phosphorus (TP) concentrations were analyzed once a month by Flow Injection Analysis (FIA, Lachat QuikChem® 8000, Lachat Instruments, U.S.A.) after acid digestion (McGill and Figueiredo, 1993).

2.3.

HRTs and plants significantly affected the DOC removal by the constructed mangrove microcosms.

3.2. N)

Nitrogen (ammonia-, nitrate- and total Kjeldahl

The average concentration of ammonia in the primary treated sewage was 20.93 mg L−1 . During the whole treatment period, ammonia was effectively removed from municipal wastewater, with >65% removal efficiency (Table 2). Lower ammonia concentrations were found in effluents from the planted microcosms than the unplanted ones (Fig. 3). In the planted microcosms, the effluent ammonia concentrations were lower

Statistical analyses

Mean and standard deviation values of three replicates for each treatment were calculated. A parametric two-way analysis of covariance (ANCOVA) was used to determine any significant differences in removal percentages between the two HRTs and between the planted and unplanted microcosms at the level of p ≤ 0.05, with the days of measurements as a covariate. All statistical analysis was performed by the computing package called Statistical Package for Social Science (SPSS 11.0 for Windows, SPSS Inc., IL, U.S.A.).

3.

Results

3.1.

Dissolved organic carbon

The mean concentration in the influent was 72.79 mg L−1 with temporal fluctuations (Fig. 2). The effluent concentrations in the planted microcosms were lower than the unplanted ones at both retention times. The DOC removal was generally higher in treatments under longer retention time; for instance, in planted microcosms, 70.43% and 76.38% removal were obtained at 5- and 10-day HRT, respectively (Table 2). According to the results of two-way ANCOVA (Table 3), both

Fig. 2 – Concentrations of dissolved organic carbon (DOC) in influent and effluent from different treatments during the 6-month wastewater application.

140

e c o l o g i c a l e n g i n e e r i n g 3 4 ( 2 0 0 8 ) 137–146

Table 3 – Two-way ANCOVA results (F-value) on concentration of DOC, NH3 –N, NO3 − –N, inorganic-N, ortho-P, TKN and TP in effluent from microcosm with different hydraulic time (HRT), with and without plants

Days of measurement HRT Plant HRT* Plant

DOC

Ammonia

7.305* 6.007* 4.034* 0.001

0.279 5.025* 12.073*** 119.903***

Nitrate 44.998*** 0.773 39.872*** 8.243**

Inorganic-N 22.507*** 32.998*** 274.203*** 43.664***

TKN 54.570*** 1.301 14.215*** 0.050

Ortho-P 46.653*** 25.117*** 44.693*** 27.326***

TP 13.237*** 2.328 1.360 0.034

*p < 0.05; **p < 0.005; ***p < 0.001 according to two-way ANCOVA test.

However, since the nitrate level for all treatments increased due to the activity of nitrification, the removal of inorganic N, a summation of ammonia and nitrate values, was lower than that of ammonia-N (Table 2). Nevertheless, positive removal percentages of inorganic nitrogen were still observed in all treatments. In general, treatments under 5-day HRT were less effective in removing inorganic nitrogen (Fig. 5). The average removal percentages were 47.89% and 69.63% for the planted microcosms under 5-day and 10-day HRTs, respectively. Significantly higher removal percentages were observed in the planted treatments (Table 2). Both HRTs and plants significantly affected the removal of inorganic N (Table 3). The average influent TKN concentration was 53.38 mg L−1 , nearly fourfold of that in the effluent (an average of 15.09 mg L−1 ). The planted microcosms were more effective in removing TKN than the unplanted ones (Fig. 6). On the other hand, hydraulic retention time did not have any significant effect on TKN removal (Table 3). The removal efficiencies of TKN were 67.62% and 65.00% for the unplanted microcosms under 10- and 5-day HRTs, respectively (Table 2).

Fig. 3 – Concentrations of ammonia in influent and effluent from different treatments during the 6-month wastewater application.

than 5 mg L−1 , the effluent discharge standard set by the Hong Kong Government. The average removal percentages of the planted mangroves under 10-day HRT were 91.91%, significantly higher than that under 5-day HRT with average of 76.38% (Table 3). The influent concentration of nitrate was very low with an average concentration of 0.27 mg L−1 , but nitrate in effluent was more than 5 mg L−1 (Fig. 4), indicating ammonia oxidation to nitrate via the nitrification process. The level of increases varied with treatments. There was significant difference between the planted and unplanted microcosms but HRT retention appeared to be less important (Tables 2 and 3). Higher nitrate effluent concentrations were detected in the unplanted microcosms than the planted ones under both 5and 10-day HRTs.

Fig. 4 – Concentrations of nitrate in influent and effluent from different treatments during the 6-month wastewater application.

e c o l o g i c a l e n g i n e e r i n g 3 4 ( 2 0 0 8 ) 137–146

4.

141

Discussion

The removal percentages in the planted systems were 70.43–76.38% for DOC, 76.16–91.83% for ammonia-N, 47.89–63.37% for inorganic-N, 75.15–79.06% for total Kjeldahl nitrogen, >97% for ortho-phosphate and 86.65–91.83% for total phosphorus. These removal efficiencies were comparable to and even higher than that reported by previous studies of commonly used perennial wetland plants (Table 4). The quality of most treated effluent was able to meet the discharged standards set the Hong Kong Government (Table 5). All effluents met all standards set for phosphate discharge, suggesting that the constructed mangrove microcosms were very effective in removing phosphorus from municipal wastewater even with only the 5-day HRT. In Hong Kong water, algal blooms or even eutrophication problems are often due to the addition of phosphorus, as phosphate is the limiting factor (Hodgkiss and Chan, 1983; Harrison et al., 1990; Hodgkiss and Ho, 1997; Yin et al., 2004). It is very important to have a treatment system effective in removing phosphorus from wastewater. For total nitrogen, 100% of the effluents discharged from the constructed mangrove wetland systems complied with the discharge standards set by the Government for coastal water of most Water Control Zones. For the more stringent standards for coastal waters in Tolo and Port Shelter Water Control Zones, the planted systems had significantly higher compliance (55.56–83.33%) than the unplanted treatFig. 5 – Concentrations of inorganic nitrogen in influent and effluent from different treatments during the 6-month wastewater application.

3.3.

Phosphorus (ortho- and total-P)

Inorganic phosphate in effluent was lower than 0.6 mg L−1 (Fig. 7). The removal reached almost 100% in all treatments. The effluent concentration of phosphate was relatively more constant than the other pollutants in wastewater, especially in the planted microcosms. Lower effluent inorganic phosphate concentration (an average of 0.09 mg L−1 ) was recorded in the planted microcosms under 5-day HRT than in the unplanted microcosms (an average of 0.32 mg L−1 ). Like other pollutants, 10-day HRT microcosms had better performance than the 5day HRT ones (Table 1). The average concentration of total phosphorus in influent was 3.56 mg L−1 and the effluent concentrations were below 0.58 mg L−1 in all treatments (Fig. 8). As the treatment continued, the concentrations of total phosphorus in the effluent dropped gradually and then became steady. The P inputs from sewage discharge were relatively small compared to those in the mangrove soils. For instance, the total amount of input P from the influent during the 6-month treatment period under 5-day HRT was around 630 mg, while the amount in the mangrove soil was more than 9000 mg. This explains why mangrove soil should have a large capacity of retaining P before saturation. Neither plants nor HRT had any significant effect on the removal of total phosphate (Table 2).

Fig. 6 – Concentrations of TKN in influent and effluent from different treatments during the 6-month wastewater application.

142

e c o l o g i c a l e n g i n e e r i n g 3 4 ( 2 0 0 8 ) 137–146

Table 4 – Removal efficiencies of different constructed wetlands for sewage treatment Location

Type of wastewater

Wetland plants used

Removal efficiencies (%) DOC

Israel Tanzania

Spain Uganda

Ontario, Canada Hong Kong

Domestic primary effluents Pre-treated wastewater from student’s hostel Rural wastewater Pre-settled municipal wastewater Municipal wastewater Municipal wastewater (after primary sedimentation)

Lemna gibba L.

68.88

Phragmites mauritianus and Typha latifolia Scirpus lacustris Cyperus papyrus

58.5

P. mauritianus Mixed T. latifolia and S. acutis Kandelia candel under 5-day HRT

K. candel under 10-day HRT

NH3

TKN



PO4 3−

Reference TP







Ran et al. (2004)

24.15







Kaseva (2004)

74 63.27

90 39.00

87 –

99 16.44

95 –

Soto et al. (1999) Okurut et al. (1999)

42.98 –

55.08 50

– 37

36.93 82

– 90

Cameron et al. (2003)

70.43

76.16

75.17

97.50

86.65

This study

76.38

91.83

79.07

97.50

91.83

(–): Not reported.

Table 5 – Percentages of effluents complied with the discharge standard at different water bodies in Hong Kong set by the Government (HKEPD, 1990) Parameters

Treatment

Mean value and range (in parentheses) in effluent (mg L−1 )

Percentages (%) of effluents complied with the discharge standarda Inland water Group A

Zone 1

Zone 2

68.63 100 13.73 98.04

NA NA NA NA

NA NA NA NA

100 100 100 98.04

100 100 100 100

NA NA NA NA

NA NA NA NA

0.09 (0.01–0.65) 0.09 (0.01–0.22) 0.32 (0.01–0.68) 0.12 (0.01–0.55)

100 100 100 100

NA NA NA NA

NA NA NA NA

NA NA NA NA

KM5 KM10 CM5 CM10

19.73 (6.23–39.89) 14.39 (2.18–32.53) 24.14 (10.57–38.36) 24.68 (10.81–47.72)

NA NA NA NA

NA NA NA NA

55.56 83.33 33.33 22.22

100 100 100 100

KM5 KM10 CM5 CM10

0.47 (0.02–1.98) 0.29 (0–1.26) 0.58 (0–0.98) 0.44 (0–1.89

NA NA NA NA

100 100 100 100

100 100 100 100

100 100 100 100

NH3

KM5 KM10 CM5 CM10

4.94 (2.13–9.04) 1.69 (0.01–3.17) 7.08 (3.36–26.73) 2.15 (0.01–10.39)

0 19.61 0 7.84

NO3 −

KM5 KM10 CM5 CM10

6.11 (0.97–12.42) 4.75 (0.85–11.05) 7.36 (2.85–13.19) 8.08 (2.30–15.50)

PO4 3−

KM5 KM10 CM5 CM10

Total N

Total P

Group B

Coastal water control

KM5, microcosms planted with Kc under 5-day HRT; KM10, microcosms planted with Kc under 10-day HRT; CM5, microcosms without plant under 5-day HRT; CM10, microcosms without plant under 10-day HRT. a

Group A: inland waters include all waters in water gathering grounds and within the boundaries of country parks and is the most stringent discharge standard for NH3 , NO3 − and PO4 3− are 1, 15 and 1 mg L−1 , respectively; Group B: inland waters are mainly those draining agricultural areas in the New Territories and standard for NH3, NO3 − and total P are 5, 30 and 10 mg L−1 , respectively. Coastal Water Control Zone 1: Tolo and Port Shelter Water Control Zones; Zone 2: Other Water Control Zones in Hong Kong with more open water. In these control zones, discharge standards are set only in terms of total N and total P and the respective values are 20 and 8 mg L−1 for Zone 1, 100 and 10 mg L−1 for Zone 2. NA: not applicable as no discharge standard is set by the government.

e c o l o g i c a l e n g i n e e r i n g 3 4 ( 2 0 0 8 ) 137–146

143

discharge would partially simulate natural tidal cycles in intertidal zones. It also produced periodic aerobic and anaerobic stages that were beneficial for nitrification and denitrification, respectively (Pell and Nyberg, 1989). The importance of plants in constructed wetland treatment systems is always in debate. It has been reported that the removal of dissolved organic carbon was not significantly related to the presence of plants (Tanner et al., 1995; Soto et al., 1999). The present study, on the contrary, shows that the planted mangrove systems were significantly more effective in removing DOC than the unplanted ones. The controversy might be caused by different plant species utilized and the different patterns of water flow (Soto et al., 1999). The rhizosphere environments varied with wetland plants, leading to differences in the degree of oxygen released from roots to rhizosphere and the types of microorganisms colonized on the surface of roots and soil particles (Stottmeister et al., 2003). Shackle et al. (2000) found that the main mechanism of reducing dissolved organic carbon relied on the activity of microorganisms in soil. Additionally, significant differences in ammonia removal were found between planted and unplanted systems, indicating that the presence of mangrove plants was important. The removal of ammonia-N in sewage treatment was mainly due to its oxidation via nitrification, partially due to its adsorption onto the negatively charged exchange colloids in sediments as well as plant assimilation (Tam and Wong, 1996; Ottova et al., 1997). Nitrification and denitrification processes are believed Fig. 7 – Concentrations of inorganic phosphorus in influent and effluent from different treatments during the 6-month wastewater application.

ments (22.22–33.33%). In Hong Kong, constructed mangrove wetland systems for wastewater treatment will be more likely to be employed in rural regions due to the rapid expansion of population sizes in the New Territories and the development of village houses. Most houses did not connect to any sanitary sewer system and it was not transported to centralized wastewater treatment works. The wastewater generated was discharged into nearby inshore waters after preliminary treatment using septic tanks, creating serious water pollution problems. The constructed mangrove treatment system, with high treatment percentages, could be an alternative for these houses. In nature, mangrove wetlands are inter-tidal ecosystems with tidal flushing once or twice a day, thus having alternate wet and dry environments. It has long been believed that mangrove plants cannot grow well in areas without tidal cycles. The present study shows that it is feasible to use constructed mangrove wetland systems planted with K. candel without tidal flushing. This mangrove plant species grew well in all microcosms: the stem heights increased from an initial 20.1 to 27.2 cm at the end of the 6-month wastewater treatment. The flow pattern employed in this study was subsurface horizontal and wastewater was discharged intermittently to the system. The sediment was wet during wastewater discharge and was allowed to dry between discharges. The alternate drying and wetting of mangrove sediment due to wastewater

Fig. 8 – Concentrations of TP in influent and effluent from different treatments during the 6-month wastewater application.

144

e c o l o g i c a l e n g i n e e r i n g 3 4 ( 2 0 0 8 ) 137–146

to be an important mechanism for nitrogen removal (Bachand and Horne, 2000; Tam et al., in press). The nitrogen removal ability for a wetland was found to increase according to the degree of coverage and biomass of the wetland plants (Tanner, 1996). In this study, decreases of ammonia in effluent were followed by increases in nitrate but at different rates. The planted systems had significantly lower effluent nitrate concentrations than the unplanted ones under both HRT. Nitrates can be removed by denitrification and plant uptake. It is obvious that the mangrove plants not only absorb nitrate for their growth, they also enhance the efficiency of both nitrification and denitrification processes. On one hand, oxygen is transported to the roots of mangrove plants creating aerobic rhizosphere for nitrification. On the other hand, the root exudates provided carbon sources for denitrification in anaerobic microhabitats (Qian et al., 1997). Bastviken et al. (2003) also found that plants could take up nitrate in the soil pore water and their roots could provide a large surface area for microbial growth. In the present study, the amounts of nitrogen accumulated in plant tissues at the end of wastewater treatment were 2.88 and 3.28% of total nitrogen in the microcosms under 5and 10-day HRTs, respectively (data not shown). Soil was still the most important deposit for nitrogen, although the concentration of soil total nitrogen after wastewater treatment was less than that before the experiment in all microcosms. Similar decreases in mangrove soil nitrogen after wastewater treatment were also reported as the added ammonium would accelerate the release of nitrogen from its soil pool for microbial transformation (Tam et al., in press). The percentages of nitrogen retained in soil at the end of wastewater treatment were 73.76% and 84.15% for planted systems under 5- and 10-day HRTs, respectively (data not shown). In terms of phosphorus removal, it is well accepted that soil adsorption is the main mechanism for the removal in constructed wetland treatment systems (Verhoeven and Meuleman, 1998; Klomjek, 2005). Over 96% of total phosphorus was kept in the soil at the end of the experiment. However, the planted microcosms in the present study had better efficiency in removing inorganic phosphate than the unplanted ones, suggesting that plant uptake (less than 1% of total P) or changes in soil properties around the rhizosphere zone may also play some roles in the removal. Li and Recknagel (2006) showed that plant uptake not only involved in the removal of inorganic P but also enhanced sediment P adsorption ability. Plants could indirectly affect P transformation in soil by positively influencing bacterial synthesis of polyphosphates in soil which can be very active under aerobic conditions (Merlin et al., 2002). The well-developed roots could release a certain amount of oxygen to the adjacent environment creating an aerobic environment around the rhizosphere (Stottmeister et al., 2003). One of the limitations in constructed wetland treatment systems is the requirement of large land areas due to long hydraulic retention time. The longer the HRT, the more space needed, but the efficiency would be better as the function of soil microorganisms in removing DOC increased with retention time (Koottatep and Polprasert, 1997). White (1995) showed that the ammonia removal efficiency by the wetland system with the 10-day HRT was two times that with 2.5-days HRT. It has been recommended that HRT for typical vegetated

submerged bed constructed wetland should fall into the range of 2–7 days (Wood, 1995), but HRT longer than 10 days had been employed in previous studies and field trials (Tanner et al., 1995; White, 1995; Schreijer et al., 1997; Dierberg et al., 2002; Hench et al., 2003). In the present study, 5-day HRT had lower removal percentages than 10-day HRT (Tables 2 and 3); however, the effluent from the shorter HRT also met the discharge standards set by the government. To make the constructed mangrove system more cost effective, a 5-day system would be more suitable. More in-depth research on further reducing HRT and enhancing the roles of plants is needed to optimize the mangrove treatment system.

5.

Conclusions

From the 6-month wastewater treatment study, it is clear that the mangrove microcosm with intermittent subsurface horizontal flow is effective in removing organic matter, nitrogen and phosphorus from primary settled municipal wastewater. The treated effluents, in terms of all forms of nitrogen and inorganic phosphate, were able to meet the effluent discharge standards for inland waters set by the Hong Kong Environmental Protection Department. The planted mangroves had higher treatment efficiency than the unplanted ones, and the higher efficiency under the 10-day HRT over that of the 5day HRT could be offset by its larger land requirement. The present research demonstrates the feasibility of using constructed mangrove wetlands planted with K. candel, without tidal flushing, and shows that they could be employed as the secondary treatment process for municipal wastewater.

Acknowledgement The work described in this paper was supported by the Areas of Excellence Scheme established under the University Grants Committee of the Hong Kong SAR, China (Project No. AoE/P04/2004).

references

Alongi, D.M., Clough, B.F., Robertson, A.I., 2005. Nutrient-use efficiency in arid-zone forests of the mangroves Rhizophora stylosa and Avicennia marina. Aquatic Botany 82 (2), 121–131. Al-Sayed, H.A., Ghanem, E.H., Saleh, K.M., 2005. Bacterial community and some physico-chemical characteristics in a subtropical mangrove environmental in Bahrain. Marine Pollution Bulletin 50 (2), 147–155. Bachand, P.A.M., Horne, A.J., 2000. Denitrification in constructed free-water surface wetlands. I. Very high nitrate removal rates in a macrocosm study. Ecological Engineering 14, 9–15. Bastviken, K.S., Eriksson, P.G., Martins, I., Neto, J.M., Leondardson, L., Tonderski, K., 2003. Potential nitrification and denitrification on different surfaces in a constructed treatment wetland. Journal of Environmental Quality 32, 2414–2420. Cameron, K., Madramootoo, C., Crolla, A., Kinsley, C., 2003. Pollutant removal from municipal sewage lagoon effluents with a free-surface wetland. Water Research 37, 2803–2812. Chen, G.Z., Miao, S.Y., Tam, N.F.Y., Wong, Y.S., Li, S.H., Lan, C.Y., 1995. Effect of synthetic wastewater on young Kandelia candel

e c o l o g i c a l e n g i n e e r i n g 3 4 ( 2 0 0 8 ) 137–146

plants growing under greenhouse conditions. Hydrobiologia 295, 263–273. Chiu, C.Y., Chou, C.H., 1993. Oxidation in the rhizosphere of mangrove Kandelia candel seedling. Soil Science Plant Nutrient 39, 725–731. Chu, H.Y., Chen, N.C., Yeung, M.C., Tam, N.F.Y., Wong, Y.S., 1998. Tide-tank system simulating mangrove wetland for removal of nutrients and heavy metals from wastewater. Water and Science Technology 38 (1), 361–368. Chung, A.K.C., Wu, Y., Tam, N.F.Y., Wong, M.H., 2008. Nitrogen and phosphate mass balance in a sub-surface flow constructed wetland for treating municipal wastewater. Ecological Engineering 32, 81–89. Clough, B.F., Boto, K.G., Attiwill, P.M., 1983. Mangroves and sewage: a revaluation. In: Teas, H.J. (Ed.), Biology and Ecology of Mangrove, Tasks for Vegetation Science Series v. 8. Dr. W. Junk Publishers, Lancaster, pp. 151–162. Comín, F.A., Romero, J.A., Astorga, V., Carcía, C., 1997. Nitrogen removal and cycling in restored wetlands used as filters of nutrients for agricultural runoff. Water and Science Technology 35 (5), 255–261. Cronk, J.K., 1995. Constructed wetlands to treat wastewater from dairy and swine operations: a review. Agriculture, Ecosystems and Environmental 58 (2–3), 97–114. Dierberg, F.E., Debusk, T.A., Jaclson, S.D., Chimney, M.J., Pietro, K., 2002. Submerged aquatic vegetation-based treatment wetlands for removing phosphorus from agricultural runoff: response to hydraulic and nutrient loading. Water Research 36, 1409–1422. Eation, A.D., Clesceri, L.S., Greenberg, A.E. (Eds.), 1995. Standard Methods for the Examination of Water and Wastewater. American Public Health Association, Washington, DC, pp. 18–25. Gopal, B., 1999. Natural and constructed wetlands for wastewater treatment: potentials and problems. Water Science and Technology 40 (3), 27–35. Gottschall, N., Boutin, C., Crolla, A., Kinsley, C., Champagne, P., 2007. The role of plants in the removal of nutrients at a constructed wetland treating agricultural (dairy) wastewater, Ontario, Canada. Ecological Engineering 29, 154–163. Gumbricht, T., 1992. Tertiary wastewater treatment using the root-zone method in temperate climates. Ecological Engineering 1, 199–212. Hammer, D.A., 1989. Constructed Wetlands for Wastewater Treatment: Municipal, Industrial and Agricultural. Lewis Publishers, Michigan, pp: 831. Harrison, P.J., Hu, M.H., Yang, Y.P., Lu, X., 1990. Phosphate limitation in estuarine and coastal waters of China. Journal of Experimental Marine Biology and Ecology 140, 79–87. Hench, K.R., Bissonnette, G.K., Sexstone, A.J., Coleman, J.G., Garbutt, K., Skousen, J.G., 2003. Fate of physical, chemical, and microbial contaminants in domestic wastewater following treatment by small constructed wetlands. Water Research 37, 921–927. HKEPD, 1990. Technical memorandum—standards for effluents discharged into drainage and sewerage systems, inland and coastal waters. Available at: http://www.epd.gov.hk/epd/english/envir standards/statutory/ esg stat.html. Hodgkiss, I.S., Chan, B.S.S., 1983. Pollution studies on Tolo Harbour, Hong Kong. Marine Environment Research 10 (1), 1–44. Hodgkiss, I.S., Ho, K.C., 1997. Are changes in N:P ratios in coastal waters the key to increased red tide blooms. Hydrobiologia 352, 141–147. Jacob, D.L., 2004. Influence of Typha latifolia and fertilization on metal mobility in two different Pb–Zn mine tailings types. Science of the Total Environment 33 (1–3), 9–24.

145

Kaseva, M.E., 2004. Performance of a sub-surface flow constructed wetland in polishing pre-treated wastewater—a tropical case study. Water Research 38 (3), 681–687. Kivaisi, A.K., 2001. The potential for constructed wetlands for wastewater treatment and reuse in developing countries: a review. Ecological Engineering 16, 545–560. Klomjek, P., 2005. Constructed treatment wetland: a study of eight plant species under saline conditions. Chemosphere 58 (5), 585–593. Knight, R.L., Victor, W.E., Payne, J., Borer Jr., R.E., Clarke, R.A., Pries, J.H., 2000. Constructed wetlands for livestock wastewater management. Ecological Engineering 15, 41–55. Koottatep, T., Polprasert, C., 1997. Role of plant uptake on nitrogen removal in constructed wetlands located in the tropics. Water Science and Technology 36 (2), 1–8. Lewis, R.R., 2005. Ecological engineering for successful management and restoration of mangrove forests. Ecological Engineering 24, 403–418. Li, M.S., 1997. Nutrient dynamics of a Futian mangrove forest in Shenzhen, South China. Estuarine, Coastal and Shelf Science 45, 463–472. Li, W., Recknagel, F., 2006. Balancing phosphorus adsorption and consumption processes in experimental treatment ponds for agricultural drainage water. Ecological Engineering 28 (1), 14–24. Mays, P.A., 2001. Comparison of heavy metal accumulation in a natural wetland and constructed wetlands receiving acid mine drainage. Ecological Engineering 16 (2), 487–500. McGill, W.B., Figueiredo, C.T., 1993. Total nitrogen. In: Cater, M.R. (Ed.), Soil Sampling and Methods of Aanalysis. CRC Press Inc., Boca Raton, FL, pp. 204–205. Merlin, G., Pajean, J.L., Lissolo, T., 2002. Performances of constructed wetlands for municipal wastewater treatment in rural mountainous area. Hydrobiologia 469 (5), 87–98. Okurut, T.O., Rijs, G.B.J., van Bruggen, J.J.A., 1999. Design and performance of experimental constructed wetlands in Uganda, planted with Cyperus papyrus and Phragmites mauritianus. Water Science and Technology 40 (3), 265–271. Ottova, V., Balcarova, J., Vymazal, J., 1997. Microbial characterization of constructed wetlands. Water Science and Technology 35 (5), 117–123. Pell, M., Nyberg, F., 1989. Infiltration of wastewater in a newly stated pilot sand filter system. III. Transformation of nitrogen. Journal of Environmental Quality 18 (4), 463–467. Qian, J.H., Doran, J.W., Walters, D.T., 1997. Maize plant contributions to root zone available carbon and microbial transformations of nitrogen. Soil Biology and Biochemistry 29 (9–10), 1451–1462. Ran, N., Agami, M., Oron, G., 2004. A pilot study of constructed wetlands using duckweed (Lemna gibba L.) for treatment domestic primary effluent in Israel. Water Research 38 (9), 2241–2248. Sansanayuth, P., Phadungchep, S., Ngdngam, N.S., Sukasem, P., Hoshino, H., Ttabucanon, M.S., 1996. Shrimp pond effluent: pollution problems and treatment by constructed wetlands. Water Science and Technology 34 (11), 93–98. Schreijer, M., Kampf, R., Toet, S., Verhoeven, J., 1997. The use of constructed wetlands to upgrade treated sewage effluents before discharge to natural surface water in Texel Island, The Netherlands—pilot study. Water Science and Technology 35 (5), 231–237. Shackle, V.J., Freeman, C., Reynolds, B., 2000. Carbon supply and the regulation of enzyme activity in constructed wetlands. Soil Biology and Biochemistry 32 (6), 1935–1940. Solano, M.L., Soriano, P., Ciria, M.P., 2003. Constructed wetlands as a sustainable solution for wastewater treatment in small villages. Biosystems Engineering 87 (1), 109–118.

146

e c o l o g i c a l e n g i n e e r i n g 3 4 ( 2 0 0 8 ) 137–146

Soto, F., Garcia, M., de Luis, E., Becares, E., 1999. Role of Scirpus lacustris in bacterial and nutrient removal from wastewater. Water Science and Technology 40, 241–247. Soukup, A., Williams, R.J., Cattell, F.C.R., Krough, M.H., 1994. The function of a coastal wetland as an efficient remover of nutrients from sewage effluent: a case study. Water Science and Technology 29 (4), 295–304. Stottmeister, U., Wießner, A., Kuschk, P., Kappelmeyer, U., Kästner, M., Bederski, O., Müller, R.A., Moormann, H., 2003. Effects of plants and microorganisms in constructed wetlands for wastewater treatment. Biotechnology Advances 22 (1–2), 93–117. Tam, F.Y., Wong, Y.S., 2000. Field guide to Hong Kong Mangroves. City University of Hong Kong Press, Kowloon, Hong Kong, pp: 6–13. Tam, N.F.Y., Wong, Y.S., 1996. Retention of wastewater-borne nitrogen and phosphorus in mangrove soils. Environmental Technology 17, 851–859. Tam, N.F.Y., Wong, A.H.Y., Wong, M.H., Wong, Y.S., in press. Mass balance of nitrogen in constructed mangrove wetlands receiving ammonia-rich wastewater: effects of tidal regime and carbon supply. Ecological Engineering. Tam, N.F.Y., 2004. Conservation of Hong Kong Natural Coastlines through Mangrove Replanting. Final Report for Environment and Conservation Fund and Woo Wheelock Green Fund, Hong Kong.

Tanner, C.C., 1996. Plants for constructed wetland treatment system—a comparison of the growth and nutrient uptake of eight emergent species. Ecological Engineering 7, 58–83. Tanner, C.C., Clayton, J.S., Upsdell, M.P., 1995. Effect of loading rate and planting on treatment of dairy farm wastewaters in constructed wetlands. II. Removal of nitrogen and phosphorus. Water Research 29 (1), 27–34. Verhoeven, J.T.A., Meuleman, A.F.M., 1998. Wetlands for wastewater treatment: opportunities and limitations. Ecological Engineering 12 (1–2), 5–12. White, K.D., 1995. Enhancement of nitrogen removal in subsurface flow constructed wetlands employing a 2-stage configuration, and unsaturated zone, and recirculation. Water Science and Technology 32 (3), 59–67. Wong, Y.S., Tam, F.Y., Chen, G.Z., Ma, H., 1997. Response of Aegiceras corniculatum to synthetic sewage under simulated tidal conditions. Hydrobiologia 352 (1–3), 89–96. Wood, A., 1995. Constructed wetlands in water pollution control: fundamentals to their understanding. Water Science and Technology 32 (3), 21–29. Yin, K., Song, X., Sun, J., Wu, M., 2004. Potential P limitation leads to excess N in the Pearl River estuarine coastal plume. Continental Shelf Research 24, 1895–1907.