XU Jiabo,SHI Yonghai,ZHANG Genyu,LIU Jianzhong,and ZHU Yazhu

Shanghai Fisheries Research Institute,Shanghai Fisheries Technical Extension Station,Shanghai 200433,P.R.China

1 Introduction

As a wastewater recycling technology,constructed wetlands (CWs) have been widely used to recycling wastewater from agricultural,industrial and municipal activities.Compared to the conventional wastewater treatment systems,CWs run at low cost,energy consumption and maintenance requirement,showing a great potential of being applied in developing countries (Kivaisi,2001;IWA,2000).

CWs treat the wastewater using natural components(e.g.,wetland vegetation,soil,and their associating microbes) through various biotic processes (e.g.,microbial mineralization,nitrification and denitrification and uptaking by vegetation) and abiotic processes (e.g.,precipitation,sedimentation and substrate adsorption) (Kadlec and Knight,1996; Reddy and D’Angelo,1997).These can be of importance to phosphorus (P) removal in industrial and agricultural wastewater treatment systems.

Several studies have been carried out in order to determine the effectiveness of CWs in aquaculture wastewater treatment.Tun?iper (2009) evaluated the effect of hydraulic loading rate (HLR) on the efficiency of CWs in nitrogen (N) removal in aquaculture wastewater.Akratos and Tsihrintzis (2007) investigated the effect of hydraulic residence time (HRT) on the performance of CWs in treating wastewater.These studies demonstrated that both HLR and HRT are crucial parameters that affect the performance of CWs in treating aquaculture wastewater.Considering the correlation between HLR and HRT,the present study chose HLR as a parameter to assess the effect of HLR on the efficiency of CWs in wastewater treatment.

At high salinity,CWs have been used to treat effluents from shrimp ponds,decreasing the concentration of total suspended solids (TSS),biochemical oxygen demand(BOD),total organic carbon (TOC),total nitrogen (TN),and total phosphorous (TP) by 84%,91%,46%,48%,and 31%,respectively (Sansanayuthet al.,1996).

Linet al.(2003) used a pilot-scale CW unit at a low HLR (0.3 m d?1) to treat wastewater from a Pacific white shrimp aquaculture system.The CWs integrated into the recirculating aquaculture system were found necessary for the removal of suspended solids,organic matter,ammonia,and nitrite.The authors concluded that CWs can improve water quality,providing a good culture environment.

Sindilariuet al.(2007,2008) studied the wastewater treatment efficiency of subsurface wetland at a high HLR during raceway runoff from a trout aquaculture system,and constructed subsurface-flow wetlands to treat effluents from an intensive trout farming system for over 6 months.The authors examined the effects of HLR on the efficiency of CWs in wastewater treatment and ascertained the most suitable area in need of commercial application.

In the present study,CWs are constructed to treat brackish water from a puffer aquaculture system using relatively high HLRs compared with those in other studies.Brackish water district has broad development prospects in aquaculture.High HLR in the CWs can raise the utilization ratio of land and the economic benefits of per unit area,and can improve environment.The objectives of this study are: 1) to evaluate the performance of CWs in recirculating brackish water in puffer aquaculture,and 2) to evaluate the effects of high HLR on the CW efficiency in effluent treatment.

2 Materials and Methods

2.1 Recirculating Aquaculture System

The recirculating aquaculture system (RAS) was established at the Shanghai Fisheries Research Institute,Aquatic Animal Breeding Technology Center,China.The RAS consisted of 4 indoor culture tanks (12.80 m × 3.43 m × 1.2 m) and 6 outdoor CWs,including 1 vertical-flow(VF) CW (19.40 m × 1.10 m × 1.68 m) and 5 connected horizontal-flow (HF) CWs (19.40 m × 2.06 m × 0.90 m).The 6 CWs were covered with euphotic nylon pellicle(Fig.1) in order to improve the warm performance of RAS.The thermal insulation property of water temperature can be improved to more than 10℃,thus solving problems related to fish overwintering.Culture tanks and wetlands were both made of brick and cement.

Fig.1 The schematic diagram of recirculating aquaculture system consisting of indoor culture tanks and 6 outdoor constructed wetlands.

The VF-CW was filled with corallites to a depth of 0.68 m,firmly supported by a lacunal concrete partition.The 5 HF-CWs were filled from bottom to top with a 0.25 m layer of stones (80–100 mm particle size),a 0.20 m layer of coarse gravel (30–50 mm particle size),and a 0.10 m layer of fine gravel (10–20 mm particle size).Water level was maintained constant in the 6 CWs throughout the study.To prevent any dominant plant species from occupying the majority of wetland areas,3 local species of salt-tolerant plant were grown in the CWs,including common reed (Phragmites australis) in CW2 and CW4,smooth cordgrass (Spartina alterniflora) in CW3 and CW5,and bulrush(Scirpus mariqueter) in CW1 and CW6 (Tilleyet al.,2002; Shiet al.,2011).The density of each plant species in the CWs was initially 20 plants m?2and reached over 100 plants m?2in all CWs by the end of the study period.

Tank water depth was maintained at 1.1 m.The water continuously flew into the CWs by gravity and was pumped into culture tanks after CW treatment.Water flow was controlled by a gate valve beside the pump in the CWs to receive different HLRs.No water was replaced or displaced in the RAS during this study,except for the water added to offset water lost through evaporation.

Post-larvae (PL) obscure puffer (Takifugu obscurus) with an initial mean length of 4–5 cm were introduced into the culture tanks (100 PL m?3) and fed with powder feeds (2%of body weight per day) during the culturing period.

2.2 Sampling and Analysis

The CWs were operated at 0.762,0.633,and 0.458 md–1for every 10 d.The average HRT equivalent to each HLR was 0.469,0.565,and 0.781 d,respectively.The CWs were allowed to operate for 3 d before water sampling for each trial.Water samples were taken in triplicate from the influent and effluent at each HLR at 13:00 every other day.To evaluate the efficiency of CW treatment at the 3 HLRs,nutrient concentrations (mg L–1) of total ammonium N (TAN),nitrite N (NO2?-N),nitrate N (NO3?-N),total Kjeldahl N (TKN),total suspended solids (TSS),chemical oxygen demand (COD) and TP were measured for water samples with conventional methods.Other water quality parameters were measured using Yellow Springs Instrument (OH,USA): YSI-100 pH electrode (0.01 unit),YSI-58 dissolved oxygen (DO)/temperature electrode(0.01/0.1 unit),and YSI-30 salinity electrode (0.1 unit).

2.3 Calculations and Statistics

Results were expressed as the arithmetic mean values of triplicate measurements.Differences (?p) in the tested nutrient concentrations and quality parameters between the inflow and outflow as well as between each pair of simultaneous samples were calculated.The relative treatment efficiency (?e) was calculated using the following formula:

where ?pis the inflow-outflow concentration (mg L?1),andCinis the inflow concentration (mg L?1).The related area loading rate (AL,g m?2d?1) and area removal rate(AR,g m?2d?1) of the applied CW area were calculated as follows:

Analysis of Variance (ANOVA) was performed to test the significance of differences in water quality between the influents and effluents (OriginLab,1996) using SPSS 13.0.

3 Results

3.1 Physical Water Parameters

During the 30-d CW treatment,water temperature increased by 0.3 and 0.2℃ at the low and high HLRs,respectively.Compared with the influent DO level,the effluent DO level decreased by 11.0% and 4.7% at the medium and low HLRs,respectively.Associated effluent pH increased by 3.13%,1.13%,and 1.48% from the high to low HLRs,respectively (Table 1).

Table 1 Physical water quality parameters of influents and effluents at different hydraulic loading rates

3.2 Effect of HLR on Water Quality

Water quality of the influents refers to the quality of water in culture tanks.Thus,the significance test of difference in water quality among the inflows at different HLRs was necessary.In terms of nutrient removal (Table 2),there were no significant difference in the TAN concentration among influents at different HLRs (P>0.05),whereas the TAN and NO2–-N concentration differed significantly between the influent and effluent at the same HLR (P<0.05).The treatment efficiency of CWs in TAN removal ranged from 81.03% to 92.81%,which increased with the decrease of HLR,reaching the highest at the lowest HLR.The CWs showed the highest efficiency in NO2–-N removal (99.68%) compared with the treatment of other nutrients.Both TAN and NO2–-N were slightly affected by HLR.There were significant difference in the NO3–-N and TKN concentration among the influents at different HLRs (P<0.05).NO3–-N concentration differed significantly between the influent and effluent (P<0.05) at the same HLR except the high HLR.TKN concentration showed significant difference between the influent and effluent at the same HLR (P<0.05).Relatively,the efficiency of CWs in NO3–-N (7.14%–17.49%) and TKN(27.76%–38.73%) removal was poor compared with those of TAN and NO2–-N.

The COD and TSS level differed significantly among influents at different HLRs (P<0.05).The mean removal efficiency of COD at the 3 HLRs was 54.22%,51.92%,and 51.23%,respectively,and that of TSS was 76.75%,86.50%,and 86.88%,respectively.The efficiency of TSS removal generally increased with the decrease of HLR,but showed no obvious difference between the medium and low HLRs.The variation trend in COD removal efficiency with HLR was in contrast to that in TSS removal.

The concentration of TP showed significant difference between the influent and effluent at the same HLR (P<0.05),but not among the influents at different HLRs (P>0.05).The treatment efficiency of TP removal was generally low,with the highest efficiency of 28.48% at the low HLR.

Table 2 Characteristics of influents and effluents and constructed wetland treatment efficiency at different hydraulic loading rates

4 Discussion

4.1 Physical Water Parameters

In this study,water temperature increased during CW treatment at the high and low HLRs.This was attributed to the euphotic nylon pellicle which overshadowed CWs and provided warmth,especially during fall and winter.In addition,the euphotic nylon pellicle could help overwintering fish in the culturing system.Alternatively,the temperature increase could be related to the fine weather during the study period,which led to temperature rise in the CWs.However,the ambient temperature of CWs at the medium HLR was very low,resulting in slight decrease in water temperature.

During CW treatment,water pH slightly increased.A similar phenomenon was observed in several other studies(Zhang and Sun,2004; Vymazal,2007) which showed that photosynthesis in marine alga and submerged macrophytes resulted in the high pH level during daytime.DO reduction was observed in influents and effluents at the medium and low HLRs.This was due to oxygen consumption by the oxidation of organic matter and nitrification (Linet al.,2003; Tun?iper,2009).Other studies have demonstrated that the DO level decreased to different degrees in the influents and effluents of wetlands(Bachand and Horne,2000; Thullenet al.,2002; Beutelet al.,2009).

In the present study,physical water parameters changed with the progress of rearing.Water temperature,one of the most influential factors in biological activity,differed among experimental treatments.Organic and inorganic loadings to CWs increased due to the increasing feed consumption dependent on fish growth.Furthermore,bacterial (and plant) biomass in CWs significantly affected the water treatment activity and increased with the progress of rearing.However,HLR showed the most significant effect on CW water treatment among all tested physical water parameters.

4.2 Nitrogen Removal

TAN and NO2–-N removal in the recirculating aquaculture system by CWs was highly effective.Associated removal efficiencies (81.03%–92.81% and 99.40%–99.68%,respectively) were higher than those obtained in other studies (IWA,2000; Linet al.,2005).The high TAN and NO2–-N removal rate,combined with their complete transformation into NO3–-N,implied a high nitrification rate in the CWs.Regarding the vegetation,the CWs with mixed plant species displayed high plant density.Kyambaddeet al.(2004) suggested that fast-growing plants with more roots are favorable for nitrifying bacteria and enhance the nitrification activity.In the CWs,effluent TAN concentration was positively correlated with AL (y= 0.1424x1.4416,r2=0.304,Fig.2a).Furthermore,the TAN concentration was quite low in effluents at the medium and low HLRs (0.180 ± 0.047 mg L–1and 0.189 ± 0.037 mg L–1,respectively).The AR for TAN showed a linear relationship with AL (y= 0.7144x+ 0.2371,r2=0.775,Fig.3a),and the maximum AR (1.635 g m–2d–1)was observed at the high HLR with an AL of 1.963 g m?2d?1.NO2–-N concentration was low in the effluents and remained at 0.001 ± 0.001 mg L–1at the highest HLR.Differently,NO3–-N concentration increased with the culturing time in the influents.The highest concentration of NO3–-N reported here (8.487 mg L?1) was lower than the toxic concentration (1000–3000 mg L?1) to most fish species (Lawson,1995).The treatment efficiency of NO3–-N removal decreased with the increase of HLR,consistent with the finding by Ingersoll and Baker (1998).However,the present study revealed a poor performance of CWs in nitrate removal (7.14%–17.49%) compared with those reported in other studies (up to 68%) (Linet al.,2003,2008).Such a difference was due to the variation in HLRs,as a low HLR might prolong the contact time of NO3–ions and denitrifying bacteria,thus enhancing the microbially mediated denitrification.The HLRs used in this study were higher than other rates used previously,causing the poor performance of nitrate removal.As for TIN(TAN+NO2–-N+NO3–-N),its concentration in the effluent of CWs (Fig.2b) was positively correlated with AL(y=1.3935x0.8507,r2=0.249).In addition,the AR of TIN had a linear relationship (y= 0.2632x+ 0.4925,r2=0.698)with AL (Fig.3b),and the maximum AR (2.448 g m–2d–1)was observed at AL of 6.961 g m–2d–1.The AL of TIN reported here was higher compared with the values reported in other studies,which decreased the rate of nitrification-denitrification.Although plant uptake could remove N,this process was of less importance than nitrification and denitrification processes.

Fig.2 The relationship between the concentration and loading rate of total ammonium nitrogen (TAN) (a) and total inorganic nitrogen (TIN) (b) in the effluent.

Fig.3 The relationship between removal rate and loading rate of total ammonium nitrogen (TAN) (a),total inorganic nitrogen (TIN) (b),total suspended solid (TSS) (c),and chemical oxygen demand (COD) (d).

4.3 Organic Matter Removal

The TSS concentration was constantly below 10 mg L–1in the effluent.At the high HLR,the treatment efficiency of TSS removal was the lowest (76.75%),but the mean treatment efficiency (83.37%) was consistent with results of previous studies (Vymazal,2005; Babatundeet al.,2008; Fountoulakiset al.,2009),which observed high treatment efficiency of TSS removal.For COD,the treatment efficiency (54.22%–51.23%) varied at different HLRs in an opposite trend compared to that in the literature.Sindilariuet al.(2008) reported that the COD treatment efficiency increased with the decreasing HLR.Schulzet al.(2003) indicated that the emergent plants used in CWs played a crucial role because their rhizomes created the necessary environment for nutrient removal processes.The reduction of COD showed no influence on the hydraulic load,and the mean treatment efficiency of COD (52.46%) was lower than the ranges reported in the literature (87%,Gomezet al.,2001).However,several researchers found the COD treatment efficiency similar to that reported here.Ansolaet al.(2003) observed that in a wetland system planted withTypha latifoliaandSalix atrocinerea,only 60% of the incoming COD was removed.Sindilariuet al.(2008) reported that the mean COD treatment efficiency was 61.4% under 3 different HLRs in an intensive trout farm.

The AR of TSS showed a linear relationship (y=1.0285x? 6.2141,r2=0.940) with AL (Fig.3c),with the maximum AR (43.52 g m–2d–1) observed at the medium HLR,corresponding to an AL of 47.44 g m–2d–1.Similarly,the AR of COD had a linear relationship (y=0.6093x?0.4880,r2=0.964) with AL (Fig.3d),with the maximum AR (3.93 g m–2d–1) corresponding to an AL of 6.87 g m–2d–1.

4.4 Phosphorous Removal

The treatment efficiency of TP removal increased significantly as HLR decreased.Schulz (2003) pointed out that the removal of phosphorus generally involves adsorption processes and chemical reactions in plant root zone systems,leading to the formation of a solid phosphate phase.In the present study,the AL of TP in influent at the 3 HLRs was low with a slight P uptake by plants,accounting for the poor performance of TP treatment.

Although the CW treatment at the low HLR had great advantages,the treatment efficiency at the high HLR was satisfactory.The high HLR was superior to the low HLR in improving water quality of the influent.The CWs operated at the 3 HLRs proved effective and were likely capable of operating at an even higher HLR.Taking into account the feasibility and safety,the cultural density of puffer was maintained at a low level.Future study will be conducted using increasing levels of the cultural density of puffer and the HLR.

5 Conclusions

This study demonstrated that the CWs planted with salt-tolerant hydrophytes and operated under brackish conditions were effective for treatment of aquaculture wastewater,as well as maintenance of high water quality and good culture environment.The efficiency of N removal increased gradually with the decreasing HLR.TAN and NO2–-N removal,in particular,had highly effective performances.Removal of TIN and TP via plant uptake was limited.The main influencing factor of the TSS removal efficiency was HLR.Moreover,the emergent plants played a crucial role in COD removal.A warm environment in the wetland during cold seasons potentially ensured normal plant growth and bacterial transformation.

Acknowledgements

The authors would like to thank Mrs.Haiming Zhang,Yongde Xie,and Xiaodong Zhu for experimental assistance.This study was supported by the Agriculture Commission and the Sciences and Technology Commission of Shanghai (No.09ZR1429000),and Shanghai University Knowledge Service Platform,Shanghai Ocean University aquatic animal breeding center (ZF1206),China.

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