Five native plant species (Typha angustifolia, Scirpus mucronatus, Lepironia articulata, Eleocharis dulcis and Phragmites karka) were investigated for their nutrient removal efficiencies in shallow pond systems in Sg Buloh Wetland Reserve with water depth 0.12-0.30 m of a total 160 m2 in area size. The project aimed to investigate nutrient removal rates, removal efficiencies, nutrient storage in plant biomass, and plant growth. Pond water quality before and after planting in fully vegetated ponds improved significantly at 24.4% TP and 64.4% TN reduction in Pond 6. Scirpus mucronatus, Typha angustifolia, Lepironia articulata and Eleocharis dulcis showed high growth rates. Phragmites karka showed low growth rate due to pest attack by aphids. Nutrient concentration in stem/leaf samples of the 5 species were within the ranges of 1% to 3% of dry weight for nitrogen and 0.1% to 0.3% of dry weight for phosphorus. Average daily mass removal rates ranged from 3.5-9.6 kg P ha−1 day−1 and 13.5-33.5 kg N ha−1 day−1. This field trial showed that the 4 species Lepironia articulata, Eleocharis dulcis, Typha angustifolia and Scirpus mucronatus are suitable species in surface flow wetland or shallow pond systems for nutrient removal in tropical environments.
The Alexandra Wetlands, part of PUB's Active, Beautiful, Clean Waters (ABC Waters) Programme, showcase a surface flow wetland, an aquatic pond and a sub-surface flow wetland on a 200 m deck built over an urban drainage canal. Water from the canal is pumped to a sedimentation basin, before flowing in parallel to the three wetlands. Water quality monitoring was carried out monthly from April 2011 to December 2012. The order of removal efficiency is sub-surface flow (81.3%) >aquatic pond (58.5%) >surface flow (50.7%) for total suspended solids (TSS); sub-surface (44.9%) >surface flow (31.9%) >aquatic pond (22.0%) for total nitrogen (TN); and surface flow (56.7%) >aquatic pond (39.8%) >sub-surface flow (5.4%) for total phosphorus (TP). All three wetlands achieved the Singapore stormwater treatment objectives (STO) for TP removal, but only the sub-surface flow wetland met the STO for TSS, and none met the STO for TN. Challenges in achieving satisfactory performance include inconsistent feed water quality, undesirable behaviour such as fishing, release of pets and feeding of animals in the wetlands, and canal dredging during part of the monitoring period. As a pilot showcase, the Alexandra Wetlands provide useful lessons for implementing multi-objective wetlands in an urban setting.
Lorong Halus, Singapore's first landfill leachate treatment system, consists of a pre-treatment system (8,000 m(2)), five constructed reed beds (38,000 m(2)), five polishing ponds (13,000 m(2)), an education centre and a learning trail for visitors. Eight species of wetland plants (total 160,000 plants) were selected for their ability to uptake nutrients, tolerance to low phosphorus concentrations and resistance to pest infestations. The wetland was launched in March 2011 and water quality monitoring started in April 2011. The removal efficiencies of the pre-treatment system from April 2011 to August 2012 are biochemical oxygen demand (BOD5) 57.4%; chemical oxygen demand (COD) 23.6%; total suspended solids (TSS) 55.1%; ammoniacal nitrogen (NH4-N) 76.8%; total phosphorus (TP) 33.3% and total nitrogen (TN) 60.2%. Removal efficiencies of the reed beds are BOD5 47.0%; COD 42.2%; TSS 57.0%; NH4-N 82.5%; TP 29.3% and TN 83.9%. Plant growth is generally satisfactory, but the lower than designed volume of leachate has adversely affected some sections of plants and resulted in uneven flow distribution in reed beds. The plant management programme includes improving plant regrowth by harvesting of alternate strips of plants and replanting. The treated effluent meets water quality limits for discharge to the public sewer and is subsequently treated by the NEWater treatment system, which recycles water for industrial and indirect potable use.
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