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Biofiltration systems, as one of the best management practices, have good potentials to improve stormwater quality and hydrology of urban catchments. While biofiltration systems are well-studied in developed countries, the majority of those studies are conducted for temperate climate and there is a lack of lab-scale and field-scale studies on such systems under tropical conditions. This paper focuses on the performance of a lab-scale prototype biofiltration systems in stormwater retention efficiency as well as pollutants removal (including heavy metals and nutrients) from synthetic stormwater reproducing tropical rainfall events. A three-layer sand-based filter media with two different native plants including Pedilanthus tithymaloides and Cyperus alternifolius was selected for this study. Results showed that the system with Cyperus has a better stormwater retention capacity compared to the one with Pedilanthus. In addition, the observed infiltration rate in Cyperus and Pedilanthus were 338 mm/h and 267 mm/h, respectively. The better hydraulic performance in the system with Cyperus was attributed to the deeper and more extensive root penetration of this plant (as deep as 800 mm) compared to Pedilanthus (as deep as 250 mm). While both systems failed to perform well in removing total nitrogen, they performed significantly better in removing total phosphorus (Cyperus and Pedilanthus removed 67.3% and 62.5% of total phosphorus, respectively). The statistical analysis of results showed that the top 100 mm layer of filter media is the main contributor to total phosphorus removal. However, no major differences were observed between the two systems in phosphorus removal. Moreover, both systems were also capable of removing the available heavy metals (i.e., Fe, Cu, Mn, Ni, Pb, and Zn) as the removal efficiencies exceeded 90%, except for Fe (76%). Similar to phosphorus, it was concluded that the top layer is the major contributor to the heavy metals removal. Overall, the biofiltration system using Cyperus was found to be a successful system for operating under tropical conditions.

Singapore, a small island city-state depends on four sources, namely local catchment, imported water from Malaysia, desalinated water, and NEWater, to provide for its daily water consumption of about 1.36 million m3 per day. NEWater, a high grade reclaimed water, is key in achieving water sustainability through the multiplying effect of water reuse. In addition to being used for direct non-potable use (DNU) in wafer fabrication processes and other industrial processes, NEWater is being injected into surface reservoirs for indirect potable use (IPU). To ensure the safety of NEWater for IPU, PUB has adopted a multi barrier safety approach. The multi safety barrier approach starts from source and extends to taps in households in stages, namely source control; greater than 85% of wastewater from domestic source; comprehensive secondary wastewater treatment; microfiltration (MF) process, reverse osmosis (RO) process and ultra-violet (UV) disinfection in NEWater production; natural attenuation in surface reservoirs; as well as conventional water treatment process of coagulation, flocculation, sand filtration and disinfection. The approach is further enhanced by a comprehensive water quality monitoring programme that includes emerging contaminants of concern and a strict operating philosophy. The water quality test results are in turn reviewed by experts from local universities as well as overseas experts of international standing. Over the years of operations, NEWater has been found to be well within WHO Drinking Water Guidelines and USEPA Drinking Water Standards and each safety barrier has a significant role to play in ensuring the safety of NEWater for IPU.