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With the popularity of swimming activities across the world, the responsibility of the pool operators and managers to ensure the microbial and chemical safety of the pool participants becomes extremely significant. Disinfection by-products (DBPs) in swimming pool waters (SPWs) have attracted considerable public attentions due to their high frequency of occurrence, considerable concentrations and potent toxicity. This study explored the appropriateness of novel approaches for the control and treatment of DBPs. Chlorine/bromine stabilized disinfectants have been attempted to fulfil the dual requirements of sufficient disinfectants and less DBP formation. Reverse osmosis and nanofiltration have been used to remove haloacetic acids (HAAs, a group of prevalent DBPs) and the mechanism governing the rejection has been well interpreted. The effect of calcium ion (a ubiquitous species in SPWs) on HAA rejection has been systematically investigated. The fundamental understanding of these DBP control protocols offers good theoretical basis for future practical applications. Doctor of Philosophy (IGS)

Forward Osmosis (FO) process is driven by osmotic energy, which is arisen from the osmotic pressure difference between the draw solution (high concentration) and the feed solution (low concentration) separated by a semi-permeable membrane. Combining anaerobic digestion with FO membrane to retain influent organic waste, this research aims to develop the integrated biological wastewater treatment technology: Anaerobic Osmosis-Membrane Reactor (AnOMBR). In the preliminary study, mixed organic fouling of the FO membrane in submerged mode was systematically investigated. Fouling behavior of cellulose triacetate (CTA) FO membrane and thin film composite (TFC) polyamide FO membranes were studied and compared. It was interesting to find under mild FO fouling conditions, TFC FO membranes could have greater fouling tendency as compared to CTA FO membranes due to their greater surface roughness. Although FO is believed to have superior fouling resistance in the AL-FS orientation, severe fouling could occur even at moderate flux levels, especially for TFC membranes or for unstable feed solutions. In this case, solution chemistries such as pH and presence of calcium ions posed remarkable effect on the cake layer composition due to the effect of foulant-foulant interaction(s); In contrast, the foulant composition was not strongly affected by the membrane type (CTA versus TFC) nor the testing mode (pressure-driven NF mode versus osmosis-driven FO mode). With the understanding of FO organic fouling mechanisms, a novel submerged AnOMBR utilizing CTA FO membrane in anaerobic bioreactor was developed and feasibility of using the AnOMBR to treat low-strength synthetic wastewater at mesophilic temperature was evaluated. Flux declined under the effect of both feed conductivity build-up and membrane fouling. Generally fouling on membrane was mild, while both organic fouling and inorganic scaling could still be observed at the edge of membrane. Bulk pH could be sustained within neutral to slightly alkaline due to the retention of alkalinity by FO membrane. The AnOMBR showed good and stable soluble chemical oxygen demand (sCOD) removal and perfect total phosphorous removal. However the removal of total nitrogen and ammonia still needed improvements. The elevated salt environment had marginal effect on bioactivity of methanogens and methane production of AnOMBR system was stable. Based on the promising results, the AnOMBR was operated at both mesophilic temperature and room temperature to compare the performance in terms of membrane flux level and mixed liquor conductivity, nutrient removal and methane production. At room temperature, the flux decreased and conductivity increased both at a slower speed than at mesophilic temperature. The membrane durability was also better and tap water cleaning was practical at room temperature with 90% flux recovery. At both temperatures, the AnOMBR showed good rejection to nutrients. However, at higher temperature, the nutrient concentration in supernatants was relatively lower, indicating the faster and efficient nutrient degradation by microbial at higher temperature. Methane production rate at mesophilic temperature was also significant higher than at room temperature. Doctor of Philosophy (CEE)

Phthalates are a group of common indoor semi-volatile organic compounds (SVOCs) that can concurrently exist in gas-phase, airborne particle-phase and settled dust phases, due to their special physicochemical properties. Human exposure to phthalates in indoor environments has raised public concerns owing to the identified associations between phthalates and a number of adverse human health effects. This study first develops sampling and analytical methods for field studies of airborne phthalates. Based on the methods, the study investigates occurrence of typical phthalates in indoor air of air-conditioned environments and assesses the impacts of air-conditioning and mechanical ventilation (ACMV) system on indoor phthalate concentrations. The study further examines phthalate concentration differences in indoor air and in personal air. More importantly, the study investigates the impacts of human induced particle resuspension and generation on airborne phthalates concentrations. Meanwhile, it evaluates gas/particle partitioning of selected phthalates when there are different levels of particle concentrations in real indoor environments. In the development of sampling and extraction method, works mainly focus on testing homemade two-bed sorbent tubes with silanized glass wool and Tenax TA as sorbents for airborne phthalate sampling. The sorbent tubes can sample airborne phthalate at low air flow rate for shorter sampling times, improve detection limits of low-volatile phthalates, and are compatible with thermal desorption (TD) method. The development of analytical method tests the extraction efficiency of the tubes. The extraction procedures and parameters are optimized. Important issues including carry-over, recovery, repeatability and linearity are considered. The study investigates occurrence of five typical phthalates in 24 ACMV indoor environments that are covered with different flooring materials. The results show that bis(2-ethylhexyl) phthalate (DEHP) has higher airborne concentrations in the involved indoor environments when compared with the literature data. There is also evidence that the rooms covered by polyvinyl chloride have apparently higher DEHP concentrations than those covered by carpets. The calculated whole room emission rates suggest that there are strong source strengths of DEHP and DBP (di-n-butyl phthalate) in the indoor environments. The study evaluates the impacts of operations of the ACMV system on indoor concentrations of DEHP and DBP in a typical office. The results indicate that indoor DEHP and DBP concentrations accumulate when the ACMV system is off, but decrease continuously to certain quasi-stable levels when the ACMV system is on. The evidence demonstrates that the ACMV system does have certain level of impacts on the indoor concentrations of the both high-volatile and low-volatile SVOCs. The study compares personally sampled phthalates with the corresponding phthalates in the bulk indoor air in three rooms. The results show that there is no detectable difference between the personal concentrations and the environmental concentrations. The explanation may be that the sedentary subjects’ activities are not strong enough to resuspend settled particles, which carry significantly higher phthalate concentrations. This study conducts intentional particle resuspension and generation in well-controlled real rooms to understand the impacts of human activities on airborne phthalate concentrations as well as their gas/particle partitioning variations. The results suggest that increased airborne particle concentration due to both particle resuspension and generation can result in obvious increases in airborne concentrations of phthalates with low volatilities. The increases are because of either resuspension of phthalates bound to settled dust or partitioning of gas-phase phthalates to the airborne particles. The findings also demonstrate that the gas fractions of the low-volatile phthalates decrease when the airborne particle concentrations increase. Doctor of Philosophy (CEE)

The continuous discharge of nutrients and micropollutants such as pharmaceutically active compounds (PhACs) potentially create a threat to urban aquatic environment and water safety. This issue is extremely important for some metropolitans that rely on the renewable/reclaimable water resources such as Singapore. This study explores new approaches to improve the current wastewater management practice particularly in terms of nutrient recovery and micropollutant mitigation from both scientific and engineering perspectives. In Chapter 2, this study first reviewed the current research on nutrient recovery and micropollutant removal from wastewaters. Cytostatic drugs were selected as target micropollutants, whose environmental occurrence and behaviors, source contributions, and potential ecotoxicity were summarized. Source separation strategy promises to improve the current wastewater management practice, especially in terms of nutrient/energy recovery and separate treatment of pharmaceutical residue. The benefits, limitations, and trends of development of different approaches to address the ‘emerging’ contamination issue are covered for urine source-separation strategy, membrane bio-reactor, reverse/forward osmosis (RO/FO) and advanced oxidation processes. Promising alternatives may essentially lie on the state-of-the-art treatment technologies based on urine source-separation strategy; it deserves further comprehensive evaluation from the technological, social-economical and administerial perspectives. In Chapter 3, the feasibility and potential limitations of applying urine source separation system in a tropical urban setting was evaluated by systematically investigating urine hydrolysis process (i.e., enzymatic ureolysis, spontaneous mineral precipitation, odor emission issue) and different nutrient recovery approaches (i.e., seawater-induced struvite precipitation and ammonia stripping). The results demonstrated that an effective and automated urine source separation system could be achieved. The entire process can be accurately monitored by measuring conductivity as a timesaving and cost-efficient indicator. Approaches were suggested to cope with pipe clogging and odor emission issues from an engineering perspective. This study also demonstrated that seawater can be used as a cost-effective magnesium source to recover phosphorus from hydrolyzed urine. Structural characterization of the precipitates confirmed struvite crystals (slow-releasing fertilizer) as the main product with certain magnesium calcite or calcite as co-precipitates. Air stripping process can be further applied to effectively recover ammonia from source-separated urine and the experimental parameters provided certain guidelines for the development of N-recovery process in industrial-scale. In Chapter 4, the feasibility of applying FO dewatering process for nutrient recovery from source-separated urine was further investigated under different conditions, using seawater or desalination brine as a low-cost draw solution. The filtration process exhibited relatively high water fluxes up to 20 L/m2·h. The process revealed relatively low rejection to neutral organic nitrogen (urea-N) in fresh urine but improved rejection of ammonium (50–80%) in hydrolyzed urine and high rejection (> 90%) of phosphate, potassium in most cases. Compared to simulation, higher water flux and solute flux were obtained using fresh or hydrolyzed urine as the feed, which was attributed to the intensive forward nutrient permeation (i.e. of urea, ammonium and potassium). Membrane fouling could be avoided by prior removal of the spontaneously precipitated crystals in urine. Compared to other urine treatment options, the current process was cost-effective and environmentally-friendly for nutrient recovery from urban wastewater at source, yet a comprehensive life-cycle impact assessment might be needed to evaluate and optimize the overall system performance at pilot and full scale operation. The FO process can be incorporated into a proposed decentralized source-separating sanitation system aiming to recycle the ‘human resource’ in agriculture with minimized environmental discharge and energy demand. In Chapter 5, a robust analytical method was developed and validated for the trace determination of multi-class cytostatic drugs in environmental matrices based on polymeric solid-phase extraction and liquid chromatography tandem mass spectrometry. The method was intensively optimized for sample pretreatments, instrumental conditions, and quantification in order to enhance the analytes’ recoveries and to mitigate matrix effects. Due to severe SPE loss and matrix effects, only twelve analytes could be reliably quantified by multiple-reaction monitoring with dual isotope-labelled internal standards. The optimized method was then applied for cytostatic screening in tropical aquatic environment where two hormone antagonists – tamoxifen and bicalutamide – were detected at 11–91 and 32–146 ng/L, respectively, indicating their incomplete removal during wastewater treatment and even trace presence in surface water. Precautionary actions are therefore needed to eliminate these detectable cytostatic drugs from the aquatic environment although their long-term fate in the environment is still beyond the knowledge of current science. In Chapter 6, the study further investigated the removal of a selected group of 13 cytostatic drugs (including 3 alkylating agents, 2 antimetabolites, 1 plant alkaloid, 3 cytotoxic antibiotics, and 4 hormone antagonists) using FO membrane process. The rejection mechanisms were studied with a thin-film composite (TFC) polyamide membrane and a cellulose triacetate (CTA) membrane under variable conditions. A high proportion of cationic (doxorubicin, epirubicin, and tamoxifen) and zwitterionic (melphalan and methotrexate) compounds were adsorbed to membrane surfaces by electrostatic attraction, resulting in generally higher solute fluxes and lower rejection efficiencies (except tamoxifen). Both membranes exhibited consistently high rejection (> 90%) to anionic cytostatic compounds. The TFC membrane showed generally better rejection to all cytostatics with a greater size-exclusion effect (as indicated by a smaller molecular weight cut-off value). Higher draw solution concentration led to enhanced FO rejection as a result of dilution effect. The presence of effluent matrix in the feed solution generally improved the FO rejection to cationic and zwitterionic compounds. On the other hand, increasing feed pH from 7 to 9 resulted in varied response in rejection as a consequence of complex interplay of size exclusion and electrostatic interaction. The results are practically important when integrating FO technology to strengthen sustainable management of local water resources and wastewater. Doctor of Philosophy (CEE)

In the tropics, the earth surface receives abundant solar radiation annually contributing significantly to building heat gain and, thus, cooling demand. An effective method that can curb the heat gains through opaque surfaces could provide significant energy savings. This study investigates the effectiveness of “high-albedo-high-emittance (or cool) roof” on reducing heat gain through opaque surfaces into buildings. It is hypothesised that building heat gain can be reduced by increasing the albedo of opaque surfaces to reflect off the incident solar radiation. At the same time, by increasing the thermal emittance to emit off the absorbed heat (as infrared radiation) to outdoor. Computational simulations (calibrated using experiments) are conducted to compare the thermal performance of a cool concrete flat roof to that of a green roof and a thermal insulation roof under the tropical climate of Singapore. The cool roof provides higher annual net heat gain reduction of about 81-83% as compared to the green roof (about 60-75%) and the thermal insulation roof (about 68-80%). A novel and general cool roof heat transfer (CRHT) model that is based on the spectral approximation method is proposed. The CRHT model provides very concise and easy-to-use expressions to evaluate the impact of cool coating on roof temperature, ceiling temperature, indoor air temperature and heat flux on single-skin roof (SSR, i.e., solid primary roof with no air-gap between roof layers) and double-skin roof (DSR, i.e., has an air-gap in between two solid roof layers). The model can handle transient outdoor and indoor boundary conditions as experienced by naturally ventilated buildings. The CRHT models are verified against analytical (the conduction transfer function) method and validated against experiments performed in real-scale apartments in Singapore. On a sunny day, cool coating reduces the peak temperatures and daily (integrated) heat gain of a concrete flat SSR by up to 14.1oC and 0.74 kWh/m2 (or 58%), respectively. The same cool coating reduces the peak temperatures and daily heat gain of a concrete flat DSR by up to 14.7oC and 0.21 kWh/m2 (or 47%), respectively. A new roof thermal transfer value (RTTV) model is proposed based on the CRHT model to accurately assimilate the effect of solar reflectance changes (due to application of cool roof) into RTTV or equivalent models for air-conditioned buildings. The proposed RTTV model incorporates new formulations for modelling the equivalent-thermal resistance increment due to the solar reflectance effect on opaque roofs. The new model shows significant improvement in accurately capturing the cool roof effect in RTTV calculation compared to existing model. This study sheds new insights on cool roof performance in tropical climate. The proposed CRHT and RTTV models provide the foundation for accurate modelling of cool roof heat transfer characteristics. These proposed models are also applicable to other climates. DOCTOR OF PHILOSOPHY (MAE)

Polyfluorinated alkyl substances (PFASs) are a large group of organic chemicals, many of which are persistent in the environment, bioaccumulative and pose potential adverse effects to human health. The presence of neutral, volatile PFASs in air are of particular concern, as air quality is an important determinant in terms of public health. However, little experimental information is available on the physicochemical properties of volatile PFASs. The short-term concentration variations of airborne PFASs also remain unknown due to the lack of a highly sensitive analytical method which can also achieve satisfying temporal resolution. Therefore, the aim of current work is to improve our understanding of the partitioning behavior, occurrence and source strength of neutral, volatile PFASs in Singapore. Firstly, air–water partition coefficient (KH) of 4 fluorotelomer alcohols (FTOHs), a subgroup of volatile PFASs, was measured using inert gas-stripping method (IGS). KH values of FTOHs showed strong propensity to partition into air. However, the expected linear dependence of KH on molecular weight was not observed. The unique molecular geometry of FTOHs, with hydrogen bonding and molecular contortion, was probably responsible for the unusual phenomenon. The results were compared to predictions made by SPARC and EPI suite. The erroneous predications given by modeling software packages appeared to confirm the unusual behavior of the compounds.Secondly, the study attempted to develop a novel method based upon thermal desorption and GC–MS for determination of indoor airborne volatile PFASs, including four FTOHs, two fluorooctane sulfonamides (FOSAs), and two fluorooctane sulfonamidoethanols (FOSEs) through low-volume active air sampling. The optimization of sorbent combination was also conducted. The method recovery exhibited significant improvement compared with other existing methods such as passive air sampling method. The approach also achieved relatively high temporal resolution and low noise level. It has been successfullyapplied to routine quantitation of targeted PFASs in indoor office environment of Singapore, which provided further insights toward the indoor source strength of PFASs and performance of building air handlings. Thirdly, the developed thermal desorption method was further applied for determination of target PFASs in urban atmosphere through low-volume active air sampling (0.108 m3). It displayed greatly improved method recovery but higher detection limits in comparison with the conventional protocol, which employs highvolume sample collection (450 m3) and solvent-based sample extraction. Comparison of their performances in the real samples of urban atmosphere showed a higher portability and temporal resolution of thermal desorption method, which would be very useful in source apportionment analysis and assessment of concentration variations in highly-urbanized regions. In addition, the concentration levels of PFASs detected in the ambient air of Singapore were more in line with the European pattern and might indicate an increasing tendency toward using FTOHs in these regions. Doctor of Philosophy (CEE)