Micropollutant removal and nutrient recovery for sustainable management of urban wastewater
Date of Issue2015
School of Civil and Environmental Engineering
Nanyang Environment and Water Research Institute
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.
DRNTU::Engineering::Environmental engineering::Waste management
DRNTU::Engineering::Environmental engineering::Environmental pollution
DRNTU::Engineering::Environmental engineering::Environmental pollution