Workshop attendees participated in a series of focused discussion sessions to assess current scientifi c knowledge gaps and identify future research needs for key areas of water purifi cation science and technology.
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10.3.1 Membrane Synthesis and Membrane Processes
Minimization of membrane fouling and preservation of membrane functionality by appropriate feed pretreatment to remove nanoparticles and foulants (organic, inorganic, and biological) are essential for successful operation of water treatment, reclamation, and desalination technologies. Research opportunities include enhanced feed characterization (suspended particle composition and morphology, particle–particle and particle–solute interactions);
biofi lm and scale formation (impact of feed water constituents, surface charge, chemistry, particle–surface eff ects on attachment and growth, role of suspended nanoparticles in scale formation) and mitigation strategies (selective separation of critical precursors, chemical and biological demineralization mechanisms);
active anti-fouling membranes incorporating functionalized surface and pore nanocatalysts; nanobased, fast-response monitors for feed water quality monitoring and membrane performance/integrity assessment.
Development of new, substantially more robust membranes will be dependent on increased understanding and capabilities in characterizing materials and aqueous interactions at the nanoscale. Areas of critical interest include nanostructure-membrane performance relationships; nanostructure impacts on membrane selectivity, transport, and membrane chemical and mechanical stability; multiscale computation methods for assessing material interactions (particle–particle, particle–solute, particle–surface, solute–surface) on water and ion transport in confi ned spaces; casting methodologies for uniform scale-up from nanostructures to commercially relevant membranes.
Residuals management plays a signifi cant role in membrane performance and residuals management is a major operational cost in membrane-based purifi cation systems. Substantive advances in residuals minimization and treatment of critical contaminants will require better understanding of the role of nanoparticles and their structure on crystallization, more robust modeling of crystal growth and characterization of crystallites, enhanced understanding of the role and mechanisms of antiscalants in growth control, and methodologies to stabilize supersaturated solutions without precipitation or scale formation.
10.3.2 Biofouling and Disinfection
Controlling the growth of biological organisms and the formation of biofi lms is critical both to sustainable operation of water purifi cation systems and to the prevention of disease. Key research opportunities identifi ed by workshop participants in understanding the causes and minimization of biofouling and the deactivation of pathogens include the stimuli and processes responsible for the formation of the initial biofi lm layers, strategies for inhibiting initial biofi lm formation; surface design and characteristics (e.g., catalytically active moieties) that are resistant to biological attachment, including structural components (e.g., membrane spacers); surface and system design and characteristics that increase shear to minimize cellular attachment; eff ect of polarization
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layers on biofi lm attachment and growth; methods to detect onset of biofi lm formation; adhesion mechanisms for antimicrobial materials on membranes and structural surfaces; strategies and mechanisms for disruption and detachment of biofi lms (e.g., acid catalysis, enzymes, biosignal substances);
formation pathways, characterization and toxicity of disinfection byproducts;
characterization of titania, titania–alumina, and other mixed-metallic oxidative catalysts as alternative antimicrobial agents, and methods for photocatalytic viral deactivation. Point-of-use design for distributed supply systems, wet- weather (e.g., stormwater, combined sewer overfl ow, sanitary sewer overfl ow) discharges, and disaster recovery applications were also identifi ed as critical research areas for successful nanotechnology diff usion.
10.3.3 Contaminant Removal
Advances beyond current strategies for removal of critical contaminants or for transformation of critical contaminants to nontoxic forms will require enhanced understanding of molecular transformations, nanostructured systems, and material synthesis, including better understanding of the relationship between nanocatalyst structure and oxidation—reduction pathways for target contaminants, and reaction site competition among contaminant mixtures.
Fast computational models are needed to evaluate new catalysis candidates.
New surface characterization techniques, for example, spectroscopically active adsorbent probes to characterize surface composition and new computation tools to interpret surface characterization will also be essential for improved nanocatalyst synthesis. Lastly, improvements in controlled synthesis of hierarchical nanostructured mixed-metal oxide catalysts, for example, CoO–TiO2 and Pd–Cu/C, are warranted for application in tricking-bed and fi xed-bed heterogeneous reactor systems.
10.3.4 Sensors
The ability to sense a wide range of environmental and system conditions, and to detect select contaminants with high specifi city and sensitivity is emerging as one of the most critical aspects for the effi cient and eff ective operation of water purifi cation technologies, for extending the useful lifetimes of water and wastewater infrastructure assets, and for protecting the public and infrastructure assets from natural and anthropogenic threats. A number of nanoscale sensor approaches were identifi ed for further research and potential development, including combinatorial selection of aptamers for sensing presence of biofi lms cells; solid-phase, polymer-fi lm extraction/laser-induced fl uorescence for capture and detection of polycyclic aromatic hydrocarbons; impedimetric and colorimetric sensors incorporating hormonal and other binding receptors in synthetic lipid bilayer membranes for detection of endocrine disruptors;
impedimetric sensors embedded directly into treatment membranes and on pipe surfaces for detection of biofi lm formation.
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