Water treatment processes using membrane technology, electrochemistry and nanomaterials have shown tremendous promise in the last several years. While pressure driven membrane treatment processes are capable of treating water sources containing a wide variety of contaminants, they suffer from several challenges such as osmotic pressure limitations and surface scaling and fouling. Membrane distillation is a vapor pressure driven process that does not suffer from osmotic pressure limitations, and hence can be used for the treatment of high salinity sources. However, fouling is still a major concern, along with the added high energy requirements. In this work, we look at a membrane distillation system that successfully separated non-volatile contaminants from a dairy farm waste stream, resulting in a concentrated stream of nutrients to be used as fertilizer, and a dilute stream of volatile compounds that can be used as the feed in fermentation processes. We also looked at treatment of high salinity brines by membrane distillation. Due to the excellent heat and electrically conductive properties of carbon nanotubes, an electrically conducting membrane fabricated by coating a polymeric membrane with a carbon nanotube suspension proved successful in mitigating inorganic scaling to a large extent. The externally applied electric potential reduced scale deposition by electrostatic repulsion and electrokinetic mixing. In another aspect of our research activities, we studied the degradation of perfluoroalkyl substances, a class of contaminants of emerging concern that are highly recalcitrant carcinogenic compounds. Typical remediation of these compounds takes place through adsorption on activated carbon followed by incineration. Studies have shown that they can be degraded by electrochemical oxidation. We utilized the excellent sorption and electrically conductive properties of carbon nanotubes to develop a novel degradation mechanism, where an externally applied potential weakened the C-F bonds, and hydrated electrons generated by UV light resulted in defluorination. We extended this study to linear perfluoroalkyl substances having different chain lengths and headgroups, studied the feasibility of this mechanism on dechlorination of chlorinated solvents commonly co-occurring in groundwater, studied the impact of mixtures of perfluoroalkyl substances and verified the two-electron mechanism by evaluating degradation rates of isotopically labeled and unlabeled compounds.

Water shortage is a pervasive problem which has plagued societies for centuries. Recently, membrane technology has offered an effective and robust water treatment method for water sustainability due to great improvements in membrane performance. Depending on the intended applications, microfiltration (MF), ultrafiltration (UF), nano-filtration (NF) or reverse osmosis (RO) membranes can be used individually or in combination to achieve a specific water quality goal. However, a major challenge facing these technologies is membrane fouling, where particulates in the feed stream are deposited or developed on the membrane surface during the filtration process. Membrane fouling results in performance deterioration, lifetime shortening, and ultimately, increased operational costs. Therefore, membrane cleaning and fouling control or prevention are critical research topics for membrane development.

The goal of this dissertation is to demonstrate fouling prevention and foulant removal on carbon nanotube-modified (CNT) electrically conductive UF, NF and RO membranes. First, mineral scaling was prevented and removed by applying an external anodic electrical potential to an electrically conducting CNT – polyamide RO membrane. The results demonstrate that CaCO3 scaling was efficiently removed by the intermittent application of 2.5V and CaSO4 scaling can be prevented by the continuous application of 1.5V with the membrane as the anode. Second, biofouling and organic fouling were prevented while treating anaerobic sequencing batch reactor effluent by using electrically conducting UF and NF membranes. The continuous application of negative 5V to the UF membrane surface prevents organic fouling and allows good membrane performance while treating complex wastewater streams for long periods of time. The application of positive potentials to the NF membranes has also been shown to increase fouling and hinder cleaning and recovery. Third, a highly conductive and anodically stable polyaniline coated UF membrane was designed and used as a flow through electrode. Results show that the polyaniline coated CNT membrane had a significantly reduced degradation rate under high anodic potentials in a pH controlled environment when compared with polyvinyl alcohol crosslinked CNT membrane. The modified membrane can be used to degrade organic compounds and perform in situ oxidative cleaning of the fouled membrane without any additional oxidizing chemical reagents.

Brines are saline wastewaters discharged by many industries. To reduce the cost of brine disposal, membrane distillation (MD) as an emerging thermal desalination technology can minimize the volume of brines. MD systems include membrane modules and heat exchangers (HX), with extensive aqueous interfaces that play a crucial role in the desalination process. At the membrane-water interfaces, water evaporation occurs whereas salt ions are rejected by membrane. HX-water interfaces enable heat recovery and reuse, and interactions with salt ions. However, these interfaces show energy inefficiencies, including reduced thermal driving force due to latent heat losses during evaporation, limitations in heat recovery and reuse caused by HX designs, and hindered heat transfer due to mineral salt precipitation. By tailoring interfacial energy transfer, the limitations above can be addressed.

In this work, we firstly used metal mesh and/or shim as thermal carriers, to directly deliver heat to the membrane-water interface of MD. Importantly, vapor fluxes up to 9 L m−2 hr−1 were achieved using artificial hypersaline solution, with fluxes increasing with the heat input. In addition, specific energy consumption (SEC) on the order of 103 kJ kg−1 was attained, highlight the need for future heat recycling. Further experiment results of direct-heated MD demonstrated over 60% water recovery of real wastewater, but mineral scale on the membrane surface limited the maximum water recovery.

We then designed HXs using distinct heating and cooling streams to capture and reuse the laten heat in a staged surface-heated MD system. Internal and external HXs use the retentate from each stage and the feed as the condensing fluids for vapor, respectively. As a metric for energy efficiency, gained output ratio (GOR) of a 6-stage surface-heated MD reached 3.28. Surface heating increased the GOR when the added heat could be effectively recovered and reused by internal and external HXs.

Finally, the mitigation of mineral crystal nucleation was explored using alternating currents (AC). Electrode was used to simulate electroactive membrane or HX. Calcium carbonate crystallization in the bulk solution and on the surface of electrode was mitigated by AC potential of 4 Vpp and 1 Hz.

Membrane technology is an advanced water treatment process that well addresses water scarcity. It provides versatile treatment processes to expand freshwater resources, including seawater and brackish water desalination, wastewater and contaminated groundwater recycling. However, membranes are prone to fouling. In addition, they suffer from low water recovery, as well as a trade-off between water permeability and salt selectivity in desalination processes. Here, we incorporated carbon nanotubes (CNT) based electro-conducting thin film into commercial membranes and evaluated their viability as an effective approach to minimize membrane fouling along with adjusting the membrane’s selectivity toward different salt ions.

We first studied the fouling of model oil (hexadecane) emulsions stabilized by anionic, cationic and nonionic surfactants in a crossflow filtration system using ultrafiltration (UF) and nanofiltration (NF) membranes. In the UF filtration experiments, emulsions stabilized with the cationic surfactant quickly fouled the negatively charged UF membranes. Anionic and non-ionic surfactants stabilized emulsions, on the other hand, experienced less fouling. NF membranes exhibited exponential fouling with all types of surfactant stabilized emulsions. When 10 mM NaCl was used as the electrolyte, the differences between the surfactants were quenched. We demonstrated that the electrostatic interaction between the membrane surface and emulsion droplets was of key importance in membrane fouling. Therefore, we utilized electro-conducting CNT membranes to treat emulsion droplets at ionic strengths as high as 100 mM. Membrane fouling was reduced dramatically when electrical potentials were applied to the membrane surface. We concluded that the reduced fouling was due to less oil coalescence, which was caused by a re-distribution of charged surfactant molecules at the oil/water interface in response to the electric field. Finally, we fabricated the electro-conducting NF membrane by incorporating multi-walled CNT (MWCNT) and single/double-walled CNT (S/DWCNT) network into the polyamide (PA) film. By tuning the electrical potentials applied to the membrane surfaces, the membrane’s ion selectivity was dramatically changed. In particular, the application of negative potentials assisted MWCNT-PA membrane to better reject NaCl but allowed more NaCl to pass through S/DWCNT-PA membranes. We concluded that the phenomena resulted from different membrane film structures.

With the limited amount of freshwater resources, water scarcity has been becoming a serious problem. Thus, alternative water resources such as seawater and wastewater have been attracting water suppliers’ attention. To exploit these alternative water resources, membrane filtration that has an ability to remove ionic species plays an important role. Although high salt removal efficiency (> 99%) can be achieved when membrane (i.e., reverse osmosis membrane) is operated under a great condition, membranes have relatively low removal efficiency of neutral and small molecules. In addition, the operation and application of membrane filtration is often limited by accumulation of undesired materials on the membrane surface, so called membrane fouling. To mitigate membrane fouling, source water must be appropriately pre-treated prior to be fed into membrane filtration unit. In addition, membrane must be intermittently cleaned to remove readily formed membrane fouling in a chemical/physical way, which will increase water production cost.

Herein, we investigated electrochemical approach to enhance removal of neutrally-charged boron ions in seawater with electrically-conductive reverse osmosis membranes. By applying cathodic potentials, water electrolysis was facilitated to elevate local pH near the membrane surface, which increased the boron removal efficiency. We also studied optimization of groundwater treatment train that includes pretreatment and membrane filtration. By evaluating performance of each water treatment technologies, obtained results was coupled into a mathematical model to identify an optimum treatment train with blending ratio that can produce potable water that meets drinking water standards. Lastly, we also studied impact of alternating current on mineral scale formation on the electrically-conductive membrane that treats natural groundwater. With application of alternating current on the membrane surface, electrokinetic mixing was induced, which resulted in a mitigated scale formation.

Measurements of heavy metal concentrations in municipal wastewater (MWW) are currently performed by obtaining a grab-sample of water, followed by analysis using inductively coupled plasma – mass spectrometry (ICP-MS) or atomic absorption spectroscopy. However, these methods require expensive instruments and dedicated technicians, which are not typically on-site. Therefore, several rapid heavy metal detection techniques have been developed. However, these methods require the metals to be in their free forms to achieve reasonable results. Unfortunately, metals in complex water streams are generally not in their free forms. Therefore, we firstly systematically investigated the evolution of Pb(II), As(III), and Cd(II) throughout the wastewater treatment train (WWTT) in terms of the size fraction these metals were found in, as well as the metal’s partitioning behaviors. Cd(II) was found to be highly mobile, with the fraction of dissolved Cd(II) gradually increasing throughout the WWTT. As(III) was also highly mobile, with its size distribution and partitioning remaining largely steady, except when FeCl3 was used as a flocculation agent, which led to formations of arsenic/iron complexes. However, Pb(II) was found primarily in complex forms or adsorbed onto inorganic particulates. The WWTT had little impact on the size and partitioning of Pb, except that the formation of the Pb/iron complex occurred after flocculation with FeCl3. An increase of water hardness slightly increased the metals in the dissolved fraction. These results provided insightful information on the evolution of size and partition distribution of metals during the wastewater treatment train.

Based on these results, we developed processing methods that enable the detection of Pb(II) and Cd(II) using anodic stripping voltammetry. We found that vacuum ultraviolet (VUV)/H2O2 could release Cd from its complex, and strong acid could release Pb(II). Pb(II) and Cd(II) are then accurately detected and quantified using anodic stripping voltammetry by utilizing a bismuth subcarbonate/reduced graphene oxide nanocomposite incorporated in a Nafion matrix electrode. Detection results were benchmarked against state-of-the-art metal detection methods, and were found to be highly accurate (>88%).

Lastly, we fabricated electrically conductive membranes that could effectively reject As(III) due to As(III)’s high mobility. Applying cathodic potentials significantly increased As(III) rejections in synthetic/real tap water, a result of locally elevated pH that is brought upon through water electrolysis at the membrane/water interface. The elevated pH conditions convert H3ASO3 to H2AsO3-/HAsO32- that are rejected by the negatively charged membranes. In addition, it was found that Mg(OH)2 that precipitates on the membrane can further trap arsenic. Importantly, almost all As(III) passing through the membranes is oxidized to As(V) by hydrogen peroxide produced on the cathode, which significantly decreased its overall toxicity and mobility. Although the high pH along the membrane surface led to mineral scaling, the fouling could be partially removed by backwashing the membrane.

Water stress, resource scarcity, and the interdependency of food, water, and energy sectors highlight the need for developing and improving efficient resource and water management practices. While current wastewater management practices treat large volumes of wastewater and can remove nutrients that would otherwise be released into and pollute the environment, they do not meet the current needs to close the nutrient cycle and effectively recover nutrients present in wastewater. Currently, biological removal and chemical precipitation are the two most common methods employed for nutrient recovery, however, these processes are highly susceptible to feed compositions, temperature, and diurnal variations, and recover nutrients such as nitrogen or phosphorus, as nitrogen gas or struvite, a slow-release fertilizer of low commercial value. To this end, membrane-based wastewatertreatment processes have emerged as an efficient alternative to remove and recover these nutrients. In this work, we develop two novel membrane-based treatment technologies to effectively recover nutrients in their usable form as NH4+ and PO43-. We first developed an electrically conducting membrane to facilitate interfacial reactions at the membrane surface and recover ammonia from wastewater, while in the second project, we developed a novel ion exchange membrane capable of selectively transporting phosphate ion over other common ions by leveraging intermolecular interactions between the ion and membrane. The results demonstrate that ammonia was effectively recovered using our novel membrane and setup. Moreover, the process developed here reduced the overall energy consumption by 50% compared to traditional processes. The results also show that the ion exchange membrane is capable of highly efficient phosphate ion separation with a selectivity of up to a 100 compared to other competitive ions in solution.

Biofilm formation is a concerning issue for many industrial processes, especially for water treatment membranes, because it leads to fouling and poor performance of the system. Combinations of physical and chemical cleaning methods are commonly used to remove the deposited biofilms. However, there is little understanding of the interactions between chemical cleaning agents and the biofilm. Therefore, companies perform a series of iterations whereby a certain cleaning product or process is applied to the surface with the hope of dislodging the biofilm. This leads to indiscriminate use of cleaning agents and inconsistent results of biofilm removal. The goal of this project is to give insights to the impact of representative cleaning agents on individual biofilm components and ultimately, to design an efficient microbial cleaning strategy for biofouled surfaces, which will help to ensure a safe and sustainable operation of water supply and enhance the performance of membrane-based water treatment processes.

In this study, the interaction between a homogeneous layer of a single biofilm component (polysaccharides, proteins, nucleic acids) and different cleaning solutions (base, oxidizer, surfactant, chelating agent) was evaluated by comparing permeate flux of an ultrafiltration membrane and frequency shift measured by a quartz crystal microbalance. The efficacy of cleaning agents towards model biofilm component mixtures designed to mimic gram-negative and gram-positive bacterial biofilms and an actual bacterial biofilm component extracted from Pseudomonas aeruginosa (a gram-negative bacteria) were tested. The presence of calcium in the feed solution hindered the ability of the cleaning solutions to completely remove the foulants except when a chelating agent was used. The usage of a base and an oxidizer showed the best performance yet did not fully removed the model biofilm component mixtures or the bacterial biofilm component. Overall, it was determined that the presence of proteins in biofilms determines their susceptibility to cleaning.

Nitrate in drinking water poses a serious public health risk which affects water sources in California and around the world. The environmental risks of nitrate are twofold: as a contaminant in drinking water it threatens the health of infants and pregnant women, and in lakes and coastal regions it contributes to algae blooms, eutrophication, and hypoxic “dead zones.” However, nitrate is an extremely stable and soluble contaminant, which makes cost-effective removal very difficult. Activated carbon adsorption of contaminants is an accessible, economical method of water purification, but activated carbon is highly inefficient at adsorbing nitrate. Electrochemical reduction of nitrate is achievable, but has excessive energy requirements and often necessitates high-pressure or high-temperature water treatment.

Electrochemically-mediated nitrate removal on granular activated carbon (GAC) provides the benefits of both adsorption and electrochemical treatment methods and produces a high-quality permeate stream under room temperature and atmospheric pressure. Application of 5V to a GAC packed-bed filter doubled nitrate removal over 24-hour filtration period with no observed generation of ammonia or nitrate. In addition, a reverse-polarity rinse of the filter system allowed for complete regeneration of the removal efficiency of the GAC.

This removal was determined to be pH-insensitive, linearly dependent on hydraulic retention time, and moderately sensitive to interference by inorganic salt species commonly observed in groundwater. As a new treatment method for nitrate-contaminated water, electrochemically-mediated removal of nitrate on GAC offers a highly efficient, green, and reusable method with significant potential for drinking water applications.

The removal of hexavalent chromium (Cr(VI)) from wastewater is an increasing area of concern. Cr(VI) is commonly found in developed water systems and is a frequent contaminant in various wastewaters. Mounting pressure for recycled water usage, along with the consideration of ingested hexavalent chromium as a human carcinogen, is driving the need to develop methods to effectively remove Cr(VI). However, the natural solubility of Cr(VI) and its toxicity at low concentrations presents a challenge to removal. This study aims to apply previously developed electrochemical reduction methods of Cr(VI) to synthetic and real wastewater solutions. Parameters such as current density were evaluated to compare Cr(VI) removal efficiencies. The results demonstrate that Cr(VI) reduction is promising in the synthetic wastewater solutions but v does not translate effectively when applied to the wastewater. Further research is needed to characterize the various parameters in the real wastewater solution inhibiting Cr(VI) removal.