A Continuous Single-Stage Dielectrophoresis Device for the Separation of Waterborne Pathogens from Water

Abstract

An estimated 748 million people around the globe still rely on unsafe drinking water to this day. Out of this population, an average 3.4 million people perish due to waterborne illness. Escherichia coli O157:H7 is one of the most notorious pathogens that has caused serious outbreaks that has led to deaths. Another pathogen of interest in waterborne contamination is norovirus. It is the leading cause of gastroenteritis in the United States and other developed countries. The multi-barrier approach being implemented in current water treatment facilities have dramatically reduced waterborne contamination. However, some waterborne pathogens continue to persist and cause outbreaks linked to contaminated water. Membrane filtration is an alternative method that is being explored in water treatment but the technology continues to be operated in a lab-scale because of cost implications. The challenges faced by the current water treatment procedure must be addressed by new technology that is capable of removing waterborne pathogens from water without the use of chemical additives while at the same time have the cost feasibility to be implemented. Dielectrophoresis (DEP) is a technique that manipulates a particle’s movement in a suspended medium. When a particle is subjected to a non-uniform electric field, a dipole is induced within the particle and a DEP force is established. The DEP force causes the polarized particle to move in the electric field gradient thus creating an opportunity to manipulate the particle’s direction and separate the cells from its liquid medium without the use of chemical additives. This thesis hypothesizes that by applying DEP to contaminated water, waterborne pathogens can be manipulated to move towards a specific waste outlet stream and away from the clean stream, thus facilitating a filtration mechanism. The objectives of this thesis are to determine the feasibility of creating a bigger DEP device, examine the efficiency of the scaled-up DEP device in manipulating waterborne pathogens to facilitate separation, and optimize the working parameters by studying the factors involved in operating the DEP device. The first study focused on Escherichia coli K12 while the second study focused on bacteriophage MS2 as a norovirus surrogate. The effect of flow rate, frequency, and electric field strength on separation were also studied to determine the optimal conditions for maximum separation. A millimeter-sized DEP device with a Y-junction channel was constructed using polydimethylsiloxane (PDMS) and titanium electrodes. A continuous flow of contaminated tap water into the device was achieved using a syringe pump. A voltage amplifier connected to a frequency generator was applied on the electrodes to produce different non-uniform electric field magnitudes in the main channel at varying frequencies. Samples were collected at the two outlet streams to determine the separation yield. The plate and plaque counting method were performed to compare the population of E. coli K12 and MS2, respectively. The highest separation efficiency obtained for E. coli K12 was found to be 47% and 42% under the experimental parameters of 40 V, 100 μl/min, and 1 MHz; and 60 V, 1 ml/min, and 1 MHz, respectively. On the other hand, the highest separation yield for MS2 was found to be 82% at the conditions of 20 V, 100 μl/min, and 100 kHz. The voltage requirement to create a suitable electric field strength was different for the two microbes. Bacteria in general are bigger than viruses, which then implies that a stronger DEP force is needed to move bacteria than viruses in the same magnitude. To generate a stronger DEP force, the voltage must be increased to amplify the electric field strength. The separation for both E. coli K12 and MS2 obtained their highest separation with a slow flow rate (100 μl/min), thus indicating that the microbes must be given ample exposure time to the electric field to enable the DEP force to manipulate its movement in the channel. The difference in frequency is also reasonable given that particles, cells, and microorganisms react to different frequencies because they all have unique dielectric properties. The two studies showed that a DEP device can be scaled up from micrometer sized channels to millimeter sized channels. Furthermore, the DEP device demonstrated its capability to separate bacteria and virus from tap water as proven by the experiments conducted on E. coli K12 and MS2, respectively. Lastly, separation efficiency was found to improve by optimizing voltage, flow rate, and frequency.