60 research outputs found
Reverse Osmosis Biofilm Dispersal by Osmotic Back-Flushing: Cleaning via Substratum Perforation
We have demonstrated the application
of osmotic back-flushing (OBF)
for the removal of biofilms from reverse osmosis (RO) membranes and
proposed a new biofilm dispersal mechanism. OBF was conducted in a
laboratory-scale RO test cell by introducing a sequence of hypersaline
solution (1.5 M NaCl) flushes into the feedwater, while still maintaining
the applied hydraulic pressure (13.8 bar). OBF resulted in significant
biofilm detachment, leaving a thin, perforated bacterial film (24
μm thickness) with vertical cavities ranging from 15 to 50 μm
in diameter. Application of OBF led to significant reductionin the
biovolume (70–79%) and substantial removal of total organic
carbon and proteins (78 and 66%, respectively), resulting in 63% permeate
water flux recovery. Our findings demonstrate the potential of this
chemical-free RO membrane cleaning method while highlighting the possible
challenges of the technique
Comparison of Energy Efficiency and Power Density in Pressure Retarded Osmosis and Reverse Electrodialysis
Pressure retarded osmosis (PRO) and
reverse electrodialysis (RED)
are emerging membrane-based technologies that can convert chemical
energy in salinity gradients to useful work. The two processes have
intrinsically different working principles: controlled mixing in PRO
is achieved by water permeation across salt-rejecting membranes, whereas
RED is driven by ion flux across charged membranes. This study compares
the energy efficiency and power density performance of PRO and RED
with simulated technologically available membranes for natural, anthropogenic,
and engineered salinity gradients (seawater–river water, desalination
brine–wastewater, and synthetic hypersaline solutions, respectively).
The analysis shows that PRO can achieve both greater efficiencies
(54–56%) and higher power densities (2.4–38 W/m<sup>2</sup>) than RED (18–38% and 0.77–1.2 W/m<sup>2</sup>). The superior efficiency is attributed to the ability of PRO membranes
to more effectively utilize the salinity difference to drive water
permeation and better suppress the detrimental leakage of salts. On
the other hand, the low conductivity of currently available ion exchange
membranes impedes RED ion flux and, thus, constrains the power density.
Both technologies exhibit a trade-off between efficiency and power
density: employing more permeable but less selective membranes can
enhance the power density, but undesired entropy production due to
uncontrolled mixing increases and some efficiency is sacrificed. When
the concentration difference is increased (i.e., natural →
anthropogenic → engineered salinity gradients), PRO osmotic
pressure difference rises proportionally but not so for RED Nernst
potential, which has logarithmic dependence on the solution concentration.
Because of this inherently different characteristic, RED is unable
to take advantage of larger salinity gradients, whereas PRO power
density is considerably enhanced. Additionally, high solution concentrations
suppress the Donnan exclusion effect of the charged RED membranes,
severely reducing the permselectivity and diminishing the energy conversion
efficiency. This study indicates that PRO is more suitable to extract
energy from a range of salinity gradients, while significant advancements
in ion exchange membranes are likely necessary for RED to be competitive
with PRO
Thermodynamic and Energy Efficiency Analysis of Power Generation from Natural Salinity Gradients by Pressure Retarded Osmosis
The Gibbs free energy of mixing dissipated when fresh
river water
flows into the sea can be harnessed for sustainable power generation.
Pressure retarded osmosis (PRO) is one of the methods proposed to
generate power from natural salinity gradients. In this study, we
carry out a thermodynamic and energy efficiency analysis of PRO work
extraction. First, we present a reversible thermodynamic model for
PRO and verify that the theoretical maximum extractable work in a
reversible PRO process is identical to the Gibbs free energy of mixing.
Work extraction in an irreversible constant-pressure PRO process is
then examined. We derive an expression for the maximum extractable
work in a constant-pressure PRO process and show that it is less than
the ideal work (i.e., Gibbs free energy of mixing) due to inefficiencies
intrinsic to the process. These inherent inefficiencies are attributed
to (i) frictional losses required to overcome hydraulic resistance
and drive water permeation and (ii) unutilized energy due to the discontinuation
of water permeation when the osmotic pressure difference becomes equal
to the applied hydraulic pressure. The highest extractable work in
constant-pressure PRO with a seawater draw solution and river water
feed solution is 0.75 kWh/m<sup>3</sup> while the free energy of mixing
is 0.81 kWh/m<sup>3</sup>î—¸a thermodynamic extraction efficiency
of 91.1%. Our analysis further reveals that the operational objective
to achieve high power density in a practical PRO process is inconsistent
with the goal of maximum energy extraction. This study demonstrates
thermodynamic and energetic approaches for PRO and offers insights
on actual energy accessible for utilization in PRO power generation
through salinity gradients
Adverse Impact of Feed Channel Spacers on the Performance of Pressure Retarded Osmosis
This article analyzes the influence of feed channel spacers
on
the performance of pressure retarded osmosis (PRO). Unlike forward
osmosis (FO), an important feature of PRO is the application of hydraulic
pressure on the high salinity (draw solution) side to retard the permeating
flow for energy conversion. We report the first observation of membrane
deformation under the action of the high hydraulic pressure on the
feed channel spacer and the resulting impact on membrane performance.
Because of this observation, reverse osmosis and FO tests that are
commonly used for measuring membrane transport properties (water and
salt permeability coefficients, <i>A</i> and <i>B</i>, respectively) and the structural parameter (<i>S</i>)
can no longer be considered appropriate for use in PRO analysis. To
accurately predict the water flux as a function of applied hydraulic
pressure difference and the resulting power density in PRO, we introduced
a new experimental protocol that accounts for membrane deformation
in a spacer-filled channel to determine the membrane properties (<i>A</i>,<i> B</i>, and <i>S</i>). PRO performance
model predictions based on these determined <i>A</i>, <i>B</i>, and <i>S</i> values closely matched experimental
data over a range of draw solution concentrations (0.5 to 2 M NaCl).
We also showed that at high pressures feed spacers block the permeation
of water through the membrane area in contact with the spacer, a phenomenon
that we term the shadow effect, thereby reducing overall water flux.
The implications of the results for power generation by PRO are evaluated
and discussed
Influence of Natural Organic Matter Fouling and Osmotic Backwash on Pressure Retarded Osmosis Energy Production from Natural Salinity Gradients
Pressure
retarded osmosis (PRO) has the potential to produce clean,
renewable energy from natural salinity gradients. However, membrane
fouling can lead to diminished water flux productivity, thus reducing
the extractable energy. This study investigates organic fouling and
osmotic backwash cleaning in PRO and the resulting impact on projected
power generation. Fabricated thin-film composite membranes were fouled
with model river water containing natural organic matter. The water
permeation carried foulants from the feed river water into the membrane
porous support layer and caused severe water flux decline of ∼46%.
Analysis of the water flux behavior revealed three phases in membrane
support layer fouling. Initial foulants of the first fouling phase
quickly adsorbed at the active-support layer interface and caused
a significantly greater increase in hydraulic resistance than the
subsequent second and third phase foulants. The water permeability
of the fouled membranes was lowered by ∼39%, causing ∼26%
decrease in projected power density. A brief, chemical-free osmotic
backwash was demonstrated to be effective in removing foulants from
the porous support layer, achieving ∼44% recovery in projected
power density. The substantial performance recovery after cleaning
was attributed to the partial restoration of the membrane water permeability.
This study shows that membrane fouling detrimentally impacts energy
production, and highlights the potential strategies to mitigate fouling
in PRO power generation with natural salinity gradients
Omniphobic Polyvinylidene Fluoride (PVDF) Membrane for Desalination of Shale Gas Produced Water by Membrane Distillation
Microporous membranes fabricated
from hydrophobic polymers such
as polyvinylidene fluoride (PVDF) have been widely used for membrane
distillation (MD). However, hydrophobic MD membranes are prone to
wetting by low surface tension substances, thereby limiting their
use in treating challenging industrial wastewaters, such as shale
gas produced water. In this study, we present a facile and scalable
approach for the fabrication of omniphobic polyvinylidene fluoride
(PVDF) membranes that repel both water and oil. Positive surface charge
was imparted to an alkaline-treated PVDF membrane by aminosilane functionalization,
which enabled irreversible binding of negatively charged silica nanoparticles
(SiNPs) to the membrane through electrostatic attraction. The membrane
with grafted SiNPs was then coated with fluoroalkylsilane (perfluorodecyltrichlorosilane)
to lower the membrane surface energy. Results from contact angle measurements
with mineral oil and surfactant solution demonstrated that overlaying
SiNPs with ultralow surface energy significantly enhanced the wetting
resistance of the membrane against low surface tension liquids. We
also evaluated desalination performance of the modified membrane in
direct contact membrane distillation with a synthetic wastewater containing
surfactant (sodium dodecyl sulfate) and mineral oil, as well as with
shale gas produced water. The omniphobic membrane exhibited a stable
MD performance, demonstrating its potential application for desalination
of challenging industrial wastewaters containing diverse low surface
tension contaminants
The Critical Need for Increased Selectivity, Not Increased Water Permeability, for Desalination Membranes
Desalination
membranes are essential for the treatment of unconventional
water sources, such as seawater and wastewater, to alleviate water
scarcity. Promising research efforts on novel membrane materials may
yield significant performance gains over state-of-the-art thin-film
composite (TFC) membranes, which are constrained by the permeability–selectivity
trade-off. However, little guidance currently exists on the practical
impact of such performance gains, namely enhanced water permeability
or enhanced water–solute selectivity. In this critical review,
we first discuss the performance of current TFC membranes. We then
highlight and provide context for recent module-scale modeling studies
that have found limited impact of increased water permeability on
the efficiency of desalination processes. Next we cover several important
examples of water treatment processes in which inadequate membrane
selectivity hinders process efficacy. We conclude with a brief discussion
of how the need for enhanced selectivity may influence the design
strategies of future membranes
Elucidating the Role of Oxidative Debris in the Antimicrobial Properties of Graphene Oxide
In
this paper, we investigate, for the first time, how oxidative
debris affects the antimicrobial activity of graphene oxide (GO).
Besides the conventional definition of GO structure, our study demonstrates
that GO is also composed of one additional component called oxidative
debris, small and highly oxidized fragments adsorbed on the GO surface.
After the removal of oxidative debris using an alkaline washing process,
the toxicity of GO sheets to <i>Escherichia coli</i> cells
significantly decreased. Raman spectroscopy measurements and acellular
oxidation of glutathione indicated that oxidative debris increase
the antimicrobial activity of GO sheets by improving their ability
to promote bacteria inactivation via cell membrane damage and oxidative
stress mechanisms. Given the influence of oxidative debris on the
antimicrobial activity of GO, our findings emphasize the need to investigate
the presence of oxidative debris before establishing correlations
between physicochemical properties and the bioreactivity of GO sheets
Module-Scale Analysis of Pressure Retarded Osmosis: Performance Limitations and Implications for Full-Scale Operation
We investigate the performance of
pressure retarded osmosis (PRO)
at the module scale, accounting for the detrimental effects of reverse
salt flux, internal concentration polarization, and external concentration
polarization. Our analysis offers insights on optimization of three
critical operation and design parametersî—¸applied hydraulic
pressure, initial feed flow rate fraction, and membrane areaî—¸to
maximize the specific energy and power density extractable in the
system. For co- and counter-current flow modules, we determine that
appropriate selection of the membrane area is critical to obtain a
high specific energy. Furthermore, we find that the optimal operating
conditions in a realistic module can be reasonably approximated using
established optima for an ideal system (i.e., an applied hydraulic
pressure equal to approximately half the osmotic pressure difference
and an initial feed flow rate fraction that provides equal amounts
of feed and draw solutions). For a system in counter-current operation
with a river water (0.015 M NaCl) and seawater (0.6 M NaCl) solution
pairing, the maximum specific energy obtainable using performance
properties of commercially available membranes was determined to be
0.147 kWh per m<sup>3</sup> of total mixed solution, which is 57%
of the Gibbs free energy of mixing. Operating to obtain a high specific
energy, however, results in very low power densities (less than 2
W/m<sup>2</sup>), indicating that the trade-off between power density
and specific energy is an inherent challenge to full-scale PRO systems.
Finally, we quantify additional losses and energetic costs in the
PRO system, which further reduce the net specific energy and indicate
serious challenges in extracting net energy in PRO with river water
and seawater solution pairings
Thin-Film Composite Polyamide Membranes Functionalized with Biocidal Graphene Oxide Nanosheets
Fouling
of membranes by microorganisms is a major limiting factor
in membrane separation processes. Novel strategies are therefore required
to decrease the extent of bacterial growth on membranes. In this study,
we confer strong antimicrobial properties to thin-film composite polyamide
membranes by a simple graphene oxide surface functionalization. Using
amide coupling between carboxyl groups of graphene oxide and carboxyl
groups of the polyamide active layer, graphene oxide is irreversibly
bound to the membrane. Surface binding of graphene oxide is demonstrated
by scanning electron microscopy and Raman spectroscopy. Direct contact
of bacteria with functionalized graphene oxide on the membrane surface
results in 65% bacterial inactivation after 1 h of contact time. This
bactericidal effect is imparted to the membrane without any detrimental
effect to the intrinsic membrane transport properties. Our results
suggest that functionalization of thin-film composite membranes with
graphene oxide nanosheets is a promising approach for the development
of novel antimicrobial membranes
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