Summary
This thesis is part of the Sewer Mining project
aimed at developing a new technological concept
to extract water from wastewater by means of
forward osmosis (FO), a novel membrane
technology. In general, greater than 99.93 % of
municipal wastewater is composed of water. If the
water can be separated from the solids (suspended
and dissolved), it can be reused, alleviating the
global water stress that currently exists.
FO is driven by osmosis and therefore differs
from other membrane processes which depend on
hydraulic pressure. FO, in combination with a
reconcentration system, e.g. reverse osmosis (RO)
is used to recover high-quality water for use in
industrial processes. Furthermore, the subsequent
concentrated wastewater (containing an inherent
energy content) can be converted into a renewable
energy source, i.e. biogas, for further use in the
system.
FO, incorporated in sewer mining applications
shows great potential, as it could lead to a more
economical and sustainable treatment of
wastewater, but before it can reach full-scale
feasibility, several research questions need to be
addressed.
Efforts to address these pending questions
culminated into this thesis. The research approach
consisted of:
Inventory of existing knowledge on FO,
specifically relating to wastewater, via data
collection from scientific literature and other
sources;
Characterisation of wastewater (primary
effluent) from wastewater treatment plants, to
assess and analyse fouling properties on the FO
membrane;
Experimental investigations on lab-scale (Utube,
cross-flow) and pilot-scale;
Validation of experimental work via existing
and newly developed FO transport models,
coupled to a technical economic model.
The major topics in this thesis, which cover
limitations experienced by FO processes during
wastewater applications, are summarised below.
Solute leakage
The draw solution (osmotic agent) is the driving
force in FO processes. Transport of draw solutes
through the membrane, i.e. via reverse solute
leakage, can pose substantial limitations to the
implementation of FO processes, lowering the
driving force and therefore the flux performance,
while recovery of the draw solution (in closedloops)
is also financially limiting.
Several alternative solutes as draw solutions
were systematically investigated on lab-scale to
enhance the FO performance and minimise the
solute loss. The highly soluble zwitterions:
glycine, L-proline, glycine betaine and the
anthropogenic amino acid, EDTA, demonstrated
comparable water fluxes to NaCl (5 L/m2h), but
with significantly lower solute losses, which is
advantageous for cost reduction. The physicochemical
properties, charge and size of the solutes
all played dominant roles in the flux efficiencies.
The FO mass transfer model furthermore verified
the experimental investigations of the solute
transport through the membrane. The use of these
draw solutions in FO for wastewater reclamation
applications also showed the benefits of the solute
leakage, in terms of energy production (biogas)
and reduction of the reconcentration costs for the
process.
Membrane Fouling
Membrane fouling concerns a process whereby an
accumulation of solutes and/or particles exists on
a membrane surface, within the membrane pores
or within the feed spacer channel. The permeate
quality and quantity of the process is subsequently
limited. Fouling has been reported to have only a
marginal effect on FO membranes, due to the lack
of hydraulic pressure. This thesis employed raw
wastewater to test the extent of fouling.
The effects of fouling on the surface
characteristics and operational conditions of FO
membranes were investigated on lab-scale. FO
treated wastewater resulted in the formation of a
fouling layer on the investigated membrane,
causing an 18 % water flux decline compared to
the baseline study. Surface properties and
rejection behaviours of virgin, fouled and
mechanically-cleaned membranes were further
compared. Interms of surface charge analyses,
fouling was found to increase the negative charge
of the membrane surface, while contact angle
measurements established an increase in
hydrophilicity compared to the virgin membrane.
The surface tensions of the cleaned membrane
differed slightly from the virgin membrane,
confirming the presence of foulant attachment on
the membrane, which may have led to irreversible
fouling. ATP measurements determined high
concentrations of active bacteria in the fouling
layer (70.9 ng ATP/cm2), while the carbohydrate
analyses, Fourier transform infrared spectroscopy
(FTIR) and liquid chromatography (LC-OCD)
ascertained the existence of polysaccharides (3.3
mg glucose/cm2), the main composition of
extracellular polymeric substances (EPS).
Biopolymers (more specifically, polysaccharides)
were found to be the main cause of fouling on the
FO membrane.
Flux enhancement
In the FO process, internal concentration
polarisation (ICP) within the porous layer is
considered a major problem, reducing the water
flux and increasing reverse solute transport. Flux
optimisation can be carried out by improving
membrane properties, i.e. designing thinner, more
porous and less tortuous support layers to reduce
ICP or varying process-related properties, e.g.
temperature and flow conditions. Improved flux
performance will allow FO to compete with fluxes
achieved by hydraulically driven membrane
processes. During this thesis, the concept of
pressure assisted osmosis (PAO) was developed.
PAO, an FO process involving the use of
hydraulic pressure on the set-up feed side, was
proposed to enhance FO performance. An FO
mass transport model (active layer to feed side
orientation) incorporating pressure was developed
to describe the fluxes in PAO. Continuous and
discontinuous PAO operations (0.1 – 0.8 bar) on
laboratory scale were evaluated using draw
solutions equivalent to 24 bar. The fluxes
increased with increasing hydraulic feed pressures
for all PAO experiments, including activated
sludge feeds, owing to the increased driving force
and membrane deformation. Discontinuous PAO
was found to have an adverse effect on the salt
fluxes, due to the occurrence of hydraulic back
pressure. This study emphasized the benefits of
PAO using diverse feeds, while illustrating the
importance of developing more rigid membranes
and better support designs.
Feasibility of a sewer mining concept
Closed-loop FO differs from osmotic dilution/
concentration, in that the draw solution is recycled
and reused by the process. The latter process tends
to be more economically feasible as no recovery
step is required, reducing the energy cost. It is
therefore more often applied in practice. If the
energy consumption of the recovery step could be
reduced, closed-loop FO would become a more
feasible technology. Sewer mining allows for
energy generation from wastewater which can be
applied in the recovery step.
In this thesis, a technical economic model
(TEM) was developed to describe the economic
aspects of a general FO-RO process and more
particularly for sewer mining concepts. The TEM
was based on the FO mass transfer model and a
mass transfer model for larger FO membrane
installations combined with RO. As such, the total
cost pertaining to the treatment of wastewater for
use in industry was also determined.
The total treatment cost of the process,
including capital and operational costs, was
determined to be 0.65 €/m3 with the FO
membrane cost significantly influencing the price.
Despite some restrictions of the TEM model,
the Sewer Mining concept was found to be
economically feasible when compared to fullscale
water treatment (seawater desalination < 1
€/m3). Further viability will increase if future FO
membranes are optimised to reduce leakage,
increase fluxes and become more economical.
Water scarcity is a global problem and waste
accumulation is a steadily growing one. By
implementing this green, self-sufficient FO
technology to extract water and energy from
wastewater, this thesis has attempted to contribute
to changing the way wastewater is perceived: not
as waste, but as a resource. In this way, water
which we use today can be reused for generations
to come.
SummaryThis thesis is part of the Sewer Mining projectaimed at developing a new technological conceptto extract water from wastewater by means offorward osmosis (FO), a novel membranetechnology. In general, greater than 99.93 % ofmunicipal wastewater is composed of water. If thewater can be separated from the solids (suspendedand dissolved), it can be reused, alleviating theglobal water stress that currently exists.FO is driven by osmosis and therefore differsfrom other membrane processes which depend onhydraulic pressure. FO, in combination with areconcentration system, e.g. reverse osmosis (RO)is used to recover high-quality water for use inindustrial processes. Furthermore, the subsequentconcentrated wastewater (containing an inherentenergy content) can be converted into a renewableenergy source, i.e. biogas, for further use in thesystem.FO, incorporated in sewer mining applicationsshows great potential, as it could lead to a moreeconomical and sustainable treatment ofwastewater, but before it can reach full-scalefeasibility, several research questions need to beaddressed.Efforts to address these pending questionsculminated into this thesis. The research approachconsisted of: Inventory of existing knowledge on FO,specifically relating to wastewater, via datacollection from scientific literature and othersources; Characterisation of wastewater (primaryeffluent) from wastewater treatment plants, toassess and analyse fouling properties on the FO
membrane;
Experimental investigations on lab-scale (Utube,
cross-flow) and pilot-scale;
Validation of experimental work via existing
and newly developed FO transport models,
coupled to a technical economic model.
The major topics in this thesis, which cover
limitations experienced by FO processes during
wastewater applications, are summarised below.
Solute leakage
The draw solution (osmotic agent) is the driving
force in FO processes. Transport of draw solutes
through the membrane, i.e. via reverse solute
leakage, can pose substantial limitations to the
implementation of FO processes, lowering the
driving force and therefore the flux performance,
while recovery of the draw solution (in closedloops)
is also financially limiting.
Several alternative solutes as draw solutions
were systematically investigated on lab-scale to
enhance the FO performance and minimise the
solute loss. The highly soluble zwitterions:
glycine, L-proline, glycine betaine and the
anthropogenic amino acid, EDTA, demonstrated
comparable water fluxes to NaCl (5 L/m2h), but
with significantly lower solute losses, which is
advantageous for cost reduction. The physicochemical
properties, charge and size of the solutes
all played dominant roles in the flux efficiencies.
The FO mass transfer model furthermore verified
the experimental investigations of the solute
transport through the membrane. The use of these
draw solutions in FO for wastewater reclamation
applications also showed the benefits of the solute
leakage, in terms of energy production (biogas)
and reduction of the reconcentration costs for the
process.
Membrane Fouling
Membrane fouling concerns a process whereby an
accumulation of solutes and/or particles exists on
a membrane surface, within the membrane pores
or within the feed spacer channel. The permeate
quality and quantity of the process is subsequently
limited. Fouling has been reported to have only a
marginal effect on FO membranes, due to the lack
of hydraulic pressure. This thesis employed raw
wastewater to test the extent of fouling.
The effects of fouling on the surface
characteristics and operational conditions of FO
membranes were investigated on lab-scale. FO
treated wastewater resulted in the formation of a
fouling layer on the investigated membrane,
causing an 18 % water flux decline compared to
the baseline study. Surface properties and
rejection behaviours of virgin, fouled and
mechanically-cleaned membranes were further
compared. Interms of surface charge analyses,
fouling was found to increase the negative charge
of the membrane surface, while contact angle
measurements established an increase in
hydrophilicity compared to the virgin membrane.
The surface tensions of the cleaned membrane
differed slightly from the virgin membrane,
confirming the presence of foulant attachment on
the membrane, which may have led to irreversible
fouling. ATP measurements determined high
concentrations of active bacteria in the fouling
layer (70.9 ng ATP/cm2), while the carbohydrate
analyses, Fourier transform infrared spectroscopy
(FTIR) and liquid chromatography (LC-OCD)
ascertained the existence of polysaccharides (3.3
mg glucose/cm2), the main composition of
extracellular polymeric substances (EPS).
Biopolymers (more specifically, polysaccharides)
were found to be the main cause of fouling on the
FO membrane.
Flux enhancement
In the FO process, internal concentration
polarisation (ICP) within the porous layer is
considered a major problem, reducing the water
flux and increasing reverse solute transport. Flux
optimisation can be carried out by improving
membrane properties, i.e. designing thinner, more
porous and less tortuous support layers to reduce
ICP or varying process-related properties, e.g.
temperature and flow conditions. Improved flux
performance will allow FO to compete with fluxes
achieved by hydraulically driven membrane
processes. During this thesis, the concept of
pressure assisted osmosis (PAO) was developed.
PAO, an FO process involving the use of
hydraulic pressure on the set-up feed side, was
proposed to enhance FO performance. An FO
mass transport model (active layer to feed side
orientation) incorporating pressure was developed
to describe the fluxes in PAO. Continuous and
discontinuous PAO operations (0.1 – 0.8 bar) on
laboratory scale were evaluated using draw
solutions equivalent to 24 bar. The fluxes
increased with increasing hydraulic feed pressures
for all PAO experiments, including activated
sludge feeds, owing to the increased driving force
and membrane deformation. Discontinuous PAO
was found to have an adverse effect on the salt
fluxes, due to the occurrence of hydraulic back
pressure. This study emphasized the benefits of
PAO using diverse feeds, while illustrating the
importance of developing more rigid membranes
and better support designs.
Feasibility of a sewer mining concept
Closed-loop FO differs from osmotic dilution/
concentration, in that the draw solution is recycled
and reused by the process. The latter process tends
to be more economically feasible as no recovery
step is required, reducing the energy cost. It is
therefore more often applied in practice. If the
energy consumption of the recovery step could be
reduced, closed-loop FO would become a more
feasible technology. Sewer mining allows for
energy generation from wastewater which can be
applied in the recovery step.
In this thesis, a technical economic model
(TEM) was developed to describe the economic
aspects of a general FO-RO process and more
particularly for sewer mining concepts. The TEM
was based on the FO mass transfer model and a
mass transfer model for larger FO membrane
installations combined with RO. As such, the total
cost pertaining to the treatment of wastewater for
use in industry was also determined.
The total treatment cost of the process,
including capital and operational costs, was
determined to be 0.65 €/m3 with the FO
membrane cost significantly influencing the price.
Despite some restrictions of the TEM model,
the Sewer Mining concept was found to be
economically feasible when compared to fullscale
water treatment (seawater desalination < 1
€/m3). Further viability will increase if future FO
membranes are optimised to reduce leakage,
increase fluxes and become more economical.
Water scarcity is a global problem and waste
accumulation is a steadily growing one. By
implementing this green, self-sufficient FO
technology to extract water and energy from
wastewater, this thesis has attempted to contribute
to changing the way wastewater is perceived: not
as waste, but as a resource. In this way, water
which we use today can be reused for generations
to come.
การแปล กรุณารอสักครู่..
