Emerging water treatment technology

1) Membrane filtration: Ultrafiltration technology is an important kind of

membrane separation technology. Membrane separation technology refers to
the technology of separation, classification, purification and enrichment of
two-component or multi-component mixed liquid or gas by means of the
promotion of external energy or chemical potential difference and the
permeation of specific memb
 Types of membrane filtration : According to the difference of pore
size (or molecular weight retained), membrane can be divided into
micro filtration (MF), nano filtration (NF), ultra filtration (UF) and
reverse osmosis (RO)
a) . Microfiltration membranes
is called fine filtration or screen mesh filtration.The pore diameter of
microfiltration membrane is generally between 0.01-10 μ M the
particles such as solvent, salt, water and macromolecular substances
which are smaller than the pore diameter of the membrane will pass
through the membrane, while the micro particles and macromolecular
substances such as some bacteria which are larger than the pore
diameter of the membrane will be intercepted by the membrane, so as
to achieve the separation effect. It is mainly used in liquid clarification,
sterilization in industry.
b) Ultrafiltration membrane
the size of membrane pore size and membrane surface properties have
different retention effect. The pore diameter of the membrane is
generally 2-100nm. solvents or substances with small molecular
weight can penetrate the membrane, while macromolecules and fine
particles will be intercepted, so as to achieve the separation effect. It is
mainly used in the purification and separation of solutions containing
macromolecules and colloidal substances
 c) . Nanofiltration membrane
:Nanofiltration is also called ultra-low pressure reverse osmosis. The pore diameter of
nanofiltration membrane is about 1nm. substances with molecular weight less than
200 such as water and solvent will pass through the membrane, while substances with
molecular weight of 200-1000 such as solute, divalent salt, sugar and dye will be
retained, so as to achieve the separation effect. The separation performance is between
microfiltration and ultrafiltration, and the working principle of both is considered. It is
mainly used for concentration and purification of macromolecular substances in
solution.

d) . Reverse osmosis membrane

Reverse osmosis is a membrane separation process which is driven by the pressure
difference to complete the separation task through the semi permeable membrane.
When the pressure (100010000mpa) applied on the upper side of the solution is
greater than the osmotic pressure of the solution, the solvent molecules reverse the
concentration gradient (opposite to the direction of natural osmosis) under the
pressure difference, so as to obtain the penetrating solvent on the low-pressure side of
the membrane and the concentrated solvent on the high-pressure side A condensed
solution.Most of them are used for desalination of seawater. Fresh water is obtained at
the low pressure side of the membrane and brine is obtained at the high pressure side.
1. Application of membrane separation technology for water treatment
1. Drinking water
purification process In the traditional water production process, chlorine is often used
for disinfection. Research shows that chlorine can cause problems such as difficult to
kill chlorine resistant pathogens, increase the possibility of carcinogen trihalomethane
(THMs) formation, and produce chemical sludge. However, ultrafiltration technology
can effectively overcome the limitations of the traditional process, provide high-
quality drinking water, reduce turbidity and reduce the use of flocculants. However,
when it is used alone, it is easy to cause membrane pollution and insufficient ability to
deal with ammonia nitrogen, metal ions or some small molecular organics. At present,
people combine ultrafiltration membrane technology with traditional technology to
improve purification capacity and reduce membrane pollution. Now scholars at home
and abroad focus on the combination process of powdered activated carbon
ultrafiltration and coagulation ultrafiltration. Coagulation ultrafiltration combined
with pre membrane coagulation can form flocs from small molecular organics, and
then use ultrafiltration membrane to intercept flocs and remove small molecules and
macromolecular compounds in water.
2. . Treatment of industrial wastewater
Industrial wastewater has the characteristics of complex composition, great change of
water quality and strong toxic effect.The purpose of wastewater treatment is to
remove the pollutants and make them meet the requirements of discharge or recovery.
Meanwhile, the separated wastewater should be fully resourced or properly treated.In
the face of complex composition and clear treatment objectives, single membrane
method or traditional technology is difficult to meet the treatment requirements, and
the integrated membrane method combining traditional treatment methods and
membrane method is often used .The research direction of integrated membrane
technology includes ceramic microfiltration membrane technology and nanofiltration
membrane technology. Ceramic microfiltration membrane has good heat resistance
and can be used stably at 400 ℃ it has high chemical stability and can resist organic
solvent and acid-base corrosion; it has stable mechanical structure and strong anti
pollution performance, can be cleaned and regenerated, and can adapt to the harsh
environment of industrial wastewater treatment. Taking the removal of chroma from
printing and dyeing wastewater as an example: Zhang Yi et al. Treated PVA desizing
wastewater with dynamic ceramic membrane with pore diameter < 1 μ m, and finally
obtained the water quality meeting the secondary standard of discharge standard of
water pollutants for textile dyeing and finishing industry . Nanofiltration technology
has the advantages of low operating pressure, easy operation and management, and no
need for additional chemical reagents in the treatment process, which will not cause
secondary pollution. Take the textile industry wastewater treatment as an example:
Zheng Yue and others used nanofiltration membrane (Dow nf90) to treat the textile
wastewater treated by activated sludge. Compared with ozone oxidation method, it
shows that the combination of nanofiltration method and ozone oxidation method can
get ideal results and solve the problem of high cost of ozone oxidation method alone.
 3. Desalination Seawater
desalination is one of the important means to solve the shortage of water resources.
The methods of seawater desalination include reverse osmosis (RO), multi-stage flash
evaporation (MED) and compressed air distillation (VC). Among these methods,
reverse osmosis technology has the advantages of low investment cost, low energy
consumption and short construction period. It has become the most economical means
of desalination. The reverse osmosis membrane method fordesalination in power
plants can be generally divided into three processes: Seawater Pretreatment and
primary and secondary reverse osmosis . In the process of seawater reverse osmosis
desalination, in order to make the reverse osmosis membrane fully play its role, it is
necessary to meet certain water inflow requirements, so the seawater must be
pretreated. Usually, coagulation sedimentation and filtration technology are used to
remove suspended solids, particles, bacteria and other magazines from seawater, so as
to reduce the pollution on the surface of reverse osmosis membrane and improve the
service life of the system. The primary reverse osmosis system uses the characteristics
of the reverse osmosis membrane to remove most of the organic matters, soluble salts,
primary colloids and other substances in the sea water. Most of the fresh water
obtained from the primary reverse osmosis system enters the secondary reverse
osmosis system for further treatment to obtain fresh water that meets the standard of
drinking water, so as to meet the daily operation needs of the power plant and the
needs of people's living water.
Advantage and disadvantage of membrane filltration
i. Membrane separation technology can effectively intercept pollutants, bacteria
and pathogenic bacteria. Compared with conventional water treatment
technology, it has the advantages of high quality, stability and safety. Experts
even call the development of membrane separation technology "the third
industrial revolution". The importance of the research and development of
membrane separation technology can be seen.
ii. Membrane separation technology, as a new technology rising rapidly after
1960s, can be seen from the application of membrane separation technology in
drinking water purification, industrial wastewater treatment and desalination.
Compared with traditional technology and other technologies, membrane
separation technology has obvious advantages and good development
prospects. However, the application of membrane separation technology in
water treatment is not a simple alternative to the traditional process, but a
process of integrated upgrading and development. At present, a series of
problems, such as high cost, membrane pollution, poor working conditions
and membrane separation performance, to be improved hinder the further
development and application of membrane technology. Therefore, the future
development direction of membrane separation technology in water treatment
should focus on the following aspects. First, this technology focus on the
research of membrane pollution so as to improve the service life of the
membrane. Second, this technology focus on the development of new material
membrane with stable structure and strong environmental adaptability similar
to ceramic microfiltration membrane to cope with water treatment in extreme
conditions. Third, adhere to membrane technology and traditional treatment
technology combined with traditional technology, pretreatment can reduce
membrane pollution, increase membrane service life and reduce cost.
2) *Advanced oxidation proccess water treatment*:(Aops)
Commonly referred to as AOPs, Advanced oxidation processes are used to
oxide complex organic contaminants that are found in wastewater and that are
difficult to degrade into simpler end products through biological processes
advanced oxidation processes (AOPs) are among the most frequently used
approaches to remove pollutants that have low biodegradability or high
chemical stability. These methods is depend on the generation of hydroxyl free
radical (HO*) as a strong oxidant for the destruction of compounds which
cannot be oxidized using conventional oxidants
2. Theory of Advanced Oxidation
The basic principle of advanced oxidation processes entails the generation of
hydroxyl free radical (HO*), non-selective chemical oxidant , as a strong
oxidant for destroying organic compounds which cannot be oxidized by
conventional oxidants such as ozone, oxygen and chlorine . Hydroxyl radicals
are effective in the destruction of organic chemicals due to the fact they are
reactive electrophiles which not only react rapidly, but also non-selectively
with almost all organic compounds that are electron-rich [. Their oxidation
potential is quantified as 2.80V and this makes them to exhibit faster rates of
oxidation reactions compared to conventional oxidations . Once the hydroxyl
radicals are generated, they are able to attack organic chemicals through
electron transfer, hydrogen abstraction and radical combination .
3. Classification of AOPs
There are several methods that are classified under the broad definition of
advanced oxidation processes. Most of these methods combine a strong
oxidizing agent such as H2O2 or O3 with a catalyst such as transition metal
ions and irradiation such UV. Evidence from literature shows titanium
dioxide/UV light process, Fenton’s reactions and hydrogen peroxide/UV light
as the most popular AOP producing hydroxyl radicals . Various AOPs are
classified as being either heterogeneous or homogeneous. The latter are
usually further subdivided on the basis of the energy
A. Homogenous AOPs that use UV radiation are normally used for
degrading compounds which absorb UV radiation within the
corresponding range of the spectrum. The compounds absorbing UV
light at lower wavelengths render themselves suitable for such kind of
photodegradation process.
 a. Photolytic Ozonation (O₃/UV) O₃/UV is an advance water treatment
method which is effectively used to oxidize and destroy the toxic
organic compounds in waste water. Typically, aqueous systems
saturated with ozone are irradiated with UV light of 254 nm. It was
found that the O₃/UV is so effective for the destruction of
chlorophenols (CPs) than UV photolysis and ozonation . However it's
founded that photolytic ozonation is only more effective than
ozonation alone in some cases.
b. Hydrogen Peroxide and Ultraviolet Radiation (H2O2/UV)
In this advanced oxidation process, hydroxyl radicals are formed and
generated by photolysis of H2O2 and a number of propagation
reactions that are corresponding to the process. The process of
photolysis of hydrogen peroxide occurs once UV radiation is applied
and is a shown in the following equation: H2O2 + hv → 2HO*
This method requires a much longer UV exposure and/ or a relatively
high dose of H2O2. Various studies showed that the rate of photolysis
of hydrogen peroxide is largely PH dependent and tends to increase
with increase in alkaline conditions . UV lamps with output that is at
lower wavelengths may be used for increasing the molar absorptivity
of hydrogen peroxide which is low at 253.7nm. One of the drawbacks
of this method is that in situations where the water to be treated has a
higher absorbance, there is a tendency for it to compete with the
hydrogen peroxide for radiation . The H2O2/UV system has the ability
to completely mineralize any organic compound, with the products
being water and CO2. However, this process is usually not necessary
as the toxicity of oxidation products is not a problem due to the fact
that their degradation is quite easy. Addition of hydrogen peroxide may
be either as multiple points in the system or as a single slug dose.
: Schematic flow diagram of Hydrogen peroxide and UV radiation advanced

oxidation process UV – Photolysis

 The process of direct photolysis involves the interaction of light with
molecules (in addition to water), to enable the molecules to be dissociated into
fragments through the mechanistic pathway shown below: R+ hv →
Intermediates……. Intermediates + hv → CO2 + H2O + R

4. Advantage of this method:

5. H2O2 is highly soluble and may therefore be added to the source water at high
concentration and the fact that H2O2/UV processes are able to generate larger
amounts of hydroxyl radicals compared to O3/UV for equal amounts of energy
used.

6. disadvantages of the method are

 that it is expensive due to the additional costs of necessary devices and

energy requirements and the fact that the presence of residual
hydrogen peroxide in the treated effluent tends to promote biological
re-growth within the distribution systemMajor disadvantages of the
method are that it is expensive due to the additional costs of necessary
devices and energy requirements and the fact that the presence of
residual hydrogen peroxide in the treated effluent tends to promote
biological re-growth within the distribution system
 The process is less effective compared to other processes in which
radiation is combined with ozone or hydrogen peroxide, or
wherehomogeneous, heterogeneous photocatalysis or catalysis are
employed.

C.Fenton and Photo Fenton’s Oxidation

Discovered by Henry J.H. Fenton, the Fenton reagent is a mixture of
iron (II) salt and hydrogen peroxide. Fenton described the oxidation
power of hydrogen peroxide on various organic molecules in which
OH- radicals are produced from H2O2 through addition of Fe(II) as
the catalystIn order to enhance the degradation velocity of organic
pollutants, UV-visible light at wavelengths that are greater than 300nm
is used in the reaction. The process differs from the Fenton process in
that photolysis of iron (III) complexes enables regeneration of iron
(II), and this can further react with more hydrogen peroxide
The process is advantageous compared to the Fenton process in that it
reduces the formation of sludge waste that occurs in the Fenton
process.
 Advantage of this method
i. The system developed by Fenton is viewed as the most
promising treatment among AOPs for remediation of waters
 that are highly contaminated. Due to its simplicity, the Fenton
reaction is the process most often applied in situations where it
becomes necessary to remove recalcitrant compounds.

ii. The Fenton and Photo Fenton processes include lower energy
requirements compared to other technologies utilizing ozone or
UV radiation, lack of vapor emissions and hence no air permits
required, lack of mass transfer limitations due to the fact that
reactions take place in the homogeneous phase and the fact that
the process is carried out at room pressure and temperature.

 Disadvantage of this method

i. The major drawback of the process is that it leads to the production of iron
sludge waste. To eliminate this drawback, the photo-Fenton process was
developed and it uses solar light or UV for reducingFe(III) oxalate back to
Fe(II) oxalate and this results in drastic reduction in sludge waste. The extent
of mineralization achieved by the Fenton process is around 60-70%

ii. Iron extraction system is required for the removal of residual iron, a very low
pH (less than 4) is needed to keep iron in the solution and the use of artificial
light for the photo-Fenton process requires additional energy consumption for
running the lamps.

d.Electro Fenton
An alternative advanced oxidation process is the electro Fenton method also
referred to as the EF technology. The technology is based on the continuous
electro generation of H2O2 at a suitable cathode that is fed with O2 or air,
with the addition of an iron catalyst to the treated solution for the production
of OH* at the bulk through a Fenton reaction [49]. The advantages of the
method include on-site production of H2O2 and higher degradation rate of
organic pollutants as a result of the continuous regeneration of Fe2+ at the
cathode, which also results to the minimization of sludge production

E.Anodic Oxidation Another advanced oxidation process

that one may use for the treatment of waste water is the anodic oxidation ,
anodic oxidation refers to an accelerated electrochemical process that is
intensified by the natural oxide skin of the aluminium. With time, the
transparent oxide layer becomes considerably thicker compared to the natural
oxide layer. A significant advantage of using the process is the long term
protection and the strong resistance to corrosion

Heterogeneous Advanced Oxidation Processes

the heterogeneous AOPs, catalysts are used for carrying out compounds’
degradation. The term ―heterogeneous‖ refers to the fact that there is
presence of contaminants in the aqueous phase while the catalyst is usually in
the solid phase. The latter results in the acceleration of chemical processes due
to the presence of electron-hole pairs. The photogenerated electrons and holes
result in oxidation and reduction processes respectively. For aqueous
solutions, oxidization of water molecules absorbed to the catalyst results in
OH- radicals. The process is normally implemented in aerobic conditions with
the species to be reduced being oxygen, a process which generates a
superoxide radical
 Types
1. *Photocatalysts*
For a photocatalyst to be perceived as good, it should be
biologically and chemically inert, photoactive, able to utilize
visible or near UV light inexpensive, photostable and non-
toxic. Examples of catalysts include Si, ZnO, WO3, TiO2,
CdS, SrTiO3, ZnS, SnO2, Fe2O3 Wse2 . The most commonly
used of the catalysts is Titanium
2. Titanium Oxide One of the most abundant elements on the
earth’s crust (ninth), titanium is found in three possible
crystalline forms in its most stable form as an oxide. The forms
include rutile, anatase, and brookite. From a photocatalytic
view, only anatase and rutile are relevant, with the latter having
the highest catalytic activity . Fundamentally, the process takes
place through TiO2 producing pairs of electrons and holes
through absorption of UV radiation, light source or sunlight.
The electrons become excited once illuminated with the excess
energy being used to promote the electron to the conduction
band of titanium dioxide, thus, leading to the creation of
positive hole and negative electron. operating parameters for
the process include amount of the catalyst, initial concentration
of the reactant and effect of pH . The amount of catalyst
concentration to be used should be up to the optimum value.
Use of excess catalyst leads to the reduction in the amount of
photo-energy that is being transferred in the medium as a result
of the opacity that is offered by the catalyst particles. The
optimum value of the catalyst to be used is dependent on the
concentration and type of pollutant, in addition to the rate at
which free radicals are generated. For effluents that are highly
concentrated, no destruction is observed and dilution is usually
essential. The manner in which pH affects rates of
photocatalytic oxidation is complicated, with the observed
effect being generally dependent on the zero-point charge of the
semiconductor use and the type of pollutant. Generally
speaking, the adsorption of the pollutant, and therefore the rates
of degradation will be maximum near the zero-point charge
 Application of Aops
AOPs Studies have shown AOPs to be more effective than any of the
individual agents (for example, hydrogen peroxide, UV or Ozone).
AOPs are normally applied to wastewaters with low COD due to the
low cost of H2O2 and/ or ozone required for the generation of
hydroxyl radicals. Materials that were previously resistant to
degradation are now able to be transformed into compounds requiring
further biological treatment. Application of AOPs is disinfection of
treated wastewater and treatment of refractory organic compounds .
The reasoning behind using AOPs for disinfection is that free radicals
that are generated from ozone are more powerful oxidants than ozone
alone and this makes them effective for oxidization of microorganisms
and refractory organic materials in wastewater. However, the fact that
the half-life of the hydroxyl free radicals is short and as a result it is
impossible to achieve high concentrations. Extremely low
concentrations imply that the required detention times for disinfection
of microorganisms are much higher and are determined on the basis of
CT concept. For this reason, hydroxyl radicals are rarely used for
disinfection and instead are more commonly applied for the oxidation
of trace elements of refractory organic compounds present in effluents
that are highly treated