Newer technologies have gained popularity and expanded over the last one decade. Effective separation is crucial in the operation of processes of any industry. A major question is how best can these processes solve the problems and what are the edges which we can push these new technologies. Achievements have been made in (waste) water treatment. Some of the successes are; low cost of operation, high efficiency, less energy consumption and smaller spaces of operation. Membrane separation processes have been adopted throughout the world. They are divided based on the size of particles they can let to pass through and the driving force that is used. Talking of pressure driven processes like microfiltration, ultrafiltration and reverse osmosis, they are processes which changed the whole history of water treatment. For example, reverse osmosis has been used in the desalination of brackish water. Advantages of reverse osmosis in drinking water treatment include: physically removal of pathogens, effective removal of substrates in the treated water, less biofilm growth, less disinfectant chemical requirement and less disinfection of the byproduct. However, there are some unanswered questions like the exact dosage of the disinfectants we can use and since the disinfectants will be of less amount, how can we compare it to classic technologies? What are the other advantages of using the reverse osmosis?

1. NATIONAL UNIVERSITY OF PUBLIC SERVICE
Institute of Water Supply and Environmental Engineering
DESCRIPTION OF A TECHONOLOGY PROCESS
Reverse Osmosis (RO)
NYABUTI VICTOR ONG’ERA
ECYQG3

2. ABSTRACT
The basis of this paper is to analyse membrane processes particularly the reverse osmosis process. This
process (RO) has been existing in nature since time immemorial, but its full utilization has not been
reached. It’s the anticipation of this paper is to shade light on this simple but important process that
changed the history of water treatment.
METHODOLOGY
I analysed a cutting-edge technology paper and referenced several others from well-known researchers.
I also consulted my teacher on the topic who gave me advice mainly out of his experience in the sector.
KEY WORDS: Reverse osmosis, pressure, concentration and osmotic potential.
INTRODUCTION
The core of membrane processes and their characteristics requires one to fully understand their
properties. They include reverse osmosis(RO), ultrafiltration(UF), electrodialysis(EDA),
microfiltration(MF) and Nano filtration(NF). Reverse osmosis is among the finest levels of filtration
available. The RO membrane generally acts as a barrier to all dissolved salts and inorganic molecules,
as well as organic molecules with a molecular weight greater than approximately 100. RO membranes
are used to remove dissolved ions in a process that does not rely on distinct pores for filtration. Instead,
RO applies diffusion to allow water molecules to readily pass through a semi permeable membrane
layer. RO can be use in the desalination of seawater or brackish water for drinking purposes, wastewater
recovery, food and beverage processing, biomedical separations, purification of home drinking water
and industrial process water. RO Can also be used in the production of ultrapure water for use in the
semiconductor industry, power industry (boiler feed water), and medical/laboratory applications.
Process scheme (single module system).
The system includes a set of membrane elements, housed in pressure vessels that are arranged in a
certain manner. A high-pressure pump is used to feed the pressure vessels. The membrane system is a
complete plant with an inlet for feed water and outlets for permeate and concentrate. RO system
performance is typically characterized by two parameters, permeate (or product) flow and permeate
quality. An entire reverse osmosis (RO) water treatment system consists of the pre-treatment section,
the membrane element section, and the post-treatment section. Post-treatment is employed to achieve
the required product quality. In seawater desalination, this is usually pH adjustment, hardening and
disinfection. In ultrapure water (UPW) production, the permeate is usually post-treated by polishing ion
exchange demineralization.

3. Feed water enters the system through the feed valve and flows through the cartridge filter to the high-
pressure pump. From the high-pressure pump, the feed water flows to the feed inlet connection of the
module. The product stream should leave the module at no more than 5 psi (0.3 bar) over atmospheric
pressure. However, higher permeate pressure is sometimes required, e.g., to feed the post-treatment
section or to distribute the product without further pumping. Then the feed pressure must be increased
by the required value of the permeate pressure, but the specified maximum feed pressure must be
observed
The concentrate leaves the concentrate outlet connection at essentially the feed pressure. Pressure drop
will usually amount to 5–30 psi (0.3–2 bar) from feed inlet to concentrate outlet, depending on the
number of membrane elements, the feed flow velocity and the temperature. The concentrate flowrate is
controlled by the concentrate flow control valve. The system recovery is controlled by this valve and
must never exceed the design set value.
Some of the most common words used in RO include:
 Recovery - the percentage of membrane system feedwater that emerges from the system as
product water or “permeate”. Membrane system design is based on expected feedwater quality
and recovery is defined through initial adjustment of valves on the concentrate stream.
Recovery is often fixed at the highest level that maximizes permeate flow while preventing
precipitation of super-saturated salts within the membrane system.
 Rejection - the percentage of solute concentration removed from system feedwater by the
membrane. In reverse osmosis, a high rejection of total dissolved solids (TDS) is important,
while in nanofiltration the solutes of interest are specific, e.g., low rejection for hardness and
high rejection for organic matter.
 Passage - the opposite of “rejection”, passage is the percentage of dissolved constituents
(contaminants) in the feedwater allowed to pass through the membrane.
 Permeate - the purified product water produced by a membrane system.
 Flow - Feed flow is the rate of feedwater introduced to the membrane element or membrane
system, usually measured in gallons per minute (gpm) or cubic meters per hour (m3/h).
Concentrate flow is the rate of flow of non-permeated feedwater that exits the membrane
element or membrane system. This concentrate contains most of the dissolved constituents
originally carried into the element or into the system from the feed source. It is usually measured
in gallons per minute (gpm) or cubic meters per hour (m3/h).
 Flux - the rate of permeate transported per unit of membrane area, usually measured in gallons
per square foot per day (gfd) or litters per square meter and hour (L/m2
h).
The principle behind it.
As a fundamental rule of nature, everything must reach equilibrium. That is, if two liquids of different
concentrations are separated by a semipermeable membrane, they will try to reach the same
concentration on both sides of the membrane. The only possible way to reach equilibrium is for water
to pass from the pure water compartment to the salt-containing compartment, to dilute the salt solution.
The osmotic potential from any fluid is determined by:
 Concentration of salts in the water
 The temperature of the water (expressed in absolute terms)
 The pressure of the solution
The concentration of salts in the water supply has an inverse effect on the chemical potential of the
solution, whereas the temperature and pressure have a direct effect. Therefore, at constant temperature
and pressure of a solution, increased salt content results in decreased osmotic potential.

4. For the removal of small particles and dissolved salts, crossflow membrane filtration is used. Crossflow
membrane uses a pressurized feed stream which flows parallel to the membrane surface. A portion of
this stream passes through the membrane, leaving behind the rejected particles in the concentrated
remainder of the stream. Since there is a continuous flow across the membrane surface, the rejected
particles do not accumulate but instead are swept away by the concentrate stream. Thus, one feed stream
is separated into two exit streams: the solution passing through the membrane surface (permeate) and
the remaining concentrate stream.
By applying pressure to a fluid on one side of a semi-permeable membrane, it is possible to reverse the
natural flow of pure water from an area of high salt concentration to one of low concentration. This
process is called Reverse Osmosis. Its only possible if the chemical potential exist.
Some factors affecting the performance of the RO are:
 Pressure
 Temperature
 recovery and;
 Feed water salt concentration.
Permeate flux and salt rejection are the key performance parameters of a reverse osmosis. These
parameters are always referenced to:
 Given feed water analysis
 Feed pressure and,
 Recovery.
The Permeate is considered before actual designing of the system. The goal of designing a RO system
for a certain required permeate flow is to minimize feed pressure and membrane costs while maximizing
permeate quality and recovery. The feed pressure needed to produce the required permeate flow for a
given membrane depends on the designed permeate flux (permeate flowrate per unit membrane area).
The higher the permeate flow per unit of active membrane area, the higher the feed pressure. With
increasing effective feed pressure, the permeate TDS will decrease while the permeate flux will
increase. If the temperature increases and all other parameters are kept constant, the permeate flux and
the salt passage will increase.
Recovery is the ratio of permeate flow to feed flow. In the case of increasing recovery, the permeate
flux will decrease and stop if the salt concentration reaches a value where the osmotic pressure of the
concentrate is as high as the applied feed pressure. Generally, one can conclude that effective pressure
and temperature increase the permeate flow while the recovery and feed salt correction reduces it. Apart
from effective pressure, the other three increase the salt permeability.
Productivity
It is an important aspect of life. In membrane processes. It is mainly affected by the design or through
fouling. Fouling has four primary mechanisms i.e. scaling plugging adsorption and biological growth.
Effective pre-treatment of the feed water is required. Selection of the proper pre-treatment will
maximize efficiency and membrane life by minimizing Fouling and Membrane degradation. It should
increase the Product flow, salt rejection, Product recovery and reduce Operating & maintenance costs.
The type of treatment that is arrived upon depends on the source of water, its composition and
application. Water from the wells needs less treatment compared to municipal waters and the surface
waters. The municipal waters can contain high levels of organic waste while the surface water can vary
extensively on its components.
Some of the pre-treatments carried out include:

5.  Scale control
Scaling occurs when sparingly soluble salts are concentrated within the element beyond their
solubility limit. We can add some inhibitors, an acid, Softening with a Strong Acid Cation Exchange
Resin, De alkalization with a Weak Acid Cation Exchange Resin. The problem are Residual
hardness and variance of the water PH. Other methods include: Colloidal and Particulate Fouling
Prevention Media 3 Oxidation–Filtration, In-Line Filtration, Coagulation-Flocculation and
Microfiltration.
 Biological control
Microorganisms can be regarded as colloidal matter and removed during pre-treatment. The most
successful approach is the limitation or removal of nutrients for microorganisms from the water to
limit biological growth. Its starts with assessment, Culture Techniques, Total Bacteria Count and
finally the treatment. The treatment can be done through: Assimilable Organic Carbon (AOC), and
chlorination.
 Bio filtration
Plants are grown that enhance a biofilm that can filter the waste water.
 Mechanical treatment:
Like screening, settling, floatation and filtration. This eliminates large particles
 Chemical treatments;
This include the PH control coagulation and flocculation, oxidation and reduction, adsorption and
ion exchange.
The permeate from an RO facility typically requires additional treatment. Feed water pH adjustment
prior to RO, along with extensive removal of divalent ions by the RO process, produces treated water
with low pH, low alkalinity, and low hardness, which are conditions that cause water to be corrosive.
Anaerobic groundwater frequently contains hydrogen sulfide, which passes through the membrane and
causes odour problems in the treated water.
Examples of post treatments that are also carried out include: Permeate stability (It includes a range of
operation from PH adjustments to water hardness regain. This is because the water has to regain its
stability to prevent it from being corrosive.), PH control and finally, residual disinfection is always
required for municipal water distribution.
Reverse osmosis is an energy-intensive process. More than 90 percent of the energy expended to
pressurize the con- concentrate stream can be recovered. It has been exhibiting a large potential in the
treatment of water water and a lot can be achieved from this unit process. The minimization of the
shortcomings and discovery of more ways to improve this process will be of great importance.
REFERENCES.
“Glossary.” Water and wastewater engineering. (1981). APHA, AWWA, WPCF, ASCE, New York,
N.Y.
AWWA. 2007. Reverse Osmosis and Nan filtration Manual of Practice. 2007. Denver,
Colorado:AWWA.
Environment and health online: http://www.ehso.com/pretreat.htm
Lewane lexes: http://lpt.lanxess.de/uploads/tx_lxsmatrix/01_lewabrane_manual_ro_theory_01.pdf