Osmotic Process

Osmosis is a natural phenomenon and a vital mechanism in biological systems known for over 100 years. For example, the walls of the living cells are natural membranes with selective permeability wherein some materials can pass through and the others cannot. Osmosis provides the primary means for the transport of fluids into and out of cells of living organisms. Eg., Plants absorbing water from the soil, Kidneys absorb water for human body.

If two solutions (Liquids) with different ionic concentrations are separated by a semi-permeable membrane, then the solution with the lower concentration of ionic substances tend to flow towards the solution with higher concentration of ionic substances and tries to equalize the solute concentrations on the two sides. This is termed as the natural Osmotic Phenomenon. The osmotic pressure is a property of the solution proportional to the amount of dissolved minerals present in it.

This Osmotic Pressure generates a noticeable head difference between the two solutions and is referred to as the Osmotic Pressure Difference. Osmotic pressure can be defined as the potential energy difference that exists between two solutions of different ionic concentration on either side of a semi-permeable membrane.

Reverse Osmosis Process

Reverse Osmosis Process is a reversal of the natural osmosis pressure by exerting sufficient external pressure to the more concentrated solution. When pressure, that is greater than the osmotic pressure difference, is applied to the solution with higher concentration of ions, the direction of the water flow through the membrane is reversed and this process is known as the Reverse Osmosis Process. In the Reverse Osmosis (RO) Process, the ability of the membrane to selectively pass water is unchanged and only the direction of the water flow is changed.

When the water with higher concentration of ions is forced through a synthetic semi-permeable membrane that allows only water (H₂O) molecules, the pure water passes through the membrane while the dissolved solids and other contaminants present in the water are rejected and left behind.

Contaminants Removed by RO Plants

Subject to certain limitations and the requirement of additional treatment schemes, Reverse Osmosis (RO) plants are capable of removing the following impurities | contaminants.

Origin of Reverse Osmosis (RO) Technology

Reverse Osmosis (RO) Filtration, also known as hyperfiltration, is a very advanced water purification technology. First developed in the late 1950s with the successful invention of semi-permeable membranes, it was initially used by the US Government for their armed forces.

This semi-permeable membrane rejected dissolved solids (salts |inorganics) present in the water in ionic form but allowed the passage of water molecules and was therefore considered a revolutionary separation process.

Initially, this technology was applied to desalinate sea water as an alternative technology to the energy-intensive, thermal-based separation (Heating-Evaporation-Condensation). With the progress of the space race, NASA invested heavily in the membrane technology as a solution for providing drinkable water during space flight.

In the early 1970s, the first commercial low-pressure semi-permeable membrane was developed and was capable of producing 1 to 5 gallons of clean and safe drinking water per day for small households. Later, as a result of technological advancements, the capacity of membrane systems was augmented to produce sufficient pure water for whole communities.

However, one issue that had remained a challenge was scaling. Scaling is the physical process that occurs when sparingly soluble salts come out of the solution to form a solid scale on the membrane surface. This occured in membrane filtration due to the fact that when salty water entered the membrane, most of the water molecules were removed, leaving behind many salts in a reduced quantity of water. As fresh water source reduced and the demand for higher recovery systems increased, the likelihood for membrane-scaling also increased.

When polyamide membranes, also known as thin film composite (TFC) membranes were introduced, Sodium Hexametaphosphate (SHMP) was used to control the scaling potential. Due to the limitations of SHMP such as quick degradation, limited control of carbonate scaling, liquid anti-scalants primarily made of polyacrylic acid (borrowed from cooling tower technology) was introduced and this was found to be much more stable than SHMP. However, this too had limitations such as limited control of carbonate scaling and precipitation in the presence of metal hydroxides.

In the mid-to-late 80s, advanced research resulted in the introduction of phosphate-based anti-scalants and many formulations even today are derived from phosphate-based materials. These too carry many limitations like overdosing (which can exacerbate phosphate scaling and have limited control over silica precipitation) along with increased prohibitions by US and many other countries.

Phosphate being the primary ingredient in many fertilizers greatly increased the risk of algal blooms and bacterial proliferation in the discharge stream.

The new generation of anti-scalants is unique in molecular structure. Phosphate-free and made in concentrated form, this technology is an evolutionary step in the operation of RO Plants. This technology eliminates the concern of membrane fouling caused by overdosing and adheres to most discharge regulations of various countries.

Today, RO Technology accounts for 80% of the desalination plants worldwide and are used for thousands of different processes and applications. With the continuing depletion of naturally clean water sources, Reverse Osmosis (RO) Plants are now most sought after than ever before.

Technology and Process Description

RO is a reliable and time-tested water treatment methodology to desalinate the water by reducing / removing total dissolved solids (TDS) and other undesirable organic and inorganic chemicals, heavy metals present in the raw water. This technology is today globally acclaimed as the most advanced level of liquid filtration system and is used for a wide variety of applications.

How does Reverse Osmosis work?

In conventional water filtration systems, the entire water is pumped through the filter medium and this is known as ‘dead-end’ filtration. In the RO process, raw water (high TDS/salt) after necessary pretreatment - is passed through a semi-permeable membrane under external pressure created using a high-pressure pump. The amount of required pressure depends on the TDS/Salt concentration of the raw water.

This filtration is called as crossflow or tangential flow filtrations since the raw water flows over and parallel to the membrane surface. In the process, this crossflow creates two streams called the Permeate (or Product) Water and the Concentrate (or Reject) Water. The permeate stream (pure water) is collected separately whereas the reject stream creates a certain amount of turbulence to sweep away the solids/contaminants filtered by the membrane to the reject port and thereby minimizing the accumulation of contaminants on the membrane surface and facilitates smooth operation of the RO Plant.

RO Membranes

The RO Membrane has a pore diameter of 0.0001 micron which is smaller than a bacteria (0.5 mm to 1 mm) and virus (0.02 mm to 0.04 mm) and these fine pores ensure effective filtration (99%) of microbial load in addition to the removal of ions in the water. The passage of water through the membrane depends on factors such as molecular size and diffusability co-efficients.

RO Membranes are made of thin, multi-layered polyamide sheets. These polyamide sheets are layered on top of a polyethersulfone or polysulfone porous layer supported by a non-woven fabric support sheet. This multi-layer configuration gives RO membranes high filtration rates and good mechanical strength. The polyamide layer is responsible for the high rejection rate of solids and is chosen for its permeability to water and relative impermeability to various dissolved impurities.

RO - Design Basis

The basic design and configuration of RO Plants and requisite pre and post treatment modules are highly formula-driven and should be primarily based on the following factors to ensure optimal performance and durability of the RO Plant.

Percentages and Equations

The basic design and configuration of RO Plants and requisite pre and post treatment modules are highly formula-driven and should be primarily based on the following factors to ensure optimal performance and durability of the RO Plant.

Rejection of Solids (TDS) in Feed Water

This equation determines the efficacy of RO Membranes in removing the desired contaminants. While it does not reveal the performance of the individual membranes, it shows the overall performance of the RO Plant.

RO Systems with high efficiency membranes are designed to reject 95-99% of the feed water contaminants. Higher the salt rejection, better is the performance of the system. A low salt rejection may be an indication that the RO Membranes either require cleaning or replacement.

Flux

Flux is used to express the rate at which water permeates a reverse osmosis membrane. ie., total water volume per area per unit of time. Flux of an RO membrane is directly proportional to temperature and pressure.

Flux differs for different sources of water and can be determined with the help of an RO Design Software.

Product Features and Technical Information of RO Plants

Unless warranted under special conditions, Reverse Osmosis (RO) Plants generally comprise the following major parts

Pre-Treatment Stage

The efficacy, long-term performance stability and lower maintenance costs for a Reverse Osmosis (RO) plant largely depend on the feed water quality. Certain contaminants in the feed water, if not eliminated, can quickly lead to a decline in membrane performance and, in worst cases, can even irreparably damage the RO membranes leading to premature membrane failure. Eg., Silt, Suspended Solids, Colloidal Particles, Chlorine, Hardness (Ca,Mg), Microbes, Algae, Organics etc. Faulty or partial pre-treatment schemes can result in frequent membrane cleaning requirements that increases the running costs, plant downtime and also adversely affects the membrane life.

Therefore, the purpose of pre-treatment of feed water is to make sure that such contaminants are reduced, neutralized or removed from the water and thereby improve the life and performance of the RO Membrane.

While the pre-treatment module for an RO plant is dependent on the feed water characteristics and the desired output water, generally it is a selection from the following treatment schemes.

Note: In case of chlorination of feed water, ORP meters need to be used to monitor the residual chlorine levels in the feed.

Additional pre-treatment may be warranted in the event of feed water having heavy metals, oil and grease, BOD-COD. For sea water applications, clarifiers and flocculators are used before the feed water is sent to the pre-treatment module.

Main Treatment Stage

This stage consists of the full RO module. ie., Reverse Osmosis (RO) Membranes with Housings, High Pressure Pumps, Control Panel with critical accessories and the skid.

The RO Membrane is chosen based on the TDS (Total Dissolved Solids) in the water, required permeate output and the desired operating pressure. Selection factors for membranes include total surface area, pH stability, fouling factors, working life, mechanical strength, operating pressure and selectivity for solutes. The RO Housings come in both SS (Stainless Steel) and FRP (Fibre Reinforced Polymer)

Mechanical Damage to RO Membrane

While proper pre-treatment schemes can prevent membrane damage, the pre and post-system plumbing and controls used in the RO Plant also play a role in the life of a membrane. For instance, a ‘hard start’ can lead to mechanical damage to the RO membranes. Excessive backpressure on the RO Plant can also result in membrane damage. Such issues can be addressed by using pumps with VFDs, installing check valve(s) and/or pressure relief valve(s) etc.

The entire RO Plant is mounted on a compact skid made up of MS or SS with easy to the control panel.

Post-Treatment Stage

While the post-treatment module required for an RO Plant depends on the permeate quality, intended applications of the product water and the distribution method chosen for the product water, generally it is a selection from the following options.

Stages and Passes in RO Plants

Reverse Osmosis (RO) Plants are configured in a variety of ways based on the permeate quality required, desired recovery ratio and related environmental issues. There are single stage RO plants and Multi-stage RO Plants with single or multiple passes. A few examples are here below.

Arrays

In an RO System, an array describes the physical arrangement of the pressure vessels in a multi-stage system. Pressure Vessels may contain multiple RO membranes (1 to 6 in a single vessel). The reject of each stage becomes the feed stream for the successive stage. In the above illustration, it is a 2:1 array which means that the concentrate of the first two RO membranes is fed to the next membrane.

Maintenance of RO Plants

The filtration units in the pre-treatment module will have to be periodically backwashed and rinsed to remove the accumulated physical impurities. The quantity of anti-scalant chemical available in the tank will have to be monitored and periodically refilled. The UV lamp has to be checked for any drop in intensity.

Even the best pre-treatment cannot completely eliminate the decline of membrane performance over time and hence periodical cleaning of the membranes is necessary to maintain the RO membrane’s peak performance. As a general rule, if there is a drop in the permeate flow or permeate quality (increase in salt passage), it is an indicator that the membrane needs to be cleaned. Membranes can also be cleaned onsite using specialized devices / cleaning skids.

Automated RO Plants are equipped with devices that backflush the membrane at a pre-set frequency. Beside this routine backflushing, chemical cleaning may be required and the periodicity of the same depends on the fouling potential of the feed stream and other related factors. Generally it is any where between 1 to 4 times a year depending on the age and condition of the membrane/s.

General Applications of Reverse Osmosis (RO) Water Treatment Plants

• Purification of drinking water for homes, institutions, commercial establishments and industries
• Ultrapure water for medical uses. Eg. Feed for Renal Dialysis Units, Instrument Sterilization, Production of Medicines and Pharmaceutical Formulations.
• Desalination of sea water
• Packaged Drinking Water for commercial sales
• Pre-treatment to Demineralizer (DM/MB) Plants
• Recirculation of industrial and process water
• Used in Cooling Towers, Boiler Plants and other process water requirements
• Tertiary treatment for STP, ETP and Recycling Plants
• Industrial Uses - Power Generation | Food & Beverage Processing | Pharmaceutical Production | Biotechnology | semiconductor manufacturing