We are pioneers in introducing the water desalination technology in Pakistan. We have designed and built scores of water desalination plants both for industrial and municipal applications. Our desalination plants treat brackish and high brackish well-water sources. Custom-built systems are designed in-house and built to the most stringent specifications.
Reverse osmosis (RO) is a water purification technology that uses a partially permeable membrane to remove ions, molecules and larger particles. In Reverse Osmosis, an applied pressure is used to overcome osmotic pressure, a colligative property, that is driven by chemical potential differences of the solvent, a thermodynamic parameter. Reverse osmosis can remove many types of dissolved and suspended chemical species as well as biological ones (principally bacteria) from water, and is used in both industrial processes and the production of potable water. The result is that the solute is retained on the pressurized side of the membrane (Reject Stream) and the pure solvent is allowed to pass to the other side (Permeate Stream). To be “selective”, this membrane should not allow large molecules or ions through the pores (holes), but should allow smaller components of the solution (such as solvent molecules, i.e., water, H2O) to pass freely.
In the normal osmosis process, the solvent naturally moves from an area of low solute concentration (high water potential), through a membrane, to an area of high solute concentration (low water potential). The driving force for the movement of the solvent is the reduction in the free energy of the system when the difference in solvent concentration on either side of a membrane is reduced, generating osmotic pressure due to the solvent moving into the more concentrated solution. Applying an external pressure to reverse the natural flow of pure solvent, thus, is reverse osmosis. The process is similar to other membrane technology applications.
Reverse osmosis differs from filtration in that the mechanism of fluid flow is by osmosis across a membrane. The predominant removal mechanism in membrane filtration is straining, or size exclusion, where the pores are 0.01 micrometers or larger, so the process can theoretically achieve perfect efficiency regardless of parameters such as the solution’s pressure and concentration. Reverse osmosis instead involves solvent diffusion across a membrane that is either nonporous or uses Nano filtration with pores 0.001 micrometers in size. The predominant removal mechanism is from differences in solubility or diffusivity, and the process is dependent on pressure, solute concentration, and other conditions.
Ultrafiltration (UF) is used for prefiltration in reverse-osmosis plants to protect the reverse-osmosis process. Ultrafiltration is an effective means of reducing the silt density index of water and removing particulates that can foul reverse osmosis membranes. Ultrafiltration is frequently used to pretreat surface water, seawater and biologically treated municipal water upstream of the reverse osmosis unit.
Ultrafiltration membranes are capable of separating larger materials such as colloids, particulates, fats, bacteria, and proteins, while allowing sugars, and other low molecular weight molecules to pass through the membrane. With a pore size range between 0.01 to 0.1µm, ultrafiltration membrane pore sizes fall between that of Nanofiltration and microfiltration. UF membranes typically operate between 50 – 120 PSI (3.4 – 8.3 bar) and are dependent on transmembrane pressure to drive the separation process.
UF can be used for removal of particulates and macromolecules from raw water, to produce potable water. It has been used to either replace existing secondary (coagulation, flocculation, sedimentation) and tertiary filtration (sand filtration and chlorination) systems employed in water-treatment plants or as standalone systems in isolated regions with growing populations. When treating water with high suspended solids, UF is often integrated into the process, using primary (screening, flotation and filtration) and some secondary treatments as pre-treatment stages. Ultrafiltration processes are preferred over traditional treatment methods for the following reasons:
No chemicals required (aside from cleaning)
Constant product quality regardless of feed quality
Compact plant size
Achieving 90-100% pathogen removal.
In addition to providing a dependable, locally controlled water supply, water recycling provides tremendous environmental benefits. Water users can fulfill their demands by using recycled water, which can free substantial amounts of water for the environment. Other environmental benefits include a reduction in wastewater discharges and reducing or preventing the potential for pollution.
Tailoring water quality to a specific water use also reduces the energy needed to treat water. The water quality required to flush a toilet is less stringent than the water quality needed for drinking water and requires less energy to achieve. Using recycled water that is of lower quality for uses that do not require high-quality water saves energy and money by reducing water or wastewater treatment requirements
Nanofiltration is a separation process characterized by organic, thin-film composite membranes with a pore size range of 0.1 to 10nm. Unlike reverse osmosis (RO) membranes, which reject all solutes, NF membranes can operate at lower pressures and offer selective solute rejection based on both size and charge
Wide range of Nanofiltration membranes offer varying degrees of ion selectivity to aid in the development of customized process solutions. Nanofiltration membranes allow water and some salts to pass through the membrane while retaining multivalent ions, low molecular weight molecules, sugars, proteins, and other organic compounds. Nanofiltration membrane products have been proven to offer great resistance to fouling, a high degree of selectivity, and the physical durability needed for application in a wide range of industrial separation processes.
The membrane separation process known as Nanofiltration is essentially a liquid phase one, because it separates a range of inorganic and organic substances from solution in a liquid – mainly, but by no means entirely, water. This is done by diffusion through a membrane, under pressure differentials that are considerable less than those for Reverse Osmosis, but still significantly greater than those for ultrafiltration. It was the development of a thin film composite membrane that gave the real impetus to Nanofiltration as a recognized process, and its remarkable growth since then is largely because of its unique ability to separate and fractionate ionic and relatively low molecular weight organic species.
NF has greatly extended their capabilities in very high or low pH environments, and in their application to non-aqueous liquids. NF membranes tend to have a slightly charged surface, with a negative charge at neutral pH. This surface charge plays an important role in the transportation mechanism and separation properties of the membrane.
As with any other membrane process, Nanofiltration is susceptible to fouling, and so Nanofiltration systems must be designed to minimize its likelihood – with proper pretreatment, with the right membrane material, with adequate cross-flow velocities to scour the membrane surface clear of accumulated slime, and by use of rotating or vibrating membrane holders.
APPLICATIONS
Industrial applications of nanofiltration are quite common in the food and dairy sector, chemical processing, pulp and paper industry and textiles, although the chief application continues to be in the treatment of fresh, process and wastewaters.
In the treatment of water, NF finds use in the polishing at the end of conventional processes. It cannot be used for water desalination, but it is an effective means of water softening, as the main hardness chemicals are divalent.
NF membranes are also used for the removal of natural organic matter from water, especially tastes, odours and colours, and in the removal of trace herbicides from large water flows. They can also be used for the removal of residual quantities of disinfectants in drinking water.
Food industry applications are quite numerous. In the dairy sector, NF is used to concentrate whey, and permeates from other whey treatments, and in the recycle of clean-in-place solutions. In the processing of sugar, dextrose syrup and thin sugar juice are concentrated by NF, while ion exchange brines are demineralized. NF is used for degumming of solutions in the edible oil processing sector, for continuous cheese production, and in the production of alternative sweeteners.
The paper pulp industry uses a very great quantity of water in its production processes, a quantity that the industry is striving to reduce, mainly by “closing the water cycle” – a system in which the purification properties of NF have a major role.
Arsenic is a common element in the earth’s crust, natural groundwater, and even the human body. It is an odorless and tasteless semi-metal (metalloid) that is naturally present in aquifers. Arsenic is typically found as an oxyanion in the environment, most commonly in the +3 and +5 oxidation states. The drinking water standard (maximum contaminant level, or MCL) for arsenic in the U.S. has been 10 ug/L. At concentrations above the MCL, arsenic can cause Skin Damage, Circulatory Problems, and an increased Risk of Cancer.
When digested with water in dangerous concentration, several conditions, including poisoning and cancer, may occur. EPA, Environmental Protection Agency has set the arsenic standard for drinking water at 0.10 parts per million , 0.01 mg/l or 10 parts per billion to protect consumers from the effects of short-term, and long-term chronic exposure to arsenic.
Many treatment technologies are available and refined to remove arsenic from water, including chemical, physical, and biological processes. Typical technologies for arsenic removal are a combination of chemical and physical processes like Lime Precipitation, Oxidation, Coagulation/ Filtration, Adsorptive Media, Ion Exchange and Reverse Osmosis.
Activated carbon filters are generally employed in the process of removing organic compounds and/or extracting free chlorine from water. Eliminating organics in potable water, such as humic and fulvic acid, prevents chlorine in the water from chemically reacting with the acids and forming carcinogens.
There are many types of Activated Carbon Filters available for industrial filtration systems. Activated carbon can exhibit varying performance characteristics depending upon the strata from which it is derived (e.g., bituminous or anthracite coal, bone char, coconut shell) and the way it is manufactured. It is critical to match up the correct activated carbon bed with the particular need.
Coconut shells and coal (anthracite or bituminous) are both organic sources of activated carbon. Carbon forms when an organic source is burned in an environment without oxygen. This process leaves only about 30% of the organic mass intact, driving off heavy organic molecules. Prior to being used for water treatment, the organic mass must then be “activated.” The process of activation opens up the carbon’s massive number of pores and further drives off unwanted molecules. The open pores are what allow the carbon to capture contaminants, known as “adsorption”.
Activated carbon filters are periodically backwash to remove trapped silt, prevent channeling & head loss and to remove carbon fines produced by friction between granules.
The ion exchange water softener is one of the most common tools of water treatment. Its function is to remove scale forming calcium and magnesium ions from hard water. In many cases soluble iron (ferrous) can also be removed with softeners. A standard water softener has four major components: a resin tank, resin, a brine tank to hold sodium chloride, and a valve or controller.
The softener resin tank contains the treated ion exchange resin –small beads of polystyrene. The resin bead exchange sites adsorb sodium ions and displace multivalent cations during regeneration with 6-10% solution of NaCl. The resin has a greater affinity for multivalent ions such as calcium and magnesium than it does for sodium. Thus, when hard water is passed through the resin tank in service, calcium and magnesium ions adhere to the resin, releasing the sodium ions until equilibrium is reached.
When most of the sodium ions have been replaced by hardness ions, the resin is exhausted and must be regenerated. Regeneration is achieved by passing a concentrated NaCl solution through the resin tanks, replacing the hardness ions with sodium ions. The resin’s affinity for the hardness ions is overcome by the concentrated NaCl solution. The regeneration process can be repeated indefinitely without damaging the resin. It solves a very common form of water contamination: hardness. Regeneration with sodium chloride is a simple, inexpensive process and can be automatic, with no strong chemicals required.
Filtration is used in addition to regular coagulation and sedimentation for removal of suspended solids from surface water or wastewater. This prepares the water for use as potable, boiler, or cooling make-up. Wastewater filtration helps users meet more stringent effluent discharge permit requirements. Flocculants / coagulants may be used upstream of the filter to induce the tiny dirt particles to join together to form particles large enough to be removed by the filter.
Pressure vessels with sand and other Filtration media are widely used in industrial filtration applications. Sand Filters are divided into two main types: (1) Gravity Filters and (2) Pressure Filters. The principles of the two types of filters are identical. The pressure filter is operated at elevated pressures, thus prolonging the filter cycle and/or increasing the rate of flow of water through the filter.
Multimedia filtration refers to a pressure filter vessel which utilizes three or more different media whereas Sand Filter typically uses one grade of sand alone as the filtration media. In a single media filter, during the “settling” cycle, the finest or smallest media particles remain on top of the media bed while the larger, and heavier particles, stratify proportional to their mass lower in the filter. This results in very limited use of the media depth since virtually all filterable particles are trapped at the very top of the filter bed or within 1-2 inches of the top where the filter media particles have the least space between them. The filter run times are thus very short before the filter “blinds” or develop so much head pressure that it must be backwashed to avoid seriously impeding or stopping the flow.
During the cleaning cycle, called “backwash”, the bed is lifted (or “fluidized”) to loosen the filter media and release trapped dirt which is removed in the backwash flow. After the backwash cycle, the bed is allowed to settle before the filter is returned to service (i.e., normal flow). A “filter-to-waste” cycle is used following the settling to assure the filtration media has sufficiently re-stratified and that any loose dirt is removed. Filter backwash may include air scour to help loosen packed dirt in the media bed. When this step is included, it is preceded in the backwash cycle by a “drain down” period for water to be bled out of the filter vessel.
EDI systems offer a highly effective alternative to mixed bed demineralization to polish water to extremely low levels of dissolved solids. EDI systems do not require the periodic regeneration and downtime of resin-based treatment approaches and thus can operate continuously. This reduces maintenance and eliminates the need for regeneration chemicals and waste neutralization. Dissolved solids (cations and anions) are considered contaminants in some industrial processes. They cause scaling and fouling in boiler tubes, inhibiting heat transfer and system efficiency.
An EDI water treatment system is a continuous electrically driven membrane and resin ion separation technology. RO product water enters the EDI module through an inlet valve and then enters stack of cells with cation and anion membranes separated by spacers. On opposite ends of the stack is a cathode and anode – each providing an electrical potential to drive the cations and anions through the respective one way membranes to a waste channel. The product and waste channels contain a thin layer of mixed bed resin to provide a conductive path to facilitate the process. The result is a highly efficient system that removes all dissolved solids with very close to neutral pH. The purified water is then piped out of the stack. Modules of different flow rates can be plumbed together to provide higher flow rate capacities.