What is a Reverse Osmosis Plant?
A reverse osmosis plant can convert seawater, rainwater, river water, well water or municipal water into fresh drinking water. It begins with a pretreatment system that filters out large particles, color, odors, chlorine, and bacteria.
It uses extreme pressure from a high-pressure pump to force water molecules against their natural osmotic gradient through the semi-permeable membrane. It removes 95% to 99% of dissolved salts, organics, and pyrogens.
Reverse Osmosis Process
The process relies on a semi-permeable membrane that lets water molecules pass through but not dissolved salts, bacteria, viruses, colloids, and other contaminants. The water is pushed through the RO membrane by a high-pressure pump. The membrane removes 99% of dissolved salts and other contaminants from the water supply, leaving behind pure water. The rejected water is known as the “reject stream.”
This method of filtration has many industrial applications, including making maple syrup from sugary sap, and concentrating milk for cheese production. Reverse osmosis is also used in the production of pharmaceuticals, and to produce concentrated ethanol for fuel.
Reverse Osmosis can reduce total dissolved solids (TDS) in city water as well as in private well water by removing chemicals and bacteria. In addition, a reverse osmosis system can reverse osmosis plant be used to reduce heavy metals such as arsenic in well water. In the case of arsenic removal, the water would need to be pretreated with a UV system to remove any bacterial contamination prior to going through the RO process.
One of the key issues with a reverse osmosis plant is the buildup of biological contaminants on the membrane, which is called biofouling. This can be reduced with the use of a carbon filter to reduce chlorine and other harmful chemicals, as well as a sediment pre-filter.
Membrane
Reverse Osmosis systems use a semipermeable membrane that allows water molecules to pass but rejects larger contaminants like ions, bacteria and organics. It’s one of the few processes that can boast a 95-99% reduction in total dissolved solids (TDS).
Pressure from the pump overcomes natural osmotic pressure to force feed water through the RO membrane, which is sized to fit inside of a pressurised storage tank. Reverse osmosis is driven by the concentration gradient across the membrane; water with a higher solute concentration moves from the area of low solute concentration, through the membrane to the area of high solute concentration.
The RO membrane is the heart of the system and the most important component for removing dissolved contaminants. To protect the membrane from contamination, most RO systems include prefilters and postfilters, which remove sediment and chlorine that can clog or damage the membrane. Biofouling is another concern; microorganisms can grow on the membrane and cause it to fail.
To prevent this, RO systems include a disinfectant such as ultraviolet radiation to kill the bacteria and other organisms. The RO membrane is also cleaned periodically to reduce the likelihood of fouling. Most residential reverse osmosis systems have a two-stage process to produce enough water for your household, with the permeate from the first stage flowing into the second stage’s storage tank.
Pumps
A reverse osmosis system relies on pressure to push water molecules through the semi-permeable membrane. The pressure required is far higher than normal, requiring a robust pump to drive the process.
Adding a permeate pump is a smart upgrade for any RO system. The pump reduces waste water by 75 to 80%, making the system more efficient and environmentally friendly. Most RO systems are plumbed for the addition of a permeate pump, but you’ll need to make sure that yours is before you buy one.
Reverse osmosis removes dissolved solids and minerals from the water, leaving you with clean, clear, healthy drinking water. It’s also the method of choice for many saltwater aquarists who rely on a combination of reverse osmosis and deionization (RO/DI water) to provide their fish with highly pure water that mimics their natural habitats.
Seawater reverse osmosis, or SWRO, is one of the most common applications for this reverse osmosis plant manufacturer technology. Marine vessels use it to produce fresh water onboard, avoiding the cost of purchasing expensive docking water at each port they stop at.
The high-pressure pumps needed for desalination applications are often designed with bronze or stainless steel to withstand the corrosive effects of salty seawater. They require more pressure than regular household pumps, and are usually multistage centrifugal or piston pumps. Depending on the application, they may be used in conjunction with a remineralizing filter to boost alkalinity and add minerals back into the water.
Controls
Reverse osmosis is one of the primary ways to desalinate water, though it also has many other applications. In addition to its obvious use in generating drinkable water, it can pull harmful chemicals out of wastewater for safer chemical disposal, and even provide energy.
The membranes themselves need a little help from the rest of the plant to function properly. They sit in a rugged frame designed to handle the vibration from the high-pressure pumps, and it’s surrounded by a carbon steel jacket that makes it resistant to corrosion.
A reverse osmosis system typically requires pretreatment to prevent the formation of foulants on the membrane surface, which can cause premature failure. A water analysis provides an estimate of the potential for mineral scale, colloidal materials like clays and silica, dead and living microorganisms, and carbon particles. It can also detect the presence of dissolved metals that can react with cationic polymers added to the feed stream as an antiscalant.
An RO pretreatment system usually involves ion exchange softening, which uses sodium to replace scale-forming calcium and magnesium ions with soluble ones. This allows the membranes to function normally and reduces clogging. The ion exchange treatment is often followed by a chemical disinfectant such as chlorine or hydrogen peroxide to further ensure the membranes will be free of contaminants. If an antiscalant is used, it’s important to follow the manufacturer’s directions when dosing it into the feed stream.