Reverse osmosis (RO) is a separation technique that is suitable for a wide range of applications, especially when salt and/or dissolved solids need to be removed from a solution.
South African boiler manufacturer and water treatment components supplier Allmech is the sole agent for Runxin water treatment system valves in the South Africa; it states the valves have proven to be more chemically resistant and robust than other valve brands on the market.
Allmech water treatment consultant Anelia Hough explains that there are several factors to be considered when selecting an RO system. Factors include the customer’s capacity requirement such as water usage, the daily production capacity of the system, and the percent rejection for specific contaminants in the source water.
“Beyond this, RO plants require proper maintenance and care to ensure they function optimally and to extend their life span. Aside from regular maintenance, the best way to protect an RO plant is by pretreating water, which reduces the strain on the RO membrane – a costly component. It also helps to avoid issues [such as] scaling and bio-fouling,” Hough says.
Common Issues with RO Plants
“Factors that can affect an RO system’s performance include temperature, operating pressure, back pressure, the equilibrium effect/TDS creep, percent recovery, and, of course, the RO membrane’s permeate production and percent rejection ratings,” says Hough.
The most common issues in RO plants include fouling, scaling, biofouling, chemical damage and mechanical damage.
Fouling occurs when contaminants accumulate on the membrane surface which effectively plugs the membrane. There are many contaminants in municipal feedwater that are harmless for human consumption, but large enough to quickly foul or plug an RO system.
Scaling occurs when particles are deposited on a membrane, causing it to plug. As certain dissolved compounds become more concentrated, scaling can occur if these compounds exceed their solubility limits and precipitate on the membrane surface as scale. Scaling calculations are usually only based on the silicate concentration in the feed water.
Biofouling reduces actual membrane performance through microbial generation in a biofilm that forms on the membrane surface. Chemical damage on an RO membrane results in a higher permeate flow and poorer quality permeate water. Dosing of oxidant agents, such as chlorine or hypochlorite, can reduce performance and ultimately result in the failure of the RO membranes. The use of aggressive cleaners can also cause chemical damage.
Mechanical damage occurs when a system is pressurised too quickly which damages the RO membrane elements. One of the most clear signs of damage on an RO membrane is the lack of salt rejection capabilities. There is also very often an increase of permeate flow rate.
However, Hough explains that pretreatment can help to avoid these problems, and that there are various options available.
“When selecting a prefilter, users should always look for a verified efficiency rating next to the micron (μm) size on the filter’s technical data sheet,” says Hough.
A multimedia filter is used to help prevent fouling of an RO system. This type of sediment filtration is ideal for a pretreatment process to any RO system, helping to ensure long life of the RO membrane elements. A well operated multimedia filter can remove particulates down to 20 μm. A multimedia filter that uses a coagulant addition can remove particulates down to 10 μm.
The filters used in microfiltration have a pore size of about 0.1 μm. Bacteria and suspended solids are the only element that can be removed through microfiltration.
There are many chemicals that can be used as antiscalants and dispersants to improve the operation of RO. Antiscalants are a family of chemicals designed to inhibit the formation and precipitation of crystallised mineral salts that form scale.
A water softener is a filtration system that removes hardness-causing calcium and magnesium minerals from water through a process called ion exchange. Standard water softeners are cation exchange devices. Cation exchange involves the replacement of the hardness ions with non-hardness ions.
Activated carbon removes residual chlorine and chloramines by a chemical reaction that involves a transfer of electrons from the surface of the Granular Activated Carbon filtration to the residual chlorine or chloramines. The chlorine or chloramines end up as a chloride ion that is no longer an oxidiser.
“It’s also important to clean the RO membrane regularly. This involves low and high pH cleaners to remove contaminants from the membrane. “We address scaling with low pH cleaners and organics, while colloidal and biofouling are treated with a high pH cleaner,” says Hough.
Allmech offers the complete range of pre-treatment and RO membrane maintenance solutions for RO plants, including filtration systems, softeners, antiscalants, chemicals and other consumables.
“At Allmech, we’re looking forward to growing this part of our business in 2022 and beyond, leveraging our experience in all things related to water treatment. We also have a boiler division and we stock a comprehensive range of Runxin valves, so we’re well positioned to be a one-stop shop for anyone needing a water treatment or boiler specialist, and we’re expecting a busy year ahead,” Hough comments.
She concludes by noting that Allmech supplies a full range of manual and automatic filter and softener valves, which are reliable, easily available, user-friendly and cost effective.