Protecting reverse osmosis plants through water pretreatment

PROTECTING REVERSE PROTECTING REVERSE OSMOSIS PLANTS OSMOSIS PLANTS THROUGH WATER THROUGH WATER PRETREATMENT PRETREATMENT

Reverse osmosis membrane material for water filtration

One of the most common types of water treatment, reverse osmosis (RO) is a separation technique that is suitable for a wide range of applications, especially when salts and/or dissolved solids need to be removed from a solution.

There are several factors to be considered when selecting an RO system: the customer’s capacity requirement (water usage), the daily production capacity of the system, and the per cent rejection for specific contaminants in the source water,” explains Anelia Hough, water treatment consultant at Allmech.

RO plants require maintenance and care to ensure they function optimally and to extend their lifespan. “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 like scaling and biofouling,” she adds.

Common issues “Factors that can affect an RO system’s performance include temperature, operating pressure, back pressure, the equilibrium effect/ TDS creep, per cent recovery and, of course, the RO membrane’s permeate production and per cent rejection ratings,” says Hough.

The most common issues in RO plants include: • Fouling: occurs when contaminants accumulate on the membrane surface, effectively plugging 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: 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 feedwater. • Biofouling: reduces actual membrane performance through microbial generation in a biofilm that forms on the membrane surface. • Chemical damage: on an RO membrane, this means 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. Use of aggressive cleaners can also cause chemical damage. • Mechanical damage: can happen when a system is pressurised too quickly, damaging the RO membrane elements. One of the clearest 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.

Reverse osmosis is one of the most common types of water treatment

Pretreatment options “Pretreatment can help to avoid these problems. When selecting a prefilter,

users should always look for a verified efficiency rating next to the micron size on the filter’s technical data sheet,” explains Hough. Pretreatment options vary: • Multimedia filtration: 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 reverse osmosis system helping to ensure long life of the

RO membrane elements. A welloperated multimedia filter can remove particulates down to 20 microns. A multimedia filter that uses a coagulant addition can remove particulates down to 10 microns. • Microfiltration: The filters used in microfiltration have a pore size of approximately 0.1 micron. Bacteria and suspended solids are the only element that can be removed through microfiltration. • Antiscalants and scale inhibitors:

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. • Softening by ion exchange: 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

ALLMECH

One of the leading boiler manufacturers in South Africa, Allmech offers a wide range of electrical, gas and oil-fired boilers and related water treatment solutions. The company began in 1995 and has an established reputation as a boiler and water treatment specialist with a strong customer support system. Allmech’s water treatment solutions comprise: • Industrial water treatment programmes and services • Water softeners • Reverse osmosis • Pre- and post-water filtration • Demin plants

exchange involves the replacement of the hardness ions with nonhardness ions. • Granular activated carbon (GAC) filtration: Activated carbon removes residual chlorine and chloramines by a chemical reaction that involves a transfer of electrons from the surface of the GAC to the residual chlorine or chloramines. The chlorine or chloramines end up as a chloride ion that is no longer an oxidiser.

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