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UV treatment is one of a range of methods for disinfecting and decontaminating water supplies, together with other long-established approaches such as ozonation, chlorination and reverse osmosis. But is it necessarily a case of ‘either/or’ when deciding on a new water treatment system? Or can the different technologies complement each other?

Ever since the 19th century, when the importance of water treatment for human health was first recognised, a number of technologies for disinfecting and cleaning water have been developed. Two early methods of disinfection that involve chemical additives, chlorination and ozonation, are still extensively used today. More recent processes such as ion exchange and reverse osmosis are also used widely in manufacturing plants to remove dissolved substances. These operations contain components that are highly sensitive to the condition of the water, and so optimising the overall process usually involves linking different treatment technologies together.

Driving water uphill

Reverse osmosis has been used since the 1960s to remove dissolved substances from water. Osmosis is a spontaneous process where pure water molecules pass across a membrane from a solution of low concentration to one of higher concentration, eventually equalising the concentration on each side of the membrane. This process can be forced into reverse by applying pressure to the more concentrated solution, which drives water molecules across the membrane to give a very dilute solution on the other side; this is how reverse osmosis (RO) purifies water.

RO diagram

The membranes in RO systems contain extremely small pores (less than a nanometre in diameter), and so it’s no surprise that they can be blocked by the growth of bacteria or other biological films. This is why it is important to use disinfected water in these systems. However, the widely-used chemical disinfectants chlorine and ozone oxidise the membrane and cause it to be less effective, and so these too must be removed before the water enters the RO unit.

Reverse osmosis membrane coil attrib

A typical reverse-osmosis membrane module

Pretreatment prevents poor performance

To pretreat chlorinated water such as that from the municipal supply before RO processing, either a chemical additive (sodium metabisulphite) is introduced to react with the chlorine molecules, or the water is passed through a bed of carbon granules which binds the chlorine to its surface. However, both these methods introduce other problems. Sodium metabisulphite can cause scaling in the membrane, and carbon filters can be troublesome to operate, with chlorine often breaking through when the bed is saturated. They can also act as breeding grounds for bacteria which are then carried through to the RO membrane, blocking the pores.

UV treatment can mitigate these problems. Not only can it be used to treat the output from carbon filters to inactivate bacteria, but the energetic UV photons can break down the chemical bonds in the chlorine-containing molecules. This requires high UV doses which exceed those needed for disinfection, so UV can in many cases handle both disinfection and dechlorination within a single treatment, with no need for the addition of further chemicals.

Pretreating water also reduces the need for maintenance. In one example, the number of cleaning operations required on an RO membrane reduced from 16 over a period of 2 months to 8 in 4 months after a UV dechlorination unit was installed in place of the previous sodium metabisulphite chemical feed. Within a year, the savings in terms of time and cleaning chemicals outweighed the cost of the UV system.

Fair exchange

A similar situation occurs with ion exchange, which is where dissolved materials that can cause problems are replaced with more innocuous substances. An example is the softening of hard water, where the dissolved calcium and magnesium ions (charged molecules) that form scale are replaced with sodium ions. The ion exchange takes place on the surface of resin beads. Chlorine can break down the structure of these beads which reduces the effectiveness of the process, and so either a chlorine removal treatment is needed followed by UV disinfection, or a single UV dechlorination/disinfection system can be used to pretreat the water for feeding into the ion exchanger.

Ion exchange resin beads attrib

Beads of resin used in ion exchange processes

Storing up trouble

Other common stages in industrial processes can cause unwelcome changes in the water. One example is storage or holding tanks. Even with good manufacturing practice and careful pretreatment of water, it is very difficult to completely eliminate the growth of bacteria in storage tanks, and these bacteria could be carried downstream into further parts of the process. It is therefore good practice to circulate the water in a tank through a UV system at regular intervals to maintain the cleanliness of the water.

Components containing pores, such as filtration units or RO membranes, are also ideal locations for the growth of bacteria. It is recommended that the liquid leaving these components is also disinfected using UV treatment before being used in other parts of the process.

Process flowchart 3

Possible locations for UV treatment in a soft drink manufacturing process

A piece of the puzzle

As the hygiene standards demanded in industrial products become ever more stringent, a range of treatment methods will be needed to control the condition of water or other liquids used in the process. UV treatment fits neatly into this armoury. It compensates for some of the weaknesses in other treatment technologies, keeping the process as efficient and effective as possible while ensuring the safety of those consuming the product.

 

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