Reverse Osmosis v De-Ionisation – Expert Spotlight on Water Recovery Technology

Our AllWater industrial water treatment experts shine a spotlight on using Reverse Osmosis and De-Ionisation in water recovery.

At AllWater Technologies, we’re experts in industrial water treatment and we use various technologies to facilitate the important job of recycling and reusing effluent for beneficial purposes. But not all systems are suitable for every situation – so it’s important to understand their strengths and weaknesses.

Water Recovery Technology 

The use of Reverse Osmosis (RO) plant as opposed to De-Ionisation, with the view to being able to do away with the need for hazardous regenerant chemicals is a popular concept. However, our many years of experience in the business of industrial water treatment have shown us that there are a number of problems associated with the use of RO for water recovery.

Read on for some of the main ones we’ve identified. 

The Problem of Organic Fouling

Organic Fouling RO membranes are considered to be “molecular sieves”, and as such they provide a barrier to ionic species. The membranes are operated very nearly “dead ended”, with very little cross flow, and we’ve found that the result can often be that they are very susceptible to particulate and organic fouling. 

Organic foulants are typically very large molecules and can create “plug fouling” of the membrane pores, which leads to increased pressure loss across the membrane and reduced flux rates. For some of our clients, if the organic fouling is not too heavy, we’ve found that membranes can normally be recovered by carrying out a Clean-In-Place (CIP), which we undertake using an alkaline pH cleaning solution. 

Although organics are sometimes present in town water supplies and require pre-treatment for removal prior to reaching the RO membranes, in industrial water treatment systems for our clients that produce waste streams from surface finishing applications, we’ve found organics are often present. These organics derive from pressing oils, which can be found in rinses, post cleaners and brighteners. 

In order to prevent organic fouling, when we consult with our clients we recommend a considerable degree of pre-treatment. Depending on the particulars of an individual client, we might use an organic scavenger, which employs an anionic ion exchange resin and requires regeneration on brine, or a combination of brine and caustic solution. In order to improve cleaning efficiency, we might recommend warming the combined brine and caustic solution and, on occasion, we use activated carbon for organics removal – although contact time is often quite long. As it is unusual to carry out reactivation of the carbon on site, we take great care to monitor organics removal efficiency and to ensure we replace the carbon upon exhaustion. 

The De-Ionising systems (DI) we recommend and employ for our clients for their water recovery often utilise anion resins, which we’ve found offer some degree of protection against organic fouling (e.g. macroporous resins). Regeneration with higher levels of caustic on resin exhaustion also helps prevent organic fouling in our clients’ industrial water treatment systems. 

The Problem of Particulate Fouling 

The construction of RO membranes with little space between membrane sheets, and the low cross flow operation, means that RO membranes are very susceptible to particulate fouling in many of our clients’ systems, based on their usage. Depending on their needs, typically, we would employ a 5 µm filter immediately prior to the membranes to provide protection. Where the level of suspended solids in the incoming water is particularly high and the pre-filter becomes blocked on a regular basis, we sometimes find it necessary to employ additional filtration, such as is offered by multi-media filtration. 

The conventional DI systems we’ve installed for clients that employ twin beds (i.e. separate cation and anion columns) and are operated co-currently (i.e. regeneration in same direction as service flow), typically have the facility for backwashing of the resin beds as the first stage of regeneration. Although this does not make the resin beds immune from particulate fouling, and we need to carry out additional pre-filtration where incoming suspended solids levels are high, it does afford some protection and helps prevent buildup, with increase in pressure losses resulting from bed fouling tending to build up over a period of time. 

As a result, fouling from particulates tends to take longer to manifest itself on DI systems that are part of the industrial water treatment solution we’ve integrated for our clients, than on RO systems, where we can see the fouling much more quickly. 

Metal Precipitation on Concentration

Ionic species present in the incoming water are concentrated up on the surface of the RO membrane. Where heavy metals such as Copper (Cu) or Nickel (Ni) are present at low levels this can lead to concentration reaching saturation point with the result being precipitation at the RO membrane surface. The resultant particulates then lead to fouling of the RO membrane. 

Because this phenomenon does not occur in DI systems, where most metals are removed by the cation resin and the high strength of acid used for regeneration ensures that metals regenerated from the resin remain in solution, we recommend DI as an industrial water treatment solution to clients where heavy metals such as Copper (Cu) or Nickel (Ni) are present.

Iron (Fe) Fouling 

Depending on the location of our clients’ facilities, in addition to the abovementioned elements, large, trivalent ionic species such as Fe and Aluminium (Al) can cause fouling of RO membranes. Fe is often present in towns waters, which originate from borehole supplies, which is why this phenomenon is more widely recognised as “Fe fouling”. 

Where Al is present, such as in clients whose facilities produce rinse waters resulting from Al surface treatment processes, a similar problem occurs. This leads to increased pressure losses across the membrane and reduced flux rates. If fouling is not too pronounced, we can normally recover the membranes by carrying out a CIP using an acidic pH cleaning solution. 

Although the issue also occurs with ion exchange resins where the trivalent metals become strongly bound to the cation resin, we implement regeneration with higher levels of acid on resin exhaustion for our clients, to help prevent loss of capacity from irreversible exchange. 

Lack of Redundancy 

RO membranes tend to perform better when in constant operation and do not require downtime for regeneration. Our RO systems are typically designed with a single stream, whereas our DI systems are often “duplex”, offering both duty and standby streams, to accommodate for regeneration. For many of our clients, the need for unscheduled downtime required for regeneration where RO is employed would cause major problems. In those cases we would always employ a duplex DI system to ensure there is no interruption to their operations. 

When it comes to the question of whether Reverse Osmosis or De-Ionisation is the most suitable solution, the many years of collective experience of our AllWater technicians allows us to create and implement sustainable, affordable water treatment solutions tailored to the unique needs of your individual business. Our deep understanding and expertise serving a wide range of sectors will ensure your industrial effluent management is not only effective, but also compliant.

If your wastewater treatment systems need updating, or you’re starting from scratch, AllWater has the knowledge, skills and expertise to design and install the right equipment and strategies perfect for your particular requirements. When it comes to water recovery technology, it’s vital for your business and for the environment that it’s done right, and you can rely on our many years of experience to do just that. Get in touch today to discuss what we can do for you.