Bubble Power: Enhancing a Weapon for RO Membrane Cleaning

July 30, 2014
Efficiently maintained membranes can reduce cleaning frequency and the amount of chemicals used. A three-year R&D project has developed a cleaning system for reverse osmosis (RO) membranes that combines chemical bubble generation with physical bubble generation to enhance foulant removal.

Efficiently maintained membranes can reduce cleaning frequency and the amount of chemicals used. A three-year R&D project has developed a cleaning system for reverse osmosis (RO) membranes that combines chemical bubble generation with physical bubble generation to enhance foulant removal.

By Steve Chesters & Matt Armstrong

More than 600 membrane autopsies carried out showed that in almost 75% of cases, fouling is the main cause of membrane failure. Data from the Genesys Membrane Products' (GMP) facility in Madrid, Spain, demonstrates that the main impact of fouling on membrane performance is damage to the polyamide layer and its salt rejection capabilities.

It's very important to know the extent of membrane damage in order to determine if membrane rejection capabilities can be recovered. As figure 1 demonstrates, it is common to find mild and slight physical damage on the membrane surface.

Chemical damage is also detected mainly as mild and slight damage on a similar percentage of membranes. In order to accurately determine the reason for RO system failure, it's essential to be able to distinguish between the two types of damage.

During normal operation, there are many different processes and events that can damage the surface of RO membranes. These include increase in differential pressure, backpressure phenomena, abrasion processes from fouling, massive or micro damage from scaling, oxidation processes or even degradation with time or increased exposure to cleaning chemical. This damage often results in an increase in permeate flow rate and a decrease in salt rejection.

Physical membrane damage can mainly be attributed to leaks in pre-treatment, the presence of fouling or operational problems. Chemical damage may be caused by incorrect use of chlorination or chemical cleaning techniques, reducing the membrane's viable life.

Membrane Cleaning History

Over the last ten years there have been significant developments in new devices for energy recovery, new membrane materials and new sizes and orientations of RO plants. All of these developments have been motivated by a drive to reduce operating costs and improve efficiency.

However, there have been relatively few developments in recent times aimed at addressing the fundamental issue of keeping membrane surfaces clean in order to ensure efficient RO plant operation. This is surprising as any fouling of the membrane surface will have a dramatic effect on energy consumption and, consequently, plant efficiency.

Around 85% of the main foulants identified on lead membrane elements constitute organic, biological and particulate or colloidal fouling. These types of fouling can be particularly difficult to remove. Over time the foulant becomes compressed under operating pressures and builds into compact layers that conventional chemical cleaning agents can't penetrate. Any delays in cleaning mean the fouling layer becomes thicker and more compressed into the membrane surface, making it significantly more difficult to remove.

When to Clean?

Cleaning membranes before the foulant reaches this intractable stage is crucial, but how can plant operators know when the time is right? The current DOW Filmtec Technical Manual advises: "Elements should be cleaned when one or more of the below mentioned parameters are applicable:

  • The normalized permeate flow drops 10%
  • The normalized salt passage increases 5 - 10%
  • The normalized pressure drop (feed pressure minus concentrate pressure) increases 10 - 15%."

In summary, the message from the membrane manufacturers that if you clean efficiently at the correct time you will need to clean less frequently.

The True Cost of Cleaning

Many researchers have focused on identifying and studying membrane foulants, however there have been few studies into how to remove them or the effects of cleaning in place (CIP) on long-term membrane life. CIP is operationally significant because, done correctly, it will restore salt rejection, operating pressures and flow rates. It's also important to recognise the influence it can have on minimising the required cleaning frequency and physical damage to the polyamide layer.

Main fouling compositions detected on membrane autopsies

CIP costs are normally calculated in terms of the direct cost of the cleaning product per kg. Due to the inherent differences in speciality cleaning chemicals and application, it is vitally important to take a longer term view when comparing the true cost of using them for CIP. For example commodity caustics and acids are often used due to their lower unit cost. However, if we consider that a membrane may require cleaning three or four times more compared to when using a specially formulated chemical, the total CIP cost will be significantly more with the 'cheaper' chemical.

Specialty blended cleaning chemicals incorporating detergents, surfactants and chelants are widely used and are increasingly accepted by the market to be economically and environmentally viable. One of the most important factors when calculating the true costs of membrane cleaning is frequency. This has a direct effect on operational availability, man hours and membrane life and therefore operating costs.

Another important factor to consider is that removing deposits by normal cleaning techniques can often result in irreparable abrasion damage to the membrane surface. This is due to using high cleaning pressures and flow rates to remove particulate foulants.

Membrane autopsy is a powerful way to work out how to optimise the CIP process for a particular plant. During an autopsy, once the foulants have been identified, optimum chemical and cleaning protocols can be assessed in the laboratory. The most effective cleaning agents can be stored on-site ready to begin CIP as soon as there is a notable change in operating parameters. The aim is to clean the membrane effectively before the fouling layer has a chance to build up, at which point it would be much more difficult and costly to clean.

Restoring Membranes to a 'Virgin' State

Membrane cleaning efficiency has a major impact on system downtime, output, chemical usage and membrane life. The more foulant is removed during the cleaning cycle, the longer the membrane takes to re-foul and the more efficient the operational system. Using the right chemicals and applying them in the best possible way will reduce cleaning frequency and chemical use while increasing membrane life and operational efficiency.

The image shows an electron microscope images of two different types of membrane damage: abrasion marks from fouling (A) and abrasion marks from the spacer material (B)

Taking these factors into account, the objective of every CIP procedure should be to completely remove the membrane foulant, restoring the membrane to a 'virgin' state. This will reduce the required cleaning frequency and increase operational efficiency.

Ultrafiltration and microfiltration systems already use air bubbles to enhance foulant removal. The Genairclean process was launched to go a step further by combining chemical bubble generation with physical bubble generation to enhance foulant removal. It was the brainchild of Genesys senior R&D chemist Max Fazel.

For many years he had been looking for a method which completely removed foulant while minimising abrasion, reducing required cleaning frequency while avoiding membrane damage. He had a theory that using carefully controlled air bubbles could increase membrane cleaning efficiency. It wasn't until he joined Genesys in 2010 that he was able to put it into practice.

Chemical Bubble Generation

The effectiveness of effervescent compounds in cleaning reagents used in the food and drinks industry and dental hygiene is well documented. Fazel and his team tested a number of effervescent reagents while refining the formulation of powdered membrane cleaning compounds. When the powder is dissolved to make up the cleaning solution the effervescent reagents produce gas as bubbles, which physically agitate the foulant during cleaning circulation. This has the dual effect of physically removing the foulant and increasing contact of the cleaning reagents with the foulant surface.

Two cleaning agents were selected for use in the Genairclean process – Genesol 704 and Genesol 701. These combine multiple chemical cleaning mechanisms to disrupt and remove foulants: micro-bubbles; normal osmosis; effervescents; detergent; surfactant and chelants.

Research proved that bubble size, volume and distribution has a major influence on foulant removal. One component of Genesol 704 and Genesol 701 optimises bubble conditions.

Bubble Generation & Testing

The Genairator uses an energy neutral method to induct air into the CIP solution from the atmosphere. This can easily be incorporated into existing CIP systems. During the 'soaking' phase the products loosen deposits at the membrane surface. The high ionic strength of the cleaning solution causes permeate water to flow to the feed side of the membrane via normal osmosis, lifting deposits from that side. Then in the 'circulation' phase the Genairator system creates an evenly distributed flow of air bubbles along the membrane surface that agitates and lifts the foulants.

Field tests were carried out at an industrial water reuse site in the UK. The plant recycles all of the factory wash down water via an aerobic bioreactor, UF and RO membranes. The RO membranes fouled rapidly with clay, biofilm and waste wash-down chemicals. Prior to the trials, membrane cleaning was carried out every seven to ten days, with an average normalised differential pressure (ndP) across the plant of 4.6 bar. The membrane elements were changed and ndP stabilised at 2.5 bar using Genesol 704.

Furthermore it is thought that biofilm removal and disruption will significantly reduce the surviving microbial population which will not then replicate at the same rate despite the on-going nutrient source.

The initial results are very encouraging showing a distinct improvement in the ability to clean these rapidly fouling membranes in a significantly shorter timescale. No loss of salt rejection has been detected and permeate flow has improved from 15 m³/hr at the beginning of 2013 to 24 m³/hr from Jan to April 2014. This is with an average feed pressure reduction from 13 bar to less than 9 bar. The frequency of cleaning has reduced from every 10.5 days in 2013 to over 30 days in the beginning of 2014 and currently cleans are done every 50-70 days.

Reduced cleaning frequency is expected to extend membrane lifespan and the reduction in feed pressure will have a significant impact on electricity requirement for pumping. These numbers are now being calculated and will be featured in future papers.

Steve Chesters is CEO and Matt Armstrong is sales & marketing manager at Genesys International. For more information email: [email protected].

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