Reverse osmosis (RO) systems have become the industry standard for purification of any feed water source – as well as wastewater and reuse applications, including several industrial processes. The typical lifetime of a RO membrane element is three to seven years depending on the application, it also depends the RO type and free chlorine level before RO membrane. In some RO systems, however, the membrane elements may only last one to two years due to biofouling and scaling.
The lifetime of RO elements is dependent on several factors, including system design, element selection, pre-treatment, system operation, membrane cleaning, and system maintenance. This article will focus on the cleaning of RO systems containing spiral wound thin film composite membrane elements.
The surface of a RO membrane is subject to fouling by foreign materials present in the feed water, such as hydrates of metal oxides, calcium precipitates, organic and biological matter. The term “fouling” includes the build-up of all kinds of layers on the membrane surface, including scaling and biofilm buildup.
Fig 1, Ro membrane autopsy, biofilm covered on surface
1 Pretreatment Indicators
Most RO systems use pretreatment to minimize the performance decline of the membrane elements due to fouling and chemical attack. The type of pretreatment is dependent on the composition of the feed water source. For instance, surface waters typically have high concentrations of colloids (reflected as high turbidity levels and/or high SDI levels – SDI meaning silt density index). Most RO elements require turbidity levels less than 1 nephelometric turbidity units (NTU) and/or less than 5 SDI units. Multimedia filtration, microfiltration and ultrafiltration are unit operations commonly applied to reduce the colloids in the raw feed water.
The raw water source is often an indication for the type of fouling that can be expected in the membrane elements. For instance, in the case of surface waters, colloidal fouling, biofouling and organic fouling are often found on the membrane surface. Well waters typically have a potential for iron oxide or iron hydroxide fouling. Municipal waste water biofouling is major problem.
RO elements are supplied in different sizes. The most common sizes are 2.5”, 4”, and 8” diameter. RO systems containing 2.5” and/or 4” diameter membrane elements are seldom equipped with a cleaning skid. These membrane elements are either replaced or sent to a location where they can be cleaned. A few membrane chemical suppliers and original equipment manufacturers (OEMs) offer an off-site membrane cleaning service. In some cases, it’s cheaper to install new membrane elements than going through the trouble of cleaning the membrane elements. In the case of 8” diameter RO elements, cleaning is standard practice.
Many RO systems are equipped with a cleaning skid also known as a clean-in-place (CIP) skid which typically consists of a cleaning tank and a cleaning pump. The design of the CIP skid is very critical to achieve optimum cleaning results. CIP tank should equip with heater to control the CIP solution temperature and should be able to add extra PH enhancer to increase solution PH level.
Fig 2 RO CIP tank
For properly designed and operated pretreatment and RO systems, the typical cleaning frequencies are two to four cleanings per year for surface water, and one to two cleanings per year for well water. But for the municipal waste water maybe need more frequency cleaning depends the pre-treatment system.
2 Cleaning Frequency
Membrane manufacturers recommend cleaning when:
• Normalized permeate flow drops 10%.
• Normalized salt passage increases 5-10%.
• Normalized pressure drop increases 10-15%.
Effective or successful cleaning is dependent on the following factors:
• A cleaning is required when one of the key cleaning criteria above is met.
• A correctly designed cleaning skid is used.
• Membrane elements can withstand acid cleanings at pH 1 and alkaline cleanings at a minimum pH of 13, which depends the CIP solution temperature, higher PH requests slightly lower temperature
For the RO biofouling, the supply pressure will increase due to biofouling start for the lead side of the membrane, so most time this DP increase first, when the feed pressure increase the higher pressure will create more flow pass the membrane, so the flow drops always behind DP increase.
The reference point for the above key criteria is the stabilized performance of the RO system established during the first 48 to 72 hours of operation. If one of the above three cleaning criteria is met, then cleaning is recommended. If one waits too long (e.g., normalized permeate flow has dropped 30%) to clean, then it will become very difficult to effectively clean the membrane elements by standard cleanings – and may necessitate repeated cleanings or extreme cleaning.
Instead of applying the 15% normalized pressure drop increase as a cleaning criterion, some OEMs and end-users are applying 15 psi (1 bar) per element or 50 psi (3.4 bar) for a multi-element pressure vessel. This should be avoided for the following reasons. A typical normalized pressure drop for a multi-element vessel is approximately 15 psi. The 50 psi pressure drop cleaning criteria would mean that the pressure drop has increased 333%. Operating at 50 psi pressure drop may cause irreversible protrusion of the membrane element scroll commonly referred to as telescoping. In addition, membrane intrusion can result when excessive operating pressure pushes the membrane into the permeate channel spacer resulting in low permeate flow and high salt passage over time.
In addition, high pressure drops typically indicate severe fouling. Cleaning or repeated cleanings are often not able to restore the membrane element performance decline caused by the fouling. In addition, cleaning won’t restore the performance decline caused by the mechanical damage.
3 Importance of Normalization
The performance of a RO system is determined by different factors like the feed water composition, feed pressure, temperature and recovery. For example, a feed temperature drop of 4°C [7.2°F] will cause a permeate flow decrease of about 10%. This, however, is a normal phenomenon. To distinguish between such normal phenomena and performance changes due to fouling or other problems, the measured permeate flow and salt passage have to be normalized. Normalization is a comparison of the actual performance to a given reference performance while the influences of operating parameters are considered. The reference performance is typically the stabilized performance of the RO system within the first 48-72 hours of operation.
Most membrane manufacturers and a few OEMs have developed software programs to normalize operating data. The normalizing software programs offered by membrane manufacturers can be downloaded or requested through their web sites.
Instead of using software programs to normalize, the normalized permeate flow, normalized rejection and pressure drop can also be calculated by hand by using ASTM standard D4516-. But this is cumbersome and labor intensive. Normalization software is commonly used and recommended. Most of the OEMs provide free software can be download from they website.
Normalization is important to assess the performance of an RO system. It requires at least once daily collection of system parameters, such as pressures, flows, temperature, conductivity etc. The data are entered into the normalization software. Normalization is not only important to determine when cleaning is needed but it also shows the behavior of the membrane system. For instance, is the performance decline gradual or very sudden?
In this specific case, the normalized salt passage and the pressure drop did not show significant increase. The main symptom of this plant is a permeate flow decline. The data indicate that the normalized permeate flow has declined steadily. The RO system seems to be fouled (assuming all process equipment is calibrated).
This system would have required cleaning on day seven, according to the cleaning criteria. Instead, the operation of the RO system continued without cleaning. As a result, the permeate flow of the system had declined 45% after 93 days of operation. With this much flow loss, routine cleaning may not be effective in restoring the membrane performance especially in cases of organic fouling, biofouling, colloidal fouling, sulfate scaling and calcium fluoride scaling. In these cases, extreme cleanings will be necessary.
Extreme cleaning is cleaning at pH and temperature conditions that are outside the guidelines of the membrane manufacturer and/or eventually combined with cleaning chemicals, such as bleach and ammonium bifluoride, that are not compatible with the membrane elements. These extreme cleanings may be able to restore the membrane performance to an acceptable level, but often the salt passage will be higher due to the harsh treatment. Applying an extreme cleaning is worthwhile, as it may delay membrane replacement.
4 Cleaning efficiency
For the RO biofouling cleaning, normal use two batch of chemical RL1500 and RL5000, the PH is important factor to successful cleaning, higher PH level which can increase the solvability of organically, let the first patch chemical cycle one hours and sock 1 hours then cycle 1 hours and sock half hours, monitory the CIP solution color change, at first the CIP solution will be become black then slowly change dark to yellow, the yellow color is the biofilm, after 4 hours cycling if the color is not change any more, to drain the CIP solution and blending another batch, which RL5000 should be higher concentration to make it reaction the harder layer of the solution continue to monitory the CIP solution color change, the goal of second batch cleaning is try to deep cleaning the biofilm , higher PH is important, if the PH is below 12, add extra PH enhancer cleaning solution
Figure 3 before CIP cycle the solution color
Figure 4 first batch chemical after 4 hours cycling color
After cleaning, rinse out the CIP solution from membrane is important, residual of the CIP solution is possible block the RO membrane pore at normal operation higher feed pressure.
Figure 5 Second batch 4 hours cycling solution color
Table 1 RO train #1 cleaning results
To compare with before and after cleaning efficiency, using the software to compare before and after the cleaning the DP change then it will find the results. Table 1 is showing the RO cleaning results, after cleaning the Normalized DP decreased 82.85%.
5 Conclusion
Normalization is recommended in order to monitor the performance of RO systems. Normalized data are critical in determining whether one of the key cleaning criteria is met. It is recommended to clean an RO system when any one of the key cleaning criteria is met. If cleaning is put off, then standard cleanings may be ineffective. This could lead to the need for extreme cleanings, which can cause damage to the membranes.
MOE’s recommendation criteria sometime slight conservative if the RO is biofouling, using the software to monitory the biofouling speed kpa/hours is another best way to identify the time to cleaning RO, for example, the new membrane in service the membrane DP increase ( biofouling) speed normal is 0.4 kpa/hours, if the software find the speed start increase faster, up to 1.5kPa, that means it is cleaning as quick as possible, otherwise, the DP will not return to base line.