Mitsubishi Heavy Industries Technical Review Vol. 60 No. 1 (March 2023)
1Rejuvenation Chemical Agent for Degraded ROMembrane

RYO KAMITO*1

YOSHIAKI ITO*2

KOKI OKUBO*3

SHO YOKOGAWA*4

TAKAYOSHI HORI*5

Seawater desalination plants, one of the solutions to the global shortage of fresh water, arenow mainly using a reverse osmosis (RO) membrane method, which has energy saving potential,
instead of the conventional evaporation method. The desalination performance of RO membranes
degrades over time, and eventually they no longer satisfy the required desalination performance. Insuch cases, the degraded RO membranes are replaced with virgin ones and disposed of in landfills.
Therefore, reduction of the membrane replacement costs and the environmental impact havebecome issues. To address these issues, Mitsubishi Heavy Industries, Ltd. (MHI) has developed achemical agent (desalination performance rejuvenation agent) that selectively repairs degradedareas of cellulose acetate RO membranes and rejuvenates the desalination performance, thereby
reducing the frequency of membrane replacement (prolonging the membrane life). This reportintroduces the characteristics and effects of the chemical agent for rejuvenating desalination
performance of cellulose acetate RO membranes.

|1. Introduction
Today, a pressing global concern is the shortage of freshwater. Desalination technology,
especially desalination from seawater as a virtually unlimited resource, is attracting attention as asolution to this problem. In recent years, desalination technology has been shifting from theconventional evaporation method, which was the mainstream method, to the membrane method,which consumes less energy. Of the current total desalination capacity of 115 Mm3/d, 88 Mm3/duses reverse osmosis (RO) membranes(1). We also have delivered a large seawater desalination
plant that employs the membrane method.
RO seawater desalination plants require appropriate design of the RO membrane feed watertreatment (sand filtration, membrane filtration, etc.) and the operation method because theoperating rate and operating costs are affected if the water permeability decreases and thedifferential pressure increases due to fouling and scaling of the RO membrane, or if thedesalination performance decreases due to membrane degradation. If fouling or scaling occurs,
cleaning the membrane with acid or alkali is effective. However, membrane degradation(membrane structure defects) is irreversible, so there is no alternative but to replace the membrane,
which increases the replacement cost and environmental impact due to disposal. Therefore, a newmaintenance method that can address this issue is strongly needed.
One of the most promising methods is to rejuvenate desalination performance by using achemical agent. This method rejuvenates desalination performance by selectively coating thedegraded areas of the membrane with a desalination performance rejuvenation agent to inhibit salt
*1

Chemical Research Department, Research & Innovation Center, Mitsubishi Heavy Industries, Ltd.

*2

Research Manager, Chemical Research Department, Research & Innovation Center, Mitsubishi Heavy Industries, Ltd.

*3

Service Engineering Department, Research & Innovation Center, Mitsubishi Heavy Industries, Ltd.

*4

Nagasaki Solution Business Department, Steam Power Maintenance Innovation Business Division, Energy Transition & Power*5Engineering Manager, Nagasaki Plant Engineering Department, Steam Power Maintenance Innovation Business Division, EnergyHeadquarters, Energy Systems, Mitsubishi Heavy Industries, Ltd.
Transition & Power Headquarters, Energy Systems, Mitsubishi Heavy Industries, Ltd.

Mitsubishi Heavy Industries Technical Review Vol. 60 No. 1 (March 2023)2

permeation through the membrane, thereby reducing the frequency of membrane replacement(extending the membrane life), which is expected to reduce membrane replacement costs andenvironmental impact. There are two types of commercially available RO membranes: polyamide
and cellulose acetate, and for the former, which has a large market share, the use of desalination
performance rejuvenation agents has already been reported(2)-(5). On the other hand, there has been
no commercial application of desalination performance rejuvenation agents for cellulose acetate
RO membranes, which are widely used in plants we have delivered. Therefore, we have
independently developed and commercialized a chemical agent to rejuvenate desalination
performance for RO membranes made of cellulose acetate. This report describes the features andeffects of this chemical agent.

|2. Development policy of desalination performance rejuvenation
agent
There are two degradation mechanisms of cellulose acetate RO membranes: hydrolysis andoxidative degradation, as shown in Figure 1(6). Hydrolysis degradation is a phenomenon in which
an acetyl group (-COCH3) in cellulose acetate is replaced by a hydroxy group (-OH), and oxidative
degradation is a phenomenon in which the cellulose acetate main chains are broken by an oxidant
such as chlorine injected as a preventive measure against biofouling. Site defects resulting fromthese phenomena become salt permeation pathways, causing a reduction in desalination
performance.
As shown in Figure 2, the concept of the desalination performance rejuvenation method wehave invented is that the desalination performance rejuvenation agent selectively coats defects
caused by hydrolysis or oxidative degradation and inhibits salt permeation through the membrane.
The desalination performance rejuvenation agent is required to have a high affinity for thedefective (degraded) part of the cellulose acetate RO membrane. We selected a promising agent
from a large number of candidate agents, and verified its effectiveness in rejuvenating desalination
performance through laboratory-scale and pilot-scale tests.

Figure 1

Degradation mechanism of cellulose acetate RO membrane

Figure 2 Rejuvenation mechanism of desalination performance rejuvenation
agent

Mitsubishi Heavy Industries Technical Review Vol. 60 No. 1 (March 2023)
3|3. Effects of desalination performance rejuvenation agent
3.1

Laboratory-scale test

Laboratory-scale tests were conducted using the procedure shown in Figure 3. Degraded RO
membranes were prepared by forcing virgin cellulose acetate RO membranes (flat membrane,effective membrane area 140 cm2) to undergo hydrolysis or oxidative degradation using chemicals.
The degraded RO membranes were rejuvenated using the selected agent, and by comparing theseparation performance before and after the degradation and rejuvenation (NaCl: 35,000 mg/L,
feed flow rate: 3 L/min, feed pressure: 3 MPa, water temperature: 30°C, evaluation time: 2 hours),
the effect of the chemical agent as a desalination performance rejuvenation agent was verified. As aresult, as shown in Figure 4, it was confirmed that the degraded desalination performance (saltrejection) of both hydrolysis and oxidative degraded membranes was increased (rejuvenated) by therejuvenation. This indicates that the chemical agent is effective in rejuvenating desalination
performance regardless of the degradation mechanism and is a promising desalination performancerejuvenation agent.

Figure 3

Laboratory-scale test procedure

Figure 4

Laboratory-scale test result

3.2

Pilot-scale test

Pilot-scale tests were conducted using the procedure shown in Figure 5. In this test,
membranes were used from an RO module used in a seawater desalination plant in commercial
operation in Saudi Arabia. The membranes have a permeate salt concentration more than 10 times
higher than that of virgin membranes. These used membranes were rejuvenated under theconditions verified in the laboratory-scale test, and the separation performance data before and after
the rejuvenation (NaCl: 35,000 mg/L, recovery rate: 30%, feed pressure: 5.4 MPa, watertemperature: 25°C, evaluation time: 2 hours) was obtained. Based on the changes in the A value(water permeability), B value (solute permeability), and the permeate conductivity, theeffectiveness of the developed method for rejuvenating desalination performance was verified. TheA and B values are defined by the following basic equations of the dissolution-diffusion theory.
A = Jw / (ΔP-Δπ)
where Jw is the water permeate flux (mass flux kg/(m2・s) or volumetric flux m3/(m2・s)), ΔPMitsubishi Heavy Industries Technical Review Vol. 60 No. 1 (March 2023)
4
2

is the pressure difference between feed water and permeate water (kgf/cm ), and Δπ is the osmoticpressure difference between feed water (membrane surface) and permeate water (kgf/cm2).
B = Js / (CM-CP)
where Js is the salt permeate flux (kg/(m2・s)), CM is the salt concentration at the membranesurface (kg/m3), and CP is the salt concentration in the permeate (kg/m3).

Figure 5

Pilot-scale test procedure

Figure 6 to Figure 8 show the A value, B value, and permeate conductivity before and after
the rejuvenation resulting from the test. It is indicated that the chemical agent was able to reducethe B value and salt concentration of permeate water without decreasing the A value. This result
confirms that the chemical agent works effectively for actual used RO modules.

Figure 6

Pilot-scale test result (A value)

Figure 7

Pilot-scale test result (B value)

Figure 8 Pilot-scale test result (permeate conductivity)

Next, Figure 9 shows the result of approximately 1,000 hours of operation to confirm theMitsubishi Heavy Industries Technical Review Vol. 60 No. 1 (March 2023)
5

long-term stability of the desalination performance rejuvenation agent. As a result, it wasconfirmed that the rejuvenation reduced the permeate conductivity from 3,600 μS/cm to about
1,200 μS/cm, and that this condition could be maintained for 1,000 hours. This confirms that thechemical agent is effective in rejuvenating desalination performance and its long-term stability, andthat it is highly applicable to actual equipment.

Figure 9Long-term stability test result of desalination
performance rejuvenation effect|4. Conclusion
This report reported on the discovery of an appropriate desalination performance
rejuvenation agent and its application conditions for degraded RO membranes made of cellulose
acetate, and the prospect of applying the chemical agent to actual equipment. This achievement wasalso reported at the IDA* 2022 World Congress. In September 2022, we received the first order forthis chemical agent for a large seawater desalination plant in Saudi Arabia and began its
commercial application. We will work to improve the desalination performance rejuvenation agent
and expand its application. We have already obtained a patent for this chemical agent.
* International Desalination AssociationReferences
(1)
(2)
(3)
(4)
(5)
(6)

Nirajan Dhakal et al., Is Desalination a Solution to Freshwater Scarcity in Developing Countries?,Membranes, Vol.12 No.381 (2022)
S.P. Chester et al., Result from 99 sea water RO membrane autopsies, IDA Journal of Desalination andWater Reuse,5 (2013), p.40-47
Jin-Yang et al., Research on refurbishing of the used RO membrane through chemical cleaning andrepairing with a new system, Desalination, 320 (2013), p.49-55.
S. T. Mitrouli et al., Application of hydrophilic macromoleculers on thin film composite polyamide
membranes for performance restoration, Desalination, 278 (2011), p.105-116.
Taeko Nakamura et. al., Innovative chemicals for deteriorated RO membranes, IDA15WC, 51420 (2015),
p.3-9
Mohammed Al-Abri et. al., Chlorination disadvantages and alternative routes for biofouling control inreverse osmosis desalination, Clean Water, 2 (2019)