ESEC RO Operation overview and Membrane befouling Reports

Abstract:

ESEC RO system has been in operation for over 6 years, through MOC projects, our RO operation is now stable.  We have replaced the first pass membranes 6 times on each train so far, and once for each second pass. Biofouling issues still exist which requires frequent chemical cleaning, and membrane life is short.

I will discuss challenges faced, and our solutions thus far.  Weight of exhausted membranes and autopsy results of the most heavily fouled membranes found that all surfaces were covered with biofilm.  We’ll look at how chlorine and de-chlorination processes can determine membrane biofilm growth, discuss several RO operation strategies, and improved RO membrane cleaning methods.

1.ESEC RO operation overview

ESEC demin water treatment uses treated wastewater from the city .   From the discharge of a rotational filter, UF filter (Turbidity<0.3NTU, SDI<0.1), chlorine addition in service tank, the water then flows through to micron cartridge filter(5µm).  Antiscalant and sodium bisulfite are then added.  The next destination is through first pass RO membranes.  Upon exit, we add caustic before the second pass.  Mix bed polishers finish the job before storage to the demin water tank.

1.1 RO Membrane type and design life 

Hydranautics Low Fouling Spiral Wound, Composite Polyamide Membrane, CPA5-LD, Active Area: 400 ft2 (37.2 m2) total active area for the train 1 and train 2 first pass is 2678.4 m2.  Feed Spacer is 34 mil (0.86 mm) [1], which was increased from 26mil.  Thicker spacers with a unique geometry have been shown to have less pressure drop and reduce the frequency of chemical cleaning when treating water of poor feed quality [2].

Reverse osmosis membranes are semipermeable with pore size < 0.001 µm (UF pore size <0.04 µm) are capable of removing total dissolved solids from high ionic strength water (waste water, seawater ground water).Pretreatment assists in reducing the contaminants that enter the RO feed. Treatment upstream of RO membranes is designed to remove competing substances to protect the RO membranes from fouling caused by particles, inorganic salts, and organic compounds.

Membrane maximum free chlorine limit is 0.1ppm, each membrane maximum DP is 103.4 kPa, so if the first stage total DP is over 620kPa, the manufacturer recommends replacing the membranes.

RO membrane design life is 1000 ppmh free chlorine operation.  At a feed water of 0.038 ppm free chlorine level, a set of membranes can last 3 years.  Our RO annual operation is around 3100 hours, so membrane designed life is 8.0 years if RO feedwater free chlorine below 0.038ppm.  Thin-film composite membrane elements can have a very long life when operated under optimal conditions, in some case over a decade [3].

1.2 RO production

Table 1 is showing yearly RO running production, total production is 693,034 cubic meters.  Production increases starting in 2019 is due to our CT evaporator inlet target temperature being decreased from 17 °C to 12 °C (so we could increase CT power output).  Average RO operation increased from 6 hours to 12 hours per day.

Table 2 is showing most recent replacement RO operation results.  RO train #1 new membranes installed on 2019-05-29, and ran until 2020-09-21, when we replaced them again.  RO train #2 new membranes installed on 2019-07-30 then running until 2020-10-14 for replacement.  The total run time for train #1 was 4032 hours and train #2 was 4239 hours between membrane replacement being required.

Table 1 RO yearly production

	Train #1	Train #2
	ton	ton
2014	30343.8552	37660.99261
2015	44093.0239	54002.93306
2016	32229.7814	37192.22209
2017	35058.8247	44460.75135
2018	53233.021	56949.87415
2019	61986.3817	64486.63496
2020	73277.744	68058.73413
train total	330222.632	362812.1424
Total production	693,034.8 ton

Table 2 Most recently replaced RO production data

		RO Train 1	RO Train 2
Operational	Days	440	477
Run Time	Hours	4032	4239
Total throughput	cubic meters	103441	103092
Cleans	# times	13	15
Feed pressure before replacement	kPa	2050	2010
Feed pressure with new membranes	kPa	1000	1000

1.3 Membrane replacement history

Figure -1     RO train #1 feed pressure life trend

Figure 1 is showing the RO life time feed pressure increase tread, new RO membrane the fast pass feed pressure is around 1100kPa, and DP is around 350kPa between feed pressure to second stage reject pressure, the feed pressure will start increase due to the RO fouling.RO cleaning can bring feed pressure down until replacement. On the RO feed line there is a safety valve installed, set point is 2070kPa, when RO in operation over 2000Kpa inlet pressure and can’t decrease it through the RO cleaning, it is time to replace the RO membranes.

Starting from May of 2014 to current date.  For the pass #1 each train replaced 6 times, and each train pass #2 replaced once.  Total of 468 membranes replaced, using current membrane prices of $786/each, calculation of total cost is over $360,000, not including cleaning chemical and labor.  The reason we need to replace the membranes is that the RO feed pressure gets too high (low flow) and could no longer recover through chemical cleaning.  

Table 3 is showing our membrane replacement history, average operation life is 1 year plus 2 months, operation which is far less than 8 years design life.     

                                                                          Table 3 RO replacement history

Salt passage for train #1 before replacement was 0.91% which means the RO can provide the desired permeate quality water, the membrane is in good shape, the replacement only due to the membrane fouling.

1.4 RO system operation problems:

• RO inlet feed pressure increasing fast, requiring lots of cleaning
• RO trip due to pass 1 permeant pressure high and low
• O-ring leakage or glue line damage
• Frequency of replacing mixed bed bottles

1.5 Completed MOCs to help RO stable operation

Past several years to improved RO operation implementing the following MOC successfully completed and bring good results for the RO operation

Install biocide injection system.  Each time the ROs stop, the logic is set up to inject DBNPA for 25 seconds each time the trains flush.  This helps to kill bacteria and then flush it out with Demin water.

To relocate the SBS to after the cartridge filter and move the ORP meter to after the SBS injection point

• Install second pass pump VFD

Installed VFD drives to adjust the pump speed to match the first pass permeate flow and control the first pass permeate pressure to avoid the RO tripping due to either high or low RO first pass permeant pressure.  Also, this keeps the second pass feed valve 100% open, saving some energy and eliminated the second pass feed valve response issue successfully, which would occasionally trip the RO train.

RO first pass feed water vale controlled by permeant flow keeps the RO in stable operation, the inlet valve will gradually open more as the RO starts fouling.  At the end of membrane life, with the feed valve open 100%, the permeate flow starts gradually decreasing due to the RO fouling.

A stable, positive pass 1 permeate pressure also provided us with a more accurate pH indication of the water prior to entering pass 2, which allowed us to:

• Caustic injection automatic control:

Caustic pump set up automatically to adjust the pump output to help the stabilize the caustic injection flow, maintaining a consistent pH.  Keeping the second pass inlet pH at 8.4 is good practice to extend the mixed bed life.  Once the caustic was set up automatically and the pH set point increased to 8.4, the mix bed throughput is largely increased to average 15,000 cubic meters flow between each change.  Caustic pump automatic operation to stabilize the RO first pass permeate pH to best neutralize any bicarbonate acid (CO2) obviously extended the mixed bed life up to three times longer.

•  Install reject pressure transmitter at first pass stage 1 and 2

At train #1 and 2, installed 4 pressure transmitters to measure the pass 1 stage 1 and stage 2 reject pressure.  These were connected to DCS to provide the DP for the first pass membranes which gives us the information required to calculate normalized flow and DP for performance monitoring.

• Increased storage tank free chorine residual

The storage tank chlorine level increased from 1.0 ppm to 2.0 ppm to provide a more effective kill of organics in the tank.

• CIP tank install hard connection pipe

Install permanent Clean In Place piping between the CIP tank with RO train first pass to minimize the operator work to connect a temporary hose for each CIP, the permanent hard pipe to prevent the connection from leaking, then less safety and environmental potential impact at CIP time.

1.6. RO performance monitory tool

Hydranaucs provided a Reverse Osmosis (RO) system performance data management and normalizing program. It is recommended that “normalize” operating data to determine if operation have a problem with RO system. “Normalizing” computer programs, such as RODataXL, graphically represent normalized permeate flow, percent salt rejection and feed-to-reject pressure drop. These normalized parameters are calculated by comparing a specific days’ operation to the first day of operation. Adjustments are made for changes in major operating variables such as temperature, feed TDS, recovery, and pressures. In this way, performance declines unrelated to operating parameters can be identified and treated. [3]

Free software download is set up on K drive, manual data input is not required, only a “double click” is needed.  Take the new membrane stable operation data after membranes are replaced as a baseline data input, then use current data from PI to compare with baseline data to provide normalized flow, normalized pressure drops and salt passage.  In the meantime, set up before cleaning and after cleaning data, manually input the RO time stamps to understand the cleaning efficiency.

2   Weighing the RO membranes

Technical Service Bulletin Criteria for Replacement of RO Membrane Elements (TSB 126) [4] suggests weighing the membranes to understand the biofouling situation.  Determining where the fouling is occurring can help determine which elements should be replaced. TSB126 recommends removing and weighing a lead and tail membrane from a pressure vessel and then draining (for 20 minutes) and weighing the elements. Elements typically weigh about 16 kg(35lbs) when 20 minutes drained. If the tail element is significantly heavier than the lead membrane, then scaling has most likely occurred.  Heavier lead elements are usually indicative of biological, colloidal, or particulate fouling.  Weighing all the elements from a pressure vessel may help to determine how many elements need to be replaced [4].

To follow the manufacturer technical Bulletin, on October 14, 2019 at RO #2 replacement time, our weighing process was to keep the old lead and tail membranes to weight them. First step was to take new membranes to weight them, the result being 29 lbs.  Next, took each array lead and tail membrane to find their weight.  Due to membrane structure it took a long time to drain the water out.  After 20 minutes of draining measured the stage 1 row 1 tail weights, the result was 40 lbs, which is 4 lbs heavier than TSB126 typically weigh 16kg (36lbs).  After 3 hours of draining, the weight decreased to 38 lbs.  After 5 hours all membrane weights are indicated in Table 4.

After 18 hours drain the most contaminated lead membrane was 40 lbs which is 11 lbs more than a new membrane.  Water was still coming out from the membranes; 18 hours was not enough to drain it all.

                                   Table- 4   RO train #2 membrane weight result

Took the heaviest membrane (train 2 tube 1 lead) to stand on its end to dry.  After three weeks, it was found to be dry.  Weighed it again, the final weight is 36 lbs, lost another 4 lbs of water over three weeks.  The more biofouling growing up inside of membrane increased the flow resistance to slowdown the water’s ability to drain.  After completely drained and dried, compared to a new membrane, the biofouling inside of the membrane weight was 7 lbs.

From table 4, the second stage tail membrane weight is 31 lbs at (18 hours), after it completely dried, the weight was close 29 lbs, so the second stage of membrane had just minimal fouling, if any.  It can be reused it to be lead membrane, it is only slightly contaminated with 1.2 years in service.

Suggestion: All pass 1 first stage membranes had biofouling, the second stage lead membrane was biofouling, but second stage trail membrane has minimum biofilm on it. For the future membrane replacement, if membrane life is less than 1.5 years, keep second stage membranes (12), let it drain 3 weeks, weight them all and compare with new membrane weight to decide which one can be reused as a lead membrane.

3 Autopsy of bio fouled RO membranes

3.1 RO structure introduction

The standard membrane used in the industry is a composite polyamide membrane.  It is made up of a three-layer structure as shown in Figure 2 and Figure 3.  The top layer, which is not clearly distinguishable, is the polyamide membrane, which is very thin, less than 200 nm.  It is composed of the reaction product of mphenylene diamine and tri mesoyl chloride.  Although this is a very durable chemical structure, oxidants can especially cause damage to this material.  This will of course limit one of the most common cleaners for organic foulants. One of the chief concerns is the effect of free chlorine, which can lead to chlorination of the aromatic ring and greatly reduce rejection.  Thus, use of halogen-based oxidants such as chlorine, chlorine dioxide, and bromine are not allowed. Likewise, strong oxidants such as ozone and peroxides are also not allowed, as these can attack the amide bond as well [5]

Figure 2 Spiral wound element construction [5]

Figure 3   Structure of a composite polyamide membrane [5]

 

3.2 Membrane Autopsy

To understand the extent of the RO biofouling situation, and familiarize the structure of membrane, I selected the heaviest train#1 tube 1 lead membrane for the autopsy, which had 7 lbs more weight than a new membrane after totally dried.

• First step is to remove the end cover of the membrane, then one can immediately see the side of the membrane (Fig 4&5).  Next, remove the fiberglass outer wrap off the membrane.  Figure 6 shows the fiberglass outer wrap on the left and membrane core on the right. The membrane core is still wrapped by the plastic cover, use a sharp knife to very lightly cut the plastic cover of membrane, to extend the membrane.

Figure 4  Anti-telescoping cap          

Figure 5   End view                   

 

Figure 6   membrane cylinder

• Now, extend the membrane, each membrane piece is attached by glue line at end side, separate each membrane piece from end of glue line, then extend it.  While extending the membrane, the strong biofouling smell comes, the entire membrane surface was covered by yellow color of the biofilm (figure 7, figure 9).  Use a gloved finger to slightly touch the biofilm, it is very easy to swipe it away to display the membrane original color (white) as seen in Figure 10.

 

Figure 7   Brine spacer                                                  

 

Figure 8 Inside of membrane

• Now, looking closely at the membrane surface (figure 9, figure 10), it is found the biofilm is thick.  It is fully covering the entire surface of all places.  Thickness is around 0.3 mm to 0.8 mm thick.  Inspection of the brine spacer found some places to be fully blocked.  This blockage makes the cleaning chemical unable to reach the biofouling, and the CIP process can’t bring the feed pressure down.

 

Figure 9     Biofilm on membrane surface                            

 

Figure 10   Thick biofilm

•    Count the membrane pieces, hold the product water tube to count, it is total 26 pieces (figure 13) of membrane to connect to the core tube.
•   Cut pieces of membrane, to check permeate carrier situation (Figure 16), then it is easy to separate the membrane pieces and the support pieces the water will pass through the membrane pieces then travel between it to reach the center core tube.                

 

Figure 11    Use figure to attach biofilm                         

 

 

Figure 12    Color of biofilm           

Figure 13   Count membrane pieces       

Figure 14   Inlet side               

Figure 15 Fine silt on the inlet                                                                                                                                                                                                               

• Step 6 Use the Demin water to flush the membrane and clean the membrane pieces then to show the original color of membrane (figure 17), which is white.  The thickness of membrane pieces is around 0.2 mm, which is similar to paper.  The brine spacer piece is around 0.8 mm, blue in color, (in figure 7 is black in color from biofilm).  As seen in figure 14&15, there is a fine silt and particulate on the inlet end, there is no scaling on it.

 

Figure 16 Check the inside of membrane                  

 Figure 17 Cleaned briner spacer and membrane

3.3 Autopsy results

• Heavy Biofilm cover on all surfaces, thickness between 0.3 to 0.8 mm, includes backside of permeant carrier, the biofilm is yellow in color
• Some brine spacer channel was blocked, making chemical cleaning difficult
• Strong biofouling smell from membrane
• No biofilm between the membrane and the permeant carrier, it was clean
• Cleaned membrane is white color, thickness like a paper, surface is not smooth
• Cleaned feed brine Spacer is blue color, it is fouling to dark color
• Small amount of black fine slit, particulate pass through cartridge filter accumulated at membrane surface

3.3 Autopsy analysis

The membrane surface is all covered by the yellow type of the biofilms through the first stage, also on the lead membrane of second stage. The thickness is between 0.3 to 0.8mm. it can be easily removed by mechanical force (using gloved finger to touch it, it does not feel hard), but it is hard to flush it out using demin water.  There is no biofilm between membrane pieces and permeate carrier.

Autopsy results confirmed RO fouling was biofouling to match the weight results, also confirmed the reason RO cleaning can’t bring the feed pressure down is due to heavy biofilm covered membrane surface and some spacer channel was blocked, preventing cleaning chemical reaction with the biofilm.

Depends on several factors, including nutrient availability and feed water pre-treatment techniques. Several studies report that the predominant genus found in virtually all RO membrane biofilms is Sphingomonas.  Its widespread distribution in the environment is due to its ability to utilize a wide range of organic compounds and to grow and survive under low-nutrient conditions.  Sphingomonas are Gram-negative, rod-shaped, aerobic bacteria in yellow color (Figure 18,19).  Sphingomonas also appear to be the foundational biofouling organisms.  Sphingomonas produce unique extracellular polysaccharides (EPS) that build and maintain the biofilm and protect the biofilm matrix against attack from cleaning chemicals.  Further, the EPS they produce provides a modified membrane surface to which other microorganisms can readily attach and proliferate.  These bacteria can consume a broad range of naturally occurring organic compounds and survive in the high-nutrient environment found in the concentration polarization layer.  Sphingomonas have adapted well to the conditions within the spiral wound membrane element, where they survive and proliferate in a low-carbon and low-oxygen environment. [14]

Figure 18. A biofilm micrograph [43]

 

Figure 19   The Color of  Sphingomonas paucimobilis, bacterium

(Picture from https://www.sciencephoto.com/)

4 Causes of RO biofouling

4.1 Nutrients

There are four categories for fouling sources scale (inorganic), particulate, biological and organic compounds.  Biofouling depends on the amount of biological, organic matter and colloidal particles in the feed water.  Eliminating these particles (through pre-treatment) in feed water is the main objective to avoid major biofouling problems in the final RO modules of the plant that are the most affected elements.

Our municipal water effluent, even after secondary treatment UF filtration, contains high concentrations of suspended particles, collides and high level of biological activity.  UF filtration can only remove the suspended solids, it can’t remove the dissolved solids.  Raw water conductivity is almost the same range with RO feedwater, average is over 1100us/cm.  Turbidity test results are consistently 0, which means there is no suspended solids after the UF filters, however, dissolved solids are still very high.

Microorganisms including bacteria are the main reason for biofouling and since bacteria is very adaptable, it is capable of colonizing almost any surface at extreme conditions such as temperatures from -12 to 110 C and pH values between 0.5 and 13. Biofouling occurs due to the deposition and growth of biofilms. The characteristics of interacting surfaces that play a significant role in biofilm formation are hydrophobicity, hydrophilicity, and surface roughness [21]

Biofouling causes permeate flux and quality decline, the feed pressure increases, membrane biodegradation, and an increase in salt passage through concentration polarization.  RO membrane biofouling can cause the membrane life is far less then design life [15]

There are 4 phases for the biofilm grow up at membrane surface, [21]

• phase 1:  Initial Adsorption of organics onto the wetted membrane surface (conditioning)
• phase 2:  Transport and attachment of the microbial cells to the conditioned surface
• phase 3:  Surface Growth (metabolism) of the attached microorganisms and biofilm development, Extracellular polymeric substances (EPS) are continuously produced and acts as a reactive transport barrier to chemical biocides and promotes nutrient concentration/storage
• phase4 (Plateau):  Detachment and limitation of biofilm growth by fluid shear forces

Factor to impact phase 1 is Hydrophobicity, hydrophilicity, and surface roughness, charge of the solution.

Nutrients in the bulk solution serve as food for microorganisms; Higher concentrations of microorganisms coupled with quicker transport to the membrane surface to support phase 2 [15]

Phase 4 is critical to the proliferation of bacteria and resultant biofouling further along the feed channel, thereby expanding the degree of membrane surface area infected up to including the entire system [15]

Assimilable Organic Carbon (AOC) concentration is one of the most important factors in controlling attached biomass and heterotrophic bacteria activity in water as AOC is the part of dissolved organic carbon (DOC) that can be easily assimilated by bacteria and converted to cell mass [14]

Evaluating AOC after various stages in pretreatment would provide the operator with the information needed to institute a control strategy for minimizing biofouling potential within the plant and before the RO membranes. AOC was a significant predictor variable for biological fouling impacts on increased differential pressure or permeate flux decline in bench-, pilot-, and full-scale studies [41]

So, control of nutrients is one way of ultimately controlling membrane biofouling. Even though we have rotation filter, UF filtration, cartridge filter, dissolved organic compounds which are the main culprit of biofouling are not easily removed by any of common pre-treatment methods!

4.2 Critical flux

For biological fouling to occur, bacteria need to have suitable low velocity conditions so they can attach to the membrane, biofouling initiates when membrane flux exceeds a certain level termed “critical flux”, when such critical flus through the membrane is reached, the velocity of the feed water /permeant flow along the surface of the membranes (cross-flow velocity) drops to level to allow bacterial attachment to the membrane surface, the most widely used operational approach to increase cross-flow velocity is to reduce system recovery.  Operation at lower recovery leaves more flow on the concentrate/feed side of the membranes which in turn creates a higher scoring velocity on the membrane surface which deters microorganisms from attaching to this surface [36].

Decreasing the RO recovery rate will slowdown the biofouling process, we have decreased ours from 75% to 68% on the first pass.  It will slightly increase operating costs.

4.3 Chlorine and de-chlorination process  

Chlorine is a strong oxidant, it is cheap, readily available, and easy to apply, so it is widely used at industrial sites.  It can destroy the cells of bacteria release easily biodegradable organic compounds which in turn become food for the remaining bacteria that have survived chlorination by being in an inactive state, the conversion of these surviving bacteria from an inactive to an active state following by their attachment and excessive growth on the membrane surface, results in accelerated membrane biofouling.  Therefore, continuous chlorine often creates more membrane fouling problems, continuous chlorine pre-treatment is not recommended [36]  

Chlorine also oxidizes organics, such as longer-chained humic acids, cleaving them into

shorter-chained molecules.  These smaller molecules are transformed into AOC that microbes can more

easily digest. This is a particularly serious problem after the point of de-chlorination [30,31,32,33].

Studies have shown increased rates of biofouling following de-chlorination [33-35]. The absence of

disinfectant, the degree of disinfection as opposed to sterilization of the system, and the availability of

more AOC post-chlorination leading to greater rates of biofouling post de-chlorination. [14]

5   Biofouling Control Discussion

Biocide direct injection disinfection to the membranes limits and controls the microorganism growth is the most proven way.  Membrane surface modification is another aspect which manufacturers are working on.  Non-chemical disinfection so far, some degree has success to control biofouling.

5.1   Chlorine

Chlorine kills cells by attacking the cell walls. Hypochlorous acid enters the cell wall with water and breaks down enzymes in the cell wall, leaving the “bug” open to attack

Chlorine gas directly apply with water:  Cl₂ + H₂O → HOCl + H⁺ + Cl^–       

Sodium hypochlorite apply with water:  NaOCl + H₂O → HOCl + NaOH

Hypochlorous acid (HOCl) is 20x more germicidal form than Hypochlorous ion[42]. So lower pH, better disinfection (HOCl is predominant)

Figure 20   Hypochlorous acid vs Hypochlorous acid Ion

Hypochlorous acid is a weak acid and will disassociate: HOCl ⇔ H⁺ + OCl ^–, at PH 7.5 Hypochlorous acid and Hypochlorous ion will reach 50% to 50%,

The strongest oxidizer for disinfection is chlorine, but the polyamide membrane can be rapidity damaged by free chlorine, 1000ppm chlorine within one hour will damage the membrane.

Membrane exposure to chlorine (50-200 mg/L of NaOCl solution) for varying exposure periods (2.5 – 10 hours) in a plate-and-frame RO system revealed measurable increase in membrane water permeability (3% – 15%) and reduced salt rejection (from 99.2 % for the intact membrane to as low as 97.4 %) [20].

5.1.1 De-chlorination process

Hypochlorous acid is then reduced by sodium bisulfite:

2NaHSO₃ + 2HOCl → H2SO₄ + 2HCl + Na₂SO₄

In theory, every ppm of free chlorine requires 1.47 ppm of sodium bisulfite or 3.0 ppm of sodium metabisulfite for reduction.  Since most sodium bisulfite solutions are about 33–37% active, the theoretical dosage would be about 3.5–4.5 ppm per ppm of free chlorine.  Often, a safety factor of 1.5 to 2 times theoretical is applied to determine the actual dosage [27].

ORP is generally used to confirm removal of free chlorine.  When using bisulfite, an ORP value is less than 300 mV is recommended to ensure protection of the membranes from chlorine attack.  When doing on site manual grab sample at the RO inlet, with ORP at the 200 mV range, the free chlorine was 0.03 ppm.

Bisulfite can be an O2 scavenger, if too much excess bisulfite is fed into the system, it creates a reducing environment friendly to proliferation of anaerobic bacteria.

2 NaHSO₃ + O₂ → 2 NaHSO₄

5.1.2 Chlorine and De-chlorine process will expedite after growth

The chlorine suppresses bacterial activity, but when the sodium metabisulfite (SBS) is added to remove chlorine, the surviving bacteria quickly take advantage of the nutrients furnished by the degradation of larger molecules and enter into a cycle of tremendous growth (termed by some as “aftergrowth”).  The significant increase in the biomass of bacteria after de-chlorination continued with slime development on the surfaces of the piping and RO membranes [13].

From recently study results, manufacture technical bulletin (TAB-110) and RO autopsy results is confirmed chlorine and dechlorane process can’t stop RO biofouling, chlorine does kill a substantial proportion of bacteria, but it also breaks cell walls and releases cell contents into water.  This makes available the carbohydrates and proteins that were previously locked with the cell.  Nutrients can be conveniently measured as Assimilable Organic Carbon (AOC).  Autopsy showed the first stage all membrane weights being heavier than new membranes, all covered in biofilm until the nutrients are consumed, reach the end tail membrane where the test is show highest bugs.

Chemical addition of SBS accounted for the increase in AOC. AOC in the RO feed led to biological growth and subsequent RO membrane fouling [41].

5.1.3   Sodium bisulfite (SBS) injection point

TSB_110 recommends the sodium bisulfite injection point to be as far away as possible from membranes [7].  With the SBS dosing point closer to membranes, chlorine and nutrients are brought closer to the membranes. Bacterial generation time is sharply shorter due to the rich nutrients.  

Another source of nutrients is thought to be dead bacterial cells from the biofilm in the piping between the SBS injection point and the membranes. Sloughing of dead cells and the degradation of larger molecules attached to piping (the conditioning surface) and free in water will furnish a good source of nutrients for bacteria in the membranes.

Cleaning the RO inlet piping between the sodium bisulfite and the RO section will help to remove the biofilm entering the membrane and prevent it from accumulating on the surface of membranes.  But this will not stop membrane growth of biofilm.  The total surface is 4 m², our pre-treatment pipe is 4” of pipe with 30 meters total length, compare to 2678.4 m2 total first pass membrane surface is a very small portion.

Reported by Mohamed O. Saeed after the SBS injection point change from before the cartridge filter to after cartridge filter, that membrane cleaning frequency is doubled following the sharp decrease in generation time. This cleaning was necessitated by a phenomenal build-up of ∆P across the permeate at a rate of approximately 0.3bar/day. It is therefore advisable to move the SBS dosing point as far as possible from the membranes.[13]

 5.1.4 RO train #1 biofouling rapidly after membrane replaced

Table 5 Train 1 operation after RO replaced

Operation time (hours)	% Salt Passage	Permeate Flow (t/h)	Flow Change (t/h)	Differential Pressure (bar)	DP Change
New	0.65	25.6	0.00%	1.23	0.00%
10	0.58	24.72	-3.44%	1.41	14.32%
20	0.63	24.49	-4.51%	1.51	22.29%
30	0.63	23.36	-9.15%	1.61	30.62%
40	0.67	24.13	-6.30%	1.73	39.87%
50	0.63	22.57	-12.57%	1.75	41.71%
60	0.63	22.34	-14.44%	2.24	81.24%
70	0.64	22.07	-15.80%	2.38	92.54%
80	0.69	22.06	-16.06%	2.61	111.06%
90	0.66	21.63	-18.00%	3.09	150.22%

                            (DP is between first stage feed pressure with first stage rejection pressure)

After September 22, 2020 replaced RO train #1 first pass membrane, used the hydranautic RO performance software to calculate the normalized flow and feed pressure change for every 10 hours, and found biofouling speed very fast (pressure and flow normalized), software asks to take first serval hours running as base data input then to compare it afterwards operation.

With in 90 hours running the feed pressure is increase 150% compared to new membranes, the feed pressure rate of increase is 2.06 kPa/hour.  Biofouling is very fast according to this speed, from 1000kPa feed pressure to 2000kPa only need 500 hours operation if not cleaning RO.       

5.1.5 Bacteria test results

From water treatment rep RO Microbiological Study [22]:

Samples were collected around the RO system and tested for the presence of microbes:

Before cartridge filter – 0cfu/mL

After cartridge filter – 0cfu/mL

RO pump suction – 2,110cfu/mL

RO Inlet Train 1 – 3,610cfu/mL

RO Train 1 Permeate – 202cfu/mL

RO Train 1 Inter-stage – 6,200cfu/mL

RO Train 1 Concentrate – >10,000cfu/mL

Raw water supply from bonnybrook—260cfu/mL

ZV filter outlet—1cfu/mL

Cooling tower –Bacteria Planktonic, 57.16 CFU/mL

Cooling tower –Bacterial Sessile, 449.33 CFU/mL

Note – no bugs were found on either side of the cartridge filters, but a significant number of bugs were recorded after the bisulfite injection leading us to suspect a biofilm is present that is continuously sloughing off microbes that concentrates up through the RO

The test results are showing free chlorine can be killed the bacteria and control bacteria growth, but after de-chlorination, the bacteria grow very quickly.  At the inlet of membranes, the bugs count is 6200cfu/ml compared to 0 cfu/ml after cartridge filters in a very short pipe run. But the raw water supply from Bonnybroke at o.1ppm chlorine level is only 260cfu/ml bugs.

Start from January 2019, to increase service tank chlorine level to 2ppm from 1 ppm to kill the bugs in the feed water, service tank water PH is 7.0 which is best for the free chlorine to form chlorinous acid. From study, test results and RO operation results confirmed chlorine can’t kill 100% bugs, the surviving bacteria will utilize the decomposition components from sodium bisulfite, antiscalant, and the assimilable Organic Carbon (AOC) which produced by chlorine process for nutrition, to regrow quickly and attach to RO supply water pipe inner surface and membrane surfaces after de-chlorination.

5.2 DBNPA (2,2-Dibromo-3-nitrilopropionamide)

Hydranautics Technical Application Bulletin TAB_110 [7] introduces the biocide DBNPA as most effective with new or sufficiently cleaned membranes that are relatively free of any biofouling.  DBNPA, which is a fast-acting, non-oxidizing biocide which is very effective at low concentrations in controlling the growth of aerobic bacteria, anaerobic bacteria, fungi and algae. The chemical formula of DBNPA is:

DBNPA is deactivated by reducing agents, so a higher concentration of DBNPA will be required if residual reducing agents are present in the feed water.  For example, Sodium Bisulfite (SBS) will deactivate DBNPA.  If SBS is dosed during service or flushing operations, additional DBNPA will be required at a suggested dose rate of 1.0 to 1.3 ppm DBNPA per 1 ppm of SBS to account for deactivation.  Excess SBS can also be used to accelerate the deactivation of DBNPA in discharged waters.

Research demonstrated that a continuous dosage of 1 ppm DBNPA could prevent accumulation of biomass and the associated increase in pressure drop during a 7-day test with chlorine dioxide pretreated water and virgin membranes.  However, for membranes already bio fouled, continuous dosages of 1 ppm and 20 ppm of DBNPA only inactivated the accumulated biomass; neither of these dosages removed the inactivated cells and biomass, nor restored the original system pressure drop [17]

Currently our site uses DBNPA as biocide to inject 25 seconds after the RO stop for backflush, these processes can show down feed pressure increase.

5.3 Chloramines

Chloramines react with organic matter less often than chlorine, little to no trihalomethanes (THM) and other disinfection by-products are formed.  Chloramines are less reactive than chlorine and thus less damaging to RO membranes under similar operating conditions, because of the low reactive rate, a longer contact time and higher concentration of chloramines are required for disinfection, chloramines remain active in the water system for a considerably longer period of time [32]

Chloramines have been used successfully for many wastewater RO systems where ammonia is present in the feed stream and chlorine is added to obtain 1-2 ppm of chloramine.  CPA membranes have an estimated chloramine tolerance from 50,000 to 200,000ppm-hours before a noticeable increase in salt passage occurs.  The 50,000 ppm-hours levels correlate to a recommended chloramine level in the RO feed of 1.9 ppm for an operating period of 3 years.

Chloramines are formed during a reaction between chlorine (Cl2) and ammonia (NH3).  During this reaction three different inorganic chloramines are formed: monochloramine (NH3Cl), dichloramine (NHCl2), and trichloramine (NCl3).  Of the three, monochloramine is the most effective disinfectant. Monochloramine is formed when the pH of water is greater than 8; at low pH dichloramine and trichloramine are dominant.  In addition, free chlorine and organic chloramines are also present during the reaction, thus the application of chloramine to RO membrane surface requires close monitoring of chlorine species for effective disinfection and prevention of membrane damage [32]
Reaction mechanism:  NH₃ (aq) + HOCI -> NH₂Cl + H₂O

5.4 ClO₂

TB115 [8] introduced ClO₂ has been considered as a potential disinfectant. ClO₂ is present as a dissolved gas in water. One advantage of ClO₂ is that it is a weaker oxidant than HOCl, HOBr and O₃.  A weaker oxidant is less damaging to the membrane, and apparently the ClO₂ gas can penetrate the biofilm better and degrade the material.  Because of the chemistry and reactivity differences between Cl₂ and ClO₂, it has been reported that approximately ¼ of the dose of ClO₂ is required to maintain an effective disinfectant concentration compared to Cl₂.  Additionally, since it is gaseous, it will not be rejected by the membrane and will thus pass into the permeate at the same concentration as the feed.  For these reasons, ClO₂ is gaining interest as a potential disinfectant which can be used with a membrane.

But, since the reaction of ClO₂ is different than OCl-, the interaction of ClO₂ with the membrane is not yet fully understood.  Hydranautics does not fully endorse the use of ClO₂ for frequent cleaning or daily dosing until more extensive studies are done, especially with the presence of transition metals [8].

5.5. Other method to disinfection RO or control biofouling

• Ozone is very aggressive to membranes and must also be removed.  The use of ozone adds to the capital cost of the water treatment system, so it is seldom used at the industrial level
• Physical and photochemical techniques, such as UV, can be very effective at disinfection
• Isothiazolones
• Hydrogen Peroxide/Peracetic Acid
• copper sulfate
• Sodium Bisulfite
• Silver for water disinfection
• Nano photocatalytic disinfection
• Balancing the Nutrient Ratio
• RO surface modification

6.0 Anti-scalant impact with biofouling

Antiscalants are surface active polyelectrolyte compounds commonly used in reverse osmosis water treatment processes to avoid membrane scaling.

The RO antiscalant we use is RL9007 (2−Phosphono−1,2,4−butane tricarboxylic acid).  Upon decomposition it will form oxides of carbon, nitrogen, and phosphorus [24], which can be food for bacteria.

A study by Sweity et al found that the use of polyacrylate and polyphosphonate based RO

antiscalants can enhance membrane biofouling.  In bench, pilot, and on-line tests at an operating

desalination plant, they found that polyacrylate-based antiscalants altered the physio-chemical

nature of the polyamide membrane by primarily enhancing hydrophobicity; this promoted initial

attachment/deposition of microbial cells.  Polyphosphonate based antiscalants sustained biomass on

the membrane by providing nutrients in the form of phosphorous  [18].

Lauren A. Weinrich in his doctor degree study found:  Antiscalants would react with chlorine to increase the amount of biodegradable carbon or other nutrients in the RO feed, which would create optimum conditions for biological growth on the membranes. Therefore, the treatment strategies could be amended to minimize additional AOC formation and phosphate liberation by removing the oxidant residual before the antiscalant is added. AOC and phosphate measurements both indicate the presence of necessary nutrients for bacterial growth and proliferation. A strategy to minimize nutrients would be useful for controlling RO membrane biofouling.[41]

From RO autopsy and weight results, SEC does not have any scaling issues at the second stage tail membranes.  I suggest we should try to minimize the amount of antiscalant feed to the ROs to avoid another potential nutrient source.

7.0 RO operation strategy discussion

As previously discussed, also confirmed by RO membrane autopsy and bacteria test, the chlorination and de-chlorination process causes bacteria to grow very fast after de-chlorination.  This paper provides four SEC RO operation strategy for discussion:

 7.1 Keep Chlorine and de-chlorination process, not continuously to inject biocide

Maintain currently Chlorine and de-chlorination process strategy, not to continuously inject biocide to inside the membranes.  Membrane average life is 1 year and 2 months, there are several other points to take care of first (as detailed below), which will extend membrane life.

• Control operation time

Control RO operation time then use biocide to flush the membranes with demin water which can slowdown the DP increase.  Prevent extended run times over design flow limits is important.  Table 6 compares the RO performance before stopping and the next restart.  The DP decreased 20%.  In the summer time, change the RO start/stop targets to between 95% to 98% (vs 88% to 98%) which will keep the RO run time to less than 6 hours.  Once the ROs stop, biocide will inject for 25 seconds and then demin water flush can remove some biofilm from the membranes to slow down the feed pressure increases. Increasing the biocide injection time to 35 seconds also can help this process.

Table 6 RO stop condition vs next restart performance comparison

		Normalized Flow	Decreased,		Normalized DP	Increased	Percent	Operation hours
		m3/h	m3/h		Bar	Bar
2020-11-26 15:30	before RO stop	29.83	-1.45	-4.65%	8.12	4.06	100.25%	350.7970162
2020-11-27 3:30	RO start	28.87	-2.41	-7.72%	7.30	3.25	80.06%	351.0200681

Prevent RO permeant flow operation over maximum design flow is another factor, comparing train 1 and train 2 operation results (table 7).  It indicates that train 1 is biofouling much faster than train 2.  This is due to train 1 running at above design flow, increasing fouling rate within the first two weeks.

Train 2 is showing a feed pressure increase rate of 1.12 kPa per hour of operation.  Table 5 is showing train 1 feed pressure rate of increase of 2.06kPa per hour.  If no cleans were performed on the RO, the train 2 feed pressure rate of increase can be used to estimate the membrane lifetime.  They would only be in operation for 900 hours, (train#1 500hours) to increase from 1000kPa to 2000kpa inlet pressure.

Table 7   RO train 2 new membrane operation results

Operation hours	Normalized Flow	Flow change (compared to new)	Flow change	Normalized DP	DP Increase (compared to new)	DP change	Salt passage
	m3/h	m3/h	Bar	Bar	Bar	Bar	%
New	31.29			4.05			1.21%
50	30.61	-0.68	-2.16%	4.08	0.03	0.68%	0.95%
100	30.20	-1.09	-3.48%	4.22	0.17	4.10%	0.81%
100	30.21	-1.08	-3.45%	4.24	0.18	4.54%	0.81%
150	29.85	-1.43	-4.58%	4.57	0.51	12.59%	0.60%
200	29.81	-1.47	-4.70%	5.29	1.24	30.51%	0.50%
250	28.97	-2.32	-7.40%	6.56	2.51	61.79%	0.47%
300	29.23	-2.06	-6.57%	7.32	3.27	80.59%	0.47%
344	29.91	-1.38	-4.41%	8.14	4.08	100.70%	0.48%

            (Note: DP is between first stage feed pressure vs second stage rejection pressure)                          

•  Chemical cleaning as early as practically possible

From membrane autopsy results, it shows that when the biofilm reaches a certain degree of thickness, it will block the spacer channel and prevent the chemical from reaching the biofilm, or slowdown the water flow to decrease crossover flow below the critical flex, so less chance to bring the biofilm out.

From past membrane cleaning experiences, if the membranes are not cleaned, the feed pressure will increase to over 2000 kPa within 4 months operation.  If pass 1 feed pressure reaches over 1800 kPa, the cleaning results can only bring the feed pressure back to 1600 kPa.  At this point, the membrane life is already reduced 50% (table 11).  

Edw. F. Sylvester, Jr. from ChemTreat had concerns regarding the RO cleaning being unable to bring the feed water pressure down [23]:

• The ROs are not being cleaned on time and they are waiting too long
• The membranes are packed with so much dead micro bio and colloidals that you’re unable to remove it
• Gaps may have been developed, causing the flow of least resistance to occur
• The RL-1500 isn’t being thoroughly rinsed out

• Automatic control of sodium bisulfite injection

When the sodium bisulfite injection feed is in manual, it is easily possible to over feed the chemical, which provides food to the bacteria.  Especially overfeeding bisulfite to the point of a negative ORP produces a reducing environment friendly to proliferation of anaerobic bacteria.

RO operation manual requests 0 free chlorine inside of membrane, CPA5-LD specification indicate maximum free chlorine tolerance is 0.1 ppm.  Set up pump in auto control with a feed water free chlorine level target of less than 0.05 ppm.

• Control antiscalant dosage

As the RO membrane weight results is show, SEC RO membrane fouling is biofouling and not due to scaling.  Studies show that antiscalants increase the chance for initial stage bacteria to attach to membrane surfaces.  After they are decomposed, they provide nutrients for bacteria to grow.  Controlling the amount injected to the ROs is necessary.

• Flush RO inlet pipe  

From the cartridge filters to the membranes, there is around 30 m of 4” pipe for a total surface area of 4 square meters.  Just after the SBS dosing point, bacteria growth time is short.  The biofilm will attach to the internal piping surface.  Periodic flushing is necessary, the better way is to directly flush it to drain, not to go though the membranes.

• Soaking the RO elements during standby with permeate

RO operation manual [26] suggested soaking the RO elements during standby with permeate plus biocide can help dissolve scale and loosen precipitates, reducing the frequency of chemical cleaning.  Current logic set up for standby is a timed flush (with 25 seconds of biocide injection), then drained.

• Decrease service tank free chlorine level from 2 ppm to 1 ppm

Free chlorine level between 0.5 ppm to 1.0 ppm in the service tank with enough contact time can effectively to deactivate bacteria.  Higher free chlorine levels in service tank need more sodium bisulfite to dechlorinate it, which provides more nutrients to bacteria.

7.2 Keep chlorine and dechlorination process, continuously dose DBNPA biocide

Continuous dosing of biocide directly to the membranes is the best way to control biofilm growth inside of the membranes.  DBNPA is not an oxidizer, which makes it compatible with membranes. It is a moderate electrophile that acts on the bacterial cell wall and on the cytoplasm within the cell; it does not penetrate EPS [14].  DBNPA is most effective with new or sufficiently cleaned membranes that are relatively free of any biofouling.

 Table 8 dosing 1 ppm DBNPA annual cost

recover rate	permeate flow	two train total feed flow	Injection 1ppm biocide request	20% concentration
	t/h	kg/h	kg/h	kg/h	$/h	$/y
68%	40	58823.52941	0.058823529	0.294117647	5.0588235	15,705.16

Higher service tank free chlorine levels require a higher ppm SBS to neutralize any free chlorine. Normally need 1.3 times more ppm sodium bisulfite vs free chlorine concentration to dechlorinate. SBS is deactivate agent for DPNBA.  Additional DBNPA will be required at a suggested dose rate of 1.0 to 1.3 ppm DBNPA per 1 ppm of SBS to account for this deactivation and keep 1ppm free DBNPA in feedwater [7].  Sodium bisulfite react with free chlorine to form Sodium sulfate and sulfuric acid, after that Sodium sulfate reacts with sulfuric acid to give the acid salt sodium bisulfate[45] then to deactivate DPNBA.

                                             2NaHSO₃ + 2HOCl → H2SO₄ + 2HCl + Na₂SO₄

Na₂SO₄ + H₂SO₄ ⇌ 2 NaHSO₄

Currently sodium bisuflite solution (RL124B) concentration is 15% to 40% concentration [44], dosage is 650ml/h, using the average concentration 27% calculation, the dosage concentration is 3.7ppm sodium bisuflite which 1.86 times than 2ppm free chlorine.

This would require dosing of at least 4ppm to inside membranes to get 1 ppm free DPNBA.

DPNBA is a very expensive chemical.  With 20% concentration, it is easily degradable.  Table 8 is showing annual cost of a continual dose of 1 ppm.  Dosing at 4 ppm, the annual cost will be over $60,000. It would cost more than two sets of new membranes (Table 9).  Chemical cleans would still need to be completed, as DBNA will not remove any biofilm membrane.  Continuously dosing the DPNBA will dramatically increase RO operation costs.

price per membrane	One tube membrane	1st train RO tube	one train	two train
$786	6	6	$28,296.00	$56,592.00

Table 9 Two train New membrane cost

Bacteria growth requires nutrients.  The process of chlorination and dechlorination provides a lot of rich nutrients after breaking down bacteria cells, release AOC.  A lot of RO plants have stopped chlorinating (and dechlorinating) their feedwater, allowing the bacteria to grow in the service water and feed water intake system.  The bacteria will consume the nutrients when the water reaches the ROs.  The RO membranes have less biofouling because most of the nutrients are already consumed by the growing bacteria in the service water tank and feed pipe system.  RO cleaning frequency would be much less than chlorine and dechlorane process and RO life longer.

From the recent ChemTreat bacterial study, the RO train 1 second pass is 10000cmu/l, highest concentration of any place.  The second pass tail membrane had the highest concentration of bacteria, but our end of life membrane weight results showed that the second pass tail membrane had almost no biofilm on it.  The reason is because the nutrients (AOC) are consumed on the lead membrane side.  The bacteria need nutrients to form EPS to attached to the membranes, if there is a lack of nutrients, there is less chance of it becoming attached on the membranes.  The bacteria will instead be flushed out.  This strategy is to let the biofilm grow up in the service tank and pre-treatment pipe system. Then use outage time to do chemical cleaning on the service tank, and periodically flush the per treatment pipe.

Most time the raw water from   Bonnybrook provides the feed water includes less than 0.1 ppm free chlorine some degree it can limit the biocide grow up, the microbiocidal testing is showing 260 cmu/cl, it is far less than currently RO Inlet Train 1 – 3,610cfu/mL, so stop chlorine and chlorine process, RO biofouling speed will slowdown.

If the raw water free chlorine level can be kept below 0.1 ppm, the RO can operate for over 10000 hours, accounting to RO annual operational hours (3000 hours), we can almost run for 3 years.  It will extend RO life over 2 times.  Consider the SBS and sodium hypochlorite savings, this is will be an economic way to run the RO system.

Periodically cleaning the pre-treatment pipe system would be necessary.  The better way would be to not allow any cleaning water to pass through the membranes, avoiding the biofilm going through the membranes.

7.4 Switch from chlorine to chloramine and directly apply to membranes as biocide

“Have you considered dosing chloramine? OCWD (one of the World’s largest wastewater reclamation facility) has continuous 2-3 residual chloramine in the system to retard the (biological) fouling and hence they are not required to clean the RO system frequently”, email from Umang Yagnik Hydranautics – Nitto Group Company to answer my RO biofouling question. [40]

Chloramines, which is where chlorine reacts with ammonia, are successful to control biofouling at many wastewater treatment RO plants.  Ammonia and chlorine are cheap products, and if successful in controlling the monochloramine levels, let 1-2 ppm chloramine directly contact the membranes.

For equivalent chlorine concentrations, monochloramine penetrated biofilm 49 times faster than free chlorine. Free chlorine biofilm penetration was limited even after being subsequently applied to a biofilm with nearly full monochloramine penetration.[46]

Free chlorine is the best oxidizer to control biofilm, but it not compatible with membranes.  Chloramines are less of an oxidizer than free chlorine, but compatible with RO membranes in wastewater treatment. Free chlorine membrane life is 1000ppmh, but chloramine tolerance is from 50,000 to 200,000 ppmh with no salt rejection increase.

Chloramines have been used successfully in many wastewater systems where ammonia is present in the feed stream and chlorine is added to obtain 1-2 ppm of chloramine.

Multiple water contain ammonia, the total chlorine minus free chlorine, will be chloramines.  This method is dosing 1.5 to 2 ppm free chlorine to service tank, then convert the antiscalant tank (or SBS tank) to store ammonia to inject it and react with the free chlorine to form 1.5-2.0 ppm chloramines.  Then use ORP probe to detect any free chlorine, if there is none, the SBS pump does not need to run. This strategy does require more future study and testing to ensure that no organic ammonia is produced.   The service water pH average is 7.1, after adding ammonia the pH would be increase to above 8.0 which is in range for the formation of monochloramines.  About 5% excess ammonia should be maintained to ensure monochloramine is predominant.  If the test shows the reaction is fast, no free chlorine is after the dosing point, maybe use the SBS tank to store ammonia instead and keep injecting antiscalant.

8   RO Cleaning Discussion

Membrane cleaning plays a vital role in biofouling control. If the feed channel is completely blocked by biomass, this can limit the transport of the cleaning chemicals to the blocked spacer and restrict the removal of biomass from the membrane module.

Chemical methods utilize chemical agents such as ethylenediamine tetraacetic acid (EDTA), formaldehyde, sodium hydroxide and ethanol for the extraction of EPS from the microorganisms.

Caustic also increases the negative charge of humic substances in the foulant layer and so would weaken the bonds between them and the membrane.

Dosing chemical biocide to control the biofilm growth inside of the membranes minimizes RO cleaning times, but RO cleaning still needed to improve RO performance.  Not all of the time can we recover RO performance which depends the cleaning method, chelating chemical, how thick of biofilm inside of membrane and cleaning time.  Manufacturer suggests if the RO fouling reaches a heavy fouling stage, cleaning is hard to recover back to base line.

Table 10   operation manual classification of the degree of biofouling [10]

It is important to clean the membranes when they are only lightly fouled, not heavily fouled. Heavy fouling can impair the effectiveness of the cleaning chemical by impeding the penetration of the chemical deep into the foulant and in the flushing of the foulant out of the elements. If normalized membrane performance drops 30-50%, it may be impossible to fully restore the performance back to baseline conditions. [11]

Table 11   RO train 2 operation 3-month feed pressure results

	Cleaning Date	Before cleaning feed pressure	After Cleaning feed pressure
		kPa	kPa
New membrane installed at	2019-07-29	989
3	2020-02-19	1138	1157
4	2020-05-17	1909	1734

Attachment 2 and Table 11 is shows recently replaced membrane cleaning history.  From 2019-02-19 running almost 3 months, the feed pressure increased from 1138 kPa to 1909 kPa. A chemical clean could only restore the inlet pressure down to 1734 kPa, the membrane has already lost 70% life.

Cleaning RO has two methods, mechanical cleaning and chemical cleaning.

8.1 Current cleaning method

Table 12 shows cleaning results of new membranes (clean determined by calculated normalized conditions).  When a chemical clean is performed at a feed pressure below 1500 kPa, recovery is easily obtained to near new membrane condition                          

Waiting for over 15% loss in performance before cleans allows deposits to become compressed and difficult to remove.  It is advocated that cleaning is conducted as soon as fouling is perceived, and deposits are less compacted and easier to remove.

Currently we use RL-1500 and RL-5000 in combination, with 50% caustic as pH enhancer to keep the pH above 12.  Higher PH and feed water temperature has better cleaning results due to organic has higher dissolvability in higher PH CIP solution. But not allow to over the membrane maximum limit (temperature <45 C° and PH <13) Through two batch cleaning, combined membrane soaks and flushing with CIP solution.

RL-1500 has 19.5% EDTA and a very strong surfactant, 4% Noma NF-10.  This product is used where heavy micro bio is the foulant.  RL-5000 is sodium percarbonate which is like Oxy-Clean and really penetrates the slime layer on the membrane surface [23]. Membrane cleaning by dual or combined cleaning agents, when used appropriately, is more effective than membrane cleaning by individual cleaning agents [42].

Table 12 New RO cleaning efficiency:

RO Train	Cleaning date		Normalized flow	Flow increase	flow change	pressure	Pressure Decrease	pressure change
			t/h	t/h	%	bar	bar	%
#1	2020-09-29 19:15	Before cleaning	20.42			3.77
2020-09-30 3:50	After cleaning	23.83	3.15	15.23%	1.40	2.53	-188.63%
#1	2020-10-09 17:00	Before cleaning	18.36			4.28
2020-10-10 3:27	After cleaning	21.65	3.29	17.90%	1.41	3.05	-221.34%
#1	2020-10-20 5:00	Before cleaning	20.28			4.94
2020-10-20 18:30	After cleaning	21.66	1.38	6.79%	3.19	-1.75	-55.04%
#1	2020-11-09 20:00	Before cleaning	20.67			7.32
2020-11-10 18:00	After cleaning	20.50	-0.17	-0.84%	3.73	-3.59	-96.17%

 Using these two chemicals to do RO train #1 cleans at early stages of fouling has shown very good performance recovery (table 12). Table 13 RO cleaning cost is each time RO cleaning cost.

                                                                            Table 13 RO cleaning cost

	Chemical name/kw	consumption per cleaning	Price	Item price
		Kg	$/kg
Cleaning solution	RL1500	30	10.05	301.5
Cleaning powder	RL5000	2	8.5	17
PH enhancer	BL1301B	10	3.6	36
CIP pump (KW)	11	6	0.033	0.198
CIP Heater (kW)	45	6	0.33	1.98
Rise water		20	2	40
Total				396.678

December 1^st 2020 RO train #2 did cleaning,  it is first time cleaning after replaced to  new membrane  after 385 hours operation, the average biofouling speed is 1.1kPa/hour for feed pressure increase. The DP between feed to second stage reject pressure is  increased 109% compare with new membrane (table 7), for this cleaning based on ChemTreat suggestion, increase the chemical on the first batch, use 25 liter RL1500, 2.5 L RL5000, add extra 2.5 liter of 50% caustic PH increase to 12.2,compare with normal cleaning chemical  used 10 liters RL1500, and 0.8 kg of RL5000, new suggestion increase more than double of cleaning chemical.

 The CIP cleaning solution color is become dark yellow (Figure 21), from the autopsy results, the color of black is fine particle (slit) , and the yellow is biofilm, so CIP solution is showing both was cleaned out. At second batch chemical use 25 liter of RL1500, 5.0 kg of RL5000, add extra 50% caustic to keep PH higher than 12. Second batch cleaning CIP color is less black slit than first batch, but more yellow portion of biofilm reacted with RL5000 was cleaned (Figure 22).

Table 14 is showing RO train #2 the cleaning performance, the DP decrease 113% and flow increase 4.19% compare with before cleaning. it is completed recovered DP to new membrane, so slightly increase the RL 5000 at second batch will have better cleaning performance. And this cleaning is demonstrated that on the early stage of biofilm can be totally removed by the chemical cleaning.

Figure 21 First batch cleaning CIP solution color   (25 liter RL1500, 2.5 L RL 5000 PH>12.0)

 

Figure 22 Second batch cleaning CIP solution color (25 liter RL1500, 2.5 L RL 5000 PH>12.0)

                          

Table 14   RO train #2 first cleaning results after replace

Cleaning results	Normalized Flow Performance	Normalized pressure performance
Flow	Flow up	change	DP	DP down	change	Salt passage
m3/h	m3/h	%	KPa	KPa	%
New membrane	31.46			396
Before cleaning	29.56			840			0.49%
After cleaning	30.80	1.24	4.19%	393	-447	-113.74%	0.70%

(Noted: DP is normalized differential pressure first stage feed pressure to second stage reject pressure)

8.2 Reactive and Predictive Cleaning

For the past several years, SEC is based on the reactive cleaning.  No cleaning of the new membranes based on feed pressure only, permeate flow still good.  When feed pressure increases to close 2000 kPa, then we start cleaning, which never gives us a chance to lower feed pressure below 1600 kPa.

Any delay in cleaning will mean the foulant will be become thicker and more compressed into the membrane surface and be much more difficult to clean.  Predictive cleaning of the plant prevents a build-up of difficult to remove deposits which reduce membrane performance and life expectancy. Membrane fouling rate and advantages of predictive cleaning is less time required to conduct the clean and a more effective, “deeper” clean can be achieved.  This means the subsequent fouling rate is lower, closer to the rate of the new membrane when installed.  The lower fouling rate reduces the frequency between subsequent cleans increasing operational efficiency and further enhancing membrane lifespan (figure 23) [19].

Predictive cleans based on RO performance software which follows the normalized DP change.  Using the train #2 DP increase rate of 1.1 kPa per running hour and winter operation with over 45 hours of run time each week, the DP will increase 12% per week, or 25% over two weeks compared to new membranes.  This is why cleaning one train every two weeks is necessary to control biofilm.  When RO flow is increased, RO permeate flow maybe not decrease proportionally due to when feed pressure increases, the permeate flow also will increase.

Figure 23. Membrane fouling rate and advantage of Predictive cleaning [19]

8.3   Different Cleaning methods to improve cleaning efficiency

8.3.1 Install air bubble generator (figure 24)

Air Scouring. It is known that two-phase (air bubbles and water) increase shear forces and improves the removal of foulants from a membrane surface.  This has been used more in the cleaning of individual elements.

Pneumatic cleaning includes air sparging, air lifting, air scouring and air bubbling. Pneumatic cleaning has the benefits of low maintenance cost, is easy to integrate into the membrane system and no chemicals are required.

Self-collapsing air micro-bubbles (with diameters of less than 50 µm) have been shown to be a potential chemical-free cleaning technology for biofilm detachment from membrane surfaces due to their unique capacity to shrink and subsequently collapse in solution [39].

Figure 24: Installation schematic for the CIP microbubble generator

8.3.2 Reverse flow cleaning membrane:

The advantage of reverse cleaning is that certain types of foulants concentrate on the feed end of the lead membrane, as shown in Figure 25.  Biological foulants, particulates, and colloidal matter are examples of foulants that usually are present on the lead end of the feed membrane.  Normal cleaning must break up these foulants and push the foulant through all of the other membranes in the pressure vessel to exit out the brine side. With reverse cleaning, it becomes more of a physical process of “pushing” the foulant out through the feed end, the shortest path to leave the element.  This method was found to be more effective than using cleaning chemicals in a normal flow direction. [5]

Figure 25 – Diagrams showing normal cleaning flow path and reverse cleaning flow path [5]

8.3.3 Other cleaning method to improve cleaning efficiency:

• Using high ionic strength cleaners to create permeate flow across the membrane during periods of soaking helps dislodge layered deposits of clay and biofilm.
• Due step chemical deep cleaning
• NaCl light salinity cleaning.
• Electro Magnetic Fields (EMF).
• Direct Osmosis at High Salinities (DO-HS) cleaning
• Osmotic flush membrane after RO stop

9   Conclusion

RO membrane biofouling is worldwide problem especially in multiple wastewater treatment.  From RO weight results and RO autopsy is showing SEC RO membrane has biofilm grow up inside of the membrane.  RO membrane life is short due to biofouling.

Chlorine and dechlorination causes bacteria to rapidly regrow after dechlorination points, as confirmed by microbiological study and operation results.  Use RO performance tool to monitor normalized differential pressure to do predictive RO cleans will increase RO lifespans.

Continuously dosing biocide (DBNPA) inside of membranes can control biofilm growth inside of membranes, but it will increase operation costs.  

Injecting ammonia into pre-treatment systems to convert free chlorine to chloramines should be considered as another economic way to control RO biofouling.

Acknowledgments:

Specially thanks SEC management and operation team to support this project .  Finally, thanks to the ChemTreat team for providing bacteria study and great support as water treatment consultants these past years.

Reports completed at 2020-12-07

Attachment 1 Most recently replaced RO membrane cleaning history

Cleaning times	RO cleaned Date	Feed pressure
		Before cleaning	After Cleaning
		kPa	kPa
New membrane	2019-05-29		1056
1	2019-08-20	1818	1446
2	2019-10-16	1902	1428
3	2019-11-19	1902	1558
4	2019-12-16	1900	1644
5	2020-01-07	1900	1570
6	2020-02-11	1723	1584
7	2020-03-18	1925	1618
8	2020-04-16	1926	1570
9	2020-05-08	1689	1525
10	2020-06-10	1963	1754
11	2020-07-07	1958	1768
12	2020-07-31	1879	1777
13	2020-08-09	1865	1840
14	2020-08-25	2062	1864
15	2020-09-07	2046	1835

Attachment 2 RO train 2 most recently replace cleaning history

Cleaning times	Cleaning Date	Before cleaning feed pressure	After Cleaning feed pressure
		kPa	kPa
New membrane installed at	2019-07-29	989
1	2019-10-17	1453	1291
2	2020-01-14	1341	1166
3	2020-02-19	1138	1157
4	2020-05-17	1909	1734
5	2020-06-29	1962	1785
6	2020-07-23	1880	1878
7	2020-08-07	1965	1818
8	2020-08-19	2005	1887
9	2020-09-06	2075	1961
10	2020-09-12	2024	1906
11	2020-09-21	2090	1940
12	2020-09-30	2027	1906
13	2020-10-07	2087	1927

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