RESOLUTION OF PERVASIVE AND RESISTANT BIOFOULING …

Wor World Congress/Perth Convention and Exhibition Centre (PCEC), Perth, Western Australia September 4-9, 2011 REF: IDAWC/PER11-316

RESOLUTION OF PERVASIVE AND RESISTANT BIOFOULING ISSUES AT W.E.B. ARUBA N.V. SWRO PLANT Authors: Ted Prato, Engr. Filomeno A. Marchena, Erik Schoepke Presenter: Ted Prato Lead Process Engineer – GE Water – USA Abstract: Because of an arid climate and a lack of local natural water supply, Aruba depends on desalination for its drinking water supply. Since 1932, various distillation technologies supplied desalinated water. However, in 2008 more energy-efficient, conventional desalination technology was introduced. W.E.B. Aruba N.V. and GE teamed up to build a seawater reverse osmosis (SWRO) plant in Aruba. Commissioned in early 2008, three SWRO trains operate to provide a target flow of 8000 M3/day. Seawater feed enters the plant from an open beach intake, and the flow passes through a pre-treatment process using multi-media filters and cartridge filters prior to entering the membranes. The RO units began to experience process issues due to higher than expected biofouling potential. The SWRO units demonstrated high feed pressures and greatly reduced production. This paper will describe the complex issues that led to fouling of the SWRO membranes as well as an innovative and aggressive approach to resolve the problem. A combination of process modifications is described as well as the investigative efforts on the nature of the foulants. The Toray membranes were at first believed to be irreversibly fouled; however, a combination of various chemicals and application methods helped to reduce the high feed pressure and restore target flow. This process work is ongoing in order to continue to protect the SWRO integrity.

IDA World Congress – Perth Convention and Exhibition Centre (PCEC), Perth, Western Australia September 4-9, 2011

REF: IDAWC/PER11-316 -2-

I. INTRODUCTION When biofouling pervades an RO membrane water treatment system, permeate water production can decrease dramatically, and rising feed pressures will increase energy costs significantly. W.E.B. Aruba N.V.’s newly-installed seawater desalination facility experiences such a problem during the interval between the spring of 2008 and 2009. This invasive biofouling took hold starting at the intake in macro form and extended past the pretreatments system and into the SWRO membranes. Evidenced by the daily decline in plant production, the well-established biofouling matrix was unaffected by conventional standard applied CIPs. [1] This paper describes the biofouling of the plant and what was done to control and recover plant performance. We will discuss how deceptively clean feed water hid a complex mix of aggressive microorganisms and therefore a strong potential for biofouling. Additionally, we discuss longer-term strategic steps which were taken once effective CIPs were applied. Because of an arid climate and a lack of local natural water supply, Aruba depends on desalination for its drinking water supply. Since the 1930’s, various distillation technologies supplied desalinated water to Aruba. However, in 2008, because of the high energy cost of distillation, more energy-efficient desalination technology was introduced. In an effort to improve the costs of producing water for the island, W.E.B. Aruba N.V. and GE teamed up to build a seawater reverse osmosis (SWRO) plant in Aruba. Commissioned in early 2008, three SWRO trains operate to provide a target flow of 8000 M3/day. The new water treatment plant worked well until the spring of 2009 when the biofouling began to significantly affect the process, creating a steady rise in feed pressures and a steady decrease in production. Believing the water was relatively free of bioactivity, a low level shock chlorination process was designed for the system. Unfortunately, it was not realized until after macro fouling was discovered that this program was not adequate for the incoming load. Furthermore, the operations team had not realized the necessity of launching advanced fouling monitoring, and the inexperienced crew did not watch for tell-tale signs. An additional complication was a delay in the commissioning of the CIP system--this prevented cleaning of the membranes sooner in the fouling process. The shock chlorination dose of once per week of the seawater feed was not sufficiently frequent to offset the incoming load. In response to the biofouling attack, a number of process modifications were applied to the plant beginning in June of 2009. Initially, the Toray RO membranes were at first believed to be irreversibly fouled; however, a unique combination of CIP chemicals (with modified application methods) helped to reduce the high feed pressure and restore target flow.



II. W.E.B. ARUBA SWRO PLANT DESIGN The W.E.B. SWRO system takes seawater from a shallow beach intake structure. Dual intake pumps pass the raw seawater along to the pretreatment train. Pretreatment, including a daily intake chlorine sanitization, consists of a set of multi-media filters that are also periodically chlorinated during the backwash cycle to suppress biogrowth. The media filter product water enters a large filtered water storage tank. Low-pressure pumps feed water from the filtered water tank to a bank of cartridge filter vessels. Product from the cartridge filters enters the high-pressure SWRO pumps and into three SWRO units. Each unit is one stage RO with 32 vessels, each vessel containing seven eight-inch elements. SWRO product water enters an interim feed tank for the subsequent downstream BWRO, and the BWRO product is then distributed to limestone beds and final product water tanks.

Figure 1: Plant schematic of SWRO-1.

Concentrate Discharge

BWROSWRO

ERDCF MMF

BW FeedTank

2nd PassFeed Tank

Filtered Water

Feed Tank

ProductStorage

Tank

Shore Based Open

Intake

To Remin

RO Building



III. THE EXTENT OF BIOFOULING Severe biofouling extended from the inlet through to the SWRO membranes. Macro bioforms accumulated on the insides of inlet piping and all surfaces including inside the media filters. Biofilms formed on all available surfaces.

Figure 2: Biogrowth on SWRO inlet piping By the beginning of the second year of operation, standard RO CIPs were having greatly diminished effectiveness. Little improvement in SWRO membrane feed pressure resulted from standard CIPs although production in (m3/d) and axial pressure drop-dP (bar) nearly always showed moderate improvement.

Figure 3: Decline in product volume over time. Target flow is 111 m3/hr. Production of permeate was trending downward on an incremental basis, failing to meet the design target of 8,000 m3/day.



Figure 4: Increase in dP over time. The breaks represent CIPs--the upward spikes indicate rapid increase in dP after CIPs. Energy consumption for the process was significantly higher than expected because of the elevated energy consumption of the high pressure SWRO feed pumps. Early CIPs, while reducing dP for short periods, did little to affect membrane TMP, and CIP solutions did not demonstrate significant color change. As an initial attempt to better understand the biofouling and its sources, a student intern (Jink Gude) was engaged by the W.E.B. seawater desalination site to work on biofouling detection methods, including a biofilm monitor and a biofilm early warning system. This work documented the extent of biofouling throughout the SWRO pretreatment system including intake piping, media filters, filtered water storage tank, cartridge filter vessels and manifolds, and SWRO feed piping. [2] The biofouling monitor was composed of a set of plastic rings that could capture, retain, and grow biofoulant materials. See below.



The presence of significant biofouling was also demonstrated by masses of stringy, gelatinous macro-forms of biofilm. [2] Some identification of biogrowth was also done through adenosine triphosphate (ATP) analysis; however, biofilm presence was confirmed by macro forms inside piping and vessels. [3] The value of this work was in confirmation of extensive biofouling, and the project was ended in June of 2009. IV. SYSTEM INSPECTION AND MEMBRANE FOULANT ANALYSIS Several lead RO membranes were pulled from the vessels and sent out for performance testing and foulant analysis to commercial and GE laboratories. Some elements were opened for direct inspection and analysis of surface biogrowth. After autopsy and inspection, membrane samples were placed in stirred cells and subjected to a range of cleaners to see what would react well with the surface foulants. The exposed membrane was observed to be coated with a uniform, sticky thin coating composed of biogrowth and very small silt particles. See photos below.

Figure 5: Biofilm Monitor showing sets of glass rings through height of column. The column was clear at start of a monitoring test, but it quickly developed a yellow-brown color, characteristic of biofilm formation observed throughout the SWRO system.



Figure 6: Disassembled W.E.B. SWRO element. Inspection of the fouled membrane surface revealed a relatively thin layer of biofilm (~5 mil) colored with fine brown silt. Since this material could be easily wiped off manually, this suggested the possibility of removal by the right combination of cleaning chemicals. More aggressive biomass can be sticky to the touch, hence the results from the subjective wipe tests were encouraging. Foulant analysis indicated LOI of average 83% with the major component being amides. The amide present is most likely the result of microbiological content. Figure 7: Membrane surface showing area of fouled membrane wiped clean physically by mechanical action.

W.E.B. SWRO ELEMENT W.E.B Aruba

disassembled RO

element showing

biofilm matrix. This



Additional foulant analysis and physical examination was done at the GE analytical lab in Woodlands, Texas. Phase contrast microscopy (at various magnifications) was used for microbiological analysis. Samples from the membrane surface consisted primarily (>75%) of biological forms including copious amounts of exopolymer-encapsulated bacteria and iron related bacteria (e.g., Leptothrix). Bacteria over-produce exopolymer when their environment contains an abundant biodegradable carbon source. In addition to the fouling on the surface of the membranes, there was extensive biofouling and biofilm formation in other parts of the SWRO plant infrastructure. This included the high-pressure piping, cartridge filters and even the PX devices, the energy recovery equipment. The porous ceramic surfaces of the PX can be ideal for biogrowth. A strong lesson learned was the realization that biogrowth in the PX devices was recontaminating the cleaned SWRO membranes. [4] The photo below shows the heavy biofilm inside the PX internals. In one instance, heavy biofilm actually stopped the PX devices from moving.

Figure 8: The extent of biofilm in the energy recovery devices was so severe that the devices had to be taken apart for full manual cleaning. The PXs were cleaned with soap and bleach.



Figure 9: A full manual cleaning was done to all parts of the PX equipment and piping. Once effective CIPs were being applied, some of the CIP solution was also circulated through the PX system and piping on a regular basis. The PXs are now opened up and physically inspected on a periodic basis to confirm that there is no recontamination. V. IT CAME FROM THE SEA In preparation for the original seawater desalination plant, a study was made of the coastal area and conditions from which the water would be drawn. These studies included water analysis, bacteria and E-coli measurements, tidal and wind patterns, particulate measurements, current and temperature variations.

Figure 10: Aerial view of site showing direction of current and wind [5]



Although various designs for the intake structure itself were proposed, the agreed-upon design was for a beach intake, at medium tide level adjacent to a shallow lagoon. The depth of the intake opening was 10 ft. with the area defined by a floating boom. In the past, W.E.B. employed a local marine intake design company to develop shallow beach intakes for drawing water for the distillation units. Unfortunately, the same approach was taken for the SWRO by GE, and it was subsequently discovered that what is good for distillation is not necessarily good for membrane treatment of seawater. The routine marine studies made without deeper analysis of marine life and bioactivity had not been sufficient. However, options were limited since development of a deeper water intake included significant concerns with on-going boat traffic in the area. Further research has revealed that during rain and strong wind, fine particles from the island were washed or blown into the strong current flowing past the intake. Rough water conditions could also kick up silt and particulates from the bottom of the seabed. The shallow water permitted penetration of sunlight, and this combined with organic nutrients to provide healthy algae growth. Figure 11: Geographic features affecting quality of feed water including creek outlet (washed out dust particles), shallow warm water and seagrass beds. [5] Current flows from right to left in the above image. Silt particles and all types of ocean biogrowth travel along the shore of the island, passing by the seawater Unit 1 intake structure shown as “Water Factory.” See below for an image of mats and tumbleweeds of algae rolling along the bottom outside the plant intake. This is a strong indication of a vigorous biofouling potential.



Figure 12: Algal mat on shallow seabed outside of plant intake. [5] The marine environment adjacent to the plant and up-current was also analyzed for microorganisms, revealing high colony counts of bacteria including Leptothrix. W.E.B-contracted analysis of seawater biological content included: Heterotrophic bacteria (HAB), aerobic, 500,000 cfu/ml Iron-related bacteria (IRB), 140,000 cfu/ml Slime-forming bacteria, (SLYM), dense slime type, 350,000 cfu/ml It is suggested that the fine silt entering the seawater infrastructure acted as nucleation sites for biogrowth, helping to create the pervasive layer of biofilm coating the membranes. This process was assisted by readily-available organic nutrients and the warm ambient temperature. Membrane autopsy results indicated only a minor percentage of inorganics, however, the fine brown silt was observed throughout the system. How did this silt pass through the cartridge filters into the membranes? Some of the filter caps were incorrectly placed, permitting by-pass of particles from pretreatment. This was corrected, but it is difficult to see a direct relationship in improvement since 1-silt was already on the membranes and 2-other system corrections and improvements were being made at the same time. See photo below.



Figure 13: View of cartridge filters showing difference in by-pass resulting from incorrect placement of endcaps VI. ATTACKING THE PROBLEM Urgent action was called for because of the steady decline in productivity, persistence of high pressure and high dPs. The deterioration of plant performance was accelerating, and standard CIPs had become even less effective. Using analysis of membrane foulants as a starting point, stirred cell tests were initiated to observe which type of cleaners and conditions provoked reaction with and/or removal of the sticky biofilm. These tests were performed on membrane samples taken from lead elements. Based on jar testing observation and on CIP results from other installations, full plant testing with the following CIP steps was optimized. Soak with SBS to scavenge oxygen out of the water within the elements and piping. This was done to stress aerobic organisms, weakening the metabolism as preparation for attack by the following chemicals. Soak with anionic surfactant. The positive anion group would attach to negatively charged organics. With surfactant kept in solution, a proprietary cleaner was used. This cleaner was a high pH chemical with a mild oxidizer present. This helped to scour out materials. pH was bumped up to a slightly higher level. While presenting some risk to membranes, the slight pH increase to 12.2 was helpful. Temperature was held to a maximum of 35 C. Other actions were also taken to provide improved conditions for a more successful cleaning experience: These include: Correction of cartridge filter end cap orientation to prevent foulants and silt from leaking past filters through to SWRO membranes. Although fouling rate declined, it is difficult to attribute the improvement to this correction since other improvements were made at the same time.



Correction of media filter macro fouling by initiating media filter backwash chlorination: Each filter undergoes backwash once per day. At the slow backwash and 1 hr. soak steps of the BW cycle, 18-20 ppm of free chlorine is pumped into the filter. After several months of backwashing under these conditions, the macro fouling and high slime conditions within the filters were essentially eliminated. Before After Figures 14 and 15: Difference in submerged piping in MM filters before and after 3 mo. of backwash chlorination. Initiating tank and piping sterilization. A procedure was developed for shock sterilization of the piping, tanks, pumps, manifolds and cartridge filter vessels—all of the delivery infrastructure. 240 PPM of free chlorine was pumped through the system and allowed to soak. The concentration was gradually reduced overnight to 0 PPM levels by a feed and bleed method. [Membranes were sealed off from the solution using valve closure and installation of blind flanges.] Another valuable lesson learned is: Watch the rate of consumption of free chlorine not only during a infrastructure sanitization by also during the daily short-term seawater intake sanitization. It is also well-known that some bacteria may encapsulate and thus resist sanitization efforts.



VII. DISCUSSION OF CIP RESULTS The optimized CIP method dropped the high feed pressure from 65 bar down to 59 bar. This was the first time that any significant pressure drop had been observed with a CIP. Good color throw was also observed in the CIP solution indicating a strong interaction with the materials on the membrane surface. The following chart shows the change in SWRO inlet feed pressure as a result of the CIPs.

Figure 16: Drop in Inlet Pressure and Recovery of Productivity

Figure 8 also shows recovery of full target productivity of 8,000 m3/day. The sequences of successful applied CIPs combined with the previously discussed process improvements enabled the SWRO units to recover design production. Additional variations in the CIP recipes provided further benefits including significant drop in axial membrane pressure drop dP. dP went from an average of 1.5 bar down to 0.90-0.98 bar. [Start-up dP was 0.8 bar] The improvement in dP indicates that foulant materials were effectively cleaned out of the feed channels; however foulants remained in membranes pores, causing a rapid increase in TMP.



Figure 17: Progressive improvement of the axial pressure drop (dP) as additional CIPs were applied. Figure 18: Change over time of the slope of dP increase.

0.7

0.8

0.9

1

1.1

1.2

1.3

1.4

1.5

0

0.01

0.02

0.03

0.04

0.05

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8/15/2009 9/4/2009 9/24/2009 10/14/2009 11/3/2009 11/23/2009 12/13/2009 1/2/2010 1/22/2010 2/11/2010

FO

UL

ING

SL

OP

E

DATE

WEB ARUBA SWRO-1: CHANGE IN SLOPE OF FOULING INCREASE vs TIMESecond graph shows drop in starting dP after successive CIPs

SLOPE DELTA

dP BAR post CIP

CHANGE IN FOULING SLOPE VS. TIME

LOWEST dP BAR POST

T. Prato 3-28-



Energy consumption of the SWRO unit was also improved as a result of the successfully-applied cleanings. Prior to effective CIPs, the pumps were operating at full pressure, and the SWRO membrane inlet pressure was constant at 65 bar with productivity slowly decreasing. There were other plant issues that also affected energy consumption including excess air conditioning and an unplanned reduction in BWRO output due to use of product water for pump seal cooling. These issues were also addressed as part of the overall improvement in operation.

Figure 19: Improvement in energy consumption over time. While energy consumption was improved, it did not return to the design energy level of 4.0 KWhr/m3. This is in part due to aging and compaction of membranes, but it also pertains to foulants driven deep into the membrane structure, fouling that may not be reversible by CIPs. VIII. THE PATH FORWARD While it is clear that the correct combination of CIP chemicals and technique helped to improve critical operating parameters of the SWRO unit, the membranes appeared to re-foul quickly. Some of this re-fouling and rapid pressure increase was no doubt because of readily available recontamination from biomass within the infrastructure such as inside the piping and inside the PX devices. However, even the recontamination does not fully explain the rapid reestablishment of high feed pressure. In addition to deeply embedded foulants, permanent compaction of the RO membranes is suspected. This is supported by the fact that even while high feed pressure is re-established, the increase in dP is minimal, thus the transmembrane pressure (TMP) becomes the dominant variable.



While the salt rejection performance of the membranes has remained excellent throughout, that is starting to decline as the membranes age and are subject to the multiple cleanings. The membranes are entering their third year of operation. Few membranes were replaced during the first two years of operation. In October of 2010, W.E.B Aruba elected to replace the lead three elements in each vessel of each of the three SWRO trains. While this is not the best recommended procedure, all concerned recognized that these lead elements were severely fouled. The lead three elements weighed an average of 3 lb. more than the lag elements. The main lesson learned here is that replacing lead elements of a set of fouled elements will only provide short term gains of months or less in this type of environment. Performance following the partial membrane changeout showed that the three new elements fouled rapidly within three months of operation, and pressures began to spike. However, W.E.B. Operations had become much more expert at applying timely CIPs and system sanitizations. While CIP frequency has settled down to become regular, the feed pressure has never spiked near the maximum of 65 bar, and the product flow has never dropped below target.

Figure 20: Increase in SWRO Unit A feed pressure after installation of 3 new lead elements Next steps include completion of element changeout, but more importantly, there will be a switch in feed source from open intake to beach wells. It is expected that although some bacteria were detected in well drilling pilots, the switch from beach intake to wells may help to reduce the biofouling potential of the incoming water. Combined with the new membranes and free of the heavy fouling and frequent CIP history, the W.E.B. SWRO Unit 1



should perform well and reliably. This will only be possible if the wells are properly maintained, and more active preventative monitoring is conducted. IX. ACKNOWLEDGEMENTS: I wish to acknowledge the assistance of Ken Irwin, Paul Clemens, Bill Musser, Melanie Blake, Randolf Wiersma, Mustansar Malik, Victorino Alberts, Franklin Rasmijn, Juliaan Rustveld, Robert Wever, and Nestor Franken. X. REFERENCES

1. Kasama, H., “Biofouling in SWRO Systems: Tackling the Biofouling Challenge,” Global Water Intelligence, April 2011, pp 41-43.

2. Gude, J., “Quantifying Serious Fouling in the SWRO Process at W.E.B. Aruba N.V., Internship

Report, Technical University Delft, 2009.

3. Webb, W.G., Paul, D.H., “Surface Water Reverse Osmosis System Biofouling,” Ultrapure Water Magazine, November, 1994, Tall Oaks Publishing, Boulder, CO.

4. Vigneron-Larosa, N., Magnier, M., Gaval, G., Rapenne, S., Breda, C., “Comparison of Surface

Water Pretreatment Technologies for Membrane-Based Drinking Water Production Processes,” American Water Works Association, 2011 Membrane Technology Conference Proceedings.

5. Eco Vision, MER Study Report, “Inventarisation of the Effects of the New RO-2 Plant of W.E.B. Aruba N.V. on the Marine Environment and the Salt Water Intake-final Report,” 2010. With permission from W.E.B. Aruba N.V.

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RESOLUTION OF PERVASIVE AND RESISTANT BIOFOULING …