Innovative Ultrafiltration (UF) Membrane for Produced Water Treatment

Authors: Lyndsey Wiles1, Elke Peirtsegaele1, Mike Snodgrass1

1TriSep Corporation, 93 South La Patera, Goleta CA 93117

Abstract Current unconventional oil & gas exploration methods both consume and generate large volumes of water that require proper treatment, handling, and management. This practice, combined with an increasing amount of water scarcity, has made water reuse a necessary focus for the oil & gas industry. As a result, oil & gas producers are seeking treatment options to use their water more effectively. Advanced treatment and reuse of produced and flowback water often requires ultrafiltration (UF), which removes suspended solids, bacteria and other microorganisms, as well as oil & grease. Traditionally, produced and flowback water has proven challenging for UF membranes due to its aggressive fouling tendencies. This paper examines three produced water case studies using an innovative UF membrane that was successfully operated with the following upstream treatment technologies: 1) coalescer, 2) walnut shell filter, and 3) electric coagulation. Despite differences in produced water quality at each test site, the UF membrane process was able to provide consistent effluent quality at a high throughput rate. Oil levels in the UF feed, regardless of upstream treatment, reached as high as 300 mg/l while suspended solids were consistently >50 mg/l. Undeterred by spikes in incoming water quality, the UF membrane provided consistent effluent quality: <1.0 mg/l oil & grease, <1.0 mg/l TSS, and <2.0 silt density index (SDI). Introduction As unconventional oil & gas production becomes increasingly prevalent, the amount of water produced via these methods will continue to grow. In the United States, produced water estimates are on the order of 20 billion barrels per year and are steadily rising (Clark and Veil, 2009). To account for ever-increasing fresh water scarcity throughout the country, oil & gas producers have realized that water must be utilized more effectively to ensure continued growth and development. Such concerns over water have pushed water reuse to the forefront of oil & gas exploration. High concentrations of various contaminants make produced water particularly challenging to treat through traditional methods. Produced water typically has oil levels in the range of 40-2000 mg/l (Clark and Veil, 2009), total suspended solids (TSS) in the range of 1.2-1000 mg/l (Guerra et al., 2011), total dissolved solids (TDS) in the range of 1,000-400,000 mg/l (Clark and Veil, 2009), and no bacteria present (Veil et al., 2004). Additionally, every oil & gas well has unique and often-changing water characteristics.

This presents a significant challenge to oil producers when determining the best treatment approach, and requires their collaboration with water industry experts in developing innovative treatment solutions. Successful produced water treatment technologies for reuse are those that can adapt to changing water quality while providing a reliable effluent stream. The size-exclusion mechanism of removing oil, suspended solids, and bacteria from a produced water stream via ultrafiltration (UF) offers an attractive method for the production of a consistent effluent stream. However, produced and flowback water have traditionally proven a challenge for UF membranes due to their aggressive fouling tendencies. Polymers used to manufacture ultrafiltration membranes are derived from oil and, historically, have been both highly oleophilic and hydrophobic. Such properties have left polymeric UF membranes vulnerable to severe and rapid fouling in applications like produced and flowback water treatment. This paper discusses an innovative UF membrane with a significantly greater oil tolerance than traditional UF membranes. This membrane has a proprietary hydrophilic polyvinylidene fluoride (PVDF) chemistry with a 0.03 micron pore size. The configuration of the discussed membrane is in a spiral-wound, submerged-style (vacuum-operated), module with air-scouring and backwashing capabilities, as shown in Figure 1. This membrane was developed for a variety of wastewater applications, and typically treats streams with about 1,000 mg/l TSS and has successfully treated streams with as high as 3,000 mg/l TSS. Due to its unique chemistry and its ability to handle tough feeds, this membrane was considered a strong candidate in exploring oily feeds.

Figure 1: UF configuration iSep™ 500-PVDF

The performance of the UF membrane was characterized by operating the same pilot unit system at three different test sites. This pilot unit allows for the membrane to be cleaned chemically or mechanically, utilizing backwashing or air scouring. Key components of this pilot unit include a permeate (vacuum) pump, backwash pump, chemical metering pumps, blower, automated valves, and other instrumentation. The pilot unit contains a touchscreen HMI for control and automation, and all process sequences, including production, backwash, and chemically-enhanced backwash. A process flow diagram of the pilot unit is shown in Figure 2.

Permeate Collection TankPump

Pump

Metering Pump

Metering Pump

Metering Pump

Blower

iSep

Module

FI

Flow

Meter

Control

Valve

Acid

OverflowChlorine

Caustic

Feed

Air

T

Temperature

Sensor

FI

Flow

Meter

Control

Valve

Drain

Automated

Valve

Control

Valve

FI

Flow

Meter

P

Pressure

Gauge

Automated

Valve

Automated

Valve

FI

Flow

Meter

Drain

Automated

Valve

Figure 2: UF pilot unit process flow diagram

Based on numerous lab scale studies demonstrating exceptional oil tolerance, the UF membrane was selected for a variety of pilots exploring the advanced treatment of produced water. When following initial oil-removal technologies, the UF membrane offers an advanced water treatment method for oil & gas producers to reuse their water. Three case studies involving a different upstream technology followed by the aforementioned UF membrane are examined. The different upstream treatment technologies for each case study are as follows: 1) coalescer, 2) walnut shell filter, and 3) electrocoagulation.

Case Study 1: Coalescer Upstream of UF

As a result of the UF membrane’s potential to tolerate high oil concentrations compared to traditional UF membranes, the membrane was selected for a pilot study on evaluating various technologies for advanced treatment of produced water. Piloting was conducted over the span of two weeks during August 2013 by an independent research organization in partnership with the various technology suppliers selected for the study. The short time frame involved was intended to demonstrate proof-of-concept for the technologies’ ability, to handle the challenges of produced water treatment. The feed water for this piloting was taken from an oil production site in the Midwest and was treated by a coalescer unit prior to being fed to the various technologies being evaluated, including the UF. Following pretreatment by the coalescer, the feed to the UF contained an average oil concentration of 22 mg/l, with a maximum of 45 mg/l, and an average turbidity of 160 NTU, with a maximum of 220 NTU, as displayed in Table 1.

Table 1: Feed water characteristics with coalescer pretreatment

Parameter Unit Value

Average Turbidity NTU 160

Maximum Turbidity NTU 220

Average Oil Concentration mg/l 22

Maximum Oil Concentration mg/l 45

Throughout this trial, the UF operated on the feed described above without experiencing any of the serious issues commonly observed when using traditional UF membranes to treat oily feeds. Typically, oil fouling of a membrane results in a high transmembrane pressure (TMP) due to oil adhering to the membrane surface and restricting flow. Despite the presence of both oil and solids, the pilot unit maintained a reasonable TMP between 1-4 psi while providing high-quality effluent, as seen in Figure 3, demonstrating significant oil tolerance and removal.

Figure 3: UF feed (left) and permeate (right) demonstrating oil removal

Further evidence of considerable oil removal was displayed by the UF. There were two different types of oil detection instruments deployed during this study: a fluorometer and an optical imaging device. When the UF permeate was examined by both of these devices, the oil concentrations were found to be below each instrument’s detection limit, indicating oil levels <1 mg/l. These results for oil level detection confirm the oil removal capabilities of the membrane. Overall, the combination of oil tolerance and oil removal qualified this as a successful initial produced water pilot for the UF. During the testing, a variety of other technologies were subjected to the same feed water that was fed to the UF, including hollow fiber membranes and different forms of media filtration. Several of these technologies simply could not continue operating due to issues such as oil fouling. The fact that the UF was able to run for the entire two weeks without a serious fouling event showcases its ability to tolerate the oily streams of produced water.

Case Study 2: Walnut Shell Filter Upstream of UF

After a successful proof-of-concept study in the Midwest, the UF was selected for another piloting opportunity at a produced water treatment research facility to take place in early 2014. The piloting was carried out by a company in the United States that wanted to evaluate various membrane technologies for produced water applications. Testing was done over the span of a month, offering more time to analyze the membrane performance. To maintain consistency throughout the entirety of testing, a synthetic produced water was used as the feed to the overall treatment process. After mixing of the synthetic produced water, the stream was fed to a walnut shell filter (WSF) for bulk oil removal. Following the WSF, the filtered stream was fed to the UF, followed by a reverse osmosis (RO) unit. To accurately duplicate a produced water stream, various constituents were added to obtain desired levels of oil, TSS, TDS, and several organic compounds, as shown in Table 2. After addition of the oil to the synthetic waste stream, the oil and other hydrocarbons were mechanically emulsified.

Table 2: Synthetic produced water composition

Chemical Compound Concentration (mg/L)

Ca 250

Fe 10

Mg 75

K 200

Na 1,700

SO4 100

CO3 (as CaCO3) 500

Cl 3,015

TSS (majority as bentonite) 20

Oil 30

Benzene 2

Toluene 0.5

Ethylbenzene 0.5

Xylenes 0.5

Upon pilot start-up, the feed water quality to the UF was mild, with oil levels <10 mg/l and turbidity levels well below 1.0 NTU. As a result, very low TMPs were observed during this time, indicating extremely efficient membrane performance. Based off this performance, an operating flux of 30 gfd was chosen for the remainder of the pilot study. As planned, the oil concentration from the WSF increased and fluctuated, with an average concentration of 34.8 mg/L and a maximum concentration of 166 mg/l. Despite the significant spikes in oil concentrations, as shown by the presence of oil on top of the module (see Figure 4), the UF continued to perform well. To understand the maximum oil tolerance of the UF, it was decided to bypass the WSF altogether. Bypassing the WSF resulted in an average UF feed oil concentration of 72 mg/l with a maximum concentration of 288 mg/l.

Figure 4: Visual presence of high oil levels in UF feed

During the third week of testing a fouling event occurred, as shown by a significant TMP spike from 3.7 to 10.7 psi in the span of a single day. Initially thought to be due to the significant oil concentrations being fed to the UF, it was later found that an unexpected algaecide containing a cationic surfactant contributed considerably to the membrane fouling. To restore UF performance, membrane cleanings were attempted with chlorine, caustic, acid, and a detergent alkaline cleaner. The detergent alkaline cleaner successfully restored the TMP to its original start-up levels. Since membrane cleanings are a common part of membrane operation and maintenance, the TMP restoration by cleaning is a valuable demonstration of this membrane’s ability to operate successfully in this application. Similarly to the pilot study with the coalescer, this pilot study demonstrates the membrane’s ability to maintain permeability (i.e. performance), despite fluctuating oil levels with relatively large spikes. Extended operation at moderate to high oil levels indicate the hydrophilic PVDF membrane has significant oleophobic properties. Graph 1 shows TMP, flux, and temperature-corrected membrane permeability during the pilot study. Since permeability decreases as membrane fouling increases, the relatively stable permeability and restoration of permeability after cleaning attest to the membrane’s resistance to oil fouling.

Graph 1: TMP, flux, and permeability of the UF over time

Oil removal capabilities were further demonstrated and quantified by this testing as well. Throughout the month of piloting, the average oil concentration fed to the UF was 53.4 mg/l. Even with the spikes in oil, the UF consistently provided effluent oil concentrations of <1 mg/l, as shown in Graph 2 below. As with the intial pilot, oil detection methods were not always accurate, but it was mutually agreed that the UF permeate had <1 mg/l oil because the RO unit running on UF permeate did not experience significant fouling issues. The UF consistently provided permeate suitable for RO feed, as demonstrated by measuring silt density index (SDI), a common feed water qualifying method in RO systems. Typically, a feed SDI of 3.0 is a common design criterion for RO systems. The UF membrane consistently produced permeate with SDI values well below 2.0, indicating very high effluent quality.

0.00

5.00

10.00

15.00

20.00

25.00

30.00

35.00

40.00

45.00

50.00

0.0 100.0 200.0 300.0 400.0 500.0 600.0 700.0

Hours of Operation

TMP (psi)

Flux (gfd)

Permeability (gfd/psi)

Graph 2: Influent and effluent water quality of the UF

Overall, this pilot study gave additional evidence for the UF’s oil tolerance and removal capabilities. A summary of the pilot study results is given in Table 3. Based on this successful pilot opportunity, additional opportunities were sought out and secured, resulting in the third case study.

Table 3: Summary of UF performance

Parameter Units Value

Operating Flux gfd 30-40

TMP (average) psi 2.85

Temperature °C 14.7-29.9

Influent Turbidity NTU 0.057-8.918

Effluent Turbidity NTU 0.020-0.100

Effluent SDI - 0.78-1.40

Influent Oil mg/l 0.8-288.0

Effluent Oil mg/l 0.0-5.8

Backwash Frequency min 15

Backwash Duration sec 60

Relaxation Duration sec 60

0.01

0.1

1

10

100

1000

0.0 100.0 200.0 300.0 400.0 500.0 600.0 700.0

Hours of Operation

Feed Oil (mg/l)

Feed Turbidity (NTU)

Permeate Oil (mg/l)

Permeate Turbidity (NTU)

Case Study 3: Electrocoagulation Upstream of UF

With about 1 of every 50 barrels of U.S. oil output coming from Colorado, its Western Slope region has become an important location for exploring produced water treatment options (U.S. EIA 2014). In mid-2014, this fact, combined with a better understanding of what the UF could handle, led to a third piloting opportunity. Piloting began in May 2014, and the raw feed water source was a producing oil well in the Western Slope. In this case, the technology utilized for bulk oil removal ahead of the UF was a multi-step electrocoagulation process. The overall goal of this pilot was to demonstrate the potential of using the electrocoagulation process followed by the UF for reuse applications, such as RO, direct reuse, or surface discharge. To accomplish this demonstration, the pilot was run for two weeks and water samples from various stages of the process were taken for analysis. The UF worked well with the electrocoagulation process and produced water that visually accomplished the treatment goals. Samples of water were taken at various stages of the electrocoagulation process and UF, and a picture of them is displayed in Figure 5. The clear water produced by the UF indicated satisfactory operation by the UF.

Figure 5: UF feed (second to right) and permeate (furthest right) demonstrating

satisfactory oil removal

In accordance with the first two piloting experiences, the UF demonstrated promising oil tolerance and removal. Throughout testing, the UF maintained a relatively constant permeability and TMP. Shortly after 20 hours of operation, the TMP spiked and the

permeability sharply decreased due to a treatment issue prior to the UF. Upon resuming normal operation of the UF, TMP and permeability were restored to a stable level, demonstrating the UF’s capability of handling stream upsets, as shown in Graph 3. Graph 3: TMP and permeability over time

Similarly to the initial piloting experience, exact oil concentration in the UF permeate was difficult to measure. Samples were taken for analysis, and it was found that the UF provided a permeate oil concentration of <1.0 mg/l, despite a significant solids loading. Since each well can have different water quality, the UF’s ability to handle both high solids and high oil concentrations is a good indication of its flexibility to tolerate a wide variety of produced waters. A summary of the water quality characteristics is provided in Table 4.

Table 4: Influent and effluent water quality of the UF

Parameter Units Value

Influent Turbidity NTU 76.2

Effluent Turbidity NTU 0.02

Influent Oil Concentration mg/l ~20

Effluent Oil Concentration mg/l <1.0 mg/l

After optimizing pilot operation, another valuable analysis was performed during this testing: an operating cost analysis. To determine total power consumption, the power usage of the permeate, backwash, and metering pumps was recorded. Based off these values, the total power consumption at this pilot was found to be 0.005 kW per bbl. To determine the overall operating cost, cleaning chemical, membrane replacement, and power costs were taken into account. Combining these factors together revealed that

0

5

10

15

20

25

0.0 10.0 20.0 30.0 40.0 50.0 60.0

Hours of Operation

TMP (psi)

Permeability (gfd/psi)

operating costs would be <$0.01 per bbl, thus characterizing the UF as an affordable method of advanced treatment for produced water. In conjunction with the oil tolerance and oil removal capabilities further demonstrated by this pilot, the low operating costs prove that the UF offers a valuable treatment option of produced water.

Conclusion

The three case studies show that the described UF provides an effective option for the advanced treatment of produced water following initial bulk oil removal technologies. Based off the piloting results discussed in this paper, the UF can tolerate up to 300 mg/l oil, demonstrating significant oil tolerance over traditional polymeric UF membranes. In addition, the piloting results show that the UF provides permeate with <1 mg/l oil, highlighting its oil removal capabilities. Along with its solids removal capabilities, the UF consistently provides SDIs below 2.0, well below the usual design specification of 3.0 for RO feed water quality. UF membranes capable of handling high levels of oil and suspended solids have become an attractive treatment option for oil producers. The ability to tolerate and remove high levels of oils greatly simplifies produced and flowback water treatment by consolidating multiple treatment steps into one. Reduction in treatment scope drives down both capital and operating costs, making advanced water reuse much more attractive to the oil industry. Since UF is a distinct barrier layer that removes oil, it also provides oil producers the opportunity to capture additional oil by processing of the UF retentate. Additional oil recovery allows UF membranes to serve as not just a wastewater treatment step, but a revenue-generating step as well. As a new and challenging source of water, produced water creates a unique challenge for oil producers that necessitates innovative treatment schemes. Flexible options such as this UF will play a vital role in turning produced water into a valuable source of water rather than a source of environmental concern for the general public. Fracking and other unconventional oil exploration methods will be a necessity if the United States wishes to reduce its dependence on foreign oil, and those involved in water treatment should rise to this challenge by developing effective and affordable treatment options.

Sources Clark, C.E. and Veil, J.A. “Produced Water Volumes and Management Practices in the United States.” Environmental Science Division, Argonne National Laboratory. September 2009. Guerra, K., Dahm, K., and Dundorf, S. “Oil and Gas Produced Water Management and Beneficial Use in the Western United States.” U.S. Department of the Interior. September 2011. Veil, J.A., Puder, M. G., Elcock, D., and Redweik, Jr., R.J. “A White Paper Describing Produced Water from Production of Crude Oil, Natural Gas, and Coal Bed Methane.” Argonne National Laboratory. January 2004. “Colorado State Energy Profile.” U.S. Energy Information Administration. August 2014.