Produced Water Treatment by Membrane Technology

Produced water is the highest volume of liquid waste stream generated by the petroleum industry which should be treated well. Quantity generated each year is so large that it acts as a remarkable component in the cost of producing oil and gas. Traditionally this treatment was limited to suspended solids and free oil which has been done by physical separation technologies, chemical processes as well as an injection in the disposal well.

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However, because of strict regulation combined with Environmental and geological restriction and water scarcity drive the produced water more greatly treated and eventually reused is increasing. Moreover, the growth in the application of water-intensive process to extract unconventional oil and gas and to reduce fresh water uptakes cost-effective treatment and reuse of produced water is increased, particularly in oil sands and shale plays.

Physical technologies traditionally used in past are, in most cases, not capable of treating produced water of suitable quality to replace fresh water uptakes, due to that petroleum industries are investing in new, highly efficient and cost-effective produced water treatment.

Produced water and refinery wastewater treatment by membrane technology is the latest phenomenon and advance research has been focused to enhance the lifetime and efficiency of the membrane during the treatment of the wastewater treatment. In this paper, we briefly focus on various advance separation technics like membrane process, membrane bio reactor(MBR), membrane distillation and ozonation as potential future technologies.

2. Introduction

The importance of oil and natural gas for humankind in modern civilization is well known. The advance technologies like hydraulic fracturing and deep drilling in petroleum rocks are useful to recover hydrocarbon energy sources, In the drilling process water is always a byproduct which is trapped with underground and brought to the surface along with the oil and gas, this water termed as produced water or formation water.

As per Estimation, global produced water generation is 250 million barrels per day in other hand oil is produced 80 million barrels per day. From that we can say that water to oil ratio is 3:1, water portion is estimated to rise in future which should be treated as per regulation for disposal [6].

Currently, the large quantity of produced water generated globally is re-injected into the deep well and because of that the treatment facility is designed in such a way that it removes Dispersed oil, grease and suspended solids from water. In the onshore facility, it is common practice to discharge produced water in the sea, in such condition main treatment objective is to remove oil and grease as per regulation disposal standards to mitigate the toxic effect on aquatic fauna and flora. Historically the treatments which were used for Produced water treatment were limited to physical separation technologies like specific gravity-based API separator and hydro cyclones. These conventional technologies are not capable of producing the desired quality of effluent which is not fit standard and regulation for recycling in the petroleum industries or reuse in, for an instance irrigation or industrial raw water.

Strict regulation and uncompromising standards by the government, geological restriction, local water scarcity are the factors which make petroleum industry operators find a new way of advance separation techniques and managing produced water that promotes the water sustainability and conservation of the environment. For an example Canadian oil industry -In 2016 Oil sands mining Industries used 182 million m³ water, Enhanced Oil Recovery (EOR) used 14 million m³ water and water used for oil sands in situ was 16 million m³ from that Industries have to treat and recycle 75% to 90% produced water generated [5]. Proper treatment technology associated with production operation is capable to treat produced water to make up a significant volume of water extracted from natural resources like lake, rivers, and aquifers.

However, treating Produced water and producing good quality effluent is challenging. The main reason is physical and chemical characteristic of produced water is varying considerably with the geographical location so, deciding suitable treatment operations or combination of one and more technologies is very first and important decision to be taken. This paper discusses some of advanced water treatment technologies which are being applied on a full scale in oil and gas operations to treat and reuse the produced water or have the potential to do so. The technologies are:

Microfiltration
Ultrafiltration
Nanofiltration and Reverse Osmosis

This paper also represents some case studies with the result of laboratory investigations carried out by different research scientist which guide us to summaries the Membrane technology usage for produced water treatment.

2.1 Major Component of Produced Water

Constituents associated with produced water is an important factor for regulatory compliances, disposal standards and management of disposal option like secondary recovery and reuse. Oil and grease are the main component of produced water which is mainly focused in treatment operation in both offshore and onshore site. In onshore site salt termed as salinity, total dissolved solids are the primary component of concern for treatment. Moreover, Organic and inorganic compounds vary from location to location. The common cations in produced water are Na, Ca, K, Mg, Fe, Al, B, Ba, Cu, Li, Zn, Ti, Mn, and toxic Cd, Cr, Pb, Hg, As, Sr, Be. The common anions in Produced water are chloride, sulfate, sulfite, (bi)carbonate, nitrate, nitrite. Produced water may contain heavy metal like cadmium, chromium, copper, lead, mercury, nickel, silver, and zinc. Sometimes the radioactive compounds like 226Ra, 228Ra, 238U and 235U also associated with Produced water, microorganisms are aerobic bacteria can also present in produced water. Crystalline substances such as SiO₂, Fe2O₃, Fe3O₄, and BaSO₄ are found in the suspended solids (SS) in produced water [3], [4].

3. Fate and Impact of Produced Water Discharge

Discharge of produced water is becoming increasingly more difficult due to space limitations and motion in oil rigs. In normal practice produced water is discharged in earth crust by injecting in a deep well or it can be disposed of in sea water, if oil producers discharge untreated produced water it causes very toxic and harmful effects, few major impacts are as follows.

If produced water discharged in soil or deep well, the most commonly reported problems are degradation of soil, surface water, groundwater. Produced water consists of salt, hydrocarbon and trace elements, so untreated produced water may harmful for surrounding environment. The lavish volume of produced water can cause erosion. Produced water consists many cations but sodium is dominant in that, high level of it compete with other cations like magnesium and potassium for uptake by plant roots, which prompt deficiency of other cations in plants and results in poor soil structure. Produced water consists of lithium, boron, radium which remains in the soil even after saline water has been flushed. These elements are toxic and absorbed in the soil which is hazardous for human health and ecosystem [2], [8].

Receiving body of produced water is a very important factor to determine environmental effect as if it discharged in the sea then it offers good dilution, while small streams offer low dilution capacity and show more effect. Oil droplets submerged with produced water does not precipitate at the bottom but remains on seawater surface which is harmful to the aquatic ecosystem. Volatile and toxic compounds evaporate, and which increase the biochemical oxygen demand of affected water. The LC50 test is used to measure acute toxicity, but long-term effects or chronic toxicity are more difficult to quantify [3], [8].

4. Membrane Process

Nowadays, most of the membranes are prepared using synthetic polymer material. Inorganic ceramic membranes are also in wide use. For water purification according to the pore size, the membranes can be classified as Microfiltration(MF), Ultrafiltration(UF), Nanofiltration(NF) and Reverse osmosis(RO) process. Pore size decreases from MF(0.1-10µm) to UF (2-100 nm) then NF (0.5-1 nm) and finally RO, with pore size decrement operating pressure is increases respectively. According to the previous study in microfiltration suspended solids and bacteria are rejected, but allow a virus, divalent ions to pass, in ultrafiltration rejects macromolecular colloids and virus but allowed to pass dissolved ionic species passes through the membrane, and remaining impurities are filtered through nanofiltration and reverse osmosis membranes.

Apart from pores main difference between MF, UF, NF and RO membranes are preparation process, as most of MF UF membranes are asymmetric in nature and consists of whole polymeric material with or without mechanical support on the other hand NF and RO membrane are composite membrane with cross-linked upper synthetic polymer layer of pore size 100-200 nm [4].

There is an important factor to be considered when we choose the membrane separation process for in real operation that, there is a large difference between theoretical membrane performance and real practice value of membrane performance, for example, MF & UF can have high pure water flux whereas, in real life application like food and dairy industries, it can be less than 10% of pure water flux because of the concentration polarization and fouling of the membrane, so we should keep in mind that in real practice membrane can’t achieve its theoretical limit due to membrane flux and fouling[4].

4.1 Micro Filtration

For produced water treatment microfiltration is considered as a pretreatment to enhance the effectiveness of UF, NF and RO, widely polymeric(polyacrylonitrile-PAN) or ceramic membranes are used in oil-water treatment by microfiltration. The ceramic MF of 0.8, 0.2µm and 0.1µm PAN membranes were used for the study and the synthetic PW was made by using heavy crude oil (density 0.972 g/cm3) in the concentration range of 250–1000 ppm. It has been observed that the increased oil concentration decreased the water flux, cross-flow velocity (CFV) and transmembrane pressure (TMP) had little impact on the final flux [4].

There are different types of membranes are used to get better separation and produced water treatment, one the previous study has been done on mullite–alumina ceramic MF membranes which are made up of kaolin clay with α-alumina by extruding the water mixture followed by room temperature drying then temperature dried membrane was calcinated at 1250^◦C for 3 h followed by strong alkali leaching to remove the free silica.in first experiment membrane used for synthetic produced water in which total organic carbon was removed by 94% and the same membrane used for real produced water in which 84% total organic carbon removal observed still it was far better in comparison to other conventional processes. If produced water contains trace element then different membranes are used, as for example hollow fiber supported liquid membrane (HFSLM) used to remove mercury from produced water It has been reported that 99.7% extraction with a recovery of 90% was possible [4], [10].

The cost is an important factor when we choose any operation for a commercial process. Thus, the cost estimation for the synthetic PW treatment using zirconium oxide ceramic MF was reported. The optimum CFV has been found at 2.0 m/s and based on the optimization, the capital and operating cost were calculated for the water recovery rate for different intervals. The cost of membrane regeneration by backwashing with 1000 mg/L of each NaOH followed by sodium hypochlorite and finally distilled water wash was also included. Based on the experimental results, the capital and operating cost was calculated for the recovery rate of 95% water for CFV from 0.3 to 4 m/s as a function of the unit volume of treated water in m3 for the PW treatment of 1000 m3/h. A total of US$ 3.21/m3was reported as the lowest possible cost [4].

To summarize, the MF can be used as cost-effective pre-treatment for the PW treatment. It can be used after removing the bulk of the oil component by using the primary treatment of sedimentation, coagulation, flocculation & sedimentation process. The MF process can effectively remove the dispersed oil droplets and other particulates with the size of more than100 nm, however, smaller particles and droplets and dissolved particles removal need UF, NF, and RO.

4.2 Ultrafiltration

UF process is mostly used along with MF for the oil removal from the petroleum industry wastewater to get the best result. In Ultrafiltration membrane Fouling is a major issue due to smaller pore and low available pressure, the most effective way of reducing fouling is to make the membrane surface more hydrophilic and reduce the roughness. Previously different types of membranes were used for produced water treatment like, but the most common and effective membrane noted for produced water is polyacrylonitrile(PAN) ultrafiltration membrane which has nominal pore size 10 nm [9].

One the observed case for performance of PAN ultrafiltration membrane for produced water treatment in the University of Regina is noted here, the raw produced water was provided from Karoon Oil and Gas Production Company, Ahwaz, Iran. It contains oil, salt, and heavy metals the details of feed shown in table-1 with the comparison of conventional method treated water and Membrane treated water. Used PAN membrane was of 10 nm and has top skin layer of 20-25µm. As a result when test performed for 1,2,3,4 and 5 bar at 40°C,the UF membranes indicated high removal of oil and grease content more than 99%, TOC around 83%, TSS around 100%, COD around 94%, turbidity around 99.2% and TDS around 65% from the produced water, It also shows high permeation flux between 80 and 180 L m^–2 h^–1 [9].

Table 1: Result taken from ultrafiltration membrane process for produced water treatment: experimental and modeling Ramin Badrnezhad and Ali Heydari Beni, IWA Publishing 2013, Journal of Water resources and desalination [9].

Parameter	Unit	Feed	Ultra-filtration membrane	Conventional treatment
Total Suspended Solids	mg L^–1	320	Trace	4
Total Dissolved solids	mg L^–1	61,364	21,445	60,212
Oil and Grease content	mg L^–1	515	<0.2	5
Chemical Oxygen demand	mg L^–1	280	18	32
Total Organic Carbon	mg L^–1	138	24	32
Turbidity	NTU	78	0.6	1.1

4.3 Nano Filtration and Reverse Osmosis Process

Nano Filtration(NF) and Reverse osmosis(RO) are Relatively high operating pressure methods then previous two and effective for inorganic minerals, RO is enough effective when it is used in multiple units in series. The membranes are highly selective because, in RO, the water permeation flux is directly proportional to the operating pressure and to maintain the pressure, oil and gas industries use fresh water for pressure maintenance. Most of the NF and nearly all the modern RO membranes are thin film composite membrane on UF membrane support. The main difference between RO and NF membrane is RO rejects all the ionic species including monovalent ions and effectively remove radium, natural organic substances, pesticides, cysts, bacteria and viruses on other hand NF are more selective for divalent ions and partially allow for Na⁺ and Cl^– [4], [14].

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These membranes are highly selective and high-pressure membranes and therefore it is easily prone to fouling.one of the case reported in given reference, the use of commercially available ceramic NF membrane and RO combination for the treatment of synthetic and real produced water. Though the ceramic membranes give more than 99% oil removal, the TOC removal was moderate [4]. In summery NF and RO membranes are highly selective due to high operating pressure, high fouling and high cost, However, operating cost of NF is less then RO as NF is operated at less operating pressure so, NF is more preferred in oil and gas industries whereas RO is only preferred for specific contaminant and ions removal.

5. Mechanism and Terminologies

The membrane is a porous barrier which separates two or more phases and restricts the transport of selective chemical and controls the exchange between two regions. Membranes are made up of Synthetic polymer for an instance polyacrylonitrile membrane or from inorganic material for an instance ceramic membrane. There are different types of membrane available such as a microporous membrane, asymmetric membrane, Thin film composite membrane, electrically charged membrane.

(5.1) (5.2) Figure-5.1: Membrane filtration overview, adopted from Review study of membrane fouling for produced water treatment. Kasper L. Jepsen, Mads Valentin Bram, Simon Pedersen ID and Zhenyu Yang [11]. Figure-5.2: Membrane filtration, Adopted from Membrane separation process by Kaushik Nath [12].

Produced water can be treated with membrane technology in many ways, as we discussed earlier physical and chemical characteristic of produced water change with respect to location so unit operations associated with treatment changes too. The simple diagram for membrane filtration shown in Figure-5.1. Commonly the feed water with chemical, contaminant, pollutant, oil, and grease pumped with a high pressure in the membrane [11].

For better understanding consider figure-5.2 where we have a feed with A+B like Water and oil, the membrane is placed on a rigid and porous support which creates two compartments in the shell. The stirrer is also there for better mixing. Feed(A+B) is pumped in the upper chamber with high pressure. Here with the perfect membrane part of feed, A only transfer through the membrane which is also known as permeate and part of feed consisting B remain in the upper chamber. In some cases of wastewater treatment either A and B both can be product [12].

Sometimes produced water is treated by the membrane with pretreatment to enhance efficiency and reduce fouling, for an instance produced water consisting high concentration of oil and other impurities can be treated first by simple API Gravity settler which reduces the load on membranes. Similarly, we can use other conventional treatment combined with membrane technology which reduces the cost of entire treatment.

There are few terminologies associated with membrane process which is required to understand with its mechanism. Few basic terminologies are membrane fouling, membrane flux, Crossflow velocities, Transmembrane pressure which are briefly explained below.

 Membrane fouling: Deposition of the solution or a particle on a membrane surface or in membrane pores which reduce the membrane’s performance. Membrane fouling is of two types one is reversible in which membrane surface can be reactivated and other is irreversible in which membrane surface cannot be reactivated.

 Membrane flux: the Flow rate of water applied per unit area of the membrane is defined as membrane flux. Higher the membrane flux, higher the permeate.

 Transmembrane pressure: The pressure required to press water through the membrane is called transmembrane pressure. In NF and RO Required transmembrane pressure is higher than UF and MF.

 Cross-flow velocity: Cross-flow velocity is the linear velocity of the flow tangential to the membrane surface. Cross-flow velocity fouling rate is important to ensure that the system is operating at optimal conditions.

6. Advantages and Disadvantages

It is fact that produced water treatment is beneficial as it protects the environment, can be reused for industrial Raw water, Irrigation water and reduce the scarcity of water. However, the membrane treatment method which we discussed here has advantages over conventional method and several disadvantages too, few of them reported here [11].

6.1 Advantages

Membrane technology is suitable for medium and large-scale offshore platform and it is less complex on other hand conventional methods are most suitable for pretreatment of wastewater in situ reuse.
In membrane technology, very less chemical and additives are required on contrast convention technology uses a large amount of energy, chemical additives required.
The greatest advantage of membrane technology is a simple operational mechanism and compactness as the conventional method are slow and complex in comparison with membrane process.
Membrane technology offers high reusability which allows recycling of selected waste stream to protect the environment on the other hand conventional method is often involved with spent chemicals that return add on to the chemical pollutant.
Last not but least Conventional technologies are rigid and exacting so it has very less hope of further improvement whereas in membrane technology allows more room for advancement and better performing treatment technology.

6.2 Disadvantages

The main disadvantage of membrane technology is capital cost as it is expensive compared to other technologies due to fouling, occasional replacement [12].
Membrane fouling is another drawback of membrane technology especially for highly contaminated feed increases the fouling rate which reduces the performance membrane [12].

7. Further Prospects of Technology

Membrane science began to emerge as an independent technology since the mid-1970s and its engineering concepts are still being defined. Initially, technology evolved by the government which now attract the industries as technology is feasible, clean and energy saving. As US national council noted frontiers of membrane technology and new membrane materials should be developed [12].

New membrane material to be used is a still big option for developing and making membrane technology more and more efficient and can be used for different types of feed. The Journal of membrane science is the best to reference for where we can get lots of information and knowledge of membrane innovation and material [12].

The other development in membrane technology is hybrid membrane system, which is the combination of different conventional process with different membrane process which gives better result in compare of solo technology, reduces energy requirement, lower capital cost and lower production cost. membrane distillation, thermal evaporation, Pervaporation, membrane bioreactor, forward osmosis is recent advance technology which is the product of membrane technology advancement. Still, many advancements in membrane material and technology are to be done with new upcoming challenges.

8. Summary

The complexity of the produced water from oil and gas industries make it one of the toughest wastewater to manage within acceptable disposable standards and affordable cost. The future development in membrane method will have to mainly focus on advanced pretreatment methods. Diagram for the overall treatment of produced water is given in figure-2. The shown unit operations are not fixed it can be varied with properties of feed.

Figure-2: diagram for overall treatment of produced water Adopted from Use of membrane technology for oil field and refinery produced water treatment-A review Selvaraj Munirasua, Mohammad Abu Haijab, Fawzi Banata [3].

It is clear from Above literature survey that if we want to treat produced water to get potable water then we must use RO as RO only can give potable and drinking quality water. Produced water can reuse in oil field itself for an instance in hydraulic fracturing or can be used in irrigation purpose and industrial process water and such quality can be obtained by NF. So, the RO can provide the better quality of water then NF and the NF can provide a more cost-effective process than RO. Fouling is a big problem in membrane treatment, but it can be overcome by selecting proper membrane and reactivating surface of the membrane.

Advanced membrane technologies like membrane distillation, thermal evaporation, and forward osmosis are the brand new developing technologies with the membrane which are taking more and more attention by wastewater industries. In summary, membrane technology is the best option for produced water treatment it has many advantages over conventional technologies and has few disadvantages which the challenge is as well as the opportunity for upcoming engineers.

9. References

Hussain, J. Minier-Matar, A. Janson, S. Gharfeh, S. Adham (ConocoPhillips), Advance technologies for produced water treatment and reuse, International petroleum technology conference 2014, IPTC 17394

Argonne National Laboratory, John A. Veil, Markus G. Puder, Deborah Elcock, Robert J. Redweik, Jr. A white paper describing produced water from production of crude oil, natural gas, and coal bed methane, January 2004
Fakhru’l-Razi Ahmaduna, Alireza Pendashteha, Luqman Chuah Abdullah, Dayang Radiah Awang Biaka, Sayed Siavash Madaenic, Zurina Zainal Abidina, Review of technologies for oil and gas produced water treatment, Journal of hazardous materials, 170 (2009) 530-551
Selvaraj Munirasua, Mohammad Abu Haijab, Fawzi Banat, use of membrane technology for oil field and refinery produced water treatment – A review, Process Safety and Environmental Protection ,100(2016) 183–202
Frequently used statistics published by Canada oil and Natural gas procedure and CAPP,2017.
E.H. Khor, Y. Samyudia, Review on Produced Water Treatment Technology, Research gate,2017
Qing Li1, Jianbang Du, Fengxiang Qiao, and Lei Yu, Open access commentary on Challenges and Opportunities in Produced Water and Drilling Waste Treatment Techniques to Mitigate the Adverse Environmental Impacts,
Katie Guerra, Katharine Dahm, Steve Dundorf, Oil and gas produced water management and beneficial use in the western united states, September 2011.
Ramin Badrnezhad and Ali Heydari Beni, Ultrafiltration membrane process for produced water treatment: experimental and modeling, Journal of water reuse and desalination, DOI: 10.2166/wrd.2013.092.
Mohsen Abbasi, Mojtaba Mirfendereski, Mahdi Nikbakht, Meysam Golshenas, Toraj Mohammadi. Performance study of mullite and mullite–alumina ceramic MF membranes for oily wastewaters treatment. Journal of Desalination, 259 (2010) 169–178
K. L. Jepsen, M. V. Bram, S. Pedersen, and Z. Yang, “Review Membrane Fouling for Produced Water Treatment: A Review Study from a Process Control Perspective,” http://www.mdpi.com/2073-4441/10/7/847/pdf, 26-Jun-2018.
Kaushik Nath, Book on Membrane separation process, second Edition, PHI learning private limited-110092,2017.
Syarifah Nazirah Wan Ikhsan, Norhaniza Yusof, Farhana Aziz, Nurasyikin Misdan, A Review of Oilfield Wastewater Treatment Using Membrane Filtration Over Conventional Technology, Malaysian Journal of Analytical Sciences, Vol 21 No 3 (2017): 643 – 658

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