Characteristics of produced water
Produced water is the water produced with oil and gas as a byproduct. It consists of the formation water that exists below the gas and oil layers in the well and the injected water with artificial chemicals. Therefore, produced water contains not only the natural hydrocarbons, but also various artificially added chemicals to the well in order to enhance extraction process and to protect equipment, which include some or all combinations of hydrate inhibitor, corrosion inhibitor, demulsifier, antifoam, biocide, wax inhibitor, scale inhibitor, etc. Though it is highly variable depending on the source and the maturity of the gas/oil well, produced water often contains high level TSS, salts, COD/TOC, heavy metals, etc. In sufficient nitrogen and phosphorous contents for biological treatment are commonly observed. Table 1 and 2 show some examples of produced water composition. When biological treatment is adopted, a few weeks to a few months of acclimation time may be required to obtain steady state performance due to the high total petroleum hydrocarbon (TPH) and high salt level.
One complexity in treating produced water is that the water quality and quantity change as the well matures. In the early phase of production, not as much produced water is produced as oil, but the volume of produced water increases as oil and gas layer depletes in the later part of the well lifecycle. The water to oil ratio is 3:1 on average over the lifetime of a typical well, but it can be even 50:1 in the end of the lifecycle. Simultaneously, as the pressure in the well decreases, more water with chemicals is injected to the well, which makes the quality of produced water changes (Igunnu and Chen, 2012).
Table 1. Key parameters of importance in produced water treatment (Fakhru’l-Razi et al., 2009)
Table 2. Wastewater characterization of TPAO basin (Turkey) brackish oil and gas field produced water (Kose et al., 2012)
Not much has been known for produced water treatment by MBR, but it has been known that the success of MBR hinges on the removal of non-emulsified free oil drops in pretreatment just like the success of municipal MBR hinges on the removal of hairy material as discuss here. The refractory dark heavy oil drops must be removed thoroughly before they reach membrane tank. Once the free oil drops leak into membrane tank, they tend to attach on membrane surface that is more hydrophobic than biosolids. The attached oils can cause immediate membrane permeability loss. The attached heavy oil on membrane can eventually make the membrane material soft and cause hollow fiber membrane breakages especially near the top header, where all the tensions caused by the moving fibers end up while the floating oil drops attach most.
Optimum design parameters
There is no consensus on design parameters due to the lack of sufficient field data. But, the limited literature shows that biological side of MBR can be designed following the guidelines for municipal MBR design including F/M and F/V ratio, MLSS, anoxic tank design, etc. While F/V ratio ranges at 0.9-2.6 kg COD/m3/d, which is close to the typical F/V ratio found in municipal MBR as shown here, COD and O&G (oil and grease) removal from produced water was 95.2-98.6% (Sharghi and Bonakdarpour, 2013).
Meanwhile, flux tends to be somewhat lower than those for municipal MBR. Sustainable flux appears 10-20 LMH instead of 20-30 LMH for municipal MBR due to the dispersed flocs at high salt and oil contents. More researches are required to draw firm guidelines for MBR design.
Total petroleum hydrocarbons (TPH) and oil and grease (O&G) removal efficiency
There are not many reports available in public domain with pilot MBR test results, but all the available data suggests removal efficiency of hydrocarbon by MBR is very high (Kwon et al., 2008; Zilverentant et al., 2009; Kose et al., 2012; Sharghi and Bonakdarpour, 2013). As shown in Table 3, total petroleum hydrocarbons (TPH) removal was nearly complete, which included monoaromatic hydrocarbons, benzene, toluene, ethylbenzene, xylene, aliphatic hydrocarbons with less than 40 carbons, aliphatic acids, aromatic acids, naphthenic acids with less than 9 carbons, polycyclic aromatic hydrocarbons (PAHs), and fatty acids. Most, if not all, of those biodegradable hydrocarbons can be reduced less than 1 µg/L (or ppb) by using serial aeration tanks with extended total retention time.
Fig. 2 shows COD removal efficiency of lab scale MBR treating produced water from TPAO basin of Turkey (Kose et al., 2012), where effluent COD is quite consistent at 200-400 mg/L regardless of the influent COD. In this lab scale test, O&G contents in influent and effluent were 41 mg/L and 12 mg/L on average, respectively, with 70% removal efficiency. O&G removal efficiency increased as SRT increased, which suggested microorganisms acclimate better with oil and grease components and actually decompose some of those. Similarly, O&G removal efficiency increased from 65% to 83% when HRT increased from 8 hours to 12 hours in treating produced water with 35.5 mg/L of oil and grease (Zhang et al., 2010), where high HRT directly cause high SRT. On the contrary, O&G removal efficiency tended to decrease as F/V ratio decreased in other study shown in Table 4, where lower F/V ratio directly means longer HRT and longer SRT. These conflicting information suggests that the effect of HRT on O&G removal efficiency might be site specific.
Table 3. Removal efficiencies of organic component of produced water in Sangachal Terminal in Azerbaijan (Zilverentant et al., 2009)
|Component||Concentration (mg/L)||Removal (%)|
|Benzene, Toluene, Ethylbenzene, Xylenes (BTEX)||12||0.0019||99.9|
|C9-C40 aliphatic hydrocarbons||36||0.046||99.9|
|C5-C9 naphthenic acids||3,900||4.3||99.9|
|Polycyclic Aromatic Hydrocarbons (PAHs)||0.21||0.00021||99.9|
|C2-C7 volatile fatty acids||1,707||<10||>98|
Fig. 2. COD removal efficiency of MBR treating produced water with the composition shown in Table 2 (Kose et al., 2012).
© Seong Hoon Yoon