Integration of Flotation Technologies and Advanced Oxidation Processes for Oil and Gas and Desalination Industries Effluents Reuse

Resumen

This thesis is based on water treatment technologies, mainly advanced oxidation processes (AOPs), applied to two types of industrial wastewater: produced water (PW), from the oil and gas industry, and cleaning waters from seawater desalination reverse osmosis (RO) membranes. PW is the aqueous effluent that is brought to the surface along with oil or gas in extraction operations. It includes formation water (trapped underground) and injection water that are extracted together with the fossil fuel during oil and gas production. PW has a complex composition that includes organic and inorganic substances. Salts, free and emulsified oil and grease (O&G), benzene, toluene, ethylbenzene and xylene (BTEX), polycyclic aromatic hydrocarbons (PAHs), phenols and organic acids are its main components. For the treatment and reuse of this PW, integrated processes consisting in a pretreatment related to flotation and settling, followed by an advanced oxidation process (AOP) have been proposed. For the pretreatment, dissolved air flotation (DAF) and settling processes were studied and compared to a novel flotation technology based on the use of glass microspheres of limited buoyancy and its combination with conventional DAF (Enhanced DAF or E-DAF). They were evaluated as pretreatments for AOPs to polish PW for reuse purposes. Settling and E-DAF without air injection showed adequate turbidity and O&G removals, with eliminations higher than 87% and 90% respectively, employing 70 mg L-1 of FeCl3 and 83 min of settling time; and 57.9 mg L-1 of FeCl3, 300 mg L-1 of microspheres and a flocculation rate of 40 rpm. A linear correlation was observed between final O&G concentration and turbidity after E-DAF. After that first stage, pretreated PW with O&G content below the requirements for discharging was obtained, which could be even further decreased through a later ultrafiltration membrane. However the resulting effluent did not satisfy quality requirements for its reuse, since the rest of the components, mainly the dissolved organic compounds, were not eliminated in those mentioned pretreatment. Therefore AOPs such as photocatalysis, photo-Fenton, Fenton and ozonation were studied to include one of these processes within the integrated treatment solution after the E-DAF or settling. The best results were obtained by ozonation combined with H2O2, where most of the components added in the synthetic PW essayed in this work were eliminated, including a high percentage of acetic acid (the most recalcitrant component, not eliminated by the rest of AOPs studied). The optimum conditions for ozonation were 4 g h-1 O3 and 1500 mg L-1 H2O2, at pH 10. After 2 h a 74% of TOC removal was achieved and the acetic acid elimination was 77.8%. Photocatalysis adding 500 mg L-1 P25 under solar radiation achieved after 240 min a TOC and phenol removals of 18% and 100%, respectively; malonic acid remained and there was no degradation of acetic acid. With the photo-Fenton process under simulated solar radiation adding 557 mg L-1 H2O2 and 55.7 mg L-1 Fe at pH 3, a 16.5% of TOC removal was achieved after 90 min, phenol was completely eliminated and malonic acid was removed up to a 89%; but the acetic acid remained intact, and additional intermediates as benzaldehyde were formed. With the Fenton process at 70ºC by adding 557 mg L-1 H2O2 and 55.7 mg L-1 Fe, a 18% of TOC removal was achieved after 60 min. As in the case of photo-Fenton, toluene, xylene, naphthalene and phenol were rapidly eliminated but no acetic acid removal occurred and intermediates compounds were also formed. As conclusion, the combination of E-DAF (or settling) followed by the ozonation process with 4 g h-1 O3 and 1500 mg L-1 H2O2 at initial pH 10 could be suggested as a suitable polishing process for the reuse of PW. If salts elimination is also required, reverse osmosis final treatment should be used. During normal operation, RO membranes found in seawater desalination plants can become fouled mainly due to the suspended or emulsified materials that may be present in the feed water. Depending on the type of fouling (if most part of the fouled material is organic or inorganic), different types of cleaning are applied. For instance, alkaline cleaning-soaking cycles with permeate water are commonly used against the organic fouling of the membranes. These operations generate a significant amount of wastewater that was treated in this thesis by different AOPs in order to be reused for irrigation or in other stages of the process and contribute to osmosis desalination plant concept of zero liquid discharge. These cleaning waters from RO membranes are expected to contain organic matter detached from the membrane together with common cleaning products: such as tetrasodium ethylenediaminetetraacetate (Na4-EDTA) and sodium dodecyl sulfate (SDS). First, Fenton process was applied for the removal of the organic load of these cleaning waters. Fenton experiments were performed at pH 3 after selecting ratios of H2O2/COD (wt) between 1.9 and 20, and H2O2/Fe2+ (wt) between 0.13 and 82.80. For the studied conditions, it was observed that the optimal reaction time was 157 min and the optimal reagents doses were above 4000 mg L-1 of H2O2 and 200 mg L-1 of Fe2+. This combination achieved a maximum TOC removal of 67% and a final biological oxygen demand (BOD5) value of 14.8 mg L-1 O2. Photo-Fenton and photocatalysis processes were also applied. Highest elimination was achieved with the photo-Fenton process at pH 3, where 79% of TOC removal was obtained with 1400 mg L-1 H2O2 and 70 mg L-1 Fe2+ after 60 min of simulated solar radiation, and 85.6% after 157 min. After this time, also total removal of SDS was achieved, what implied obtaining suitable effluent for its reuse on irrigation. This photo-Fenton reaction could be also performed at pH 7, because at 157 min, the same elimination was reached. The addition of P25 accelerated the photo-Fenton process, although after one hour, similar elimination was reached for both conditions. 77% of TOC removal after 180 min was obtained under solar radiation at pH 3 by adding 1200 mg L-1 of H2O2 with 250 mg L-1 of P25 instead of Fe. Under UV-C radiation and by adding 1200 mg L-1 of H2O2 at free pH, 61% of TOC removal was achieved at 157 min. Life Cycle Assessment was also applied as a sustainability tool to assess the best treatment processes for both type of wastewaters from an environmental point of view. For the PW pretreatments, in general, impacts were slighter minor in the DAF process followed by settling process. Concerning the AOPs in the PW treatment, that was the next step, photo-Fenton was the process that caused less environmental impacts and the one that generated the higher impacts was Fenton with temperature. Among the AOPs applied in the treatment of RO membranes cleaning waters, Fenton process was the least polluting, although similar results were obtained by the photo-Fenton process. The one that generated the higher impacts was UV-C/H2O2 due to the energy consumption of the lamps.