Article pubs.acs.org/IECR
Targeted Removal of Dissolved Organic Matter in Boiler-Blowdown Wastewater: Integrated Membrane Filtration for Produced Water Reuse Gil Hurwitz,*,† David J. Pernitsky,‡ Subir Bhattacharjee,† and Eric M.V. Hoek† †
Water Planet, Inc., 721 Glasgow Avenue, Unit D, Los Angeles, California 90301, United States Suncor Energy Inc., P.O. Box 2844, 150 Sixth Avenue SW, Calgary Alberta T2P 3E3, Canada
‡
S Supporting Information *
ABSTRACT: The efficacy of coagulation and membrane filtration was studied for the treatment of boiler-blowdown (BBD) wastewater to enable reuse and minimize the overall water consumption in steam-assisted-gravity-drainage (SAGD), thermally enhanced, oil recovery operations. Direct nanofiltration of chemically unadjusted BBD at its original pH was the optimal treatment option with respect to the flux stability and the removal of dissolved organic material and salinity, which if not removed would result in the fouling and failure of downstream process equipment. The naturally high solute hydrophilicity allowed for prolonged operation with an elevated flux of 60 L m−2 h−1 (LMH) and recovery up to 85% while maintaining solute removal as high as 80% and 45% for dissolved organic carbon and total dissolved solids, respectively. Comparatively, neither precoagulation nor preacidification improved the rejection of dissolved organic material or salinity and consistently resulted in increased membrane surface fouling and flux decline. The proposed filtration treatment solution would result inasmuch as a 4-fold reduction in the volume of makeup water required and BBD wastewater disposed compared to a conventional SAGD facility.
1. INTRODUCTION Steam-assisted-gravity-drainage (SAGD) operations are used extensively for bitumen extraction from oilsands in Alberta, Canada. In this process, steam is injected through a horizontal well into the formation to reduce the bitumen viscosity and aid in its mobilization and extraction.1−3 An emulsion of steam condensate and heated bitumen flows down along the periphery of the steam chamber to the production well, which is located below the injection well. This emulsion is then pumped to the surface, where the bitumen is separated and the produced water is treated and reused for additional steam generation. After separation of the steam, about 20% of the initial volume remains for disposal as highly concentrated boiler-blowdown (BBD) wastewater, characterized with high pH, elevated temperature, and excessive concentrations of total dissolved solids (TDS) and dissolved organic matter (DOM).4 BBD wastewaters possess extremely challenging water quality, in part, because of the properties of the geological formation as well as the residual effects of upstream unit processes unique to the SAGD method. It is common for most BBD waters to have near-boiling temperatures caused by residual heat from steam generation. The pH is also elevated to values typically well above 11 because of the addition of caustic chemicals upstream of the boilers for hardness removal and prevention of silica-scale formation. In addition, BBD water has exceedingly high levels of DOM and TDS ranging from 2000 to 5000 mg/L and from 10000 to 35000 mg/L, respectively.5 Produced waters generated in the oilsands region already possess elevated concentrations of TDS and DOM because of the unique composition of the geological formation. These levels are then increased by up to a factor of 5 during the steam generation process. Numerous associated operational issues © 2015 American Chemical Society
have been reported in direct produced water reuse and disposal operations, such as equipment fouling.6−8 Additionally, extreme conditions present in the existing treatment train, such as within the steam generator, transform the DOM into more stable and recalcitrant derivatives, making conventional chemical and biological treatment processes less effective.9−11 In general, a wide variety of organic constituents are present, including significant amounts of polar compounds with carboxylic and phenolic functional groups.12−14 Therefore, effective treatment techniques will be required that can handle high concentrations of highly stable and recalcitrant organics with varied properties and chemical functionality while withstanding the naturally high pH, salinity, and temperature.15 Varying degrees of DOM removal from SAGD-produced water can be achieved through many processes, such as thermal evaporation, nanofiltration, chemical coagulation, biological degradation, and media filtration or some combination thereof. However, in this study, the focus was on the implementation of membrane filtration, chemical coagulation/precipitation, and in-line coagulation/filtration for the primary removal of dissolved organics and secondary removal of TDS. Because of the lack of TDS and DOM removal options in conventional SAGD operations, minimum removal of 50% DOM and 30% TDS has been targeted for proof-of-concept viability of the tested technologies. These degrees of removal are targeted to prevent and/or minimize downstream fouling and scaling of process equipment during BBD reuse.16,17 Received: Revised: Accepted: Published: 9431
June 3, 2015 September 4, 2015 September 9, 2015 September 9, 2015 DOI: 10.1021/acs.iecr.5b02035 Ind. Eng. Chem. Res. 2015, 54, 9431−9439
Article
Industrial & Engineering Chemistry Research
supernatant provided a measure of the acid-extractable organic matter present and provided a baseline for estimation of the qualitative removal of problematic HoA constituents by a given treatment method. 2.3. BBD Wastewater Characterization and Coagulant/Membrane Selection. All BBD samples were highly colored and possessed a large concentration of organic matter, which was primarily dissolved because of the high pH of the solution (Table 1). The BBD samples were very similar to most
The solution pH is an important variable and can affect the efficacy of coagulation by dictating the coagulant solubility and particle surface charge, which has dramatic effects on both the separation and economic efficiencies. Similarly, the pH can affect membrane separation by influencing the surface charge and chemical stability.18−21 For this reason, the pH was taken into special consideration with respect to the selection of commercial coagulants and membranes as well as system optimization. The present study was designed to determine the appropriate treatment conditions to allow the direct reuse of BBD as boiler feedwater. A variety of BBD samples were collected from three operating production plants to determine what effect variations in the BBD water quality, such as TOC and TDS, have on the treatment efficacy. A detailed review of prior work suggests that these treatment options have never been studied with respect to BBD wastewater treatment.
Table 1. BBD Feed Water Quality Measured at Room Temperature conductivity (mS/cm) pH turbidity (NTU) TDS105 °C (mg/L) color (CU) SUVA (L mg−1 M−1) TOC (ppm of carbon) DOC (ppm of carbon) [HoA] (ppm of carbon) SDI TSS (mg/L) alkalinity (mg/L as CaCO3) calcium (mg/L) iron, total (mg/L) magnesium (mg/L) silica, total (mg/L) sodium (mg/L)
2. MATERIALS AND METHODS 2.1. BBD Sample Collection and Expected Water Quality. BBD samples were collected from three different production plants in Alberta, Canada, representative of Athabasca Oilsands bitumen-bearing formations. Samples were collected from hot process streams using a sample cooler and put into epoxy-lined steel containers with zero headspace. Once received, samples were characterized, chilled, and stored for further testing. Any samples that were opened and exposed to air were discarded if not used within 2 h to minimize sample degradation due to aging.22 2.2. Laboratory Characterization. TDS was determined gravimetrically as nonfilterable solids by measuring the residual weight of a known volume of a sample after filtration and oven drying at 180 °C (EPA method 160.1). The total suspended solids concentration was measured as filterable solids by measuring the weight of the solids collected on a glass filter after filtration of a known volume of sample and oven drying at 105 °C (ESS method 340.2/EPA method 160.2). Specific ion concentrations were determined using an inductively coupled plasma optical emission spectrometry (ICP-OES; TJA Radial Iris 1000) analysis. Total organic carbon (TOC) and dissolved organic carbon (DOC; the TOC concentration after filtration through a 0.22 μm filter) were measured using a hightemperature combustion method (TOC-V, Shimadzu Corp., Japan) with data reported as nonpurgeable organic carbon after automatic acidification and inorganic carbon purging. The conductivity and pH were measured using standard pH and conductivity probes (XL 60, Accumet, USA). The color, alkalinity, and UV absorbance, from which the specific UV absorbance (SUVA) was calculated, were measured using a Hach DR 5000 spectrophotometer. The silt density index (SDI) was measured using a portable SDI kit (model 18200105SC, Sterlitech, USA) and reported as SDI15 min (ASTM D4189). The turbidity was measured using a Hach 2100AN turbidimeter. Because standard procedures were applied for each type of measurement,23 detailed operating procedures for the experiments are not provided here. The hydrophobic acid (HoA) content was quantified as the fraction of organics that precipitated out of solution after acidification to pH 2. Aliquots of 15 mL were acidified using 0.1 N HCl to a pH of 2. Following acidification, the samples were left standing for at least 12 h to allow precipitation and settling of the solids. The supernatant was carefully decanted and filtered through a 0.22 μm filter, after which the DOC was measured. The difference between the DOC of the original sample and the acidified
plant 1
plant 2
plant 3
15.4 11.9 53 14900 28800 2.38 2890 2524 940 6.62 2.0 3230 490 11.4 212 331 2980
32.0 11.6 51 19000 63900 4.15 5060 4940 1000 6.64 2.2 95500
