The suspended solids in wastewater from Rekabak oilfield, Kazakhstan, were characterized and treated with flocculants to enhance settling. The wastewater contained a high concentration of total dissolved solids and calcium ion. Scanning electron microscopy and energy dispersive X-ray analyses showed that suspended solids were mainly composed of corrosion products (iron oxides) and silicon dioxide particles. Also, much salt deposition from wastewater caused a large increase in the suspended solids value. The settling of solid particles in wastewater was investigated by turbidity decrease within 60 min. The particle settling was enhanced by adding polyaluminum chloride (PAC) as coagulant and hydrolyzed polyacryamide (HPAM) or cationic polyacrylamide (CPAM) as flocculant. At optimal dose, the particle settling ability with PAC and CPAM was better than that with PAC and HPAM. Particle size analysis showed that HPAM or CPAM with high molecular weight played an important role for enlarging the particle size. The experiments with simulated wastewater showed that particle settling by using HPAM deteriorated significantly compared to that by CPAM at high calcium ion. This study provides further understanding about the effect of high salinity and Ca2+ on solids formation, flocculant performance and particle settling. Meanwhile, the results are also helpful to develop novel flocculants used for high salinity wastewater.
Oilfield wastewater with dissolved salts and various pollutants is produced mainly during oil production and is called produced water. The common pollutants usually include crude oil, suspended solids (SS), scale, corrosion products, bacteria and residual chemicals. The wastewater is often reinjected in most cases due to shortage of fresh water, treatment cost or environmental consideration. It is necessary to remove these pollutants from the re-injected wastewater to avoid formation damage (Ahmadun et al. 2009). Nevertheless, formation damage caused by solid particles introduced with the injected water still occurred in some regions. The suspended solids (SS) are usually present in the form of fine colloidal particles with negative charge and are usually difficult to settle through gravity separation.
It is necessary to identify the composition, size and quantity of suspended solids for selecting appreciate treatment technique and chemicals. The characteristic of suspended solids is influenced mainly by formation brine, mineral composition and injected chemicals. It was reported that FeS contaminant in unfiltered produced water caused downhole blockage and decline in injectivity during fracturing water injection (Ojukwu & van den Hoek 2004). Shubo et al. (2009) investigated the suspended solids samples from chemical flooding and confirmed the presence of organic substances (anionic polyacrylamide (PAM) and crude oil) as well as inorganic substance of SiO2, Fe2O3, Fe3O4 and BaSO4. Corrosion products, i.e., siderite (FeCO3), iron oxide (Fe2O3) and pyrite (FeS2), were also found in the deposition on a steam-injection boiler (Dong et al. 2015). Li et al. (2016) characterized the solids in produced water from fractured wells through X-ray photoelectron spectroscopy analysis and confirmed the presence of carbonate-based minerals and iron oxides.
To enhance the settling and removal of the solid particles, coagulants and flocculants are often used in the settling and filtration process for treating large volumes of oilfield wastewater. The widely used coagulants are polymeric inorganic salts, including polyaluminum chloride (PAC), polyferric sulfate (PFS) and their derivatives. Also, the inorganic salts of divalent and multivalent metal ions (such as Ca2+, Mg2+, Al3+, Fe3+, Fe2+, Ti4+) are important coagulants (Ozkan & Yekeler 2004; Bella et al. 2014; Chekli et al. 2015,). The flocculants are mainly water-soluble polymers based on the PAM backbone with high molecular weight (MW) (Bolto & Gregory 2007). There are many reports about the application of coagulants and flocculants in oilfield, mineral, metallurgical and papermaking wastewater processing (Pan et al. 2005; Prasad & Kumar 2013; Lee et al. 2014).
It is generally known that multivalent metal ions are widely present in the oilfield wastewater and play an important role in the flocculation and settling of particles. Much work shows that the influence of metal ions on the flocculation process is very complicated, depending on ion concentrations, pH value, particle properties, flocculant types, dosage and mixed conditions (O'Shea & Tallón 2011; Ghimici & Nichifor 2012; Nasser 2014). It was reported that the flocculation of kaolin suspension with anionic PAM was inhibited by Ca2+ and Al3+ because of adsorption and hydro-complexation between multivalent metal ions and COO− group on PAM (Peng & Di 1994). Ozkan & Yekeler (2004) showed that Mg2+ ion was more effective than Ca2+ and Al3+ ions for the coagulation of celestite mineral suspension at pH range of about 8.5–11.0 and adding Mg2+ or Ca2+ could improve the flocculation performance of anionic PAM at pH 11. Witham et al. (2012) indicated that the effect of Ca2+ on the performance of anionic acrylamide/acrylate copolymers for a calcite slurry was higher than that of Mg2+ and Na+ at 0.05 M concentration and the effect increased with increasing anionic character and ion concentration. Ji et al. (2013) showed that the settling of mineral tailings using acrylamide/sodium acrylate copolymer with 27% charge density was enhanced by saline water containing 679 ppm Ca2+ and 701 ppm Mg2+. In summary, the metal ions significantly influence the electrostatic repulsion and the effective bridging between particle–particle or particle–polymer. Consequently, the size, density and settling rate of the produced aggregates (flocs) in the flocculated water are altered remarkably.
However, very limited research is available on the characteristics of suspended solids and the settling process in oilfield wastewater with high salinity and multivalent metal ions. This paper mainly investigated the enhanced settling of solid particles with PAC and PAM-based flocculants experimentally. Furthermore, the effect of multivalent metal ions on particle settling was studied using a simulated wastewater. This can provide technical support for oilfield wastewater treatment in large scale with low cost and high efficiency.
PAC with 30.1% Al2O3 content and 80.2% basicity (provided by Gongyi Yimin Co., China) was used as coagulant. Furthermore, two different PAM-based flocculants (provided by Beijing Hengju Chemical Group Co., China) were used in flocculation and settling tests. The cationic PAM (CPAM) type ‘HENGFLOC 87410’ had 6–8 × 106 average MW and 42.1% cationicity. The anionic PAM flocculant (hydrolyzed polyacryamide, HPAM) type ‘HENGFLOC 64018’ had 6–8 × 106 MW and 40.5% anionicity. Inorganic salts (NaCl, CaCl2, MgCl2, etc.) and petroleum ether were purchased from Sinopharm Chemical Reagent Co., Ltd, Beijing. All solutions were prepared with deionized water.
Water properties analysis
The wastewater samples were preserved at 4 °C in sealed glass bottles. The supernatant of the samples was taken through a syringe from the middle liquid. Prior to water quality analysis, the supernatant needed to be diluted with deionized water until the ion concentration was within the detection range. The metal ion concentrations (Na+, K+, Ca2+ and Mg2+) were measured by inductively coupled plasma mass spectrometry (NexION 300X ICP-MS, Perkin Elmer, USA). The chloride ion content was determined by titration with AgNO3 aqueous solution. The sample turbidity was measured with an AQ4500 turbidity meter (Thermo Scientific Corp., USA).
Firstly, the suspended solids were separated by filtering the wastewater supernatant through a microporous membrane with a 0.45 μm pore size under vacuum. Then petroleum ether and deionized water were used in turn to clean the membrane for removing oil and inorganic salts. The suspended solids in oilfield wastewater were collected on the membrane. After drying at 105° and spraying with carbon, the membrane containing suspended solids was analyzed by scanning electron microscopy (SEM) equipped with energy dispersive X-ray (EDX) spectroscopy (JSM 6510A, JEOL Ltd, Japan).
Particle size measurement
The particle size analysis was conducted by a Mastersizer 2000 analyzer (Malvern Instruments, UK) with a detection range of 0.02–2,000 μm. The Hydro 2000A sampler was equipped with a stirrer, circulating pump, and 1,000 mL beaker. After starting the stirrer and pump, the wastewater sample, after being coagulated or flocculated, was added into the beaker until the appropriate shading degree was obtained. The results of the particle size distribution were recorded by the software in the computer automatically.
An RT10 multi station digital magnetic hotplate stirrer (IKA Corp., German) was used to carry out the settling tests. The 200 mL of wastewater in six identical beakers with high form was homogenized for 20 min under magnetic stirring at room temperature (22 ± 1 °C). The coagulant solution was added to the beaker at a rapid stirring speed of 200 rpm for 2 min and then the flocculant solution was added at a slow stirring speed of 50 rpm for 4 min. Then 10 mL of flocculated wastewater was collected using a syringe from a depth of 3 cm below the liquid surface and transferred to the sample bottle of the AQ4500. The bottle was inversed four times, placed into AQ4500 turbidity meter and allowed to settle. The turbidity (in NTU) of flocculated wastewater was measured at 1–5 min intervals for a duration of 60 min.
Each settling test was replicated three times and the average value of measured turbidity was adopted as a function of the settling time. The settling test of raw wastewater was performed with the same procedure except that no chemicals were used.
The synthetic brine was made by dissolving inorganic salts (NaCl, CaCl2, MgCl2.6H2O, etc.) in deionized water according to the ion content of the oilfield wastewater. To separate the solid particles, the wastewater was centrifuged (TDL-5-A, Shanghai Anting Co., China) and poured into a 200 mL cylinder for settling until the turbidity was below 5 NTU. The liquid in the cylinder was drawn off by a syringe and the solid particles were separated in the bottom. The simulated wastewater was prepared by mixing the separated particles with synthetic bine and sonicating the mixture using an ultrasonicator in Mastersizer 2000 to disperse the particles.
RESULTS AND DISCUSSION
Characteristics of the oilfield wastewater
The wastewater was characterized as having extremely high salinity (total dissolved solids, TDS) of 148,736 mg/L, as shown in Table 1. The data show that the most abundant anion ion is Cl− and the most predominant metal ion is Na+, indicating that there is a great amount of dissolved sodium chloride in the wastewater. Also, the divalent metal ions are abundant with calcium ion concentration of 2,459 mg/L and magnesium ion of 526 mg/L. The high conductivity (172.3 mS/cm) could be related to the large amount of dissolved inorganic salts in the oilfield wastewater. The turbidity is 189.3 NTU, which is mainly due to the presence of the particles and oil drops. According to the salinity and chemistry, the wastewater from Rekabak oilfield was CaCl2 water type, which is common in the Atyrau region and Caspian Sea basin. The formation of CaCl2 brines is related to evaporation and accumulation of early period seawater rich in CaCl2. The wastewater is weak acidic with pH value about 6.6, which is related to the hydrolysis of calcium and magnesium ions.
|Items .||Value .|
|Na+, K+ (mg/L)||55,012.3|
|Items .||Value .|
|Na+, K+ (mg/L)||55,012.3|
Morphology and main elements of the suspended solids
SEM-EDX analysis for suspended solids on the membrane was conducted to identify the solid morphology and elemental composition.
Effect of PAC and different flocculants on particle settling and size distribution
During wastewater treatment, the combination of coagulants and flocculants is expected to enhance the particle removal and reduce the dosages. The chief function of coagulants is achieving particle destabilization through charge neutralization. The flocculants can prompt particles to form bigger and denser aggregates through adsorption and bridging between particles, increasing the efficiency of particle settling and subsequent processes. Usually, the flocculation efficiency increases with the MW.
Compared to traditional coagulants such as alum or iron salts, the PAC hydrolysis products are characterized by higher positive electric charges and MW. PAC can bring a rapid and better floc formation. Also, PAC is pre-neutralized and effective in a wide pH range, minimizing pH adjustment. The polymers based on PAM are mostly used as flocculants in wastewater treatment. The MW of PAM flocculants can reach up to 10–25 millions. Also, PAMs with various charge properties can be prepared by copolymerizing acrylamide with other monomers. CPAM and anionic polyacrylamide (HPAM) are two representative PAM-based flocculants and are used in this study.
Turbidity is the key property to describe the settling activity of particles or flocs for coagulation and flocculation. The settling efficiency can be evaluated and compared through turbidity change with settling time.
It is apparent that the turbidity decrease of wastewater treated with PAC and additional flocculants was much faster than for the above two cases. At the beginning there was a rapid decrease in turbidity for wastewater with PAC and HPAM and the turbidity reached 25.7 NTU at 30 min. Thereafter the turbidity decreased slowly and reached 15.1 NTU at 60 min.
For wastewater with PAC and CPAM, the turbidity decreased rapidly at the initial settling and reached 12.5 NTU at 30 min. Thereafter the turbidity decreased more slowly and reached 7.5 NTU at 60 min. Visual observations showed that the particle size produced by additional flocculants was much larger than that by PAC. In summary, the particle settling for PAC-coagulated wastewater was enhanced significantly by the addition of PAM-based flocculants due to the bridging of polymer. Moreover, there was almost no turbidity increase during settling in flocculated wastewater, which indicated the flocs had higher strength and were not easily broken into small flocs during the settling process.
PAC could cause some small particles to coalesce to form large ones through charge neutralization, but the bridging was insufficient. However, PAM flocculants played an important role in causing larger particle formation due to the strong adsorption bridging of high MW polymer. Consequently, the settling rate of particles with larger size was improved according to Stokes' law, which can be confirmed by the turbidity curve shown in Figure 6.
Effect of Ca2+ concentration on particle settling for PAC and different flocculants
Simulated wastewater was used to investigate the effect of calcium ion concentration on the flocculation and settling of particles with PAC and HPAM or CPAM.
It can be found from turbidity curves that the influence of high concentration of Ca2+ on HPAM performance was higher than that on CPAM. The HPAM molecule containing COO− groups with negative charge could interact with positively charged Ca2+. The interaction between Ca2+ and HPAM included mainly three cases: (a) single or multiple Ca2+ was bonded to single HPAM molecule; (b) single Ca2+ was bonded to two HPAM molecules; (c) multiple Ca2+ was bonded to multiple HPAM molecules. At low concentration of Ca2+ and HPAM, the predominant cases were (a) and (b), which can improve the bridging between particles and polymers (Mpofu et al. 2005). However, the harmful effect occurred at high concentration of Ca2+ and HPAM because the case (c) was predominant and caused extensive formation of complexes, gel or precipitation with deterioration of flocculation and settling process (Mortimer 1991; Grima 2003). On the other hand, the high divalent ions or salinity could change the conformation and dimension for PAM-based flocculants, causing polymer coiling and decrease in adsorption and bridging between particles and polymer molecules (Bilanovic et al. 1988; Boisvert et al. 2002). As the salinity and multivalent ions are of frequently high concentration in the Kazakhstan area, their effect on the performance of polymeric flocculants and other flocculants needs to be carefully considered.
The characteristics and enhanced settling of suspended solids in oilfield wastewater with high salinity and Ca2+ were investigated. The TDS and Ca2+ concentration of wastewater reached up to 14.87% and 2,459 mg/L, respectively. SEM-EDX analysis showed that the suspended solids mainly contained corrosion products (iron oxides) and silicon dioxide particles. High salinity in wastewater led to a large increase in the suspended solids value by 330.2 mg/L due to salt deposition, which is a potential problem to be considered for high salinity wastewater. Settling tests and particle size distribution showed that HPAM or CPAM flocculants played an important role in enhancing the particle settling and enlarging the particle size compared to PAC. Moreover, staged turbidity increase occurred during settling in PAC-coagulated wastewater due to breakup of large aggregates but it did not appear in flocculated wastewater. The turbidity decrease with PAC and CPAM was faster than that with PAC and HPAM at the optimal dose (i.e. 80 mg/L PAC and 4 mg/L flocculant). The high concentration of Ca2+ (6,000 mg/L) could bring a harmful effect on performance of HPAM and CPAM, especially the former due to the bonding between Ca2+ and COO− in HPAM. Moreover, high salinity and Ca2+ may cause polymer coiling and reduce the adsorption bridging between particles and polymer molecules. The above results would provide guidelines not only for effective application of flocculants in high salinity wastewater, but also for potential salt deposition and corrosion leading to solids increase.
The research has been supported under the Oversea Basic Research Development Program, sponsored by Petrochina Company Limited. The authors wish to express thanks to the Testing Center of USTB Co., Ltd for performing SEM-EDX analysis. The authors are also thankful to Zijun Ma, Shubai Ding and Junhan Ma for experiment advice.