Experiments and modeling of Komvophoron sp. Growth in hydraulic fracturing wastewater
Graphical abstract
Introduction
Hydraulic fracturing (fracking) is used to stimulate oil and gas production in low-permeability reservoirs such as coal beds, tight sands, and shale formations. Large amount of wastewater (WW) produced by the process may threaten the environment and health of nearby communities [1]. The lack of information on the ultimate fate of these WW furthers the apprehension. Flowback water (FW) and produced water (PW) are two WW streams that require immediate action to reduce long term adverse impact of this industry on environment and human health. FW is the water returning to the surface after fracking before oil or gas production starts and mainly consists of hydraulic fracturing fluids and formation brines. After a flowback period of about 1–2 weeks, wells start producing oil or gas along with PW [2]. Considering that larger amounts of PW are produced, and it is more contaminated with oil, grease and formation water than FW, handling and management of PW should be given priority for treatment. The presence and amounts of various organic compounds including hydrocarbons [3] in PW depends on the geologic formation fractured. Total dissolved solids (TDS) concentration in PW shows geographic variability with average values around 100,000 mg L−1 in Europe and America [4]. The highest TDS concentration of 360,000 mg L−1 reported for PW was from the Marcellus Shale in Pennsylvania, US [4]. An extensive list of chemical compositions of WW streams generated during hydraulic fracturing has recently been published by Ferrer and Thurman [5]. Currently, reinjecting of PWs and FWs into newly drilled wells or their disposal into deep underground injection wells in approved sites are the two main methods for disposal of these WWs [6].
Although techno-economic feasibility of various PW treatment techniques has been assessed to a degree [6], [7], the need for minimizing environmental impact, cost reduction, and compliance with the national and local WW discharge regulations emphasizes the importance of further research and development work in the field.
Biological treatment of FW and PW by microalgae has gained attention during the last decade [8], [9] as a viable option for processing and reuse of these WWs. Recently, effects of growing algae in FW [10], [11] and PW [12], [13] on effluent water quality have been reported. Among microalgae, cyanobacteria have been reported to better tolerate heavy oil pollution and degrade petroleum components [8]. In particular, this ability is well developed in the Order of Oscillatoriales [14]. Considering that there are significant differences in chemical composition of FW and PW [5] generated at different geographic locations, further research and development studies are vital for a better understanding of algae growth in these WW streams and determine economic and technical viability of algal remediation processes.
Description of a process by means of a mathematical model is very helpful for evaluating its economic and technical feasibility [15]. Once validated by experimental data, mathematical models are excellent tools to optimize operating conditions, design and scale up of any process [16]. Several mathematical models have been proposed to simulate microalgae growth under various operating conditions [17], [18]. However, to the best of our knowledge, a mathematical model describing growth profile of an Oklahoma native cyanobacterium strain, Komvophoron sp., in very complex systems, such as PWs or FWs, has not been attempted yet.
Hence, the main goal of this study was to investigate potential of Konmvophoron sp., for remediation of PW and FW collected from different wells in the same region. This is the first study examining growth of this cyanobacterium in these harsh WWs. A comprehensive mathematical model was developed to describe the growth profile of the algae strain and determine the optimum conditions for biomass production and contaminant removal.
Section snippets
Inoculum and culture medium preparation
The strain Komvophoron sp., identified as UTEX SP33, was obtained from the culture collection of the University of Texas at Austin. The strain which was originally collected from Great Salt Plains, OK, US was maintained in the A+ culture medium at room temperature. The chemical composition of the medium is available on the UTEX official website [19]. All the chemicals used in this study were reagent grade unless otherwise specified. Two 32 W white fluorescent tubes (F32T8/SP65/ECO, General
Mathematical model
Mass and energy conservation equations for a stirred reactor operating in a semi-batch mode (gas is in continuous phase and liquid and solid contents of the reactor are in batch mode) were utilized for simulation of the experimental data to develop a mathematical model to describe the algae growth. Light intensity (I), nutrient concentrations (C) and medium pH were identified as the main limiting factors affecting microalgae growth in the model. The following mass balance equation describes the
Initial wastewater composition
The PW samples were collected from wells producing oil at the time of sampling. As reported in Table 2, Na+ and Cl- were the main ions in all the WW samples indicating the high salinity conditions at the sample collection site.
Overall, the order of relative abundance of the ions in PWs were as follows: Na+, Cl-, Ca2+, Mg2+, K+, SO42-, Br- and HCO3–. Though many of these elements can contribute to the aquatic toxicity of PW [36], they may also be exploited as nutrients for microalgae growth. The
Wastewater composition after pre-treatment
WW samples were subjected to pre-treatment before using as microalgae growth media. The goal was to reduce the amounts of suspended solids and dark colored grease in the samples. About 21 to 59 g of oil per kg of PW or FW could be removed from WW using a mechanical skimmer. The reason for oil removal from WW prior to algae cultivation was that very dark colored oil forms a film on the surface of WW hindering light penetration, consequently inhibiting photosynthesis and cell growth [37].
Algae growth in produced water and flowback water
Komvophoron sp. (SP33) is a filamentous motile cyanobacterium [39] originally isolated from surface soil and brine pools of the Great Salt Plains (GSP) in Northwestern Oklahoma by the Salt Plains Microbial Observatory program [40].
Growth profile of Komvophoron sp. in PW-A, biomass concertation, and the change in pH of the medium in time are shown in Fig. 2. After a lag-time of about 4 days, biomass concentration in the culture increased from 1.1 g L-1 to 1.5 g L-1 in the stationary phase. This
Biomass characteristics
Once harvested, the microalgal biomass produced in WW was characterized by a thermogravimetric method to evaluate its moisture (M), volatile matter (VM), fixed carbon (FC), ash (A) contents and high heating value (HHV). The VM contents of the algal biomass generated by Komvophoron sp. varied between 56 and 77.0% depending on the specific WW being considered (Table 6). Considering that higher VM in biomass is desirable for its conversion to bio-products via pyrolysis, biomass generated in FW-A
Model aided identification of optimal operating conditions
The mathematical model proposed in this study was also used to simulate the effect of CO2 concentration and light intensity on algae growth and pollutant removal efficiency and determine optimum growth conditions. Fig. 6 shows the effect of simulated light intensity vs CO2 concentration on biomass productivity in PW and FW samples. The maximum achievable biomass productivities estimated by the model were 11, 7.9 and 5.9 g m−3 day−1 in PW-B, FW-A and FW-B, respectively.
Similar simulations were
Conclusions
An Oklahoma native microalgae strain, Komvophoron sp., was cultivated in four different types of WW produced during hydraulic fracturing process employed for oil and gas production. The experimental data obtained in this study demonstrated that despite the low concentrations of macronutrients, this strain survived and grew in these WW that created a harsh environment for cell growth. Biomass productivities ranging from 6 to 14 g m−3 day−1 were obtained depending on the WW used as a growth
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This research was supported by the Oklahoma Center for the Advancement of Science and Technology, Basic Plant Science Program, Project # PS13-007 and Oklahoma Water Resources Center.
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