Application of an integrated dissolved ozone fl otation process in centralised fracturing wastewater treatment plant

To solve the problems of unstable chemical oxygen demand (COD), turbidity and suspended solid (SS) removal for the electrocatalytic process and unstable operation of the subsequent ultrafiltration membrane–reverse osmosis membrane in a centralised fracturing wastewater treatment plant in Inner Mongolia, the integrated dissolved ozone flotation (DOF) process was proposed to replace the original electrocatalytic process. Multiple processes, such as ozonation, flotation, coagulation and decolourisation, can be achieved in one integrated DOF reactor. The results showed that the removal efficiency of COD, colour, turbidity and SS in the DOF process could reach 25.4, 49.9, 95 and 96%, respectively. Meanwhile, the treatment cost was reduced by 47% (i.e., 1.77 RMB/m for the DOF process) compared with the electrocatalytic process.


GRAPHICAL ABSTRACT INTRODUCTION
The fracturing wastewater (FWW) comes mainly from the fracturing fluid that flows back to the surface from the wellbore during the fracturing process (Shih et al. ).
Hydraulic fracturing is a technique used to extract oil or natural gas from impermeable host rocks, which involves horizontal drilling into rock formations and injection of high-pressure fracturing fluid (Williams et al. ; Schultz et al. ). In the hydraulic fracturing process, up to 24,500 m 3 of water-based fluid can be injected into a single well, and 70% of the fracturing fluid is returned to the surface for harmless disposal (Wang et al. a). Because FWW has the characteristics of high salt, high organic matter and high viscosity, the effluent from the biological treatment cannot meet the reuse water standard (GB/T18920-2002), which can be used as a compound drilling fluid and for miscellaneous municipal purposes (Lira-Barragán et al. ). If the AAO effluent can be used for miscellaneous municipal purposes, it must pass through an ultrafiltration membrane and a reverse osmosis membrane (Qurie et al. ). Therefore, advanced treatment is necessary for the AAO effluent to meet the requirements of reclaimed water.
The existing advanced treatment process for this CFWTP is an electrocatalytic system (ECS) and dualmedia filtration. However, due to the large number of chloride ions in the raw water during the actual operation, the chloride ions in the water were oxidised to chlorine gas under the action of the electric current, and the chlorine gas further reacted with water to form hypochlorous acid and hydrochloric acid (Zodi et al. ; Gao et al. ).
This reaction not only causes a waste of electric energy but also generates oxidising substances that need to be neutralised by adding reducing substances; otherwise, the presence of these miscellaneous will cause irreversible pollution in the subsequent processing system (Dixon et al. ; Jin et al. a). At the same time, partially generated chlorine gas emissions to the air would also threaten the health of on-site operators (Clark et al. ). To solve the above problems and maintain favourable dissolved organic matter removal efficiency, the proposed integrated DOF process was applied in this study (Jin et al. ). In this process, ozonation and flocculation are carried out simultaneously during the air flotation process, and the flocs are Therefore, it is necessary to carry out long-term experimental research on the application of the DOF process to FWW advanced treatment to evaluate the feasibility of the process.
The aim of this study was to investigate the removal performance of the DOF process under long-term operating conditions. In addition, a comparison between the DOF and the original ECS process was conducted to better understand the advantage of the DOF process in terms of removal performance and cost.

Raw water quality
The DOF reactor was fed with the effluent from the sedimen-

Data analysis
In this study, two software programmes, origin (OriginPro 2019b) and SPSS (IBM SPSS Statistics 26), were used for data analysis. Between these programmes, Origin is mainly used to analyse the long-term operation data of the DOFintegrated reactor, and the Mann-Whitney tests in SPSS

RESULTS AND DISCUSSION
Organic matter removal performance of the DOF process COD and colour removal performance The COD variation in the influent and effluent of the DOF reactor is shown in Figure 2

Comparison between the DOF and ECS processes
Removal performance Table 2 shows the SPSS outputs for the Mann-Whitney tests. Table 2 shows that the significance level is less than 0.05 (p ¼ 0.000 for COD, colour, turbidity and SS). Therefore, the test results are statistically significant. The decisions from the hypothesis testing were that the null hypotheses were rejected for all four cases. Therefore, the distribution of the above four detection indicators in the effluent of the DOF and ECS processes is significantly different, showing that the advanced treatment effluent of CFWTP has a significant impact due to changes in the process.

Economic evaluation
The present tests were designed to determine the difference in operating costs between the DOF and the ECS processes.
The operating cost of the DOF process applied to the DFWTP secondary effluent (without considering labour costs and equipment depreciation costs) includes the following main parts: flocculant agent (i.e., PAC) cost, polymer coagulant aid (i.e., PAM) cost and power consumption. The operating cost of the original ECS process includes the following main parts: reducing agent (i.e., sodium bisulfite) cost and power consumption.
The unit price of PAC solid is 2,000 RMB/t, the unit price  Table 3.  The reduced operating cost of the DOF process comes from the following two aspects. First, compared with the ECS process, the energy consumption of the DOF process exhibited a significant decrease. At the same time, because the DOF process does not produce Cl 2 and HClO, the DOF process will not cause equipment corrosion and personnel health threats. Second, since HClO is not produced, there is no need to add reducing agents before the membrane treatment process is entered. Improvements in the above two aspects led to an improvement in the working environment of workers, removal performance and treatment economy.

CONCLUSIONS
To solve the problems of unstable COD, turbidity and SS removal for electrocatalytic processes with unstable operation of the subsequent ultrafiltration membrane-reverse osmosis membrane in CFWTPs, the study indicated that the DOF process exhibited favourable COD, colour, turbidity and SS removal efficiency, and the removal efficiency reached 25.4, 49.9, 95 and 96%, respectively. Meanwhile, the DOF process effluent can meet the influent requirements of the subsequent dual membrane system. The treatment cost was apparently reduced by 47% (i.e., 1.77 RMB/m 3 for the DOF process) compared with the original ECS process. This work offers valuable insights into the application of the DOF process in the enhanced treatment of the CFWTP secondary effluent.

CONFLICT OF 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.