The reverse osmosis concentrates (ROCs) with high concentration of chemical oxygen demand and salts are produced by a double-membrane technology, which has been widely used to treat the wastewaters from petroleum refinery enterprises. The ROCs are prohibited from directly discharging from environmental point of view and must be treated to comply with the tighter discharge limits of wastewater and solid wastes per the upgrading national emission standard. Therefore, effective technologies for the removal of organic matter and recovery of salts from ROCs have been committed to be developed. In this review, the recent progress on the treatment technologies of ROCs from petroleum refinery waste waters are summarized and discussed. Firstly, the compositions of ROCs, the basis of developing the treatment process, from several petroleum refinery plants are collected and analyzed. Subsequently, the removal methods of organic contaminants, such as physicochemical method, advanced oxidation method, and biochemical method, are elaborated in detail. Especially for some innovative advanced oxidation processes, including O3 oxidation, Fenton method, electrochemical oxidation, photocatalytic oxidation, and ozonation-based combination processes, and their mechanisms, advantages and disadvantages for the treatment of ROCs from industrial sources are emphasized comprehensively. Finally, the methods of salts’ recovery from ROCs are also discussed briefly.
Contents
1 Introduction
2 The compositions of ROC from petroleum refinery wastewater
3 Organic matter removal in ROC from petroleum refinery wastewater
3.1 Physicochemical method
3. 2 Advanced oxidation processes ( AOPs)
3.3 Biochemical method
4 Recovery of salts in ROCs from petroleum refinery wastewater
Petroleum refinery wastewater (PRW) is one of the largest wastewater generated in the manufacturing operations. The wastewater typically contains high levels of total dissolved solids (TDS) and total organic compounds (TOC), and many kinds of soluble organic compounds, most of which are non-biodegradable[1]. The total discharge amount of PRW from refining and petrochemical enterprises in China is more than 3.8 × 104 m3/h at present. Such large amount of PRW shall be treated and recovered before discharge. Recently, the attention on the reclaim and reuse of PRW from governments and enterprises is rising in order to implement energy conservation and emission reduction seriously, as well as to utilize water resources reasonably.
For the methods of PRW treatment, a well-known combined process of pre-treatment+ double-membrane (microfiltration (MF)/ ultrafiltration (UF)-reverse osmosis (RO)) is widely used[2,3,4,5,6,7,8,9,10,11,12]. Sinopec Yanshan Plant in China was the earliest adaptor to use UF and RO technology to treat the wastewater, and followed by other petroleum refinery enterprises including Daqing, Qilu, Jinan, Jinzhou, Dushanzi, etc.[1]. Reverse osmosis concentrates (ROCs) are thereby generated from the last RO step of the combined process. Hereafter a full picture for technologies employed in the combined process are summarized and discussed firstly to give a better understanding of generation of ROCs.
The pre-treatment process is employed to protect the double-membrane systems from suspended substances and organic compounds in the combined process. The technologies, such as biological aerated filter (BAF)[2,3,4], sand filtration (SF)[3], multi-media filter (MMF)[5], activated carbon (AC)[6,7,8],electric coagulation (EC)[9,10,11], and flocculation[12]were employed to remove the undesired substances in PRW to allow the treated water to meet the requirement for the feed of double-membrane process. Dai et al.[2] found that BAF was effective at treating biodegradable organic contaminants. It achieved the mean degradation efficiencies for biological oxygen demand (BOD5) of 83.64% and chemical oxygen demand (CODCr) of 54.63%. Xie et al.[3] used a novel industrial-scale combination process of BAF and SF to treat PRW. The results showed that the average removal efficiency for CODCr of 62.6%, oil pollutants of 71.7%, NH3-N of 92.6% and suspended solids (SS) of 97.0%, respectively. Xu et al.[4] found that most of the sulphides in refinery wastewater can be removed by biodegradation with isolated BAF. Fang et al.[5] treated refinery wastewater using the MMF technology which can remove the suspended solids, colloidal particles, algae, etc. CODCr rejection was about 27%~54%, and the turbidity of MMF effluent was about 3~6 NTU. Ibrahim et al.[6] used the granular AC column to remove phenol, parachlorophenol, and benzene from the wastewater of Baiji oil refinery in Iraq. They found that the breakthrough and exhaustion time were directly proportional with the thickness of granular AC, and inversely proportional with the concentration of pollutants and liquid hourly space velocity. Under the optimal experimental conditions, these pollutants could be removed completely. El-Naas et al.[9] used a batch EC setup to remove sulfate and CODCr from PRW using three types of electrodes. The results demonstrated the feasibility of EC technology for the pre-treatment of heavily contaminated PRW. Ibrahim et al.[10] developed an EC process for the treatment of PRW. They found that the EC could provide better quality of treated water than the conventional methods, and was an efficient process for recycling of PRW. Zhong et al.[12] investigated the effect of flocculation conditions on flocculation results by orthogonal experiments, and the optimized conditions were dosage of 70 mg/L, temperature of 40 ℃, stirring time of 90 min and holding time of 90 min. The results showed that the UF membrane fouling decreased meanwhile the permeate flux and permeate quality increased after pretreatment with flocculation.
For the double-membrane system in the combined process, MF/UF can be an economical option for RO pre-treatment to remove most of the potential elements leading to membranes’ fouling such as turbidity, particulates, total SS (TSS), bacteria, viruses, and other organic matters with large molecular weight. A pilot-scale hybrid membrane system was utilized to decrease harmful and damaging components in PRW from Tehran Refinery[13]. In the optimum operating conditions, MF-UF system reduced oil by 99.7%, CODCr by 82%, BOD5 by 79.3%, conductivity by 60.5%, TDS by 52.6%, turbidity by 99.7%, total hardness by 73.2%. Al-Malack et al.[14] investigated the performance of constant-flux immersed UF membrane treating PRW. They found that the membrane performance was pronouncedly depended on oil content of PRW and permeate flux values. Norouzbahari et al.[15] used UF to remove the emulsified oil droplets from PRW followed by the removal of TDS through reverse osmosis process, and experimental results showed that the UF membrane removed more than 75% of the oil and can be considered as an effective pretreatment. Fang et al.[5] applied the UF technology to treat refinery wastewater. CODCr concentration of the UF permeate water was lower than 32 mg/L, and both the turbidity and silt density index (SDI) of the UF permeate water met the quality requirement for RO feed water (Turbidity < 1 NTU and SDI < 5).
For RO process in the double-membrane technology, about 70~80 wt% of the incoming effluent can be reclaimed and used as the high-quality feed water for steam production/cooling water makeup. The refinery wastewater reuse system of Sinopec Yanshan Plant in China composed of biological treatment and media filtration with a combination of UF and RO has been in operation[1]. The salt rejection efficiency of current RO system is still over 97% after more than 30 times of chemical cleaning. Abbasi et al.[16] used a treatment scheme comprising MF, UF, and RO to treat the PRW from Tehran refinery. They reported that the RO process was necessary to reduce salinity to an acceptable level, and that the reclaimed water can be used as boiler feed. Venzke et al.[17] applied a RO process to treat the industrial wastewater from a large petrochemical complex in Southern Brazil. It was found that the components higher than 90% were removed, demonstrating that the RO process was viable for reuse of process-produced wastewater since reused water met the requirements for boiler and cooling tower waters. However, for the remaining 20~30wt% of the incoming effluent, i.e. the RO concentrate (ROC)obtained by RO process, the contaminants in it could be 3~4 times more concentrated than in the feed water. Therefore, it is very hard to be treated due to its high CODCr content, poor biodegradability, high salinity, etc.[18].
The huge amount of ROC, if discharged directly, may potentially cause not only human and ecological health problems, but also a considerable loss of water resources. To implement the maximum water recovery and avoid liquid effluent disposal, an evaporation-crystallization method has been employed to directly re-concentrate the ROCs to crystallize salts in solar/thermal evaporators. Shenhua Group Company in China[19] has been using the evaporation-crystallization method to treat the ROC from the project of coal refining for clean fuel. However, ROC with high CODCr concentration hardly meets the requirements of the feed water of evaporator, resulting in a residue which can only be stored to avoid groundwater contamination due to its high content of both organic and inorganic salts. In addition, the evaporation-crystallization technology is limited due to its high energy consumption, expensive equipment investment and operating cost, especially scaling and corrosion of equipment caused by the organic matter in ROC, etc.
With the further upgrading of national emission standard for ROC, wastewater treatment plants are facing new challenges to follow tighter wastewater emission limits. For example, the concentrations of CODCr and BOD5 must be decreased to 50 and 10 mg/L, respectively, to meet the first grade A emission standard of GB31570-2015 and GB31571-2015 in China. To meet the emission limits imposed by legislation, solve the problems of environment pollution, and achieve the maximum water recovery from ROC, several other treatment processes of ROC from petroleum refinery wastewater have been strongly explored.
Treatment technologies put forward by the researchers for the treatment of ROC are mainly classified into two categories: one for the removal of organic compound and another for the recovery of salts. This review was mainly dedicated to the recent development in these technologies. The compositions of ROCs from petroleum refinery wastewaters were analyzed as basis firstly. Organic matter removal methods, such as physicochemical method, advanced oxidation method, and biochemical method were elaborated in detail. Among the advanced oxidation methods, the mechanisms, advantages and disadvantages of ozonation, Fenton, electrochemical oxidation, photocatalytic oxidation, and ozonation-based combination processes were highlighted, respectively. The recovery of salts was also discussed finally.
2 The compositions of ROC from petroleum refinery wastewater
To eliminate the problems associated with the compositions of ROC, treatment technologies must be adapted to the source of ROC. The characteristics of ROC from the RO units of wastewater treatment plant in different petroleum refinery companies were listed in Table 1. The ranges of pH values, BOD5, CODCr, and electric conductivity were 6~9, 3~17 mg/L, 60~350 mg/L, and 1~9 mS/cm, respectively. It was obvious that these ROCs had a higher concentration of CODCr and a wider range of salt concentration. In particular, most of ROCs were associated with the poor biodegradability due to the BOD5/CODCr ratios less than 0.1.
Therefore, it is of great significance to determine the compositions of organic compound in ROCs for the development of appropriate wastewater recycling techniques and the evaluation of the potential environment risk of wastewater effluents. Generally, the organic compounds in ROCs could be divided into two parts: (1) one part from the refinery processes, such as electric desalting, which generates the oil-containing wastewaters[30,31,32], (2) the other from the wastewater treatment processes, which includes the scale inhibitors and dispersants, non-oxidizing biocide and flocculants[32,33,34,35,36,37,38]. For example, the scale inhibitors and dispersants of which the dosage was 3 ~ 5 ppm practically to prevent the CaCO3 scaling on NF/RO membranes contributes about 10% of CODs. Practically, the organic compounds in ROCs were analyzed and categorized per their structures and/or properties. Li et al.[39] analyzed the concentration of dissolved organic compounds (DOCs) in ROC obtained by the process of oil-water separation, flotation, anaerobic/aerobic biodegradation, sedimentation, lagoon and RO processes. Four kinds of carbon structures, namely aliphatic, aromatic, heteroaliphatic and carboxylic carbons were found in ROC by solid-state cross polarization magic angle spinning 13C nuclear magnetic resonance (NMR) spectrometry analysis. Elemental and specific ultraviolet light (UV) absorbance at 254 nm (UV254) analyses shown relative high contents of aliphatic structures and low contents of aromatic structures in refinery effluent-derived humic substances. Also, the DOCs in ROCs from oil refining were separated by XAD-8 resins and characterized by UV254, Fourier Infrared Spectroscopy (FT-IR) and three dimensional fluorescence spectroscopy (3D-EEM)[40]. The DOCs were classified as drainage water acidic organics, hydrophobic basic organics, hydrophobic neutral organics and hydrophilic substances. Zhuo et al.[20] analyzed the organic matters in ROC from a petroleum refinery in China by gas chromatography-mass spectrometry (GC-MS). Before analysis, the organic compounds were firstly enriched by solid phase extraction (SPE) and solid phase micro-extraction, respectively. Some kinds of organic matters were found, and the contents of benzene and methyl substituted naphthalenes were a bit higher than the others. Li et al.[41] investigated the molecular composition and transformation of DOCs dissolved in a refinery wastewater. The DOCs in organic phase and aqueous phase obtained by solvent extraction and SPE were analyzed by negative ion electrospray (ESI) coupled with FT-IR mass spectrometry, and 76 compound class species were identified in a single mass spectrum. These organic compounds are associated with toxicity and poor biodegradability, and their discharge would lead to serious environment pollution.
The current emission standard of CODCr pollutants of ROC from PRW is lower than 50 mg/L (GB31570-2015). ROCs from different PRWs present higher organic pollutant load but lower salinity than RO concentrates from desalination plants, thus efficient treatment methods to destroy the most persistent pollutants in ROC from PRW are desired.
3 Organic matter removal in ROC from petroleum refinery wastewater
The methods of organic contaminants removal in ROC from petroleum refinery wastewater mainly includes physicochemical method, advanced oxidation processes, and biochemical method. Fig. 1 shows the applicability of water treatment technologies based on the amount of organic load[42]. It was clear that advanced oxidation processes have a wider application range for the removal of organic compounds.
3.1 Physicochemical method
The physicochemical treatment used to treat ROCs from PRW comprises neutralization and coagulation, for pH correction and elimination of suspended and dissolved contaminants. The commonly coagulants used for wastewater treatment are inorganic salts of iron and aluminum. As of now, coagulation mechanisms are still unclear because it can be affected by several factors, such as the composition and concentration of impurities, temperature, pH, alkalinity, type and dosage of coagulant, mixing conditions, as well as ionic strength[42,43]. Generally, the addition of coagulants results in the form of certain amounts of multinuclear ions due to the hydrolysis of coagulants, leading to the destabilization of negative charged impurities due to the specific adsorption of hydrolysis species onto their surfaces. Therefore, when the attractive surface forces overcome the repulsive forces, large flocs can be formed by the means of interparticle bridging, charge neutralization and binding of aggregation products via van der Waals forces. Also, during the process of large flocs precipitation, small particles can be physically enmeshed by metal precipitates[44]. Subsequently, these flocs can be efficiently removed in the subsequent separation process.
Coagulation process is well known to remove the organic high-molecular-weight compounds[45]. Bagastyo et al.[46] found that positively charged aluminum ions selectively bound large size of negatively charged organics. Organic compounds with large molecular weights (>3 kDa) were preferentially removed by alum coagulation compared to smaller molecules. Iron-based coagulants is proven to be more effective than aluminum-based ones for the treatment of ROC from PRW, although the removal efficiency of organic compound is influenced significantly by the characteristics of the ROC. Yu et al.[47] selected polyethylene oxide and FeCl3 as coagulants to treat ROC from a refinery. The removal efficiency of CODCr attained 68.35% at the reaction time of 30 min, temperature of 30 ℃, and pH of 11.06. Shen et al.[48] applied coagulation and adsorption methods to treat ROC. Experimental results showed that FeCl3 was the best coagulant compared to the other 4 kinds of inorganic compounds, achieving 56.9% removal of CODCr. The effluent obtained by the coagulation-adsorption integrated process was colorless and transparent. Unfortunately, this process wasn’t suitable for large-scale industrialization due to its high treatment cost.
In ROCs from PRW, most of organic compounds are low molecular weight and have the poor biodegradability. The coagulation process showed a low removal efficiency of organic matters with lower molecular weight. Furthermore, the addition of coagulants generated a large quantity of sludges and increased the salinity of effluents due to the introduction of salts. All of these led to the complexity of subsequent treatments of effluents and sludges.
3.2 Advanced oxidation processes (AOPs)
A treatment process for ROCs from PRW was needed to promote biodegradability and meet the emission standards of a petroleum refinery plant. AOPs involving various types of oxidants, such as fluorine (F2), hydroxyl radical (·OH), ozone (O3), hydrogen peroxide (H2O2), chlorine (Cl2), and oxygen (O2) have been proven to be effective on the degradation of recalcitrant and chemically complicated contaminants. Table 2 listed the oxidation-reduction potentials of these oxidants.
·OH with the high oxidation potential is an extraordinarily reactive chemical species, and can non-selectively oxidize almost all electron-rich organic molecules because of its electrophilic nature. It attacks organic compounds by abstracting a hydrogen atom or by adding to the double bonds. Also it can carry out the mineralization of organic compounds by transformation into more oxidized intermediates, CO2, H2O, and inorganic ions. These reactions of OH radicals with organic compounds can be expressed as follows:
$\bullet OH+RH \to R \bullet +H_{2}O$
$\bullet OH+C=C\to HO-C-C\bullet$
$\bullet OH+RH\to CO_{2}+H_{2}O+Inorganicions$
where R refers to the organic compounds. In recent years, AOPs are attracting a lot of attentions due to the fact that degradation of various organics by generation of a large number of highly reactive free radicals surpasses the conventional oxidants in efficiency. Several advanced oxidation treatment methods, such as ozonation, Fenton, electrochemical oxidation, photocatalytic oxidation, and ozonation-based combination processes, showed good performance for removing the organic contaminants in ROCs from petroleum refinery wastewaters[40].
3.2.1 O3 oxidation
Single O3 oxidation has been extensively utilized for wastewater treatment due to its advantages, such as disinfection, deodorization, decolorization, oxidation ability to the poor degradable organics, improving flocculation effect, the rapid degradability for organic contaminants at only mild temperature and pressure conditions, as well as no secondary pollution. Generally, by the direct reactions or indirect reactions after formation of ·OH[49], O3 can lead to the decomposition of some organic contaminants through conversion into CO2, H2O, and/or organics with low molecular weight which are easy to be degraded, due to its high oxidizing power (2.07 V listed in Table 2).
As of the directly reactive process, organic contaminants are directly oxidized by an O3 molecular reaction under acidic conditions, as shown in Eq. (4). This reaction is slow and selective. Ozone molecule as dipolar reagent can cleave double bonds of many organic compounds.
As of the indirectly reactive process, the organic contaminant in the wastewater can be oxidized under alkaline conditions by the radicals which are produced by the decomposition of ozone[49]. The mechanisms of radical production are as follows:
Zhu et al.[50] treated the ROC from a wastewater treatment plant in Liaohe Petrochemical Co., China National Petroleum Corp, using the ozone oxidation method. CODCr removal efficiency was only about 30%. Li et al.[40] treated the ROC from oil refining, in which the concentration of dissolved organics was 46.1 mg/L. After the ozonation treatment, the DOC removal ratio was about 2.5%, but aromatic structures and unsaturated bonds were destroyed and a large number of hydrophobic substances were transformed into hydrophilic substances. She and Han[51] employed the O3 oxidation + BAF to remove the COD of ROCs in 450 m3/h plant. The results showed that the COD decreased from 120 mg/L to 115 mg/L after O3 oxidation, i.e., merely 4.21% for the removal efficiency. It was clear from these results that the O3 ozonation led to a high degree of mineralization, due to its disadvantages, such as low solubility and stability of O3 in water leading to incomplete oxidation or mineralization of organic compounds, limit its application. Therefore, it is necessary to combine it with other oxidation processes. The ozonation-based combination technology will be introduced in the section of 3.2.5. Another disadvantage of O3 oxidation, as well as the following oxidation process, is that the precipitation of CaCO3 may occur due the decomposition of scale inhibitors during the oxidization process, which should be seriously taken into consideration during the process development and equipment selection.
3.2.2 Fenton method
The Fenton method is one of the most popular chemical AOPs and has also been successfully used to degrade organic compounds from wastewater in the past decade[52]. The Fenton process is a relatively economical method as its no requirement of additional energy compared to many other AOPs. It was mainly classified into the conventional Fenton method and the Fenton-like process (the electro-Fenton (EF) method and photo-Fenton (PF) method, etc.).
For the conventional Fenton process, Fe2+ and hydrogen peroxide (H2O2) as raw materials can produce very highly reactive ·OH by Fenton reaction (Eq. (15)) under acidic condition, which is the second strongest oxidizing agent after fluorine[53]. ·OH can react with target organic contaminants, resulting in the decomposition of some organic contaminants via converting into CO2 and H2O, as well as the mineralization of some organic compounds via converting into inorganic ions (Eq. (16)). Fe2+ can also be oxidized by the generated ·OH radicals (Eq. (17)) and the dissolved oxygen (Eq. (18))[54].
Fe3+ formed by Eqs. (15), (17) and (18) can be converted into Fe(OH)3 colloids by Eq. (19), which acts as the functions of coagulation and adsorption. In addition, Fe3+ may decompose H2O2 into water and oxygen by Eqs. (20) and (21), in which Fe2+ ions and radicals are produced. During the Fenton process, other reactions involving radicals are expressed by Eqs. (22) and (23). It is obvious that the species, i.e. Fe2+, Fe3+ and Fe(OH)2+ coexist in a Fenton oxidation system[54].
Zhang et al.[55] used the Fenton method to treat the ROC from a refinery plant. Under the optimum technology conditions, the CODCr removal efficiency was up to 49.8%, and the effluent met the requirements of wastewater discharge standard. Sun et al.[56] studied the removal ability of non-degradation organic compounds in ROC from a refinery in the Northeast of China by the Fenton method. The results indicated that the influence of factors followed the order of H2O2 dosage > initial pH > treatment time > volume ratio of H2O2 to Fe2+, and the CODCr removal ratio attained 87.42% at the H2O2 concentration of 30%. With an increase in chemical dosages, the removal efficiency of dissolved organic contaminants might be higher. The same phenomenon has also been found by Fan et al.[54].They developed a homogeneous Fenton process to treat the petrochemical ROC from a typical petrochemical industrial park in China. Under the optimum reaction conditions, i.e. H2O2 concentration of 75 mmol/L, Fe2 + concentration of 7.5 mmol/L, and initial pH of 3, TOC concentration decreased from 198.4 mg/L to 111.6 mg/L, and the ratio of BOD5 to CODCr increased from 0.11 to 0.28, meaning that the biodegradability of ROC was significantly improved. Wang[57] reported that the COD decreased from 100 mg/L to 50 mg/L by use of Fenton process in a 420 m3/d ROC treatment facilities in a refinery plant.
However, the conventional Fenton process for the treatment of ROCs has not been widely studied due to the high cost and easy decomposition of raw material, complex transport and storage processes of H2O2, large amount of Fe2+ dosage, high metal concentrations of the final effluent resulted from the introduction of Fe2+, as well as large amounts of ferrous iron sludge. In order to solve the problems existing in the conventional Fenton process and increase the removal of organic compounds, the Fenton-like processes, such as the EF process and the PF process have been developed, and will be introduced in this work due to the fact that they have been widely used to treat the ROC from PRW.
Compared to the conventional Fenton process, the electro-Fenton method seems to be more attractive owing to its high mineralization effectiveness. The electro-Fenton process is an indirect electrochemical process due to the Fenton reagent partially or completely produced from electrode reactions. Generally, H2O2 is generated at the cathode by the reduction of dissolved O2 (Eq. (24)). In comparison with the conventional Fenton process, the reaction of EF process is more powerful because ·OH can be fulfilled by the electro-regeneration of Fe2+, which is generated by the oxidation of a sacrificial iron anode or an iron elementary substance exists in acidic solution (Eq. (25)), as well as the cathodic reduction of Fe3+ (Eq. (26)). It is clear for the EF process that Fenton’s reagent is electrochemically produced in situ, thus avoiding the use of large amount of H2O2 and Fe2+ salt.
In addition, a small amount of Cl- in the ROC can be oxidized and form strong oxidants by Eqs. (27)~(29), i.e. chlorine (Cl2), hypochlorite (HOCl), and hypochlorite ion (ClO-), which facilitates an effective indirect oxidation of organic compounds.
It is clear from the above equations that efficiency of EF technology depends mainly on the factors, such as the quantities of H2O2 and Fe2+, pH, current density and supporting electrolyte concentration[58]. An EF process by using iron electrode were studied for the treatment of PRW[59]. The removal efficiencies of CODCr and phenol were up to 75.71% and 98.74% at less than 10 min, respectively. However, their removal rates decreased when H2O2 in media consumed after a given time period. Yan et al.[60] used EF technology to treat the refinery wastewater from the Yulin refinery plant located in Shaanxi province, China. The results indicated that, when fine Fe particle and air were simultaneously used in the electrochemical process, the removal efficiencies of CODCr and NH3-N at pH 3 attained 89.91% and 99.47%, respectively. Li et al.[26] used the spectroscopically pure graphite electrodes as cathode and anode to treat the ROC from a wastewater reuse unit in the Lanzhou petroleum refinery, China. With the introducing FeOOH-based solid catalyst, under the optimum conditions, the removal ratio of CODCr reached 82.1%, and the CODCr mass concentration in effluent water was 42 mg/L, meeting the local wastewater discharge requirements. Du et al.[61] developed an EF process using graphite as cathode and iron as anode to treat the ROC discharged from advanced wastewater treatment process of a refinery in Northeast China. It was found that the CODCr removal efficiency was up to 80.71 % under optimum conditions, and the removal of non-degradation organic compounds was achieved by a combination of oxidation, flocculation and pulsed electric field.
The conventional Fenton process can be improved by the assistance of UV-light. In the presence of short UV light (180~290 nm), photo-Fenton is a highly efficient homogeneous oxidation as a consequence of the higher quantum yields, which involves the catalytic breakdown of H2O2 in reaction with Fe2+ by Eq. (15) in an acidic medium to form ·OH at the presence of ultraviolet light. The formation rate of ·OH radicals in PF process is higher than that in the conventional Fenton process. Fig. 1 shows the cycle of PF reactions, including the conversion of Fe2+ into Fe3+ and the regeneration of Fe2+ from Fe3+[62]
The conversion of Fe2+ into Fe3+ includes Fenton reaction (Eq. (15)), oxidation of Fe2+ (Eq. (18)), and oxidation of Fe2+-contaminants complex (Eq. (30)). The regeneration of Fe2+ from Fe3+including PF reaction (Eq. (31)), Fenton-like reaction (Eq. (20)), and reduction of Fe2+-contaminants complex (Eq. (32)), is a rate-limiting step in the cycle of PF reactions, leading to the dependence of the PF technology on the generated Fe3+ concentration in solution.
Guo et al.[63] used the PF technology to treat the ROC. The results showed that UV alone could not effectively remove the organic contaminants from wastewater. But, at the presence of UV, the removal efficiency of CODCr in ROC reached 30% by the addition of H2O2 and Fe powder, and the decrease of organic contaminant concentration by increasing of reaction time. Mohadesi et al.[64] treated the wastewater of Shaz and Oil Refinery Company in Iran via photo-Fenton method. Under the optimum parameters, 75% more or less of initial CODCr was removed. BOD5 and TSS were also reduced significantly. The ratio of BOD5 to CODCr ratio of the effluent was increased significantly.
Based on the above, it was obvious that the removal efficiencies of organic compounds the EF process and the PF process were superior to that of the conventional Fenton process. As of the EF process, to further increase current density and decrease the energy consumption, the structure of electrolysis cell still needs to be optimized, as well as highly efficient and low cost electrodes should be developed. Furthermore, the EF process and the PF process might be combined to further improve the removal efficiency of organic compounds in ROC from PRW[65].
3.2.3 Electrochemical oxidation
Electrochemical oxidation technology has been attracting great attention for treatment of the ROC obtained from petrochemical wastewater due to its many distinctive advantages, such as versatility, high energy efficiency, safety, efficient control of reaction conditions, amenability of automation, no chemical requirements, and environmental compatibility[66].The electrochemical oxidation processes can be classified into the direct oxidation reaction in which the poor degradable organic compounds on the anode surface are directly converted into products, and the indirect oxidation reaction in which the organic compounds in the solution are oxidized by the oxidants accumulated in the bulk liquid. Generally, during the process of electrochemical oxidation, the two reactions simultaneously happen. However, the direct oxidation mechanisms require a high electrode surface area, and the reaction itself is limited by mass transfer[67]. The indirect oxidation of organic contaminants was carried out mainly by ·OH obtained from the electrolysis of water (Eq. (33)), the formed reactive chlorine and oxygen species (Eqs. (27) and (34)),as well as hypochlorite oxidants (Eqs. (35)~(38)) produced on the anode surface[66]. Therefore, electrodes material which can significantly impact on the mechanisms and consequently the products of the anodic reaction is one of the most important parameters in the electrochemical studies and applications[68,69].
Several electrode materials, such as boron-doped diamond on silicon substrate (Si/BDD), BDD on niobium substrate (Nb/BDD), lead dioxide on tantalum substrate (Ta/PbO2), and ruthenium oxide on titanium substrate (Ti/RuO2) have been investigated for the treatment of ROCs from petroleum refinery plants. BDD anode can produce directly ·OH by the electrolysis of H2O with very high current efficiency, due to its wide electrochemical window (>3 V) before the generation of hydrogen at the cathode and oxygen at the anode. Bagastyo et al.[70] evaluated the performance of electrochemical oxidation using Si/BDD electrodes for the treatment of ROC from wastewater effluents obtained through MF/RO membranes. It was found that a high concentration of Cl- in the ROC enhanced the oxidation, and about 11% of Coulombic efficiency for CODCr removal was achieved with 5.2 Ah/L of specific electrical charge. Compared to Si/BDD electrode, Nb/BDD can save more energy due to the high conductivity nature of Nb[71]. da Silva et al.[72] used Nb/BDD electrodes to evaluate the performance of an electrochemical oxidation process on treating of a ROC generated from a petrochemical wastewater. Under the optimum conditions, the removal efficiency of total organic carbon and the energy consumption value of CODCr were 71% and 66.5 kW·h·kg-1, respectively, showing satisfactory results. Yavuz et al.[59] also used Nb/BDD electrodes to study the ability of an electrochemical oxidation process for the treatment of ROC from a petroleum refinery. The concentrations of CODCr and phenol decreased from 590 mg/L and 192.9 mg/L to 266 mg/L and 38.8 mg/L at electrolysis time of 10 min, respectively. With the further increase of electrolysis time to 40 min, their concentrations decreased to 60 mg/L and 0.91 mg/L, respectively. However, the extension of electrolysis time led to a great increase in energy consumption. Gargouri et al.[73] conducted anodic oxidation of wastewater from the petroleum exploration of a petroleum plant in Tunisia using Ta/PbO2 and BDD anodes. Compared to Ta/PbO2, the BDD anode showed a better performance in removing hydrocarbon compounds due to the lower adsorption enthalpy of ·OH radicals on its surface. The efficiency of Ti/RuO2 anode for treating organic compounds in wastewaters from petroleum industry was investigated by Santos et al.[74]. The CODCr removal of combination method of electrochemical oxidation with flotation process was up to above 96% after 120 min of electrolysis. Yavuz et al.[59] also found that, in comparison with methods including electro-coagulation, electro-Fenton and electrochemical oxidation with ruthenium mixed metal oxide electrode, the electrochemical oxidation technology with BDD electrode was the most efficient method to remove phenol of 99.53% and CODCr of 96.04% at the current density of 5 mA/cm2. An industrial application of the electrochemical oxidation process was reported for treatment of ROCs from Daxie petroleum plant to reduce the COD from 100 ~ 150 mg/L to lower than 60 mg/L[75].
In addition, mixed metal oxide coated electrode has been used for the treatment of organic petroleum wastewater in recent years. Li et al.[76] used the electrochemical oxidation process with ruthenium oxide (RuO2) and iridium oxide (IrO2) coated titanium (Ti/RuO2-IrO2) as anode and titanium as cathode to treat the ROC from a wastewater treatment plant in an oil refinery enterprise. Over the CODCr concentration range from 50 to 70 mg/L, under the optimum conditions, CODCr removal efficiency was up to about 60%, and the effluent met the wastewater discharge requirements. Ramalho et al.[77] applied electrochemical oxidation technology with RuO2, titanium oxide (TiO2) and tin dioxide (SnO2) coated titanium (Ti/RuO2-TiO2-SnO2) as anode to remove organic compounds in ROC obtained by petroleum exploration of petroleum plant in Brazil. It was found that the oxidation rate of the organic contaminants was strongly affected by higher electrolyte flow rate, suggesting the indirect oxidation of organic compounds on the Ti/RuO2-TiO2-SnO2 anodes as the principal mechanism. Some works have been carried out to compare the ability of BDD and mixed metal oxide coated electrode to treat ROCs from petrochemical industry. da Silva et al.[78] found that higher removal of DOC and COD were achieved using the BDD anode. However, the IrO2 and tantalum oxide (Ta2O5) on tantalum substrate (Ti/IrO2-Ta2O5) anode gave a lower energy consumption and thus operating cost.
For the electrochemical oxidation technology, in-situ generated active chlorine species can effectively oxidize organic pollutants and achieve the removal of color, but the possible formation of chlorinated byproducts could lead to an increase in toxicity level of the effluent. The removal of halide ions, particularly chloride, ahead of electrooxidation may avoid these issues. Other strategies, such as active carbon (AC) treatment, continuous operation and short residence time, and over saturation of the solution by chlorine were also proposed for minimizing the formation of chlorinated organic compounds or increasing the degradation rate of chlorinated intermediates.
3.2.4 Photocatalytic oxidation
In recent year, the photocatalytic oxidation technology has been considered as an efficient and economical method to remove the organic contaminants in wastewater due to its advantages, such as non-toxic, nonselective oxidizing a broad range of organic compounds, non-corrosive, on-place use, no extra energy consumption, and harmless to the environment[25]. And it is based on the use of a semiconductor and ultraviolet-visible (UV-vis) radiation. Some semiconductor materials, such as titanium dioxide (TiO2), zinc oxide (ZnO), and tungsten trioxide (WO3) have been selected as catalysts to degrade the organic compounds in wastewater. Among these semiconductors, TiO2 is currently the most popular photocatalyst due to its high photocatalytic efficiency, strong oxidizing power, high chemical stability, relative inexpensiveness, and antibacterial property. Fig. 3 shows the mechanisms of photocatalytic oxidation[79].
It is clear from Fig. 3 that the fundamental step of the process is generation of electron-hole pairs which have a high redox ability to degrade organic contaminants. First, TiO2 absorbs photons with adequate energy under the irradiation of UV light, and then the electrons move up to the conduction band from the valence band, finally resulting in the generation of electron-hole pairs in the catalyst by Eq. (39). In the meantime, the oxidative radicals are generated on the interface between the catalyst and H2O. First of all, O2 adsorbed onto the TiO2 surface will be reduced to superoxide radical anion (· O2-) due to the strong reduction ability of photogenerated electrons by Eq. (40). And then ·2-can further reacted with H+ to form HO2· radical and H2O2 by Eqs. (41) and (42), respectively. Residual electron-holes within the valence band of TiO2 react with OH- and H2O to form hydroxyl radicals (·OH) by Eqs. (43) and (44). Finally, organic contaminants in ROC can be degraded by ·OH, electron-hole pairs, and superoxide radicals through intermediates, and then finally to CO2 and H2O.
Some researchers applied the photocatalytic oxidation technology with TiO2 as catalyst to deal with the organic contaminants in ROCs. Ding et al.[21] studied the dynamic characteristics and the influencing factors of photocatalytic degradation of ROC from a refinery plant in Tianjin, China. They found that the removal rate of CODCr was only 47.51% at the nano TiO2 dose of 1 g/L, pH value of 4.5, and UV lamp irradiation of 8 h, but the biodegradability of the ROC was improved with the BOD5/CODCr value of the ROC increasing from 0.01 to 0.22. The photocatalytic degradation process followed the Langmuir-Hinshelwood kinetics model and the reaction accorded with the pseudo-first-order reaction kinetics. Base on the results, it was obvious that, in the practical application, the photocatalytic efficiency of TiO2 alone was very low. This was attributed to its wide band gap (Eg=3.2 eV), being active only under UV light, low quantum yield resulted from rapid recombination of photogenerated electron-hole pairs, and low affinity for organic contaminants[79].
To overcome the restrictions on the TiO2 application and enhance the photocatalytic ability of TiO2, TiO2-based composites, such as doping with metal or nonmetallic elements (neodymium-modified TiO2 and N-doped TiO2), and coupling with the semiconductor or incorporating nanostructured materials (TiO2/ZnO and TiO2/zeolite), have been developed by modifying TiO2[80,81]. Li et al.[82] investigated the effect of TiO2/zeolite catalyst on the removal ability of organic compounds in ROC from the Shengli Oil Field in China. It was found that the removal efficiency of total organic compounds was up to 67.4% at the reaction time of 2.5 h and TiO2/zeolite catalyst dose of 1.5 g/L, and the chemical properties of TiO2/zeolite catalyst remained stable under the optimum experimental conditions.
In addition, to further improve the CODCr removal efficiency of photocatalytic oxidation can be improved by the addition of other oxidants and the combination with other oxidation methods. Zhao et al.[25] treated the ROC from petrochemical wastewater with the addition of H2O2. The removal efficiency of CODCr were 93.63% at reaction time of 2 h, suspended TiO2 dose of 0.6 g/L, H2O2 concentration of 0.8 ml/L, pH value of 4, aeration quantity of 0.75 L/min, and irradiation of mercury-lamp of 500 W. It was found that the pH value and H2O2 concentration had an important effect on the removal efficiency of CODCr. The removal ability of CODCr decreased with the increase of pH value from 4 to 12, and was only 24.03% at pH 12 due to the fact that pH could affect the charge distribution on the surface of TiO2 and charge properties of the interface. In addition, the removal ability of CODCr was only 70% without the addition of H2O2. The addition of H2O2 dramatically increased the photolysis efficiency because a certain concentration of H2O2 can capture electrons and hinder the recombination of electron-holes on the surface of TiO2, leading to the quick generation of ·OH radicals. However, the further increase of H2O2 concentration resulted in the termination of radical chain reactions. Wang et al.[27] treated the ROC from a petroleum refinery plant by the photoelectron-catalytic oxidation with Ti/TiO2 as anodes. In the ROC, the concentrations of CODCr, NH3-N, salts, and Cl- were 204 mg/L, 44.61 mg/L, 4490 mg/L, and 1943.25 mg/L, respectively. The removal efficiencies of CODCr and NH3-N at reaction time of 150 min, current density of 2.0 mA/cm2, and UV light intensity of 30 μW/cm2 were 92.06% and 93.68%, respectively.
For the photocatalytic oxidation technology, the separation and recycling of suspended catalyst particles might be an inconvenient, time-consuming, and expensive process[82], and the solution with the suspended catalysts will be less penetrable by UV radiation, indicating that the dosage of catalyst is limited. Furthermore, the wide band gap for the TiO2 photocatalytic system limits its large-scale application.
3.2.5 Ozonation-based combination processes
The combination processes of ozonation with other technologies, such as O3/UV, O3/AC, O3/catalysis, O3/photocatalysis, and O3/electrochemical oxidation, have been proven to be effective ways to degrade organic pollutants and they have been used for the treatment of a wide variety of industrial wastewaters.
O3/UV oxidation process: Under the irritation of UV light, the dissolved O3 can be easily converted into H2O2, which can further generate ·OH radicals. Synergetic effects of O3 and UV light irritated a series of chain reactions, and generated the powerful radicals, which can rapidly degrade the organic pollutants in ROCs. Ozone in combination with UV irradiation has been studied for the degradation of the organic content of ROCs from PRW.
Fu et al.[83] studied the removal efficiency of organic pollutants in wastewater produced from a petroleum plant in Fujian, China, by the combination technology of O3 and UV oxidation. They found that the combination technology was superior to the conventional O3 oxidation, and it only took 7.5 min to decrease the CODCr concentration to 33.2 mg/L. Zhang et al.[84] also used the O3/UV oxidation method to treat the alkaline wastewater obtained from a refinery plant. The experimental results indicated that the O3 concentration and reaction time were the important factors that influenced the treatment effect of wastewater. In comparison with the single ozone oxidation, the removal efficiencies of CODCr and phenol increased by 24% and 28%, respectively. Although the biodegradability of wastewater was greatly improved, the removal efficiency of CODCr was still low.
O3/AC oxidation process: Single O3 oxidation ability can be improved through the addition of AC in an ozonation system. When O3 was adsorbed to the surface of active carbon, a series of free radical chain reactions happened to generate ·OH radicals which can effectively degrade the organic pollutants in ROCs. AC has a strong adsorption ability to O3, and thus quickly promote the generation of ·OH.
Zhang[85] treated the ROC from a petroleum refinery company by an ozone oxidation method with the addition of AC into the ROC. It was found that CODCr removal ratio attained the maximum value of 53.6% at the O3 concentration of 5.5 mg/L. The increase of initial CODCr concentration in the ROC to 80 mg/L led to the increase of the CODCr removal efficiency. This might be attributed to the fact that the suspended AC in the wastewater containing O3 initiated the chain reactions and promoted the generation of ·OH. Gu[86] treated refinery wastewater by combining ozone and AC. The results indicated that the oxidation ability of ozone can be intensified by the adsorption and catalysis of AC. The BOD5/CODCr value of the effluent was greatly increased to 0.53 from 0.07, but the removal efficiency of CODCr was only 47%. Bai et al.[23] used the same method to treat ROC from a refinery in Tianjin, China. Before experiments, AC was modified by three methods. Among these methods, the best performance was of AC modified by H2O2, with which the BOD5/CODCr value of the effluent attained 0.38. Although the synergistic effect of combining ozone and AC showed a good potential in increasing the BOD5/CODCr value of the ROCs, the removal efficiency of CODCr was still low. The effluent cannot meet the requirements of local wastewater emission standard.
O3/catalysis oxidation process: Single O3 oxidation ability can be further improved through the addition of catalyst in an ozonation system. In the system of O3 oxidation, the addition of transition metal ions can promote the self-decomposition of O3 molecule, resulting in the rapid generation of ·OH radicals. According to the existing form of catalyst in the reaction system, the O3/catalysis oxidation technology can be classified as the homogenous catalytic ozonation and the heterogeneous catalytic ozonation. For the homogenous catalytic ozonation technology, the soluble transition metal ions play a positive effect on the ability of O3 oxidation, but this kind of catalysts are easy to be lost, resulting in a new pollution of effluent due to the introduction of catalyst. For the heterogeneous catalytic ozonation, the supported catalysts with metal oxide or active carbon as carriers are used to increase the self-decomposition efficiency of O3. In comparison with the homogenous catalytic ozonation, the supported catalysts are easy to be recycled and avoid the introduction of new pollutants. Therefore, this technology has been widely used to treat ROCs from PRW.
Application of heterogeneous catalytic ozonation system requires to develop highly efficient catalysts with characteristics of balanced support and catalytic[87]. Shen and Li[75] reported an industrial application of the O3/catalysis oxidation process with ROCs treatment capacity of 100 m3/h, which reduced the COD from 250 ~ 350 mg/L to 100 ~ 150 mg/L, i.e. 50%~60% of the removal efficiency. Cao et al.[31] reported the similar results in another plant, meanwhile the COD decreasing from ~170 mg/L to ~ 55 mg/L. Li et al.[88] developed a kind of catalyst with aluminum oxide as carrier, which was used to treat the ROC of oil refining wastewater by the heterogeneous catalytic ozonation method. It was found that, at optimum conditions, the specific surface area and pore structure of the catalyst did not have any changes, and the effluent met the requirements of wastewater emission standard. Gong et al.[22] treated the ROC produced in the reuse of wastewater from secondary sedimentation tank in a refinery by the catalytic ozonation technology. The supported catalysts were prepared on the basis of Al2O3 carrier and the active components of Mn, Fe, Ti and Si. The CODCr of the treated wastewater was decreased to below 60 mg/L, and the biodegradability of the effluent increased from 0.07 to 0.34. Jiang et al.[89] treated the ROC from a refinery in Shandong province, China, by the ozone catalytic oxidation method. Several catalysts were prepared with AC, Al2O3, or AC-based composite as carriers, and their activities followed the order of Cu-Ce/ACNT > Cu-Ce/AC > Cu-Ce/Al2O3. Compared with the individual ozonation, the CODCr removal efficiency of catalytic ozonation with Cu-Ce/ACNT as catalyst was increased to 78.6% from 22.6%. The CODCr concentration of effluent met the requirements of wastewater treatment, and there was no obvious decrease in activity of catalyst within 30 days. Chen et al.[28] prepared two kinds of novel composite Mn-Fe-Mg/Al2O3 and Mn-Fe-Ce/Al2O3 catalysts, which exhibiter higher catalytic activity during the ozonation of ROC from a wastewater treatment plant in Liaohe Petrochemical Co., China National Petroleum Corp. In comparison with the single ozone oxidation, their CODCr removal ratios increased by 23.9% and 23.2%, respectively. However, most of the synthesized catalysts are costly, as well as scale and agglomeration easily occur in catalyst packed bed at high salt concentration, which limits their industrial application.
O3/photocatalysis oxidation process: The integration of ozonation and photocatalysis oxidation seems to be an advanced ozonation route by combining the beneficial effects of ozonation with the generation of ·OH radicals via the electron-hole pairs[90]. The electron affinity of O3 is higher than that of O2, which can promote photocatalytic reactions in the presence of O3.The dissolved O3 can easily get electrons on the TiO2 surface. The recombination of electrons and holes can be interfered by the reactions between O3and electrons on the TiO2 surface, and thus a larger number of radicals, such as oxygen atom radicals (·O), superoxide and ozonide radical anions (·O2-and ·O3-) can be produced. In addition, a larger amount of H2O2molecules which are produced from the dissolved O3 under the irradiation of UV light, can also react with electrons on the TiO2 surface to form ·OH radicals.
Based on the above, photocatalytic ozonation could be an effective alternative way for oxidation and elimination of organic pollutants under specific conditions for treating industrial wastewater. Corrêa et al.[91] evaluated the efficiency of batch O3/photocatalysis oxidation technology in the degradation of pollutants in the wastewater of petroleum refineries. Photocatalytic ozonation led to lower degradation times for CODCr and TOC removal. After 5 min, O3/photocatalytic oxidation treatment could remove phenol of 99%, sulfide of 53%, CODCr of 37.7%, and ammonia of 1.9%. With the reaction time increasing to 60 min, the removal efficiencies of phenol, sulfide, CODCr and ammonia were up to 99.9%, 97.2%, 89.2% and 15%, respectively. The percentage of mineralization was nearly 100% in phenol. As far as the oxidation of organic contaminants in wastewater were concerned, the combination of ozone and appropriate photocatalysts could lead to higher degradation efficiencies compared to those of ozonation and photocatalysis separately. The synergistic effects of O3 and photocatalysis oxidation were mainly attributed to the intensive generation of powerful oxidative reagents in the system of photocatalytic ozonation. But this combined method still existed the problems of scale and agglomeration in catalyst packed bed at high salt concentration.
O3/electrochemical oxidation process: For the combination of O3 and electrolysis, synergistic degradation of organic pollutants could be obtained. Reaction products during the electrolysis process can irritate the self-decomposition of O3 molecules to generate radicals. pH plays an important role during the process of O3 decomposition. When pH value is between 6 and 10, the O3 molecules in water can rapidly be decomposed into ·OH radicals, and OH- absorbed on the anode is effectively converted into ·OH. The indirect oxidation of organic contaminants is dominant. Over the pH range from 3 to 6, the O3 molecules in water slowly are decomposed into ·OH radicals, and a large amount of H2 are produced on the surface of cathode, leading to the increase of adverse reactions. However, the strong alkaline condition with pH value more than 10 is not favorable for O3/electrochemical oxidation because a large amount of scale is formed on the anode surface, interfering the further oxidation.
To control the environmental pollution from the refinery wastewater, Xie et al.[92] treated the ROC from a refinery wastewater treatment system by the O3/electrochemical oxidation method with metal oxide coated titanium as anode. The results showed that the organic contaminants could be effectively degraded by the combination process. The removal rates of CODCr increased first and then declined with the increase of pH. The CODCr concentration of effluent decreased to 50 mg/L from above 200 mg/L at the pH of 7.5, O3 dosage of 8 mg/L, current density of 50 mA/cm2 and hydraulic retention time of 30 min. Bernal-Martínez et al.[93] evaluated the effectiveness of the integrated ozone/electrochemical oxidation process on wastewater from a treatment plant located at the end of an industrial park. At pH value of 7 and current density of 40 mA/cm2, electrochemical oxidation method reduced CODCr by 43% and BOD5 by 42%. With the introduction of O3, at the same experimental conditions, the removal efficiencies of CODCr, BOD5, color, turbidity, and total coliforms were 84%, 79%, 95%, 96%, and 99%, respectively. García-Morales et al.[94] evaluated the coupling technology of ozonation and electrochemical oxidation to remove the organic pollutants in wastewater from the treatment plant of an industrial park. Electrochemical oxidation using BDD electrodes reduced the CODCr by 99.9%, but required over 2 h per 0.7 L batch. However, through the coupling technology of ozonation and electrochemical oxidation, it only took about one hour to remove the CODCr of 99.9%. In addition, color and turbidity could be eliminated without the addition of chemical reagents, and no sludge to be generated also. The addition of O3 obviously intensified the oxidation ability of electrochemical method for wastewater treatment and shortened the reaction time of electrochemical oxidation, resulting in the decreasing of the energy consumption of wastewater treatment. Therefore, the coupled process of ozonation together with electrochemical oxidation is a promising wastewater treatment technology due to its high efficiency and environmentally friendly.
3.3 Biochemical method
The biological treatment is the conventional activated sludge process for the removal of biodegradable organics. However, most of the organic compounds in ROCs from petroleum refinery wastewater are bio-refractory. Therefore, before biochemical process, some other advanced oxidation processes, such as biological activated carbon adsorption, ozonation, and O3/catalysis oxidation, need to be used to increase the biodegradability of the ROCs by decomposing or dissociating refractory organic pollutants into small molecules which can be easily handled by microorganisms, and mineralize a part of degradable organic pollutants in ROCs.
For the biochemical process, the salt concentration of ROCs showed an important effect on the removal efficiency of organic contaminants in petroleum refinery wastewater[95]. When the salt concentration was less than 25 000 ppm, the wastewater had a better biodegradability, and the CODCr removal efficiency was up to about 92%. With the salt concentration of wastewater increased to 35 000 ppm, sludge activity was obviously inhibited. With the salt concentration further increased to 60 000 ppm, sludge activity tended to collapse, sludge flocs were fragmented and dispersed, and the CODCr removal efficiency was only 45%. It was mainly attributed to the fact that the inorganic salts with high concentration in wastewater had an inhibition effect on dehydrogenase in microorganisms, decreased the activity of cells in organisms due to the high osmotic pressure, and caused the loss of active sludge because of an increase in wastewater density.
To break through these limitations of the biochemical method, some works have been done to improve the treatment effect of petroleum refinery wastewater, such as the domestication and cultivation of halophilic microorganisms, and the development of membrane bio-reactor (MBR). The ROC produced by a petroleum refinery plant in Harbin, China, was treated by MBR[96,97]. Over the wastewater conductivity range from 3000 to 5000 μS/cm, the halophilic microorganisms used can effectively degrade the organic pollutants in ROC. But as the conductivity was increased to 6000 μS/cm, the activity of microorganisms was inhibited. And as the conductivity was less than 2000 μS/cm, the prevalent bacteria of microorganisms were converted into non-halophilic bacteria, and the removal efficiency of contaminants decreased obviously. Wang[98] applied a kind of sewage treatment technology based on fluidized bed, namely moving bed biofilm reactor which was an efficient new reactor between active sludge method and the biological membrane method, to treat the ROC produced by the refinery wastewater treatment plant. Before the biochemical process, the ROC was firstly treated by ozonation process, and the CODCr of 43.9% was removed. Subsequently, the removal rate of CODCr for the biochemical process was up to 49.9%, and the CODCr concentration of the effluent was less than 50 mg/L, meeting the national emission standard. The MBR technology has a better separation performance of solid-liquid, and can effectively solve the problems of active sludge loss and high content of suspended solids in the effluent. However, this technology also exists some disadvantages, such as membrane fouling resulted from a large number of extracellular polymers formed by the secretion of microorganisms at high salt concentration, and high cost of membrane maintenance.
4 Recovery of salts in ROCs from petroleum refinery wastewater
Recovery and commercialization of salts in ROCs from petroleum refinery wastewaters is the ultimate challenge to improve the management of ROCs. The components of salts in ROCs from petroleum refinery wastewaters were mainly sodium chloride, sodium sulfate, etc. If these substances can be recovered from ROCs, the environmental impact of ROCs disposal will be greatly reduced, the economy of the total treatment process to be improved, and the maximum water recovery, namely zero liquid discharge, to be achieved. Unfortunately, as of now, only the mixtures containing organic and inorganic compounds in the ROCs from petroleum refinery wastewaters are obtained by the evaporation-crystallization process, and then disposed in a landfill.
Liu et al.[99] applied the simple evaporation-crystallization method to treat the ROC from a petroleum refinery plant in the North of China. The results indicated that conductivity and TOC content of effluent were 66 μS/cm and 9.3 mg/L, respectively. The mass concentration of total solid was increased from 13.29 g/L to 172.16 g/L, and its water content was less than 40%. Aquatech company[100] also used the evaporation-crystallization method to treat the ROC obtained by a high pressure RO device. Zero liquid discharge was achieved with the effluent beneficially reused. However, the obtained solids were disposed in a landfill. For the evaporation-crystallization process, the ROCs with high CODCr content cannot meet the requirements of the influent. Therefore, this technology exists some disadvantages, such as high energy consumption, easy scaling and high cost of equipment investment, thus could not be actually utilized by the enterprises. It is of necessary to find an economic and environmental-friendly treatment technology of the ROCs from PRWs.
Multi-effect distillation technology, in which the evaporation and the condensation is in the negative pressure, showed a good industrialization future due to its low energy consumption, low risk of scaling on the surface of pipe, high thermal efficiency, and effective utilization of low-quality energy sources such as discharged smoke heat, cooling water, and solar energy[101]. Zhao et al.[102] established a lab-scale vertical tube multi-effect distillation system with a top brine temperature lower than 70 ℃. They used the developed device to treat the salinity wastewater of petrochemical enterprises, and optimal operation schemes were determined. It was found that this technology was suitable for dealing with the low salinity wastewater, meaning that it could be used as the pretreatment before the wastewater crystallization. To improve the efficiency of multi-effect evaporation device to concentrate ROCs of petrochemical enterprises, Zhao et al.[103] developed a kind of thermodynamic model for treatment of wastewater from petroleum refinery enterprises, and studied the effect of operating parameters on the evaporation system. Liu et al.[104] established the mathematical model for multi-effect evaporation on the basis of the characteristics of ROC evaporating process. In their work, some key parameters affecting the performance of multi-effect evaporation technology, such as effect number, concentration ratio, and temperature have been optimized. The results indicated that the effect number showed the most important effect on the performance of multi-effect evaporation of ROC, the concentration ratio only had a slight effect on both the output ratio and the total heat transfer area, and temperature in last effect influenced slightly the output ration but remarkably the total heat transfer area.
The ultimate objective of ROCs from PRW must be the zero discharge of liquid and recovery of salt compounds from ROCs of PRW. Zero liquid discharge of ROCs can be achieved through the above methods. For the recovery of salt compounds, before the combination process of evaporation and crystallization technologies, membrane technology may be used to achieve the separation of monovalent salt and polyvalent salt. And then valuable salts might be recovered, and can be used by industries, such as chemical industries and steel production. However, the main problem is the feasibility of application to industrial level due to their investing and operating costs.
5 Conclusions
With the further upgrading of national emission standard, the ROCs from petroleum refinery wastewater cannot meet the discharge requirements. It is of necessary to find an innovative technology to solve the problems of the ROCs management. The final objective of ROCs treatment technology must be the zero discharge of liquid and the recovery of valuable inorganic compounds from RO concentrates. As of now, the treatment of ROCs from PRW were mainly focused on the reduction of organic pollutants by applying advanced oxidation technologies, especially ozonation-based combination technologies, which are probably the most promising technologies to remove organic compounds. However, the method for the qualitative and quantitative analysis for the organic contaminants in ROCs was still not well established, which is so important to optimize the technology for the CODCr removal. Recovery of salts from ROCs is a promising investigation interest. Although the evaporation-crystallization technology is well established, they are energy intensive. Moreover, the salt mixtures containing organic and inorganic compounds obtained were only disposed in a landfill. Therefore, the zero discharge of liquid and the recovery of salts from ROCs are hard to be applied to industrial level due to the high investing and operating costs. To implement the recovery and reuse of salts, ahead of the evaporation- crystallization process, some processes might be used to achieve the separation of monovalent salt (such as NaCl) and polyvalent salt (such as Na2SO4).
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