# Purification of Produced Water by Solvents to Enhance Oil Recovery and Reuse Separated Droplets
### Traditional Industrial Methods for Produced Water Treatment  
Primary treatment can be carried out using skimming tanks, API separators, corrugated plate interceptors (CPIs), and coalescence. Secondary treatment may employ hydrocyclones, induced gas flotation (IGF), dissolved gas flotation (DGF), compact flotation units (CFUs), or membranes, among other technologies. The final cleaning or refining treatment uses walnut shell filters or dual media filters [3].  
Hydrocyclones require energy to pressurise the inlet, do not separate solids, are prone to fouling, and incur high maintenance costs [4]. Corrugated plate interceptors are inefficient due to the time required to retain the fine oil particles and their maintenance requirements [5]. Dissolved air flotation involves dissolving pressurised air into produced water and subsequently depressurising it under atmospheric pressure, releasing air bubbles that attach to oil droplets and make them float. This technique has the drawback of requiring a large volume of air, long retention times for separation, and producing a high volume of skimmed waste [6]. Flotation has the disadvantage that, at high temperatures, high pressure is needed to dissolve the gas in the water [7; 8].  
Hydrophilic membrane filtration (MF) suffers from fouling caused by the formation of colloidal scaling, requiring elaborate multi-stage pretreatment, large base areas, and complex designs for the treatment system. It consumes a significant amount of energy due to the high power per unit surface area of the membrane.  
Antifouling chemicals and other pretreatment agents require handling and storage [7; 9].  
Microfiltration (MF) demands high energy, exhibits low efficiency for divalent and monovalent salts and viruses, and is prone to fouling from iron deposits [7]. Ultrafiltration (UF) also requires high energy; iron fouling and membrane fouling are problematic, as is its lower efficiency for low-molecular-weight organic compounds. The salt rejections may also contain radioactive materials [2; 7]. Nanofiltration similarly requires high energy, is less effective for monovalent salts and low-molecular-weight organic compounds, and, like ultrafiltration, its salt rejections may contain radioactive materials [7].  
The catalytic method for treating produced water requires high temperatures and pressures, which can promote emulsification [10].  
Using produced water from oil fields for irrigation also requires desalination methods, to address the high salt concentrations, as well as the removal of crude oil droplets [11; 12].  
Media formed from alternating hydrophilic and hydrophobic fibres are convenient for the purification of liquids containing certain organic materials, such as oil, but the interactions between crude oil droplets and water present complex correlations [13].  
Ceramic membranes suffer from internal scaling [11; 14].  
Oxidation processes treat only small volumes of water, which is disproportionate to the massive quantities requiring purification [15]. A similar issue arises with methods utilising microalgae [16] or activated carbon: the volumes of water requiring treatment are so large that proportionality is unfeasible [11; 17].