# Purification of Produced Water by Solvents to Enhance Oil Recovery and Reuse Separated Droplets
### Laboratory Methods for Produced Water Treatment  
In addition to the traditional processes for separating crude oil droplets from produced water, there are numerous laboratory-based methods and attempts characterised by promising ideas. However, these are not without their pros and cons when scaled to real industrial dimensions. Below, we summarise some of these methods. The method of using melamine sponges with silica granules has the drawback of a complex manufacturing process for these sponges. Furthermore, crude oil is removed from produced water by absorption into the sponge, making subsequent separation of the crude oil from the sponge, as well as its recovery and commercial utilisation, difficult [17; 18; 19; 20; 21; 22; 23; 24]. Another method involves flooding the oil well with a thickening polymer to reduce water leakage and improve crude oil extraction. The required polymer concentrations are on the order of thousands of parts per million, which is excessive. Moreover, this method is not specifically designed for the removal of oil droplets from produced water, which would be preferable [25]. Other separation operations rely on the use of cotton and lotus plants, which are limited to specific operations such as offshore oil spills. Porous cotton fabrics are used to absorb oil from spills. However, the subsequent treatment of the oil-saturated cotton involves burning it, which generates another type of environmental pollution [26; 27]. Membrane-based methods using carbon and graphene do not mention the percentage of crude oil recovered, which is a crucial metric for evaluating a treatment's effectiveness [28; 29]. Another alternative is the use of cationic surfactants with a double fatty chain, such as esterquats. These have limited applications and achieve a recovery rate of 81.31% to 83.75% of oil, both for removing crude oil from produced water and for extracting water traces from crude oil [30]. Another absorption method involves using sawdust at an ideal temperature of 27 \({}^{\circ}\)C. This is not feasible for seawater or large volumes of water. This method, cited in [31], does not indicate the percentage of oil separated. While sawdust absorbs oil effectively [32], it provides poor separation rates, not exceeding 70%. Additionally, separating the crude oil from the saturated sawdust is difficult. Saturated sawdust could be burned to produce energy, but this would shift the pollution problem elsewhere. Sawdust is useful for cleaning stains from essentially dry surfaces but has limited utility when mixed with produced water. The use of superhydrophobic Al\({}_{2}\)O\({}_{3}\) particle beds is another proposed method. If the pore width in the layers is approximately 0.3 \(\upmu\)m, they are effective for removing crude oil from oil-in-water (O/W) emulsions. Pores of approximately 40 \(\upmu\)m are effective for removing water from water-in-oil (W/O) emulsions. However, the issue with produced water is that it is not a simple emulsion, but rather a complex mixture of multiple emulsions, rendering this system ineffective [33]. The carbon steel method is suitable for treating produced water but has the disadvantage of high manufacturing costs and suboptimal oil removal efficiency [34]. Polyaluminium chloride has also been proposed as a method for treating produced water. High salinity and an increase in temperature can improve oil separation [35]. However, this is illogical, as increasing the temperature enhances molecular movement. It has been demonstrated that emulsification begins at as low as 47 \({}^{\circ}\)C, which is detrimental to the oil separation process [32]. In summary, both traditional industrial processes and recent laboratory-scale attempts at purifying produced water have significant flaws. These include low efficiency, high procedural costs, the inability to recover crude oil for commercialisation (and thus offset the purification costs), or the fact that solving one environmental issue may create another. Laboratory methods often lack scalability to industrial levels.