# Experimental Study on the Desalination of Waste Leachate Using the Combined Freezing Method
## 3 Results and Discussion  
### Freezing Parameter Settings  
During the freezing process, the water in the solution freezes in solid form, while the solutes are more inclined to be retained in the non-ice phase of the concentrate, and with the prolongation of the freezing time, the ice crystal structure grows and gradually excludes the solutes, thus improving the salt rejection [14]. Figure 1 shows the variation in the water quality parameters at different freezing times. The experiment found that the salinity of the concentrated solution increased with the increasing freezing time, increasing from 6.66% to 9.3%. The increase in salinity of the concentrated solution is mainly due to the longer freezing time, which increases the density and hardness of the ice due to the mutual compression between the ice crystals. During the freezing process, it is difficult for heat to penetrate the interior of the ice, and the melting rate of the ice is relatively slow, which reduces the dilution of the concentrated solution in the stainless steel bucket under the funnel [15]. As the freezing time increases, the ice production rate for the same gravity time tends to increase. This is mainly because prolonged freezing compacts and hardens the ice body, making it more difficult for brine channels to form. The salinity of the ice meltwater decreased and then increased with the freezing time to 2.65%, 2.54%, 2.41%, 2.76%, and 2.83%, respectively, which were significantly lower than the initial salinity of 5%. This result indicates that the salts in the wastewater were effectively removed by the freezing treatment, and the salinity of the ice meltwater fluctuated with the increase in the freezing time. The main reason is that increasing the freezing time means that the solution absorbs more cold energy, causing more water molecules to freeze and reducing the amount of water molecules lost during separation so that the salinity of the ice meltwater decreases, but beyond a certain freezing time, the salinity of the ice meltwater increases as the concentrate becomes more saline and viscous, more likely to adhere to the ice crystals and more difficult to separate. The salt rejection and conductivity removal rates had approximately the same trend, which was increasing with the freezing time first and decreasing after the freezing time reached 18 h. The salt rejections were 42.14%, 44.18%, 47.38%, 38.39%, and 36.97%, respectively. This result shows that the salt rejection is the highest when the freezing time is 18 h. After 18 h, the ice production rate reaches a higher degree, which will inevitably entrain more salt impurities and lead to a decrease in salt rejection. To summarize, the freezing time of 18 h is the best desalination time. The freezing temperature is one of the important parameters in the freezing process (Figure 2), and as the freezing temperature was reduced from \(-10\)\({}^{\circ}\)C to \(-30\)\({}^{\circ}\)C, the lower the freezing temperature at the same freezing time, the higher the ice production rate, which increased from 41.61% at the beginning to 94.48%. There is no particular pattern in the change in salt rejection throughout the process, but overall, the salt rejection decreases from 34.71% to 14.09% with the decreasing freezing temperature, and the salinity of the ice meltwater increases from 2.99% to 3.78% initially. Such a result is not unexpected. Concentrates cannot be produced in large quantities at lower temperatures, and the high ice production rate leads to a significant reduction in the migration capacity of the salt ions, which are more transiently held in the ice phase, resulting in a reduction in the salt rejection rate. [PERSON] et al.