Effect of freezing-air thawing pretreatment of cutting fluid wastewater on the organics' distribution, transfer and transformation

The cutting fluid wastewater was treated by freezing-air thawing method. After thawing, stable stratification occurred in the thawing wastewater, and the froth floated on the liquid surface, and the liquid from top to bottom was divided into the first layer, the second layer, the third layer and the fourth layer. The molecular weight (MW) distribution and organics characteristics of the froth layer and each layer wastewater were analyzed by ultrafiltration membrane filtration combined with mass balance, three-dimensional fluorescence excitation-emission matrix spectroscopy, ultraviolet absorption spectroscopy, flourier transform infrared spectroscopy and droplet size analysis. The results showed that the cutting fluid wastewater contained 66.74 % organics with MW > 100 kDa and 17.04 % organics with MW < 1 kDa, and the organics might contain aromatic and heterocyclic compounds. After three times of freezing-air thawing: 27.53 % of organic matter was volatilized into the air; the froth layer was mainly macromolecular organic matter with MW > 100 kDa, accounting for 94.59 %; from the froth layer to the fourth layer wastewater, the content of macromolecules with MW > 100 kDa gradually decreased, while the content of small molecules with MW < 1 kDa gradually increased. The results of molecular weight distribution analysis, spectroscopy analysis and droplet size analysis indicated that during the freezing-air thawing process, there was not only the demulsification maybe but also the destruction of molecular structure. The molecular weight distribution of the mixture of the froth layer and the fourth layer wastewater were close to that of the raw cutting fluid wastewater, so the mixture of the froth layer and the fourth layer wastewater could be reused in metal process. The wastewater in the first, second and third layers had better biodegradability, which could be treated by biological method.