results a partial hydrolysis of the triglycerides producing mono-and di-glycerides which will act as surfactants.
The mono-and di-glycerides will increase the solubility of oils in water producing suspensions that are difficult to separate from the aqueous phase that contains the glycerol.
On the other hand, the impurities in the mixture of used oils and the amount of methanol used could produce small flocs of glycerol (this is optically observed) which reduce the settling rate of glycerol.