1. The paper analyses the decolorizing mechanism of biofiltration and the hardening of filter materials and finally concludes that the mixed argil grain and spongy iron is the best.

2. The present invention relates to a low temperature, solvent-free trans-acidolysis process for preparing surface-active carbohydrate fatty-acid esters comprising the steps of reacting acylated carbohydrate with free fatty acid in the presence of acid catalyst, under reduced pressure, and without adding any solvent; decolorizing and separating the reaction mixture thus obtained into unreacted fatty acid and carbohydrate fatty-esters by solvent extraction and stage cooling.