Vodka production: Rectification
Purifying or 'rectification' columns remove any remaining off notes or odours from an alcoholic spirit. It is common practice in vodka production for a distiller to purchase neutral spirit from an agricultural distiller and then to further rectify that spirit in their own stills.
As the quality of neutral spirit available from agricultural distillers has greatly improved with modern still technology it is now possible to buy base alcohol that does not require further rectification. Hence, many so-called "vodka distillers" are merely compounders who purchase high-quality neutral spirit and blend it with water to reduce it to bottling strength, often also adding small amounts of sugar or flavouring according to their vodka recipe.