Happily, these sake lees are not wasted. They are used in the Japanese kitchen as a marinade for fish or meat, made into a sweet rice drink (amazake), or re-used for brewing table sake.
Sake kasu are also used as a pickling agent.
[Sakekasu from Daishichi]
In fact, sake kasu are rich in proteins and very nutritious. They take the form of a thick rice paste sold in plastic bags from which the fragrance of sake still wafts up. Another type is sold in dry form. A more expensive kind are sake lees made from Ginjo sake.
In Japan, you will especially find them in winter, when all breweries are operating. Sake kasu has a distinct umami taste.
[Sakekasu from Daishichi]
Photos own work.
Japanese Food Dictionary