What is spent slurry and its importance?

Slurry is the organic matter that is used in the production of  bio-gas.  Spent slurry is the slurry that has been used to produce bio-gas.

Kindly refer to the following link for uses of spent slurry:

https://www.meritnation.com/ask-answer/question/what-is-the-use-of-spent-slurry-in-a-biogas-plant/science/2809660