Simply I want a procedure of nitrogen purging to remove that oxygen from the container containing animal manure.

If you can pump out most of the air and replace it with N2, and repeat that several times the O2 partial pressure will be low. If you want a particular pp you can achieve that by continuous purging with the required gas composition produced using mass flow controllers

There are some ways to remove oxygen from the container such as.,

Flushing pure Nitrogen Gas into the container removes Oxygen consequently.

For the container you just need to add a layer of zinc metal powder to the container. The zinc metal powder will react the oxygen and remove the atmospheric oxygen in the container.