A method for recovering fibers from fibrous products comprising the steps of: providing a reaction solution comprising water for hydrolyzing the fibrous products; dividing the fibrous products into a plurality of batches, each batch consisting of a portion of the fibrous products; and processing the plurality of batches to recover the fibers, comprising the steps of: in a first reactor selected from a plurality of hydrothermal reactors to deposit the reaction solution and an individual batch, pressurizing the reaction solution and maintaining a temperature thereof at a temperature predetermined to form a superheated reaction solution that contains subcritical water, whereby the individual batch is hydrolyzed to yield a collection of recovered fibers; separating the collection of recovered fibers from the superheated reaction solution by filtering, whereby the superheated reaction solution is recovered; and in a next reactor selected from the plurality of hydrothermal reactors to deposit a next batch selected from the plurality of batches where the next reactor is different from the first reactor and the next batch has not been hydrolyzed, transferring the recovered superheated reaction solution to the next reactor for recycling the superheated reaction solution in hydrolyzing the next batch. providing a reaction solution comprising water for hydrolyzing the fibrous products; dividing the fibrous products into a plurality of batches, each batch consisting of a portion of the fibrous products; and processing the plurality of batches to recover the fibers, comprising the steps of: in a first reactor selected from a plurality of hydrothermal reactors to deposit the reaction solution and an individual batch, pressurizing the reaction solution and maintaining a temperature thereof at a temperature predetermined to form a superheated reaction solution that contains subcritical water, whereby the individual batch is hydrolyzed to yield a collection of recovered fibers; separating the collection of recovered fibers from the superheated reaction solution by filtering, whereby the superheated reaction solution is recovered; and in a next reactor selected from the plurality of hydrothermal reactors to deposit a next batch selected from the plurality of batches where the next reactor is different from the first reactor and the next batch has not been hydrolyzed, transferring the recovered superheated reaction solution to the next reactor for recycling the superheated reaction solution in hydrolyzing the next batch. in a first reactor selected from a plurality of hydrothermal reactors to deposit the reaction solution and an individual batch, pressurizing the reaction solution and maintaining a temperature thereof at a temperature predetermined to form a superheated reaction solution that contains subcritical water, whereby the individual batch is hydrolyzed to yield a collection of recovered fibers; separating the collection of recovered fibers from the superheated reaction solution by filtering, whereby the superheated reaction solution is recovered; and in a next reactor selected from the plurality of hydrothermal reactors to deposit a next batch selected from the plurality of batches where the next reactor is different from the first reactor and the next batch has not been hydrolyzed, transferring the recovered superheated reaction solution to the next reactor for recycling the superheated reaction solution in hydrolyzing the next batch.