Abstract: Disclosed herein is a treatment unit comprising a housing and a mesh container. The housing has one or more than one inlet and one or more than one outlet. The mesh container is received in the housing and comprises a mesh surface with a plurality of holes therethrough. The mesh container is configured to receive a solid material and solution flowing into the housing through the inlet passes through the plurality of holes and contacts the solid material before flowing out of the housing through the outlet. The treatment unit may be used to produce treated biochar.