Abstract: A submersible pump and filter unit for pumping and filtering acids comprises a removable pump cartridge (13) and filter unit (14) housed in respective cylindrical cavities (11, 12) in a housing (10). The pump cartridge (13) is a gas-driven double acting bellows unit with a cylindrical body (13A), the bellows (21, 22) and the body (13A) being formed of PTFE. Each bellows (21, 22) is at a respective end of a reciprocating rod (36) which is journaled coaxially in the body (13A) and which extends through a central aperture (16) in the body (13A). Control gear including a logic control valve (41) is housed in the aperture (16) and is thereby encased in PTFE structure and shielded from acid.