Provided are: a sludge treatment system which is capable of reducing sludge without using a chemical agent such as an aqueous hypochlorous acid solution; a wastewater treatment system using the sludge treatment system; a sludge treatment method; and a wastewater treatment method. The sludge treatment system is provided with: an aeration tank (2) which serves as a biological treatment tank wherein wastewater such as sewage water or industrial wastewater that contains organic matter is subjected to organic matter decomposition by means of microorganisms; a sedimentation tank (3) for separating the mixture of water and sludge, said mixture being treated in the aeration tank (2), into sludge and treated water; and a sludge return path (4) which serves as a returning means for returning the sludge separated by the sedimentation tank (3) to the aeration tank (2), while applying a water hammer pressure to the sludge by a water hammer device (8) so as to destroy cell membranes of microorganisms in the sludge.
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