To provide a method for cleaning barn effluent whereby a large amount of effluent can be treated; humic substances can be oxidatively decomposed economically; and the concentration of trihalometanes as a by-product is kept low enough to meet the effluent standard by adding a chloride and causing electricity to flow through while the pH is being adjusted to a specified value with an alkali agent when decomposing organic substances contained in an effluent formed by the activated sludge treatment of a barn sewage.

A barn effluent 2 after an activated sludge treatment is transferred to a filtration vessel 4 to be adsorbed by a filtering material 5 and then is put into an electrolysis vessel 6 and a reaction vessel 13. An aqueous halogen compound solution, e.g. an aqueous common salt solution, from a chemical liquid vessel 9, is injected into the effluent 2 to be treated; some alkali agent is added to an aqueous solution adjusted so that the concentration of common salt is kept higher than 3% to prevent an electrode from being damaged, thus adjusting the pH to 6-7; and a rectifier 12 is switched on. After 30 min of electrolysis, an electrolytic solution 8 is injected into 10-50 times as much effluent 2, which is transferred from the filtration vessel 4 and stored in a reaction vessel 13, and is stirred with a stirrer 17 to be homogeneously mixed. Sodium hypochlorite synthesized in the electrolysis vessel 6 emits nascent oxygen as an oxidizing agent to oxidatively decolor huminic substances, a coloring component.