Process for thermal hydrolysis of sludge, characterised in that it is carried out in at least three reactors operating in parallel in each of which sludge is subjected to a complete thermal hydrolysis cycle, the said cycle comprising steps consisting of feeding the said sludge into on of said reactors, injecting live steam into it to bring it to a pressure P and a temperature T to enable hydrolysis, and keeping the sludge at the said pressure P and the said temperature T for a given time, and then suddenly returning the said sludge to atmospheric pressure releasing flash steam, and emptying the said sludge thus hydrolysed, from the reactor, and in that it consists of offsetting the time of the said cycle in the different reactors to that flash steam produced from one reactor can be injected into another one of said reactors, the said live steam being continuously injected alternately in the said three reactors and characterised in that it includes three phases: - a first phase during which: • sludge is fed into a first reactor and the flash steam originating from a third reactor is injected into the said first reactor, • live steam is injected into a second reactor containing sludge, • a third reactor containing sludge is kept at the hydrolysis temperature and pressure and is then returned to atmospheric pressure, the emitted flash steam being transferred into the said first reactor just before it is emptied; - a second phase during which: • live steam is injected into the said first reactor, • the second reactor is kept at the hydrolysis temperature and pressure and is then returned to atmospheric pressure, the emitted flash steam being transferred into the said third reactor just before it is emptied, • the sludge is fed into the said third reactor and the flash steam from the second reactor is injected into the said third reactor; - a third phase during which: • the first reactor is kept at the hydrolysis temperature and pressure and is then returned to atmospheric pressure, the emitted flash steam then being transferred into the said second reactor just before it is emptied, • sludge is fed into the said second reactor and flash steam originating from the first reactor is injected into the said second reactor, • live steam is injected into the said third reactéor. - a first phase during which: • sludge is fed into a first reactor and the flash steam originating from a third reactor is injected into the said first reactor, • live steam is injected into a second reactor containing sludge, • a third reactor containing sludge is kept at the hydrolysis temperature and pressure and is then returned to atmospheric pressure, the emitted flash steam being transferred into the said first reactor just before it is emptied; • sludge is fed into a first reactor and the flash steam originating from a third reactor is injected into the said first reactor, • live steam is injected into a second reactor containing sludge, • a third reactor containing sludge is kept at the hydrolysis temperature and pressure and is then returned to atmospheric pressure, the emitted flash steam being transferred into the said first reactor just before it is emptied; - a second phase during which: • live steam is injected into the said first reactor, • the second reactor is kept at the hydrolysis temperature and pressure and is then returned to atmospheric pressure, the emitted flash steam being transferred into the said third reactor just before it is emptied, • the sludge is fed into the said third reactor and the flash steam from the second reactor is injected into the said third reactor; • live steam is injected into the said first reactor, • the second reactor is kept at the hydrolysis temperature and pressure and is then returned to atmospheric pressure, the emitted flash steam being transferred into the said third reactor just before it is emptied, • the sludge is fed into the said third reactor and the flash steam from the second reactor is injected into the said third reactor; - a third phase during which: • the first reactor is kept at the hydrolysis temperature and pressure and is then returned to atmospheric pressure, the emitted flash steam then being transferred into the said second reactor just before it is emptied, • sludge is fed into the said second reactor and flash steam originating from the first reactor is injected into the said second reactor, • live steam is injected into the said third reactéor. • the first reactor is kept at the hydrolysis temperature and pressure and is then returned to atmospheric pressure, the emitted flash steam then being transferred into the said second reactor just before it is emptied, • sludge is fed into the said second reactor and flash steam originating from the first reactor is injected into the said second reactor, • live steam is injected into the said third reactéor.