To convert sulfate to sulfide and simply and inexpensively recover org. substances as heat capacity by mixing a waste liq. of photographic development with a digestion waste liq. in a Kraft pulp manufacturing process and performing burning processing of it in a Kraft recovery boiler.

When a waste liq. of photographic development contg. fixing renewal liq. discharged from sensitized material and printing industries is processed, the waste liq. of photographic development is mixed with a black liq. (a waste liq. of digestion) in the Kraft pulp manufacturing process and the mixture is burnt in a Kraft recovery boiler. This mixing is pref. performed in a filtrate tank of a washer for separating and cleaning the pulp and the black liq. after digestion or a diluted black liq. tank for storing the black liq. before concentration in a vacuum evaporation can or a mixing tank wherein the black liq. concentrated in the vacuum evaporation can and fume such as sodium sulfate produced from a recovery boiler. In addition, in this case, to minimize deviation of loading to the recovery boiler, as a place where the waste liq. of photographic development is mixed, a more preceding process than the vacuum evaporation can is set.