Patent ID: 7264733

Claim:
A method of treating heavy-metal-containing wastewater using a sulfidizing agent, the method comprising the steps of: adding the sulfidizing agent to the heavy-metal-containing wastewater to produce a heavy metal sulfide precipitate at a pH not exceeding 8, while detecting a concentration of hydrogen sulfide gas generated from the wastewater; regulating the amount of sulfidizing agent that is added to the wastewater based upon the detected concentration of hydrogen sulfide gas such that a trace amount of hydrogen sulfide gas is generated, the trace amount of hydrogen sulfide gas corresponding to a detected concentration of hydrogen sulfide gas that exceeds 0 ppm but is not more than a hydrogen sulfide gas concentration corresponding to a liquid reaction endpoint for the heavy metal, the generation of the trace amount of hydrogen sulfide gas indicating a state in which there is a very slight excess of the sulfidizing agent in the wastewater and signifying an endpoint of a reaction between heavy metal ions and sulfide ions in the wastewater; and the regulation of the amount of sulfidizing agent that is added to the wastewater further including ceasing to add the sulfidizing agent when the endpoint of the reaction between the heavy metal ions and sulfide ions is reached; whereby the heavy metal ions are sufficiently removed from the wastewater, generation of excessive hydrogen sulfide gas is suppressed, and redissolution of heavy metal sulfide precipitate caused by polysulfide production, which hampers solid-liquid separation, is prevented.