During the flue gas desulphurization process of the boiler (lime stone/gypsum method), in order to maintain the balance of substances in the slurry circulation system of the desulphurization device, desulphurization wastewater is mainly the wastewater discharged from the absorption tower system to prevent the concentration of the soluble parts (i.e., chlorine) in the smoke from exceeding the regulated value and also to ensure the quality of the gypsum. It generally comes from the gypsum dehydration and cleaning system, or the overflow water hydrocyclone and filtrate from the belt filter.

 Water Quality Characteristics
The main impurities in the desulphurization wastewater are suspended substances, supersaturated sulfites, sulfates and heavy metal, among which, many are class-one pollutants required to be controlled by the national environmental protection standards. The main characteristics of the wastewater are :(1) It is mildly acidic, and it has a pH between 4～6; (2) Its content of heavy metal exceeds the standard, and it mainly contains heavy metals such as mercury, lead, nickel and zinc as well as non-metal pollutants such as arsenic and fluorine; (3) Its concentration of suspended substances is as high as (9000～12700) mg/L; (4) It also has a high concentration of chloride ion, and generally, its concentration of chloride ion is smaller than 2000mg/L.

Thought for Treatment
Desulphurization wastewater enters the regulation aeration tank, where certain CODs are removed from the wastewater, and then it enters the neutralization tank; add milk of lime into the neutralization tank, regulate the pH of the wastewater to 8.5~9.5, certain heavy metals are precipitated, and then it enters the settling tank; add organic sulfur, flocculating agent and coagulant aids into the settling tank, and further precipitate the heavy metal ions that cannot be precipitated by hydroxides. The desulphurization wastewater flows from the flocculation tank to the clarifier, and flocculates in the wastewater are settled at the bottom of the clarifier under gravity, where they are concentrated to sludge and cleaned by the mud-scraping device; clean water is lifted to the top and flows to the clean water pond through the ring triangular overflow weir, the sludge is transported by vehicles after pressure filtration, and the filtrate flows back to the regulation aeration tank for re-treatment.