NOVELTY - Flue gas is desulfurized by contacting with absorption solution. Carbon source is added to the resultant to reduce sulfite and sulfate ion into sulfide and sulfureted hydrogen. The sulfide and sulfureted hydrogen are oxidized into elemental sulfur by sulfur bacteria. The oxidized wastewater is separated from the active sludge and recycled. The concentrated solution is subjected to solid-liquid separation and filtration. After filtration, a sand layer is scrapped off from filter surface, extracted and distilled using carbon bisulfide, to obtain high-purity elemental sulfur.    USE - Flue gas alkali-free adsorption biological desulfurization process.    ADVANTAGE - The flue gas desulfurization process is carried out economically with no desulfurized byproducts.    DETAILED DESCRIPTION - Desulphurization of flue gas is performed by supplying the to-be-desulfurized flue gas into an absorption tower through a pipeline, contacting with absorption solution under a pressure of 3 Kg/cm2, and spraying from a non-blocking nozzle, where pH of the absorption solution is maintained at 7-8, such that sulfur dioxide and sulfur trioxide in the to-be-desulfurized flue gas are adsorbed by the absorption solution. The resulting absorption solution is discharged to form desulfurized wastewater which is supplied into the sloping-plate sedimentation tank with surface loading of 0.5 m3/(m2.hour) and then supplied into a regulating reservoir whose volume is capable of accommodating desulfurized wastewater collected in five days. The regulating reservoir is divided into ten small regulating reservoirs having inflow-pushing structures so as to decrease the temperature of desulfurized wastewater gradually, and each small regulating reservoir is equipped with a sewage pump. Carbon source is added to the desulfurized wastewater such that the ratio of chemical oxygen demand (COD) to the sum of sulfite ion and sulfate ion in the desulfurized wastewater is 2 or more. The carbon source is starch, glucose, ethanol, methanol, acetic acid, molasses and acid-ferment-extract of household garbage. The desulfurized wastewater is then supplied into up flow anaerobic sludge bed reactor at 35-40 degrees C by lifting the pump, where wastewater has an alkalinity of 1500-2500 mg/L, pH of 7.5-8, COD volume load of 6-8 KgCOD/(m3.day) and sulfate ion and sulfite ion load of 3-4 KgCOD/(m3.day). The sulfite and sulfate ion are reduced into sulfide and sulfureted hydrogen by action of carbon. The wastewater discharged from the up flow anaerobic sludge bed reactor is supplied into the intermediate sedimentation tank, and then into the contact oxidation pond under the conditions including temperature of 22-30 degrees C, pH of 7.5-8, sulfide load of 9-12 Kg/(m3.day) and hydrogen sulfide load of 4-6 Kg/(m3.day). The sulfide and sulfureted hydrogen are oxidized into elemental sulfur by action of aerobic microbial colorless sulfur bacteria. The wastewater flowed from the contact oxidation pond is separated from the active sludge so as to supply the wastewater flowed from the secondary sedimentation tank into the ultrafiltration concentrated tank, and water is extracted by curtain-type ultrafiltration membrane. The extracted water is supplied into the cycle water tank to get recycled in the absorption tower continuously. The concentrated solution in the ultrafiltration concentrated tank is supplied into a slow filter chamber for solid-liquid separation by the lifting of pump, where the filtering velocity is 0.14-0.27 m/hour, and the filtered water is returned back to the cycle water tank for recycling. After filtration, a sand layer of 5-6 cm is scrapped off from the surface of the slow filter chamber for trapping elemental sulfur particles, the mixture of sand elemental sulfur is extracted and distilled by carbon bisulfide, to obtain high-purity elemental sulfur, in which the extractant carbon bisulfide is recycled.