| The chemical precipitation and flocculation were used to treat Dual-Alkali Flue Gas Desulfurization wastewater in this paper which was derived from the national 863 program "Large and medium-sized industry boiler FGD technology and equipment industrialization".It took neutralization, sulfide precipitation, chitosan and abio-macromolecule flocculant to treat Dual-Alkali Flue Gas Desulfurization wastewater and other wastewater such as single heavy metal wastewater, mixed heavy metal wastewater, simulated wastewater which was manually prepared.Compared of the efficiency of treating heavy metals in FGD wastewater with NaOH, Ca(OH)2 and Mg(OH)2. It indicated that NaOH was much efficient for precipitated waste water; Ca(OH)2 was much efficient for unprecipitated waste water, the removal efficiency reached 98.4%. With the increasing of the main ion of Ca2+, Cl- in FGD wastewater, the efficiency decreased. It decreased 60% for the removing of Mn and 10% for Zn, Cd and Ni. The rectification coefficient token the Ksp of practical wastewater because it quite different with theoretic Ksp at different pH; the theoretic and practice addition were proportioned and it commonly reached 1:300.The precipitation efficiency of Na2S at different pH and the effect of Ca2+ was determined. It indicated that with the pH increased the removal efficiency increased and it reached 99.3%. Ca2+ and CY in FGD waste water decreased the precipitation efficiency. The proportion between theoretic and practice addition was changed with pH and the general ratio was 1:5.The flocculation efficiency of chitosan at different pH and the effect of Ca2+ was estimated. It indicate that the removal efficiency of heavy metals increased with the pH increasing by chitosan; the efficiency sequence was ηZn>ηNi> ηCd >ηMn The removal efficiency was 99.6%, 99.1 % , 98.7 % and 96.2%. With the increasing of Ca2+and Cl- the efficiency decreased. It decreased 5% for each addition of 2.77g/l CaCl2.Compared of the efficiency of treating heavy metals in FGD wastewater with PFS and PAC. The PFS was super to PAC. When the dosage of PFS was 20mg/l, the sedimentation time was 30mins then the removal efficiency of SS and S2+ reached 90% and 85%.With the increased of ORP the concentration of heavy metals decreased. When the ORP≥ 120mv, the removal efficiency was above 99.5%. Thus, it obtained heavy metals concentration by monitor ORP simply.Compared with the flows of Na2S precipitation -Flocuulation -Flocculation , Neutralization-Na2S precipitation. Neutralization- Na2S precipitation-Flocuulation and Na2S precipitation -Flocuulation based on the research of neutralization, Na2S precipitation and flocuulation. It indicated that flow 1 and flow 3 treated FGD wasterwater economically and effectively and reached the national wastewater discharge standard.
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