Treatment of flue gas desulfurization wastewater by a coupled precipitation-ultrafiltration process

Flue gas desulfurization (FGD) wastewater is characterized by low pH, high content of hardness ions and COD. In this work, chemical precipitation and ultrafiltration (UF) membrane separation were corporately coupled to treat the FGD wastewater on a lab scale to remove the hardness as well as some organic compounds for further advanced treatment. Firstly, influential factors for precipitation to deal with the simulated wastewater, including reaction time, stirring speed, and dosage, were investigated by batch tests, while CaO and Na2CO3 were used as the first and second precipitants. The batch results showed that the removal rates of Mg2+ and Ca2+ could be both higher than 98% under the optimal reaction condition. Then a polyvinylidene fluoride tubular UF membrane was used to remove the precipitates without adding flocculants. Thirdly, the coupled process with the optimal operation parameters was adopted to treat real FGD wastewater. Almost all the hardness ions and most COD could be removed, while rapid permeability decline was observed in the UF process due to the high COD, which could be recovered by a simple cleaning. In summary, when the mass ratios of [CaO]/[Mg2+] and [Na2CO3]/[Ca2+] were 2.5 and 2.7, the flow velocity was around 5 m/s, the removal rates of the hardness ions can be above 98% and the membrane flux can be recovered by 98% with a backwash with pure water and an HCl solution of pH = 2. This work proved that the coupled process is a technically feasible technology for treating FGD wastewater to realize the ZLD. •Two-step precipitation and UF separation are coupled to treat the FGD wastewater.•Influential factors for precipitation and the UF process are investigated.•The removal rates of the hardness ions can be above 98%.•The membrane flux can be recovered by 98% with the cleaning method.