Removal Of AOX From Dye Wastewater And Excess Activated Sludge

Dye wastewater contains a variety of halogenated organic synthetic intermediates which are difficult for bio-degradation.These halogenated organic compounds are readily adsorbed onto activated sludge in a wastewater biological treatment process,and thereby pose risks to ecosystems and human health if discharged without proper treatment.In this study,all the treatment methods were based on the inexpensive and easily obtained zero-valent iron?ZVI?.For the dye wastewater,ZVI was used as a Fenton-like catalyst to remove the AOX?adsorbable organic halogens?,and the AOX removal rate as well as the removal mechanisms were investigated.The excess activated sludge of a dye wastewater treatment plant was treated with ZVI reduction and ZVI reduction followed with Fenton oxidation,and the AOX removal rate was compared.The AOX of dye wastewater was treated with Fe/H₂O₂ Fenton-like system,the results showed that 83.37% of AOX and 68.04% of TOC?total organic carbon?was removed after reaction for 1.5 h with 3 g·L-1 of ZVI powder,0.28 mol·L-1 of H₂O₂ and without the initial pH adjustment.Meanwhile,about 50% of chroma was removed as well.After recycling for 4 times of ZVI powder,71.56% of AOX and 58.03% of TOC was removed by the Fe/H₂O₂ system.Most of the halogenated organic compounds and anilines were removed with Fe/H₂O₂ system except 2,4-Dinitrochlorobenzene,which was difficult for complete removal by direct oxidation due to its stable structure.Compared to Fe/H₂O₂ Fenton-like system,the AOX removal rate was increased 8.47% when the dye wastewater was reduced for 1.5 h and then oxidized with H₂O₂.The removal mechanisms of AOX in dye wastewater by Fe/H₂O₂ Fenton-like system was investigated.Both alone Fe and H₂O₂ systems showed low removal rate of AOX.Fe/H₂O₂ Fenton-like system could effectively degrade AOX,AOX and the intermediate products would compete for access to the ·OH,resulting in the AOX removal rate and Clrelease efficiencies becoming more different as the reaction time increased.In Fe/H₂O₂ Fenton-like system,AOX was mainly removed by the oxidation of ·OH radicals,especially the ZVI surface-bounded ·OH.Excess activated sludge of a dye wastewater treatment plant was treated with ZVI reduction,Fenton oxidation and ZVI reduction followed with Fenton oxidation.The AOX removal rate was compared,the operation conditions were optimized and the removal mechanisms were investigated.The results showed that only 24.7% of AOX was removed when the sludge was reduced for 30 d with 5 g·L-1 of ZVI powder under anaerobic environment.The removal rate of AOX was increased to 73.7% when 0.059 mol·L-1 of Fe²⁺ and 0.89 mol·L-1 of H₂O₂ was dosed for 1.5 h of Fenton oxidation.The AOX removal rate was further increased to as high as 90.3% when the sludge was reduced in an anaerobic environment for 30 d with 2 g·L-1 of ZVI powder and then oxidized with Fenton reagent.GC-MS analysis revealed that 2,6-Dichloro-4-nitroaniline was of the principal AOX component,which was efficiently removed by the combined process of ZVI reduction and Fenton oxidation.