Study On The Adsorption Performance Of Activated Carbon On Dissolved Organic Matter In Secondary Effluent Of Urban Wastewater Treatment Plants

Based on the demand for water environment improvement and recycled water reuse,the discharge standards of China’s urban wastewater treatment plants are becoming increasingly stringent,with many areas requiring the implementation of the surface water environmental quality standard（GB3838-2002）quasi-IV standard for effluent quality.Therefore,many wastewater treatment plants need to be upgraded,of which further reduction of chemical oxygen demand（COD）is one of the key points and difficulties.After the secondary biochemical treatment of municipal wastewater,the remaining COD is mostly dissolved and hard to degrade organic matter,which is difficult to be removed by traditional coagulation and sedimentation and filtration processes.Activated carbon adsorption process can effectively remove dissolved organic matter in water,which has been widely used in drinking water treatment,but is less applied in the field of municipal wastewater deep treatment,and its removal characteristics of dissolved organic matter in secondary effluent need further study.Therefore,in this study,the adsorption performance and influencing factors of granular activated carbon on dissolved organic compounds were investigated in the secondary effluent of municipal wastewater plants,and the modification methods of granular activated carbon were explored to enhance its adsorption effect on dissolved organic compounds.Firstly,the removal effect of granular activated carbon（without modified treatment）on secondary dissolved organic matter in municipal wastewater treatment plants was investigated by static tests.The results showed that when the initial concentration of TOC is 12.98 mg/L,the initial content of UV₂₅₄ is 0.1641 cm^-1,and the dosage is 0.8 g/L,the removal rates of TOC and UV₂₅₄ by granular activated carbon were 52.81%and77.30%,respectively,with good removal effects.The effects of adsorbent dosage,activated carbon particle size,p H and temperature on the adsorption performance of activated carbon were investigated,and the optimal adsorption conditions were determined as follows:adsorbent dosage of 0.6～0.8 g/L,activated carbon particle size of 24～28 mesh,p H of 5～6 and temperature of 20±0.5℃.The kinetic study showed that the adsorption process of dissolved organic matter in secondary effluent by granular activated carbon is suitable to be described using the pseudo-second-order kinetics model indicating that the adsorption of dissolved organic matter in secondary effluent by activated carbon is dominated by chemisorption.In addition,Langmuir and Freundlich adsorption isotherms were used to fit the adsorption process,and the results showed that the Freundlich adsorption isotherm was a better fit,indicating that the adsorption of dissolved organic compounds by activated carbon was multilayer adsorption.Subsequently,two surface oxidation（nitric acid modification and potassium permanganate modification）,two surface reduction（Na OH modification and KOH modification）and two loading metal（iron loading modification and copper loading modification）modification methods were used to modify the granular activated carbon,and the adsorption performance of the six modified activated carbons on dissolved organic compounds in the secondary effluent was tested.The results showed that the best adsorption performance of the iron-loaded modified Fe–AC was achieved with 62.34%and 86.80%removal of TOC and UV₂₅₄,respectively,when the adsorbent was dosed at0.7 g/L.It was characterized that Fe-AC was successfully loaded with iron oxide active substance（Fe₂O₃）,and the total specific surface area was increased by 29.3%compared with the unmodified activated carbon,and the adsorbent surface contained a large amount of oxygen-containing functional groups.Three-dimensional fluorescence spectroscopy and UV–Vis spectroscopy analysis showed that Fe–AC had a good effect on the removal of humus-like macromolecular organic matter from the secondary effluent.The kinetic study showed that the adsorption process of dissolved organic matter in secondary effluent by Fe–AC is suitable to be described by the pseudo-second-order kinetics model,which indicates that the adsorption process is dominated by chemisorption.Finally,the optimal adsorption conditions for Fe–AC were determined:the p H=6,the temperature was 35±0.5°C,and the adsorbent dosing was 0.7 g/L.This study shows that granular activated carbon has good adsorption performance for dissolved organic matter in secondary effluent of urban wastewater treatment plants,and the adsorption performance of activated carbon can be further improved after modified by loaded iron.Therefore,activated carbon adsorption is an effective method for deep treatment of municipal wastewater treatment plants.