Energy-saving optimization of coking wastewater treated by aerobic bio-treatment integrating two-stage activated carbon adsorption

Contaminants removal efficiency and energy consumption are of practical importance in wastewater treatment. Herein, an energy saving system including aerobic bio-treatment combined with a two-stage adsorption (AB-2A) was proposed and applied to the treatment of coking wastewater. The adsorption before bio-treatment partially extracts energy and organic constituents from the wastewater, making the subsequent bio-treatment more effective. The adsorption after bio-treatment brings the effluent to the discharge requirements. In comparison with the biological treatment combined with subsequent ozonation (MO mode), the proposed system rationalizes the material flows in the wastewater treatment by optimized activated carbon utilization targeting higher adsorption and energy extraction efficiencies. The combined system AB-2A aimed at reducing the chemical oxygen demand (COD) to the national discharge standard at maximized net energy benefit (NEB) and benefit cost ratio (BCR). The modified cost benefit analysis was applied to energy balance evaluation of the system. Two combustible adsorbents, a commercial activated carbon (CAC) and a sludge-derived activated carbon (SAC), were used for pollutants adsorption. The results showed that the average effluent COD of the combined system AB-2A reached 78.8 mg/L when subjected to the activated carbon adsorption after biodegradation, with 7.0 g/L CAC added sequentially to the biologically treated and then to the raw coking wastewater (M2(CAC) mode). COD removal efficiency of M2(CAC) mode exceeded 96% being equal to the reference result of MO mode. Additionally, the NEB and BCR reached -11.81 kWh/m(3) and 0.80 in M2(CAC) mode, respectively. Moreover, the M2 mode applying SAC (M2(SAC) mode) in amount of 7.0 g/L demonstrated the highest energy recovery compared to MO and the separate adsorbent application to the raw influent and the bio-treated effluent (M1 mode). The difference values of NEB and BCR in M2(SAC) compared to MO were shown as big as 28.87 kWh/m(3) and 0.07, respectively. The excess sludge recycling in the wastewater treatment resulted in the positive benefit in energy saving. Summarizing, the AB-2A system using SAC appears to be high COD removal efficiency and energy saving in the future sustainable wastewater treatment. (C) 2017 Elsevier Ltd. All rights reserved.