^aThe first flush was treated with the upflow clarifying filter using chemicals.

^bThe previous treatment systems were replaced with the efficient technologies and equipment for reducing the energy consumption (Seoul 2021).

^cConsidering the results of EQPS (simulation of the treatment performance according to the daily maximum inflow that meets the effluent standards in 2040), the capacity of the bioreactors must be increased. To increase the concentration of MLSS in the bioreactors, Gould-type sludge collectors in the secondary sedimentation systems were replaced, and a bypass line from the primary clarifier was constructed. The capacity (2,800–3,500 mg/L) of the MLSS could be increased to 4,000 mg/L (Seoul 2021).

^dTo increase the removal efficiency of nitrogen, a four-stage BNR process was introduced in plant #3 (Seoul 2021). In addition, by introducing an advanced treatment process at the end of the secondary sedimentation, micro-pollutants such as CECs, PPCPs, EDCs, ARBs, and ARGs could be managed. Particle pollutants such as microplastics could be treated by fine filtration, and the other micro-pollutants could be treated by the ozone and activated carbon processes (Seoul 2021).

^aThe sludge treatment systems consists of three plants. Plant #1 of the sludge systems treats the sludge from plants #1-1 and #1-2 in the wastewater system.

^bThirty-five percent of sludge in the WWTP Seo-Nam is disposed of at a drying facility and coal-fired power plants, 22% is incinerated, and 43% is landfilled and treated via consignment. To independently and sustainably dispose of the waste sludge, the municipality planned to expand the drying facilities. Notably, the use of sludge pellets fuel must be prohibited in coal-fired power plants because its ash produces fine and ultra-fine particulate matter leading to air pollution.

^cThe old motors in the thickening and dewatering processes were changed, reducing electricity consumption by 46% (Seoul 2021).

^dTo increase the biogas and energy yields, two-phase anaerobic digestion systems were introduced. A mechanical stirrer was used to replace the existing gaseous stirrer to improve biogas production by 10% (Seoul 2021). Sludge solubilization with an ultrasonic equipment was introduced to increase the sludge treatment and biogas production by 8% (Seoul 2021).

^eThe previous sludge treatment scheme was retained in this alternative, except the sludge incinerator was expanded to limit the operation of drying facilities producing sludge pellet fuel.

^fThe two-step anaerobic digestion system was undesirable because of its low biogas yield. To address this problem, the two-phase anaerobic digestion system was introduced in plant #2 (Seoul 2021). To address the issues related with the sludge drying facilities and pellet fuel, the capacity of the sludge incinerator was expanded.