[A-OS21] Coastal ocean circulation and material cycle

[AOS21-12] Impact of Effluent Density Adjustment through Seawater Mixing Operations at a Sewage Treatment Plant on Coastal Transport Dynamics

*Yusuke Uchiyama¹, Yuta Asai², Ryo Masutani³ (1.Graduate School of Engineering, Kobe University, 2.School of Environment and Society, Institute of Science Tokyo, 3.Osaka Metro)

Keywords:Sewage treatment plant, Effluent oceanic dispersal, Eulerian advection-dispersion model, ROMS

A sewage treatment plant located along the Kobe in Osaka Bay, Japan, initiated a seawater mixing operation in 2021 to mitigate the impact of treated effluent on an adjacent seaweed farm. This study aims to quantitatively evaluate the effects of the seawater mixing operation on effluent dispersal and its subsequent impact on the seaweed farm using a quadruple-nested, 3D ocean circulation model coupled with a tracer advection-dispersion model. Numerical experiments were conducted for two scenarios: one representing normal operation with freshwater effluent only (no mixing), and the other with the effluent mixed with seawater at a 1:1 ratio. Results indicate that the reduced buoyancy and doubled volume flux caused by the mixing operation alter the near-field flow field, which subsequently induces detectable changes in far-field temperature, salinity, effluent tracer concentrations, and 3D residual currents. These changes also affect the flow and tracer around the seaweed farm located near the eastern shore of the plant. A tracer flux budget analysis in the vicinity of the farm revealed that the seawater mixing operation reduces the accumulation of effluent from the plant within the farm area by approximately 12%.