On relaxation of the influences of treated sewage effluent on an adjacent seaweedfarm in a tidal strait
Introduction
Osaka Bay (OB) suffers from the anthropogenic loads from the densely populated hinterland, even though the water
quality has been improved since 1980s. Tarumi Sewage Treatment Plant (TSTP) is a major wastewater treatment plantin OB near the narrow Akashi Strait (AS), where energetic tidal currents occur. The area around TSTP is well known forseaweed farming, in which the spores are quite vulnerable to density and nutrient conditions during the hatch in fall.
The local government (Kobe City) has constructed a new western outfall in TSTP aimed at seasonal diversions of theeffluent release in fall to reduce possible impact on the farm upon the claims from the local fishery. A previous study(Uchiyama et al., 2018) concluded the western diversion operation successfully decreased the accumulation of TSTP
effluents in the farm by on average ~28%, while the instantaneous effluent concentration was reduced to ~50%.
However, the farmers are still demanding that the local government should further diminish TSTP influence. Therefore,
in the present study, we conduct several high-resolution numerical model experiments with different operation scenarios
suggested by the local government to provide qualitative engineering guidelines on the alternative discharge operations
optimized for reducing TSTP influence on the seaweed farm.
Methodology
The assessment is performed using a quadruple nested JCOPE2-ROMS oceanic downscaling model in a high-resolution
configuration (Fig. 1). The lateral grid size refinement occurs from ~10 km (JCOPE2) to 2 km (ROMS-L1), 600 m (L2),
100 m (L3), and further down to 20 m (L4). We take into account the surface momentum (wind), heat, and freshwater
fluxes, tides, and major river discharge, etc, as realistically as possible. The sewage effluent is implemented in theROMS-L4 model as an additional point source and 3D Eulerian passive tracer model of Uchiyama et al. (2014, 2018).
The sewage effluent is applied to the coastal area near TSTP as a bottom-released freshwater plume at the volume rateof 180,000 m3 d-1 for the baseline case (case 1) mimicking the standard operation. The first alternative (case 2) is simply
a reduced discharged volume rate from 1.8 × 105 m3 d-1 to 1.5 × 105 m3 d-1. In the second alternative (case 3), theeffluent density is intentionally increased by mixing with 0.8 × 105 m3 d-1 of the ambient seawater to suppress thebuoyant plume from its immediate surfacing during the initial dilution.
Results and Discussion
The surface residual currents in Fig. 2a represent that strong tidal currents intrude into OB through AS. A pair ofclockwise and counterclockwise rotating gyres are formed near AS; the former is in the south whereas the latter is justoff TSTP. Two selected enlarged regions shown in Fig. 2b-c demonstrate the influences of man-made marine
constructions in the red dotted boxes labeled with z1 and z2 in Fig. 2a. The complex residual current appears around thefishing pier (Fig. 2b). The breakwater provokes a bifurcation of the westward coastal residual current with noticeable
velocity weakening (Fig. 2c). The time-averaged non-dimensional tracer (effluent) concentration c (color) and theseawater density (contours) at the surface are depicted in Fig. 3. Consistent with the residual current field (Fig. 2a), forthe baseline case shown in Fig. 3a, the mean effluent concentration c is mostly distributed in the alongshore direction
near TSTP. Similar to the c distribution, slight decreases in the surface density occur because of the fresh effluent
discharge from the western diversion outfall. The effluent concentration and density surface deviations observed forCase 2 are generally consistent although with opposite signs (Fig. 3b). The surface density ρ slightly increases around
TSTP due to the reduced fresh effluent, while c is diminished. By contrast, the adjusted effluent density with theunchanged total tracer flux (Case 3) does not result in obvious changes in ρ and c (Fig. 3c). Fig. 4a shows the net
cumulative tracer in the seaweed farm for three cases. Fig. 4b investigates that the released flux reduction of 16.7% (1.8× 105 m3d-1 à 1.5 × 105 m3d-1) at the diversion outfall in Case 2 results in a 25.4% reduction (2.01 × 104 m3 à 1.50 ×104 m3) in the time-averaged effluent accumulation in the farm relative to Case 1 (black curve), while the reduction inCase 3 is subtle (ΔQ31 ~ 0, red curve). Although Case 3 was believed to be a favorable operation option claimed by thegovernment and seaweed farmers, the model solidly declines it and suggests simply reducing the discharge.

Figure 1. (a) ROMS-L1-L4
domains. (b) ROMS-L4
domain with bathymetry incolor (m). The blue box isthe seaweed farm area;
red star is the location ofTSTP; the red dots are thetwo outfalls (W: western
diversion, C: central).

Figure 2. (a) Time-averaged surface velocity

vectors (white arrows) on their own
magnitude (color). (b) Same as (a) but for theenlarged region shown by the red dotted box(z1) in Figure 2a. (c) Same as (a) but for theenlarged region shown by the red dotted box(z2) in Figure 2a. The red solid box shows theseaweed farm area.
cumulative tracer (m3)c (color, non-dimensional) at the surface for the baseline
case (Case 1). The black contours denote the sea surfacedensity at 0.02 kg m-3 intervals. (b) The differences in c(color, non-dimensional) and density (contours) between
Case 2 and Case 1 (Case 2 – Case 1). The red and blue
contours are positive and negative values (intervals:0.001 kg m-3). (c) Same as (b), but for Case 3 – Case 1.

(a)48Figure 3. (a) Time-averaged passive tracer concentration,x 10

Q1

6Q2(b)

4Q3

1x 10

∆Q21

∆Q31

042-10-2
21/Sep1/Oct

11/Oct21/Oct-2
31/Oct 21/Sep

1/Oct11/Oct21/Oct31/Oct

Figure 4. (a) Time series of the temporally cumulative net tracer fluxes stored in the seaweed farm area,Qfarm (m3). (b) The temporal changes in the Qfarm (m3) differences for each case. The black curve ΔQ21 isfor Case 2 – Case 1, while the red curve ΔQ31 is for Case 3 – Case 1.

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Manuscript information
Name of the Journal: Marine Pollution Bulletin
Volume and page: Volume 144, pp. 265-274
Accepted date or planned submission date: April 2019
DOI: 10.1016/j.marpolbul.2019.04.050

Title
On relaxation of the influences of treated sewage effluent on an adjacent seaweed farm in a tidal strait

Authors and affiliation
Xu Zhang a, Yusuke Uchiyama a, b, and Akihiko Nakayama ca Department of Civil Engineering, Kobe University, Kobe, Japan, 167t142t@stu.kobe-u.ac.jp
b Coastal and Estuarine Environmental Department, Port and Airport Research Institute, Yokosuka, Japanc Department of Environmental Engineering, Universiti Tunku Abdul Rahman, Kampar, Perak, Malaysia