*Discussion on Model Results*

Model results for near-field simulations in Tables 2 and 3 show that the criteria for all constituents are met except for Ptot in Case 1 and Ntot and Ptot for Case 2. With the current dilution factor for Case 1 and an ambient concentration of 0.01 mg/L, the Ptot effluent criteria could be met with an effluent concentration of 0.0135 mg/L. Ptot effluent concentration should not exceed 0.34 mg/L (instead of 0.78 mg/L) if the standard is to be met through the tidal signal. It should be noted that the criteria for Ptot could be met several hours from slack tide when stronger tidal currents exist in the outfall location.

**Figure 6.** Variation of dilution over the spring-neap tidal cycle for Case 1.

**Table 3.** Effluent constituent concentration at the end of mixing zone.


Note: The effluent TDS concentration for Case 2 is smaller than the ambient concentration.

For Ntot, the effluent criteria would be met in Case 1. However, for Case 2, the high effluent concentration (35 mg/L) would exceed the mixing zone criteria for some tidal phases. Ntot effluent concentration should not exceed 8 mg/L if the standard is to be met through the tidal cycle.

Delft3D model simulations were performed with CORMIX results as input to the model for design verification in the far field. The far-field Delft3D model employs a nested rectilinear grid model with horizontal grid spacing varying as 180 m, 60 m and 20 m. The fine mesh grid covers the diffuser region with the effluent concentrations from CORMIX lumped over one Delft3D grid to represent the near field mixing. A sigma grid vertical coordinate system is selected for the model with the coarse, medium and fine grid domains represented with 1, 3 and 6 layers, respectively. The model results show that, in general, the impact of the effluent discharge on the intake is insignificant for all cases considered. For the high TDS case, the dredged bathymetry appears to guide the effluent toward the intake area. The time-series of the TDS concentration shows above ambient at the intake area; however, the maximum above ambient concentration is shown to be far below the near-field criteria for the TDS constituent. As noted before, the far-field model only provides a confirmation of the near filed mixing analysis, which governed the diffuser design including the type, size and number of diffuser ports, port discharge angle and spacing.

It should be noted that the approach to combine the results of near field simulations into the far field model is not dynamic and treated as one-way offline input. Changes in ambient conditions as a result of the near field mixing after each tidal phase are not accounted for in the far field model. Also, quasi transient treatment of the tidal cycle in the near field model only to a limited extent accounts for the accumulation of constituent concentration during tidal reversal [10]. The design of the diffuser system therefore is dependent on the conservatively selected dilution factor in the near-field model. Also, after each tidal phase the near-field model did not update he ambient flow field from the far-field model results. Therefore, accumulation of concentration built-up during tidal reversal over several tidal cycles has not been accurately modeled in the in the near-field CORMIX model.

#### **6. Conclusions**

This paper describes typical wastewater process streams from an LNG plant and presents a diffuser system design case study in a meso-tidal coast to meet the effluent mixing zone criteria. CORMIX mixing model is employed to evaluate the near field mixing process of the combined plant effluent in marine environment. Three dimensional Delft3D model is used for the far field mixing only as a confirmatory analysis taking input from the near-field model. The neap-spring tidal cycle is discretized on a quasi-steady approach to evaluate dilution in the Cormix model. A staged fanned diffuser with 36 ports and 2.5 m spacing was used to achieve significant dilution within the mixing zone boundary. The length of the diffuser is 92.5 m.

The dilution factor at the boundary of the mixing zone was obtained by taken the lowest possible dilution around various slack tide conditions for neap and spring tides from the CORMIX simulations. The analysis showed that although not all effluent constituents would meet the regulatory mixing zone criteria for all tidal phases for the selected diffuser design and marine environment, the dilution criteria would be met for all constituents for most tidal phases. Also, the large tidal range and persistent surface wind govern the conditions for the diffuser design.

#### **Author Contributions**

Conceived and developed concepts: KTE MAS. Performed model simulations and analyzed results: KTE MAS. Wrote the paper: MAS KTE.

#### **Conflicts of Interest**

The authors declare no conflict of interest.

#### **References**

1. Sankey, P.; Hermann, L.; Clark, D.T.; Micheloto, S.; Nip, W. *Gorgon & the Global LNG Monster, Deutsche Bank Markets Research: Global LNG*; Deutsche Bank Securities Inc.: New York, NY, USA, 2012; p. 82.

