Mixing Criteria

The criteria for effluent constituents are based on regulatory requirements at the edge of the defined mixing zone (70 m from the outfall location). The TDS and salinity values are typically correlated and the mixing zone criterion adopted was +5 percent of ambient for both. The criterion for TDS was then estimated as 39,480 mg/L (+5 percent of 37,600 mg/L). As the background concentrations of Ntot are above the regulatory guideline of 0.1 mg/L, the 80th percentile of the background concentrations was used as the site specific standard, which was estimate to be 0.187 mg/L. The Ptot, TSS and AL+ criteria were taken as 0.015 mg/L, 8 mg/L and 0.32 mg/L, respectively, following regulatory guidelines.

The geometric characteristics of the diffuser includes a diffuser length of 92.5 m with an orientation of 90° to current direction, 36 Duckbill valve type ports each with an area of 0.00082 m2 with equivalent diameter of 0.03231 m at 45° vertical angle from horizontal. A spacing of 2.5 m between ports is assumed with the port height of 1.0 m above seabed. The seabed slope at the diffuser is assumed horizontal. Based on the flow rate (750 m3 /h) and port opening (0.00082 m2 ), the exit velocity at the port/duckbill valve is 7.16 m/s.

#### *4.4. Model Simulation Cases*

Two cases of effluent composition are evaluated, as can be seen in Table 1. Case 1 represents effluent discharge condition with high TDS and Case 2 with low TDS magnitudes. The two cases represent various stages of the plant construction and operation to bind the expected range of effluent properties and expected environmental impacts. A number of CORMIX simulations were performed for the two cases to arrive at a suitable diffuser design meeting the following restrictions:



**Table 1.** Outfall discharge evaluation cases.

Slack tide is typically the most conservative conditions for dilution analysis as ambient velocity becomes at or near stagnation conditions producing the lowest dilutions.
