Brine Dilution Analysis for DeepWater Desal, LLC, Monterey Bay Regional Water Project at Moss Landing, CA

Transcription

1 Brine Dilution Analysis for DeepWater Desal, LLC, Monterey Bay Regional Water Project at Moss Landing, CA by Scott A. Jenkins, Ph.D. Deep Water Desal s Science Symposium 16 June 2014

2 Discharge Compliance Issues Present Standard: Requirement III.C.4(b) of the California Ocean Plan * dilute to chronic toxicity threshold within 1000 ft. from discharge (zone of initial dilution) * chronic toxicity occurs at 20% over ambient salinity for locally relevant species Proposed Future Standard : The 5% Rule * dilute to 5% over ambient ocean salinity within 100 m from discharge (regulatory mixing zone) The 5% Rule is more stringent

3 Ambient Ocean Salinity in Monterey Bay varies by as much as 8%. The SWRO plant at the MBRWP will operate at 48% maximum recovery and produce brine having an average end-of-pipe salinity of ppt. The brine will be heated to a delta-t =20 0 C over ambient ocean temperature because the source water is used to cool a data center before entering the SWRO plant. Positive buoyancy of this heated water is diminished by the addition of brine. The equation of state teaches that it takes a delta-t = +4 o C of discharge temperature to offset buoyancy effects of 1 ppt of salinity increase due to brine. Therefore the combined effluent will be heavier than the ambient ocean receiving waters.

4 Two Discharge Sites Evaluated offshore of the proposed Tenera intake UTM coordinates: N, E: 1) Deep Water 35 m of local water depth UTM coordinates: N, E 2) Intermediate 25 m of local water depth UTM coordinates: N, E.

5 Diffuser Design: Five 10 inch discharge risers at 10 m spacing will extend above the seabed from a 36 inch manifold pipe. Each riser pipe is fitted with a Tideflex duckbill nozzle. Five such nozzles will discharge a maximum of 5.45 mgd each of brine, or an ultimate maximum total discharge of mgd of brine at ppt end-of-pipe salinity.

6 Hydrodynamic simulation of outflow and entrainment regions in the nearfield of one of 5 ea. diffuser and risers that comprise the MBRWP linear diffuser system.

7 Period of record of historic Ocean mixing variables at Moss Landing, as used in time-stepped simulations of brine dilution at the MBRWP project discharge sites. Three distinct synoptic current regimes: a) Up-welling Period b) Relaxation Period c) Davidson Current Period

8 Canonical Circulation Pattern near Monterey Bay during the Upwelling Period

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10 MBRWP Worst Case at Deep Water Site during The Up-welling Period. 5-jet linear diffuser Brine Discharge ppt end-of-pipe. Ambient ocean salinity = ppt Depth contours (black) in meters MSL Discharge pipeline (red) Intake pipeline (green)

11 MBRWP Worst Case at The Intermediate Site during The Up-welling Period. 5-jet linear diffuser Brine Discharge ppt end-of-pipe. Ambient ocean salinity = ppt Depth contours (black) in meters MSL Discharge pipeline (red) Intake pipeline (green) Easement boundary (cyan)

12 Canonical Circulation Pattern near Monterey Bay during the Relaxation Period

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14 MBRWP Worst Case at Deep Water Site during The Relaxation Period. 5-jet linear diffuser Brine Discharge ppt end-of-pipe. Ambient ocean salinity = ppt Depth contours (black) in meters MSL Discharge pipeline (red) Intake pipeline (green)

15 Canonical Circulation Pattern near Monterey Bay during the Davidson Current Period

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17 MBRWP Worst Case at The Intermediate Site during The Davidson Current Period. 5-jet linear diffuser Brine Discharge ppt end-of-pipe. Ambient ocean salinity = ppt Depth contours (black) in meters MSL Discharge pipeline (red) Intake pipeline (green)

18 Fully Built-Out Scenario: Probability density and cumulative probability of daily mean of maximum seafloor salinity at 100 m from the discharge (limit of the Regulatory Mixing Zone under the 5% Rule*) for: (a) deep water discharge site; and (b) intermediate discharge site. Based on 8,149 modeled outcomes using heated source water from the data center. *5% over ong-term mean = 35.1 ppt

19 Dilution performance is better at the Intermediate site at 16m depth than at the deep water discharge site at 31 m depth. The reason is because the last phase of dilution down to the limit of the 5% Rule occurs on the seafloor, where the brine plume disperses as a turbulent bottom spreading layer. Much of the dilution in the bottom spreading layer is promoted by the scrubbing action of oscillatory wave motion. This wave scrubbing action is stronger in shallow water due to wave shoaling, and vanishingly small in deep water, particularly at depths on the order of 30 m to 40 m.

20 Start-up Scenario: Probability density and cumulative probability of daily mean of maximum seafloor salinity at 100 m from the discharge (limit of the Regulatory Mixing Zone under the 5% Rule*) for: (a) deep water discharge site; and (b) intermediate discharge site. Based on 8,149 modeled outcomes using unheated source water from the data center. *5% over Long-term mean = 35.1 ppt

21 Conclusions 1) The diffuser dilution strategy at both the deep water and intermediate MBRWP discharge sites satisfies any of the presently permitted or potential future dilution standards for all foreseeable long-term ocean conditions at MBRWP Moss Landing. 2) The magnitude of occasional over-limit model results at either discharge site are within sampling error of standard oceanographic temperature/conductivity measurements used for determination of practical salinity units (psu). 3) The unheated start-up scenario finds slightly larger, more frequent over-limit discharge results at both the deep and intermediate discharge sites. However the preponderance of outcomes easily satisfies the 5% Rule, and the over-limit cases are still not statistically significant, particularly sense these cases represent start-up transients.