Second International Conference on Coastal
Zone Engineering and Management (Arabian
Coast 2010), November 1-3, 2010, Muscat, Oman
ISSN: 2219-3596
A CORMIX Model Study of an Effluent Plume from a Marine
Oufall at Minal Al Fahal, Oman
Anton Purnama*
Department of Mathematics and Statistics, Sultan Qaboos University, Sultanate of Oman
MS Baawain
Department of Civil and Architectural Engineering, Sultan Qaboos University, Sultanate of Oman
YVB Sarma
Department of Marine Sciences and Fisheries, Sultan Qaboos University, Sultanate of Oman
Abstract:
A submerged marine outfall at a distance of 150 m from the beach has recently been installed for
discharging the drained water from crude oil tanks at the effluent treatment facility at Minal Al Fahal terminal. In order
to meet the marine disposal limits of Oman, CORMIX was applied to simulate effluent plumes to predict, within a
circular mixing zone with a radius of 150 m from the outfall, the concentration levels of certain components such as
biochemical oxygen demand, chemical oxygen demand, dissolved oxygen, mercury, pH, phenol, total petroleum
hydrocarbon, temperature and salinity.
The effluent discharge density is greater than the surrounding ambient water density, and thus the effluent plume is
negatively buoyant and will tend to sink towards the seabed. The potential benthic impact due to a concentrated effluent
plume should be considered. Due to uncertainty in the input data, CORMIX simulations were carried out by varying the
ambient current velocity, the effluent discharge density, and the effluent flow rate. The results showed that the
permissible concentration levels are met within the regulatory mixing zone, except phenol. Hence, further treatment of
effluent at the terminal is still needed in order to meet the marine discharge limits.
Key words: CORMIX, effluent discharges, marine outfall, Minal Al Fahal, Oman.
*
Corresponding author. Email: antonp@squ.edu.om
PDO has modified its marine discharge of treated
effluents at MAF outfall system from an open gutter over
the beach to a submerged outlet system, at a distance of 150
m from the beach (Baawain et al., 2009). The effluents are
heterogenous and contain a wide range of pollutants,
mainly oil and heavy metals in a matrix of highly saline
wastewater. Therefore the impact of components of the
effluent, such as biochemical oxygen demand (BOD),
dissolved oxygen (DO), phenol, total petroleum
hydrocarbon (TPH, also referred to as oil and grease),
chemical oxygen demand (COD) and (heavy metal)
mercury, should be studied.
1. Introduction
Mina Al Fahal (MAF), an open sandy bay within the
capital area, Muscat, is the site of Petroleum Development
Oman's (PDO) oil storage facility (tank farm), tanker
loading operations and the Oman Refinery Company (Jupp,
1998). Contaminated oily water, which is allowed to settle
out and is drained from the crude storage tanks, is then
treated in an effluent treatment plant (ETP). The plant is
designed to remove the saline production water by the
dehydration of settled oily water and then dispose of this
treated water as an effluent into MAF, and also to recover
any skimmed off oil in this process.
The bathymetry of MAF bay shows a relatively flat
bottom to the 30 m contour then a gentle slope towards the
1
60 m contour in the north-east. Tides in the Gulf of Oman
have a strong diurnal component with a spring-tide range of
2.6 m or more (National Hydrographic Office, 2008). Other
reported data and other observations (Jupp, 1998) showed
that the nearshore currents in and around MAF are
generally weak of up to 0.1 m/s with the predominant
current running to the north-west (2700-3150) for 40-50% of
the time and in the opposite direction (90 0-1350) for 1520% of the time. High but variable current speeds occur
further offshore with maxima at spring tides of up to 0.8
m/s. The overall conclusion would appear to be that MAF
experiences strong horizontal and mainly westerly water
movements.
point. The legislation also defines acceptable limits of the
effluent release for various parameters, and the near-field
dispersion modelling will show whether the outfall
discharges are sufficient to achieve these standards. The
design of the outfall is an important component to predict
the fate of an effluent in the immediate vicinity of the
outfall, and this will provide the starting point for the farfield (up to 1000 m) modelling for different ambient
conditions of the various release scenarios.
3. CORMIX v6.0 Base Simulations
The marine outfall at MAF is located at 150 m offshore
on the uniformly sloping beach, and 3.5 m below the sea
surface. Since the height of the port is 0.5 m above the
seabed, CORMIX classifies the outfall as a deeply
submerged discharge. The effluent density 1034.54 kg/m3
(typically of salinity 52 ppt and temperature 30 0C) is
greater than the surrounding ambient density 1022.72
kg/m3. Therefore, this brine type of effluent plume is
negatively buoyant and will tend to sink at the seabed. The
base simulations with brine concentration (above ambient)
set as 100% at the discharge point will be terminated when
the boundary limits as specified by the regulatory mixing
zone (RMZ) at 150 m radius from the discharge point or the
region of interest (ROI) at 1000 m downstream are met.
The input data are summarized in Table 1.
A numerical modeling is required by Omani law to
study the impacts of the MAF effluent disposal in the sea.
The model should be applied to the worst initial mitigation
conditions, i.e. the lowest wind speed concurrent with the
diminishing high and low tides, the lowest recorded current
speed in the location and the tidal reflection in view of such
conditions. It should also be applied to the effluent
discharge conditions of normal flow rate at 800 m3/day and
peak flow at 1950 m3/day.
2. Cornell Mixing Zone Model Expert
(CORMIX) System
CORMIX is a U.S. EPA approved software system for
the analysis, prediction, and design of outfall mixing zones
resulting from the discharge of aqueous pollutants into
diverse water bodies (Akar and Jirka, 1991; Del Bene et al.,
1994; Jones et al., 1996). It employs a rule-based expert
system to screen input data and select the appropriate
hydrodynamic simulation model to simulate the physical
mixing processes contained within a given dischargeenvironment interaction, ranging from internally trapped
plumes, buoyant plumes in uniform density layers, and
sinking of negatively-buoyant plumes. Boundary
interaction, upstream intrusion, buoyant spreading, and
passive diffusion in the far field are also considered.
Table 1. Input data for the CORMIX base simulations
Parameter Base value
Ambient (unbounded coastal environment)
Velocity of the currents
1.0
Depth at discharge
3.5
Bottom slope
1.34
Wind speed
1.0
Average Density
1022.72
Temperature
27
(Single port) Discharge
Distance to nearest bank
150.0
Port diameter
0.1
Port height above bottom
0.5
Theta = Vertical angle
-22.5
Sigma = Horizontal angle
90
Effluent flow rate
0.01
Salinity (above ambient)
17.0
Temperature (above ambient)
3.0
Average Density
1034.54
Mixing zone
RMZ = Regulatory mixing zone
150
ROI = Region of interest
1000
Discharge type Single Port
CORMIX has been successfully applied by regulators,
engineers, and environmental scientists worldwide to the
design and monitoring of wastewater disposal systems in
oceans, rivers, lakes, and estuaries, and it is also recognized
by regulatory authorities in all continents for environmental
impact assessment (US EPA, 1999). Extensive comparison
with field and laboratory data has shown that the CORMIX
predictions on dilutions and concentrations (with associated
plume geometries) are reliable for the majority of cases
(Roberts and Tian, 2004). CORMIX results include
contemporary three dimensional plume and diffuser
visualizations, design recommendations, flow class
descriptions and reporting oriented on discharge zone
analysis (Doneker and Jirka, 2001).
According to the Omani regulation for mixing zones,
the modelling process will be limited to a domain of
approximately 300 m in diameter centred on the outfall
2
Unit
m/s
m
o
m/s
kg/m3
°C
m
m
m
o
o
m3/s
mg/l
°C
kg/m3
m
m
-
Table 2. The regulatory mixing zone characteristics
The dynamic of MAF brine effluent plume motion is
classified as (CORMIX flow class NH4) the near bottom,
negatively buoyant flows in a uniform density layer. As the
concentrated brine effluent is released from the outfall, the
jet plume becomes strongly deflected by the ambient
current. The deflected plume slowly descends toward the
bottom and once it reaches the seabed within 8 m
downstream, the plume will remain at the seabed due to its
negative buoyancy. The height of the top part of the plume
slowly rises and reaches its maximum within 200 m
downstream, and the concentration distribution becomes
relatively uniform across the plume width and thickness
(Figure 1). After the concentrated plume is attached to the
seabed, it continues to spread laterally due to bottom
density current while it is being advected by the ambient
current. In the absence of ambient stratification, the
concentrated plume will proceed down the slope until it
reaches the ROI. Therefore, the potential benthic impact
can no longer be ruled out.
Concentration at the edge of RMZ
Dilution at the edge of RMZ
RMZ location
(centerline coordinates)
RMZ plume dimensions
Cumulative travel time
0.28 %
357
X=150 m
Y=1.30 m
Z= -3.53 m
Half-width= 2.30 m
Thickness= 1.11 m
127.53 sec
The plume conditions at the boundary of the RMZ are
presented in Table 2. The concentrated plume dilution,
defined as the ratio of the initial concentration (above
ambient) at the discharge point to that at a given location, is
given in Figure 3. A plume dilution value of 562 is obtained
at the ROI.
Figure 1. Side view of the negatively buoyant plume of
MAF effluent
The three dimensional view of the concentrated
effluent plume within the RMZ is shown in Figure 2. The
origin is located at the sea surface directly above the
submerged outfall, and 150 m offshore. The x-axis points
downstream, the y-axis points to the left (in the flow
direction), and the z-axis points upward. The CORMIX
base simulation results show that the overall concentration
increase is less than 0.3% at the edge of the RMZ.
Figure 3. The MAF effluent plume dilution within RMZ
Other CORMIX base simulations were carried out for
other components of the MAF effluent, and the results at
the boundary of the RMZ are summarized in Table 3,
together with the maximum permissible limits (or Water
Quality Standard=WQS) set by the Oman government (MD
159/2005) that should be met within the RMZ. The results
show that WQS for all components are met for the normal
flow rate for the effluent discharge conditions of 0.01 m3/s.
4. CORMIX Sensitivity Study
CorSens is the CORMIX sensitivity analysis tool that
generates a sensitivity study case to address model
performance due to inherent uncertainty in the input data
(Alameddine and El-Fadel, 2007). Therefore, instead of the
tedious repetition of manual data entry, it automatically
Figure 2. The negatively buoyant plume of MAF
effluent
3
Table 3. Summary of MAF outfall effluents
Effluent
MAF Outfall
RMZ
DO (mg/l)
0.71
5.985
pH
6.8
7.997
Salinity (ppt)
52
35.05
Temperature (oC)
30
27.01
BOD (mg/l)
COD (mg/l)
Mercury (mg/l)
Phenol (mg/l)
TPH (mg/l)
155
1400
0.004
0.7
90
0.434
3.919
0.0001
0.002
0.252
Table 4. The CorSens results summary on the ambient
velocity
RMZ centerline (in m)
X
Y
Z
150.0
1.30
-3.53
150.0
1.42
-3.53
150.0
1.61
-3.54
150.0
1.89
-3.54
150.0
2.35
-3.56
150.0
3.11
-3.57
150.0
4.57
-3.61
150.0
8.57
-3.70
150.0
18.78 -3.94
150.0
65.68 -5.02
Dilution
S
357
310
263
218
181
153
116
64
27
15
Dilution
Ambient
10.6
6.0
6
8.0
8.5
35.0
3.0
27.0
7.75
7
4
350
6
0
0
0
0
0
of the RMZ, the elongated effluent plume grows and
becomes wider in the y-axis. The RMZ half-width plume
dimensions at velocity 0.2 m/s equals to 7.11 m. The
uncertainty in sea conditions has big effects on effluent
plume dilution (or concentration at the edge of RMZ), and
thus the potential benthic impact of a concentrated plume
should be considered. It is found that the plume dilution
value at ambient velocity 0.2 m/s is 13 times smaller than
that at 1.0 m/s.
increments data to analyze mixing zone conditions.
Sensitivity studies are also motivated by the fact that there
are no user-adjustable model “tuning” coefficients within
CORMIX. The basis for this restriction is that normal
variation in ambient conditions are likely to have greater
influence over mixing zone behavior than “tuning” a model
parameter to obtain a “desired” result.
Velocity
(m/s)
1.0
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
WQS
Concentration
5.5
(0.5 below ambient)
7.8
(0.2 below ambient)
37.0
(2 above ambient)
28.0
(1 above ambient)
20.0
200.0
0.001
0.002
15.0
Table 5. The CorSens results summary on the discharge
density
%
C
0.28
0.323
0.38
0.459
0.551
0.655
0.866
1.56
3.70
6.68
Density
(kg/m3)
1034.54
1032.54
1030.54
1028.54
1026.54
1024.54
RMZ centerline (in m)
X
Y
Z
150.0
1.30
-3.53
150.0
1.30
-3.53
150.0
1.30
-3.53
150.0
1.31
-3.53
150.0
1.32
-3.53
150.0
1.35
-3.53
Dilution
S
357
369
383
399
419
451
%
C
0.28
0.271
0.261
0.251
0.239
0.222
Next, further simulations were also carried out by
varying the discharge density (an effluent parameter),
which reflect the uncertainty on the MAF effluent property,
and the results summary of reducing discharge density from
1038.54 kg/m3 to 1024.54 kg/m3 are presented in Table 5.
The density variations represent the base effluent with a
temperature rise up to 50 0C, and a salinity decrease down
to 39 ppt. Since the plume dilution values are all bigger
than the base dilution, reducing the discharge density
improves the effluent plume characteristics at the edge of
the RMZ.
First, the CorSens simulations were carried out by
varying the ambient current velocity (a coastal environment
parameter), which reflect the uncertainty in the sea
conditions, while holding the other input parameters the
same as the base simulation given in Table 1, and the
results summary of reducing the ambient velocity from 1.0
m/s to 0.1 m/s are presented in Table 4.
For calm sea conditions (i.e. when the ambient velocity
is less than 0.2 m/s), also perhaps due to its negative
buoyancy, the concentrated brine plume becomes
immediately attached to the seabed as it leaves the outfall,
and then spreads due to bottom density current. At the edge
Lastly, the simulations were carried out by varying the
effluent flow rate (a discharge parameter), which reflect the
uncertainty on the effluent treatment plant's operation, and
4
the results summary of increasing effluent discharge rate
from 0.01 m3/s to 0.025 m3/s are presented in Table 6. The
normal flow rate for the effluent discharge conditions at
MAF outfall are 800 m3/day which is equivalent to 0.01
m3/s, and the peak flow rate conditions are 1950 m3/day or
about 0.023 m3/s. At the edge of RMZ, the effluent plume
dilution values decrease but they are still much higher than
the base plume dilution, except for phenol.
ambient velocity reduces from 1.0 m/s to 0.2 m/s. Although
all the concentration values are increasing, they are still
below the WQS values, with the only exception being
phenol, where all values are higher than the maximum
permissible limit of 0.002 mg/l.
5. Conclusions
CORMIX base simulation results showed that the
permissible concentration levels are met within the
regulatory mixing zone. However, phenol concentration
levels are found to be higher than the permissible limits
under the calm sea conditions and when the effluent
discharge rate is bigger than the normal flow rate for the
effluent discharge conditions of 800 m3/day. Hence, further
reduction of phenol concentration at the discharge point is
required by upgrading the treatment plant. In order to
reduce the plume dilution to below 50 for the calm sea
conditions, the concentration of phenol at MAF outfall
should be less than 0.1 mg/l.
Table 6. The CorSens results summary on the effluent
flow rate
Flow rate
(m3/s)
0.01
0.0125
0.015
0.0175
0.02
0.0225
0.025
RMZ centerline (in m)
X
Y
Z
150.0
1.30
-3.53
150.0
1.58
-3.54
150.0
1.90
-3.54
150.0
2.25
-3.55
150.0
2.63
-3.56
150.0
3.05
-3.57
150.0
3.51
-3.58
Dilution
S
357
292
251
230
221
208
195
%
C
0.28
0.343
0.399
0.434
0.453
0.481
0.512
As the concentrated effluent plume is attached to the
seabed and spreads due to bottom density current, the
potential benthic impact should also be considered.
The predicted concentration values of MAF effluent
plumes at the edge of RMZ are given in Table 7 as the
Table 7. Summary of the MAF effluents plume at RMZ on the ambient velocity
1.0
0.8
0.6
Effluent
WQS
Dilution
357
263
181
DO (mg/l)
pH
Salinity (ppt)
Temperature (oC)
BOD (mg/l)
COD (mg/l)
Mercury (mg/l)
Phenol (mg/l)
TPH (mg/l)
5.5
7.796
37.0
28.0
20.0
200.0
0.001
0.002
15.0
10.6
5.9
8.5
3.0
7.75
7
4
350
6
5.985
7.997
35.048
27.008
0.434
3.922
0.0001
0.002
0.252
5.980
7.995
35.065
27.011
0.589
5.323
0.0002
0.0027
0.342
5.971
7.993
35.094
27.017
0.856
7.735
0.0002
0.0039
0.497
0.4
0.2
116
27
5.954
7.990
35.147
27.026
1.336
12.069
0.0003
0.006
0.776
5.804
7.956
35.630
27.111
5.741
51.852
0.0015
0.0259
3.333
Alameddine, I. and El-Fadel, M. 2007. “Brine discharge
from desalination plants: a modelling approach to an
optimized outfall design”, Desalination, 214, 241-260.
Acknowledgments
The funding of this study was provided by Sultan
Qaboos University and PDO under the support service
agreement AS/ENG/CIVL/09/02.
Baawain, M.S., Ahmed, M., Mansour, M., Marikar, F., AlHaddabi, M., Bahadori, A., Vuthaluru, H.B.,
Sathasivan, A. and Badruzzaman, B. 2009. “Feasibility
study to upgrade the effluent water treatment facility
and water disposal system at the PDO MAF terminal in
Oman”, Final Report, Sultan Qaboos University,
Oman.
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