Modeling wastewater discharge at the planning stage of a marine ...

Environ Monit Assess (2012) 184:3165–3184 DOI 10.1007/s10661-011-2180-x

Modeling wastewater discharge at the planning stage of a marine outfall system Esin Esen & Erdem Sayin & Orhan Uslu & Canan Eronat

Received: 10 November 2010 / Accepted: 8 June 2011 / Published online: 29 June 2011 # Springer Science+Business Media B.V. 2011

Abstract The possibility of marine discharge of a negatively buoyant industrial waste was evaluated by a modeling study using Killworth 3-D, which is the first version of the Modular Ocean Model (MOM). The Model was run with the recorded wind direction and speed on the cruise dates and the circulation patterns for surface and subsurface were found to be similar with the current meter measurements. Model scenarios have been set-up in order to estimate the intensity and direction of the currents in the Nemrut Bay under the condition of wind blowing from a definite direction for a long time. MOM model has been run for four major wind directions, each having duration of 10 days and the behavior of the discharge plume in the worst case has been traced. Also, the behavior of the discharge plume in the real case has been estimated by using the wind data of the region. According to the model results, impact of trace elements that compose the discharge effluent is limited both in time and space. It is concluded that trace elements will leave the Bay in a short time due to the short residence times. E. Esen (*) : E. Sayin : C. Eronat Institute of Marine Sciences and Technology, Dokuz Eylul University, Baku Bul. No:100, Inciralti, 35340 Izmir, Turkey e-mail: [email protected] O. Uslu Department of Environmental Engineering, Bahcesehir University, Ciragan Caddesi, Besiktas, 34353 Istanbul, Turkey

Keywords Discharge modeling . Sea outfall design . Hydrodynamics . Residence time . Marine pollution . Environmental impact assessment

Introduction Competitive demands for natural resources in coastal systems can lead to a serious deterioration of the environment. Most of the large cities located at coastal areas worldwide prefer to discharge their treated or untreated wastewaters into the ocean through marine outfall systems for convenient and economic reasons (Vijay et al. 2010; Alameddine and El-Fadel 2007; Andreakis 1997). It is believed that oceans have a good capacity to assimilate waste discharge. Enclosed water bodies like bays have lower assimilative capacities (Torres et al. 2009). Uncontrolled and excessive waste disposal may create unacceptable levels of sea water pollution. This will deteriorate the economical value and the ecological state of the coastal water with regard to other uses. The amount of waste which can be received by a water body without significant consequences and where and how the waste discharge should be made in order to minimize the resulting pollution level in sea water are the main questions to be answered before the ocean discharge (Alameddine and El-Fadel 2007; Malcangio and Petrillo 2010). Sufficient dilution of discharged effluent to reduce contaminant concentrations well below established water quality standards under most circumstances can be achieved with a properly designed marine outfall system (Roberts et al. 2010). A minimum initial

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dilution of 100:1 is common, making the marine outfall competitive with conventional waste treatment plants (Fisher et al. 1979). Minimal adverse impacts are expected if rapid mixing and dilution are ensured in the discharge zone (Alameddine and El-Fadel 2007). These optimal mixing conditions can be attained by the careful design and construction of outfalls that account for local circulation patterns, hydrographic currents, and the hydrodynamic characteristics of the discharge area (Alameddine and El-Fadel 2007; Roberts et al. 2010; Lattemann and Höpner 2008). There are several mechanisms which govern the dilution characteristics of a marine outfall. These are usually considered separately in three phases: (1) initial dilution which occurs in the first few minutes as the waste stream leaves the outfall diffuser; (2) horizontal transport and dispersion of the discharge field; and (3) biogeochemical reactions which take place in the sea (Alameddine and El-Fadel 2007; Lattemann and Höpner 2008; Bleninger and Jirka 2008). For conservative substances (non-degradable) initial dilution is the most important factor. Initial dilution occurs due to three effects: jet mixing due to the momentum of the discharge stream as it leaves the diffuser port; the buoyancy effect resulting from density differences between the discharge and the ambient sea water (temperature and salinity differences) which causes the discharge field to rise upward in the water column as an expanding plume unless the discharge density is higher than the ambient density, thus mixing with the sea water; and finally the current effect causing the lateral entrainment of fresh sea water into the discharge plume (Malcangio and Petrillo 2010; Lattemann and Höpner 2008; Bleninger and Jirka 2008). The rate of water exchange between a coastal system and the open sea plays a critical role in controlling the chemical and biological processes within the basin (Yuan et al. 2007). The water exchange has been increasingly investigated in coastal water quality studies (Yuan et al. 2007; Wang et al. 2004; Dabrowski and Hartnett 2008; Ribbe et al. 2008; Silverman and Gildor 2008; Sandery and Kampf 2007). Flushing time, residence time, total exchange time, turnover time, or detention time are commonly used to represent the time scale of physical transport in estuaries that residence time refers to the time a dissolved or suspended matter resides in an

Environ Monit Assess (2012) 184:3165–3184

estuary before it is carried out into the open sea (Wang et al. 2004). Water exchange in coastal areas can be studied via mathematical models being characterized by major physical processes (i.e., advection, turbulent diffusion, and dispersion) (Chen et al. 2010; Yuan et al. 2007; Wang et al. 2004; Dabrowski and Hartnett 2008; Ribbe et al. 2008; Silverman and Gildor 2008; Sandery and Kampf 2007). The basic idea is that a mass of hypothetical conservative tracer is instantaneously introduced into a region of interest. A unit tracer concentration is prescribed initially inside that region, and the subsequent advective dispersion of this mass is then obtained by solving the mass transport equation numerically and the time variation of the tracer mass inside the region is tracked (Wu et al. 2005; Murdoch et al. 2010; Kuhrts et al. 2004). The work in this is paper is based on a part of the investigations carried out for the assessment of environmental impact of the proposed Aliaga Zinc Recovery plant. The facility aims to recover zinc oxide from waste material produced by steel plants in the Aliaga Heavy Industrial Zone. The process of recovery produces a “wash effluent” which is composed of elemental ions of different chemicals. Aliaga town located 50 km northwest of Izmir Municipality (Fig. 1) has been subjected to extensive industrial development. There are iron-steel factories, coal storage yards, fuel storage yards, fertilizer factory, natural gas power plant, electrical substation, small industrial areas, and other medium and small establishments in the region, which cause heavy metal pollution (Sponza and Karaoglu 2002). Nemrut Bay, neighboring marine environment of Aliaga town is a subsystem within the Candarli Bay ecosystem and connects to the northern Aegean Sea.

Field measurements and sampling Field measurements for this study were carried out by RV/K. Piri Reis (February, July, and September, 2005). Oceanographic data were collected by lowering Seabird CTD (Conductivity/Temperature/Depth) instrument through water column in each of the fixed 15 stations (Fig. 2). The stations—except station 15—are located in a 1-km2 area where the station 15 was fixed to represent the offshore conditions. Aanderaa type (RCM9) current meter is used to detect the flow intensity and direction.


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Fig. 1 Study area

Marine outfall design The purpose of the project was the investigation of possibility of disposal of the “wash effluent” that would be produced by the process of the proposed recovery plant to marine environment, namely the Nemrut Bay, conceptually. The flowrate of the discharge effluent was assumed as 28.13 m3/h. The proposed discharge location was at 30 m depth at

station 9 shown in Fig. 2. The dilution estimations and diffuser design have been based on equations provided by Fischer et al. (1979) in Mixing in Inland and Coastal Waters. For vertical single port discharges, dilutions at the end of the initial mixing region can be calculated with the following equations where Q is the volume flux, M is the specific momentum flux, and B is the specific buoyancy flux at the diffuser discharge nozzle. The discharged fluid

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Fig. 2 Locations of the measurement stations in the study area

forms a jet in the ambient water. For a round jet, Q and M are given by: Q¼

1 2 pD W 4

1 M ¼ pD2 W 2 4

L3 =T

4 2 L =T

ð1Þ

A characteristic length scale is defined in terms of Q and M as: Q I0 ¼ pffiffiffiffiffi M

ð4Þ

The mean dilution μ/Q is: ð2Þ

in which D is the jet diameter and W, the mean outfall velocity assumed uniform across the jet. For a round jet, the initial specific buoyancy flux is: 0 ð3Þ B ¼ gðΔr0 =rÞQ ¼ g0 Q L4 =T 3 in which Δρ0 is the difference in density between the receiving fluid and the fluid being discharged or alternatively g0′ is the initial apparent gravitational acceleration.

m z ¼ 0:25 Q I0

ð5Þ

where, μ is the volume flux of the jet and z is the distance from the jet orifice. For diffuser diameter of 50 mm, dilutions of 112.84 and 169.26 times are achieved at 20 m and 30 m from the jet orifice, respectively. As the discharge volume is very small (even for the full operation conditions), multiport diffuser design is not feasible. As the discharge fluid is denser than the sea water, the buoyancy flux is calculated as negative and the


plume will tend to sink to the sea floor after initial dilution. Considering the achieved dilution due to the initial momentum of the jet, the mixture of the discharged fluid and the ambient sea water will achieve a density that will be practically equal to that of the ambient sea water. Consequently the negative buoyancy effects can be regarded as negligible. The plume will be further diluted and carried away by the prevailing currents.

Model description The behavior of the Bay’s water has been estimated by a modeling study. Killworth 3-D, which is the first version of the model Modular Ocean Model (MOM), is used for this purpose. The model is using the real wind data, stratification of the seawater, and the real bathymetry for the solution of the hydrodynamic equations. The Killworth numerical Ocean General Circulation Model (MOM) is based on the primitive equations as described by Bryan (1969) and Cox (1984). The specific model configuration was developed by Killworth et al. (1989). The free surface version of the Princeton model is a modification of the Bryan–Cox–Semtner numerical ocean general circulation model. It has been adapted to include a free surface explicitly. The Killworth model has been widely used in oceanographic studies. Among others, Lehman (2000) applied Killworth model on the wind-driven circulation of the Baltic Sea and Omstedt et

Fig. 3 Evolution of stratification in Nemrut Bay

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al. (2004) used it in the BALTEX research program. Sayin (2003) applied the general circulation model on the Izmir Bay. The continuous equations for an ocean obeying the hydrostatic, incompressible and Boussinesq assumptions solved by MOM are: p ut þ Γ ðuÞ fv ¼ ma1 þ Fu ð6Þ r0 l vt þ Γ ðvÞ þ fu ¼ a

1

p r0

ϕ

þ Fv

ð7Þ

where ϕ is latitude, λ is longitude, and a is the radius of the earth. The velocity field is given by (u/east, v/ north, and w/up). ρ0 is a reference density, Fu, Fv represent effects of horizontal turbulence as detailed by COX (1984). The continuity equation is given by Γ ð1Þ ¼ 0:

ð8Þ

Here, the operator Γ is an advective operator, defined by Γ ðmÞ ¼ ma1 ðumÞl þ vmm1 þ ðwmÞz

ð9Þ

and μ represents any scalar quantity. m ¼ sf

ð10Þ

f ¼ 2Ω sin f

ð11Þ

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u¼


al m

ð12Þ

and (u’, v’) is the baroclinic flow which has no depth average. Zz

:

v ¼ af

ð13Þ

The local pressure p is given by the hydrostatic relation Z0 ð14Þ p ¼ ps þ rgdz

0

Zz

u dz ¼ 0 H

v0 dz ¼ 0

ð23Þ

H

Integration of the continuity equation with respect to z from −H to η together with the kinematic

z

where ps is the pressure at the surface (z=0). It is defined as ps ¼ r0 ghðl; ϕ; t Þ

ð15Þ

where η is the free surface elevation. The conservation of a tracer T is given by Tt þ Γ ðT Þ ¼ F T

ð16Þ

where FT represents diffusive and other effects acting on T. The equation of state is r ¼ rðq; S; zÞ

ð17Þ

where θ is potential temperature and S is salinity. A polynomial fit is used for the equation of state. The lateral boundary conditions are u ¼ v ¼ Tn ¼ 0

ð18Þ

where n is a coordinate normal to the wall. Kinematic boundary conditions are required at the surface and at the bottom, respectively w ¼ ht þ uma1 hl þ va1 hf

z¼0

at surface w ¼ uma1 Hl þ va1 Hf

z ¼ H

at bottom

ð19Þ

ð20Þ

The barotropic mode is defined by u¼

U þ u0 H

v¼

V þ v0 H

ð21Þ

where (U, V) is the vertically integrated (barotropic) mass flux Zz U¼

Zz udz

H

V ¼

vdz H

ð22Þ

Fig. 4 The wind rose showing dominant wind direction and intensity in the Nemrut Bay area: a annual data, b in February 2005, and c in July 2005

Environ Monit Assess (2012) 184:3165–3184 Table 1 The chosen model parameters for the winddriven circulation experiments

Fig. 5 Model current patterns showing the average current intensities and direction along the depth in February

Parameters

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Horizontal resolution

50 m

Number of vertical layers

6

Layer thickness (m)

5, 10, 15, 15, 15, and 10 m

Horizontal eddy coefficient for momentum

1,000 cm2/s

Vertical eddy coefficient for momentum

1.0 cm2/s

Horizontal eddy coefficient for heat

1,000 cm2/s

Vertical eddy coefficient for heat

0.1 cm2/s

Baroclinic time step

20 s

Barotropic time step

0.5 s

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boundary conditions and the approximation η