**Abstract**

Recently proposed options for desalination brine management involve blending of brine with a lighter effluent or concentrating the brine prior to discharge, either of which can significantly alter the discharge concentrations of contaminants. We evaluate the effect of these brine management strategies on the design of submerged outfalls used to discharge brine. Optimization of outfall design is considered such that adequate mixing can be provided with minimum cost. Designs with submerged and surfacing plume are considered for outfalls located in shallow coastal regions with small currents (quiescent receiving water is assumed). Pre-dilution with treated wastewater is shown to reduce the outfall cost, whereas pre-dilution with seawater or pre-concentration are shown to result in higher costs than the discharge of brine alone. The effect of bottom slope is also explored and the results suggest that multiport diffusers are better suited than single jets at locations with a mild bottom slope.

**Keywords:** brine disposal, desalination, outfall, optimization, brine management, multiport diffuser

## **1. Introduction**

Reject brine from desalination plants can have twice as high salinity as seawater [1] as well as high concentrations of other contaminants such as anti-fouling agents, anti-scalants, products of corrosion, etc., which can be harmful to benthic organisms. Thus, brine is usually discharged as a dense submerged jet which provides rapid mixing with ambient water. However, at locations that are characterized by shallow water depth and mild tidal currents, such as the north-western Arabian Gulf [2], diffusers with multiple jets are preferred as they can generate the required amount of mixing in smaller water depths.

Various options have been proposed for better management of reject brine from seawater reverse osmosis (SWRO) desalination plants [3, 4]. Processes such as pressure retarded osmosis (PRO) [3, 5] and reverse electrodialysis (RED) [6, 7] utilize the salinity difference between brine and treated wastewater effluent (TWE) to recover energy. On the other hand, processes such as electrodialysis (ED) [8] and ion-concentration polarization (ICP) [9] concentrate brine further to increase freshwater recovery [4] or lead to a zero discharge scenario. These options for brine management (pre-dilution with TWE or concentration) affect the discharge concentrations of contaminants present in brine, and can affect the design of outfall used to discharge brine.

Coastal desalination plants are often co-located with power plants which provide them with low-grade heat, used in the distillation of seawater (for multistage flash desalination plants) [10], or electricity (for reverse osmosis plants). Brine is often blended with condenser cooling water (CW) from the power plant before being discharged. TWE can also be used for pre-dilution (mixing with brine before discharge) if a treatment plant is nearby. Pre-dilution helps in reducing concentrations of salt and other contaminants present in brine as well as contaminants in the prediluting stream (e.g., condenser cooling water or treated wastewater effluent). It also results in increased discharge flow rate (due to blending of the two streams) and reduced discharge salinity which, in turn, reduces the density of the blended effluent. This leads to progression towards shallow or vertically mixed conditions [11].

If treated wastewater effluent from a treatment plant or condenser cooling water from a coastal power plant are not utilized for pre-dilution, they are usually discharged separately and need an outfall. Thus, in addition to the reduction in discharge concentrations of contaminants, pre-dilution also leads to a reduction in total outfall cost by eliminating the need for two separate outfalls which would cost more than one outfall for the blended stream. Thus, blending of brine with cooling water or wastewater is often recommended [12].

While concentration of brine prior to discharge using submerged outfalls (which result in dilution) is not environmentally desirable in its own right, brine can be concentrated to increase freshwater recovery or harvest salts. In order to increase freshwater recovery, brine can be desalinated in two steps involving ICP and reverse osmosis (RO) [4]. ICP is used to separate brine into two streams: 1) a lighter stream with salinity of about 35 ppt, which is then desalinated using RO; and 2) a concentrated brine stream, which is either used to harvest salts or discharged using an outfall. The concentrations of contaminants present in brine increase due to concentration. Due to the high concentrations of contaminants in concentrated brine, the near-field mixing required to dilute contaminants to desirable levels is also high.

From an environmental standpoint, one is interested in reducing concentrations of contaminants in receiving water beyond a certain mixing zone. Environmental regulations usually specify the size of a mixing zone and require outfall designs that ensure that contaminant concentrations at the edge of the mixing zone are lower than specified threshold concentrations. To dilute a contaminant to a desired concentration, the outfall needs a certain water depth. At a location with offshore sloping bottom, this means going offshore to a certain distance which has an associated capital cost. Also, the cost for pumping the effluent constitutes an operating cost. The design parameters can be optimized to achieve the right balance of these two costs and design an outfall which provides desired dilutions at the end of the mixing zone with minimum cost.

We look at the effects of four brine management strategies – pre-dilution with seawater, power plant cooling water, treated wastewater effluent and preconcentration on the design of submerged single and multiport outfalls. Outfall design variables (discharge velocity, number of ports, receiving water depth, etc.) are optimized for four different designs such that contaminants can be diluted to satisfy environmental objectives. Effect of brine management strategies on outfall cost is investigated and discussed using examples. Recommendations regarding the cost-effectiveness of different brine management options are presented.

### **2. Review of near-field mixing concepts for dense discharges**

High velocity submerged jets are often used for the discharge of brine from desalination plants as they induce rapid mixing with ambient water and lead to *Desalination Brine Management: Effect on Outfall Design DOI: http://dx.doi.org/10.5772/intechopen.99180*

reduction of contaminant concentrations. Inclined jets located near the sea floor are commonly used to discharge dense effluents as they increase the jet trajectory (and, in turn, dilution). Such jets rise to a maximum (terminal rise) height equal to *yT* before the negative buoyancy causes the jets to return to the seafloor at the impact point. For a jet (with diameter *D*0) discharging an effluent of density *ρ*<sup>0</sup> with a velocity of *u*<sup>0</sup> in an ambient of density *ρ<sup>a</sup>* and uniform depth *H*, one of three regimes – deep, shallow or vertically mixed can be identified depending on the value of the shallowness parameter *<sup>D</sup>*0*F*0*=<sup>H</sup>* [11, 13]. Here, *<sup>F</sup>*<sup>0</sup> <sup>¼</sup> *<sup>u</sup>*0*<sup>=</sup>* ffiffiffiffiffiffiffiffiffiffiffiffi *g*0 0 *D*<sup>0</sup> p is the densimetric Froude number of the jet, *g*<sup>0</sup> <sup>0</sup> ¼ Δ*ρ=ρ<sup>a</sup>* ð Þ*g* ¼ *ρ*<sup>0</sup> � *ρ<sup>a</sup>* f g ð Þ*=ρ<sup>a</sup> g* is the reduced gravity and *g* is the acceleration due to gravity.

The receiving water is considered "deep" if its depth is sufficiently large and the dense effluent does not interact with the surface. "Shallow" conditions occur if the effluent interacts with the surface but it forms a bottom layer in the vicinity of the discharge. If the depth is small enough, the effluent can be mixed over the entire water column for large distances. Such a situation is categorized as being "vertically mixed". Increase in the value of *D*0*F*0*=H* leads to a progression towards vertically mixed conditions. For a jet inclined at 30o, the transition between deep and shallow conditions is observed at *D*0*F*0*=H* ¼ 0*:*72 and that between shallow and vertically mixed conditions is observed at *D*0*F*0*=H* ¼ 7*:*36 [11].

#### **2.1 Negatively buoyant submerged jet**

In deep water, the impact point dilution, which is the minimum dilution along the seafloor, of an inclined submerged jet is proportional to *F*<sup>0</sup> [14–16]. In shallow water and vertically mixed conditions, the dilution is independent of *F*<sup>0</sup> and is proportional to *H=D*<sup>0</sup> [11, 17]. The constants of proportionality depend on the discharge angle (*θ*0). In deep receiving water, an inclination of 60<sup>o</sup> provides the highest dilution (for fixed value of *F*0). However, smaller angles are preferred in shallow conditions [13, 17]. An inclination of 30<sup>o</sup> is chosen for further analysis which is suitable for shallow regions. For this choice of *θ*0, the impact point dilutions in deep and shallow (and vertically mixed) conditions are given by Eqs. (1) and (2), respectively [11, 13].

$$\mathbf{S}\_{i,dep} = \mathbf{1} \mathbf{2} F\_0 \tag{1}$$

$$\mathbf{S}\_{i,shallow} = \mathbf{0}.8\mathbf{\tilde{G}}H/D\_0\tag{2}$$

#### **2.2 Unidirectional diffuser**

A unidirectional (or tee) diffuser is an outfall which consists of an array of submerged jets (number of jets ¼ *N*) arranged in parallel with all jets pointing in one direction perpendicular to the manifold. Use of a unidirectional diffuser is suitable in locations with mild bi-directional currents [18]. Individual jets of a unidirectional diffuser interact with each other in shallow water and lead to mixing that is different from a mere superposition of individual jets [19].

In deep water (*D*0*F*0*=H* < 0*:*72) and with adequate port spacing, there is no interaction among individual jets of a unidirectional diffuser [20] and the dilution is the same as that of a single jet (given by Eq. (1) for *<sup>θ</sup>*<sup>0</sup> <sup>¼</sup> 30o).

In shallow water (*D*0*F*0*=H* between 0.72 and 7.36), there is more interaction among individual jets and the impact point dilution of a unidirectional diffuser with port spacing equal to water depth (*l* ¼ *H*) is given by:

$$\mathbf{S}\_{i,shallow,ud} = \mathbf{0}.\mathbf{82}F\_0^{-0.15}(\mathbf{H}/D\_0)^{1.15} \tag{3}$$

In vertically mixed conditions (*D*0*F*0*=H* > 7*:*36), the dilution is independent of the discharge buoyancy (or *F*0). The impact point dilution of a unidirectional diffuser with port spacing equal to water depth (*l* ¼ *H*) in vertically mixed conditions is:

$$\mathcal{S}\_{i, \text{mixed}, \text{ud}} = \mathbf{0}. \mathbf{\mathcal{G}1H}/D\_0 \tag{4}$$

For a unidirectional diffuser discharging in quiescent shallow or vertically mixed conditions, proximity to shoreline can result in a reduction in dilution [21]. However, the reduction in dilution is less than 15% if the separation between the diffuser and the shoreline (in constant water depth) is more than 60% of the diffuser length. At a location with uniformly sloping bottom, this is roughly equivalent to an offshore distance equal to 1.2 times the diffuser length [21]. In the presence of moderate to high crossflow, Shrivastava and Adams [22] observed no significant reduction in dilution if the separation between the diffuser and the shoreline is at least 15% of the diffuser length for a diffuser discharging in uniform water depth. This corresponds to a shoreline separation of 30% or more of the diffuser length at a location with uniformly sloping bottom.
