Seawater Intake & Brine Outfall Systems

Seawater intake and outfall systems are used worldwide to draw in seawater and disperse brine for desalination and power plants. These systems produce potable water for drinking and serve as a source of cooling and steam to aid onshore processes.

1 Main Components of Seawater Intake & Outfall Systems

The following are the main items of seawater intake and outfall system:

• Water transfer pipeline
• Pre-treatment & Desalination plant
• Brine reservoir
• Pump station
• Preliminary screening
• Main outfall pipeline
• Diffuser
• Diluted brine
• Intake Pipeline
• Intake headworks/ structure

2 Seawater Intake & Outfall Systems: Design Considerations

• Intake System/Type
• Site selection/location
• Coastal Processes
• Environmental considerations
• Operational & Maintenance considerations
• Hydraulic Design
• Structural Stability

2.1 Systems/Types

2.2 Seawater Intake & Brine Outfall Systems Type: Direct Intakes

2.2.1 Direct Surface Intake

• Normally it is located next to the shore, protected by coastal structures such as breakwaters or in an estuary
• Requires coastal structures (breakwaters, groynes)
• Wave action & sediment
• Extract great volumes of feedwater
• WQ – extract floating debris
• Expensive
• Impact on the shoreline (construction and operational phase)
• On-shore maintenance
• Co-location plants

2.2.2 Direct sub-surface intake

• Located offshore, with feedwater transported to the desalination plant via an offshore marine pipeline
• Suitable for large volumes of water, but requires large, expensive pipelines
• Better for exposed coastline and rough seas
• Entrainment & Impingement
• Limits the impact on the coastline
• Requires offshore maintenance

2.3 Seawater Intake & Brine Outfall Systems: Coastal Processes

2.3.1 Wind

For shallow outfalls (typically less than 20m water depth), the diurnal land and sea breezes will result in diurnal changes in the surface waste fields’ transport (onshore/offshore).

2.3.2 Tides

Water levels and associated currents govern the hydraulic design of the intake and outfall, as well as the dispersion and transport of the effluent plumes.

2.3.3 Waves

Waves are a critical environmental factor essential for the design and construction of marine structures.

2.3.4 Currents

Initial and secondary dilutions.

2.3.5 Stratification

Stratified conditions, characterized by layering in the water column due to a density gradient between the surface and the bottom, can prevent a buoyant plume from rising, leading to reduced initial dilution and the formation of a submerged waste field beneath the surface. However, since the effluent to be discharged offshore is denser than the surrounding seawater, the potential impact of stratification on initial dilution is considered negligible.

2.4 Seawater Intake & Brine Outfall Systems: Environmental Considerations

2.4.1 Impingement

Larger marine life is trapped in or against the intake screens in the intake openings due to the velocity and force of the water flowing through them.

2.4.2 Entrainment

Very small and microscopic organisms (e.g. phytoplankton, zooplankton, eggs and larva) are pulled through the screens and into the abstraction system.

Measures to reduce impingement and entrainment associated with direct intake systems – recommended by the United States Environmental Protection Agency (EPA 1985, 2001 and 2004 DESALINATION ISSUES ASSESSMENT REPORT 2003 and SYNTHESIS PAPER )

• Intake location
• Intake velocity
• Velocity cap
• Screening methods (mainly applicable to direct, open intakes located at the coastline where re-circulation of seawater for cooling water is used – i.e. a closed system)
• Physical barriers
• Behavioural systems (i.e. electrical impulses to distract fish – not considered to have a high success rate)

2.5 Seawater Intake & Outfall Systems: Site Selection

The balance between proximity to desalination plant versus environmental & physical requirements & characteristics.

Generally, the deeper the intake structure (furthest offshore) and with the extraction point raised a few meters above the seafloor, the cleaner the feedwater tends to be, as it typically contains less sediment in suspension. This setup also reduces the impact of wave forces on the structure while ensuring that the extraction point remains submerged at all times.

The primary treatment processes at the plant increase in complexity and cost as the feedwater quality decreases. However, the further offshore the extraction location, the more expensive the initial construction costs.

The location of existing ocean outfalls should be considered, as this could affect the feedwater quality, particularly the location of the brine outfall, which could lead to re-circulation.

From an environmental point of view, for a subsurface direct offshore intake, the deeper the extraction point is located, the less oxygen in the water and, subsequently, marine life. When selecting a location for a direct surface intake at the shore, the beneficial uses, environmentally sensitive areas and possible negative effects on the coastline (alongshore sediment regime) have to be considered.

Finally, the proposed location’s proximity to ports and fishing activities, together with popular navigation ship routes, should be taken into consideration. Ship anchors can cause major damage to subsurface structures, fishing nets can block the intake screens, and pollution caused by vessels or port activities could impact feedwater quality.

2.6 Hydraulic Design

The following design guidelines, which are specified in the Coastal Engineering Manual (EM 1110-2-3001, 1995), should be taken into account to ensure the optimum hydraulic performance of a seawater intake structure:

• The water flow path between the inlet openings in the intake structure and the flow pipeline to shore should be streamlined to ensure water velocities increase gradually;
• Abrupt changes in flow cross-section areas should be avoided to minimize turbulence and consequent power loss;
• The flow section between the intake opening (circular screen structure in this instance) and the main intake pipe section is particularly important: The transition should be made over a flow distance equal to one or more conduit diameters, and
• Model tests (flow dynamics in the intake system) are very useful in determining the direction of the flow path between the intake opening and the main intake pipeline.

2.7 Structural Stability

The intake structure should be designed to withstand wave stresses and current forces on the intake head during adverse sea conditions. Provision should be made for safe navigation of seafaring vessels.

2.8 Design Approach

3 Brine Outfall: Design Considerations

The effluent discharge system is designed to minimize environmental impact while ensuring full compliance with relevant environmental guidelines, regulations, and legislation.

It is always better to discharge continuously.

When fluctuations occur due to unforeseen production or operational issues, the outfall system should be designed to discharge intermittently at design flow rates.

The outfall system is designed to comply with environmental criteria and hydraulic requirements for a specific design flow rate and effluent composition. Therefore, the system will not perform according to the design (environmental and physical) requirements if the effluent is discharged at a reduced flow rate.

3.1 Water Quality Objectives

3.1.1 Legislation

• Integrated Coastal Management Act
• Since the National Environmental Management Integrated Coastal Management Act (Act 24 of 2008) came into force on 1 December 2009, the Department of Environmental Affairs is responsible for and regulates the use of coastal waters, including the discharge of effluents from land-based activities.
• Coastal Water Discharge Permits (CWDP)

3.1.2 Policies on Disposal of Waste Water

• Operational Policy For The Disposal Of Land-Derived Water Containing Waste To The Marine Environment Of South Africa (DWAF, 2004)
• Assessment framework for the management of effluent from land-based sources discharged to the marine environment (DEA, 2015)

3.1.3 Water Quality Guidelines:

• The South African Water Quality Guidelines for Coastal Marine Waters provide recommended target values for various water quality constituents to prevent negative impacts on the marine ecosystem (DWAF, 2004).

3.1.4 Required Dilutions(more below)

3.2 Required Dilutions

The term dilution describes the process of reducing the concentration of effluent constituents by mixing the effluent with uncontaminated ambient seawater. This process aims to achieve acceptable concentration levels that support ecosystem functioning and allow for safe recreational human activities, such as swimming. The required dilution is a function of the effluent concentration and the “buffer capacity”, which is the difference between a guideline value (target value) and the ambient concentration of the specific water quality variables.

The required initial dilution for the concentration of conservative constituents can be estimated by the conservation of mass as follows (DWAF, 2004):

S = (Ce – Cb) / (Cg – Cb)

Where:

S = Required dilution

Ce = Concentration of constituent in wastewater

Cb = Concentration of constituent in receiving marine environment (ambient concentration)

Cg = Recommended concentration (guideline)

3.3 Achievable Dilutions

The total dilution of conservative constituents at a distant location can be considered as two distinct processes: the initial dilution when the effluent stream is injected into the receiving water body, and the secondary dilution where the waste field is transported to a distant location.

For an offshore outfall (deep water), the initial dilution is brought about by the entrainment of clean seawater when effluent is jetted out into the receiving water body. The degree of entrainment is related to the shear between the plume and the adjacent water, a function of the effluent jet’s momentum and buoyancy. The initial dilution process will cease when the plume’s vertical velocity reaches zero, or the plume reaches the water’s surface.

The effluent field will then be further diluted by diffusion (eddy) while being transported away by ocean currents—secondary dilution. The vertical behaviour of the effluent plume will be affected by layering (stratification) in the water column, depending on the effluent’s relative density relative to the density of the receiving water body.

3.4 Achievable Initial Dilutions

The physical properties of brine plume (negatively buoyant) limit the initial dilutions which can be achieved. Dense plume will sink to the seafloor.

• The salinity concentration of brine stream from a Reverse Osmosis (RO) plant will normally be double that of the ambient seawater.
• International MWQG: Allowable salinity of a diluted effluent plume should normally be within 33 to 36 ppt.

3.5 Secondary Dilution

3.6 Diffuser Configuration

3.7 Brine Outfall: Diffuser Configuration – Untapered Diffuser

3.8 Brine Outfall: Diffuser Configuration – Tapered Diffuser

3.9 Optimization of Diffuser Configuration

To optimize the diffuser configuration, consider the following influencing aspects (controlling parameters):

Effluent density: which is denser than seawater for a brine effluent.

The required dilution necessary to meet the environmental objectives is typically around 20, depending on site-specific requirements.

The diameter of the main pipeline, initially determined for the outfall pipe, will determine the diffuser configuration, including port diameters and the number of ports.

Additional momentum is needed to ensure the jet plume has a long enough path to entrain seawater and achieve the required dilutions. Therefore, the momentum flux for each port must be increased. However, it is advisable not to raise port velocities too high, as increased velocities can lead to greater forces on diffuser components.

Although a brine diffuser does not require great depth due to the limiting rising height of the effluent plume, more inshore marine structures are more vulnerable to nearshore physical processes, such as wave forces and unstable seabed conditions.

The discharge angle of the ports should be inclined relative to the horizontal to maximize the plume’s path length.

Initial modelling, using numerous configurations, is required to optimize the diffuser design.

To provide developers with an initial estimate of the diffuser configuration requirements, a method was developed based on scientific principles and widely accepted environmental regulations. This approach offers a rough idea of the required number of ports for a specific discharge flow rate and port diameters.

3.10 Initial Dilution Prediction Model

Numerous prediction theories and techniques are available. The choice of the technique (“model”) to be applied is to be decided upon by the design engineer, taking the following into account:

• Confidence in the “accuracy” of the dilution prediction estimates.
• Project/client requirements and specifications.
• The control that the engineer has on the technique (“model”), and the thorough understanding of the applied theories.

Not one of the theories/ prediction techniques available can be considered inaccurate because they were not developed in isolation but were part of the “evolution” of an overall concept, supported and verified by numerous field and laboratory experiments.

The essential issue is that the user of any “model” must be fully aware of the estimation’s sensitivity to the complexity of the continuously varying processes in the receiving environment.

4 Seawater Intake & Outfall Systems Technical Terms Glossary

µg/L (micrograms per litre)	Micrograms per litre; a measurement describing the amount of a substance (such as a mineral, chemical or contaminant) in one litre of water. It is expressed in terms of weight per volume. One µg/L is equal to one part per billion.
Beneficial use area	Desired uses of the marine and estuarine areas
Biocide	A chemical (e.g. chlorine) used to kill biological organisms.
Brine	Water that contains a high concentration of salt. Brine discharges from desalination plants may also include constituents used in pre-treatment processes, in addition to the high salt concentration seawater.
Bromide	An element that is present in desalinated seawater.
Coagulation	A pre-treatment process used in some desalination plants. A substance (e.g., ferric chloride) is added to a solution to cause certain elements to thicken into a coherent mass, so that they may be removed.
Coastal area	The area where the land is influenced by its proximity to the sea, and where the sea is influenced by its proximity to the land.
Cogeneration	A power plant designed to conserve energy by using “waste heat” from generating electricity for another purpose.
Concentrate	Water that contains a high concentration of salt. Concentrate discharges from desalination plants may include constituents used in pre-treatment processes, in addition to the high salt concentration seawater.
Conventional treatment	A method of treating water which consists of mixing, coagulation-flocculation, sedimentation, filtration, and disinfection. Similar to direct filtration with the addition of flocculation and sedimentation.
Deaeration	Removal of oxygen. A pre-treatment process in desalination plants to reduce corrosion.
Desalination	Desalination is the process of removing dissolved salt and other minerals from seawater to create fresh water.
Diffuser	The offshore component of an outfall system, consisting of the main pipe (with or without tapers) equipped with discharge ports placed at specific intervals. It is designed to provide an even distribution of port flows along with the diffuser.
Dilution	The lessening in the concentration of a substance due to mixing with water.
Direct seawater intake	Open water intake extracts water directly from the sea.
Discharge	A return stream from the desalination plant that is released back into the environment through dilution and mixing.
Disinfection	Water treatment which destroys potentially harmful bacteria.
Distillation	A process of desalination where the intake water is heated to produce steam. The steam is then condensed to produce product water with low salt concentration.
Ecosystem	A community of plants, animals, and organisms interacting with each other and their environment’s non-living (physical and chemical) components.
Eddies	The movement of a stream of water in which the current doubles back on itself, causing a type of “whirlpool”. This is typically caused by promontories along a coastline or counteractions from driving forces such as wind shear and an ambient current.
Electrodialysis	Most of the impurities in water are present in an ionized (electrically charged) state. When an electric current is applied, the impurities migrate towards the positive and negative electrodes. This leaves the intermediate area with reduced impurity levels, resulting in a purified stream of product water. While effective for treating brackish water, this technology is not yet used commercially for desalting seawater on a large scale.
Environmental impact	A positive or negative environmental change caused by human action.
EPA	The United States Environmental Protection Agency
Estuary	A partially or fully enclosed body of water open to the sea permanently or periodically, and within which the seawater can be diluted, to a measurable extent, with freshwater drained from land or a river. The upstream boundary of an estuary is the extent of tidal influence.
Euphotic zone	The depth of the water body in an ocean that is exposed to sufficient sunlight for photosynthesis to occur.
Far-field dilution	As an effluent plume moves away from the initial mixing zone, it undergoes secondary dilution through processes such as dispersion, entrainment, and mixing with seawater. This dispersion is driven by factors like currents, turbulence, eddies, and shears. Additionally, the chemical and biological dispersion of non-conservative substances and the decay of certain organisms contribute to what is known as the “far-field dilution process”.
Feedwater	Water fed to the desalination equipment. This can be source water with or without pre-treatment.
Filtration	A process that separates small particles from water by using a porous barrier to trap the particles and allow the water through.
GRP	Glass Reinforced Polyester/Plastic
HDPE	High-Density Polyethylene
Head loss	The drop in the sum of pressure head, velocity head, and potential head between two points along a path.
Hydraulic grade line (HGL)	The height to which the water would rise in a piezometer tube attached vertically to the water conveyance pipeline.
Indirect seawater intake	Intake water filtered through the seabed (e.g. via beach wells).
Infiltration Gallery	A method used for seawater intake. Perforated pipes are arranged in a radial pattern onshore sand below the water level. Water in the saturated sand enters the perforated pipes.
Initial dilutions for dense plume	The dilution of the wastewater plume generated by jet momentum and the negative buoyancy effect that occurs, causing the plume to descend on the seabed.
Initial dilutions for buoyant plume	The dilution of the wastewater plume generated by jet momentum and the positive buoyancy effects that occur between the outlet ports of a marine outfall’s diffuser and the sea surface.
Intake	The physical facilities through which the seawater enters the plant.
Marine discharge	Discharging wastewater to the marine environment either to an estuary or the surf zone or through a marine outfall (i.e. to the offshore marine environment).
MarineEnvironment	The marine environment includes estuaries, coastal marine and near-shore zones, and open-ocean-deep-sea regions.
Marine outfall pipeline	A submarine pipeline originating onshore conveys wastewater from a headworks to a submerged discharge location on or near the seabed beyond the surf zone (i.e. to the offshore marine environment). Also referred to in the literature as a long sea outfall/pipeline and ocean outfall/pipeline.
Mean sea level	The average elevation of the sea surface for all stages of the tides over a long period.
Membrane desalination	The use of membranes to remove salts from seawater.
Meteorological conditions	The prevailing environmental conditions as they influence the prediction of weather.
mg/L	Milligrams per litre is a measurement describing the amount of a substance (such as a mineral, chemical or contaminant) in one litre of water. One milligram per litre is equal to one part per million.
Micro-filtration (MF)	A water filtration method utilizing a pressure-driven membrane process that incorporates particle filters capable of rejecting particles larger than 1.0 microns. This process produces a less refined effluent compared to ultra-filtration.
Micro-layer	The upper few millimetres of the ocean. Fish eggs are sometimes concentrated in the micro-layer.
Mitigation	Preventing damage or repairing an area after construction or creating environmental improvements (sometimes in a different location).
Multi-effect Distillation (MED)	A form of distillation. Evaporators are in series, and vapour from one series evaporates water in the next. This technology has several forms, one of the most common of which is the Vertical Tube Evaporator (VTE).
Multi-stage Flash Distillation (MSF)	A form of distillation. The intake water is pressurized and heated. It is discharged into a chamber maintained slightly below the saturation vapour pressure of the water, and a fraction of the water content flashes into steam. The steam condenses on the exterior surface of heat transfer tubing and becomes product water. The unflashed brine enters a second chamber, where the brine flashes to steam at a lower temperature. Each evaporation and condensation series is called a stage.
MWQG	Marine Water Quality Guidelines
Nearshoredischarge	Diluting and mixing the concentrate with a large water flow and returning it to the nearshore area.
Ocean Thermal Energy Conversion (OTEC)	A solar, ocean thermal desalination approach where electricity is produced by using the temperature differential between cold, deep waters and warm, shallow surface waters. Water at the ocean surface (at about 70°F) is used to heat liquid ammonia, which vaporizes at this temperature in a vacuum chamber. The ammonia vapour is used to turn a turbine to produce electricity. The vapour is then condensed using cold water pumped up from the ocean depths (at about 35°F).
Offshore	Within the context of ocean outfalls, this is the zone in the sea in which wave action has an insignificant effect on water circulation and shoreline processes (erosion and accretion).
Offshore discharge	Discharge to the offshore areas.
Pollution	The direct or indirect alteration of the physical, chemical, or biological properties of the natural environment, including marine environments. This alteration makes the environment less suitable for its intended beneficial purposes or renders it harmful, or potentially harmful, to human health, safety, and welfare, as well as to aquatic and non-aquatic organisms.
Potable	Water that does not contain pollution, contamination, objectionable minerals or infective agents and is considered safe for domestic consumption.
PP	Polypropylene
Product Water	The desalinated water delivered to the water distribution system.
Reverse Osmosis (RO)	A desalination process where pressure is continuously applied to the feedwater, forcing water molecules through a semi-permeable membrane. Water that passes through the membrane leaves the unit as product water; most dissolved impurities remain behind and are discharged in a waste stream.
Rhodamine-B dye	A fluorescent red basic xanthene dye used in the marine environment to determine transport and dispersion patterns.
Saline water	Water that contains a significant concentration of dissolved salts (NaCl).
Salinity	Generally, the concentration of mineral salts dissolved in water. Salinity may be measured by weight (total dissolved solids – TDS), electrical conductivity, or osmotic pressure. Where seawater is known to be the major source of salt, salinity is often used to refer to the concentration of chlorides in the water.
SDI	Swartz’s Dominance Index is used to evaluate benthic community assemblages and is defined as the minimum number of species comprising 75% of the total abundance in a given sample.
Secondary dilutions	The further dilution that occurs after initial dilution when a wastewater plume is transported away from the discharge area.
Stagnant stratified conditions	The absence of currents and with stratification of the seawater (density gradient between the surface and the bottom).
Stagnant un-stratified conditions	The absence of currents and with no stratification of the seawater.
Stratification	When denser seawater underlies lighter seawater causing a vertical density gradient in the water column, depending on the vertical temperature gradient between warmer upper water layers and colder deeper water layers and the salinity gradient.
Surf zone	Also referred to as the “breaker zone”, where water depths are such that the incoming waves collapse and breakers are formed.
Suspended solids (SS)	Small solid particles that remain in suspension in water as a colloid or due to the motion of the water. It is used as one indicator of water quality. Sometimes abbreviated as SS, it should not be confused with settleable solids, also sometimes abbreviated SS, which contribute to the blocking of sewer pipes.
Seawater Reverse Osmosis (SWRO)	A desalination process where pressure is continuously applied to seawater, forcing water molecules through a semi-permeable membrane. Water that passes through the membrane leaves the unit as product water; most of the dissolved impurities remain behind and are discharged in a waste stream.
Thermaldesalination process	It involves heating seawater, generating water vapour, which is then condensed to produce fresh water.
Total Dissolved Solids (TDS)	Total salt and calcium carbonate concentration in a sample of water, is usually expressed in milligrams per litre (mg/L) or parts per million (ppm). The state- recommended Maximum Contaminant Level (MCL) drinking water standard for total dissolved solids is 500 mg/L, the upper MCL is 1,000 mg/L, and the short-term permitted level is 1,500 mg/L.
Total dynamic head	The summation of the hydraulic head (elevation, pressure, and/or friction losses) that the flow of water must overcome to move forward.
TSS	Total Suspended Solids
Turbidity	A measure of suspended solids concentration in water.
Ultra-filtration(UF)	A membrane filtration process that falls between RO and MF in terms of the size of particles removed.
Ultraviolet Treatment (UV)	The use of ultraviolet light for disinfection.
Vacuum Freezing (VF)	A process of desalination where the temperature and pressure of the seawater are lowered so that the pure water forms ice crystals. The ice is then washed and melted to produce the product water. This technology is still being developed and is not yet commercially competitive.
Vapour Compression	A form of distillation. A portion of feedwater is evaporated, and the vapour is sent to a compressor. Mechanical or thermal energy is used to compress the vapour, which increases its temperature. The vapour is then condensed to form product water, and the released heat evaporates the feedwater.

Contact EPCM