Seawater intake & outfall systems are used across the world, to draw in seawater and dispersion of brine, for desalination and power plants – producing potable water for drinking and as a source for cooling and steam to aid onshore processes.
Table of Contents
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
Overview of pipeline 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
Seawater Intake & Outfall Systems
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
Plan view Surface intake
2.2.2 Direct sub-surface intake
- Is located offshore and feedwater transported to the desalination plant via an offshore marine pipeline
- For large volumes – large pipelines – expensive
- Better for exposed coastline & rough seas
- Entrainment & Impingement
- Limits impact on the coastline
- Requires offshore maintenance
Side view graph of what Sub-surface intake
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 of the transport (onshore/offshore) of surface waste fields.
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 an important environmental parameter required for the design and construction of structures in the marine environment
2.3.4 Currents
Initial and secondary dilutions
2.3.5 Stratification
Stratified conditions (layering in the water column) occur due to a density gradient between the surface and the bottom, subsequently inhibiting a buoyant plume to rise with subsequent reduced initial dilution, resulting in a submerged waste field below the surface, however, the density of the effluent which will be discharged offshore will be denser than the receiving environment (seawater) and therefore the possible effect of stratification on the initial dilution are considered insignificant.
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 VS environmental & physical requirements & characteristics
Generally, the deeper the intake structure (furthest offshore) with the extraction point raised a few metres from the seafloor, the cleaner the feedwater (e.g. less sediment in suspension) and the less impact of wave forces on the structure, while ensuring the extraction point is 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 and especially the location of the brine outfall which could lead to re-circulation.
From an environmental point of view for a sub-surface 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 and environmentally sensitive areas and possible negative effect on the coastline (alongshore sediment regime) have to be considered.
Finally, the proximity of the proposed location with regards to port and fishing activities together with popular navigation ship routes should be taken into account. Ship anchors can cause major damage to sub-surface structures, fishing nets can block the intake screens and pollution caused by vessels or port activities could impact the 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 of great value 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
Seawater Intake & Outfall Systems
3 Brine Outfall: Design Considerations
Discharge effluent while ensuring the impact to the environment is minimized and the system adheres to the appropriate environmental guidelines, regulations and legislation
It is always better to discharge continuously
When possible fluctuations 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. Subsequently, the system will not perform to the design (environmental and physical) requirements should 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 to regulate 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 provides recommended target values for a range of 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 and therefore achieving acceptable concentration levels for maintaining ecosystems functioning and recreational human activities (e.g. 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 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 in the receiving water body. The degree of entrainment is related to the shear between the plume and the adjacent water, which is a function of the momentum and the buoyancy of the effluent jet. The initial dilution process will cease when the vertical velocity of the plume reaches zero or when the plume reaches the surface of the water.
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 relative density of the effluent regarding the density of the receiving water body.
3.4 Achievable Initial Dilutions
Physical properties of brine plume (negatively buoyant) limit the initial dilutions which can be achieved
Dense plume will sink to the seafloor
Outfall Systems
- Salinity concentration of brine stream from a 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
Seawater Intake & Outfall Systems
3.6 Diffuser Configuration
Seawater Intake & Outfall Systems
3.7 Brine Outfall: Diffuser Configuration – Untapered Diffuser
Seawater Intake & Outfall Systems
3.8 Brine Outfall: Diffuser Configuration – Tapered Diffuser
Seawater Intake & Outfall Systems
3.9 Optimization of Diffuser Configuration
Determine influencing aspects (controlling parameters):
Effluent density (which is denser than seawater for a brine effluent)
The required dilution to meet the environmental objectives (which would normally be in the order of 20, depending on the site-specific requirements
The diameter of the main pipeline, which would first be determined for the outfall pipe itself, which subsequently will determine the diffuser configuration (i.e. the port diameters and the number of ports).
Additional momentum is required for a “long” enough path of the jet plume to entrain seawater for achieving the required dilutions, thus the momentum flux for each port has to be increased. It is not advisable to raise the port velocities too high, since the forces on diffuser components become greater as the port velocities increase.
Although a brine diffuser does not require great depth due to the limiting rising height of the effluent plume, the more inshore any marine structure, the more vulnerable to nearshore physical processes (wave forces and unstable seabed conditions).
The discharge angle of the ports should be inclined to the horizontal to achieve the maximum path length of the plume.
Initial modelling, using numerous configurations, is required to optimize the diffuser design.
To provide developers with an initial estimation of the diffuser configuration requirements, a method was provided, based on scientific theories and generally accepted environmental regulations, which will provide 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, which the engineer has on the technique (‘model’) and the thorough understanding of the theories that are applied.
Not one of the theories/ prediction techniques available can be considered as inaccurate because of these were not developed in isolation and was 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 sensitivity of the estimation to the complexity of the continuously varying processes in the receiving environment.
Seawater Intake & Outfall Systems
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 a 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 part of the land affected by its proximity to the sea, and that part of the sea affected by its proximity to the land. |
| Cogeneration | A power plant that is 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 freshwater |
| Diffuser | The offshore end (part) of an outfall, consisting of the main pipe (with or without tapers) with discharge ports at specific distances apart, 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 extraction 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 with the non-living (physical and chemical) components of their environment |
| 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 due to 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. The intermediate area becomes depleted of impurities and discharges a purified stream of product water. This technology is used for brackish waters but is not currently available for desalting seawater on a commercial 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 which is 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 the upstream boundary of an estuary is the extent of tidal influence. |
| Euphotic zone | The euphotic zone is the depth of the water body in an ocean that is exposed to sufficient sunlight for photosynthesis to occur. |
| Far-field dilution | When an effluent plume is transported away from the initial mixing zone, dispersion, entrainment and mixing with seawater is brought about by currents, turbulence, eddies and shears, a process generally referred to as secondary dilution which together with the chemical/biological ‘dispersion’ of non-conservative substances and the decay of certain organisms, can be described 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 allowing 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 | 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 seabed (e.g. via beach wells) |
| Infiltration Gallery | A method used for seawater intake. Perforated pipes are arranged in a radial pattern in the sand onshore 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 which causes 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) |
| Marine Environment | 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, which 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 | 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; a measurement describing the amount of a substance (such as a mineral, chemical or contaminant) in a litre of water. One milligram per litre is equal to one part per million |
| Micro-filtration | A method of water filtration, using a pressure-driven membrane process, which includes particle filters that reject particles larger than 1.0 micron in size. Provides a less refined effluent than ultra-filtration |
| Micro-layer | The upper few millimetres of the ocean. Fish eggs are sometimes concentrated in the micro-layer |
| Mitigation | The process of 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 is used to evaporate water in the next one. 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 brine flashes to steam at a lower temperature. Each evaporation and condensation series is called a stage |
| MWQG | Marine Water Quality Guidelines |
| Nearshore discharge | Diluting and mixing the concentrate with a large flow of water 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 by 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 the marine environment, to make it less fit for any beneficial purpose for which it may reasonably be expected to be used, or to make it harmful or potentially harmful to the welfare, health or safety of human beings or to any aquatic or 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 process of desalination where pressure is applied continuously to the feedwater, 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 |
| 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: used to evaluate benthic community assemblages and 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) | The term “Suspended solids” refers to small solid particles which 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. It is sometimes abbreviated SS but is not to be confused with settleable solids, also sometimes abbreviated SS, which contribute to the blocking of sewer pipes. |
| Seawater Reverse Osmosis (SWRO) | A process of desalination where pressure is applied continuously 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 |
| Thermal desalination process | Involves the heating of seawater, generating water vapour, which in turn is then condensed to produce fresh water. |
| Total Dissolved Solids (TDS) | Total salt and calcium carbonate concentration in a sample of water 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 the permitted level is 1,500 mg/L |
| Total dynamic head | The summation of the hydraulic head (elevation, pressure, and/or friction losses) that a 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 reverse osmosis (RO) and micro- filtration (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 is used to evaporate the feedwater |
