Design of Large Scale Seawater Intake & Brine Outfall Systems
Seawater intake and 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.
Impact on shoreline (construction and operational phase)
Direct sub-surface intake
Located offshore and feedwater transported to the desalination plant via an offshore marine pipeline
Large volumes – large pipelines – expensive
Better for exposed coastline & rough seas
Entrainment & Impingement
Limited impact to coastline
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.
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
Waves are an important environmental parameter required for the design and construction of structures in the marine environment
Initial and secondary dilutions
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.
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
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 ):
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)
Behavioural systems (i.e. electrical impulses to distract fish – not considered to have a high successful rate)
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 environmental sensitive areas and possible negative affect 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.
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 in order 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.
The intake structure should be designed in order 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.
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
Always better to discharge continuously
If possible fluctuations due to unforeseen production or operational issues: outfall system should be designed to discharge intermittently at design flow rates.
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 according to the design (environmental and physical) requirements if effluent is discharged at a reduced flow rate
Water Quality Objectives
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)
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)
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).
Required Dilutions: more below.
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)
S = Required dilution
Ce = Concentration of constituent in wastewater
Cb = Concentration of constituent in receiving marine environment (ambient concentration)
Cg = Recommended concentration (guideline)
The total dilution of conservative constituents at a distant location can be considered as two distinctive processes that is 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 sea water when an 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 with reference to density of the receiving water body.
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
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
Effluent density (which is more dense 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 in order to achieve the maximum path length of the plume.
Initial modelling, using numerous configurations, is required to optimize the diffuser design.
In order 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
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.