The intake sets the ceiling on pre-treatment performance and the floor on membrane life. This guide covers open versus subsurface intakes, velocity caps, screening selection and impingement/entrainment mitigation for reliable seawater supply.
Turnkey SWRO plants from intake screening through pre-treatment to membrane racks.
Harmful algal bloom protocols that begin at the intake with siting and source control.
Fouling indices and pre-treatment that depend on a clean, stable intake water quality.
The single most consequential decision for downstream pre-treatment
Offshore screened intakes or shoreline structures draw large flows from any site, but deliver raw seawater with the full burden of turbidity, algae, organics and seasonal HAB events — placing the entire load on DAF and filtration pre-treatment.
Beach wells, slant wells and infiltration galleries use the seabed as a natural filter, delivering low-SDI, algae-free water that can halve pre-treatment cost. Limited to suitable hydrogeology and moderate capacities, with a risk of reducing conditions and manganese.
Capacity, coastal geology, water-quality variability, environmental permitting and capital budget decide the route. Large plants on exposed coasts almost always default to open intakes with robust multi-barrier pre-treatment.
Protecting marine life and protecting the plant are the same design problem
Impingement velocity cap: regulators commonly limit through-screen velocity to ≤ 0.15 m/s (0.5 ft/s) so that fish and larvae can swim away from the screen face. This drives the open area, and therefore the physical size, of the intake screens.
Screening train: coarse bar racks (50–100 mm) remove debris; travelling band or drum screens (1–10 mm) follow; fine wedge-wire or passive cylindrical screens (0.5–3 mm) with airburst cleaning protect the pumps and reduce entrainment ahead of the pre-treatment plant.
Keeping the intake compliant, reliable and low-maintenance
Low approach velocities, fish-friendly travelling screens with fish-return troughs, and passive wedge-wire screens minimise the organisms pinned against the screen face.
Fine cylindrical screens with 0.5–2 mm slots and subsurface intakes cut the eggs and larvae drawn into the plant — a key permitting metric on sensitive coastlines.
Intermittent or low-dose chlorination, periodic thermal/mechanical cleaning and antifouling coatings keep mussels and barnacles from constricting intake tunnels and pipelines.
Intake depth and offset are chosen to draw below surface algal layers, reducing the organic and TEP load that reaches DAF during bloom season.
Through-screen velocity limits, impingement and entrainment physics, and velocity cap design for marine life protection and reliable flow.
Regulatory frameworks worldwide cap through-screen velocity at ≤0.15 m/s (0.5 ft/s) to allow fish and mobile invertebrates to escape the intake zone. For non-fish-bearing waters or cooling-water intakes, a relaxed limit of ≤0.3 m/s (1.0 ft/s) is sometimes permitted. These limits directly determine the required open screen area: Ascreen = Q / vmax. A 100,000 m³/day plant at 0.15 m/s requires >7.7 m² of effective screen area, driving the physical footprint and structural design of the intake headworks.
Impingement occurs when organisms lack the sustained swimming speed to overcome the local velocity field at the screen face. Critical escape velocities vary by species: juvenile fish 0.1–0.3 m/s; adult fish 0.3–0.8 m/s; crustaceans 0.05–0.2 m/s. Approach velocity (the vector sum of ambient current and intake suction) must remain below escape thresholds. Velocity caps — bell-mouth or hooded structures — redirect flow vertically from below, reducing the horizontal approach velocity and creating a velocity refuge near the seabed.
Entrainment affects eggs and larvae too small to avoid screens passively. Egg diameters range from 0.5–2 mm (fish) to 0.1–0.5 mm (invertebrates). Screen slot width must exceed organism size by a safety factor to retain organisms while passing water. Wedge-wire slots of 0.5–1 mm retain >95% of fish eggs; 2–3 mm slots retain >80% but allow higher hydraulic capacity. Passive intake screens with airburst or brush cleaning maintain slot integrity without manual intervention.
Modern velocity caps use reinforced concrete or steel bell-mouth profiles with intake ports oriented downward or horizontally to draw from mid-water rather than the surface. Key parameters: cap diameter 2–5 m; intake elevation 2–5 m above seabed to avoid sediment resuspension; offset 50–200 m from shore to access stable salinity and temperature. Computational fluid dynamics (CFD) modelling optimises port orientation and spacing to minimise vortex formation and ensure uniform velocity distribution.
Mussel growth, barnacle settlement, microbiologically influenced corrosion, and the biocide strategies that control them.
Blue mussels (Mytilus edulis) and zebra mussels (Dreissena polymorpha) settle on intake walls, tunnels and pump casings when water temperature exceeds 10–12 °C and calcium availability is adequate. Veliger larvae prefer surfaces with low shear (<0.5 m/s) and positive electrochemical potential. Attachment strength reaches 1–2 MPa, making mechanical removal difficult. Once established, mussel colonies constrict flow area, increase headloss, and shed larvae that colonise downstream pre-treatment equipment.
Barnacles (Semibalanus balanoides, Balanus amphitrite) progress through a free-swimming nauplius stage to a non-feeding cyprid stage that actively explores surfaces for settlement. Cyprids prefer rough, high-energy surfaces with biofilm presence and avoid surfaces with high shear (>2 m/s) or low salinity (<20 ppt). Adult barnacles create protrusions that increase drag and turbulence, accelerating localised corrosion under their basal plates.
Sulphate-reducing bacteria (SRB) and acid-producing bacteria (APB) colonise intake pipelines under anaerobic deposits, generating H₂S and organic acids that pit carbon steel and degrade concrete. SRB activity is highest in stagnant zones and under mussel/barnacle beds where oxygen is depleted. Typical attack rates for unprotected carbon steel in MIC-active seawater intakes reach 0.2–0.5 mm/year. Cathodic protection, biocide dosing, and epoxy coatings are essential mitigation measures.
Continuous chlorination at 0.5–2 ppm free chlorine prevents mussel and barnacle settlement by oxidising the proteinaceous adhesion threads. Intermittent shock chlorination (5–10 ppm, 30–60 min daily) is effective for established colonies but requires dechlorination before RO. Non-oxidising alternatives include DBNPA (100–200 ppm, 2–4 hours contact), glutaraldehyde (50–100 ppm), and isothiazolinones — useful where chlorine discharge is restricted by environmental permits. Thermal treatment (water >40 °C for 1–2 hours) provides a chemical-free option for periodic descaling of intake tunnels.
Band screens, drum screens, wedge-wire and microstrainers — mesh sizes, capacities, maintenance regimes and selection criteria.
| Technology | Mesh / Slot Size | Typical Capacity | Cleaning Method | Maintenance | Best Application |
|---|---|---|---|---|---|
| Travelling Band Screens | 1–10 mm | 5–50 m³/s per unit | High-pressure spray (0.5–1.5 MPa) with fish-return troughs | Moderate: chain, bearings, spray nozzles | Large open intakes with high debris and fish loads |
| Rotary Drum Screens | 1–6 mm | 1–20 m³/s per unit | Internal backwash spray or brush | Low–moderate: seals, drive mechanism | Compact installations, moderate debris, space-constrained sites |
| Wedge-Wire (Passive) Screens | 0.5–3 mm | 0.5–10 m³/s per unit | Airburst or high-pressure backwash (no moving parts in flow) | Very low: periodic air system check | Offshore intakes, low-debris stable waters, fish-sensitive sites |
| Microstrainers | 20–100 μm | 0.1–2 m³/s per unit | Rotating drum with high-pressure spray | High: frequent mesh replacement, high headloss | Final protection before pumps or very sensitive ecosystems |
Selection guidance: For large SWRO plants (>50,000 m³/day) on exposed coasts, travelling band screens (3–6 mm) followed by wedge-wire passive screens (1–2 mm) provide the optimal balance of capacity, fish protection and maintenance burden. Drum screens are preferred for compact onshore intakes. Microstrainers are rarely used for raw seawater due to rapid blinding; they find application only where entrainment of eggs <0.5 mm is a strict permitting condition.
Species-specific intake challenges: cell size, buoyancy, and biomass loading
20–40 µm cells pass screens but are removed by DAF. Surface accumulations during stratification.
View Guide20–40 µm solitary cells. Vertical migration brings cells to surface intakes at night.
View Guide0.5–2 mm cells retained by drum/wedge-wire screens. Physical overload is the primary challenge.
View GuideCm-scale surface colonies. Coarse screens capture aggregates; fragments challenge fine screens.
View GuideReynolds & Bauhm designs intake screening and pumping that protect both marine life and your membranes, matched to your site and regulatory regime.
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