Membrane filtration solutions remove unwanted components and ensure stable water quality.

A membrane is a material that allows some substances to pass through and retains others – a selective barrier. A specific membrane material will retain components of a certain size or with certain chemical properties. For water filtration, pressure is used to get purified water through the membrane, while retaining unwanted components.

Water filtration membranes can be characterised by pore size. The pore size determines which components are retained (as shown in the figure below). The type of membrane filtration process is chosen based on desired water quality.

What are membranes able to remove?

1. Particle removal:

Turbidity, totally suspended substances
Mineral particles, sand, clay and soil, colloids (0.1–1 μm)
Organic particles derived from living microorganisms; algae, protozoa, etc. (1–100 μm)

2. Organic pollution; colour and humus/ natural organic matter (NOM)

3. Biological pollution; parasites, bacteria and viruses:

Salmon lice (Lepeophtherius salmonis) 540–850 μm up to 5000–10,000 μm
Gyrodactulus salaris 500 μm
Paramoeba perurans (AGD) 20–30 μm
Ichthyobodo sp. 7 μm
Aeromonas salmonicida 0.5–6 μm
Yersinia ruckeri (Yersiniose) 1–3 μm
Moritella viscosa (Winter ulcers) 0.5–2.5 μm
SGP-virus (Salmon Gill Poxvirus) 0.3 μm
ISA-virus 0.09–0.13 μm
IPN-virus 0.06 μm

FiiZK Aqua offers facilities for:

Ultrafiltration (UF)
Nanofiltration (NF)
Reversed osmosis (RO)

The main components in membrane filtration plants are pre-filtration / pre-treatment, pump(s), and membrane modules with a total membrane area given by the needed water capacity.

Our plants have fully automated processes with programmable logic control (PLC).

Ultrafiltration (UF)

Pore size: 0.01-0.1 µm

UF renders sea water and fresh water almost particle, bacteria and virus free.

Ultrafiltration (UF) purifies sea water and fresh water, removing suspended solids and most bacteria and viruses. UF-membranes have pore sizes from 0.01 µm to 0.1 µm, and components in the water are separated out based on size and shape. Pressure is the driving force for the separation. Ultrafiltration is operated at 1-2 bar.

No desalination takes place by ultrafiltration of sea water; the salinity will be the same before and after filtration.
For fresh water, ultrafiltration can be combined with coagulation to remove natural organic matter (NOM). NOM is the component that gives water its colour, and humus is a large part of NOM.

Ultrafiltration can be used to achieve stable intake water quality for land-based plants in aquaculture. UF provides excellent conditions for UV treatment, which is recommended as a double barrier against pathogens.

Metals are often tied to NOM in fresh water. Removal of this fraction with UF in combination with coagulation will reduce the content of metal in the water and the risk of releasing toxic metals into fish farming plants.

Ultrafiltration illustration (*by: Suspended solids)*

Water quality ultrafiltered sea water

Particles, parasites,
bacteria and viruses: ≈ 0

Application areas

Intake water for land-based
aquaculture facilities

Water quality ultrafiltered fresh water

Particles, parasites, bacteria and
viruses: ≈ 0
Colour: 5 (with coagulation)
TOC: 2 mg / l (with coagulation)
Reduced level of metals by coagulation

Application areas

Intake water for land-based
aquaculture facilities

Nanofiltration (NF)

Pore size: 0.01-0.001 µm

NF is used for the partial desalination of sea water, brackish water and salty groundwater. It can also be used to cleanse fresh water.

In NF membranes, the pores are between 0.001 and 0.01 µm. Separation based on size and shape, solubility and diffusion in the membrane material itself and effects of the surface properties of the membrane – mainly the membrane charge. Pressure is the driving force for the separation. Nanofiltration plants are operated at 8-40 bar, based on how dense the membrane is.

Nanofiltration can be used to produce low salinity water for use in the prevention and treatment of fish parasites, such as lice and AGD.

NF can also be used to produce low salinity water as intake water for RAS (Resirculating Aquaculture Systems) plants, especially for post-smolt production. Intake water with the desired salinity can be produced directly from sea water, no freshwater source is needed. The sulphate content is greatly reduced by nanofiltration, which can prevent H₂S formation. At the same time, release of metals by mixing sea water and fresh water is avoided, and the filtration provides increased biosecurity.

Nanofiltration illustration (*by: Suspended solids)*

Water quality low salinity water

Salinity: 3-22 ‰
NaCl: 2900-21900 mg/l
Ca² +: 5-50 mg/l
Mg² +: 3-50 mg/l
SO₄²-: <15 mg/l
Suspended solids, parasites, bacteria, virus: ≈ 0

Application areas

Prevention and treatment of fish parasites
Intake water for land-based aquaculture facilities
Intake water for post-smolt production

Reverse osmosis (RO)

Pore size: < 0.001 µm (1nm)

RO removes virtually all salt and other impurities from sea water. This yields ion-modified water with extremely reduced content of salt.

Reverse osmosis (RO) is used for desalination of sea water, brackish water and salty groundwater. Reverse osmosis is proven technology for production of potable water from sea water and brackish water.

RO membranes are dense films, with pore sizes less than 1 nm (0.001 µm). Practically all dissolved solids are retained, what passes through the membrane is based on solubility and diffusion in the membrane material itself. Pressure is the driving force for the separation, and in RO plants pressure from 30 to 85 bar must be applied.

Reverse osmosis can be used for the production of fresh water from seawater for use in the prevention and treatment of fish parasites, such as lice and AGD.

RO can also be used to produce fresh water for land-based plants. With the help of RO, you can transform sea water to fresh water and be independent of a freshwater source on land.