Ultrafiltration (UF) is part of the family of pressurised membrane filtration systems that can purify water from undissolved and most dissolved substances. UF is used as a final treatment step and can be applied in the acute response as well as in the stabilisation and recovery phases of emergencies.

UF uses membranes to filter water under pressure and functions the same as Microfiltration T.3 . UF units can be prefabricated and skid-mounted or available as singlemembrane modules. Most UF membrane modules in skidmounted systems are made of small, string-like hollow fibres (polymer materials) that are mounted in cylindrical (pipe-like) vessels or tanks due to the high packing density (2,000–15,000 m²/m³ depending on system type). The main difference between MF and UF is the exclusion size, which for UF is 0.1–0.01 μm and for MF is 0.5–0.1 μm. This leads to similar filtration results for particles, protozoa, bacteria (3-log to 6-log) and a significantly better removal of viruses (1-log to 5-log) with UF membranes. Additional proteins and polysaccharides are removed by around 80 % and humic substances by 40–60 %. Post-treatment usually includes disinfection, such as Chlorination T.6 , to provide residual protection.

Design Considerations

Membrane-based filters can be operated as dead-end-filters (feed is pushed completely through the membrane), or cross-flow filters (feed flows over the membrane, not all of the feed is filtered). Typical UF membranes are run as dead-end-filters. Depending on the intake water quality, particularly the turbidity (above 500 NTU), a pre-filtration T.2 or T.3 or pre-treatment T.5 should be considered to avoid membrane plugging. Pre-treatment always includes a protective pre-screen (typically an auto-backwashing type rated at about 300 micron). Additional pre-treatment, such as Assisted Sedimentation T.4 , can improve the removal of dissolved materials and reduce the fouling potential of the water. Automatic in-line coagulation followed by direct microfiltration is also used for water with a high fouling potential. Regular backwashing removes particles accumulated on the membrane surface. Prefabricated and skid-mounted UF systems usually include a control system to regulate operating conditions, including pump-driven filtration, backwash frequency, chemical cleaning (typically once a month), and integrity tests (to ensure the membranes are not damaged). Typically, systems backwash themselves with filtered water every 20–30 minutes depending on the raw water quality. During the 2–3 minutes of backwashing, the unit does not produce filtrate. Overall, about 85–95 % of the feed water becomes usable filtrate, and the rest is discharged as spent backwash or chemical cleaning waste. Typically, UF systems using the principle of dead-end filtration are operated in a constant flow mode with the transmembrane pressure in the range of 0.5–1 bar. Some systems are designed to work with Gravity S.7 , so are under constant pressure and variable flux. To avoid reversible or irreversible fouling, regular chemical cleaning must be carried out. To avoid damage, UF membranes should never run dry. It is possible to store some UF membranes after a preservation process, and there is no need for a filter-to-waste step after backwashing.

Materials

Typically in emergency situations, the UF system is bought as one unit, not just the membrane elements. Ancillary equipment, including support racks, pumps, valves, pre-screen(s) and a computer control system (for backwash and water quality monitoring) are just as important as the membranes themselves. Consumables include membrane elements (8–10-year service life if operated correctly), membrane repair kits, electricity and chemicals (e.g. citric acid, sodium hypochlorite for cleaning and disinfection; caustic sodium hydroxide and sodium bisulfide for neutralisation).

Applicability

UF technology can be used in a wide variety of contexts due to its modular functionality, giving it a flexible filter performance. It can be a one-step treatment, as it has excellent filtration, though can also be used as a pre-treatment step to reduce turbidity for Reverse Osmosis T.15 . Automated small-scale, skid-mounted systems are available and can be set up in a few hours. UF is sometimes applied in remote locations, though is typically reserved for a village or city. UF membrane filtration can be used in the acute response (as smaller units) and in the stabilisation and recovery phases of emergencies. UF membrane elements are modular, though adapting the number of modules in skid-mounted systems is not easy due to limitations of the auxiliary equipment (pumps, control systems).

Operation and Maintenance

Well-trained operators are required for a long, reliable service life. Although the systems are usually automated or semi-automated, operating mistakes can cause major damage to membrane elements (broken fibres, fouling). Regular O & M tasks include the daily verification of instrument accuracy and an integrity test, a daily check on chemical levels, a weekly calibration of chemical feed pumps, instrument cleaning, weekly review of data and consideration of revisions to operating parameters like flux, monthly (or sometimes more often) chemical cleaning and a volt-amp check on electric motors. Gravity-driven systems usually require regular manual backwashing (daily or weekly) and flow monitoring.

Health and Safety

Retentate disposal must be carefully considered, as it contains the contaminants found in the feed water. Depending on the makeup and local regulations, retentate can be directed back to the source, disposed of in the municipal sewer, diluted and used for irrigation or treated on-site before disposal. Treatment before disposal and reuse is recommended when disposal in municipal sewers is not possible. Cleaning chemicals can be corrosive and require trained operators and personal protective equipment.

Costs

Initial investment costs are comparatively high due to the cost of membrane modules and the need for advanced auxiliary equipment. While the UF membrane alone is relatively cheap (10–20 USD/m² of the membrane), the cost of the entire module varies between 70–120 USD/m² of the membrane, depending on the producer and membrane type. Regular maintenance will ensure a service life of up to 10 years (depending on the manufacturer), resulting in relatively low costs per user over time. A constant investment in cleaning agents, repairs and trained personnel is necessary and varies according to country and region.

Social and Environmental Considerations

Acceptance is high as the water produced is safe and clear, the colour is partially removed and there is no change in taste.