Ultrafiltration is a membrane stage that retains everything larger than the pores of the membrane: suspended solids, colloids and micro-organisms. Separation is by size, and the retention threshold is from 0.1 down to 0.01 µm. Dissolved matter — hardness salts, nitrates, the salinity as a whole — passes through such a membrane unchanged.
Principle
The working element of the unit is a hollow-fibre membrane module: a bundle of hollow fibres whose wall is a porous partition. In the units of the catalogue the fibres work inside-out: feed water is delivered into the inner channel of the fibre and leaves through its wall, and the filtrate is collected in the module housing. The governing parameter of the range is not the size of the vessel but the total membrane area: the smaller models carry two modules of some 27 m² in total, the 5 m³/h models four modules of 54.8 m², and the area grows further along with capacity.
The driving force is the pressure difference across the fibre wall, known as the transmembrane pressure. It is modest: the working pressure of the catalogue models lies within 0.2–0.4 MPa, whereas reverse osmosis on fresh water calls for 1.0–1.3 MPa. The reason lies not in the strength of the membrane but in the physics of the process: there is no osmotic pressure to overcome here at all, since dissolved salts pass through the pores and create no difference in concentration across the membrane. Hence the several-fold difference in energy consumption between the two membrane technologies.
What is retained does not leave the apparatus by itself: it builds up on the fibre wall as a layer that raises the transmembrane pressure. The layer is removed by backwashing — filtrate is driven through the wall in the reverse direction and flushes the deposit to drain. A backwash is started by timer or on reaching a set differential, takes tens of seconds and repeats in every cycle. During a backwash the unit delivers no filtrate, and that is covered either by a storage tank or by spare capacity.
Flux through a porous membrane is inversely proportional to the viscosity of the water, and viscosity depends on temperature: at one and the same transmembrane pressure cold water yields less filtrate than warm water. The permissible water temperature for the catalogue models is from 5 to 40 °C, and the size is chosen for the winter minimum rather than for the annual mean: a unit sized on summer water does not reach the required flow in winter.
Fouling divides into reversible and irreversible, and the maintenance regime follows from that. A loose layer of solids is removed by backwashing with water; mineral scale and biofilm are not, and they call for chemical regeneration — a backwash with reagent added, or a full clean with circulating solutions. Acid dissolves the mineral part, alkali and sodium hypochlorite lift organics and biological growth. The pH permitted during cleaning is wider than in service: from 2 to 12, against a working range of 3 to 9.
Principle
A hollow-fibre module works in two fundamentally different hydraulic modes, and the choice of mode changes both the water spent on the unit's own needs and the limiting turbidity of the feed. In dead-end mode all the water supplied leaves as filtrate while what is retained accumulates on the fibre and is removed by periodic backwashing: there is no continuous reject, the share of water spent on own needs is small, but the feed water has to be comparatively clean. In cross-flow mode part of the stream travels along the fibre and carries the retained matter away continuously: a continuous reject appears, water and energy consumption rise, but the apparatus tolerates markedly more turbid and more heavily loaded water. One and the same size delivers a different filtrate capacity in these two modes, so the mode is fixed by calculation before ordering rather than settled during commissioning.
An ultrafiltration membrane is cleaned with sodium hypochlorite — the very reagent that irreversibly destroys the polyamide active layer of a reverse osmosis element at concentrations of the order of 0.1 mg/l. The difference in the chemical resistance of the materials is fundamental here, and two practical conclusions follow from it. Ultrafiltration tolerates water with residual free chlorine and is therefore applicable directly downstream of a chlorinated source. At the same time the pairing of ultrafiltration with reverse osmosis does not remove the need for dechlorination: the resistance of the first membrane does not extend to the second, and an adsorption stage or the dosing of a reducing agent between them remains obligatory.
How clear the water looks says nothing about its suitability for membrane desalination. The danger comes from colloids — particles of a fraction of a micrometre that neither settle nor are visible; their content is assessed by the silt density index SDI, measured on a separate 0.45 µm membrane, and for polyamide elements it is usually kept below 5. A 5 µm cartridge does not lower that index at all — colloids pass straight through it — while granular filtration of surface water gives a result that shifts with floods and algal blooms. Ultrafiltration does lower SDI, because its retention threshold is two orders of magnitude finer, and that is precisely its role as pretreatment: it does not improve the water in general, it makes one specific parameter predictable at the inlet of the membrane unit.
Practice
Ultrafiltration closes the undissolved part of the task entirely and in terms of result replaces the pairing of settling and microfiltration — two units that take up floor space and require attention. The quality of the filtrate is set by the retention threshold rather than by a settling regime, so seasonal changes in the source do not show at the outlet: a flood, an algal bloom or stirred-up sediment after works on the mains change the frequency of backwashes, not the turbidity of the treated water.
The second typical role of the stage is pretreatment ahead of reverse osmosis on difficult water. The economics of such a scheme are counted not by the cost of the unit but by the service life of the osmosis elements and by the number of chemical cleans per year: the less stable the source, the sooner ultrafiltration pays for itself. One reservation is essential: it removes the load of suspended solids and colloids only, while hardness salts, silica and alkalinity reach the osmosis stage unchanged, so softening or antiscalant dosing does not disappear from the scheme.
The microbiological barrier here is mechanical and calls for neither reagent nor contact time. It is not, however, disinfection in the regulatory sense: the barrier exists exactly as long as the fibres are intact, and a single broken fibre barely shows in flow or pressure and is detected only by monitoring the turbidity of the filtrate. The line downstream of ultrafiltration is therefore closed by ultraviolet disinfection rather than treated as complete.
The requirements on the feed water look different from those of reverse osmosis, but they exist. A coarse strainer ahead of the module is obligatory: sand and large solids damage the fibre mechanically. Iron and manganese must either be within limits or fully oxidised at a preceding stage — in dissolved form they pass through the membrane, and as they oxidise inside the pores and on the fibre wall they leave deposits that backwashing no longer removes.
The unit is supplied assembled on a frame and comprises the membrane modules, a pump, a prefilter, the backwash assembly, instrumentation and a mode controller. The site has to provide a heated room at 5 to 35 °C, a feed water connection of the design flow, a drain line for backwash water, power supply and access for servicing the modules. We do not manufacture this equipment: our part of the work is the water analysis, the calculation of the scheme and of the membrane area, the choice of size and mode, supply, commissioning and subsequent service.
Limits
Practice
The cases are typical examples, not site reports.
Innovations
Scheme
Unlike reverse osmosis, ultrafiltration can be a stage in its own right: where the task is confined to turbidity, colour of suspended origin and microbiology, the scheme consists of a strainer, the unit, a storage tank and disinfection. A coarse strainer is obligatory in every case: it protects the fibre from sand and large solids.
The requirements on the feed water are stated in figures rather than in words: limiting turbidity and suspended solids to suit the chosen mode of operation, temperature from 5 to 40 °C, pH from 3 to 9, iron and manganese either within limits or fully oxidised at a preceding stage. Free chlorine, unlike in schemes built around polyamide membranes, is permissible here.
In combination with desalination the order of the stages admits no rearrangement: ultrafiltration comes first and removes suspended solids and colloids, while dechlorination and antiscalant dosing or softening remain between it and the osmosis stage. The reverse order makes no sense: the osmosis stage would receive the whole colloidal load for the removal of which the stage is installed.
The backwash water line is treated as part of the scheme rather than as an auxiliary drain. It is sized on the instantaneous backwash flow rather than on the average share, because the discharge comes in bursts. Cleans with reagent call for neutralisation of the solution discharged; the discharge point and its limits are agreed with the water and sewerage authority before the equipment is ordered.
Scheme
Operation
Energy
The specific power consumption is set by the working pressure and is therefore modest: at 0.2–0.4 MPa and with backwashes taken into account it comes to some 0.1–0.2 kWh per cubic metre of filtrate. The exact figure depends on pump efficiency, on losses in the pipework and on the frequency of backwashes, which rises with the turbidity of the feed. For comparison, reverse osmosis on fresh water consumes 0.3–0.8 kWh per cubic metre of permeate, and the difference is explained not by the design but by the fact that no osmotic pressure has to be overcome here.
Consumables
What goes to drain
Backwash water contains everything retained in a reduced volume: suspended solids, colloids, biomass and the organics bound up with them. The discharge comes in bursts, so the line is sized on the instantaneous backwash flow rather than on the averaged share: that instantaneous flow is of the order of 10–20 % of the feed. After cleans with reagent the discharge additionally calls for neutralisation — the working solutions of chemical regeneration have a pH of between 2 and 12. The discharge point and its limits are agreed in advance, and they differ from site to site.
Recovery
What ultrafiltration spends on its own needs is backwash water. In dead-end mode a backwash is short and periodic, so the share averaged over the cycle usually comes to a few per cent, even though the instantaneous flow of the backwash itself reaches 10–20 % of the feed. The figure is not a constant: it rises with the turbidity of the feed water, since dirty water calls for more frequent backwashes, and it is markedly higher in cross-flow mode, where a continuous reject is added to the backwashes. The flow should be counted at the inlet rather than at the filtrate, and that requirement falls on the source rather than on the unit.
Sizing
What we account for
What a wrong choice costs
Sizing
Questions
These are different tasks, not different levels of quality. Ultrafiltration is a porous barrier with a retention threshold from 0.1 to 0.01 µm: it retains suspended solids, colloids and micro-organisms, that is, everything larger than the pores. A reverse osmosis membrane has no through pores at all, transport goes through a dense polymer and separation takes place at the level of ions, which is why salinity falls. The order of decision is simple: take the analysis report and look at two groups of parameters separately. Turbidity, colour, suspended solids and microbiology are the domain of ultrafiltration. Salinity, hardness, nitrates and fluorides are the domain of osmosis. If there are exceedances in both groups the scheme will have two stages, and ultrafiltration will be the first of them.
No, and this is not a question of the degree of treatment. Calcium, magnesium, sodium, nitrates and chlorides are in the dissolved state, whereas separation at this stage is by size, so dissolved matter passes the membrane unchanged. After ultrafiltration brackish water becomes clear and stays brackish, and scale forms in the same amount as before. For these parameters ion exchange or reverse osmosis is used.
It does not, although it does create a barrier by size against bacteria. Viruses are retained the more completely the lower the retention threshold of the module, and at 0.1 µm retention is incomplete. Beyond that, the barrier exists exactly as long as the fibres are intact: a single broken fibre barely shows in flow or pressure and is detected only by monitoring filtrate turbidity. The line downstream of the unit is therefore closed by ultraviolet disinfection, and the membrane is regarded as a stage that reduces the load on it rather than one that removes the need for it.
A coarse strainer at the very least: sand and large solids damage the fibre mechanically and irreversibly. What else is needed follows from the analysis. Where the water carries dissolved iron and manganese, their oxidation is completed before the membrane — otherwise they pass through it in ionic form and settle on the fibre from the other side. Free chlorine, unlike in schemes with polyamide reverse osmosis elements, is permissible: ultrafiltration membranes are themselves cleaned with sodium hypochlorite.
In dead-end mode it is backwash water. The instantaneous flow of a backwash is of the order of 10–20 % of the feed, but the backwash itself takes tens of seconds and repeats periodically, so the share averaged over the cycle usually comes to a few per cent. It rises with the turbidity of the feed water, and in cross-flow mode a continuous reject is added to the backwashes, which makes the share markedly higher. The drain line, meanwhile, is sized on the instantaneous flow, since the discharge comes in bursts.
In terms of result, yes, and on surface water that is often the preferable decision: the quality of the filtrate is set by the retention threshold of the membrane and therefore does not depend on the seasonal state of the source, whereas granular media give a variable result during a flood. The two decisions differ in operation: a membrane stage calls for a controller, a regime of chemical cleans and a drain line, but it takes up less floor space and lowers the silt density index SDI, which a granular filter does not do. The choice is made on the analysis and on the requirements of the stages that follow.
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