Granular Microfiltration: Where It Sits Between Coarse Filtration and Ultrafiltration

Filtration is the operation in which the liquid in a suspension passes through a medium under a driving force, and solid particles and other substances are intercepted by that medium, separating solids from liquid. Granular microfiltration — also called granular microporous filtration, and sometimes high-efficiency microfiltration — applies that definition using nano-composite particles as the filtration medium instead of a membrane sheet. Developed and promoted over the past ten-plus years using natural or synthetic nano-composite compounds as the filtration material, it fills a gap that single microfiltration-membrane filtration never covered well.

What It Catches and What It Lets Through

Granular microfiltration intercepts particles between 0.1 and 2 microns. Macromolecules and dissolved solids such as inorganic salts pass through; suspended solids, bacteria and large-molecular-weight colloids do not. The same selectivity holds for the second description of the medium: macromolecular organics and inorganic salts pass, while suspended solids, bacteria, some viruses and large-scale colloids are blocked.

That selectivity is a design feature rather than a limitation. A medium which also retained dissolved salts would be a nanofiltration membrane, and would arrive with the flux penalty and the cleaning regime that come with it. Its effective separation range is particles of 0.1-10 um, and it is classified as precision filtration — high efficiency, convenient and economical. The name is accurate: microfiltration placed between coarse filtration and ultrafiltration.

Speed Comes From Porosity

Surface porosity is high, generally reaching above 70%, which makes the medium at least tens of times faster than a filter membrane of equivalent interception capacity. That ratio is the practical argument for the technology. Where a membrane of the same cut-off would need a large installed area and a careful flux policy, a granular bed delivers the same separation through a much smaller vessel.

There is a purity argument too. Nano-composite particles form a high-hardness continuum, so during filtration no medium detaches and no secondary pollution is caused. The filtrate is high-purity in a way that a shedding medium cannot match.

Four Filtration Principles

Granular microfiltration operates on four principles: sieving, filter-cake-layer filtration, deep-bed filtration and adsorption. The separation mechanism is generally held to be sieving, with the physical structure of the particles playing the decisive role, though adsorption and electrical properties also affect the interception rate.

Broken down by where the particle ends up, there are three interception mechanisms:

• Sieving — microporous filter particles retain particles larger than or comparable to the pore size. Also called mechanical interception.
• Adsorption — suspended particles are held by physicochemical adsorption, so matter smaller than the particle pores can still be intercepted.
• Bridging — suspended particles pile up and push against each other, so many cannot enter the pores or become stuck inside them, completing the interception.

In service these do not operate in isolation. A freshly backwashed bed starts with sieving and deep-bed capture; as the cake builds, cake-layer filtration takes over and the effective cut-off drifts finer. Operators who expect a constant cut-off across a run tend to misread their own trend data. Bridging is the mechanism that surprises people. It means the medium can retain particles nominally smaller than its pores, which is useful in practice and inconvenient in modelling.

Operating Conditions

The operating pressure of granular microfiltration is 3-6 bar. The operating static pressure difference is 3-6 MPa. Both figures should be read in context: the first describes normal running, the second the pressure the bed can be driven to before the differential becomes the limiting factor. For design purposes, the approach is the same one used for any deep bed — start from raw-water quality and quantity, size the equipment and structure accordingly, then size the water-distribution plate, branch pipe and vessel around the chosen flux.

Where It Is Used

The application list is broad because the separation range is useful in most process water:

• Water treatment — removal of suspended solids, fine particles and bacteria, and pre-treatment ahead of reverse osmosis and nanofiltration.
• Electronics — terminal treatment of ultrapure water for semiconductors and cleaning water for integrated circuits.
• Pharmaceuticals — bacteria removal, pyrogen removal and drug sterilisation in medical pure water.
• Medical — removal of bacteria from tissue fluid, antibiotics, serum and plasma proteins.
• Food and beverage — removal of suspended matter, microorganisms, odour impurities, yeast and mould, plus clarification of fruit juice.
• Chemical industry — filtration and clarification of various chemicals.
• Surface water — removal of algae and particulate impurities from reservoirs, lakes and rivers.
• Household — filter membrane modules for domestic water dispensers.

The reverse-osmosis pre-treatment duty is where the economics usually close. Protecting an RO membrane from particulate load is worth more than the filtration step costs, and a medium that runs tens of times faster than an equivalent membrane makes the skid small enough to fit.

As an effective supplement to traditional single microfiltration-membrane filtration, granular microfiltration is worth costing whenever the duty falls between what a sand filter can hold and what an ultrafiltration membrane is being asked to do.