Filter Media and Contact Media Compared: Honeycomb, Soft Fibre, Quartz Sand, Carbon and Cloth

In a biological aerated filter, the medium is not a consumable bolted onto the process — it is the process. Surface area per cubic metre sets the biomass inventory, void geometry sets the flow regime, and mechanical strength sets how long the bed survives before it has to be replaced. Yet media selection is frequently made from a supplier catalogue, without reference to the failure modes that show up two or three years later.

What goes wrong with the conventional choices

The contact media commonly used in China mainly include FRP or plastic honeycomb media, three-dimensional corrugated media, soft fibre media, semi-soft media and irregular granular media such as sand, gravel, slag, coke and anthracite.

• FRP or plastic media. Smooth surfaces give poor biofilm adhesion and the material tends to age; in practice they often suffer clogging of varying degrees.
• Soft fibre media. The flow regime is not ideal, and the media are easily bound together by biofilm to form balls. Those balls greatly reduce the available surface area and create anaerobic conditions inside, which impairs treatment performance in a way that is hard to see from the effluent until it is severe.
• Irregular granular media. High flow resistance and a strong tendency to clog the oxidation tank.

Flaky ceramsite has also been studied in China for treating slightly polluted source water. It is an irregular granular medium: biofilm formation is good, but flow resistance is high, it clogs easily, its strength is poor and it breaks under water scouring. Those limits confine it to slightly polluted source water and keep it out of mainstream wastewater duty. It is precisely these shortcomings of traditional contact media that have limited the uptake of biological aerated filters in China — which makes a good contact medium the key to whether the process succeeds here at all.

Quartz sand: the reference medium

Quartz sand grains are polygonal or spherical in appearance and pure white, with high mechanical strength, a strong solids-capture capacity and good acid resistance. It is the most widely used filter medium and performs the filtration function in the most literal sense: much like water percolating through sand and gravel into the ground, it intercepts suspended solids, mainly the fine suspended particles.

Sand filtration is a rapid method for separating insoluble substances from water in both water supply and wastewater treatment. Since the 1960s, severe water shortages put wastewater reuse on the agenda, and filtration technology spread into advanced treatment as plants sought higher effluent quality. The usual position in a train is after chemical coagulation and biological treatment: water passes through a bed of porous media such as sand, coal particles or diatomite, and the operation removes dispersed suspended inorganic and organic particles, including plankton, bacteria, filterable viruses, floating oil and emulsified oil.

Granular activated carbon: adsorption rather than straining

Activated carbon filtration is a different function dressed in the same equipment. The abundant pores give the material a very large specific surface area — 500-1700 m2 per gram of material, with a specific gravity of 1.9-2.1 — so it adsorbs fine substances from liquids rather than simply trapping them between grains. Beds come as fixed, moving or fluidised configurations. For a fixed carbon layer the thickness is 1.5-2.0 m, the filtration rate is 8-20 m/h, the backwash water rate is 28-32 m/h and the backwash time is 4-10 min. Regeneration is by cleaning with high-temperature steam or by microwave heating, with a recovery rate of 80%-94%.

In advanced treatment of water supply and wastewater, activated carbon is mainly used for decolourisation, deodorisation, dechlorination and removal of organics, heavy metals, synthetic detergents and viruses. The design consequence is that carbon has a finite adsorption capacity and therefore a replacement or regeneration cycle — unlike sand, whose life is limited by attrition rather than saturation.

Cloth and fabric media: the dewatering end of the family

Cloth media use canvas, felt or nylon cloth as the filter medium. In water treatment they remove fine suspended solids, typically particles of 10-100 um, such as lint in textile wastewater, and they also serve to dewater industrial wastewater sediment such as gypsum and the sludge produced in treatment. Common sludge dewatering machines include vacuum rotary drum filters and plate-and-frame filter presses, applied to sludge from textile, printing and dyeing, chemical, metallurgical and papermaking wastewater treatment.

Comparing on the parameters that decide the design

• Capture size. Sand takes fine suspended solids; carbon takes dissolved organics by adsorption; cloth takes 10-100 um particles and dewatered cake. Choosing across these categories is a chemistry decision, not a preference.
• Bed depth and rate. A fixed carbon layer at 1.5-2.0 m with 8-20 m/h sets the vessel size; sand beds trade depth against rate in the same way. Get the rate wrong and the bed either blinds or fails to capture.
• Backwash. The 28-32 m/h backwash water rate and 4-10 min backwash time for carbon define the wash water demand and the surge tank volume. Soft fibre media that ball up effectively lose their backwash path entirely.
• Service life. Sand wears, carbon saturates, plastic media ages and soft fibre collapses. Each failure mode has a different cost curve, and only one of them can be restored by cleaning.

Selection guidance

For biological duty, prioritise surface area that stays available over years, not the figure on the datasheet at commissioning; a medium that balls up has promised area it cannot deliver. For tertiary solids removal, quartz sand remains hard to beat on cost per cubic metre of water treated. For dissolved organics, colour or trace contaminants, carbon is the tool, but budget the regeneration. For sludge, cloth and fabric media belong to the dewatering stage and should be specified against cake dryness rather than filtrate clarity. Treat media as an engineered component with a defined failure mode, and the specification writes itself.