Fourth Treatment Stage & Micropollutant Removal
Powdered activated carbon adsorbs micropollutants — but it must also be taken back out of the water. Pile cloth filtration provides that reliable activated carbon retention: in continuous operation, compact and retrofittable.
Why a Fourth Treatment Stage?
Conventional mechanical and biological treatment is designed for carbon, nitrogen and solids. Yet an entire group of dissolved substances passes through these stages largely unchanged: pharmaceutical residues and other organic micropollutants. Across Europe, their removal is moving up the water-protection agenda — making the fourth treatment stage a planning topic for a growing number of plants.
The most widely used route to remove these micropollutants is adsorption onto powdered activated carbon (PAC). The carbon binds the dissolved molecules on its vast internal surface. But that solves only half the task — the loaded carbon must then be completely separated from the water again.
The Real Bottleneck: Retaining the Activated Carbon
Every carbon particle that leaves the plant with the effluent carries its adsorbed micropollutant load back out with it — straight into the receiving water. The performance of the fourth stage therefore stands or falls on the quality of the solids separation. This is exactly where pile cloth filtration comes in.
The dense, three-dimensional pile of the filter cloth forms a depth-filtration bed. During filtration the flexible fibres lie flat and retain even fine carbon particles throughout the fibre volume — not just on a surface. For cleaning, the fibres stand up during suction and release the collected carbon in concentrated form. Filtration and cleaning run at the same time, so carbon can be dosed upstream continuously.
Why Pile Cloth Rather Than a Micro-Screen or Sand Filter?
Fine powdered activated carbon tends to blind rigid micro-screens quickly — retention drops off or the cleaning effort rises sharply. A sand filter bed adds head loss, backwash pumping and a large footprint.
The self-cleaning pile cloth, by contrast, stays open even under a continuous carbon load, works under gravity with low head loss and needs only very little rinse water. An existing filter tank can often take over the retention duty without additional pumping energy.
Operational Advantages at a Glance
Compact footprint
A submerged pile cloth stage replaces large filter beds — ideal for retrofitting a fourth stage into an existing plant.
Low energy demand
Gravity operation with minimal head loss; only the cloth is cleaned by suction, not the whole bed.
Very little rinse water
Suction cleaning uses only a fraction of throughput as rinse water — the separated carbon stays concentrated.
Continuous & load-robust
Filtration and cleaning run at the same time; solids and flow peaks are absorbed without interrupting the cycle.
Multi-Barrier Approach: One Stage, Several Targets
Pile cloth filtration is already used as a tertiary stage and for flocculation filtration in phosphorus removal. The same filtration stage can additionally take on activated carbon retention — a multi-barrier approach in which a single stage contributes to solids, phosphorus and micropollutant targets at once. That is especially valuable where plant space is scarce.
Cloth grade, dosing regime and mixing energy are matched to the combined load. As a retention stage for fine carbon, our finer filtration grades are the natural choice — the supermicrofibre pile cloth for the highest fineness and the Bioactive pile cloth where biofouling within the filter itself should also be limited.
On microplastic retention we stay deliberately precise: as a fine depth-filtration stage, pile cloth filtration retains a significant share of the solids and the microplastic particles bound to them; retention rises with a finer filtration grade. Actual performance depends on particle size, cloth grade and operating conditions and is assessed case by case — so we do not quote a blanket percentage.
Frequently Asked Questions
Planning or Retrofitting a Fourth Treatment Stage
Whether new build or retrofit of a micropollutant stage — we contribute media selection, load case analysis and measured filtration data to the design of the activated carbon retention.