Flocculation Filtration for Phosphorus Removal
Coagulant choice, beta factor, dosing point and filtration speed — the design parameters that decide whether a pile cloth filtration stage reaches low phosphorus values.
The Principle: Turning Dissolved Phosphorus Into Particles
After the biological stage, a substantial share of the remaining phosphorus is present in dissolved form as orthophosphate. A filter cannot separate this fraction. Flocculation filtration solves the problem in two steps: a coagulant binds the orthophosphate into insoluble compounds that agglomerate into flocs — and the pile cloth filtration stage retains those flocs.
The decisive point: the achievable effluent value ultimately depends not on chemistry alone but on the quality of the solids separation. Every floc that passes the filter carries its phosphorus with it. In practice, therefore, the total phosphorus effluent value correlates closely with the TSS effluent value of the filtration stage — which makes filter media selection part of the phosphorus strategy, not an accessory to it.
Design Parameters From a Pilot Trial
At a municipal wastewater treatment plant, flocculation filtration with fine R+F pile cloth was piloted in continuous operation over several months. The following test conditions provide a solid reference for designing comparable installations:
| Integration | Classic flocculation filtration in the secondary clarifier effluent |
| Operating mode | Continuous operation over roughly three months, without interruption |
| Target | Total phosphorus < 0.2 mg/l |
| Coagulants tested | Ferric chloride (FeCl₃, 40 %) and polyaluminium chloride (PAC) |
| Beta factors | 3 — 4 — 5.5 (increased systematically) |
| Peak factor | 1 |
| Throughput | 24 m³/h |
| Filtration speed | 5.0 m/h |
| Dosing point | Directly into the inlet pipe shortly upstream of the filter, under high turbulence via static mixer |
| Filter media | Fine R+F pile cloth |
| Parameters monitored | Total P, PO₄-P, turbidity |
We deliberately publish only the test conditions here, not effluent concentrations. The pilot measurement series are available to us in graphical form only, and we quote figures for effluent total phosphorus only where we can document them unambiguously. Statements about achievable effluent quality are instead based on our own measured TSS data.
Coagulant Choice: Iron or Aluminium
Ferric chloride is the workhorse of phosphorus precipitation: inexpensive, widely available and robust across a broad pH range. It produces compact, readily filterable flocs but raises the iron content in the effluent — relevant where an iron consent must also be met. Polyaluminium chloride often gives faster floc formation with less sludge production, but is more expensive and more sensitive to low temperatures and pH fluctuation.
For the filtration stage, floc structure matters more than the metal itself. Flocs that are too small and shear-sensitive pass through any medium; flocs that are too large and water-rich shorten the cycle length and drive up rinse water volume. Dosing rate, mixing energy and filter media must therefore be considered together.
The Beta Factor — and Why You Cannot Simply Raise It
The beta factor describes the molar ratio of dosed precipitant metal to the phosphorus to be precipitated. At beta = 1, exactly one metal ion per phosphorus atom would be dosed stoichiometrically — in practice this is never sufficient, because side reactions consume part of the metal. Practical values are therefore considerably higher; in the pilot trial described, beta was raised systematically from 3 to 4 and on to 5.5.
The higher the beta factor, the lower the achievable phosphorus value — but also the higher the chemical cost, the sludge production and the solids load on the filtration stage. Overdosing merely shifts the problem: the filter then has to separate more floc mass, cleans more frequently and produces more rinse water. A well-chosen filter medium therefore often makes it possible to lower the beta factor rather than raise it.
Operational Monitoring: Turbidity as an Early Indicator
Total phosphorus and PO₄-P are determined in the laboratory — with the corresponding delay. Turbidity, by contrast, can be measured continuously online and responds immediately to floc carry-over. In the pilot trial, turbidity was therefore recorded alongside total phosphorus and PO₄-P. For routine operation we recommend the same combination: laboratory values to demonstrate compliance, online turbidity for day-to-day process control and to verify dosing.
Designing or Optimising a Phosphorus Stage
Whether new build, retrofit or optimisation of an existing flocculation filtration stage — we contribute media selection, load case calculation and measured filtration data to the design.