What is phosphorus and why do I need to remove it from wastewater?
Phosphorus is a chemical element with the symbol P. It is highly reactive and is therefore never found naturally as a free element, occurring instead as inorganic and organic phosphates (oxidised) in water, soil and sediments.
These phosphates occur in three types:
The simplest form of phosphate, available for biological metabolism without further breakdown
Complex molecules which require hydrolysis to revert to orthophosphates
Esters of phosphoric acid; of minor importance in municipal wastewaters, but can be significant in industrial applications.
Phosphorus is a vital element for all life, with phosphate compounds being a component of DNA, RNA and ATP, and it is essential for plant growth. This is the reason why we apply phosphate fertilisers in our gardens and on farmland.
Phosphates are released from rocks by weathering and the action of plant roots. These phosphates are then assimilated from the soil by plants, with the help of bacteria.
All the phosphorus that the human body needs is obtained by eating the plants or plant-eating animals that have already absorbed and utilised it.
Whilst phosphorus is very useful and effective as a fertiliser to encourage plant growth, it can cause catastrophic problems if too much gets into our streams, rivers, lakes and seas. Phosphorus reaches bodies of water via runoff over land, leaching through soil from fertilisers and discharges from sewage treatment works. In fact, the UK Water Industry Research (UKWIR) cites phosphorus as a major reason that UK rivers are not achieving ‘good’ status under the Water Framework Directive (WFD).
This is the reason why:
Where levels of phosphates in water bodies are too high, it can trigger algal blooms which block sunlight from reaching lower waters, thereby causing plants to die. As the plants and also the algae die and decay they cause depletion of oxygen levels – this is known as eutrophication. Oxygen depletion can lead to fish dying, impaired reproduction of remaining fish and a reduction in biodiversity. The algal blooms themselves can also be toxic to people, plants and animals.
Fertilisers, human waste and detergents are the main sources of phosphate pollution. One person produces an average of 2 g of phosphorus a day which ends up at municipal treatment works. At the end of the treatment process, municipal wastewaters can contain 2 – 5 mg/l of total phosphorus.
To maximise phosphate removal there can be up to three processes applied:
1) Phosphate in solid form can be removed using filtration/solids settlement processes. This generally occurs in the primary treatment stage.
However, much of the phosphate will be dissolved in the wastewater stream. These dissolved phosphates can be removed by the secondary and tertiary processes:
2) A proportion of the dissolved phosphates will be removed by the biological process at the treatment works. This is done by growing microorganisms that can absorb and store phosphorus as polyphosphate. The phosphorus is incorporated into the biomass which is then separated from the treated water at the end of the process. This stage is associated with the secondary treatment processes.
3) The remaining dissolved phosphates can be chemically precipitated to convert into solids that can be removed with filtration processes. This is normally done using addition of metal salts to react with the soluble phosphate to form solid precipitates. This is explicitly applied as a tertiary treatment process. Dosing can be performed at various stages in the wastewater treatment process.
We have a range of continuous upflow vertical sand filtration technologies available in the UK which, with the addition of a coagulant such as ferric sulfate, can remove phosphates from wastewater at the tertiary treatment stage.
DynaSand® - The DynaSand® system uses sand as the filter media and provides high-quality suspended particle filtration from a small-footprint filter bed with no moving parts or backwash equipment requirements.
DynaSand® can provide removals down to 0.1 mg/l whilst also removing effluent BOD and TSS down to 10 mg/l.
DynaSand® Oxy - The DynaSand® Oxy system uses sand as the filter media, plus the addition of air and provides small-footprint, continuous sand filtration with air addition to facilitate removal of ammonia from wastewater.
Primarily designed for ammonia removal, for which it delivers removals down to 2 mg/l, the DyanSand® Oxy can also provide removals of effluent phosphorus down to 0.2 mg/l as well as effluent BOD and TSS down to 10 mg/l.
DynaSand® Deni - The DynaSand® Deni uses sand as the filter media with carbon addition to provide small-footprint removal of nitrates from wastewater.
Primarily designed for nitrate removal, for which it delivers down to 10 mg/l total N, the DynaSand® Deni can also provide removals of effluent phosphorus down to 0.2 mg/l, effluent BOD and TSS down to 10 mg/l.
We also supply a high-performance, high-volume, low headloss disc filter for tertiary wastewater applications, the DynaDisc® system.
DynaDisc® Filter - Primarily designed for TSS and BOD removal, for which it delivers removals down to 10 mg/l for each, the DynaDisc® can also provide removals of effluent phosphorus down to 1 mg/l in its standard configuration and 0.25 mg/l with the addition of metal salts.
DynaSand® has a proven track record of phosphorus removal going back over 20 years. We have included a case study for each of the DynaSand® series and one for DynaDisc®
DynaSand® - Read about how DynaSand® has been reducing phosphorus levels in lakes at Växjö, southern Sweden since 1994.
DynaSand® Oxy - Find out about the performance of DynaSand® Oxy filters in a year-long trial at Watton Water Recycling Centre, UK.
DynaSand® Deni - Find out how DynaSand® Deni has provided 3-in-1 removals for Ratzeburg in Germany.
DynaDisc® - Read about the single stage phosphorus and TSS removal at Pietersaari WWTP in Finland.
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