Phosphorus is an essential element for all living things. Agriculture needs considerable amounts of it for crop fertilisation. In the EU there are almost no natural sources and farmers are dependent on imports.
Sewage sludge is an important source of phosphorus. However, distributing it directly onto the fields as fertiliser entails not only hygiene problems. Its potential content of heavy metals and organic pollutants is leading more and more countries to restrict or even ban its use. For this reason, efforts are being made to recycle phosphorus in a plant-available form from sludge.
There are two approaches:
Recycling of phosphorus from sewage sludge ash
These processes usually start from the sewage sludge ash following a mono-incineration. The technologies for the recovery of plant-available phosphorus are all still in development. They will only be used directly in the sewage treatment plants in exceptional cases, and are restricted to large facilities.
Sludge mineralisation with PYREG®can be the alternative for the recovery of phosphate from sludge ash following mono-incineration.
Recycling of phosphorus by precipitation from the sludge stream
Currently, chemical processes appear to be the main method in use in treatment plants: they crystallise phosphorus through targeted chemical precipitation as magnesium ammonium phosphate ("struvite") from the digested sludge or from the sludge water from the dewatering process and separate it.
The EloPhos® process, developed by ELIQUO STULZ, uses struvit separation in the digested sludge is advantageous in that the reduction of free phosphate in the sludge simultaneously improves the dewatering process and prevents the blockage and caking problems that result from struvit deposits.
A method such as the Pearl® technology from Ostara, separates the struvit from the sludge water, whilst having no effect on the sludge dewatering process and struvit deposits. The process benefits are significantly higher struvit yield and a guaranteed quality of the generated struvit "prills". They are marketed as a commercial fertiliser under the trade name "Crystal Green®".
Phosphorus recovery by struvite precipitation
ELIQUO STULZ works closely with it’s Dutch sister company ELIQUO WATER & ENERGY BV, who have large-scale experience in the creation of systems that separate struvit from sludge. ELIQUO WATER & ENERGY has installed the first Pearl® plant in continental Europe, at the Amersfoort wastewater treatment plant.
ELIQUO WATER GROUP is a member of the association Deutsche Phosphor Plattform (DPP e.V.). DPP e.V. has set itself the goal of establishing a sustainable and environmentally compatible phosphorus management in Germany by means of efficient phosphorus utilisation, as well as through effective recycling and reprocessing.