TECHNICALLY SPEAKING
The PHOSPAQ process has been applied since 2006 in Olburgen, Netherlands, treating a combination of potato processing wastewater and centrate from a municipal sludge digester, for 1.2t/d of struvite
A second full-scale plant, in Lomm, Netherlands, treats potato processing effluent to produce 800kg/d struvite
Recovered struvite, in the form of easily handled granules or crystals with a high phosphorus and a low heavy metal content, are easily filtered and require no further drying. Therefore, struvite remains an intriguing opportunity, since it forms itself spontaneously in sewage treatment works and therefore is in principle very harvestable. Whilst struvite undoubtedly has some value as a fertiliser (including its ammonia and magnesium content), it is difficult to transform into other phosphate derivatives using any existing technology at the disposal of the industry. While not usable in a traditional wet acid route, struvite could be processed by electrothermal reduction, but the presence of ammonia requires drastic changes in furnace feed preparation or a radically different process flowsheet.
The same cannot be said of the other option: calcium phosphate is ideal for onward industrial processing, as the recovered material is indistinguishable, in most respects, from mineral phosphate rock. Several full scale processes are already recovering phosphates from wastewater streams through calcium phosphate formation: DHV Crystalactors at the Dutch wastewater plants of Westerbork (12,000 PE), Heemstede (35,000 PE) and (Geesmerambacht 230,000 PE) since 1994
Pilot plant at Essex & Suffolk Water in Chelmsford from 1997-1998
Reactor tested by Karlsruhe University at Darmstadt Süd, Germany, since 1997
Demonstration plant at Warriewood, Australia, by Sydney Water (50,000 PE) since 1995
Three plants constructed by Kurita, Japan Fixed bed precipitation at Mercedes factory at Gagenau, Germany, (160m3/h) since 1998 The recovered pellets drain readily to below 5-10% water and contain 5-15% phosphorus. The calcium phosphates deposit by amorphous precipitation around the seed material, rather than true crystallisation, forming compounds with different hydration complexes (calcium hydroxyapatite, dicalcium phosphate dihydrate, octacalcium phosphate, tricalcium phosphate). The solubility and crystallisation properties of these different molecules vary and the balance between them will modify the overall behaviour of a recovery reactor. Some recovered calcium phosphate has comparatively high residual organics, which would pose no problems for electro-thermal reduction, but for the wet acid recovery would need a simple calcining step, as employed on many natural rocks.
Slick solution in Slough
Thames Water is incorporating a new technology at Slough Sewage Treatment Works, one of the UK’s first biological phosphorus removal plants, commissioned in 1993 with a capacity of 257,000 PE and flow (FFT) of 118.4Ml/d. As a consequence of the rich phosphorus streams, struvite can form scale on the inside of pipes and valves, increasing maintenance.
To reduce the costly maintenance resulting from the damaging build-up of struvite, and
simultaneously extract and recover phosphorus and ammonia from its wastewater stream and transform them into a commercial fertiliser, a pilot scale nutrient recovery facility began operating in Slough in March 2010 (see WWT October). After demonstrating the technology’s potential to support efficient operation of the plant’s biological phosphorus removal process, and confirming its full scale viability, this first installation in Europe also marks a unique partnership: The process supplier, Ostara will build and finance the nutrient recovery facility, expected to yield 150t/year of Crystal Green fertiliser
Thames Water has agreed to pay a monthly fee for the treatment capacity provided, expected to be completed in mid-2011
Without having to make any capital investment, estimated at £2M, this project will help the operator to efficiently meet nutrient limits, optimise the plant’s efficiency and reduce operational and maintenance cost between £128,260 and £202,000 annually, while recovering a valuable resource. A second facility is being considered in the Netherlands, while others already operate in North America: A reactor in Edmonton, Canada, produces 500kg/d of fertilizer, open since May 2007
Tigard, Oregon, is the first US plant, with a capacity of 1t/d, open since May 2009
In Suffolk, Virginia, the Nutrient Recovery Facility was launched in May 2010
The York, Pennsylvania WWTP implemented a unit in summer 2010
These and other related topics will be presented and discussed at the upcoming WEF IWA Conference on Nutrient Recovery in the US in January:
www.nutrient2011.org.
Phosphorus recovery facility implemented by Thames Water in Slough December 2010 Water & Wastewater Treatment 35
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