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ABSTRACT: The fixation of phosphorus (FIX-Phos) combines struvite prevention and phosphorus recovery by the addition of calciumsilicatehydrate (CSH) particles into the anaerobic digester. The CSH fixates phosphorus as calcium phosphate and reduces the phosphorus concentration in the sludge water that allows for control of struvite formation. The phosphorus-containing recovery product can be separated and recovered from the digested sludge. In pilot plant experiments, 21% to 31% of phosphorus contained in digested sludge could be recovered when CSH was added at concentrations of 2 g/L to 3.5 g/L to a mixture of primary sludge and waste activated sludge (WAS) from enhanced biological phosphorus removal. The recovery product contained few heavy metals and a phosphorus content of 18 wt % P2O5, which allows for recycling as fertilizer. The fixation of phosphorus within the digester may increase wastewater sludge dewaterability. The phosphorus recycle stream to the headworks of the wastewater treatment plant is reduced. Water Environ. Res., 84, 220 (2012).
KEYWORDS: phosphorus recovery, calcium phosphate, calciumsilicatehydrate, struvite scaling, anaerobic digestion, wastewater sludge, EBPR, FIX-Phos.
doi:10.2175/106143012X13347678384125
(ProQuest: ... denotes formulae omitted.)
Introduction
Phosphorus is essential for all life on earth. Without the continuous input of phosphorus fertilizer to agriculture, the growing world population could not be nourished. The global phosphorus reserves that are concentrated in a few countries (Mineral Commodities Summary, 2010) are finite; at some time in the future, the world will run out of cheap, lowcontaminated, and the easily accessible mineral, phosphorus fertilizer. Hence, it is an uttermost need to decelerate the global phosphorus cycle.
Phosphorus in Wastewater Treatment. During anaerobic digestion of wastewater sludge, biologically bound phosphorus is released into the liquid phase (sludge liquor) and then partly refixated into the sludge; mainly by (a) precipitation with ferrous or ferric, aluminium, calcium, and magnesium, or (b) adsorption (Jardin and Pöpel, 2001; Frossard et al., 1997; Wild et al., 1996). Phosphorus that is not fixated within the sludge digester is returned to the wastewater treatment plant (WWTP) with the centrate. In case of enhanced biological phosphorus removal (EBPR) this can be up to 30% of the WWTP phosphorus-load (Jardin and Pöpel, 2001).
Formation of Struvite in WWTPs. The scaling of struvite (MgNH3PO4 3 6 H2O) in digesters, pipe works, pumps, or centrifuges is a common nuisance...