Pilot scale recovery of phosphorus as calcium phosphate from nitrified UASB effluent of a potato processor and subsequent reuse in the wet process for phosphoric acid production

Abstract

In the frame of the sustainable use of phosphorus, a pilot scale experiment ran for more than one year on the effluent of an upstream anaerobic sludge blanket reactor (UASB) of a potato processor to recover phosphorus as calcium phosphate. Calcium phosphate may be mixed with phosphorous ore and as such used in phosphoric acid production. The effluent was first nitrified and - at neutral pH - adding calcium ions from calcium chloride in a ratio of about 4 or 8 relatives to phosphate-P, resulted in 80 % respectively 90 % phosphorous recovery as calcium phosphate. In the P-reactor a brown colour developed, a colour that remained in the precipitates. After ultrafiltration of the nitrified UASB effluent the recovered precipitates still contained organic material. The phosphorous content of the recovered precipitates expressed as P2O5, was 35–40 m%; the precipitates also contained 4–8 m% calcium carbonate. Except for the criteria for organic material (best 1.67 m% TOC vs 0.40 m%)) and magnesium (0.56 m% as MgO vs 0.11 m%), the precipitates fulfilled all the criteria for phosphorous ore. In a two-step calcination process milled colored precipitate was converted over a light grey (600 °C) to a pure white product (900 C°). This calcination enhanced the crystallinity of the precipitates. A mixture of non-calcinated precipitates (total 30 kg) was used for phosphoric acid production. Both the acid and the isolated calcium sulphate were colored but fulfilled most other chemical criteria. Not the magnesium contamination but either the contamination with organic material appeared to be a problem.