Recovery of phosphorous

Abstract

A method for recovery of phosphorous, in particular of phosphorous from a waste stream, and a product obtained thereby. The product is in a form wherein phosphorous can be released to, e.g., the soil and plants at a desired amount per interval of time.

Claims

1. An improved (semi)continuous method of recovering phosphorous from an aqueous solution comprising the steps of: providing the aqueous solution comprising soluble phosphorous, adding a composition, the composition comprising as mixed ingredients 5-40% CaO (w/w, relative to a total weight of the composition), 10-40% clay mineral, and 25-60% CaCO.sub.3, wherein 0.5-2 mole CaO/mole soluble P is added, and recovering the phosphorous, by forming a product comprising brushite (CaHPO.sub.4.2H.sub.2O), calcite (CaCO.sub.3), and meta-kaoline (Al.sub.2Si.sub.2O.sub.7), wherein the composition is not a calcium silicate hydrate (CSH).

2. The method according to claim 1, wherein the solution is an acidic solution, the acidic aqueous solution having a pH of 1-6, wherein the pH is increased to 5.5-8 by adding the composition.

3. The method according to claim 1, wherein the solution is a basic solution.

4. The method according to claim 1, wherein an amount of soluble phosphorous present is in a range of 1-50 ppm (mg/l).

5. The method according to claim 1, wherein an amount of soluble phosphorous present is in a range of 100-1000 ppm (mg/l).

6. The method according to claim 1, wherein an amount of soluble phosphorous present is in a range of 10-250 g/l.

7. The method according to claim 1, wherein the clay mineral is selected from a natural or artificial clay.

8. The method according to claim 1, wherein the composition comprises 20-30% CaO (w/w), 20-35% (meta)kaolin, and 25-50% CaCO.sub.3 (w/w), and wherein 0.8-1.2 mole CaO/mole soluble P is added.

9. The method according to claim 1, wherein the composition is obtained by thermal conversion of a material chosen from paper waste and residue from the paper production.

10. The method according to claim 1, wherein the aqueous solution is selected from the group consisting of waste water, treated waste water, sludge, organic acid digest solution, acid digest solution, animal manure, pig manure, cow manure, and combinations thereof.

11. The method according to claim 1, wherein the aqueous solution comprises 0.01-6 wt. % solids and impurities.

12. The method according to claim 1, wherein in the composition comprising CaO, the clay mineral, and CaCO.sub.3 are homogeneously distributed.

13. The method according to claim 1, wherein the composition has a BET surface area of 5-100 m.sup.2/gr.

14. The method according to claim 1, wherein the product precipitates, and wherein the precipitate is recovered.

Description

SUMMARY OF FIGURE

(1) FIG. 1. Experimental configuration of a brushite recovery process.

DETAILED DESCRIPTION OF THE FIGURE

(2) FIG. 1 is further detailed, in as far as relevant, below.

EXPERIMENTS

(3) The below relates to experiments performed on a specific type of waste stream. Organic acid digest is a product of an initial step of a multi-phase, multi-temperature anaerobic digestion process used at some wastewater treatment plants, often with the intention of reducing solids loading and increasing biogas generation. Within the 1 to 4-day retention time, acidogenesis and acetogenesis occur, resulting in a digest containing high concentrations of volatile fatty acids, an acid pH, and high concentrations of soluble phosphorus.

(4) Methodology

(5) TopCrete was tested in the second of five batch brushite recovery trials at Madison Metropolitan Sewerage District (MMSD). Organic acid digest was produced in a custom-built 208-L digester with a 50/50 (v/v) mixture of primary sludge and activated sludge. The sludge was held for ten days at a four-day residence time until the pH stabilized. The digest having a pH of 5.46, a temperature of 37 C., and 4.38 wt. % solids, was then mixed with a flocculent and pumped through a decanter centrifuge (Table 1). See FIG. 1 for process configuration.

(6) A first experiment relates to laboratory jar tests: such provides an initial assessment of how TopCrete could perform on a larger scale. Organic acid centrate was collected in triplicate 1-L jars and sufficient TopCrete was added to raise the pH to 6.5. Initial and final samples of the centrate were analyzed by ICP.

(7) As a second experiment, in a scaled up version, a batch trial was performed: The above centrate was collected in a 190-L mixing tank, after which 312.5 g of TopCrete was added in a 250 g/l slurry, bringing the pH of the digest to 6.5. The precipitate was settled, collected, and dried in an oven at 50 C. Initial and final samples of the centrate were analyzed by ICP. The settled material was collected and analyzed by ICP and XRD. Citrate-soluble P (which corresponds to available P) and heavy metals were measured as well. All samples were collected and measured in triplicate.

(8) Results and Discussion

(9) The centrate used for the jar tests initially contained 731 ppm (mg/l) of soluble phosphorus with 0.40% solids (Table 3 and Table 4). Addition of TopCrete to the centrate reduced the soluble phosphorus to a final 215 ppm, a 71% reduction. The TopCrete and brushite settled out of solution easily. For an initial test such a reduction is considered promising. Chemical analysis indicated 11.3% P in the recovered precipitate of the jar test, equivalent to 25.9% P.sub.2O.sub.5 (Table 5) if expressed in oxide format.

(10) Centrate for the batch trial initially contained 715 ppm phosphorus with 0.46% solids (Table 3 and Table 4). TopCrete successfully precipitated brushite and settled as easy as in the jar tests. Centrate phosphate was reduced from an initial value of 715 ppm P to 2 (sic!) ppm, a 99% reduction in the soluble phosphorus (Table 4). The phosphorus content of the precipitate was 8.8% P, or 20.2% P.sub.2O.sub.5 (Table 5) if expressed in oxide format.

(11) Conclusion

(12) TopCrete performed well in reducing soluble P in the organic acid digests, with a 71% reduction in soluble phosphorus during the jar test and a 99% reduction in the batch trial. The resulting material settled well and had a P content of 11.3 and 8.8% in the jar and batch tests, respectively. It is noted that pure brushite, CaHPO.sub.4.H.sub.2O, has a P content of 18%, indicating some dilution of brushite with the adjunct minerals in TopCrete that carried through to the precipitate. These results indicate very promising potential use of TopCrete as a source material for a phosphorus recovery and upcycling process.

(13) TABLE-US-00002 TABLE 1 Organic acid digest composition for Trial 2. Trial Date Composition pH Temp C. % Solids Jar test 21 Oct. 2013 Primary/WAS 5.48 36.5 4.05 Batch 23 Oct. 2013 Primary/WAS 5.46 37.0 4.38

(14) TABLE-US-00003 TABLE 2 Phosphorus in acid digest pre-centrifugation (mg kg1). Trial Soluble Total P Soluble Jar test 969 1166 83% Batch 1118 1298 86%

(15) TABLE-US-00004 TABLE 3 Solids in cake and centrate. Trial Cake Centrate Jar test 0.40% Batch trial 20% 0.46%

(16) TABLE-US-00005 TABLE 4 Phosphorus in centrate pre and post precipitation (mg kg1). Soluble P Centrate Soluble P Centrate P Reduction in Trial Pre Precipitation Post Precipitation Centrate Jar test 731 215 71% Batch 715 2 99%

(17) Comparable tests using Ca(OH).sub.2 show a reduction in phosphorous of 81%, 86%, 89% and 61% respectively, hence between 10-38% lower (based on 100% recovery possible).

(18) TABLE-US-00006 TABLE 5 Percent of phosphorus in recovered precipitate. Sample Total Phosphate (P.sub.2O.sub.5) Jar test precipitate 25.9% Batch trial precipitate 20.2% Comparative data P- reduction Method pH Remarks EP 243.sup.1 35-90% Stirred reactor >8 .sup.3 Berg.sup.2 30-95% fixed bed/ ? .sup.4 expanded bed Present >99% Mixing & 6.5 .sup.5 precipitating .sup.1FIGS. 2 and 3. .sup.2FIGS. 2-5. .sup.3Highest efficiency @ smallest volume, due to extreme high dosage. .sup.4Highest efficiency with new bed; gradual decrease thereafter. .sup.5performs well at acidic and almost neutral conditions, contrary to prior art methods relying on precipitation at a higher pH. Such is considered beneficial to a final product (in terms of e.g. yield and availability), to the product formed (at present mainly brushite), and for reaction conditions being relatively mild.

(19) TABLE-US-00007 TABLE 6 Percent of phosphorus in recovered precipitate and percent available. Citrate-soluble Total Phosphate Percent Sample P.sub.2O.sub.5 (P.sub.2O.sub.5) Available Jar test 25.9% precipitate Batch Trials Trial 1 Trial 2 17.0% 20.2% 84% (CDEM) Trial 3 30.7% 35.3% 87% Trial 4 21.6% 32.0% 68% Trial 5 32.2% 38.1% 84%

(20) Trials 1 and 3-5 relate to a Ca(OH).sub.2 composition, whereas trial 2 relates to use of Toperete (a composition according to claim 1), giving besides the highest recovery (99%) also almost the highest availability.

(21) Application of the present fertilizer, comprising 48.9 atom % brushite, 22.5 atom % calcite, 6.2 atom % meta-kaolinite, and 11.6 atom % Mg phosphate, to crops showed improved yields. For instance, addition of 25-100 ppm (or mg/kg soil) showed an increase in dry yield (g/pot) of 6-10% relative; hence a beneficial effect is shown as well as the availability to plants of the present fertilizer. These results are comparable to improvements obtained with other typically used phosphate sources, of which some showed slightly higher improvements under similar conditions; however, as the present precipitate is effectively diluted, such as by a factor 2, in fact better results are obtained.

(22) The invention is elucidated through the examples and figures which are exemplary and explanatory of nature and are not intended to be considered limiting of the invention. To the person skilled in the art it may be clear that many variants, being obvious or not, and combinations of the examples and embodiments, may be conceivable falling within the scope of protection, defined by the present claims.