PROCESS AND FACILITY FOR RECOVERING PHOSPHORUS AT A WASTEWATER TREATMENT PLANT WITH ADVANCED SLUDGE TREATMENT

Abstract

Disclosed is a process and a facility for recovering phosphorus present in an effluent to be treated, including the following steps: a step of biological removal of phosphorus from the effluent to be treated, a step of separating the treated water from step i and the sludge, a step of anaerobic hydrolysis of at least one portion of the sludge from step ii, a step of liquid/solid separation of the effluents from step iii, a step of advanced treatment of at least one portion of the sludge from step iv, a step of recirculation to step iii of at least one portion of the effluent from step v, and a step of recovering the phosphorus present in the effluent from step iv.

Claims

1. A process for recovering the phosphorus present in an effluent to be treated, comprising the following steps: i. a step of biological dephosphatation of the effluent to be treated, said step comprising: i.1. at least one step of treatment under anaerobic conditions, and i.2. at least one step of treatment under aerobic conditions; ii. a step of separating said treated effluent from step i, in particular the treated water from step i and the sludge; iii. a step of anaerobic hydrolysis of at least one portion of the sludge from step ii; iv. a step of liquid/solid separation of the effluent from step iii; v. a step of advanced treatment of at least one portion of the sludge from step iv; vi. a step of recirculating, to step iii, at least one portion of the liquid effluent from step v; and vii. a step of recovering the phosphorus present in the liquid effluent from step iv.

2. The process as claimed in claim 1, further comprising an anaerobic digestion step between steps iv and v.

3. The process as claimed in claim 2, further comprising a biogas exploitation step after the anaerobic digestion step.

4. The process as claimed in claim 1, wherein the liquid from step vii is recirculated to step i.

5. The process as claimed in claim 1, wherein the residence time of the effluent from step ii is less than 4 h.

6. The process as claimed in claim 1, wherein the degree of phosphorus recovery is greater than approximately 30% of the total phosphorus entering.

7. A facility for recovering the phosphorus present in an effluent to be treated, comprising at least one anaerobic reactor (1), at least one aerobic reactor (2), a separator (5), at least one phosphorus release reactor (10), at least one advanced sludge treatment unit (9), at least one means for communication between the at least one phosphorus release reactor and the at least one advanced treatment unit, and at least one phosphorus recovery unit (8).

8. The facility as claimed in claim 7, further comprising at least one means for communication between the at least one phosphorus recovery unit and the at least one anaerobic reactor and/or the at least one aerobic reactor.

9. The facility as claimed in claim 7, further comprising an anaerobic digester.

10. The facility as claimed in claim 7, wherein the facility is coupled with a facility for biogas exploitation.

11. The process of claim 5, wherein the residence time of the effluent from step ii is between 30 min and 4 h.

12. The process of claim 5, wherein the residence time of the effluent from step ii is between 30 min and 2 h.

13. The process of claim 6, wherein the degree of phosphorus recovery is greater than approximately 40% of the total phosphorus entering.

14. The process of claim 6, wherein the degree of phosphorus recovery is between 40% and 80% of the total phosphorus entering.

15. The process of claim 6, wherein the degree of phosphorus recovery is between 45% and 60% of the total phosphorus entering.

16. The process as claimed in claim 2, wherein the liquid from step vii is recirculated to step i.

17. The process as claimed in claim 3, wherein the liquid from step vii is recirculated to step i.

18. The process as claimed in claim 2, wherein the residence time of the effluent from step ii is less than 4 h.

19. The process as claimed in claim 3, wherein the residence time of the effluent from step ii is less than 4 h.

20. The process as claimed in claim 4, wherein the residence time of the effluent from step ii is less than 4 h.

Description

[0108] In the conventional systems, illustrated in FIGS. 1 to 4b, the water to be treated enters an aerobic/anoxic and aerobic reactor (2), it being possible for said reactor to be preceded by an anaerobic reactor (1). The effluents from (2) are then separated by a separator (4), following which at least one portion of the sludge is recirculated (3), while the other may undergo liquid/solid separation in a separator (5). Once the effluents have been separated and/or thickened, they can undergo dehydration (6) and/or anaerobic digestion (7) (FIG. 2). Some conventional systems also use a phosphorus-recovering device (8) at the end of treatment (FIG. 2).

[0109] FIGS. 3, 4a and 4b represent conventional systems which also have an advanced treatment unit (9) from which the advanced treatment liquid returns, or centrates (11), are sent back to the head of the treatment, preferably: [0110] in the anaerobic reactor (1) in order to supply VFAs for the biological removal of the phosphorus; [0111] in the anaerobic digester (7) for increasing the biogas production; or [0112] in the aerobic reactor (2) for oxidizing the VFAs.

[0113] In one system according to the present invention, two embodiments of which are illustrated in FIGS. 5 and 6, the effluents from the separator (4) and also the effluents (11) originating from the advanced treatment unit (9) feed the phosphorus release reactor (10).

[0114] The reactor (10) receives the effluents that have previously spent time in the reactor (2), said effluents comprising PAOs which will use the high concentrations of VFAs present in the advanced treatment returns (11) to produce PHAs and to release the phosphorus.

[0115] This amount of phosphorus resulting from the release will add to the amount of phosphorus originating from the advanced treatment returns (11).

[0116] The effluent from the reactor (10) can be subjected to a liquid/solid separation (5), for example via a thickener. The liquid effluent, loaded with phosphorus, from the reactor (10) or from the separator (5) passes into a phosphorus recovery unit (8); the phosphorus recovery can be carried out for example by precipitation of struvite. The nitrogen present in the advanced treatment returns (11) also promotes this precipitation.

[0117] The major portion of the effluent from the separator (5) is then treated in at least one advanced treatment unit (9) in the presence or absence of an anaerobic digester (7).

[0118] The example below makes it possible to compare the treatment according to the invention with the conventional treatments.

EXAMPLE

[0119] Table 1 below expresses the loads received by a wastewater purification plant for an equivalent of 360 000 inhabitants.

TABLE-US-00001 TABLE 1 Loads received by the purification plant Daily flow rate 100 000 m.sup.3/d PUPL Entry COD 140 g/IE BOD5 60 g/IE TKN 14 g/IE Pt 2.5 g/IE COD 50.4 t/d BOD5 21.6 t/d TKN 5.0 t/d Pt 0.9 t/d g/IE: grams of pollution per inhabitant equivalent PUPL: purification plant COD: chemical oxygen demand, represents the carbon-based pollution BOD5: biochemical oxygen demand TKN: total Kjeldahl nitrogen, represents the nitrogenous pollution Pt: total phosphate, represents the phosphorus-bearing pollution t/d: tonnes/day corresponds to the multiplication of the grams of pollution per inhabitant equivalent by the m.sup.3/d.

[0120] In the case of a phosphorus recovery from the digestion centrates (FIG. 2), the amount of phosphorus in the centrates is about 0.14 t/d (the centrate flow rate is about 540 m.sup.3/d for a phosphorus concentration of 250 mg/l). The phosphorus recovery reactor enables a recovery of about 0.12 tP/d, i.e. a 90% yield. Over the whole of the purification plant, this gives a degree of phosphorus recovery of 14%.

[0121] In the case of a conventional system with heat treatment where the phosphorus recovery is carried out on the sludge heat treatment returns (FIG. 4b), the amount of phosphorus recovered is higher by virtue of the new dissolution of the phosphorus contained in the sludge during the heat treatment and the phosphorus concentration is about 0.29 t/d.

[0122] The phosphorus concentration in the heat treatment returns is about 2.6 g/l and the flow rate of said returns is about 110 m.sup.3/d. The phosphorus recovery yield over the whole of the purification plant is then 32%.

[0123] This type of system is purely theoretical because of the high concentrations of COD (in particular VFAs), of nitrogen but also of phosphorus that may in practice impair the phosphorus precipitation. In addition, a dilution would be required, which is not economically favorable.

[0124] In the case of the systems according to the invention illustrated in FIGS. 5 and 6, where the heat treatment returns supply phosphorus, VFAs and nitrogen, whilst the excess biological sludge releases the overaccumulated phosphorus in the presence of VFAs, the phosphorus concentration in the heat treatment returns is about 2.6 g/l for a flow rate of 110 m.sup.3/d. The amount of phosphorus recovered is 0.45 t/d, the phosphorus recovery yield over the whole of the purification plant is then 41%.

[0125] Thus, the process according to the invention makes it possible to achieve high phosphorus recovery yields even in the absence of anaerobic digestion.