PROCESS FOR PRODUCING POLYHYDROXYALKANOATE

Abstract

High levels of polyhydroxyalkanoates (PHA) can be produced from wastewater comprising Readily Biodegradable COD (RBCOD) using activated sludge comprising microorganisms capable of accumulating PHA by contacting the wastewater with the activated sludge in the presence of dissolved oxygen during a first period of time, to obtain PHA-loaded activated sludge, and then supplying elements essential for growth such as nitrogen and phosphorus and allowing up-take of these elements and limited growth during a second period of time, the supplied amount of at least of one of said essential elements compared to the amount of RBCOD supplied in step a) limiting the growth to an extent that not all PHA is used for growth, to obtain grown activated sludge; and removing or harvesting part of the PHA-loaded activated sludge and/or part of the grown activated sludge, so that the total average retention time of the sludge is less than 72 h.

Claims

1.-15. (canceled)

16. A cyclic process for producing a polyhydroxyalkanoate (PHA) from wastewater comprising Readily Biodegradable COD (RBCOD) using activated sludge comprising bacteria capable of accumulating PHA in the presence of elements essential for growth including nitrogen and phosphorus, the process comprising: a) supplying a stream of the wastewater to a first reactor and contacting the wastewater with the activated sludge under PHA-accumulating conditions during a first period of time, the PHA-accumulating conditions comprising the presence of dissolved oxygen, wherein the wastewater supplied has a weight ratio of nitrogen to RBCOD-carbon on element basis (N.sub.a/C.sub.a) of below 1/20, and/or a weight ratio of phosphorus to RBCOD-carbon on element basis (P.sub.a/C.sub.a) of below 1/100, to obtain PHA-loaded activated sludge, and treated wastewater; b) providing at least part of the PHA-loaded activated sludge and at least part of the treated wastewater obtained in the first reactor in a second reactor; c) supplying nitrogen and/or phosphorus to the second reactor and contacting the wastewater with the activated sludge under growth conditions during a second period of time, the growth conditions comprising the presence of dissolved oxygen, wherein the weight ratio, on element basis, of the amount of nitrogen or phosphorus supplied in step c) together with any dissolved amount of nitrogen and phosphorus, respectively, supplied with the wastewater in step a), to the amount of RBCOD-carbon supplied in step a) for nitrogen (N.sub.c/C.sub.a) is between 1/20 and 1/100, and for phosphorus (P.sub.c/C.sub.a) is between 1/100 and 1/500, to obtain grown activated sludge comprising residual PHA; d) providing at least part of the grown activated sludge produced in the second reactor in the first reactor; e) removing part of the treated wastewater from the first reactor during or after step (a) and/or from the second reactor during or after step (c) and removing part of the PHA-loaded activated sludge during or after step (a) and/or part of the grown activated sludge during or after step (c), wherein the activated sludge removed comprises PHA at a level of at least 60 wt. % based on dry weight of the organic part of the sludge, and wherein the removed parts are such that the average retention time of the activated sludge (SRT) in the first and second reactor together is less than 72 h.

17. The process according to claim 16, in which N.sub.c/C.sub.a is between 1/20 and 1/75 and/or P.sub.c/C.sub.a is between 1/100 and 1/375.

18. The process according to claim 16, in which in the wastewater supplied in step (a), the weight ratio of at least one element essential for growth to RBCOD-carbon is limited, the limiting ratio, on element basis, for nitrogen (N.sub.a/C.sub.a) being between 1/20 and 1/1000, and for phosphorus (P.sub.a/C.sub.a) being between 1/100 and 1/5000.

19. The process according to claim 18, in which the limiting ratio, on element basis, for N.sub.a/C.sub.a is between 1/25 and 1/500 and/or for P.sub.a/C.sub.a is between 1/125 and 1/2500.

20. The process according to claim 16, which is preceded by a step of lowering the weight ratio of N or P to RBCOD, by at least partially N or P, and/or by adding RBCOD.

21. The process according to claim 16, in which the treated wastewater that is removed from the first and/or second reactor is separated from at least part of the activated sludge.

22. The process according to any claim 16, in which in step (e) part of the PHA-loaded activated sludge is removed during or after step (a) and the removed activated sludge removed comprises PHA at a level of at least 65 wt. %.

23. The process according to claim 16, in which the part of the PHA-loaded activated sludge and/or grown activated sludge removed in step e) contains at least 70%, preferably at least 80% PHA based on dry weight of the organic part of the sludge.

24. The process according to claim 23, in which the part of the PHA-loaded activated sludge and/or grown activated sludge removed in step e) contains at least 80% PHA based on dry weight of the organic part of the sludge

25. The process according to claim 16, in which the second reactor has a volume of between 10 and 90% of the volume of the first reactor and the reactors are operated in a continuous or semi-continuous mode or pulse-feed mode.

26. The process according to claim 16, in which in the wastewater supplied in step a) the amount of Other Biodegradable COD (OBCOD) is more than 0.2 times the level of RBCOD in step a).

27. The process according to claim 16, in which the level of RBCOD maintained in the first reactor stage in step a) is at least 20 mg/l.

28. The process according to claim 16, in which the level of RBCOD maintained in the first reactor stage in step a) is at least 100 mg/l.

29. The process according to claim 16, in which the first period of time (step a) is between 0.5 and 8 h, and the second period of time (step c) is between 0.1 and 6 h.

30. The process according to claim 29, in which the first period of time (step a) is between 1 and 4 h, and the second period of time (step c) is between 0.2 and 2 h.

31. The process according to claim 16, in which the bacteria comprise bacteria of the genus Plasticicumulans.

32. The process according to claim 31, in which the bacteria comprise bacteria of the species P. acidivorans.

33. The process according to claim 16, which is preceded by a step of anaerobically fermenting the waste water to increase the level of RBCOD.

34. The process according to claim 33, which is preceded by a step of anaerobically fermenting the waste water to increase the level of volatile fatty acids, medium-chain fatty acids, lactate, ethanol and/or MCFA.

Description

DESCRIPTION OF THE DRAWINGS

[0087] FIG. 1 schematically shows a one-reactor system according to the present invention. Accumulation and growth reactor 1 is provided with an air supply 2 with air distribution means. Wastewater containing RBCOD and limited in nutrients can be fed continuously or batch-wise through inlet line 3. Nutrients can be fed pulse-wise or batch-wise through inlet line 4. The reactor can be provided with sensors for measuring dissolved oxygen (DO), nutrients (e.g. nitrogen), (RB)COD, especially Volatile Fatty Acids (VFA), temperature, pH etc. The feed of RBCOD can be interrupted and nutrient can then be added when the RBCOD level in the reactor has dropped to a minimum level, e.g. 10 mg/l, as derived from the decreased oxygen consumption rate based on an increased DO concentration measured. Exit line 5 allows discharge of sludge-containing effluent. Effluent is separated in separator 6, from which clarified effluent is discharged through line 7. Alternatively, an internal settler can be provided in the reactor 1 (not shown). Sludge exits the separator through line 8 and is divided over optional return line 9 and discharge (product) line 10 in a controllable way. Optional exit line 11 allows to discharge sludge directly from the reactor 1. In a Sequential Batch Reactor system, sludge separation can be achieved in the reactor 1 by settling of the sludge after the air supply is temporary stopped and separator 6 can be dispensed with.

[0088] FIG. 2 schematically shows a two-reactor system according to the present invention. Accumulation reactor 1 is provided with an air supply 2 with air distribution means. Wastewater containing RBCOD and limited in nutrients can be fed continuously or semi-continuously through inlet line 3. Exit line 5 allows transfer of sludge-containing effluent to separator 6 (which also may be internal, not shown), from which clarified effluent is discharged through line 7. Sludge exits through line 8 and is divided over discharge (product) line 10, cycle line 15, and optional return line 9, in a controllable way. Line 15 carries sludge to growth reactor 12, which is provided with an air supply 13 with air distribution means. Nutrients can be fed to the growth reactor continuously or pulsed through inlet line 14. Optional exit line 11 allows to discharge sludge directly from the reactor 1 for removal or harvesting through exit line 17 and/or direct sludge feed to growth reactor 12 through line 18. Line 16 transfers the sludge containing effluent of reactor 12 back to reactor 1.

[0089] FIG. 3 represents a graph showing dissolved oxygen (% saturation) during about three process cycles in a reactor operated as a SBR, as further described in Example 1. The graph shows the dissolved oxygen concentration as function of the addition of acetate (Step 1), acetate depletion, nutrient addition (Step 2) and partial removal of reactor content (Step 3).

[0090] FIG. 4 shows PHA levels (% of dry organic matter) of the sludge during the process cycle in an SBR system. The diamonds (.diamond-solid.) show the PHA level at the end of the accumulation period and the squares (.square-solid.) show the PHA level at the end of the growth period.

[0091] FIG. 5 shows PHA levels (% of dry organic matter) of the sludge during the process cycle in a continuously operated two-reactor system. The diamonds (.diamond-solid.) show the PHA level in the accumulation reactor and the spheres () show the PHA level in the growth reactor.

EXAMPLE 1

[0092] A 6 litre double walled glass reactor was kept at 30 C.1 C. The pH was kept between 6.8 and 7.0 by continuously adding a small amount of CO.sub.2 gas. The reactor was operated as a Sequencing Batch Reactor with a cycle of 3.5 hours (210 minutes). Throughout the cycle the reactor was aerated with a fixed amount of air. The cycle consisted of: [0093] Step 1 at t=0 minutes: Addition of 750 ml synthetic medium containing 10 g/l acetate (50% mol NaAc and 50% mol HAc), 200 mg/l K (as KCl), 100 mg/l Ca (as CaCl.sub.2) and 50 mg/l Mg (as MgCl.sub.2). [0094] Step 2 at t=160 minutes: Addition of a commercial Nutrient mixture containing N (as urea), P (as H.sub.3PO.sub.4) and trace metals, where N is the limiting compound. Every cycle 90 mg/l of N is added. [0095] Step 3 at t=200 minutes: Removal of 750 ml reactor content.

[0096] The system characteristics are as follows:

[0097] Hydraulic retention time (HRT): 28 hours

[0098] Average sludge retention time (SRT): 28 hours

[0099] First period of time: 160 minutes

[0100] Second period of time: 40 minutes

[0101] N/C in weight based on elements: 1/33)

[0102] When starting the reactor under the above conditions, the sludge already contained a mixture of PHA-accumulating organisms resulting from previous research. Initially the lab reactor was seeded with aerobic sludge from a municipal wastewater treatment plant.

[0103] FIG. 3 shows the measured dissolved oxygen concentration for about three cycles. It shows that as soon as acetate is fed to the reactor (Step 1) the dissolved oxygen concentration drops, indicating oxygen consumption required for acetate up-take and PHA production. After about 100 minutes the acetate is depleted and the dissolved oxygen concentration suddenly increases back to the base line level. As soon as nutrients are added (Step 2) the dissolved oxygen concentration decreases again and within 20 minutes the dissolved oxygen concentration increases back to the base line level, with a slight variation at removal of reactor content (Step 3). During the accumulation period (the low DO period after Step 1), the dissolved nitrogen concentration is low and the acetate concentration is depleted at the end of this period (not shown). During the nutrient up-take and limited growth period (after Step 2) the dissolved N concentration increases directly after nutrient addition and dropped again within the 20 minutes growth period.

[0104] The SBR reactor was operated for more than a month (which equals around 200 cycles or 25 times the sludge retention time (SRT)) at these settings and the PHA content was analysed at the end of the PHA accumulation period and at the end of the growth period. The PHA content in the sludge was determined by Thermogravimetric Analysis (TGA) in which the PHA content of the sludge samples is lost between 200 and 300 C. This results in a distinctive peak for weight loss based on which the PHA content can be determined. Results are shown FIG. 4.

[0105] FIG. 4 shows that the average PHA content in the sludge cycled between 70 and 78% wt based on organic content where the average PHA content after the PHA accumulation period is only slightly higher than the average PHA content after the growth period.

EXAMPLE 2

[0106] A continuous set-up including two double walled glass reactors was operated for more than two months according to the flow sheet in FIG. 2 (with omission of lines 9, 11, 17 and 18) and under the following settings: [0107] PHA accumulation reactor (1): 3.5 litre volume, temperature controlled at 30 C., pH controlled at pH 7.0 using an additional supply of carbon dioxide and aerated using air ensuring a dissolved oxygen concentration above 20% saturation. [0108] Feed (3): 1.4 I/h synthetic medium containing 1.8 g/l acetate (50% mol NaAc and 50% mol HAc), 100 mg/l K (as KCl), 50 mg/l Ca (as CaCl.sub.2) and 30 mg/l Mg (as MgCl.sub.2) [0109] Settler (6): 10 cm diameter with conical bottom [0110] Growth reactor (12): 3 litre volume, temperature controlled at 30 C., pH controlled at pH 7.0 using an additional supply of carbon dioxide and aerated using air ensuring a dissolved oxygen concentration above 20% saturation. Pulse addition (14) every 30 minutes of a commercial Nutrient mixture containing N (as urea), P (as H.sub.3PO.sub.4) and trace metals, where N is the limiting compound. Every pulse 19 mg of N is added. [0111] Sludge discharge (10) from settler: 0.4 l/h [0112] Sludge recycle (15) from settler to the Growth reactor: 0.6 l/h.

[0113] The system characteristics are as follows: [0114] First period of time equals the hydraulic retention time (HRT) in the PHA accumulation reactor: 1.8 hours [0115] Second period of time equals the HRT in the Growth reactor: 5 hours [0116] Average sludge retention time (SRT): around 17 hours [0117] N/C in weight based on elements: 1/27 [0118] Organic loading rate of the first reactor: 17 kg/m.sup.3.d acetate.

[0119] When starting the system under the above conditions the reactors were seeded with sludge already containing a mixture of PHA-accumulating organisms taken from the reactor mentioned in Example 1. Initially the SBR lab reactor from example 1 was seeded with aerobic sludge from a municipal waste water treatment plant

[0120] Under these conditions a steady state was achieved after a few weeks. It was confirmed by analysis that normally the acetate concentration in the PHA accumulation reactor and the settler overflow was between 50 and 100 mg/l. The dissolved oxygen concentration in the Growth reactor dropped directly after the pulse nutrient addition and increased again before the next pulse was supplied.

[0121] PHA content was analysed in samples taken from both the PHA accumulation reactor and the Growth reactor. Results are shown in FIG. 5. The average PHA content in the sludge in the PHA accumulation reactor was 70% by wt. based on organic content whereas the average PHA content in the Growth reactor was between 50 and 60% by wt.