High flux anaerobic membrane bioreactor

Abstract

A method for treatment of wastewater includes passing influent wastewater through an anaerobic, anoxic, or bioelectrochemical bioreactor to produce an effluent. The membrane bioreactor includes a membrane with pores having a nominal pore size less than the smallest measured biopolymers and organic nanoparticles in the influent wastewater, thereby preventing them from entering and blocking membrane pores, and further comprising degrading dissolved organics smaller than 20 nm in the influent wastewater within the membrane bioreactor before entering membrane pores.

Claims

1. A method for treatment of wastewater, comprising passing influent wastewater through a membrane bioreactor to produce an effluent, where the membrane bioreactor is an anaerobic, anoxic, or bioelectrochemical bioreactor, where the membrane bioreactor comprises a membrane with pores having a nominal pore size is less than the smallest measured biopolymers and organic nanoparticles in the influent wastewater, thereby preventing them from entering and blocking membrane pores, and further comprising degrading dissolved organics smaller than 20 nm in the influent wastewater within the membrane bioreactor before entering membrane pores.

2. The method of claim 1 wherein the nominal pore size of the membrane is 20 nm or less.

3. The method of claim 1 wherein biopolymers and/or organic nanoparticles with hydrolytic enzymes are concentrated in the membrane bioreactor retentate, enabling more efficient and rapid hydrolysis.

4. The method of claim 1 wherein the bioreactor is anaerobic and produces methane.

5. The method of claim 1 wherein the bioreactor is anoxic and produces molecular nitrogen (N.sub.2).

6. The method of claim 1 wherein the bioreactor is a bioelectrochemical system incorporating exoelectrogens.

7. The method of claim 1 wherein the bioreactor contains biofilms.

8. The method of claim 1 wherein the wastewater is municipal wastewater.

9. The method of claim 1 wherein the bioreactor does not contain flocculant microbial biomass.

10. The method of claim 1 further comprising operating the bioreactor to undergo alternating periods of membrane relaxation and surface turbulence (e.g., gas sparging) such that foulants are removed from the membrane surface.

11. The method of claim 1 wherein the effluent from the membrane bioreactor has a net flux greater than 6 L/m.sup.2/h.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0010] FIG. 1 is a schematic diagram illustrating a conventional aerobic membrane bioreactor (AeMBR) used for secondary treatment of wastewater.

[0011] FIG. 2 is a schematic diagram illustrating a conventional anaerobic membrane bioreactor (AnMBR) used for secondary treatment of wastewater.

[0012] FIG. 3 is a schematic diagram illustrating a microbial bioreactor used in a method for treatment of wastewater according to embodiment of the present invention.

[0013] FIGS. 4A-B show isometric and longitudinal cross-sectional views of a hollow fiber membrane used in a bioreactor according to embodiments of the present invention.

[0014] FIG. 5A is a cross-sectional view of a portion of a conventional ultrafiltration membrane, illustrating ultrafine COD (UFCOD) foulant particles attached to the interior portions of the pores, causing irreversible fouling of the membrane.

[0015] FIG. 5B is a cross-sectional view of a portion of an ultrafine membrane according to an embodiment of the present invention, illustrating prevention of irreversible fouling by blocking ultrafine COD (UFCOD) foulants from entering the membrane pores.

[0016] FIG. 6A is a schematic diagram of a microbial bioreactor incorporating a moving media for biofilm formation, according to an embodiment of the invention.

[0017] FIG. 6B is a schematic diagram of a microbial bioreactor incorporating a fixed electrode for biofilm formation with exoelectrogens, according to an embodiment of the invention.

[0018] FIG. 7 is a graph summarizing the size range of ultrafine colloidal organic matter (15-30 nm) in domestic wastewater.

[0019] FIG. 8 is a graph summarizing different permeate COD (mg/L) depending upon membrane pore size (nm) for a pilot-scale AnMBR treating domestic wastewater.

[0020] FIG. 9 is a graph that compares decreasing ratios of permeability for 40 nm and 10 nm membrane pores when filtering the same AnMBR concentrate.

[0021] FIG. 10 is a graph summarizing membrane flux (L/m.sup.2/h) conditions depending upon membrane pore size (nm).

DETAILED DESCRIPTION OF THE INVENTION

[0022] Herein we disclose, in one embodiment of the invention, a high-flux AnMBR that that makes use of small pore size membranes (at most 0.02 μm) to enable high-flux operation, high quality effluent, and increased energy production as biogas methane. Unlike previous methods that used membranes with pore sizes of 40 nm or more, the present methods use an anaerobic microbial bioreactor incorporating membranes with a nominal pore size less than 20 nm. This was not obvious before because it was not realized that anaerobic bioreactors cannot remove organic nanoparticles (16˜40 nm) and thus, while previous approaches were able to address membrane cake layer fouling (on the surface of membranes), they were not able to address pore blocking. The inventors have demonstrated that a finer pore size membrane can reduce membrane irreversible fouling because ultrafine colloidal substrates (0.02˜0.03 μm) are rejected at the membrane surface, forming a cake that is readily controllable by conventional fouling control methods.

[0023] In one embodiment of the invention, a method for treatment of wastewater is implemented using microbial bioreactor, as shown in FIG. 3. Influent wastewater 300 is passed through the microbial bioreactor 302 to produce effluent 304 and biosolids waste 306. Influent wastewater 300 may be, for example, domestic or municipal wastewater. The microbial bioreactor 302 contains non-flocculant biomass (i.e., anaerobic or anoxic microorganisms, or exoelectrogens) and preferably does not contain any flocculant organisms. The microbial bioreactor 302 includes at least one membrane 308 with pores having a nominal pore size at a surface of the membrane less than a predetermined value selected to be the smallest measured particle diameter of biopolymers and organic nanoparticles contained in the influent wastewater 300 which the bioreactor was designed to treat. As a result, the biopolymers and organic nanoparticles are prevented from entering the pores of the membranes, thereby mitigating pore blockage while enhancing permeate quality. Preferably, the nominal pore size of membrane is less than 20 nm, or more preferably less than 15 nm. Such membranes may be obtained, for example, from Toray Membrane USA, Inc. More generally, the size of pores at the membrane surface are sufficiently small to prevent blockage of the internal portions of the pores. Surprisingly, despite the reduced pore size compared to conventional anaerobic microbial bioreactors, the reactor disclosed here can provide a net flux greater than 6 L/m.sup.2/h.

[0024] An example of a hollow fiber membrane 400 is shown in FIGS. 4A-B. A hollow cylindrical support layer 402 that allows effluent to freely pass through it provides structural support for a surrounding cylindrical layer of membrane polymer 404 (e.g., polyvinylidene difluoride). The wastewater 406 flows radially from outside, through the membrane layer 404 and support layer 402, and into a central channel 408 inside the hollow cylindrical support layer 402. Once in the central channel, the effluent 410 flows longitudinally and exits the filter through one end.

[0025] Conventional AnMBR membranes have a nominal pore size of 100 to 200 nm for microfiltration (MF) and 30 to 40 nm for ultrafiltration (UF). Organic nanoparticles smaller than this nominal membrane pore size may not be retained. Accordingly, we define COD that can pass through ultrafiltration membranes as ultrafine COD (UFCOD). UFCOD nanoparticles ranging in size from 16-30 nm typically have a peak size close to 20 nm, smaller than the nominal pore size of conventional UF membranes (40 nm). The 20 nm peak is close to the size range of humic polymer colloids, organic nanoparticles, and phage.

[0026] FIG. 5A shows a close up of a conventional ultrafiltration membrane 500 having pores 502 with nominal size at the membrane surface of 40 nm or more. Wastewater 504 flows through the membrane from left to right. As the wastewater 504 is filtered to produce filtered effluent 510, foulant particles 506 with size greater than the membrane pore size at the surface are caked at the membrane surface. This fouling at the surface is reversible. However, as the present inventors have discovered, UFCOD foulant particles 508 and 512 with size less than the membrane pore size at the surface can enter the pores and become attached to the interior portions of the pores, blocking the pores and causing irreversible fouling of the membrane.

[0027] FIG. 5B shows a close up of a membrane 550 according to the present invention having pores 552 with nominal size at the membrane surface of 20 nm or less. Wastewater 554 flows through the membrane from left to right. As the wastewater 554 is filtered to produce filtered effluent 560, foulant particles 556 and 558 with size greater than the membrane pore size at the surface are caked at the membrane surface. In contrast to the membrane of FIG. 5A, this membrane prevents smaller ultrafine COD foulant particles 558 from entering the pores and causing irreversible fouling. The UFCOD particles form cake layer fouling that is controllable with gas sparging of the membranes with recycled biogas blown at the base of the membranes. Although some UFCOD particles 562 with size less than the membrane pore size can enter the pores and become attached to the interior portions of the pores, these are far fewer and smaller, and do not substantially block the flow through the pores.

[0028] In some embodiments, the membranes with a nominal pore size less than 20 nm 308 reject and concentrate biopolymers and/or organic nanoparticles with hydrolytic enzymes in the retentate, enabling more efficient and rapid hydrolysis of biopolymers and/or organic nanoparticles.

[0029] In a preferred embodiment of the invention, the bioreactor 302 is anaerobic and produces methane. In such an embodiment the reactor includes a methane exhaust, as shown in FIG. 2. In another embodiment, the bioreactor 302 is anoxic and produces molecular nitrogen (N.sub.2).

[0030] In embodiments of the invention where the bioreactor 302 is an anaerobic microbial bioreactor, a membrane fouling control strategy is preferably performed, e.g., alternating periods of membrane relaxation and membrane surface turbulence (e.g., gas sparging) to detach foulants.

[0031] In an alternate embodiment, the bioreactor 302 is anoxic. In such embodiment, the bioreactor produces molecular nitrogen (N.sub.2).

[0032] In some embodiments, the bioreactor comprises biofilms. For example, a method for treatment of wastewater is implemented using microbial bioreactor incorporating a moving media for biofilm formation, as shown in FIG. 6A. Influent wastewater 600 is passed through the microbial bioreactor 610, 602 to produce effluent 604 and biosolids waste 606. Influent wastewater 600 may be, for example, domestic or municipal wastewater. The microbial bioreactor first stage 610 contains moving media 612 covered with microbial biomass. The wastewater recirculates between first stage 610 and second stage 602 through external recirculation lines 614, 615. The microbial bioreactor second stage 602 is a membrane tank that includes at least one membrane 608 with pores having a nominal pore size at a surface of the membrane less than the smallest measured particle diameter of biopolymers and organic nanoparticles contained in the influent wastewater 600.

[0033] In another example, a method for treatment of wastewater is implemented using microbial bioreactor incorporating a fixed electrode for biofilm formation with exoelectrogens, as shown in FIG. 6B. Influent wastewater 600 is passed through the microbial bioreactor 616, 602 to produce effluent 604 and biosolids waste 606. Influent wastewater 600 may be, for example, domestic or municipal wastewater. The microbial bioreactor first stage 616 is an electrochemical bioreactor that contains a fixed electrode 618 covered with exoelectrogens 620. The wastewater recirculates between first stage 616 and second stage 602 through external recirculation lines 614, 615. The microbial bioreactor second stage 602 is a membrane tank that includes at least one membrane 608 with pores having a nominal pore size at a surface of the membrane less than the smallest measured particle diameter of biopolymers and organic nanoparticles contained in the influent wastewater 600.

[0034] We now present experimental data demonstrating innovative features of the present invention.

[0035] FIG. 7 is a graph summarizing the size range of ultrafine colloidal organic matter (15-30 nm) in domestic wastewater 700, which is smaller than the nominal pore size range of membranes in conventional membrane bioreactors (MBRs) 702. The concentration of ultrafine colloidal organic matter (S.sub.UF) is governed by hydraulic retention time (HRT) and first-order kinetics for hydrolysis (k.sub.hyd.sup.UF)

[00001]$\frac{{\mathrm{dS}}_{\mathrm{UF}}}{\mathrm{dt}}=\frac{S_{\mathrm{UF}}^0}{\mathrm{HRT}}-\frac{S_{\mathrm{UF}}}{\mathrm{HRT}}-k_{\mathrm{hyd}}^{\mathrm{UF}}{S}_{\mathrm{UF}}$

[0036] In conventional aerobic MBRs, k.sub.hud.sup.UF is faster than the rate at which water passes through the membrane (1/HRT): k.sub.hyd.sup.UF>>1/HRT. This is because aerobic systems bio-flocculate with colloids and have high rate of hydrolysis, enabling rapid biological consumption of S.sub.UF, high-quality permeate and low membrane pore blockage due to low S.sub.UF.

[0037] Anaerobic MBRs (AnMBRs) lack bio-flocculation, and, as a result, the rate of hydrolysis is much slower (k.sub.hyd.sup.UF, 1.9 1/d) than the rate at which water passes through the membrane (1/HRT, 4.8 1/d), resulting in ineffective biological degradation of S.sub.UF, higher permeate COD, and more membrane pore blockage due to high S.sub.UF.

[0038] Counterintuitively, ultrafiltration membranes with smaller pores (nominal pore size less than 20 nm, preferably smaller than 15 nm) prevent passage of ultrafine colloidal organic matter through the membranes 704. By doing so, the hydrolysis of ultrafine colloidal organic matter is governed by solids retention time SRT (>20 days), which is much longer than HRT (˜5 hours), enabling low S.sub.UF within the system and permeate (FIG. 8), with less membrane pore blockage (FIG. 9) and higher-flux (FIG. 10).

[0039] FIG. 8 is a graph summarizing different permeate COD (mg/L) depending upon membrane pore size (nm) for a pilot-scale AnMBR treating domestic wastewater. As shown in FIG. 8, permeate that passes through 40 nm membranes in an AnMBR treating domestic wastewater had higher COD (96 mg/L) than permeate that passes through 10 nm membranes in the same system (41 mg/L). Lower COD in the permeate translates to higher quality effluent and more methane production.

[0040] FIG. 9 is a graph that compares decreasing ratios of permeability for 40 nm and 10 nm membrane pores when filtering the same AnMBR concentrate: permeability decreased by 30-35% in 40 nm membranes, and by 20% in 10 nm membranes.

[0041] FIG. 10 is a graph summarizing membrane flux (L/m.sup.2/h) conditions depending upon membrane pore size (nm). Under the same operating conditions (i.e., same wastewater, same temperature, and same TMP of ˜0.4 bar), the 10 nm membrane enabled higher flux operation (˜30 L/m.sup.2/h) than the 40 nm membrane (˜16 L/m.sup.2/h).

[0042] High flux reactors according to the present invention could be employed for municipal wastewater treatment but also in numerous other industrial wastewater applications, e.g., food and beverage, textiles, and agricultural applications.