Patent Application
20180093908
Embodiments relate to improvement of membrane bioreactor process by addition of polymeric media along with membrane module(s). This may reduce membrane fouling, improve flux, and reduce overall footprint and cost of the treatment system.
Membrane bioreactor (“MBR”) technology is a well-established wastewater treatment process. It is accepted for use with municipal as well as industrial wastewater treatment. The technology has many advantages, including small footprint, exceptional organic removal and very good consistent effluent quality. Different types of polymeric membranes like flat membrane, hollow fiber and tubular type membranes may be used in the process. The membrane is the heart of MBR process, and correct operation and maintenance of membrane and the biological process is extremely important for longterm sustainable performance.
In a conventional MBR process operates in a cyclic process and a specific time for relaxation and backwashing provides the best results in terms of sustained flux and trans-membrane pressure (“TMP”). Maintaining a consistent flux requires a relaxation period for a minute after every 9-10 minutes of operation. Backwashing of the membrane with the permeate water and periodical chemical cleanings are imperative. During the relaxation period and backwashing process the permeate production from the membrane modules is stopped for certain time. Because of this the membrane system needs to be upsized to meet water production needs during the time remaining for operation. Also during the 9 minute operation in a cycle, there is a decline in the flux. This process affects overall productivity and efficiency of system. The requirement of periodic chemical cleaning increases operational cost, and the use of frequent chemicals for cleaning may reduce the membrane life.
In addition, current MBR is limited by the flux and needs to be operated at lower flux as increase in the flux increases solid loading at the membrane surface hence membrane gets fouled and thereby requires frequent cleaning. Furthermore, during the 9-10 minutes operating period we may see decline in the flux, which is recovered during the relaxation period and gradually when this process continues the recovery of flux may not always happen and the system requires chemical cleaning, which results in loss of productivity.
Existing membrane bioreactor systems have limitations related to the content of Mixed Liquor Suspended Solids (“MLSS”). Typically a system gives best results within a certain band of MLSS values. Especially handling of low MLSS conditions is quite difficult due to increases of number of filamentous bacteria in sludge. The filamentous bacteria have poor filtration characteristics foul the membrane surface immediately. Simple air bubble impact may not able to clean the membrane surface, and with time it leads to thick solids layer formation over the membrane surface. In a conventional membrane bioreactor, when operation is suspended, there is a possibility that sludge may get washed out, and microorganisms developed for removal of certain organic compound may get removed from the system. This leads to further system upset and reduces productivity.
While the MBR process works well, it has its limitations as described above. There is another process known as Moving Bed Bio Reactor (MBBR), which is used to improve the performance of activated sludge. This process works through attached growth on the huge surface area offered by the media in place. This reduces the hydraulic retention time and consequently the reactor volume reduces. But in this process one does not achieve the quality of water that is achievable through a MBR process, and the chances of upset conditions disrupting the steady state conditions cannot be totally ruled out. The conventional MBBR uses plastic media like PVC or polypropylene, which are hydrophobic in nature and water or bio mass cannot penetrate through the media. So the impact of media is largely a surface phenomenon, which takes place on the surface area provided by the media.
It would be helpful to provide a more efficient, higher-performing MBR process. In embodiments presented herein, conventional MBR process is made efficient in performance by integrating with a non-bio degradable media, which is kept dynamic and fluidized within the membrane system to enhance the performance of both the biological process and the membrane material. Typically the media is a polymer media. In one embodiment the media comprises crosslinked polyvinyl alcohol (PVA) spheres. In another embodiment the media consists of crosslinked PVA spheres. This process can use other porous polymeric material in porous structures. These may be, for example, polyurethane, PVDF, poly-lactic acid, poly acrylic acid, polymethacrylate, polyethylene glycol, natural biopolymers like alginates, chitosan, and Carboxy methylcellulose, etc, or other porous polymers where the bacteria can be cultivated and encapsulated water and organic material which act as food can penetrate through the polymeric structure due to the pore size and void space available therein.
This media is added in a wastewater treatment system along with an MBR unit. The gaps between the membranes of MBR are kept such that these beads can easily move and flow through the gaps with air in system and scrub the membrane surface, and keep the membrane surface clean all the time. This is in contrast with the operation of air bubbles, which typically do not assist with cleaning of the membrane surface. The polymeric media remains circulating within the system as the permeate is drawn through the membranes and any sludge removal happens through a recirculation pump with has a suction strainer, which does not allow the media to come out but allows the sludge to drain out for filtration. The filtered mother liquor is circulated back into the reactor. The media can also be encapsulated within the module by caging the module with a wire mesh such that media cannot move to the bulk solution outside the membrane modules in the bio-reactor.
The soft media, which accommodates millions of bacteria, treats chemical oxygen demand (“COD”) and in a fluidized dynamic motion removes foulants from the membrane surface or prevents the fouling process from getting initiated. The media keeps the membrane surface clean without the need for providing any rest time or back wash, and at the same time allows the membrane to produce higher volume of permeate at higher flux. Therefore the overall productivity of membrane and the biological process improves by 25-40%. The rate of biological digestion of organics is also enhanced by the same margin while reducing sludge production by more than 40-50%. The media has a three dimensional role, because it is not the surface phenomenon—the inside porous structure is also activated by bacteria, which plays an important role in enhancing the reaction kinetics and reducing the sludge mass generated due to high density of active bacteria.
Inclusion of the media also offers advantages of conditioning the sludge such that the sludge volume will reduce due to compactness of the sludge. This facilitates nitrification due to the presence of nutrients and formation of nitrifying bacteria inside the media. For a given membrane area one obtains higher permeate due to non-stop permeate production at a higher flux and without any loss of flux. Consequently there is a reduction in the cost of power, which reduces the overall operating cost.
The process also works in through elegant combination of conventional MBBR and MBR in a single operating unit while maintaining the advantages of both processes and also giving unexpected results on overcoming their disadvantages i.e. delivering MBR quality water which is possible in MBBR and not having fouling problems encountered in MBR. Also the unexpected results are seen in eliminating rest time or cyclic operation where production needs to be sacrificed, at a lower TMP without suffering any flux loss and simultaneously reducing sludge production. This approach also enables increase in organic loading without compromising treatment capacity or quality.
Embodiments may have, but are not required to have, one or more of the following aspects, where comparisons are made to conventional MBR systems:
1. More productivity due to reduced or no physical cleaning required;
Typically combination of membrane filtration and suspended biological activity is known as a membrane bioreactor. Membrane bioreactor technology is widely used for treatment of municipal and industrial wastewater. MBR process could produce effluent of high quality, which can be discharged or recycled/reused. However membrane bioreactor technology has an inherent flaw, namely membrane fouling. Despite its contribution to solid rejection, membrane fouling has been generally recognized as the cause of permeate flux decline requiring cyclic operation and frequent physical/chemical cleaning. Membrane fouling is largely dependent on process parameters and biological condition. Because of operational limitations, it is necessity to improve conventional membrane bioreactor process with the following objectives, which may or may not be achieved, of:
1. Reduced membrane fouling;
2. Improved hydraulic flux;
3. Reduced or removed physical cleaning;
4. Reduced chemical cleaning;
5. Increased capacity of biological system in terms of COD load handling;
6. Protected bio culture for consistent performance of biological system;
Embodiments as presented herein may have one or more of the above advantages, thereby enhancing membrane bioreactor process for its wide application for treatment of municipal and industrial wastewater.
Generally, embodiments are presented herein are invention is related to an aerobic submerged membrane bioreactor process. A membrane bioreactor process may be made more efficient and user-friendly by integrating a membrane bioreactor process with a media capable of serving as a structure for biological growth.
A preferred media for use in embodiments of the invention is cross-linking polyvinyl alcohol. This may be prepared, for example, by a copolymerization process or cross-linking of PVA with PVA in a boric acid medium. Typically components of the media are spherical or nearly spherical. In some embodiments the size of the media is 2 to 8 mm, preferably an average of 3 mm. The size can be varied and customized for certain sizes and configuration of MBR membranes. The criteria for selection mainly depends on style of membranes that is hollow fiber of plate type. The size of the media is based on allowing free movement of media within the reactor volume and ability to keep it fluidized while allowing access to most of the membrane surface to allow it to scrub the membrane constantly. Typically the beads are highly porous, having an interior surface area of between 3000 to 5000 m2/m3.
We provide a system including a submerged membrane module along with the media reported herein used for treatment of wastewater by an aerobic process. In a specific embodiment, a membrane module with specific gaps between plates is being used for filtration of sludge and water. These inter-plate gaps may be, for example 2 to 10 mm, preferably 4-6 mm.
Along with air supplied from bottom of the module the media is kept in dynamic condition between the gaps to scrub the membrane surface area to clean it in-situ. Continued cleaning of membrane surfaces by the media results in benefits of requirement of no cyclic cleaning, reduced or even no physical and chemical cleaning requirement, high flux, low TMP and reduced frequency of chemical cleaning.
The use of highly porous media having large internal surface area provides the advantage of retention of microbiological culture for a longer time without disturbances causing an upset condition. Due to the presence of large numbers of microorganism like aerobic bacteria, protozoa, metazoan, filamentous bacteria and others the biodegradation of organic compound present in wastewater gets accelerated. Bacteria are able to reside within the porous polymeric media and able to develop the appropriate mixed culture which accelerate the metabolism process of the organic materials. In addition, the sludge generation gets reduced to about 50% of that produced by a conventional MBR process due to autolysis taking place in the reactor. This reduces solid sludge waste generation and reduces sludge handling cost. Overall efficiency of biological system gets enhanced with the combination of membrane and polymeric media in single aerobic reactor.
The following describes an embodiment of the invention. In typical embodiments a membrane module may be prepared as reported in U.S. Pat. No. 8,753,509, “Advanced Filtration Device for Water and Wastewater Treatment,” which is incorporated by reference herein. For an experimental trial a bioreactor of 150 liters capacity was made. Flat sheet ultrafiltration polyvinylidene fluoride (PVDF) cartridges supplied by Qua group were used for the trial. A membrane module was prepared placing membrane plate one by one with specific gap of 3 to 6 mm in between. Two membrane modules having surface area of 1.8 m2 are kept one above other. Total surface area of module was 3.6 m2. Air diffusers were provided at the bottom of the module to supply air. The air supplied by diffusers was used for both purposes; biological supply as well as circulating media within the reactor and in between the membrane gaps. A vacuum or permeate pump was provided for collection of product from the membrane module.
Simulated feed water having COD and nutrient was used for study. A feed pump with flow meter was provided for supply of feed water to the system. This was tested continually at different operational and process parameters. Analysis of feed and product water was done for Chemical Oxygen Demand (COD), Biological Oxygen Demand (BOD), pH, and alkalinity. Biological parameters like Mixed Liquor Suspended Solids (MLSS), Hydraulic Retention Time (HRT), Solid Retention Time (SRT) and Sludge Volume Index (SVI) were measured and monitored to control biological system. Microbiological analysis was performed by an external laboratory by colony formation method. Turbidity of product water was measure by HACH 2100N Turbidity meter. A flow scheme for this trial is shown in
In this trial media was added to the reactor equivalent to 18% of reactor volume along with activated sludge. The activated sludge was collected from an operating sewage treatment plant. Polymeric media and activated sludge were added to reactor to initiate activation for the first time. In other embodiments of the invention media may be added in amounts between 15-25% of the reactor volume, 10-30% of the reactor volume, or 16-20% of the reactor volume.
Prior to operation the media and sludge were aerated for some time with addition of food and nutrient to activate the media. For 8-10 days. a membrane cartridge was submerged in the bioreactor, and the system was connected to the vacuum assembly. The reactor was fed with simulated water having a character equivalent to municipal wastewater.
Operation of the reactor began, with an effort made to maintain certain biological parameters like Mixed Liquor Suspended Solids (MLSS) and Chemical Oxygen Demand (COD) loading, tested at different hydraulic flux. During testing membrane performance was studied in terms of change in Trans Membrane Pressure (TMP) and product turbidity while bio degradation performance was judged by COD and BOD rejection from the system.
In a typical embodiment, an enhanced media-enabled MBR has two main components: media and at least one membrane module. Detail description of both is given below
Media
Growth media referred to herein is a porous polymeric media. Typically this media includes a plurality of units of PVA as base polymer that is cross-linked. It is spherical or generally spherical in shape. The highly porous media holds >90% moisture in its structure. Media can accommodate millions of bacteria in porous structure. When added to the wastewater system, the media is activated with biomass in the environment it is exposed to and resides in. It acts as a media attached growth process. Once particular microbial activity develops in the porous area, media itself is capable of treating the wastewater and reduce COD and BOD from it. It will be appreciated that the media is not present as a single, agglomerated unit, but is instead present as a plurality of tiny units.
As microorganisms get embedded into the media, bacteria remain protected from washout or removal from system by upset conditions. Media can be fluidized and can become dynamic in aerated or agitated environment. As the media remains in fluidized condition within the membrane system or between gaps of membrane, it continually scrubs the membrane surface, which helps in removal of thin sludge layer formed over the media during filtration process. This results in in-situ cleaning of the membrane surface and avoids fouling on the membrane surface is eliminated or reduced significantly. So the membrane remains virtually clean all the time in spite of residing in highly fouling environment.
When the media gets fully activated in biological process, it converts into a brown color. SEM of activated media also indicates that a bio culture sits inside the porous structure. Microbiological analysis indicates presence of millions of bacteria. Fresh and activated media are compared in
As shown in
Membrane Module
PVDF flat sheet ultrafiltration membrane was used for making submerged membrane module. Modules were prepared by assembly of number of membrane plates with specific gap of 2-10 mm in between. Module made for testing was having 10 plates. Two such modules are placed one above other. Modules were operated in outside-in mode, where water to be purified is drawn into the modules by a vacuum or suction pump. Product is collected by vacuum or suction pump from the modules. Operation of a typical module is shown in
In a typical embodiment a module is operated in submerged mode. Reference numbers refer to
Detailed flow scheme of module operation is given in
In a second trial media (13) was added to the bioreactor tank (1) at 15-25% of reactor volume. The system was operated with a changed sequence of operation without involving cyclic operation. Module was operated without rest time. Product suction pump (7) was operated continually. Also backwashing of the module was not performed because no decline in flux was seen. Backwash tank (9) was used as chemical cleaning tank to do maintenance cleaning of the module. As media (13) kept moving between membrane gaps of module (2) along with air, it kept the membrane surface clean. Due to this in-situ cleaning effect no physical, cleaning was needed to maintain flux. Also frequency of chemical cleaning gets reduced to ⅓ of the operation without media.
Results
As discussed above a laboratory reactor was made for trial and different trials were conducted to confirm process sustainability.
In this trial we operated the Enhanced Membrane Bioreactor process at different hydraulic flux to understand the maximum critical flux process can be operated.
For this trial process was operated with simulated water having COD in the range of 200-300 mg/lit. Biological process parameters were maintained to achieve more than 90% COD and BOD reduction. During operation hydraulic flux was increased gradually from 20 lmh to 40 lmh and observed performance of reactor.
Operational Parameters
Table 1 and
This trial was conducted to validate the low TMP operation of process. For this example two trials were conduced one with and other without media addition. One set was run without media addition to the bioreactor while another was run with addition of media to the system at biological condition. Both the systems were operated at same flux. The data was collected in terms of change of suction pressure and requirement of cleaning to maintain constant flux values.
Case 1—without Addition of Media to the System
Table 2 and
Case 2—with Media Addition to the Reactor
Media added to the reactor. Along with air, media was fluidized between the gap of the membrane. The system was operated at a constant 30 LMH flux as that of case-I.
As shown in
Conventional activated sludge system operates at COD loading of 0.5 to 1.5 Kg/m3·day. Low COD loading of the process is due to limitation of excess sludge handling and difficulty experienced in keeping membranes clean in a highly fouling environment. In the conventional mode, high COD loading generates excess sludge, which has to be removed from the bioreactor to maintain biological parameters and process efficiency. The present innovation of porous media helps to increase loading capacity of the reactor. As media can hold very high amount of mass within its porous structure, the active media also take part in degradation of organic matter from wastewater. The presence of active biomass is confirmed by micro-biological testing which shows that millions of bacteria are present in each Bio-bead. Micro biological analysis results are shown in Table 5
As shown in Table 6 and
As noted above, media when used herein can hold millions of bacteria in its porous structure. During operation and continuous aeration the sludge formed in the process gets degraded by auto-lysis process. This process helps to reduce overall sludge formation in the system. In conventional activated sludge 0.5 Kg sludge formed per Kg of COD degraded. In embodiments reported herein sludge formation may be reduced by up to 50%. Sludge formed in Enhanced Membrane Bioreactor is 0.23 Kg VSS/Kg. COD degraded. Practical value of sludge generated was calculated based on actual Mixed Liquor Suspended Solids (MLSS) generated after degradation of specific amount of COD as shown in Table 7.
Overall study shows that, the present invention has potential to overcome limitation of the conventional membrane bioreactor system. Experimental data shows that use of media along with membrane for treatment of wastewater by aerobic process has a number of benefits. Due to the reduced or no physical cleaning required it is possible to operate process continually to increase productivity. As the sludge formation gets reduced by 50%, there is a reduction in sludge handling cost. The innovative process has lower capital as well as operating cost. As porous media hold microorganism in its structure, it helps to increase loading capacity of system. The presence of active biomass within the media structure has been proven by scanning electron micrograph and also by colony formation method. It is possible to operate an Enhanced Membrane Bioreactor at higher flux reducing the reduces overall foot print of the system. Product quality is maintained during various trials, indicating that consistent product quality can be achieved with added advantages.
| Number | Date | Country | Kind |
|---|---|---|---|
| 919/DEL/2015 | Mar 2015 | IN | national |
This application claims priority to Indian Application No. 919/DEL/2015, filed on Mar. 31, 2015, and to U.S. Provisional Patent Application No. 62/191,748 filed on Jul. 13, 2015. Both of those applications are incorporated by reference herein.
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/US2016/025331 | 3/31/2016 | WO | 00 |
| Number | Date | Country | |
|---|---|---|---|
| 62191748 | Jul 2015 | US |
