Patent Application
20070095754
This invention involves enhancement of a process already known and used in the sewage treatment industry. More specifically the enhancement relates to efficient operation of membrane separators which are increasingly used in sewage treatment plants for dewatering and concentration of sludge.
Membrane separators, often called membrane bioreactors or MBRs, are used in an increasing number of sewage treatment facilities because of their compactness and efficiency in removing a nearly-pure permeate from sludge or mixed liquor. The membrane separators require maintenance. They become partially clogged by accumulated solids separated out by the micropore filters of the membranes; air scouring, constant or intermittent, typically is used to agitate and loosen the accumulated solids from the surfaces of the membranes.
It is known that a procedure called relaxation improves the overall hydraulic performance of membranes submerged in mixed liquor. In simple terms relaxation is generally described as a procedure whereby filtration is suspended for a time but air scouring (cleaning) is continued. Relaxation removes solids that have accumulated at the membrane surface during filtration (dewatering).
In the literature, relaxation frequencies and duration are on the order of hours or minutes. The primary limitation is the effective derating of a plant that occurs as membranes are taken out of service more frequently or for longer periods of time to increase the benefits of relaxation.
Typical membrane bioreactor (MBR) wastewater treatment plants are designed to handle daily (diurnal) and seasonal variation in both hydraulic and waste loading. However, for design and operational purposes, such variations must fall within pre-defined limits and average daily flow rate (ADF) over each 24-hour cycle of operation further limited to minimize maintenance requirements and extend membrane service life.
Air scour in MBR basins, as typically practiced in MBR basins using relaxation, is at a constant rate through the filtration and relaxation cycle, although in some cases air may have been cycled on and off during the filtration.
It is an object of the invention to greatly improve efficiency of operation of membrane separators, by optimizing the relaxation procedure, controlling the scouring air flow rate and enhancing the removal of accumulated solids from the membranes during relaxation, all of which reduces cost of operation and can also reduce capital cost.
Pursuant to this invention, filtration/relaxation cycles are much more frequent, and can be on the order of seconds rather than minutes. This can improve the benefits of relaxation without further derating plant performance if relative filtration/relaxation times are kept approximately constant. For example, a typical relaxation cycle (1 min. relax/9 min. filtration) effectively derates a plant by 10%. Relaxing for 10 seconds every 100 seconds should increase sustainable flow rate by removing accumulated solids more frequently and preventing buildup at the membrane surface.
Perceived limitations on the frequency of relaxation are controllability and valve wear. Controllability refers to the time it takes a valve to open or a pump to turn on and what happens to the flow after such action occurs. For example, electrically actuated permeate flow control valves (FCVs) generally have a throw time of 30 to 60 seconds. This means it takes up to 60 seconds for the valve to fully open or fully close from the opposite condition. Such a throw time limits how frequent relaxation can occur because time spent opening after each cycle further reduces plant throughput. In addition, if an FCV opens too fast, a phenomenon called overshoot can occur. During overshoot, the valve opens too far allowing filtration rates temporarily to spike. Spikes in filtration bring large amounts of solids to the membranes surface effectively undoing the benefits of relaxation. Constant opening and closing of valves during frequent relaxation also increases wear causing valves to fail and operating costs to increase.
The invention encompasses two methods to address the concerns of controllability and wear. The first method involves adding a second, pneumatically actuated valve upstream of the FCV. The pneumatically operated valve opens and closes during relax while the FCV stays in the same position. Pneumatic valve throw times are on the order of 6-15 seconds. In this configuration, the FCV does not close during relaxation and overshoot is reduced after each relax cycle.
The second method and configuration is generally more cost effective and easier to operate. Similar to the method proposed above, inexpensive quarter turn valves (butterfly, ball, plug) are installed upstream of each FCV and operated using a camshaft connected to a motor. As the motor turns at an adjustable frequency, the cams open and close these isolation valves while the FCV stays in position to limit overshoot following each relax cycle. This method improves controllability and eliminates the need for actuators that can wear.
The drastic reduction in cycle lengths, i.e. the much more frequent relaxation of the membranes from filtration, while retaining a roughly similar ratio between total filtration time and total relaxation time, tends to remove accumulated solids from membrane surfaces in an overall more efficient scheme, not allowing clogging to proceed as far as with the typical longer relaxation cycling used previously, with the result that overall air requirements for a given permeate flow rate can be reduced, or, on the other hand, a greater flow rate can be achieved from a given total surface area of membrane separators.
In another aspect of the invention, air scour requirements are decreased through the use of proportional aeration. Air-scouring rates can be varied proportionally to match the permeate (filtrated water) flow rate. Current practice calls for constant aeration at any permeate flow rate and or what is referred to as air cycling. Air cycling is a patented method whereby valves are used to alternately feed the same air between two membranes units or cassettes operating in parallel. Air cycling subjects membranes to intense air scouring followed by equal periods of non-aeration. Using proportional aeration, filtration must be accompanied by some amount of aeration and the method of apportioning that aeration is not limited to a given apparatus.
Although aeration requirements are a function of numerous factors including equipment geometry and membrane properties, general practice requires operation at some minimum value, constant value. Below are quantifiable guidelines for establishing aeration requirements as a function of ADF:
Proportional aeration simply follows the particular need for flow rate of air scour based on the conditions of flux and loading against the MBRs. Air flow is not wasted when not needed. In contrast, current practice has been simply to keep air scour flow rate constant, without variation during filtration (except, as noted above, that the air has sometimes been switched on and off during filtration).
A third aspect of the invention is to enhance efficiency in MBR operation through enhanced relaxation. During filtration air scour rates are reduced, but are at much higher flow rate during relaxation. The overall air scour flow is reduced. Typically, membranes are relaxed one minute every ten minutes to remove solids that have accumulated at the membrane surface. Using the proposed innovation, the air scour requirement would be reduced from 25%-50% during the nine-minute filtration cycle and increased to 100% or more of nominal flow for the one-minute relaxation cycle. The net decrease in aeration requirement would be approximately 22.5% assuming a 25% reduction in air during filtration. The actual benefit could be more depending on actual operating conditions.
Each of the three modifications discussed above can be used individually or in combination with either of the others. It is therefore among the objects of the invention to improve efficiency in membrane separator operation achieving savings in overall air scour requirements and more efficient operation of the MBRs. These and other objects, advantages and features of the invention will be apparent from the following description of preferred embodiments, considered along with the accompanying drawings.
In the drawings,
The block 14 indicates initiation of a membrane relaxation cycle. The following block 16 denotes closing of an in-line valve, a valve additional to the flow control 12, in the liquid line, located preferably downstream of the MBRs and thus shutting off flow of effluent permeate. As indicated, this valve can be pneumatically or mechanically controlled. At the same time, the air scour rate is significantly raised, as noted in the block 18. The scour rate can be raised by about thirty percent, fifty percent, or even about one hundred percent.
A break 20 shown in the time line between the block 18 and the following block 22 represents the duration of time the high air scour is on and the inline valve is actually closed.
At 22 and 24, air scour is switched back to the low level and the inline valve is opened to resume MBR separation. After the prescribed filtration time, the system goes back to the relax cycle, as indicated at 26.
The drawing indicates that the total cycle of filtration/relaxation can be, for example, about ten seconds to several minutes, e.g. five minutes. The relaxation portion of the cycle can be, for example, about one second to about one minute, more preferably about one second to about ten seconds for short cycling. Total cycle times can be about 10 to 30 seconds, with about one to five seconds relaxation; or, for very short cycling, total cycle time of about 10 seconds, with about one second relaxation. This is about one or more orders of magnitude more frequent cycling than in the prior art, where the full cycle was typically measured in hours, and more recently, the assignee of this invention initiated shorter cycles of about ten minutes, about one minute of which was the relaxation cycle.
Assuming a constant pressure on the blower that provides aeration, a nineteen percent reduction in volumetric flow rate equates to the same reduction in power costs.
In
These numbers are only examples. The air scour flow rate at relaxation might be as much as twice the air flow rate during filtration, or even more if desired. As discussed above, this method can greatly increase the efficiency of the MBR system, achieving better cleaning during relaxation and, again, saving on air scour and blower operation costs.
As shown in
In the arrangement shown, a motor 62, which may be geared down internally as needed, has a sprocket 64 engaged with a chain 66 that rotates another sprocket 68 when the valve has to be opened or closed. The sprocket 68 is connected to the worm gear shaft 58 discussed above. The sprockets 64 and 68 can be sized appropriately to achieve the desired speed of valve opening and closing. Also, the motor 62 preferably has variable frequency drive (VFD) responsive to a variable current control signal. For example, an appropriate duration for opening or closing the valve, for the purposes of this invention, would be about one second. This could be appropriate for a one second relaxation period, although faster opening/closing times could be used, or longer duration opening/closing especially for a longer relax period. Often field optimization will determine relax frequency and duration as well as valve cycle time.
By using a separate valve 54 (which could have another type of actuation, such as a pneumatically-driven valve), the function of the existing FCV 52 is not affected, and flow settings in the FCV are retained during periods of relaxation. The overshoot problem discussed above can be avoided by use of this separate valve because the separate valve need not reach a control set point when re-opened and the FCV will need little or no movement to reach the control point. Reducing overshoot reduces the frequency of needed maintenance cleaning, thereby increasing life of the membranes.
If air and permeate lines are conveniently located, the same controller used for the permeate valve 54 can be used to increase air rate for relaxation when the permeate valve is closed and vice versa. If pneumatic control is used, common control can be used for air and permeate flow switching regardless of pipe line locations.
The above described preferred embodiments are intended to illustrate the principles of the invention, but not to limit its scope. Other embodiments and variations to these preferred embodiments will be apparent to those skilled in the art and may be made without departing from the spirit and scope of the invention as defined in the following claims.
