AU2005284686B2

AU2005284686B2 - Continuously variable aeration

Info

Publication number: AU2005284686B2
Application number: AU2005284686A
Authority: AU
Inventors: Thomas William Beck; Etienne Ulysee Brois; Warrren Thomas Johnson; Roger William Phelps; Fufang Zha
Original assignee: Evoqua Water Technologies LLC
Current assignee: Evoqua Water Technologies LLC
Priority date: 2004-09-15
Filing date: 2005-09-15
Publication date: 2011-02-24
Anticipated expiration: 2025-09-15
Also published as: AU2005284686A1

Description

- I TITLE: Continuously Variable Aeration TECHNICAL FIELD The present invention relates to membrane filtration systems and more 5 particularly, to the use of aeration to control fouling of the membranes in such systems. BACKGROUND OF THE INVENTION Any discussion of the prior art throughout the specification should in no way be considered as an admission that such prior art is widely known or forms 10 part of common general knowledge in the field. In submerged or immersed membrane filtration systems aeration using gas, typically air, is often employed on a continuous basis to control fouling. This is particularly the case for wastewater bioreactors. The prior art utilises high gas flows followed by a lower gas flow or no gas flow, the purpose of which 15 is to reduce the overall gas consumption whilst providing sufficient gas flow to scour the membrane surfaces. Mixtures of liquid/gas have also been used to scour the membranes and provide a reduction in gas consumption. There are several problems with this approach: 1. The cycle frequencies are typically very high, as it is not good to 20 have the gas off in the process for too long as fouling increases rapidly during this phase. High cycle frequencies increase wear on valves that are required to cycle between multiple cells or membrane tanks, so as to keep the overall gas flow more or less constant from the blower. There are also practical limits to how fast the valves can be cycled between on and off caused by limitations of 25 valve design and the time it takes to accelerate the gas flow.

-2 2. When the gas is off for one cell it provides an opportunity for sludge to flow back into the aerators which can cause clogging of the aerators over time. 3. The longer the time between cycles the more likely it is for fouling 5 to increase and the sludge to go anoxic. Thus, there is a lower and upper limit to the cycle time for this type of approach, and issues with valve wear. DISCLOSURE OF THE INVENTION It is an object of the present invention to overcome or ameliorate at least 1o one of the disadvantages of the prior art, or to provide a useful alternative According to one aspect, the present invention provides a method of controlling fouling in a membrane filtration system of the type where gas is used to clean or scour the membranes wherein the method includes supplying the gas to the system with a continuously variable flow rate using a variable speed 15 blower. For preference the system includes a number of modules and the supply of gas to each module has a continuously variable flow rate. Preferably the continuously variable gas flow rate, ranges from a high flow to a low flow (or to zero). For preference, the flow rate pattern of the gas flow 20 follows any continuously variable curve that achieves this result such as a sinusoidal or triangular wave. The varying gas flow not only provides the normal cleaning effect but the unstable nature of the gas flow has been found to provide an additional cleaning effect resulting from the inherent surges in flow. According to another aspect, the present invention provides a membrane 25 filtration system including one or more membranes, a source of gas for providing -3 a gas flow to form bubbles which scour the surfaces of the membrane, wherein said source of gas provides said gas flow at a continuously variable flow rate, wherein said gas source is a variable speed blower. In one form, the gas supply itself is varied, for example by varying the drive 5 speed in the gas blower. Various combinations of known valve arrangements may also be used to provide the continuously varying flow rate. The invention may be applied to a variety of membrane filtration system and is particularly applicable to submerged or immersed membranes and may be used in pressurised and non-pressurised systems. 10 According to another aspect, the present invention provides a flow control device for use in the control of the flow of aerating gas in a membrane filtration system, said flow control device including a housing having an inlet port and one or more outlet ports, a movable valve member for providing a continuously variable flow from said inlet port to said one or more outlet ports. 15 The housing may also be provided with a plurality of inlet ports. Where a number of membrane modules or cells are used within the membrane filtration system, a flow control device may be used having separate outlets connected to associated modules such that as the flow rate is being increased to one module or modules it is being reduced to another module or 20 modules. Preferably, the flow control device is a rotating or reciprocating valve. The flow control device may also provide a variable distribution of gas to one location. Distribution of the gas to each outlet port can be of continuously varying flow or an on/off flow. A feature of the flow control device is that it 25 operates in a continuously rotating manner, not an open/close static manner. In WO 2006/029465 PCT/AU2005/001405 -4 the present application, the flow control device is used to provide a continuously variable flow rate of gas to one or more locations. In a preferred form, the rotating valve is made up of three main components, the valve housing, the rotating distributor and the drive motor. The 5 valve housing is mounted to the motor and contains the inlet and outlet ports of the valve. The distributor is located within the valve housing and is driven by the motor. As the distributor rotates within the valve housing, it closes and opens the housing outlets and thereby directs gas out one outlet or out another. The number of outlet ports within the housing can vary, thus providing 10 control of the airflow to any desired number of locations. The shape of the distributor can be varied to suit the number of outlet ports and to control the rate of airflow distributed through each outlet port. Hence the valve may be used to control gas flow to a number of membrane modules or cells and optimise the total volume of gas used. 15 The valve is able to complete the function that would normally be achieved by a combination of standard valves. The valve has negligible operating friction and therefore has a long operating life expectancy. The valve operates in a continuous rotating manner and can therefore 20 cycle at a wide range of frequencies; typically 60 to 0.05Hz. BRIEF DESCRIPTION OF DRAWINGS Preferred embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings in which: Figure 1 shows a schematic drawing of one embodiment of a filtration 25 system according to the invention; WO 2006/029465 PCT/AU2005/001405 -5 Figure 2 shows a graph of air flow-rate over time for continuous variation versus cyclic variation of airflow; Figure 3 shows a perspective cutaway view of the rotating valve which may be employed in the invention; 5 Figure 4 shows a graph of air flow rate over time for the rotating valve of the time illustrated in Figure 3 against the ideal continuous flow pattern; Figure 5 shows a schematic sectional elevation view of a further type of valve which may be used to produce continuous variation in air flow rate; Figure 6 shows a graph of air flow rate over time for a valve of the type 10 illustrated in Figure 5; and Figure 7 shows a graph of transmembrane pressure (TMP) over time for a filtration module first having scouring by continuous variation in air flow rate and secondly having a cyclic flow rate. DESCRIPTION OF PREFERRED EMBODIMENTS 15 Referring to Figure 1, a pair of typical membrane filtration modules 5 is shown. Each module 5 includes a plurality of hollow fibre membranes 6 extending between upper and lower headers 7 and 8. Filtrate is withdrawn from the upper header 7 through the open ends 9 of fibre membranes 6 which open into collection chamber 10 connected to outlet 11. The lower header 8 has a 20 plurality of openings 12 connected to a chamber 13 which in turn is connected to an inlet pipe 14. Each pipe 14 is connected to a respective outlet port 15 and 16 of a rotating valve 17. The valve 17 has an inlet port 18 connected to a source of pressurised gas, typically an air blower or pump (not shown).

WO 2006/029465 PCT/AU2005/001405 -6 The ideal gas flow pattern for continuous variation of the gas flow rate of gas used in scouring the membranes against a cyclic gas flow pattern is shown in Figure 2. Figure 3 shows the rotating valve 17 in more detail. The valve 17 5 comprises a valve housing 19 with a rotating distributor 20 rotatably mounted therein and driven by a motor 21. The housing 19, in this embodiment has a centrally located inlet port 18 and two outlet ports 15 and 16, though it will be appreciated that the number of ports can be varied to suit different applications. In operation, gas is supplied to inlet port 18 of the rotating valve 17. 10 The distributor 20 continuously rotates within the housing 19 slowly opening one outlet port to the inlet pipe 14 of one module 5, increasing to full flow, then slowly restricting the flow and eventually closing it at the same time as the flow path reaches fully open on the next module 5. It will be appreciated that any number of modules can be used and the appropriate number of outlet ports can 15 be provided in the valve housing 19. The cycling period of the gas flow can be readily adjusted by altering the speed of rotation of the distributor 20. The gas supplied to the inlet pipe 14 is fed through openings 12 to scour the membranes in the normal manner. Figure 4 shows a graph of the gas flow rate produced by the rotating valve 20 of Figure 3. The small plateaus in the pattern were produced by minor leakage of the cylinder and the pattern will approximate the ideal flow rate if this leakage is minimised. Figure 5 shows an alternate embodiment of a continuous flow rate valve. In this embodiment a piston 25 is slidably mounted in a tubular valve body 26 25 having outlet ports 27 and 28 at either end and an inlet port or opening 29 in the WO 2006/029465 PCT/AU2005/001405 -7 wall of the valve body 26. Typically the valve body 26 would be cylindrical in shape though other cross-sectional shapes could be used. In use, the piston 25 reciprocates within the valve body 26 so as alternately fully open the input port 29 to output ports 27 and 28, respectively. The range of 5 piston movement is arranged such that when port 29 is fully open to port 27 it is fully closed to port 28 and vice versa. As the piston 25 moves between its limits the gas flow to one outlet port is gradually increased while the gas flow to the other outlet port is correspondingly reduced. Figure 6 illustrates the pattern of continuously variable gas flow rate 10 produced by the valve arrangement of Figure 5. Figure 7 shows the effect of continuous variable gas flow rate on the scouring process. The first part of the graph shows change in transmembrane pressure (TMP) with a continuously varying gas flow rate (pulsatile flow) to the membranes while the second portion of the graph illustrates change in TMP with 15 a cyclic gas flow rate. As more efficient scouring produces a more stable TMP over time, it is clear from this graph that the continuously variable gas flow rate produces a more efficient scouring effect and a more stable TMP over time than the cyclic flow rate scouring. The advantages which may be typically provided by the method and 20 system according the embodiments of the invention are: 1. The gas flow is only at the low or zero point for a very short period of time. This minimises the opportunity for backflow of sludge within the filtration system. 2. The essentially continuous flow of gas also means that the period of the cleaning/scouring cycles may be extended over the conventional "alternating" 25 approach. This is achievable because the gas flow is not in the low or no flow WO 2006/029465 PCT/AU2005/001405 -8 condition for very long so there is less likelihood of sludge settling and for anoxic conditions to develop. 3. The process may be employed by using a variable speed drive on a blower and programming the flow cycle profile, in which case there would be no valve 5 issues, as they are not required to cycle. Or alternatively, if conventional valves are used to control the cycles, then the longer period that the process enables means less wear of the valves. 4. The process may also be employed by using a rotary valve described in the preferred embodiment. In this embodiment the blower runs at essentially 10 constant speed, and the valve rotates continuously. There is no sudden open close action on the valve and only the one valve is required. 5. Using the rotary valve above also enables much shorter cycle periods to be obtainable by increasing the speed of the valve rotation. 6. There is a potential saving in gas requirements of up to 50%. 15 It will be appreciated that further embodiments and exemplifications of the invention are possible without departing from the spirit or scope of the invention described.

Claims

1. A method of controlling fouling in a membrane filtration system of the type where gas is used to clean or scour the membranes wherein the method 5 includes supplying the gas to the system with a continuously variable flow rate using a variable speed blower.

2. A method according to claim 1 wherein the system includes a number of modules and the supply of gas to each module has a continuously variable flow rate. 10

3. A method according to claim 1 or claim 2 wherein the continuously variable gas flow rate ranges from a high flow to a low flow, wherein the flow rate pattern of the gas flow follows a continuously variable curve in the form of a sinusoid or in the form a triangular wave.

4. A method according to claim 3 wherein the low flow is zero. 15

5. A membrane filtration system including one or more membranes, a source of gas for providing a gas flow to form bubbles which scour the surfaces of the membrane, wherein said source of gas provides said gas flow at a continuously variable flow rate, wherein said gas source is a variable speed blower.

6. A method of controlling fouling in a membrane filtration system 20 substantially as herein described with reference to any one of the embodiments of the invention illustrated in the accompanying drawings and/or examples.

7. A membrane filtration system substantially as herein described with reference to any one of the embodiments of the invention illustrated in the accompanying drawings and/or examples.