Reduction of backwash liquid waste
View Patent ↗A method of filtering solids from a liquid suspension comprising providing a pressure differential across the walls of permeable, hollow membranes ( 5 ) immersed in the liquid suspension contained in a vessel ( 8 ), the liquid suspension being applied to the outer surface of the porous hollow membrane ( 5 ) to induce and sustain filtration through the membrane walls. Some of the liquid suspension passes through the walls of the membranes ( 5 ) to be drawn off as permeate from the hollow membrane lumens, and at least some of the solids are retained on or in the hollow membranes or otherwise as suspended solids within the liquid surrounding the membranes ( 5 ). The method includes the steps of suspending the flow of the liquid suspension to the vessel ( 8 ); reducing the volume of liquid suspension within the vessel ( 8 ); suspending the filtration process; cleaning the membranes ( 5 ) by dislodging at least some of the solids retained on or in the membranes ( 5 ); and removing the liquid containing dislodged solids from the vessel ( 8 ).
1. A method of filtering solids from a liquid suspension in a membrane filtration vessel comprising the steps of:
promoting filtration of the liquid suspension through membrane walls of permeable, hollow membranes in the vessel, the permeable, hollow membranes including lumens and outer surfaces;
drawing off permeate from the hollow membrane lumens;
retaining at least some solids on or in the membranes or within the liquid suspension;
suspending a flow of the liquid suspension to the vessel;
reducing a volume of liquid suspension within the vessel to a predetermined volume by injecting aeration gas from an aeration gas line into the vessel on a feed side of the membranes while the flow of liquid suspension to the vessel is suspended, forcing filtration to continue until the predetermined volume is achieved;
suspending the filtration process when the volume of liquid suspension within the vessel has been reduced to the predetermined volume, the injection of the aeration gas continuing while the filtration process is suspended;
dislodging at least some of the solids retained on or in the membranes; and
removing the liquid suspension containing the dislodged solids from the vessel.
2. The method according to claim 1 wherein the dislodging step includes backwashing the membranes.
3. The method according to claim 1 wherein the dislodging step includes scouring the membrane surfaces with gas bubbles.
4. The method according to claim 1 , further comprising controlling the reduction in the volume of the liquid suspension by totalizing the filtering down volume and providing feedback regarding the totalized filtering down volume.
5. The method according to claim 4 , wherein suspending the filtration process comprises suspending the filtration process in response to the feedback.
6. A method of filtering solids from a liquid suspension in a membrane filtration vessel comprising the steps of:
providing feed to the filtration vessel through a feed pump;
promoting filtration of the liquid suspension through membrane walls of permeable, hollow membranes suspended within a feed vessel, the permeable, hollow membranes including lumens and outer surfaces;
drawing off permeate from the hollow membrane lumens;
retaining at least some solids on or in the membranes or within the liquid suspension;
suspending a flow of the liquid suspension to the vessel;
cleaning the feed pump by providing a flow of permeate through the feed pump from a permeate side of the membrane walls into the liquid suspension, wherein the permeate is flowed into the liquid suspension after passing through the feed pump and a feed valve and directly into the feed vessel;
reducing a volume of liquid suspension within the vessel to a predetermined volume by providing a flow of permeate through the feed pump to an outlet;
suspending the filtration process when the reduction in volume of liquid suspension within the vessel has reduced the volume of liquid suspension to the predetermined volume;
dislodging at least some of the solids retained on or in the membranes; and
removing the liquid suspension containing the dislodged solids from the vessel.
7. The method according to claim 6 , wherein the flow of permeate through the feed pump is sent to product.
8. The method according to claim 6 , wherein the flow of permeate through the feed pump is sent to a feed manifold.
9. The method according to claim 6 , further comprising controlling the reduction in volume of liquid suspension within the vessel to the predetermined level by a method including totalizing the filtering down volume, providing feedback regarding the filtering down volume, and suspending the filtration process in response to the feedback.
10. The method according to claim 6 , further comprising controlling the reduction in volume of liquid suspension within the vessel to the predetermined level by a method including measuring membrane permeability and transmembrane pressure and calculating the filtering down volume.
11. A method of filtering solids from a liquid suspension in a membrane filtration vessel comprising the steps of:
promoting filtration of the liquid suspension through membrane walls of permeable, hollow membranes, the permeable, hollow membranes including lumens and outer surfaces;
drawing off permeate from the hollow membrane lumens;
retaining at least some solids on or in the membranes or within the liquid suspension;
suspending a flow of the liquid suspension to the vessel;
introducing high pressure process gas supplied from a source separate from that of a scouring gas at a regulated pressure of between 50 and 250 kPa into a top section of the vessel on a feed side of the membranes, the high pressure process gas pushing liquid through the membrane walls;
controlling a transmembrane pressure of the membranes with a filtrate valve on a filtrate removal line during the introduction of the process gas;
reducing a volume of liquid suspension within the vessel to a predetermined volume by suspending a flow of the process gas into the vessel and providing for process gas remaining in the vessel to continue to push liquid through the membrane walls until a gas pressure on a feed side of the vessel drops and the predetermined volume is achieved;
dislodging at least some of the solids retained on or in the membranes by scouring the membrane surfaces with gas bubbles; and
removing the liquid suspension containing the dislodged solids from the vessel.
12. The method according to claim 11 , further comprising controlling the reduction in volume of liquid suspension within the vessel to the predetermined level by a method including totalizing the filtering down volume, providing feedback regarding the filtering down volume, and suspending the filtration process in response to the feedback.
13. The method according to claim 11 , further comprising controlling the reduction in volume of liquid suspension within the vessel to the predetermined level by a method including measuring membrane permeability and transmembrane pressure and calculating the filtering down volume.
14. The method of claim 1 , wherein the predetermined volume is sufficient to just allow the membranes to be fully immersed in the feed liquid during a subsequent gas scouring step.
15. The method of claim 11 , wherein the predetermined volume is sufficient to just allow the membranes to be fully immersed in the feed liquid during a subsequent gas scouring step.