UBC Theses and Dissertations
Passive membrane filtration systems Jain, Rajat
Ultrafiltration is a well-established technology for drinking water treatment. However, it is generally considered to be too complex and costly for use in resource-limited communities. Most of this complexity is associated with the auxiliary processes used for fouling control, notably backwashing, air sparging, and chemical cleaning. Recently, attempts have been made to develop passive membrane filtration systems which significantly reduce the complexity of conventional systems. With gravity-driven permeation, these systems are operated at sub-critical permeate flux with no to minimal fouling control. The biological communities that establish on the membrane surface under such conditions promote the formation of a porous foulant layer, thus decreasing its resistance and helping sustain the permeate flux. The sustained permeate flux can be further increased by retaining some simple fouling control measures which do not increase the complexity of the system. This study investigated two such measures: passive (gravity-driven) backwash and passive air sparging (with appropriate system design, the hydraulically generated vacuum during tank drain draws ambient air into the membrane tank, resulting in sparging). Passive backwashing for a duration of 5 minutes was observed to result in negligible fouling. For conditions when passive backwash could not effectively control fouling (backwash duration of 1-3 minutes), coupling it with passive air sparging also did not improve its efficacy for fouling control. These results were further validated at pilot-scale during operation spanning ~16 months. A passive membrane filtration system with a commercial ZeeWeed 1500 membrane module (filtration area of 50 m²), when exposed to passive backwash coupled with passive air sparging for 5 min/day, was able to sustain an average throughput of approximately 4650 l/day for the first 70 days of operation. The rate of decline in the throughput was minimal for the next 70 days of operation. It was only after 140 days of operation that the rate of decline in throughput was significant enough to warrant a recovery cleaning. Passive cleaning also prevented solids build-up in the system and aided in the removal of natural organic matter from raw water. A recovery cleaning approach to recover the permeability lost during long-term operation was also determined.
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