Comparison of Biofouling Control in a Membrane Bioreactor Via Two Coarse Bubble Aeration Cycles

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In the membrane bioreactor (MBR) process, coarse bubble aeration is used to scour the surface of the membranes to help control biofouling. Energy required for coarse bubble aeration represents a significant fraction of the total power demand for these systems. To reduce this demand, MBR systems manufactured by Zenon Environmental Inc. operate with cyclic aeration, using a cycle of 10 seconds on and 10 seconds off (10/10 aeration cycle). The objective of this study was to evaluate the potential to reduce power requirements for a full-scale Zenon MBR facility that is currently under construction by application of an alternative aeration cycle. The alternative cycle consists of 10 seconds on and 30 seconds off (10/30 aeration cycle), which reduces scouring air requirements by 50% from the 10/10 cycle. To achieve this objective, a pilot-scale evaluation using an existing 30-gallon per minute MBR plant was conducted. The
specific goal of the evaluation was to compare the ability of the 10/10 and 10/30 cycles to control biofouling while operating over a range of flux values. In addition to the pilot-scale testing, an estimate of anticipated power costs savings resulting from implementation of the 10/30 cycle was determined for the full-scale facility. Key findings include: (1) While operating with the 10/10 cycle at a flux of 12 gallons/ft2-d (gfd) over a period of two months, there was no measurable fouling observed. The fouling rates at 15 and 18 gfd were approximately 0.1 and 0.3 psi/day, respectively; (2) While operating with the 10/30 cycle at 12 gfd, a relatively significant amount of fouling was observed after only 14 days of operation, indicating that long operating periods could not be sustained between membrane cleanings; (3) 10/30 cycle fouling, at a flux of 12 gfd, was significantly reduced by modifying the permeate production cycle of the system from 12 minutes permeation/30 seconds relaxation to 12 minutes/45 seconds. As a result of this modification, it was estimated that the system could be operated for 17 months before the membranes would need to be cleaned; (4) 10/30 cycle fouling rates at fluxes of 15 and 18 gfd were approximately 1.1 psi/day and 9.4 psi/day, respectively. These rates are significantly higher than what was observed while operating with the 10/10 cycle; (5) It was estimated that implementation of the 10/30 cycle would result in an approximate 20% reduction of the total power requirements, and thus costs, for the full-scale MBR facility.

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