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

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.