Analysis of the Gas Holdup Evolution in a Circulating Jet-Loop Nitrifying MBR

This paper presents an analysis of the gas holdup evolution in a novel type of
jet-loop membrane bioreactor (JLMBR), designed for nitrogen removal through
the nitrite route application. Its configuration is inspired from airlift systems. It
consists of a 60-l reactor made of an internal airlift system coupled to an external
liquid recirculation loop. Hollow fiber membranes are submerged in the riser
compartment. The process was intermittently fed with a synthetic ammonia solution
and the gas holdup evolution was monitored for 500 to 600 days. Experiments
were performed using flowrates ranging from 0.4 to 1.03 Nm3/h, and from 0 to
0.6 m3/h for air and water, respectively. This corresponded to superficial velocities
from 0.004 to 0.03 m.s􀀀1 for air and 0 to 0.011 m.s􀀀1 for water. The gas holdup
g was directly measured by the volume expansion method, using a tubular level
meter located on the plant. The reported results showed that, in the absence of
microorganisms, g ranged between 0.5 and 5.5% for the investigated range of
gas liquid superficial velocities, whilst increasing from 0.5 to 4.8% only in the
presence of gas (no liquid recirculation). This double influence of the air and the
liquid velocities on the gas holdup was described by a multilinear correlation.
However in the presence of biosolids in the reactor, the gas holdup raised up to
6.5%, corresponding to an increase of ca. 48% (in average, with respect to data
recorded on day 0). This increase in g was attributed to both a gas entrainment
effect and an impact of the bioparticles recirculated into the reactor. Under experimental
conditions investigated, the gas holdup increased linearly with the air and
the liquid velocities, what corresponded to the bubbly flow regime in the system.
This showed that, according to investigated conditions, the impact of circulated
biomass was not enough to change the bubble gas flow regime.