[65] Application of immobilized captured microorganisms in water purification: An overview

Abstract Table IV provides a comparison of the general operating details of the various immobilized cell systems; it is by no means comprehensive. BEC and BRP systems capitalize on mixed cultures of microorganisms, predominantly bacteria, to adhere to inert supports and form biofilms or biomass aggregates. The advantages of such systems are the following: (1) cheap, easily obtained support material (i.e., sand, quartz, plastic), (2) high reactor biomass concentrations, (3) proven capability to handle fullscale flow rates, and (4) freedom to operate with mixed cultures and nonsterile feed conditions. This last advantage allows BEC and BRP systems to operate in a growing biomass mode and to carry out a variety of biological reactions simultaneously (i.e., carbon oxidation and nitrification, carbon oxidation-methane production). Disadvantages of BEC systems are the lack of control over biofilm thickness which contributes to possible mass transfer limitations and which can lead to excessive biofilm sloughing. BRPs to a certain extent alleviate this lack of control over biomass growth. In both BRP and BEC systems, slow growing microorganisms (e.g., autotrophic nitrifiers, methane formers) do create the problem of lengthy (several months) reactor start-up times. Artificially captured cell systems (ACCs) have the unique advantage that a biological process can be “stored” indefinitely then used, when desired, without wait. Thus, ACCs provide the advantage of relatively immediate use of the microorganism (once they are cultured and captured), thus eliminating system start-up problems. It may be a fruitful avenue of research to investigate the use of ACC systems to aid in the start-up of BEC and BRP that experience long initiation times. Disadvantages of ACCs are relatively short half-lifes (around 20–40 days), the requirement of alternating nongrowth and growth periods of operation to maintain reactivity, and the physical breakage of encapsulating materials and the resultant contamination of downstream systems. All ACC systems reviewed here were pure culture systems. One possible means of placing ACC systems on a more competitive basis with BECs and BRPs would be more research and development into defined mixed culture-multiple reaction ACC systems. For example, simultaneous nitrification-denitrification could be accomplished by first naturally attaching autotrophic denitrifiers to elemental sulfur, encapsulating the resultant particle, attaching a second culture of autotrophic nitrifiers, and finally encapsulating the entire aggregate a second time. Such research possibilities of bilayer and defined mixed culture microbial catalysts have been ignored. In general, BEC and BRP systems are used more in all scales of water purification—from fundamental laboratory systems to full-scale municipal/industrial treatment facilities. Conversely, ACC systems find their most significant potential in the analysis of specific water/wastewater analysis using microbial probe devices. To extend the potential of ACC systems, serious research to answer questions of scale-up and particle longevity are sorely needed.