A Summary of Recent Microbial Discoveries in Biological Nutrient Removal from Wastewater|
Linda L. Blackall
A number of recent studies has focussed on discovering the microorganisms responsible for various nutrient removal processes in the wastewater treatment industry. This is despite the fact that many wastewater personnel would think that these microorganisms were already known. The sample dilution and plating methods, previously the major ones used by microbiologists, have lead to the isolation of various microorganisms with specific phenotypes of interest. Consequently, Nitrosomonas and Nitrobacter have been described as the major ammonia and nitrite oxidisers, respectively. Acinetobacter has been attributed with the enhanced biological phosphorus removal (EBPR) phenotype due to its predominance in isolations from EBPR plants. However, the true situation is not so clear and even worse, the above organisms may not even play any role in the transformations for which they have achieved such acclaim. The introduction of methods that allow microbiologists to investigate the microbial composition in mixed cultures has begun to resolve the questions on the true identity of microorganisms responsible for nutrient transformations. These methods also allow the ready in situ confirmation of such hypotheses on organism identity and allow this to be linked with organism phenotype. To answer the question on organism identity, enrichment culture around specific phenotypes can still be employed, however, instead of the enrichment of one microorganism over all others, a consortium is obtained. The enriched consortium is investigated by in situ identification methods like fluorescence in situ hybridisation (FISH) with domain, division, and sub-division level probes. A fairly clear picture of the "global" microbial community structure is thus obtained. Evolutionarily conserved genes like those for the 16S rRNA are extracted from the enriched culture and analysed to place them in the phylogenetic tree. Sequences from groups known to dominate the enrichment culture from FISH, can be focussed upon and more intensively studied, including the design of novel probes for FISH on the enrichment culture. The same approach using FISH and analysis of conserved genes from full-scale operations has also been employed to identify the organisms of relevance to specific nutrient transformations. The final product is a suite of FISH probes that can be used with any mixed culture biomass for enumeration of organisms of interest and correlation of their abundance with process performance. Once the identity of the organisms truly responsible for nutrient transformations is known, their physiological parameters can be studied.