Production of Soluble Microbial Products (SMP) in Anaerobic Chemostats Under Nutrient Deficiency

It is known that in well-operated anaerobic systems the majority of the residual chemical oxygen demand (COD) present in the effluent is due to soluble microbial products (SMP) generated during treatment. SMP production is affected by many factors, and this study investigated the effect of nutrient deficiency on SMP production in anaerobic chemostats fed on glucose as the sole carbon source. The results showed that under steady-state conditions the SMP/CODout ratio was as high as 95% and the normalized SMP production (SMP/So) averaged 3%. During nutrient deficiency the SMP/CODout ratio averaged 45% indicating that the importance of SMP in the effluent was reduced due to the enhanced production of VFAs. Nevertheless, under such stressed conditions the normalized production of SMP (SMP/So) increased reaching up to 37%. Analysis of extracellular polymers (ECP) indicated that ECPc production was enhanced after nutrient cessation and chemical analyses of the effluent suggest that most of the SMP was not protein- or carbohydrate-like material. DNA analysis indicates that part of the SMP produced during nutrient deficiency might be due to enhanced cell lysis, although some organics might have been deliberately excreted to scavenge metal nutrients. Biochemical methane potential (BMP) assays showed that the lack of N and P resulted in the highest SMP production which might have been released to dump electrons that could not be used in cell synthesis due to the lack of these macronutrients.