Polyhydroxyalkanoates can be produced using surplus activated sludge from biological wastewater treatment processes. This direct accumulation requires that the WWTP imposes inherent selection pressures to promote for PHA accumulating microorganisms. Direct accumulation may reach PHA contents of 0.50 gPHA/gVSS which is similar to outcomes with open culture enrichment accumulation processes. Distinction between the fraction of PHA storing microorganisms in a biomass, versus the amount of polymer on average per storing microorganism is generally not made. Therefore, methods were developed for selectively staining PHA and biomass components coupled to quantitative image analysis with confocal laser scanning microscopy towards insight on the distribution of PHA storing activity in activated sludge.
PHA accumulation was performed at pilot and laboratory scales with full-scale municipal activated sludge sourced from different WWTPs. In preliminary studies, for method development, and based on replicate pilot-scale accumulation experiments over 48 hours, biomass reached consistently up to 0.49 ± 0.03 gPHA/gVSS. This polymer content is an average mass of PHA in a biomass that contains both PHA accumulating microorganisms (PAMs) and non-accumulating microorganisms (NAMs). Image analysis enabled quantitative distinction between PAM and NAM fractions to suggest a degree of phenotype enrichment for this activated sludge, and the resultant average PHA content of the PAMs. It was estimated, in this case, that on average 54% of the activated sludge was engaged, in PHA storage activity (Fig. 1). Thus, it could be estimated that even if the average biomass content reached 0.49 gPHA/gVSS, the PAMs attained nominally 0.64 gPHA/gVSS.
Similar evaluations were subsequently undertaken with laboratory scale PHA accumulation experiments using 8 other full-scale WWTP activate sludge sources. The outcomes and insights of this screening study towards understanding distribution and performance of PAMs in municipal activated sludge including dynamics of the microbial community are to be discussed in this presentation.
