Zoogleal clusters and sludge dewatering potential in an industrial activated-sludge wastewater treatment plant

Microscopic observations at an industrial wastewater treatment facility were conducted over a period of approximately 4 years to determine the relationship between the abundance of zoogleal clusters and sludge dewatering potential. Dewatering potential, measured as centrifuged solids content, was determined in sludge samples from an aeration basin. The abundance of zoogleal clusters and centrifuged solids content were negatively correlated, as determined by linear regression (r(2) = 0.62). Curve-fitting using an exponentially weighted moving average and a polynomial equation yielded r(2) = 0.82. Probing with small ribosomal subunit RNA (16S rRNA) oligonucleotide probes (ZRA and ZEE) indicated that the microorganisms forming these zoogleal clusters were not the same as previously classified Zoogloea ramigera strains (members of the beta subclass of Proteobacteria) isolated from other wastewater treatment systems. The microorganisms responsible for zoogleal cluster formation were identified using micromanipulator separation, cultivation, and 16S rRNA analysis. Distance matrix tree analysis of isolated strains revealed the presence of two types of microorganisms, referred to as mzt and mzl strains. The mzt isolates grouped most closely with members of the genus Thauera, whereas the mzl isolates grouped more closely with members of the genus Hydrogenophaga. Using 16S rRNA methods, a probe complementary to members of the genus Thauera (MZ1; S-G-Thau-0646-a-A-19) was designed to target mzt strains. Whole-cell hybridization with a fluorescein-labeled probe, coupled with epifluorescence microscopy, was used to verify the identity of the zoogleal cluster-forming organisms. Results indicated that the MZ1 probe hybridized specifically with zoogleal clusters present in sludge.