Title page for ETD etd-10042006-143851
|Type of Document
||Higgins, Matthew J.
||The roles and interactions of cations, proteins, and polysaccharides in the settling and dewatering of activated sludge
|Novak, John T.
|Benoit, Robert E.
|Little, L. C.
|Love, Nancy G.
|Randall, Clifford W.
|Date of Defense
The roles of cations and exocellular biopolymer on the settling and dewatering of
activated sludge was investigated. Both laboratory and full-scale activated sludge
systems were examined. The results of the study showed the settling and dewatering
properties of the activated sludge were dependent on the calcium, magnesium, potassium
and sodium concentrations added to the feed. A minimum of 0.72 meq/L each of calcium
and magnesium in the feed was necessary for acceptable settling and dewatering. Two
types of microbial cultures were observed, one required both calcium and magnesium in
the feed while the other required either calcium or magnesium, but not both, for
optimization of settling and dewatering. Sodium addition to the feed improved the settling
of activated sludge when the ratio of sodium to calcium plus magnesium was equal to
approximately one on a meq/L basis. When this ratio was greater than two, the settling
and dewatering properties deteriorated, but the deterioration could be reversed by
increasing the calcium and magnesium concentration in the feed. In general, the data
supported the cation bridging model for bioflocculation, and the cations act to bind
protein to the biofloc structure.
Results from the full-scale activated sludge plants correlated well with results
from laboratory activated sludge systems and demonstrated the cation content in these
systems had a direct impact on the settling and dewatering properties. Field trials in
which divalent cations were added to activated sludge systems resulted in dramatic
improvements in the settling properties of these systems.
Characterization of the exocellular protein extracted from laboratory, industrial
and municipal activated sludges revealed the presence of a single protein, which appears
to be a lectin. The molecular weight of the protein measured by SDS PAGE was
approximately 16,000 Daltons with similar amino acid composition as microbial
lectins. Also, amino acid sequencing analysis indicated the N-terminal sequence of the
protein was consistent with those of microbial lectins. In addition, the activated sludge
cultures exhibited lectin activity as demonstrated by binding site inhibition
experiments. A model of bioflocculation that includes the role of protein was proposed.
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