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Factors influencing dewaterability of thermophilic aerobically digested biosolids Zhou, Jianpeng
Abstract
This dissertation reports on research findings from a study investigating factors that influence the dewaterability and other characteristics of thermophilic aerobically digested biosolids. Thermophilic aerobic digestion is a high temperature (50-65°C) sludge treatment process that produces Class A biosolids. Experience from operating full-scale facilities has revealed that dewatering thermophilically digested sludge requires substantially higher dosages of polymers for conditioning, compared to dewatering of mesophilically digested sludge. The objectives of this research program were to investigate how feed sludge composition, digestion temperature, digestion time, and mixing induced shear affect the dewatering properties of digested sludge. The characteristics of thermophilically digested sludge and mechanisms related to dewatering thermophilically digested sludge were also studied. The experimental work was carried out at laboratory scale, using batch operated aerobic digesters. Dewaterability was measured as specific capillary suction time (SCST). This research found that feed sludge composition is an important factor affecting dewaterability (measured as SCST) of the digested sludge. Regardless of how the sludge was digested, the measured SCST exponentially correlated to the weight proportion of the secondary sludge contained in the feed. A higher proportion of secondary sludge in the feed resulted in a higher SCST in the sludge. Dewaterability did not correlate to pH, volatile solids, concentrations of ammonia and phosphate in digested sludge, but correlated to concentrations of soluble extracellular proteins and polysaccharides. Digestion temperature had a significant effect on dewaterability (measured as SCST) of the digested sludge. When the sludge, containing 100% secondary sludge, was digested at 55°C or higher temperatures, digestion resulted in immediate and significant increases in SCST. When the same type of sludge was digested at 40 or 50°C, digestion also resulted in a significant increase in SCST, but the rate of increase in SCST was lower than the rate when the sludge was digested at 55°C or higher. Following the initial surge in SCST, continued digestion at 55°C or higher temperatures for more than 1 d resulted in a reduction in SCST; while continued digestion at 40°C or 50°C did not result in much change in SCST. Mesophilic digestion resulted in a progressive increase in SCST over the entire duration of 10-12 d digestion. Thermophilic digestion did not result in much change in floe charge, but did cause an immediate reduction in floe size. Digestion at all temperatures resulted in a reduction in solids, changes in pH, conductivity, concentrations of ammonia and phosphate, and phosphorus distribution among the solid and liquid phases of the sludge. However, the deterioration in dewaterability of digested sludge did not correlate to changes in these parameters; instead, it correlated to concentrations of soluble extracellular proteins and polysaccharides in the digested sludge. The substances affecting dewaterability of thermophilically digested sludge was mainly associated with the liquid phase of the digested sludge. The soluble extracellular proteins had small sizes, with 86% of such materials less than 7,000 Daltons. These proteins could not be effectively stained by Coomassie Brilliant Blue dye, and were not affected by boiling treatment. Protease treatment confirmed that protein played a role in affecting dewaterability. The deterioration in dewaterability, due to thermophilic digestion, was a physical-chemical phenomenon, not a microbiological phenomenon, although the substances resulting in deterioration of dewaterability (such as extracellular proteins) were originated from bacterial cells. Mechanical shear applied to the digested sludge had a significant effect on dewaterability, regardless of digestion temperatures. The effect of mechanical shear was due to the reductions in floe charge and size. Digestion and mixing induced shear resulted in changes in distribution of cations among the solid and liquid phases of the digested sludge. However, dewaterability was not associated with changes in ratios of monovalent to divalent cations. Thermophilic digestion and mechanical shear resulted in a reduction in the initial yield and adhesion coefficients of digested sludge. Digested sludge showed non-Newtonian characteristics, in particular, the shear-thinning property. For thermophilically digested sludge, an initial polymer demand needed to be satisfied first, before the SCST could be substantially reduced by polymer conditioning.
Item Metadata
Title |
Factors influencing dewaterability of thermophilic aerobically digested biosolids
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Creator | |
Publisher |
University of British Columbia
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Date Issued |
2003
|
Description |
This dissertation reports on research findings from a study investigating factors that influence
the dewaterability and other characteristics of thermophilic aerobically digested biosolids.
Thermophilic aerobic digestion is a high temperature (50-65°C) sludge treatment process that
produces Class A biosolids. Experience from operating full-scale facilities has revealed that
dewatering thermophilically digested sludge requires substantially higher dosages of polymers
for conditioning, compared to dewatering of mesophilically digested sludge.
The objectives of this research program were to investigate how feed sludge composition,
digestion temperature, digestion time, and mixing induced shear affect the dewatering properties
of digested sludge. The characteristics of thermophilically digested sludge and mechanisms
related to dewatering thermophilically digested sludge were also studied. The experimental
work was carried out at laboratory scale, using batch operated aerobic digesters. Dewaterability
was measured as specific capillary suction time (SCST).
This research found that feed sludge composition is an important factor affecting dewaterability
(measured as SCST) of the digested sludge. Regardless of how the sludge was digested, the
measured SCST exponentially correlated to the weight proportion of the secondary sludge
contained in the feed. A higher proportion of secondary sludge in the feed resulted in a higher
SCST in the sludge. Dewaterability did not correlate to pH, volatile solids, concentrations of
ammonia and phosphate in digested sludge, but correlated to concentrations of soluble
extracellular proteins and polysaccharides.
Digestion temperature had a significant effect on dewaterability (measured as SCST) of the
digested sludge. When the sludge, containing 100% secondary sludge, was digested at 55°C or
higher temperatures, digestion resulted in immediate and significant increases in SCST. When
the same type of sludge was digested at 40 or 50°C, digestion also resulted in a significant
increase in SCST, but the rate of increase in SCST was lower than the rate when the sludge was
digested at 55°C or higher. Following the initial surge in SCST, continued digestion at 55°C or
higher temperatures for more than 1 d resulted in a reduction in SCST; while continued
digestion at 40°C or 50°C did not result in much change in SCST. Mesophilic digestion resulted
in a progressive increase in SCST over the entire duration of 10-12 d digestion. Thermophilic digestion did not result in much change in floe charge, but did cause an immediate reduction in
floe size. Digestion at all temperatures resulted in a reduction in solids, changes in pH,
conductivity, concentrations of ammonia and phosphate, and phosphorus distribution among the
solid and liquid phases of the sludge. However, the deterioration in dewaterability of digested
sludge did not correlate to changes in these parameters; instead, it correlated to concentrations
of soluble extracellular proteins and polysaccharides in the digested sludge.
The substances affecting dewaterability of thermophilically digested sludge was mainly
associated with the liquid phase of the digested sludge. The soluble extracellular proteins had
small sizes, with 86% of such materials less than 7,000 Daltons. These proteins could not be
effectively stained by Coomassie Brilliant Blue dye, and were not affected by boiling treatment.
Protease treatment confirmed that protein played a role in affecting dewaterability.
The deterioration in dewaterability, due to thermophilic digestion, was a physical-chemical
phenomenon, not a microbiological phenomenon, although the substances resulting in
deterioration of dewaterability (such as extracellular proteins) were originated from bacterial
cells. Mechanical shear applied to the digested sludge had a significant effect on dewaterability,
regardless of digestion temperatures. The effect of mechanical shear was due to the reductions
in floe charge and size.
Digestion and mixing induced shear resulted in changes in distribution of cations among the
solid and liquid phases of the digested sludge. However, dewaterability was not associated with
changes in ratios of monovalent to divalent cations. Thermophilic digestion and mechanical
shear resulted in a reduction in the initial yield and adhesion coefficients of digested sludge.
Digested sludge showed non-Newtonian characteristics, in particular, the shear-thinning
property. For thermophilically digested sludge, an initial polymer demand needed to be satisfied
first, before the SCST could be substantially reduced by polymer conditioning.
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Extent |
13642889 bytes
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Genre | |
Type | |
File Format |
application/pdf
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Language |
eng
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Date Available |
2009-11-13
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Provider |
Vancouver : University of British Columbia Library
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Rights |
For non-commercial purposes only, such as research, private study and education. Additional conditions apply, see Terms of Use https://open.library.ubc.ca/terms_of_use.
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DOI |
10.14288/1.0063816
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URI | |
Degree | |
Program | |
Affiliation | |
Degree Grantor |
University of British Columbia
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Graduation Date |
2003-05
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Campus | |
Scholarly Level |
Graduate
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Aggregated Source Repository |
DSpace
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Item Media
Item Citations and Data
Rights
For non-commercial purposes only, such as research, private study and education. Additional conditions apply, see Terms of Use https://open.library.ubc.ca/terms_of_use.