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Please use this identifier to cite or link to this item:
http://hdl.handle.net/2282/381
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| Title: | Degradation of polymeric and particulate organic carbon in biofilms |
| Authors: | Kommedal, Roald |
| Issue Date: | 2003 |
| Abstract: | Polymeric and particulate organic carbon (POM) are fundamental compounds in the global
cycling of carbon, and constitute significant amounts of BOD in municipal wastewater.
The main objective of this work is to study molecular size effects on degradation dynamics
in biofilm systems. Specifically, the effect of substrate molecular weight on degradation
kinetics and transport dynamics, location of depolymerisation enzyme activity and
depolymerisation intermediate formation dynamics are assessed. A mathematical model
for biofilm degradation dynamics is presented, and used for data interpretation and
simulations.
Dextran, an a-1,6 Glucan, was used as model substrate during batch degradation in a
Rototorque biofilm reactor, in addition to batch tests on biofilm sub samples retrieved from
the Rototorque, and during pure endo- and exo-Dextranase studies. Oxygen utilisation rate
(OUR) estimates and bulk phase TOC mass balances were used to evaluate the effect of
variable initial molecular weight on the observed half order removal coefficient
(Harremoës, 1978; Rittmann and McCarty, 1980). Size exclusion-HPLC analysis for
determination of bulk phase depolymerisation intermediates, and specific enzyme assays
were used to evaluate transport dynamics of polymers and location of enzyme activity in
the enhanced mixed population biofilm system.
Dextran removal rate decrease with increasing Dextran molecular weight. The observed
areal half order removal rate coefficient, k1/2,A, demonstrate an approximate 10-fold
decrease in the 1-500 kDa range, showing negative logarithmic correlation to the initial
MW of Dextran. A less distinct correlation is observed above this transition limit (1-10
MDa). Evaluation of the Thiele moduli, from one step depolymerisation modelling,
suggests that the logarithmic reduction in observed removal rate is caused by combined
reaction rate and transport limitations. Transport limitations dominates as the polymeric
substrate size increase and hinders biofilm matrix diffusion, and the removal rate becomes
a surface limited process. Removal of Dextran is biomass dependent in what appears to be
a non-linear dependency on biofilm thickness. Expressed as biomass areal density (g/m2),
no depolymerisation is observed for thin biofilms (0.7 g/m2), slow for medium (3.7 g/m2)
and high for thicker biofilms (5.2 g/m2). Depolymerisation intermediates accumulated in the bulk phase over the entire Dextran size
range during pure Dexranase studies, with even size distributions. Final products were
oligo-isomaltoses (DP 2-6). Dextran was not depolymerised by a-Glucosidase nor Oligoa-
1,6 Glucosidase. During biofilm reactor and slide sub-sample tests, low MW Dextran
intermediates (1-10 kDa) accumulated in the bulk during depolymerisation of 160 kDa
Dextran at 250 and 200 mg/l initial concentrations, but were not detected during
experiments with 100 mg/l initial concentrations. Intermediate range Dextran (10-100 kDa)
did not accumulate in either case. At the same conditions, some assimilable range Dextran
(0.2-0.9 kDa) accumulated in the bulk liquid during initial 250 and 200 mg/l batches, but
was not detected during 100 mg/l initial Dextran concentrations. The extent of bulk phase
accumulation seems to depend on the biofilm growth rate, where more bulk phase
accumulation is observed during experiments with starved compared to more actively
growing biofilms. More intermediates accumulate during low MW initial standards,
compared to higher. These observations indicate that the extent of bulk phase intermediate
accumulation is balanced by the rate of depolymerisation, and the substrate uptake rate
(growth). Accumulation of intermediate hydrolysis products in biofilm systems is therefore
dependent on the slowly biodegradable organic (SBCOD) loading rate.
Dextranase was detected in the cellular fraction of the biofilms. The enzyme activity was
not detected in any other biofilm sub compartments, implying that the exogenous enzyme
remains attached to the cells while working on polymers. These findings support the
conceptual model of Confer and Logan (1998), implying that bulk phase intermediate
accumulation observed in this study and by others, is not a result of enzymatic activity in
the bulk phase, but transport of intermediates from the biofilm matrix. |
| Keywords: | Biofilm
Organic compounds
Polymeric compounds |
| Publisher: | Høgskolen i Telemark |
| Document type: | PhD thesis |
| URI: | http://hdl.handle.net/2282/381 |
| ISBN: | 82-471-5655-5 |
| Appears in Collections: | Doktorgradsavhandlinger i prosess- energi og automatiseringsteknikk
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