Biomedical Engineering Reference
In-Depth Information
carbon source for energy generation was found, with more carbon dioxide being evolved per
unit substrate removed and less carbon being incorporated into biomass with continuously
feeding substrate at low dilution rates to a culture. In the study by Sun et al. [47], a pilot scale
submerged membrane bioreactor (MBR) was used to treat high strength industrial wastewater
with a higher sludge concentration of 14,500 mg L
-1
at prolonged SRT. The results showed
that the sludge yield and endogenous decay rate were 0.115 g VSS g
-1
COD and 0.024 d
-1
,
respectively, which was almost half the lower value reported for CAS process. The specific
oxygen utilization rate (SOUR) was also lowered due to the less energy required by
microorganisms during the endogenous phase. It was demonstrated that the energy usage by
microorganisms during endogenous phase was much small and most of the incoming energy
(oxygen and carbon sources) could be used for cell maintenance instead of cell synthesis. For
the purpose of reducing excess sludge production in the wastewater treatment process, an
MBR system has also been introduced by Canales et al. [48] to study the maintenance and
cryptic growth phenomena of
Pseudomonas fluorescens
culture, where a thermal treatment
was added in line on a recirculation loop to increase the death and the lysis of the biomass.
The results of this study showed that the biomass with high sludge age had a low viability
level and COD removal associated with requirements for maintenance was higher than for
growth. A maintenance coefficient of 0.035 g COD g
-1
biomass h
-1
was obtained.
MBR can work with high biomass concentration that may eventually lower the ratio of
the food/microorganism concentrations, and subsequently amplify the maintenance
phenomena. However, effluent quality from the MBR system is still dependent on the
biodegradability directly. Moreover, membrane fouling remains an important issue in MBR
operation.
2.4. Uncoupled Energy Metabolism
Senez [49] suggested that a separation between the catabolic and the anabolic chain of
reactions exists in the case of organisms and culture conditions where the source of energy is
distinct from the materials utilized for cell biosynthesis. In order to consume this part of free
energy, several possibilities have been considered, including the dissipation of energy as heat
or futile cycles by adenosine triphosphatase systems, reducing the efficiency of ATP
generation by deletion of oxidative phosphorylation sites, the activation of alternative
metabolic pathways by-passing free energy conserving reactions, disposing of intracellular
energy by dissipation of membrane potential, the accumulation of polymerized products in
storage form or as secreted waste, branching of the respiratory chain and ATP hydrolysis
[34]. In uncoupling process, energy generation from catabolism-associated substrate
consumption is beyond that required for anabolism, including growth and maintenance. The
portion of excess energy will be wasted as heat [50]. The following sections will especially
focus on the mechanism of uncoupled energy metabolism and their applications in excess
sludge reduction.
