Biomedical Engineering Reference
In-Depth Information
consumption of small, dissociative flocs due to feeding activity and (ii) improved settling due
to the function of triggering flocs aggregation to settle out of suspension. In their study,
particle size was in the range of 0.2-10 μm, corresponding to the size range of particles
consumed by rotifers. As a result, rotifers might be exploited as biotechnological tools for
reduction of bio-solids from wastewater by grazing on dispersed bacteria.
Lee et al. [17] investigated the possibility of reducing sludge production in aerobic
treatment using a continuous-flow two-stage laboratory reactor system with two different
carbon sources (acetic acid and methanol): the first stage working as a completely mixed,
aerobic reactor at a short solid retention time (SRT) without biomass retention to prevent the
growth of predators and to favor the growth of fast-growing, dispersed bacteria that consume
the readily biodegradable organic matter in the wastewater, and the second stage being
designed to be a biofilm reactor as a predator stage, in order to provide a long sludge age and
optimal conditions for the growth of the protozoa and metazoa feeding on the bacteria. Their
results showed that the sludge yield decreased considerably both with acetic acid (61%
reduction) and methanol (80% reduction). Generally, for the two-stage process, a large
volume of bioreactor is needed in the first stage because of long hydraulic retention time
(HRT), resulting in increasing the capital investment to install such a chamber compared to
CAS process. Moreover, the increased mineralization of organic matter in the process leading
to a decreased sludge yield will involve higher operation costs in the form of oxygen
requirements. It was reported that the oxygen consumption of the process was almost doubled
due to the increase in oxidation of organic matter [42]. Furthermore, with these systems,
significant amount of ammonium, nitrate and phosphate as consequences of biomass
mineralization released in the predator stage is another limitation of the two-stage principle
[15,17].
So far, the relationship between operation conditions and predators growth is still not
very clear. It is challenging to create favorable niches for their maturation and successful
sexual reproduction [43]. Further research needs to be focused on the connection between
food chain (prey-predator) and the sludge flocs formation-disintegration for bacteriovoric
metabolism-associated sludge reduction [5].
2.3. Maintenance Metabolism
It is known that microorganisms satisfy their maintenance energy requirements in
preference to producing additional biomass [44]. The utilization of carbon source by cells to
provide energy requirements for non-growth activities (in particular maintenance functions
and survival functions) would divert the organic substrates from assimilation for biosynthesis
of new biomass. Exothermic maintenance functions include the turnover of enzyme, osmotic
work to maintain concentration gradients, cell viability, nutrient storage and cell motility
[34,45]. Selection of microorganisms present in the wastewater treatment process with high
maintenance energy, i.e., substrate is utilized for cellular maintenance and endogenous
metabolism rather than for biomass synthesis could reduce the excess sludge production. This
selection can be achieved by lowering the substrate to biomass ratio [42,46].
In order to reduce biomass production during wastewater treatment with respect to
maintenance metabolism, Low et al. [44] looked into how microorganisms would allocate
organic substrates between maintenance and biosynthesis, and additional utilization of the
