Biomedical Engineering Reference
In-Depth Information
2.2. Bacteriovoric Metabolism
Appropriate practice to maximize, encourage and optimize the growth of higher
organisms in the food chain which predate on bacteria would be one possible way to prevent
excess sludge accumulation. During energy transfer from a low trophic level (bacteria) to a
high level (protozoa or metazoa), energy is reduced due to respiration, growth, reproduction,
defecation and nonpredatory death (organisms that die but are not consumed by consumers).
Typically, on average, about 10% of net energy production at one trophic level is passed on to
the next level. Such inefficient biomass conversion induces energy lost and subsequently
sludge reduction in the process of bacteriovoric metabolism [15,31]. Promotion of
macroinvertebrates can minimize the amount of assimilated energy in the system [32].
Bacteriovoric metabolism in excess sludge reduction process can be achieved via
bacteriovory by higher organisms, such as protozoa and metazoa (e.g. nematodes, rotifers and
oligochaete worms), which play an important role in CAS process because they maintain the
density of bacterial populations by consuming dispersed bacteria and contribute to the
flocculation process, keeping the effluent clear and with high quality [33]. Predation by
phagotrophic protists, the protozoa, are considered to be the most common predators and a
major mortality factor of bacteria, constituting around 5% of the total dry weight of a
wastewater biomass [34,35]. Bacteriovoric metazoa are also widely distributed in various
aquatic and terrestrial habitats and play a part in enhancement of excess sludge reduction. It
has been demonstrated that increased protozoa/metazoa grazing on bacteria results in shifts in
bacterial communities from edible to grazing-resistant species or ecotypes [36]. In aerobic
biological wastewater treatment processes, floc-/film-growing bacteria or granules are more
resistant from the predators. In order to achieve an efficient excess sludge reduction, it is
necessary to fully understand the specificity and population dynamics of prey-predator
relationships and identify predators capable of grazing on the present biomass.
Investigations of bacterial predation by protozoa and metazoa showed that grazing
efficiency was limited by their physical/mechanistic properties, such as the size of the mouth
and pharynx of protozoa and metazoa [37], thus different grade of sludge yield reduction
would be possible when different kinds of protozoa and metazoa were predominant in the
system. Worms are almost the largest organisms observed during the microscopic
investigation of activated sludge including
Aeolosomatidae
,
Naididae
,
Pristina
,
Dero
and
Tubificidae
[38], and may have more potential on sludge reduction in practical application
due to their bigger sizes.
T. tubifex
, which has the largest size among oligochaete worms, was
chosen to be a sludge predator by Huang et al. [39], demonstrating that sludge minimization
could be triggered by
T. tubifex
's predation on sludge in the recycled sludge reactor where
T.
tubifex
was introduced and grew. The sludge reduction rate of
T. tubifex
R was from 0.18 to
0.81 mg VSS mg
-1
Tubifex
d
-1
. The sludge reduction capacity of the recycled sludge reactor E
was from 650 to 1080 mg VSS L
-1
d
-1
. The optimum density of
T. tubifex
and the optimum
sludge recycled ratio were 2500 mg L
-1
and 1, respectively. Although the total amount of
sludge can be reduced substantially by the introduction of oligochaete worms, the practical
application is still uncontrollable because of unpredictable factors affecting worm growth
making them unstable, e.g. how to stimulate the initiation of worm bloom and maintain a
stable worm density under controlled full-scale conditions are still not well understood [40].
Lapinski et al. [41] reported that cultured bdelloid rotifers can significantly improve
municipal wastewater clarification through two-fold mechanism on suspended particles: (i)
