Biomedical Engineering Reference
In-Depth Information
fusca
specific growth rate. For Cr(VI) < 1 mg l
-1
, μ varies between 0.08 and 0.17 h
-1
,
depending on the algal species. The growth of
C. vulgaris
is severely inhibited by Cr(VI) = 5
mg l
-1
. The production of metabolites is small compared with biomass production, for all
Cr(VI) concentrations. The organic carbon content of algae is about 40%-50% (dry basis),
except for
S. obliquus
(around 30%). The biomass of
C. fusca
and
S. acutus
presents the
greatest sedimentation rates. The presence of high Cr(VI) concentrations negatively affects the
sedimentation.
Keywords:
Microalgae;
Chlorella fusca
;
Chlorella vulgaris
;
Scenedesmus acutus
;
Scenedesmus obliquus
; growth kinetics; chromium (VI)
1. Introduction
As population, population density, and development level increase, the issue of
wastewater-related nutrient loading has grown as one of the major concerns in environmental
management. On the other side, wastewaters resulting from human activities often contain
heavy metals [1]. These elements make part of a list of dangerous substances with toxicity
characteristics and a high potential of persistence and accumulation.
There has been considerable interest in using aquatic organisms for wastewater treatment.
Portugal presents, mainly in Centre and South areas, climatic conditions ecologically
favourable to green microalgae development, so the subject has been faced with increasing
attention. Because of the fact that, in those areas, the limiting factor to agriculture is water, a
judicious management of the hydric resources available is necessary, and so water reuse
constitutes an adequate contribution for solving ecological and water shortage problems
simultaneously.
Microalgae are often used for wastewater treatment due to their high growth rate and
simple growth requirements. Several works refer to nitrogen and phosphorous removal by
microalgae [2-6]. Lau et al
.
[7] show that carrageenan-immobilized
Chlorella vulgaris
cells
efficiently removed N and P (95 and 100%, respectively) from wastewater. Meiring et al. [4]
reported nitrogen removals of about 85%. Robinson [5] has obtained P removal efficiencies
of 80% to 95% using
Chlorella emersonii
in suspension. Continuous-flow culture studies
were also carried out using small-scale packed-bed reactors containing
Chlorella
immobilized
in alginate beads. Reactors were maintained at 25ºC and
illumination was provided at 60 μE
m
-2
s
-1
. Gonzalez et al. [2] run 216 h-experimental cycles of batch cultures of
Chlorella
and
Scenedesmus
and concluded that
Chlorella vulgaris
was less efficient than
Scenedesmus
dimorphus
in removing ammonia. Twist et al.
[6] used
Scenedesmus subspicatus
cultures
incubated at 20ºC with an incident photon flux density of 40 μE m
-2
s
-1
. The interaction of air
flow rate with nutrient supply was tested under controlled temperature (19±1ºC) and light
(85-95 μE m
-2
s
-1
).
The pH of the culture medium determines CO
2
and minerals solubility and influences
directly or indirectly the algal metabolism. Becker [8] states that stopping agitation and/or
CO
2
supply can cause an increase in pH, leading to the formation of algal flocs, which will
settle down. Experiments with
Scenedesmus
sp. to study the effect of pH showed that no
flocculation could be obtained for pH values between 5.0 and 7.5, whereas at pH values
above 8.5 almost 95% of the algal biomass could be removed.
