Biomedical Engineering Reference
In-Depth Information
shown in Figure 7. The percentage of organic carbon in the algal biomass varies from 29% (
S.
obliquus
) to 53% (
C. vulgaris
). These results are in agreement with those reported by several
authors that present values of the same order of magnitude by using empirical formulas [23,
30-33].
4.5. Effect of Cr(VI) on Biomass Settling
The ability of algal biomass to settle down has been expressed in terms of the decrease of
the suspended biomass concentration after a 6 h-settling period. Results of the biomass-
settling rate for each algae species and different Cr(VI) concentrations are presented in Table
5. In the absence of Cr(VI) and for Cr(VI) = 1 mg l
-1
, the settling rate is similar for
C. fusca
,
S. acutus
and
S. obliquus
but much lower for
C. vulgaris
. As Cr(VI) concentration increases,
the settling rate decreases for all algal cultures but the effect is particularly evident for
C.
vulgaris
and
S. acutus
. Results suggest that Cr(VI) ≥ 5 mg l
-1
impairs algae autoflocculation.
Table 5. Biomass settling rate (mg l
-1
h
-1
)
Concentration of
Cr(VI) (mg l
-1
)
Chlorella fusca
(
CF
)
Chlorella
vulgaris
Scenedesmus
acutus
Scenedesmus
obliquus
0
55.1
13.4
41.1
42.7
1
45.8
12.3
46.4
54.6
5
26.5
1.4
16.0
33.2
25 (50 for
CF
)
nd
0.52
nd
6.5
nd - not determined.
5. Conclusions
Microalgae growth kinetics is adequately described by the logistic equation. Experiments
carried out at 24.6 ºC and initial pH = 6.5 led to values of the specific growth rate of biomass,
μ,
of 0.0162, 0.0284, 0.0359 and
0.0370 h
-1
for
Scenedesmus obliquus
,
Chlorella vulgaris
,
Scenedesmus acutus
and
Chlorella fusca
, respectively. The corresponding maximum biomass
concentration,
K
, was 604.1, 369.2, 542.9 and 403.3 mg l
-1
for the same algal species. In spite
of the low growth rate,
Scenedesmus obliquus
culture reaches the highest biomass
concentration. A decrease in temperature operation to around 21 ºC originated lower μ
(between 0.0241 and 0.0357 h
-1
) and
K
(between 169.9 and 327.8 mg l
-1
) values. An increase
in pH to about 7.9 led to drastic decreases in the specific growth rate of biomass (below
0.0177 h
-1
) and the maximum biomass concentration (below 54.7 mg l
-1
). Results show that
pH plays a central role in microalgae growth.
Chlorella vulgaris
seems to be the less resistant
microalgae to changes in pH and temperature. Looking simultaneously at μ and
K
values, the
best performance was obtained with
Scenedesmus acutus
and
Chlorella fusca.
Growth inhibition of
Chlorella fusca
,
Chlorella vulgaris
,
Scenedesmus acutus
and
Scenedesmus obliquus
by Cr(VI) occurs for concentrations ≥ 5 mg l
-1
, but concentrations up
to 1 mg l
-1
do not seem to seriously affect growth or even may increase it slightly. The more
marked effect of Cr(VI) on algal growth is observed in
C. vulgaris
culture. Metabolites
production has proved to be in small quantity either in presence or absence of chromium, thus
