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Abstract
This thesis addressed important issues related the environmental and
human health including: 1) the presence of toxic cyanobacteria in the source
water (Nile River) of drinking water treatment plant (WTP), 2) the efficiency of
conventional methods in a representative treatment plant for the removal of
cyanobacteria and microcystin toxins, 3) potential breakthrough of
cyanobacteria and microcystins toxins into domestic reservoirs, and 4)
heterotrophic bacteria associated with cyanobacteria in raw and treated water of
this treatment plant. The results are summarized as follows:
The present study revealed the presence of toxic cyanobacterial bloom
dominated by M. aeruginosa in the Nile River Delta, Damietta branch, which is
used as a water source for all drinking WTPs in this region. The cell density
(1.08 X104 - 6.61 X 104 cells mL-1) at the intake surpassed the WHO Alert Level
(> 2000 cells mL-1).
Our data also showed that this bloom can produce MC-LR and –RR, and
the concentrations of these toxins exhibited substantial variability throughout the
duration of the study. Such differences may be caused by changes in
environmental conditions favoring the growth and toxin production of
cyanobacteria. High nutrient concentrations, particularly NO3, NH4 and PO4,the
most important factors correlating with the outgrowths of Microcystis
aeruginosa and microcystin production, were in the source water (Nile River
water).
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In addition to intracellular microcystins, extracellular (dissolved) microcystins
were also detected in the source water at concentrations (1.2-4.5 μg L-1)
surpassed the WHO guideline value of 1 μg L-1 for MC-LR in drinking water .
The results also showed that conventional methods such as
coagulation/flocculation/sedimentation (C/F/S), sand filtration and chlorination
used in Damietta WTP were not only inefficient for complete removal of
cyanobacterial cells and toxin degradation, but also caused cell lysis of
cyanobacteria and release of high amounts of MCs into the ambient water.
Overall, even the whole treatment process in Damietta treatment plant has
led to the removal of cyanobacterial cells by 98.5% maximum, final water
(outflow water) still contained > 2000 cells ml-1 (i.e. more than acceptable level).
Concentrations of dissolved MCs in outflow basin (water distributed to the
public) were also high and exceeded the 1 μg L-1 limit.
Cyanobacterial cells and microcystin toxins were also detected in
domestic reservoirs receiving water from Damietta treatment plant. The
cyanobacteria found in these reservoirs were similar to those found in the
finished drinking water of Damietta treatment plant (M. aeruginosa and
Gloeocapsa sanguinea), but with high cell densities indicating that the
conditions in reservoirs were suitable for the proliferation of M. aeruginosa.
Concentrations of extracellular MCs in reservoir waters did not show any
significant difference from those detected in outflow water received from
Damietta treatment plant (1.1-7.4 μg L-1). This implies that the source of
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extracellular MCs in reservoir water that had been received from this treatment
plant, and no toxin released from cyanobacterial cells into reservoir waters.
The results also revealed the presence of heterotrophic bacteria associating with
cyanobacteria in raw and treated waters of Damietta treatment plant. The most
common bacteria detected in these waters (Aeromonas sp., Bacillus, Proteus sp.)
are related to species, which are considered a nuisance and associated with
adverse health effects.
Recommendations
Since the Damietta WTP is a representative Egyptian WTP using the Nile
River as source water and using similar conventional water treatment processes,
the results in this study may likely occur in other WTPs in Egypt. Although, the
guideline value for MC-LR recommended by WHO in drinking water has been
included in Egyptian legislation by the Ministry of Health in the last decade,
formal regulations are not yet in place because the capacity of proper equipment
and laboratories with qualified staff able to determine MCs and other
cyanotoxins in drinking water is still limited. Consequently, the documentation
of toxicity episodes of cyanotoxins are lacking in Egypt, due to a lack of
awareness and knowledge to properly correlate the toxicity with these toxins.
Taken this scenario and the findings of our study, an urgent need is necessary
for regular monitoring of cyanobacteria and their cyanotoxins in the intake raw
water of Egyptian drinking WTPs. Furthermore, these WTPs need to be
upgraded to improve their performance in cyanobacterial cell and cyanotoxin
removal.