During conventional treatment processes, a large variation of
pH conditions can occur in water, particularly during coagulation.
In this study, the impact of different pH conditions on cyanobacteria
and the subsequent water quality was evaluated on three
commonly found bloom species in Australia. It was demonstrated
that low pH stress (pH < 5) can cause substantial cell lysis and
metabolite release for Anabaena circinalis and Cylindrospermopsis
raciborskii whereas the acid tolerance of Microcystis aeruginosa was
higher. Although the exposure time to low pH is usually brief
during coagulation, it was shown that at pH’s below 5 water
quality can be impaired due to the release of toxins and T&O
compounds into the treated water. Meanwhile, high pH conditions
(up to 11) are found to have minor impact on the cells. Therefore,
increasing the pH prior to the application of coagulant has the
potential to minimise the impact. Overall, when handling waters
that are laden with A. circinalis and C. raciborskii, it is recommended
that the pH is closely monitored at all times to ensure that the risk
of cell damage, even localised damage, by low pH stress is kept to a
minimum. This will in turn reduce the requirements for, and the
cost of, subsequent water treatment.
The cyanobacterial cells used in this study were harvested
during the exponential phase. During natural blooms, cells can be
found at various stages of the life cycle. Aged cells may react
differently under pH stress and whilst this is acknowledged, the
need to isolate mechanisms of metabolite release restricted this
work to the exponential growth stage.