than the standard guideline, except for cycle 2, which took 7 days.
The increase of pH from 7 to 9 under the control sets might have
been caused by algae growing in the systems. Since photosynthesis
of algae required CO2 from the bicarbonate in the water,
high concentrations of OH were found in the system (Azov, 1982).
However, the system pH under plant conditions remained around
7e8.