Cyanobacteria, more commonly known as blue-green algae, are becoming more prevalent in waters affected by increasing nutrient levels. This report investigates the causes, effects and management of the blue green algal blooms, which occur periodically in Swanbank Power Station's Cooling Water Make Up Lake. Every summer the lake suffers from severe blooms which decrease water quality, adversely affecting the lake's appearance and decreases the efficiency of the Cooling Water (CW) system, which in turn increases the potential health risks for employees and the public.
There are several health and safety and operational issues that surround the increased presence of blue green algae in the lake. These are:
• Monitoring for blue-green algae is increased. If the outbreak turns toxic, health and safety issues for workers and the public must be addressed;
• After chlorination there is an increased amount of sediments in the CW system, which therefore increases the blowdowns and cleaning required on each system;
• Blowdowns increase the suspended solids in the discharge to Oakey Creek, which may cause violations of environmental limits; and
• Decreased efficiency of the cooling water system due to increased sediment load.
To investigate the problem, a nutrient mass balance was performed over Swanbank Lake to ascertain whether nutrient loads entering and leaving the lake were significant. The water samples were analysed for TP, orthophosphate, ammonia, nitrate and nitrite. It was proposed that the blue green algae were obtaining significant amounts of orthophosphate from incoming water, which together with warmer water temperatures and other environmental factors were aiding blooms. It was found that the Power Station does contribute significant amounts of orthophosphate into the lake, a mass balance calculation revealed approximately 0.7 kg/day is being added to the lake. This figure does not include any additions from the Power Station.
The majority of the lake’s water is pumped in from Berry’s Lagoon or Kholo. The average orthophosphate loads in the water from Kholo are seven times lower than the water being obtained from the current major source, Berry's Lagoon. The levels are still above the suitable growth conditions for blue green algae but decreasing the nutrient load into the lake would slow eutrophication of the water and possibly restrict the current periods of suitable growth.
Ammonia, nitrate and nitrite results were found to be within ranges for suitable blue green algal growth given by Moisander et al (2000). However, all the major species of blue green algae in the lake are able to fix nitrogen therefore these results, though important, can not be used to limit their growth.
Small laboratory growth experiments were also performed to investigate the effects of phosphorus on lake water samples. It was found that phosphorus addition to lake water samples did aid in the growth of the blue green algae.