Algae can be problematic in nutrient-rich source waters, especially those containing phosphorus, in which case they often reproduce rapidly and produce colored water and mats of algae known as algae blooms. In natural waters or source water, algae blooms are problematic because they can change many water characteristics.
One of the primary factors that algae blooms influence is the dissolved oxygen content of the water. During the day, the masses of algae produce so much oxygen that the water becomes supersaturated. Then at night, the algae actually use up oxygen in the water and can cause such extremely low dissolved oxygen levels that fish kills may result.
Algae blooms can also cause elevated pH levels in the water. They may raise the pH levels as high as 9.5, which will influence many of the natural processes occurring in the water.
In some cases, an algae bloom will consume itself. As the algae grow and reproduce, they use up nutrients in the water. Eventually, the nutrient levels will drop so low that the algae will have no nutrients and die back. In this case, the dead algae bodies will often promote a bacterial bloom as the bacteria respond to the abundance of food. This overabundance of bacteria can cause yet more problems, depleting the dissolved oxygen levels in the source water.
Algae can cause taste and odor problems in source water and also raise the pH and alkalinity levels that may cause problems with treatment. It may also raise the total organic carbon (TOC) concentration causing a problem with disinfection by-products such as trihalomethanes. To avoid these problems, controls may be necessary.
The best method of control when dealing with algae blooms is prevention. By limiting the nutrient levels in source water, algae blooms can be avoided. The limitation on phosphorus in some wastewater effluents stems directly from this concern.
Another method of controlling algae in a water source, such as a lake or reservoir, is to treat in the early spring with a measured dose of copper sulfate. Aeration is another method of controlling algae as is a new technology
All of the treatment technologies mentioned above are used to treat the symptoms and not the disease. The real problem is the nutrients that are entering the source water through agricultural wastes, over-fertilization, wastewater treatment effluents and urban fertilizer usage in landscaping. A watershed approach is a good method to use in trying to control the nutrient load to the source water for the drinking water treatment plant. It all comes down to less pollutants in the water equals less cost to make drinking water.
The following links provide some information concerning problematic algae.