Decaying algae and other micro-organisms give off a variety of organic chemical compounds, any or all of which could make water taste moldy. None of those decay products has been found to pose a health threat, especially at the low concentrations found in open bodies of water. They may not be dangerous, but they are powerful in the effect they produce. Sensitive palates make it difficult to eliminate the taste completely for everyone, but there is a straight-forward way to treat raw lake water to get rid of most of the taste. Finely ground particles of carbon are added to water as soon as it arrives in the plant.
Carbon attracts molecules which adhere to it like steel to a magnet.
The carbon settles out of the water later in the treatment process. The only problem is that carbon is not cheap. A seasonal problem with earthy water is nothing new. In the late fall, as the temperatures drop, the top layer of the lake cools first. The process, called the fall turnover, stirs up sediment and brings up decaying alagae.