Index >> Microbes and Hydrosphere>>Eutrophication

Most secondary sewage treatment plants, though, precipitate solids and inactivate most bacteria in domestic sewage, yet they do not remove the basic nutrients such as ammonia, nitrogen, nitrates, nitrites and phosphates. These nutrients stimulate algal growth and lead to plankton blooms. Some plankton blooms, particularly those of blue-green algae produce obnoxious odours and tastes in waters. Others, such as the dinoflagellate blooms or ' the red tide' of southern coastal regions, produce toxic metabolic products which can result in major fish kills. Plankton blooms of green algae do not always produce undesirable odours or toxic product, but still create problems of oxygen supply in the water.

Excessive nutrient levels in aquatic systems can also cause two other kinds of ecological problems. Primarily, they may lead to extensive growth of aquatic weeds. Excessive growth of these weeds can impair fishing, bathing, fish spawning, shell fish production, and even navigation. Secondarily, nitrates can be converted in the human digestive tract by certain bacteria to nitrites, and the same transition may occur in opened cans of food even if they are subsequently refrigerated. Nitrites react with haemoglobin, forming methaemoglobin which will not take up oxygen. Laboured breathing and occasional suffocation result most severely in human infants. Nitrites may also react with creatinine (present in the vertebrate muscles) to form nitrosarcosine which can be carcinogenic.