Wastewater may contain high levels of [[nutrients]] such as nitrogen and phosphorus. The excessive release of these nutrients to the environment can cause the overgrowth of weeds, algae, and cyanobacteria (blue-green algae). A rapid growth in the population of algae can result is an algal bloom of a magnitude such that the numbers are unsustainable. Eventually most of the algea die. The decomposition of the dead algae by bacteria depletes the dissolved oxygen in the water. As a result most or all of the animals living in the water die. This creates more organic matter for the bacteria to decompose, continuing the cycle. In addition to causing deoxygenation, some algal species produce toxins that contaminate drinking water supplies. This scenario is typical of the Nitrogen cycle. Nitrogen is typically excreted or released by aquatic organisms in the form of amonia. This in turn is converted to amonium by micro-organisms which in turn is a nutrient that plants use for growth. The plants are then in turn eaten by creatures, thus completing the cycle (Figure 3).
Different treatment processes are required to remove nitrogen and phosphorus. Phosphorous is generally removed using chemicals such as alum, or ferric chloride which precipitate to form a salt and hydronium or hydroxide ions (Figure 2). [[Image:Phosphorous removal.gif| thumb| 1000px| left| Figure 2. Phosphorus Removal (Awaiting Permission)]] These ions can cause the waters pH to fluctuate. Generally lime is used to stabilize the pH of the effluent. These reactions require a settling basin to remove the precipitants.