Biological wastewater treatment is often described as occurring in three steps. In step one, Transfer, food from the wastewater is transferred to the cell. Adequate mixing and detention time are needed to assure that the organism comes into contact with the food source. Step two, Conversion, occurs as the organism metabolizes the food supply, converting it to new cells. In order for this to occur the food supply must be a usable type and in a usable form. Some compounds are easily degraded by the bacteria, while others are metabolized more slowly. Some pollutants may not be metabolized until the organisms become acclimated to it, producing the right kinds of enzymes. A proper D.O. environment must be present; aerobic organisms will not efficiently remove pollutants in an anaerobic environment. The nutrient balance must be proper for conversion to take Food Storage Oxygen Soluble Organics Cell Membrane Adsorbed Particle Enzymes (Absorption) 9 Activated Sludge Manual place. Like other life forms, the organism needs nitrogen and phosphorus, among other minor nutrients, in order to metabolize food and build new cells. The ratio of carbon to nitrogen to phosphorus is generally taken as 100:5:1. In the third step of treatment, Flocculation and Separation, the microorganisms stick together to form large particles that will settle out of the purified wastewater in the secondary clarifier. Flocculation occurs when mixing allows the organisms to contact one another, but does not cause conditions so turbulent that the flocculated material is torn apart. Settleability and compaction of the floc particles depends on the density, size and shape of the particles as well as the efficiency of the clarifier. Settleability is affected by the abundance of filamentous bacteria, those that form strings as they grow rather than forming floc. An excessive growth rate of these bacteria may cause a bulking condition in which the mixed liquor does not compact well, taking up much more volume in the clarifier. This condition may be caused by many factors, among which are improper D.O. environment and nutrient imbalance, and may result in solids loss in the clarifier effluent. More about bulking and control of filamentous organisms later.