All municipal wastewater treatment plants produce biosolids which are the stabilized residuals that settle from the water during the various treatment processes. Solids are produced during primary treatment, as heavy suspended solids settle out. In secondary treatment, microbes eat the dissolved and remaining suspended solids; then, being heavier than water, they also settle out in quiet water. In some cases, tertiary treatment can be used to clean the water even further, and a third type of solids is produced - one that normally involves chemical and physical treatment. Following wastewater treatment, the collected solids are treated to stabilize the readily putrecible materials, reduce volume and destroy pathogens. Solids treatments are generally biologically based with microbes using the organic carbon in the solids as an energy source. Material produced as a result of solids treatment is called biosolids. Biosolids are generally about 50% organic at the end of typical anaerobic digestion. All biosolids can be used for their fertilizer value. Total nitrogen (N) in biosolids varies with the treatment process, but generally ranges from 1-6% N. Used in agriculture, they are generally applied at rates sufficient to meet the nitrogen requirements of the crop. Because of the high content of organic matter, biosolids are also good soil conditioners. Biosolids can generally be divided into two categories: high N and low N biosolids.