A biosolid, more commonly referred to in the plural as biosolids, is a material produced after sewage sludge has been treated and processed in a wastewater treatment plant, sometimes called a sewage treatment plant. Domestic sewage, mainly composed of human waste and sewer water from households, is the raw material most commonly used to produce a biosolid. Biosolids processing usually includes the removal of most of the water contained in the sewage, chemical treatments such as the application of lime, and various other chemical and biological treatments done to remove odor, disease-causing material, and various toxins and heavy metals. Biosolid material can be used for many different purposes, including as a fertilizer and soil conditioner. Regulations for the use and application of biosolids vary depending on the country and jurisdiction, and it is important to know the local rules and regulations before using biosolid material.
Wastewater biosolids are very different than the sewage sludge used to produce this biosolid material. One important difference is that the biosolid contains less water, since most of the water is removed from the sludge when it enters the biosolids processing facility. The separated solid material is then processed in various ways. For example, it can be treated with lime to reduce its smell and raise its pH value, and sanitized in various ways to remove disease-causing microbes like parasites, bacteria, and viruses. Sometimes, the material is stabilized, meaning it is put into tanks where microorganisms called digesters consume much of the organic material and potentially disease-causing organisms.
Biosolids can be put into landfills, incinerated, or recycled in various ways. This material contains significant amounts of phosphorous and nitrogen, chemical compounds that are important plant nutrients, and one of the main uses for recycled biosolids is as fertilizer for agriculture, gardening, and forestry. Biosolids land application also includes spreading biosolid material on previously strip-mined sites to enable plant regrowth and using it to add to and improve the soil in recreational areas such as parks and golf courses.
Many countries are trying to increase their use of recycled biosolid material and also improve their biosolids management, including instituting better rules and regulations for its production and use. In order for recycled biosolids to be safe to use, it is important that toxic compounds and potentially hazardous contaminants like heavy metals are removed in the wastewater treatment process. This is especially important because of biosolids application for agricultural purposes.