The term biosolids was coined by the Water Environment Foundation, formerly known as the Federation of Sewage Works Associations, to lessen public objection to spreading sewage sludge on the land. Now it is used to differentiate between raw, untreated sewage sludge and treated sludge (biosolids) that might be suitable as a soil amendment of fertilizer.
Anaerobic digestion is the process by which bacteria decompose organic material into methane and carbon dioxide in the absence of air. The resulting solid product, called the digestate, has less odor and fewer pathogens. When the process of digestion is conducted at approximately 122–150 °F (50–60 °C), the thermophilic temperature range, pathogens are killed and the biosolids can attain a Class A or even Class AA rating [2, 14].
Another process for treating wastewater is aerobic digestion, which is conducted at ambient temperatures and requires air to be pumped into the water. Aerobic digestion typically requires less time and produces Class B biosolids, which contain more pathogens than Class A or Class AA biosolids.
Methane extracted from the anaerobic digestion process can be used to generate electricity. A rough estimate of the methane produced from wastewater is 18,250 kg per 1,000 persons per year. Another estimate is that the wastewater from one person can produce enough methane to generate 2.2 watts of electricity. Within the thermophilic temperature range, one study found that methane output is maximized at approximately 127 °F at which point methane output is approximately three times what it would be at 135.5 °F and double what it would be at the 95 °F (35 °C) second-stage temperature higher than the 42°C primary digestion temperature.
The large amount of methane produced from wastewater can be problematic if released into the atmosphere because of its global warming potential. Both the carbon dioxide and the uncaptured methane produced during digestion can contribute to global warming, with each unit of methane producing approximately 23 times the global warming potential of the same amount of carbon dioxide over a 100-year period. These gasses absorb relatively more solar radiation than other gasses do, and the result is a relative warming of the atmosphere as we're all painfully aware. The Intergovernmental Panel on Climate Change has determined that these warming effects threaten to increase average temperatures, change precipitation patterns, raise sea levels, and increase the frequency and intensity of extreme weather events.
When used as fertilizer, biosolids must be evaluated to determine their levels of metals; organic compounds (such as PCBs, dioxin, pharmaceuticals, and cleaners); and biological pathogens. In the Clean Water Act of 1993, the EPA defined the standards for the allowable levels of these contaminants.