Organic residuals contain energy: dried biosolids has about as much energy as low-grade coal (lignite).
This energy is commonly harvested by:
- anaerobic digestion - producing biogas, a renewable methane-rich fuel used in combined heat & power (CHP) systems
- incineration / thermal conversion - with energy recovery (pyrolysis & gasification of biosolids are being developed)
- composting - biological activity releases energy during composting, providing low-grade heat
Water resource recovery facilities (WRRFs) are striving to reduce their net energy use - striving for "net zero." As of 2014, several have attained net zero, some even producing more electricity than they use. Research has shown that achieving net zero requires use of anaerobic digestion (AD) and, in most cases, importation of additional wastes, such as fats, oils, grease (FOG), glycerin, food processing wastes, and/or food scraps. In New England and eastern Canada, there are fewer AD facilities than in other areas. But that is changing. AD is expanding dramatically across this region and around North America - at WRRFs, on farms, and in industry.
Burning wastewater solids (sewage sludge) to reduce them to an innocuous ash is the leading biosolids management method in southern New England and the largest cities in Quebec. These incineration / thermal oxidation systems usually use natural gas. If the solids fed into the incinerator are dry enough (~28% solids or more), once it reaches operating temperature, the incinerator will continue its burning without additional natural gas, relying on the energy in the solids (autogenous burn). In recent years, the waste heat from these incinerators is being recognized as a resource and captured for electricity generation and other uses.
Although it is not common, the low-grade heat released naturally during the composting process is sometimes captured and put to use for heating work spaces. For example, Transaqua of Moncton, NB, uses compost heat in some work spaces and to keep its compost operation free of ice and snow.