What is a Anaerobic digester?
Anaerobic digester: Sludge stabilisation with biogas recovery
An anaerobic digester is a sealed, heated tank in which bacteria break down sewage sludge without oxygen over two to four weeks, reducing its volume and odor, killing most pathogens and producing biogas that is 60 to 65 percent methane. The gas is burned to heat the digester, run engines or turbines for electricity, or cleaned to pipeline-quality renewable natural gas. Klarifi’s aerial imagery review identifies 1,548 US publicly owned plants with digester silos or gas holders, and the city capital plans it tracks contain 136 digester and biogas line items, from tank cleaning and cover replacement to combined heat and power.
How digestion works
Thickened primary and waste activated sludge is pumped into the digester, mixed and held at about 35 °C (mesophilic) or 55 °C (thermophilic). Hydrolysis, acid formation and methanogenesis happen in sequence in the same tank; a two-stage layout separates them for a more stable process and Class A biosolids. Digested sludge is dewatered and the digester gas is stored in a gas holder before use. Pre-treatment such as thermal hydrolysis increases gas yield and cake dryness at large plants.
Which plants have digesters
Digestion pays back through gas energy and lower hauling volume, so it is found mainly at plants above about 5 MGD; smaller plants use aerobic digestion or lagoons. From the air, digesters appear as tall cylindrical silos or egg-shaped tanks, often with a domed or floating cover and a separate gas holder, which is how Klarifi counts them for every plant.
The capital cycle
Digesters need cleaning and cover, mixer and heat-exchanger replacement every 15 to 25 years, and gas systems are upgraded as engines and gas-cleaning technology improve. Co-digestion of food waste and FOG, and renewable natural gas projects supported by fuel credits, are the growth areas.