Thickened waste activated sludge (WAS) from DAF, along with fermented primary sludge and scum, are pumped to the digesters. The digesters are closed circular tanks 30 m in diameter and up to 12 m deep. The digestion uses the naturally-occurring anaerobic (i.e. living without oxygen) microorganisms to break down organic materials into methane and carbon dioxide. The sludge is heated to 37 degrees C in the primary digester to improve the rate of digestion. The sludge then enters the secondary digester.
Anaerobic digestion is a biological process which breaks down a significant amount of organic solids in the sludge and at the same time produces methane gas that is used as a boiler fuel for plant heating. Consequently, the volume of final sludge is significantly reduced. So is the cost for sludge disposal.
The mixture of sludge and liquid in the secondary digester is hauled by truck to Edmonton’s Clover Bar Sludge Lagoons for disposal. The digesters are equipped with gas compressors and sludge recirculation pumps to provide adequate mixing capability for better digestion of sludge and production of methane gas. In the summer season when the production of methane gas exceeds the plant use, the extra biogas is burned through a gas flare.