INTRODUCTION TO DIGESTER SYSTEM
Stabilization of thickened sludge at the Littleton/Englewood WWTP is currently achieved through anaerobic digestion. The five anaerobic digestion units are each approximately 80 feet in diameter and 30 feet deep with a sludge volume of approximately 145,800 cubic feet each.
Anaerobic digestion is a residual solids treatment process. Solids removed from raw wastewater, known as primary sludge, and solids removed from the biological treatment processes, known as secondary sludge, are treated, after thickening in Dissolved Air Floatation Thickeners, in the anaerobic digestion process. The anaerobic digestion process stabilizes the biodegradable solids concentrated from wastewater which in turn, provides the following benefits:
- protects public health and the environment
- makes sludge relatively inert
- reduces odor generation from undigested sludge
- reduces bacteria and pathogenic organisms,
- reduces the volume and weight of sludge, and
- reduces the cost of sludge handling and ultimate disposal.
Stabilization of sludge also reduces the possibility of sludge becoming a food source and breeding ground for disease-spreading insects and rodents (vectors).
The anaerobic digestion process itself is a multistage biological process that occurs in the absence of oxygen. Complex organic substances are solubilized and fermented to methane, carbon dioxide, trace gases, cells, and stabilized sludge solids.
Anaerobic digestion at the Littleton/Englewood Wastewater Treatment Plant (WWTP) is accomplished in five conventionally shaped anaerobic digesters. The total facility is comprised of five major components:
- thickened sludge loop and dedicated digester mixing and circulated sludge pumps
- five fixed cover anaerobic digests
- boilers, heat exchangers, cogeneration and a heat reservoir system to maintain a warm environment for the digestion process
- digested sludge recirculation pumps for continual heating and foam suppression and mixing pumps to distribute feed sludge and toxic digestion products
- associated gas handling equipment, including a waste gas burner
Fixed submerged cover digesters operate as a constant liquid level system. Digested sludge is removed by displacement of the digester contents by feeding new sludge into the digester through the DAFT loop feed pumps that discharges into the center of the digester dome. The center digester mixer combines the fresh sludge with the digester contents. This causes hydraulic displacement of sludge from the bottom of the digester up and over the sludge transfer overflow weir and into the sludge withdrawal line. This overflow line carries the digested sludge into a 24-inch diameter standpipe. The level of sludge in the standpipe is measured, which controls the operation of the sludge transfer pumps. On a high level in the standpipe, the pump is turned on until the sludge reaches a predetermined low level, where the pump ceases operation. The digested sludge is transferred into either Digester No. 1 or No. 2 where it is held until it can be dewatered. Since all of the solids are sent through grinders prior to thickening in the DAFTs, material accumulation in the digesters such as grit, rags, and plastic is minimized.
The anaerobic digestion process is a single unit process in the overall treatment of wastewater sludges. Gas produced by the digestion process is used to fire boilers and cogeneration units to produce both heat and electrical power. The heat source provides hot water that is used to heat the facility as well as maintain the temperature in the digesters. These units will burn methane or natural gas.
Mixing is currently provided by 25 horsepower sludge circulation pumps. This system circulates digested sludge continuously to the DAFT gallery, returning through the heat exchangers to the digester tanks. Thickened sludge from the DAFTs is pumped into the circulating sludge loop to be transported into the digester system.