| Contents Notes |
"A thermophilic aerobic-anaerobic digestion system with a nominal secondary sludge capacity of 16,400 gallons per day was designed and constructed at the Hagerstown, Maryland, Wastewater Treatment Plant. This project establishes the process performance of the dual digestion system in a full-scale design. The system included a short (approximately 1 day) retention time aerobic digester followed by a high-rate anaerobic digester. Thickened, air-activated sludge was autothermally heated by the aerobic oxidation of organic substrates in the first step and then fed to the anaerobic second step, where the stabilization process was completed with the formation, of methane gas. Data were collected to evaluate the system's performance regarding volatile solids destruction, oxygen consumption, power draw, heat production, and process stability. Analysis of pathogens and indicator organisms were also made to determine the effectiveness of the aerobic step to inactivate pathogenic bacteria, viruses, and parasites. Thermophilic temperatures (greater than 45ÀC) were rapidly achieved upon start-up of the dual digestion system and were maintained in the aerobic reactor at a hydraulic retention time of approximately 1 day. The high shear aeration device demonstrated sufficient oxygen transfer capacity to achieve and maintain these high temperatures at reasonable power densities in the aerobic reactor. The system responded well to variations in feed flow and solids concentration as well as to operational upsets. Analyses were performed that illustrate the ability of the dual digestion system to achieve significant reductions in the level of pathogenic organisms in sewage sludge. Finally, over the course of some 20 weeks of operation, the dual digestion system proved itself to be an effective sludge stabilization process, achieving an overall volatile solids reduction of 41.6 percent, with weekly averages in the 24 to 58 percent range." |
