A new preconditioning process enables wastewater treatment plant operators to upgrade existing anaerobic sludge digesters to produce Class A biosolids, suitable for land application.
The process is known as the DuoTherm(tm) Process. It solubilizes (dissolves) some of the biosolids and destroys pathogens in a preconditioning tank at 150 degrees F for 1 hour. This tank is located upstream of the anaerobic digester. The solubilized solids, finished in the preconditioning tank, enhance the anaerobic digester by being readily degraded in liquid form. The anaerobic digester provides stabilization and energy recovery through biogas generation. This process should not be confused with another pretreatment process that achieves Class A biosolids and uses anaerobic digestion for stabilization. Kruger and CBI Walker are both manufacturers of the ATP (Aerobic Thermophilic Pretreatment) process. Like the DuoTherm Process, the main objective of the ATP process is to destroy pathogens with a secondary benefit being solids solubilization for enhanced mesophilic anaerobic digestion.
However, the preconditioning of the DuoTherm Process is a prepasteurization step (simply relying on heat), while the ATP process is a biological treatment step requiring the maintenance of an aerobic thermophilic population of bacteria. The heat recovery system for the process is also unique. It uses a tube-in-tube heat exchanger. Tube-in-tube heat exchangers are not new to the industry. However, the heat exchanger for the DuoTherm Process has been modified to accommodate solids flow through the annular space as well as the inside tube.
The heat associated with the preconditioned biosolids is recovered using the modified exchanger. The temperature of the biosolids leaving the preconditioning tank is reduced by using it to warm cold, unconditioned solids. This heat recovery reduces the heat duty associated with the incoming, unconditioned solids and cools the conditioned solids (at 150 degrees F) before they are discharged to the anaerobic digester (95 degrees F).
Data shows that using aerobic thermophilic digestion prior to mesophilic anaerobic digestion can result in higher gas yield through improved volatile solids destruction; less organic material and fewer odors in the stabilized solids.
Methane generated in the anaerobic digestion phase is used to produce hot water. The hot water is used to heat the biosolids in the anaerobic digester, maintaining its operating temperature at 95 degrees. Piping modifications can allow the same heater and heat exchanger to maintain the first-stage preconditioning at 150 degrees F.
Preconditioning reduces pathogens in the solids prior to digestion. Pathogen destruction requires less time with increased process temperature. A temperature of 150 degrees F requires less then one hour of detention to achieve sufficient pathogen reduction to meet the Class A standard.
The one-hour detention time allows for a small preconditioning tank, and makes batch feed feasible. The benefit of batch feed is the assurance that pathogen levels will not be exceeded due to short-circuiting.
Four bench scale tests of preconditioning were performed. The consisted of heating biosolids mixtures of 66 percent primary sludge and 33 percent waste-activated sludge (v/v) to 150 degrees F for one hour in a 2-liter beaker. All four tests resulted in pathogen reduction below the Class A fecal coliform requirement of 1,000 MPN per dry gram. Influent fecal coliform averaged 11.9 million MPN per dry gram. Another advantage of the preconditioning is the solubilization and breaking down of the sludge.
In testing, anaerobic digestion by itself and thermal conditioning followed by anaerobic digestion each required about .92 MBtu/hr total heat. While 1.53 MBtu/hr was consumed heating the incoming solids in the thermal conditioning process compared to .76 MBtu/hr for anaerobic digestion alone, the thermal conditioning process was able to recover .77 MBtu/hr. Both processes hat total convective heat loss of .16 MBtu/hr.
