TWO CLASS “A” PROCESSES THREE YEARS LATER: A LONG-TERM EVALUATION OF THE DUAL “A” PROCESS

Class A biosolids processes are perceived to be more expensive than traditional Class B processes. This perception is not always true. The City of Three Rivers, Michigan chained together two Class A biosolids processes: a 2nd generation autothermal thermophilic aerobic digestion (ATAD) and an indirectly heated batch dryer to lower construction and operating costs, provide more reliable operation and add process flexibility.The 2.75 mgd City of Three Rivers WWTP was beset by numerous problems, including organic overloading, septage receiving, failing anaerobic digesters and tremendous odor problems. Sludge drying was selected to replace anaerobic digestion, due to the small volume of end product and a potential local market for dried biosolids. Potential problems were odors, lack of redundant units and staffing for continuous dryer “campaigns”. Additional research and analysis indicated that biosolids stabilization before drying would allow the use of a smaller, much less expensive batch dryer and eliminate the need for redundant units. The most cost effective stabilization process in this “Dual A” concept was a 2nd generation ATAD. It emits no odorous reduced sulfur compounds, produces VS reduction in excess of 60% and could be placed in two existing sludge-holding tanks and a garage. The dryer is an indirectly-heated batch dryer that automatically loads and unloads, is much less expensive than other dryers and can be operated daily and shut down.The “Dual A” design produces three types of Class A biosolids: 1) liquid biosolids from the ATAD, 2) dewatered cake, and 3) dried biosolids. This flexibility eliminates the need for redundant dewatering and drying, reduces greatly the equipment, and operating costs.The process was put on-line on schedule and with a few problems noted in this paper. The ATAD feed flow has been greater than design and the feed solids far below the design value. Despite this, the ATAD has maintained thermophilic temperatures, provides almost 50% TS reduction and an easily dewatered sludge at a reasonable polymer dose. The ATAD has been trouble-free. The ATAD enables the plant to accept septage and generates 125,000/yr of added revenue. The dryer has continual small problems but operates at higher than anticipated thermal efficiency and produces a non-dusty, large particle size, dried biosolid.