TWO CLASS A PROCESSES ARE BETTER THAN ONE: OR, HOW THE “DUAL A” PROCESS PROVIDES RELIABILITY AND FLEXIBILITY AT LESS COST

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, 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 design 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 with odors, cost of redundant units and staffing issues to run the continuously operating dryers in round the clock “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, can produce 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 can produce three types of Class A biosolids: 1) liquid biosolids from the ATAD cool-down tank, 2) Dewatered cake at 26-28% TS, and 3) dried biosolids at 75-90% TS. This flexibility eliminates the need for redundant dewatering and drying, greatly reducing the equipment, construction and operating cost.The process was put on-line on schedule and with a few problems noted in this paper. The ATAD provides almost 50% TS reduction and produces an easily dewatered sludge at a reasonable polymer dose. An unexpected benefit was septage receiving. All septage is screened, combined with primary and secondary sludges and pumped once a day to the ATAD. The ATAD digests the combined sludges and eliminates the effects that septage may have on secondary treatment. The trouble-free septage operation provides in excess of 150,000/yr of added revenue with little added cost. The dryer has operated satisfactorily, producing a relatively non-dusty, large particle size, dried biosolid.