Advanced Digestion Process Makes High Grade Sludge in Tennessee
At the Franklin, Tennessee, municipal wastewater treatment plant, a process with a complicated-sounding name is turning sewage sludge into "Class A biosolids" that can be applied to land. The efficient process is known as Autothermal Thermophilic Aerobic Digestion, or ATAD for short. It uses natural digestion to kill pathogens, parasites and viruses, and to reduce vector attraction, and does this so effectively that its end product meets the U.S. Environmental Protection Agency's requirements for beneficial reuse in land application. Vector attraction refers to characteristics of sludge that attract insects and pests.
The system was designed and installed by U.S. Filter/Jet Tech of Edwardsville, Kansas and is in the final stages of startup, according to Jerry Bolen, director of development. "The startup has gone well," says Bolen. "We have operated through the winter months producing biosolids that meet the reuse requirements."
A complete ATAD system includes: a raw sludge holding tank; thickener feed pumps; sludge thickening system; thickened sludge holding tank; reactor feed pumps; two or more insulated reactor vessels with aspirators, jet aerators and mixing pumps; foam dispersion system; odor control system; control instrumentation; and finished biosolids holding tank. All external hot piping, valves, and the reactors are insulated to conserve heat.
The system operates under thermophilic temperature conditions of 45 to 70 C (113 to 158 F) without any supplemental heat. System temperature is maintained by heat generated by the digestion process. Total retention time for the sludge is six to nine days. The reactors are designed so the A'I'AD process can operate for a minimum four-day retention time with one of the reactors out of service.
How ATAD Works
In this process, a thickening unit receives raw sludge from the holding tank once a day, and the sludge is thickened to a solids concentration of from 0.75 to four percent or higher. Then, the concentrated sludge is pumped to the thickened sludge holding tank and/or the first reactor. ATAD's reactor operates on a continuous duty cycle. Over 23 hours each day are devoted to mixing and aerating the sludge.
Processed sludge is isolated from partially processed or raw sludge by a series of reactor configurations and reverse-order filling procedures. On a daily basis, aeration and mixing equipment is stopped and biosolids are discharged from the last reactor into a biosolids holding tank. Each reactor in turn is refilled from the preceding reactor. Undigested sludge is then fed to the first reactor. Transfer and feeding take place once a day.
To dislodge any debris from the aspirator, the manufacturer has developed a patented Vacflush reverse sludge flow system which is activated each time the system transfers sludge out of or between reactors. Finished biosolids are transferred to a final holding tank for cooling and storage until it is time for the disposal step.
The Franklin plant has a capacity of 5.5 million gallons per day (mgd). The ATAD system includes three 60,000 gallon reactors, and is sized to handle flows of 13,000 to 25,200 gallons per day of thickened sludge. Provision was made in the plant's design for the addition of a fourth reactor should the influent flow increase beyond the plant's capacity to handle it in the future.
When U.S. Filter/Jet Tech installs one of these systems, every effort is made to incorporate existing equipment in the process train whenever it is appropriate. At the Franklin site an existing dissolved air flotation (DAF) unit is used to thicken the influent sludge from 0.75 percent solids concentration to 4 to 6 percent. Then the thickened sludge is pumped by two disc pumps from the DAF wet well to a 25,200-gal raw sludge storage tank, and subsequently to the first reactor by a pair of 350-gpm progressive cavity feed pumps.
Aeration, mixing, and tank-to-tank sludge transfer is handled by three 2,700-gpm screw centrifugal motive pumps. Automatic and manual knife gate valves control the sludge flow between tanks. Odors are treated in a bio-filter which draws air from all three reactors and the raw sludge holding tank. And the foam in the reactors is controlled by the manufacturer's specially designed Foam Abater.
Treated sludge is directed to two existing 460,000-gal tanks which provide storage capacity for up to 36 days. A boiler system was installed to supply supplementary heat to second and third reactors should that be required during extremely cold weather.
Editor's Note: This article first appeared in the June, 1997 issue of The Wave, published by U. S Filter Corporation. It was modified slightly to fit Water Online/Public Works Online's journalistic style.