AUTOTHERMAL THERMOPHILIC AEROBIC DIGESTION
IN THE FEDERAL REPUBLIC OF GERMANY
Kevin Deeny, Senior Operations Specialist
Roy F. Weston Engineers Inc.
West Chester, Pennsylvania 19380
James Heidman, Senior Environmental Engineer
USEPA (MERL)
Cincinnati, Ohio 45268
James Smith, Environmental Engineer
USEPA (CERI)
Cincinnati, Ohio 45268
INTRODUCTION AND BACKGROUND
The Autothermal Thermophilic Aerobic Digestion system is an aerobic digestion process that operates within a thermophilic temperature range (40 C to 80 C) without the introduction of supplemental heat.
The potential for conducting autothermal thermophilic aerobic digestion (ATAD) of wastewater
sludges has been known for some time. In 1969, Kambhu and Andrews [1] reported the results from
computer simulations which showed that ATAD could be self-sustaining with respect to temperature.
In simulations with air aeration, it was predicted [2] that sludge temperatures could increase by more
than 40% at residence times greater than 7.5 days.
Much of the early experimental work on which full-scale ATAD systems have been based was
performed by Popel and coworkers [3,4,5,6]. The process was demonstrated both with animal manure
and sludge having high concentrations (10-60 kg/cum) of organic solids and using the "Umwalz-
belufter" developed by Fuchs to provide proper aeration. Reactor temperatures of 50 C were obtained
even at low ambient temperatures.
The empirical operating characteristics of this aerator type (self-aspirating aerator) were reported
in the late 1960's [7]. The aerator and several comparable designs were successfully marketed and
implemented for industrial and agricultural waste treatment in the US by the DeLaval Company in
a patented process called the LICOM (from liquid compositing) system. Studies on dairy, beef and
swine wastes in the US indicated that autoheating to thermophilic temperatures was possible.
In 1971, substantial increases in temperature were inadvertently obtained at the Hamilton, OH
wastewater treatment plant when the covered digesters were converted to aerobic digesters [8]. Temperatures of 38 C were obtained when the 4% feed solids concentration was fed to the digester at a
rate of 2 kg/cu m/day. Insufficient air was available to maintain a positive DO concentration and
odors were present.
Other ATAD studies were conducted in the early to mid 1970's in the US with high purity oxygen
[10]. Union Carbide was reported to have been working on ATAD systems utilizing high purity
oxygen since 1972. Studies at the Speedway, IN treatment plant indicated that substantial increases
in temperature were possible although thermophilic conditions were not attained. Pilot plant studies
at Tonawanda, NY on both single and two-stage digestion systems were conducted under autothermal
thermophilic conditions. Based on these studies, it was concluded that an ATAD system operating
at a 5 day HRT could achieve volatile solids destructions equivalent to a conventional system at a
15 a to 20 day HRT. Nitrification was observed to be inhibited under thermophilic conditions. It
was also concluded that reductions in pathogenic organisms to less than detectable levels can be
achieved if the temperature of the sludge is maintained at 50 C or greater for 5 or more hours. Similar
reductions could be achieved at temperatures as low as 45 C with longer residence times. Sludge
produced from an ATAD system was shown to dewater as well as an anaerobically digested sludge.
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