where X0 and X20 are, respectively, the initial and final (after 20 days) volatile solids concentrations found within the reactor. However, despite this theoretical level of performance, 503 Rule regulations require solids retention times (SRTs) of 40 days at 20°C and 60 days at 15°C before the product of this mesophilic aerobic digestion process is considered suitable for widespread use as a soil amendment based on desired VSS destruction. Here again, the latter increase in SRT reflects the fact that metabolic solids degradation slows down at colder temperatures.

Yet another important factor with aerobic digestion is that of its higher energy requirement, for both mixing and aeration, vs. that of anaerobic digesters, which not only require far less energy for mixing but also generate an energy-rich methane gas product. Aeration and mixing within an aerobic digester must therefore be provided using either mixers that entrain oxygen mechanically or compressed air blowers that diffuse oxygen into these tanks through bubble transfer. Mixing intensities with mechanical mixers are usually designed to provide 10 to 100 W/m3 (0.4 to 0.5 hp per 1000 gal), while the diffused aeration systems are typically sized to provide 20 to 40 m3/min of air per 1000-m3 tank volume.

As mentioned previously, one variation to conventional aerobic digestion that has recently drawn considerable attention due to its inherent ability to pasteurize sludge effectively is that of the autothermal thermophilic aerobic digestion (ATAD) process, by which the heat released exothermically from the digesting sludge naturally causes the reactor temperature to rise into the thermophilic range (i.e., to values typically between 55 and 65°C, and sometimes well in the 70s). Federal regulations specifically stipulate a holding time of only 24 hours for those systems able consistently to maintain a temperature at or above 55°C.

In comparison to standard mesophilic operations, these ATAD systems are able to take advantage of a considerable increase in the available rate of endogenous decay, with values believed to be many times greater than those experienced at mesophilic temperatures. A properly designed and operated ATAD system does not need external heating. Heat is provided by the exothermic biodegradation reaction. A clear advantage of these ATAD systems is that their solids loading rates are considerably higher than either the mesophilic aerobic or anaerobic options, with values typically in the range 4.5 to 5 kg TSS/m3 • day. In turn, the design and operating SRT values used with ATAD systems can be decreased sizably, to values of a few weeks and possibly even less.

Example 16.6: Preliminary Autothermal Thermophilic Aerobic Digester Design Due to the volume estimated for the standard anaerobic digester system in Example 16.5, yet another request is then presented to the Deer Creek, Illinois, consulting engineer to prepare a second preliminary sludge processing design for a more advanced, and assumedly smaller, autothermal thermophilic aerobic digester. This ATAD design makes use of all four assumptions regarding sludge production and character (that were presented in Example 16.5).

Preliminary Design Details

Sludge solids mass and volume, reactor sizing, and retention time

Related notes

Estimated daily total sludge solids mass: 151.43 kg TSS/day Estimated raw wet sludge volume: 2.5 m3/day Design anaerobic digester solids loading rate: 4.8 kg TSS/m3 • day Design ATAD digester volume:

31.5 m3 = (151.4kg/day)/(4.8kg/m3 • day) Design anaerobic digester HRT:

1. ATAD loading rates are sizably higher than those of even the high-rate anaerobic digester, and as a result these systems will require a distinctly smaller volume (i.e., in this case, the projected ATAD sizing is only one-third that of the anaerobic digester).

2. ATAD solids loading rates are generally based on the total vs. the volatile solids content of the sludge.

3. Prethickening of the incoming to sludge to values at or above 6% total suspended solids levels is very important with ATAD systems in order to secure autothermal (i.e., self-heating) operations.

4. Here again, this single-reactor design does not afford the desired system redundancy associated with multiple units.

To generate the level of heat output required to incur this temperature increase (i.e., raising the reactor temperature autothermally), the incoming total solids content of the raw sludge supply must be routinely prethickened (using thickeners, gravity belt thickeners, etc.) to values of 6% or higher. The corresponding density of these solids, and the fact that oxygen solubility drops considerably at higher temperatures, presents a distinct challenge in terms of providing the oxygen necessary to maintain aerobic conditions without unacceptably stripping heat away from the reactor at a rate that would negate the thermophilic condition desired. However, high-efficiency aerators (e.g., jet-type mixing units) have proven to be a suitable aeration technology for this type of application.

Despite the apparent technical benefits afforded by ATAD processing, a number of important operating details have yet to be fully resolved. First, instrumentation for measuring dissolved oxygen levels at these thermophilic operating temperatures has only recently been developed and has limited field experience. Preliminary testing has therefore been conducted using measurement of oxidation-reduction potential (ORP) as an alternative indication of the apparent aerobic nature of these reactors, and it does appear that holding this parameter within an approximate range of about —50 to —350 mV can subsequently be used to regulate aeration rates in a fashion that will obviate, or at least minimize, undesired shifts toward fully anaerobic conditions (with ORP dropping much below —400 mV), especially following intermittent loading events. Second, excessive foaming events have been observed at a number of early full-scale plants. Here again, this phenomenon is not fully understood, but it does appear that low-SRT operations are particularly prone to this problem, perhaps due to load-related swings in the cyclic level of soluble proteins. On the one hand, a limited amount of foam (i.e., ~10 to 30 cm) is actually beneficial, in that it helps to provide an insulating blanket across the top of the tank. However, excessive foam production can lead to undesirable reactor overfoaming conditions, to the point where foam-cutter (essentially a coarsely toothed disk rotating just above the desired foam height) or foam-aspirator mechanisms are used to minimize this condition. Finally, it is important to note that the microbial behavior, temperature range, and environmental circumstance of ATAD processes is equivalent to that of bioso-lids composting operations, or at least the interior zones of the actively composting piles, as described in the following section.

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