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Fecal Coliform Reductions in Mesophilic Anaerobic Digestion are NOT What You Might Think!

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The U.S. Federal regulation, 40 CFR Part 503, controlling application of Class B biosolids to the land, requires that biosolids that are anaerobically digested must either conform to the specific requirements of a “Process to Significantly Reduce Pathogens” (PSRP) or, if not, to have a fecal coliform (FC) density less than 2 million CFU or MPN per gram of total solids. An extensive and thorough survey by Stukenberg et al. (1994) indicated that few facilities practicing mesophilic anaerobic digestion failed to meet one or the other of these standards. These authors estimated that the average FC reduction was two logs (a 100-fold reduction). A designer or plant operator might be led to believe that if the FC density in his typical primary solids-waste activated sludge (WAS) mixture was under 200 million MPN or CFU per gram, well-operated single-stage mesophilic digestion would meet the regulatory standard without difficulty. Unfortunately, a close look at available information indicates that this may not be the case.

Stukenberg et al. reported FC reductions for 47 digestion systems, and observed that the average FC reduction was 2 logs. Excluding three systems that utilized two heated digesters in series, 22 out of 44 digestion systems achieved less than a reduction of 2.0 and 14 had reductions less than 1.5. The extensive nature of the study did not allow the authors to probe into all the reasons for the high and low reductions. They were able to observe that systems with two heated stages of digestion in series produced superior FC reductions. The authors observed that about half of the systems were not single stage but actually were two-stage, but in all but five cases, the second stage was not heated. The authors did not separate the results on the basis of the presence or absence of a second unheated stage. Since it is well-known that simple storage of biosolids reduces fecal coliform densities, the log FC reduction of a single stage of anaerobic digestion can be expected to average below 2.0 by an unknown amount.

In the present investigation, close scrutiny was made of published and some unpublished extensive studies carried out to establish fecal coliform reductions in single and two-stage mesophilic anaerobic digestion. Close reading of some widely cited papers revealed that fecal coliform reductions were reported on a volume basis. Since solids content is reduced in digestion, the fecal coliform reductions reported were substantially greater than they would have been if they had been reported on a mass basis. Considering all available data, a conservative estimate of the expected log FC reduction of no more than 1.5 logs seems appropriate.

The fundamental factors that produce reductions of fecal coliform density are considered. Estimates are made of the effect of temperature and residence time, based on information available in the literature. The conclusion is drawn that, outside of converting a digester to the thermophilic mode, probably the most effective procedures for increasing fecal coliform reductions are to use two (or more) digesters in series, convert to draw-and-fill feeding once a day, or follow single-stage digestion with anaerobic storage.
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Document Type: Research Article

Publication date: 01 January 2006

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