Increases in Indicator Bacteria Densities after Digestion and Dewatering
In the last few years, a number of utilities have reported substantial, sudden increases in fecal coliform and/or E. coli densities in anaerobically digested biosolids immediately after dewatering. This sudden increase has been termed “reactivation”. Storage of
the cake samples results in further increases in the indicator densities, with peak densities approaching 107 to 108 organisms per gram dry solids (DS). This additional increase during storage has been termed “regrowth”. The reactivation and regrowth phenomenon
has been found to be more prevalent in plants that utilize centrifuge dewatering. A number of different process types have reported reactivation and regrowth, including mesophilic, thermophilic, temperature phased anaerobic digestion, pre-pasteurization, and autothermal aerobic digestion.
The sudden increase is generally low in mesophilic anaerobic digestion (MAD) processes, with typical increases in densities of 0–1 order of magnitude being reported. However, during regrowth, the density can increase by another 1–2 orders of magnitude. For thermophilic and Class
A processes, the densities have increased up to 5 orders of magnitude when comparing pre- and post-dewatering samples, with additional increases of several orders of magnitudes during regrowth. Plants utilizing belt filter press dewatering have not reported reactivation and regrowth to the
extent that plants with centrifuge dewatering have reported.
The reasons for the sudden increase are not clear, but several proposed theories include: resuscitation of non-culturable bacteria; centrifuge contamination; and release of bacteria due to floc-breakup/shear. On-going work
is examining these different mechanisms to better understand this problem. Regrowth appears to be due to the release of substrate during the dewatering process which supports bacterial growth. The shear imparted to the floc releases biomolecules that can be utilized for growth. An interesting
parallel to this work is research on odors produced by cakes. This research has shown that shear during centrifugation releases protein, which supports bacterial growth and the biodegradation of this protein leads to the production of odorous compounds. Odors produced by belt filter press
cakes have been shown to be much lower compared to centrifuge cakes.
The increases in indicator bacteria are a concern since regulatory requirements for Class A and B biosolids utilize, in part, indicator bacteria limits. From a public health perspective, pathogen reactivation and/or
regrowth is a significant concern. On-going research is examining whether pathogens are also being reactivated and are regrowing, and data will hopefully be available in the form of a WERF report during the summer of 2007.
For additional references on this topic, please see the attached
list of references.
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