Does Biosolids Quality Improve During Storage?
Regrowth of pathogens in stored biosolids and the resulting odors are an ongoing cause for concern among utility operators. Any techniques that can help reduce and/or control pathogens are a welcome addition to biosolids management, especially if the techniques are simple ones.
Two recent studies report research that shows that temperature modification alone can result in a reduction in fecal coliform (FC) and E. coli growth in biosolids which were exposed either to freeze-thaw conditions (Meingast) or temperatures above 25 degrees C (Fane). Both of these techniques present promising, low-tech opportunities for control of indicators and pathogens in stored biosolids.
In the article by Meingast et al, the issue of over-wintering of biosolids was addressed and studied. Since winter land application of biosolids is not always an option in some areas of the country, biosolids are often stored until the spring. A concern about storing treated biosolids is the opportunity for regrowth of pathogens, resulting in a product that is no longer considered Class B and which produces increased odors. However, a preliminary study that monitored FC numbers through winter storage found that culturable FC levels decreased significantly after the stored biosolids went through cycles of freezing and thawing. Meingast’s field study of biosolids that were stored indoors over the winter showed that FC numbers dropped to non-detectable levels at all depths of biosolids that underwent FTCs (freeze thaw cycles). Follow up lab studies found that just one episode of freeze-drying biosolids at -20 degrees C for 48 hours, followed by a 24 hour thaw, resulted in a reduction of FCs of up to 93%. Since the moisture content of biosolids from WWTPs can vary significantly due to different dewatering techniques, the researchers also tested various TS samples (total solids). They found that freeze drying biosolids with TS in the range of 12-20% produced the greatest reduction in pathogens. Upon further analysis it was found that FTCs controlled FC population by damaging the cell walls, cytoplasm, and DNA of non-surviving FC cells.
They suggest that further studies are needed to: (a) confirm the ideal range of temperature and moisture content of biosolids needed to optimize pathogen destruction, (b) confirm FC reduction with one FTC versus multiple FTCs, and (c) test the effect of FTCs on other indicator organisms besides FC.
The second study by Fane et al took place during the summer months and addressed the issue of regrowth by focusing on E. coli resurgence in dewatered biosolids cake storage. They presented evidence showing the effect of modifying the temperature in the biosolids storage environment. Raising the temperature of the stored biosolids above 25 degrees C resulted in a reduction of E. coli numbers. This was confirmed by both field studies and lab experiments. The field trials compared biosolids kept in storage areas that were uncovered with those that were kept in insulated covered conditions. The biosolids were simply covered with tarps, black in color, in order to absorb as much heat as possible during day light hours. The success of this pilot trial may be shown to decrease indicator organisms to a level that meets industrial compliance requirements. The researchers suggested that further work should be done to understand all characteristics affecting E. coli growth. If E. coli is going to be utilized as an indicator organism, then a more comprehensive data set of environmental precursors to the growth of E. coli is the key to understanding, and improving pathogen control.
Fane states; “This study presents evidence of the effect on fecal indicator bacteria of the modification of temperature in the biosolids storage environment. (>25 degrees C).” The treatment consisted of biosolids stored in the field, covered with a black tarp, which allowed the temperature to rise. Trials were conducted in summer months for a period of 28 days.
“Effects of Biosolids Moisture Content on Indicator Organism Reduction Via Freezing and Thawing” Christa L. Meingast, Jennifer G. Becker, Eric A. Seagren, Michigan Technological University, WEFTEC 2015
“ Modification of Physical and Chemical Parameters in the Biosolids Storage Environment Contributing to E. coli Die-Off “ Sarah Fane1, Andreas Nocker1, Andrea Wilson2, Elise Cartmell1, Monica Rivas Casado1, Sean Tyrrel1 WEFTEC 2015
1.)Water Science Institute, School of Energy, Environment and Agrifood, Cranfield University, Bedford, MK43 0AL, UK
2.)Severn Trent Water Plc., Severn Trent Centre, St. John’s Street, Coventry, CV1 2LZ, UK
 E Coli is used as an indicator organism and considered as a surrogate for levels of pathogens within the final biosolids product before land application - Sidhu and Toze, 2009 per Fane)
 It is theorized that the FTCs damaged the cytoplasmic membrane, cell wall, and/or DNA in cells that did not survive (Gunnarsdóttir et al., 2012 per Meingast)
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