Biosolids Safety and Odor Research Underway

Article for Keystone Water Quality Manager, the PWEA Magazine

In July 2009, WERF embarked on a new study addressing the problem of regrowth, odors, and sudden increase (ROSI) after anaerobic digestion. The two-year research project, led by Matthew J. Higgins, Ph.D., Department of Civil and Environmental Engineering, Bucknell University, and Sudhir N. Murthy, Ph.D., P.E., District of Columbia Water and Sewer Authority, is entitled, “Wastewater Treatment Plant Design and Operation Modifications to Improve Management of Biosolids Odors and Sudden Increases in Indicator Organisms” (WERF SRSK4T08).   The study is evaluating current mesophilic and thermophilic anaerobic digestion processes; E. coli quantification methods; and persistent odors.


The importance of this study can’t be underestimated.  Two of the most significant public acceptance issues facing biosolids managers are odors and the perceived “safety” of biosolids. Odors create nuisance conditions, resulting in local opposition to biosolids recycling programs and the perceived potential risk of pathogens in biosolids also can generate public opposition.


Over the last few years, tests at several anaerobic digestion facilities have shown that densities of indicator bacteria, fecal coliform, in biosolids are very low or undetectable after thermophilic anaerobic digestion but, immediately after digested biosolids are dewatered using high-solids centrifuges, the densities often increased by 100-10,000 times, a phenomenon known as “sudden increase”.


A WERF study in 2005 suggested that bacteria entered a non-culturable state during thermophilic treatment. This meant that the bacteria would not grow on standard culturing media when tested, so the actual concentration of viable indicator organisms was underestimated. Then, under certain conditions (mainly centrifuge dewatering), the bacteria “reactivated,” meaning they became culturable again. Once reactivated, the bacteria could grow during cake storage and further increase in density, a process called “regrowth”.  The sudden increase and/or regrowth of indicator bacteria can result in levels above regulatory limits for Class A and Class B biosolids at the time of use or disposal.  


Sudden increase generally has not been observed in mesophilic digestion processes after centrifuge dewatering, although regrowth during cake storage has been measured often in plants with centrifuge dewatering.  Sudden increase has not been observed for Salmonella, for thermophilic, Class A processes.  And while sudden increase has not been observed for Salmonella after mesophilic anaerobic treatment, regrowth of Salmonella has been observed during cake storage with mesophilic systems.


This important research project is broken into two tracks. Phase I is focused on filling critical research gaps that will provide fundamental insights for developing monitoring, laboratory methods, and mechanisms for needed solutions for controlling ROSI .  The initial research in 2009 and 2010 focused on finding an efficient and appropriate method to culture the indicator organisms in the quantities needed, as well as evaluating odorants and the mechanisms for their production. 


Phase II is focused on developing, implementing and evaluating solutions to ROSI.  As part of this research effort, new approaches and solutions are being field tested where biosolids are land-applied.  This phase began in summer 2010 and builds on approaches and methods already in use by the project team for related projects and from earlier research.


Several treatment methods have been tested by adding amendments directly to the dewatered biosolids cake for both odor and regrowth control. Two of the amendments were able to significantly reduce odors and/or regrowth. Both are natural products that could be used in full-scale applications. Additional testing is underway to explore the use of these chemicals, but preliminary results look promising.  In one test, incubation of anaerobically digested biosolids with protein-degrading enzymes yielded the positive effect of additional usable methane gas production as well as a reduction in odors after dewatering.


Field testing and evaluation of the new operational methods is expected to begin in 2011. Here is where WERF needs your help.  This project depends on the support of WERF subscribers and of wastewater treatment plants with anaerobic digestion processes. Data from these facilities is needed, including biosolids samples. Who is eligible?  Any utility that operates anaerobic digesters followed by high solids centrifugation may be considered for inclusion in this research.  Each plant that is sampled will be thoroughly documented in terms of design criteria and operational protocol. This information will then be used to develop design criteria and operating procedures that will avoid or minimize ROSI.


Utilities may participate in either a baseline characterization or the solution implementation level. 


  • Baseline- Characterization of your processes and biosolids in terms of ROSI using the standardized protocols developed through WERF research. 
  • Solution Implementation- After the baseline characterization, WERF could implement appropriate pilot and full-scale projects at your plant with the goal of mitigating biosolids cake odors, regrowth and/or sudden increase.


The benefits of participating now;

  • Participating utilities will learn how their biosolids characteristics compare to similar plants.  All the procedures will be uniform so the results will provide a true benchmark.
  • If the baseline study shows a need for improvement, participants will learn what can be done to mitigate ROSI.  Also, the samples will be coded to preserve anonymity.
  • The WERF ROSI project will share some of the costs with the utility.


Wastewater treatment plants interested in participating in the ROSI study, may contact Dr. Matthew Higgins at


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