Recently, a WERF-sponsored research study addressed the challenge of developing effective ways for utilities to manage the issues of regrowth, odors, and sudden increase (ROSI) of indicator and pathogenic bacteria in biosolids. A draft of the study’s final report entitled, “Wastewater Treatment Plant Design And Operation Modifications To Improve Management Of Biosolids Regrowth, Odors And Sudden Increase (ROSI) In Indicator Organisms” (WERF SRSK4T08), was released on June 8, 2012. The principal investigators for this project were Matthew J. Higgins, Ph.D. from BucknellUniversity and Sudhir N. Murthy, Ph.D., P.E. from DC Water. Garvey Resources was one of several team members who worked on this project.
A previous WERF study in 2005 had suggested that, after thermophilic treatment/anaerobic digestion, some indicator organisms in biosolids were not destroyed but had actually entered a non-culturable state. It was postulated that this “suspended animation” could be reversed by centrifuge dewatering, resulting in a “sudden increase” (SI) in bacterial densities to a level often exceeding regulatory requirements for acceptable levels of Class A biosolids.
Based on an additional observation that SI occurs with fecal coliform (a primary indicator organism) but not Salmonella, the new testing focused on E. coli, the main fecal coliform organism. The researchers found that tests using higher temperatures and longer time frames than are currently recommended were able to eliminate SI. Indicator organisms were much more robust than actual pathogens.
The other focus of the current ROSI study is on reducing odors, which are mainly associated with primary sludge, rather than waste activated sludge. The main persistent odorants in centrifuged biosolids were compounds identified as the breakdown products of organics such as proteins, carbohydrates and fats. It was suggested that eliminating these precursors would help reduce odors. Field tests in which several amendments were added to the solids either during or after the dewatering process resulted in significantly reduced odors with the added benefit of reduced regrowth in the biosolids. Also, incubating digested biosolids with protein degrading enzymes reduced odor after dewatering. This technique had the added benefit of an increase in gas production, which could be recovered for reuse. However, these results are preliminary and more research is needed in this area.
The beneficial reuse of biosolids is often a key component in biosolids management plans. The demand for beneficial reuse methods is expected to grow as wastewater treatment plants incorporate more sustainable practices into their management plans. With this in mind, the recently reported results of the ongoing WERF study addressing the problem of ROSI after anaerobic digestion are important for biosolids managers who are dealing with issues of public acceptance, especially in the areas of odor control and the perceived safety risk of biosolids reuse. The researchers hope is that the newly developed approaches can be easily implemented for better biosolids management.
NOTE: This blog was based on the final report of the WERF ROSI study. At the time of publication of this blog, the final report may be available online. To view the final report, go to www.werf.org