Coliform Reactivation

Organizations Respond Quickly to Evidence of Coliform Reactivation

According to a recent Water Environment Research Foundation (WERF; Alexandria, Va.) study, fecal coliform counts can increase dramatically in anaerobically digested biosolids, but the reason is unclear.

Utility managers, microbiologists, design engineers, policy analysts, and regulators came together to discuss this unexpected phenomenon at a special technical session at WEFTEC®.06 in October.

“Basically, what happens is when you come out of digestion and go into a dewatering device, you have one density of fecal coliform and then, immediately after dewatering, that level increases by one, two, three, or four orders of magnitude,” said Matthew Higgins, associate professor in the Department of Civil and Environmental Engineering at Bucknell University (Lewisburg, Pa.), and a principal investigator for the WERF study.

Because the bacteria cannot reproduce that quickly, researchers have concluded that this phenomenon is due to the reactivation of bacteria. That is, the fecal coliforms were always present in the samples but were invisible to standard culturing methods until dewatering somehow spurred them to function fully.

Higgins described several mechanisms that could account for this nonculturable state. The first theory is that the bacteria could have been injured but not destroyed during digestion and that dewatering enabled them to repair themselves and become active again. The second theory points to the bacteria entering a viable but nonculturable state; this state would be a defense mechanism to the conditions of the digestion process. The third theory, Higgins explained, is that an inhibitor present in the digested solids is removed by the dewatering process.

Whichever of these mechanisms is at play, dewatering, frequently centrifuge dewatering, often triggers the bacteria to reactivate.

Fecal coliform concentrations are an integral part of the U.S. Environmental Protection Agency (EPA) Part 503 regulations that govern biosolids treatment and use. Class B and A biosolids must contain fewer than 2 million colony forming units of fecal coliform per gram of dry solids and less than 1000 CFU/g dry solids, respectively, at the time of use or disposal to comply with the pathogen criteria of the Part 503 regulations.

However, the regulations also include process-based equivalents for Class A and B that require no subsequent testing. In these cases, the observed reactivation and regrowth could push the fecal coliform concentrations above the allowed levels unbeknownst to utilities.

Molecular Methods
To find out how many nonculturable fecal coliform bacteria are present in digested solids, the researchers used polymerase chain reaction (PCR) testing to count Escherichia coli cells. PCR enumerates bacteria based on copies of DNA present, and E. coli is one of the main fecal coliform detected in culturing methods, Higgins said.

The testing showed that anaerobic thermophilic digestion followed by centrifuge dewatering led to reactivation most consistently and with the greatest magnitude of increase, Higgins said. About 87% of samples dewatered by centrifuge showed reactivation.

However, substantial increases in fecal coliform and E. coli density can occur after any type of dewatering of anaerobically digested biosolids. Such reactivation occurs about 70% of the time, according to WERF’s findings.

Regulatory Effects
At the technical session, Mark Meckes, an EPA microbiologist, stated that much more work needed to be done before any actions were taken. “We still feel that the 503 [regulations] are appropriate and protective,” Meckes said. “There’s a lot more research necessary in this area if we are to discern the public health impacts.” However, he added, that as data comes to light, EPA will make changes to the part 503 rules as needed.

Meckes emphasized that reactivation and regrowth have only been seen in bacterial indicator organisms, not pathogens. Moreover, he said that this issue is strictly a bacterial one. Viruses, protozoa, and other monitored organisms require a living host to multiply. Meckes said the chances of those pathogens reactivating or regrowing after digestion are “slim to none.”

He also noted that reactivation has not been found at every plant using anaerobic thermophilic digestion with centrifuge dewatering. “Don’t jump the gun and assume you have a problem because you have that digestion process,” Meckes added.

Quick Response
To provide a level basis for seeking answers, the Water Environment Federation (WEF; Alexandria, Va.) in September issued a technical practice update (TPU) on reactivation and regrowth issues. The TPU includes an overview of related regulatory issues for biosolids stabilization and testing protocols, recommendations for communication strategies, and a description of known methods to address reactivation and regrowth.

“There was a lot of concern that once [WERF’s] report was released there could be some backlash in the biosolids community from a lot of the opponents of biosolids,” said Rhonda Bowen, recycling manager at Hampton Roads Sanitation District (Virginia Beach, Va.) and a co-chair of the WEF subcommittee that compiled the TPU.

Even though it might open a big can of worms, asking these tough questions is the only way to ensure that land application remains a viable option, said Chris Hornback, senior director for regulatory affairs at the National Association of Clean Water Agencies (NACWA; Washington, D.C.).

“The court of public opinion rules the day, and that’s ultimately where we will be judged on our response to this situation,” Hornback said.

To that end, the speed and extent with which the initial data was shared with stakeholders and the ongoing commitment toward research and communication are almost as important as the research results itself, Hornback added.

Hornback applauded WEF, WERF, and NACWA for moving so quickly and disbursing the information so widely.

“In fact, this project and our industry’s response, I think, have highlighted what we can accomplish when we all work together,” he said.

Mitigation Strategies
There are steps that treatment facilities can take to either avoid the reactivation or mitigate it. John Willis, vice president of Brown and Caldwell (Walnut Creek, Calif.), suggested four options for Class B systems. The first is land application of liquid biosolids, as there has been no documented reactivation or regrowth without dewatering, he said. Second, he suggested switching to a different dewatering technology, such as belt filter presses. Third, some form of post-dewatering treatment, such as lime addition, can be used to keep coliform levels in the acceptable range. Last, he suggested simply storing the dewatered solids until the fecal coliform reactivate and regrow and then die off.

To prevent reactivation and regrowth for Class A systems, he suggested keeping the solids hot from digestion through land application, using liquid land application, switching dewatering technologies, dosing dewatered biosolids with lime, and exploring multistage Class A thermophilic digestion. To date, systems with multistage anaerobic thermophilic digestion and centrifuge dewatering have shown no reactivation and regrowth, he said.

Ongoing Research
At press time, the second phase of research was expected to be completed by the end of the year, with a report on the findings due in spring 2007.

The objectives of the second phase are to verify and to better understand reactivation and regrowth and to develop mitigation strategies by answering the following questions:

What combinations of wastewater process technologies lead to this observed increase in fecal coliforms?
What specific process designs and operating conditions contribute to this increase in fecal coliforms?
What are the mechanisms for reactivation?
What conditions contribute to regrowth?

The study report, Examination of Reactivation and Regrowth of Fecal Coliforms in Centrifuge Dewatered, Anaerobically Digested Sludges (03-CTS-13T), was published in June. It can be obtained by searching WERF’s publication database at www.werf.org. The WEF TPU can be obtained through the WEF online bookstore at www.wef.org/marketplace. The TPU, free to WEF members, costs $55 for nonmembers.

— Steve Spicer, WE&T
http://www.wef.org/ScienceTechnologyResources/Publications/WET/
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