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Upgrading to Class A anaerobic digestion: is your biosolids program ready to make the move?

More utilities are looking at producing Class A biosolids from municipal wastewater sludges as a way to reduce their regulatory requirements and improve their program's image with the public. The question is, what is the best way to convert an existing Class B biosolids digestion system into a Class A program?

Instead of abandoning their current processes and investing heavily in new technology, some utilities are successfully retrofitting and adapting their existing mesophilic anaerobic digestion systems to produce Class A product. Using this approach, it is possible--depending on the size of the treatment plant--to construct a new Class A system for a slightly larger investment than conventional high-rate mesophilic digestion.

There are currently four plants in the United States producing Class A biosolids using thermophilic anaerobic digestion, with another five either due to come online soon or slated for future upgrades. Utilities have converted their anaerobic digestion operations from mesophilic temperatures to temperatures in the thermophilic range (50[degrees] to 60[degrees] C) over the past five to 10 years for a variety of reasons, including increased volatile solids destruction or overall process capacity and improved pathogen destruction.

While there is no dispute about improvements in pathogen destruction at the higher temperatures when maintained for a given contact time, the ability of these converted systems to produce biosolids meeting Class A pathogen requirements--as defined in the 40 CFR, Part 503 regulations--has been addressed on an application-by-application basis.

While Part 503 provides six alternatives for achieving Class A pathogen destruction, the most commonly used alternatives for thermophilic anaerobic digestion have been Alternative 1--Time and Temperature; Alternative 3--Documented Virus and Helminth Ova Destruction; and Alternative 6--Treatment with Processes that are equivalent to a Process to Further Reduce Pathogens (PFRP).

In general, Alternative 1 requires the least amount of additional investigation and proof of efficacy. Conversely, Alternative 1 typically requires the most conservative combinations of time and temperature batch operation of the three alternatives. It also is not surprising that the earliest plants that achieved Class A operation did so using Alternative 1.

More recently, plants have used full-scale operating data to obtain U.S. Environmental Protection Agency Regional Office approval to operate Class A systems by qualifying under Alternative 3. Often, full-scale testing does not fully demonstrate the process' effectiveness on helminth and virus reduction. As a result, all plants currently qualifying under Alternative 3 have ongoing monitoring requirements that are more extensive than those for Alternative 1. Plants currently operating with Alternative 3 authorization are required to periodically test raw and treated biosolids for helminth and enteric viruses.


Even more recently, utilities have explored qualification as PFRP equivalent using Alternative 6. These investigations have used pilot-scale pathogen spiking studies and process definitions that address scale-up issues. The data collected from the pathogen spiking tests have shown that Class A performance can be achieved reliably using less conservative combinations of batch time and temperature. While no Class A plants are currently operating under Alternative 6, some are under design or construction at this point.


Four wastewater treatment plants (WWTPs) that are either operating or under development as Class A--in Chapel Hill, N.C.; Los Angeles; Columbus, Ga.; and Washington, D.C.--provide illuminating examples of thermophilic anaerobic digestion system upgrades.


In California, Los Angeles' Hyperion WWTP was required to upgrade to a Class A system by Jan. 1, 2003, in order to maintain its biosolids land application program in Kern County. The Hyperion Class A system has been operating under an Alternative 3 arrangement via EPA's Region 9 since the end of 2002. The fully thermophilic operation has batteries of thermophilic digesters operating as a first stage of digestion. The continuous discharge from the first stage is cycled through four second-stage digesters operated in fill/hold/hold/draw modes. The second-stage digesters' operating mode cycles every eight hours.

In addition to meeting the EPA's process requirements, Hyperion must monitor for additional compliance criteria. As with all operating Class A processes, fecal coliform must always be less than 1000 most probable number per gram total solids, as monitored on a weekly basis. In addition, monthly enteric virus samples must be less than 1 plaque forming unit per 4 grams total solids and helminth ova must be monitored on a monthly basis. Every other month, samples prior to digestion are tested. In the other months, digested biosolids are tested. The viable helminth ova concentrations after the treatment process must be less than 1 per 4 grams total solids.

Los Angeles also is developing testing plans to certify the Hyperion operation as a PFRP equivalent under either Alternative 1 or Alternative 6. Under Alternative 1, the plant may be capable of achieving the required time-and-temperature batch times once thickening enhancements are brought online in late 2006 or 2007. Through further thickening, the volume fed to digestion will be reduced to achieve batch times in excess of 20 hours. Under Alternative 6, in the case of developing an equivalent PFRP, the city would prove that batch times considerably shorter than Hyperion's current 16-hour batch could accomplish the required levels of pathogen reduction. If pursued, the testing would be performed in 2006, with the EPA's Pathogen Equivalency Committee (PEC) negotiation following test completion.


The Blue Plains Advanced WWTP, operated by the District of Columbia's Water and Sewer Authority (DCWASA), is adding the world's largest egg-shaped digesters to its skyline with plans to produce Class A biosolids using a combination of Alternatives 1, 3, and 6. DCWASA will convert the biosolids production at the Blue Plains Advanced WWTP from lime stabilization to anaerobic digestion within the next five to six years. The facility will consist of eight 4 1/2 million-gallon egg-shaped digesters and four 2 1/2 million-gallon silo-shaped digesters.

The facility also will use a temperature-phased digestion flow pattern with two options for meeting Class A criteria. Under the preferred Class A operating option, DCWASA will use a flow-through configuration that consists of multiple thermophilic digesters in series to approximate plug flow and eliminate short-circuiting.

The egg-shaped digesters also may be operated in a manner satisfying the time-and-temperature requirements. Under this mode, three silo-shaped digesters would be operated in fill/hold/draw mode in order to satisfy an 18-hour, greater than 56[degrees] C batch requirement.


Columbus Water Works (CWW) in Columbus, Ga., has developed and patented a PFRP equivalent called Columbus Biosolids Flow-Through Thermophilic Treatment, or [CBFT.sup.3]. The utility has undertaken pathogen-spiked pilot-scale tests to document pathogen destruction at time-and-temperature combinations that are considerably less conservative than those defined by the Alternative 1 time-and-temperature equation.

The [CBFT.sup.3] process also takes advantage of the pathogen destruction that occurs in non-batch thermophilic digesters. As with all PEC-approved equivalent PFRPs, compliance with a bacterial indicator test (either fecal coliform or salmonella) is still required by the Part 503 regulations.


The [CBFT.sup.3] process combines a continuous-feed digester followed by a 30-minute or equivalent batch such as a plug-flow digester. The [CBFT.sup.3] implementation at the South Columbus Water Resource Facility will use a plug-flow reactor to achieve the 30-minute batch requirement; the Class A process will be followed by mesophilic digestion (resulting in a temperature-phased digestion configuration similar to Blue Plains).

[CBFT.sup.3] process development has included Ascaris- and poliovirus-spiked, laboratory-scale testing. The PEC has required demonstration of a 2-log reduction of viable helminth ova density and a 3-log reduction of enteric virus density to prove equivalency. CWW has received a confirmation from the PEC that the laboratory-scale work demonstrated a PFRP equivalent operation. As of today, the PEC has granted conditional PFRP equivalency to the 53[degrees]/60[degrees] C [CBFT.sup.3] process. As a condition of the full-scale operation, CWW will be required to monitor the finished biosolids for helminth ovum and enteric viruses in a manner similar to the requirements for Hyperion. The utility intends to accelerate virus testing after start-up in order to remove the condition for equivalency designation at the earliest opportunity. Discussions and negotiations with the PEC are ongoing regarding equivalency criteria for operating under the 55[degrees]/55[degrees] C mode.

On Nov. 1, 2005, at the WEFTEC conference in Washington, D.C., CWW gave its patent for the [CBFT.sup.3] process to the Water Environment Research Foundation for maintenance in the public domain in order to make it available to all utilities.


The Mason Farm WWTP in Chapel Hill, N.C., operated by the Orange Water and Sewer Authority (OWASA), was perhaps the first treatment plant in the United States to be brought online with a configuration to produce Class A biosolids using thermophilic-anaerobic digestion. OWASA originally investigated configurations that would have qualified as an equivalent PFRP before settling on a time-and-temperature configuration.

In the mid-1990s, the EPA's PEC denied OWASA's initial application for a 51[degrees] C operation. The basis for that application was mathematical modeling of fecal coliform kinetics in the reactors, with rate constants consistent with the Part 503 time-and-temperature equation. The application was denied based principally on three factors: a lack of empirical data supporting the process description, perceived likelihood of pathogen short-circuiting in the described operational scheme, and concern over the ability of a 51[degrees] C process to adequately inactivate Ascaris.

OWASA subsequently built the facilities based on a plan to operate the process near 56[degrees] C. This allowed the second reactor, with a greater than 20-hour batch time, to reliably provide the necessary batch time to meet the Part 503 time-and-temperature requirements and qualify under Alternative 1. Most plants that have considered time and temperature options have gravitated to operational criteria near 55[degrees] C, with approximately 24 hours of batch detention time. This is due largely to the operational stability of thermophilic systems at temperatures in the 55[degrees] to 56[degrees] C range, while allowing the batch times to be maintained at practical times and volumes.

In the Mason Farm Class A digestion system, the thermophilic stages are followed by a mesophilic stage creating a temperature-phased system and increasing volatile solids reduction and gas production. The additional gas produced at OWASA is used to fuel an engine-driven aeration blower.

The digestion system at OWASA was started up in late 2000 and reached stable, Class A operation in the spring of 2001. The plant currently is undergoing an upgrade from 12 to 14 1/2 mgd capacity. As part of that project, the digestion system is being enhanced to eliminate odors and improve operational reliability.

--Willis is a vice president and southeast wastewater practice leader with Brown and Caldwell Consulting Engineers in Atlanta. Schafer is a vice president and Brown and Caldwell Consultants' National Biosolids practice leader in Sacramento, Calif.

Portions of this article were part of a 2005 WEFTEC presentation titled "The State of the practice of Class A anaerobic digestion: Update for 2005."

By John L. Willis, P.E., DEE, and Perry Schafer
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Title Annotation:Wastewater treatment / Biosolids reuse
Author:Willis, John L.; Schafer, Perry
Publication:Public Works
Date:Jan 1, 2006
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