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° to 60° 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.

Hyperion Plant

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.

Blue Plains WWTP

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° C batch requirement.

Columbus Water Works

Columbus Water Works (CWW) in Columbus, Ga., has developed and patented a PFRP equivalent called Columbus Biosolids Flow-Through Thermophilic Treatment, or CBFT3. 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 CBFT3 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 CBFT3 process combines a continuous-feed digester followed by a 30-minute or equivalent batch such as a plug-flow digester. The CBFT3 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).

CBFT3 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°/60° C CBFT3 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°/55° C mode.

On Nov. 1,2005, at the WEFTEC conference in Washington, D.C., CWW gave its patent for the CBFT3 process to the Water Environment Research Foundation for maintenance in the public domain in order to make it available to all utilities.