ADVANCES IN THERMOPHILIC ANAEROBIC DIGESTION

John Willis* and Perry SchaferA * Brown and Caldwell

990 Hammond Drive, Suite 400 Atlanta, GA 30328

A Brown and Caldwell – Rancho Cordova, CA

ABSTRACT Significant advances have been made in the knowledge base and practical options for improving thermophilic anaerobic digestion over the past 10 to 15 years. Today, more options exist that promise to enhance the degree of stabilization achieved and finished product quality in new and retrofitted existing processes. In parallel, the body of knowledge regarding pathogen inactivation in these systems has also been advanced, well beyond that available at the time that the 40 CFR Part 503 regulations1were promulgated. This paper summarizes recent work with thermophilic anaerobic systems. This paper is a literature review and comparison of recent publications and work from a number of sources. KEYWORDS Anaerobic Digestion, Thermophilic, Vector Attraction Reduction, and Pathogen Inactivation. DEVELOPMENT OF THERMOPHILIC ANAEROBIC DIGESTION Thermophilic anaerobic digestion of municipal wastewater sludge has been investigated from the 1920s onward, initial work being completed at laboratory scale. However, full-scale work began by the 1940s and 1950s.

• Garber’s work at the City of Los Angeles in the 1950s and 1970s (Garber et al, 1975)2 was aimed primarily at improved dewatering, but the work indicated process concerns and instability at temperatures above about 49 degrees C.

• Popova and Bolotina (1963) 3identified thermophilic digestion for Moscow (Russia). The primary purpose was better pathogen reduction and operation at lower Solids Retention Time (SRT) than mesophilic digestion.

• Research by Goluecke (1958)4 and Malina (1961)5 was instrumental. • Chicago work (Rimkus et al, 1982)6 at the Stickney Plant showed thermophilic digestion

success at limited SRTs and the ability to reduce digester foaming. By the late 1980s, thermophilic anaerobic digestion was generally thought to achieve greater Volatile Solids Reductions (VSRs) than comparable mesophilic digestion, and perhaps be

#1114

operated at reduced SRTs. However, there was major concern for process stability and concern for product odor. Dewatering recycle characteristics were known to be strong. Pathogen reduction benefits were recognized; however the US EPA had not recognized thermophilic anaerobic digestion as a PFRP – Process to Further Reduce Pathogens. By the 1990s, the situation began to change significantly. Work in Germany showed that thermophilic digestion worked well, especially in combination with mesophilic digestion (i.e., thermophilic, then mesophilic, staged digestion), and that thermophilic temperatures of 55 degrees C provided stable operation. At Vancouver, Canada, the Greater Vancouver Regional District (GVRD) operated its Lions Gate Plant first with single-stage thermophilic digestion, then with 2-stage thermophilic digestion and showed the significant disinfection improvement (and disinfection reliability) with a 2-stage configuration (Krugel et al, 1998). 7 Work by Professor Richard Dague at Iowa State University in the mid-1990s showed that thermophilic operation at 55 degrees C was quite reliable and, in combination with a second stage mesophilic digester, produced a high-quality, well-stabilized product with low odor (Han and Dague, 1996).8 Engineers working for GVRD then designed, in the mid-1990s, the Annacis Island Plant digestion facility using 4-stage thermophilic digestion and modeled the pathogen reductions based on the US EPA’s research work for Class A biosolids that had evolved from development of the Part 503 regulations. By 1999, this facility proved that it could produce a continuous, Class A digested (and dewatered) product that was not odorous and operated in a stable and reliable manner at 55 to 56 degrees C (Schafer et al, 2002). 9 Total system SRT was about 25 days. With the publication of the US EPA’s Part 503 rules in 1993 and especially the time/temperature equations that could be used to produce Class A biosolids, engineers developed various process configurations to meet the requirements. There was some confusion initially about the time/temperature equations until it was recognized that they were developed with the intention that every particle of sludge was to be subjected to the time/temperature requirements (i.e., operated in a batch or plug-flow arrangement). PROCESS OPTIONS The thermophilic process options and configurations for wastewater sludge in the US are largely driven at this time by the desire to achieve Class A digestion and Class A biosolids. The most common approaches that have evolved are summarized in Figure 1, although there are several variations on these concepts. However, some agencies have implemented thermophilic digestion, or more normally the thermo-meso staged arrangement, to achieve greater VSR, more gas production, and a more stable digested product (i.e., not necessarily a Class A objective). Pre-pasteurization of sludge prior to anaerobic digestion, is a proven PFRP process, but is not discussed in this paper because the pre-pasteurization process is usually conducted at higher temperatures (65 to 70 degrees C) which are not conducive to a biological anaerobic digestion

#1114

Figure 1: Options for Class A ThermophilicAnaerobic Digestion Process Configurations

Class A Product

Option 1: Batch digestion with rapid sludge transfer

Class A Product

Option 2: Sequencing batch digestion usingfill/hold/draw rotation through at least 3 reactors

Class A Product

Option 3: Thermophilic digesters in series with not batch(none yet approved in US)

Class A Product

Option 4: Acid/Gas with pathogen destruction in first thermophilic stage

ThermophilicCFSTR

ThermophilicBatch

Fill/Hold/DrawDigester

Fill/Hold/DrawDigester

Fill/Hold/DrawDigester

ThermoCFSTR

ThermoCFSTR

ThermoCFSTR

MesophilicMethane Phase

ThermoAcid-Phase

Digester

process. This paper evaluates only thermophilic anaerobic digestion processes that are successfully operated with a viable thermophilic anaerobic biological population.

For any of the process configurations shown in Figure 1, a mesophilic stage can be added at the end to obtain a product with reduced odor level and a product with greater overall stability. PROCESS AND SENSITIVITY ISSUES Thermophilic digestion is proving to be a robust process even at high loading rates. Thermophilic digesters are often loaded at 0.15 to 0.2 lb VS/cubic foot/day and in some cases are being loaded at higher rates. At Wilhelmshaven, Germany, the thermophilic digester at 55 degrees C (first stage of a thermo-meso staged system) has a 3-day average SRT and loading rates are about 0.7 lb VS/cubic foot/day. Few agencies are willing to push thermophilic anaerobic digestion to this degree, but its successful performance at Wilhemshaven gives credence to claimed high reaction rates and ability to handle very high loading rates (Schafer, 1999). 10

Operating alkalinities for thermophilic anaerobic digestion are somewhat higher than comparable mesophilic digester alkalinities. This is due to higher reaction rates and greater destruction of organic material. Ammonia concentrations are also higher in the thermophilic digesters and there is concern at some facilities (especially plants with very high percentage of WAS feedrate) of achieving levels where ammonia toxicity could occur. Digester pH is often above 7.5 (even approaching pH of 8.0 on occasion) in thermophilic anaerobic digesters, which is higher pH than comparable mesophilic digesters (usually closer to 7.0 to 7.2). Volatile fatty acid (VFA) concentrations are higher in thermophilic, over mesophilic, digesters. VFA levels are often over 500 mg/l (as acetic acid), and levels as high as 1000 mg/l are not

#1114

unusual. This is one of the issues of concern with thermophilic digestion products – the odor from VFAs. However, with a high degree of dewatering, this odor is largely an issue for the dewatering recycle stream and less of an issue in the dewatered cake product. Thermophilic anaerobic digestion requires additional heating to achieve the temperatures required. This need pushes agencies to provide thicker feedstock sludge, typically at least 5 percent solids and in most cases toward 6 percent solids or higher, if possible. Others recover heat off of the discharged biosolids to preheat the raw sludge feedstock and reduce the total fuel required to reach thermophilic temperatures. Digestion Solids Retention Times (SRTs) The total SRTs of the configurations shown in Figure 1 can vary. Only about two systems are known to be operating with SRTs at or slightly below 15 days, and there is concern for product stability when SRTs drop below about 15 days. Most thermophilic digestion systems provide total system SRTs of 20 to 25 days. Many thermo-meso staged systems also have well over 20 days of total system SRT, although the thermo-meso system at Wilhelmshaven, Germany has as little as 15 to 18 days of total SRT. Temperature Sensitivity The ability to obtain consistent temperature has been identified as a key issue for successful thermophilic anaerobic digestion. The thermophilic biological population appears to be more sensitive to temperature changes than mesophilic populations. Modern temperature control features on digestion heating systems provide much more consistent digester temperatures than historically. Reactor temperature fluctuations at modern thermophilic digesters typically show only minor variations on a daily basis (such as 0.1 to 0.2 degrees C variation throughout the day). The requirement for consistent thermophilic temperature means that mixing within the digester must be very good, or there will be pockets of sludge at reduced temperature. Agencies are using various mixing technologies including mechanical draft tube mixers, gas mixing, and hydraulic mixing, with the common theme to insure that all portions of the tanks are well-mixed. The experience at the City of Los Angeles (Iranpour, 2003 and 2005) is instructive for temperature control. Thermophilic digester temperatures were raised rapidly perhaps in an inadvertent manner. A several-degree C temperature increase occurred, bringing the digester quickly into the range of 58 degrees C or higher. This rapid increase negatively affected the biology and the digestion process suffered. Reduced sulfur emissions caused odor problems. At GVRD’s Annacis Island Plant, digestion operation at 56 degrees C over most of the past 6 years has provided stable digestion operation with no odor or process problems. At least 25 wastewater agencies in North America and Europe operate thermophilic anaerobic digesters at 54 to 55 degrees C with no reported problems in process stability. Therefore, the City of Los Angeles experience shows that rapid increase in thermophilic temperature must be avoided.

#1114

However, consistent operation at 55 degrees C (and even 56 degrees C) is providing stable operation. Laboratory-scale work has shown that thermophilic digestion of wastewater sludge becomes less stable at temperatures toward 60 degrees C, and, therefore to date, agencies have felt most comfortable operating in the range of about 53 to 56 degrees. Thermophilic Acid Phase Digestion The operation of an acid-phase thermophilic digestion phase has received much less research and evaluation than thermophilic gas phase digestion. Lab-scale and pilot-scale work has been conducted for thermophilic acid-phase, and the DuPage County, Illinois plant was operated in this arrangement for a short period of time. Controlling the short SRT to the required degree (such as 1 to 1.5 days) is expected to be challenging at full-scale. The combination of very high VFA concentrations (such as 5000 to 10,000 mg/l), coupled with thermophilic temperatures, and low pH (6 or below), creates a tremendously odorous gas. Managing the gas from a thermophilic, acid-phase reactor is expected to be additionally challenging because of its extreme odor and because the gas is not expected to support combustion on its own due to low methane content. Any leakage of this gas, even in minute amounts prior to its combustion, is bound to cause an odor problem. A treatment plant in Lakeland, FL is in the process of implementing the Infilco Degremont 2PAD process which employs the thermophilic acid phase step with a 6-hour batch. This plant is expected to be operational by 2007. Gas Management for Thermophilic Digestion Gas production rates can be much higher in thermophilic digestion than in mesophilic digestion. For fairly low-SRT thermophilic digestion (say 4 to 8 day SRT), much higher gas production rates will occur than from a 20-day SRT mesophilic digester. Therefore, gas piping is likely to need upsizing and gas management systems need to be able to handle high gas flowrates. Thermophilic digester gas contains much greater moisture content than mesophilic digester gas, and, therefore, condensing this moisture becomes a much more important need. STABILIZATION WITHIN THERMOPHILIC DIGESTION It is now well-accepted that thermophilic anaerobic digestion systems will destroy more volatile solids than mesophilic anaerobic digestion systems at the same total SRT. Comparative work at full-scale and pilot-scale supports this statement and Schafer et al, 2002 provides a summary of recent comparisons. The amount of additional VSR depends on several factors. At relatively low SRT (say 15 to 20 days), the thermophilic system is likely to achieve about 4 to 8 percentage points of additional VSR. For example, if the mesophilic anaerobic system achieved 50 percent VSR, the thermophilic anaerobic system (at the same SRT) is likely to achieve between 54 and 58 percent VSR. At high total SRT (say 30 days) there will tend to be less difference between the two systems. Another comparison is that the same VSR (say 50 percent) can be achieved in

#1114

less total SRT with thermophilic systems, over mesophilic systems. The additional VSR that is achieved within the thermophilic system creates commensurately more digester gas. PATHOGEN REDUCTION TO ACCHIEVE CLASS-A PERFORMANCE A number of plants have converted their anaerobic digestion operations from mesophilic temperatures to temperatures in the thermophilic range over the past 15 years. While the improvement in pathogen destruction at the higher temperatures when maintained for a given contact time is not disputed, the ability for these converted systems to produce biosolids meeting Class A pathogen requirements as defined in the Part 503 regulations has been addressed on an application-by-application basis. While Part 503 provides 6 alternatives to achieve Class-A pathogen destruction designation, the most commonly used alternatives for thermophilic anaerobic digestion have been:

• Alternative 1: Time and Temperature • Alternative 3: Documented Virus and Helminth Ova Destruction • Alternative 6: Treatment with an PFRP-Equivalent Process

In general, Alternative 1 requires the least amount of additional investigation and additional proof of a process’s efficacy. Conversely, Alternative 1 typically requires the most conservative combinations of time and temperature batch operation of the three alternatives. It is also not surprising that the earliest plants that achieved Class A operation did so using Alternative 1. More recently, plants have used full-scale data to obtain EPA Regional approval to operate Class-A systems by qualifying under Alternative 3. Often, the full-scale testing does not fully demonstrate the process’ effectiveness on helminths and virus reduction so that plants operating under Alternative 3 typically have monitoring requirements that are more extensive than for Alternative 1. There are currently plants operating with Alternative 3 authorization and all have requirements to periodically test feed and treated biosolids for helminths and enteric viruses. Even more recently, utilities and engineers have explored processes using Alternative 6. The investigations have used pilot-scale pathogen spiking studies and have required process definitions that address scale-up issues. The data collected from the pathogen spiking tests have shown that Class A performance can be reliably achieved at pilot scale using considerably less conservative combinations of batch time and temperature. While there are not any Class A plants currently operating under Alternative 6, this is likely to change as more data become available from similar tests and upgrades currently under construction are brought on line. OVERVIEW OF METHODS TO ACHIEVE CLASS-A OPERATION Alternative 1 – Time and Temperature

#1114

Utilities have used the time and temperature batch requirements defined under Class-A Alternative 1 of the Part 503 Regulations to modify their digestion process to produce Class-A biosolids. Figure 2 shows a plot of the time and temperature curve for sludge with total solids concentrations of less than seven percent. Plants that have considered time and temperature options have gravitated to operational criteria near 55 degrees C which require approximately 24 hours of batch detention time. This is due to reported operational stability of thermophilic systems at temperatures in the 55 degrees C range, while allowing the batch times to be maintained at practical times/volumes. The time/temperature equations between 50 and 70 degrees C within Alternative 1 were developed from the following information which included prior guidance of US and European rules. Also, the equations were developed within a framework of conservatism and safety (Farrell, 2003): 11

• The prior 40 CFR Part 257 rule contained the PFRP definition for sludge pasteurization – i.e., 70 degrees C for 30 minutes.

• Feachem12(Feachem, 1983) developed time/temperature plots for pathogen reduction including ascaris ova, enterovirus, and salmonellae. However, Feachem did not explain in detail how he developed his time/temperature plots from the raw data, so EPA placed some additional conservatism on Feachem’s work.

• The US Food and Drug Administration pasteurization requirements for milk products were used. Eggnog pasteurization requirements, in particular, were important because eggnog more closely represents a slurry situation. Extending the time/temperature plot for eggnog pasteurization (i.e., extending to lower temperatures and longer times) fit well with being on the safe side of Feachem’s plots. The FDA believes the pathogen reduction capability of its pasteurization rules provides many logs – at least 7 log10 reduction, and probably more.

• Pathogen reduction rates tended to follow straight lines when plotted as temperature versus the log of time. Therefore, EPA equations were developed on this basis.

• All pathogen reduction time/temperature information assumed batch-type operation, whereby the entire mass is subjected to the required temperature for the associated time.

Figure 2: Time and Temperature Relationship

Plants have used the ~1 day at ~55 degrees C for compliance

with time and temperature

#1114

• A 3-degree C benefit was provided for “less than 7 percent solids” slurries since heat transfer is clearly better in a slurry than within bulk materials such as compost, and data on pathogen reduction supported this approach.

• EPA used the lower limiting temperature of 50 degrees C for these equations since reduction of ascaris ova, in particular, below this temperature was not considered sufficiently reliable based on available data in the early 1990s.

Dr. Joseph Farrell, who developed the pathogen reduction information and the time/temperature equations for the Part 503 rule, indicates he believes these equations in Alternative 1 probably provide at least 5 logs of pathogen density reduction. The following plants are either online, producing Class-A biosolids or in start-up and plan to qualify under Alternative 1:

• Orange Water and Sewer Authority (OWASA) – Chapel Hill, NC: Mason Farm WWTP. The system uses a batch held at greater than 55.5 degrees C for at least 21 hours (Willis and Gottschalk, 2001).13.

• City of Los Angeles, CA: Terminal Island WWTP. System uses a batch held at 55 degrees C for 24 hours (Iranpour, 2002, 2005). 14, 15

• Metropolitan Sewerage District – Madison, WI: Nine Springs WWTP: process is in start-up and plans to use batches held at g 55 degrees C for 24 hours (Schimp, 2003).16

• Chattanooga, TN: starting up an upgraded system, and they plan to utilize a 59.3-degree C, 6-hour batch.

Alternative 3 – Full-Scale Testing for Helminths and Viruses In 2001 and 2002, two plants (in California) were allowed Class-A operation by qualifying under Alternative 3. Alternative 3 requires that testing of process influent and effluent show that, if there are enteric viruses and helminths in the process feedstock, that the process effluent have concentrations below the permitted limits. The perception by both regulators and the regulated community is that Alternative 3 allows for a less rigorous proof of a process’ efficacy than either Alternative 1 or 6. The processes that are operating under an Alternative 3 justification are typically less conservative than the time and temperature equation. Regulators have required ongoing monitoring for helminth ova and enteric viruses in the treated biosolids to provide further data in support of the operation of these systems. Plants currently operating under Alternative 3 include:

• Inland Empire Utilities Agency (IEUA) – Rancho Cucamonga, CA: Regional Plant Number 1. Plant uses a three-phase system that includes mesophilic acid phase, thermophilic methane phase, followed by an intermediate-temperature third phase (Fondahl, 2001).17

• City of Los Angeles, CA: Hyperion WWTP. System uses a 16-hour, 53-degree-C batch (Iranpour, 2005). 18, 19, 20.

#1114

Alternative 6 – Demonstration the Process is Equivalent to a PFRP The PEC requires that a 2-log10 reduction of viable helminth ova density and a 3-log10 reduction of enteric virus density be demonstrated in order to prove PFRP equivalency. Even with PEC-approved Equivalent PFRPs, compliance with a bacterial indicator test (either fecal coliform or salmonella) is still required by the Part 503 regulations. Researchers who have undertaken pathogen-spiked, pilot-scale tests have documented pathogen destruction at time and temperature combinations that are considerably less conservative than those defined by the Alternative 1 time and temperature equation. Some of the proposed PFRP-Equivalent processes (if based on time and temperature criteria) also attempt to take advantage of the pathogen destruction that occurs in non-batch thermophilic systems. Currently, only the following two processes have pursued Qualification using Alternative 6;

• Ondeo Degremont’s 2PAD process21 is scheduled to be started at a treatment plant in Lakeland, FL some time in 2007. The process uses a thermophilic, acid-phase digester with a roughly 6-hour batch with at least 4-hours at greater than 55 degrees C followed by a mesophilic, methane-phase digester. (Ferran, 2002)

• Columbus Water Works’ first CBFT3 process first implementation will be at the South Columbus Water Resource Facility and is currently scheduled for start-up in 2008. The process consists of a 6-day-MCRT thermophilic digester without a batch followed by a 30-minute batch at either 55 degrees C (still under review by the PEC) or 60 degrees C (currently conditionally approved as a PFRP Equivalent).

Each of the above processes has been granted “Conditional” PFRP Equivalency by the PEC based on laboratory- or pilot-scale testing. The conditional PFRP equivalency is dependent on full-scale testing to prove that helminths and enteric viruses can be reduced by 2- and 3-log10, respectively, without exceeding the allowable pathogen densities in the effluent. While the sampling for these organisms in the feed and finished biosolids is occurring, the finished biosolids may be treated as Class-A provided all identified operational criteria are met. CURRENT DATA ON PATHOGEN DESTRUCTION CAPACITY OF THERMOPHILIC ANAEROBIC DIGESTION While the plants that are operational in Class-A mode have qualified under either Alternative 1 or Alternative 3, it has been the new processes attempting to qualify under Alternative 6 that have performed more robust testing to achieve Class-A compliance using the shortest/lowest combinations of batch time and temperature.22(Willis, 2005) These same tests have developed data on the ability of thermophilic anaerobic digestion systems’ ability to inactivate helminths and viruses, in addition to fecal coliform or other bacterial indicators. The Alternative 6 investigations have used pilot-scale pathogen spiking studies and have required process definitions that address scale-up issues. While there are not any Class A plants currently operating under Alternative 6, this is likely to change as more data become available from similar tests.

#1114

In discussions with the Pathogen Equivalency Committee (PEC) of the USEPA, three criteria have been identified for PFRP Equivalency:

• Systems must prove capable of affecting a 2-log10 reduction of Helminth ova while having no viable ova after treatment.

• Systems must prove capable of affecting a 3-log10 reduction of enteric viruses while having no viable viruses after treatment.

• Systems must be capable of producing a product with fecal coliform densities of less than 1,000 MPN/gram of dry solids as ongoing fecal coliform or Salmonella testing will be required, as with all Class-A processes. There has been less emphasis placed on the bacterial reduction performance of pilot-scale systems because this ongoing monitoring requirement remains in place regardless of the Alternative used to qualify. Bacteria are gaining more attention recently, however, as they may represent the most resistant of the current indicator organisms to the conditions present in thermophilic anaerobic digesters.

Infilco Degremont 2PAD Pilot-Scale Testing The 2PAD system consists of a thermophilic, acid-phase digester followed by a considerably larger, mesophilic methane-phase digester. During the pilot testing, the acid-phase digester was fed four times per day with each feeding depressing the reactor temperature to between 49 and 50 degrees C. The temperature would then recover to above 55 degrees C for the last 2.5 to 3.0 hours prior to the next set of transfers. Naturally occurring fecal coliform were present in the feedstock at an average concentration of 6.35 log10 MPN/g TS. While treated biosolids fecal coliform concentration data were not presented in the references, Ferran (2002) states that the Class-A fecal coliform levels were achieved. Ascaris, which traditionally have been considered the most thermally resistant of the indicator organisms, were spiked in the process feed at an average concentration of 2.61 log10 PFU/4g TS. No viable ascaris ova were detected in eleven 4+ g TS samples. Poliovirus were spiked in the feedstock at an average concentration of 4.02 log10 PFU/4g TS. Poliovirus were reduced by between 2.82 and 3.15 log10 without detecting a viable plaque forming unit in the finished product in seven samples. Of interest are the thermophilic and mesophilic digester average pH and chemical concentrations shown in the table to the right. Ferran cites anti-pathogenic enhancements from non-ionized VFAs at pH lower than 5.0 and from free ammonia at pH greater than 7.5; He also notes, however, that the operating ranges of the 2PAD system do not fall within the ranges identified. In addressing the observed Class-A performance of the 2PAD system, Ferran compares the much shorter holding time (roughly 2.5 hours) at 55 degrees C compared to the 24 hours required by the time and temperature equation. The following is a condensed excerpt from that paper:

#1114

Table 1: pH and Chemical Concentrations During 2PAD Pilot Testing with 4 feedings/day at 55OC

pH Free Ammonia, mg/l

Non-Ionized VFA, mg/l

Thermophilic, Acid-Phase Digester

6.08 2.1 138

Mesophilic, Methane-Phase Digester

7.22 13.4 0.7

“Based on the US EPA guidance for the production of Class A biosolids, a minimum of 24 hours at 55 degrees C should be maintained between the feedings of “draw and fill” thermophilic digester to ensure freedom from pathogens. Only 6 hours separated feedings of the pilot unit and the inside temperature was not steady at 55 degrees C. Therefore, parameters other than temperature may have intervened to achieve the observed pathogen destruction. The presence of a toxic substance in the feed was rapidly ruled out since the seeding recoveries were acceptable and absence of cytotoxicity to poliovirus was demonstrated. The presence of substances such as VFA and free ammonia in the digesters was investigated next. By comparison with the data from literature the (observed) free ammonia and VFA are too low to trigger any significant pathogen decay.”(Ferran, 2002)

CBFT3 Laboratory-Scale Testing The CBFT3 testing was conducted at laboratory-scale at the University of North Carolina – Chapel Hill. A continuous-feed, stirred tank reactor (CFSTR) was continuously Ascaris-suum- and poliovirus-spiked sludge. After a stable operation was achieved, a portion of the CFSTR contents would be transferred to a second vessel so that samples could be taken to simulate various batch times downstream of a CFSTR. Both the CFSTR and the batch tank were maintained at the same temperature. The tests were also conducted with sludge from three different sources:

1. Co-thickened primary and secondary sludge from the South Columbus Water Resource Facility in Columbus, GA;

2. Fermented primary sludge from the Orange Water and Sewer Authority’s (OWASA) Mason Farm WWTP in Chapel Hill, NC; and

3. Pure-oxygen waste activated sludge from the Western Lake Superior Sanitary District (WLSSD) in Duluth MN.

Table 2 shows the ascaris and poliovirus reductions observed across the CFSTR alone operating at an SRT of between 4.5 and 6 days. Only the 51 degrees C operation detected any viable ascaris downstream of the CFSTR. Poliovirus was not detected in any of the effluent samples at any of the temperatures tested. Similarly, neither organism was detected at any batch detention time downstream of the CFSTR either.

#1114

Table 3: Ascaris and Poliovirus Reduction in Batch Digester during CBFT3 Tests

Test

Ascaris at T=0, log10/g

TS

Time to Reduce ascaris below

Detection Limit, hr

Poliovirus at T=0,

log10/g TS

Time to Reduce poliovirus

below Detection Limit, hr

49OC 3.20 >6 2.18 2 51OC – 1 3.15 2 4.71 0.5 51OC – 2 3.25 1 3.43 0.5 53OC – 1 3.35 0.5 3.21 0.17 53OC – 2 3.29 0.5 4.85 0.5 55OC 2.56 0.5 Not done Not done

Fecal coliform were detected in the CFSTR effluent in excess of the Class-A limit of 1,000 MPN/g TS. In fact, it was observed to take as long as a 2-hour batch downstream of the CFSTR at 55 degrees C to reach Class-A levels. Counter to what would be expected these results were not repeated at lower temperatures:

• Five out of six tests conducted at 53 degrees C produced Class-A fecal coliform levels leaving the CFSTR. The sixth produced Class-A levels at less than a 1-hour batch (interpolated from the reductions to be 30 minutes) downstream of the CFSTR.

• Finally, both tests at 51 degrees C failed to meet the fecal coliform criterion immediately after discharge from the CFSTR but did achieve the Class A levels after a 1-hour downstream batch.

Because the CFSTR proved to be extremely effective in reducing the ascaris and poliovirus, a separate test was conducted to determine the effectiveness of the batch reactor. Spiked concentrations of ascaris and poliovirus were monitored in a batch digester to observe the rate of destruction. Table 3 shows the results from this effort. D-Factors Support Results Obtained in Alternative 6 Testing

Table 2: Ascaris and Poliovirus Reduction in CBFT3 Pathogen-Spiked CFSTR

Test (number of replicates)

Ascaris Reduction,

log10

Viable Ascaris

?

Poliovirus Reduction,

log10

Viable Poliovirus

?

Avg. pH

Free Ammonia/

Non-ionized VFA, mg/l

SCWRF-55OC A (2) >2.07 N >2.07 N 7.38 73 / 6

SCWRF-55OC B (2) >2.35 N >4.29 N 7.37 77 / 6

SCWRF-53OC (2) >2.18 N >4.36 N 7.34 73 / 8 SCWRF-51OC (2) 2.04 Y >3.37 N 7.52 102 / 2 OWASA-53OC (2) >2.10 N >5.76 N 7.02 19 / 8 WLSSD-53OC (2) >2.85 N >3.26 N 7.44 110 / 4

#1114

In looking at research that predated the Part 503 promulgation, a number of references support the more recent findings. In particular the following are relevant and indicate ascaris destruction times that are much more similar to those observed by Ferran and Aitken. In particular:

• R. J. Barnard23(1987) enumerated dead and live ascaris ova at various batch times and temperatures. His research identified that a greater than 3-log10 reduction and complete inactivation of ascaris was reached after:

o 287 minutes at 50OC o 46.7 minutes at 52OC o 6.5 minutes at 55OC

• E. G. Carrington24(1985) performed similar experiments and observed the destruction of ascaris in sludge pasteurization scenarios at relatively low thermophilic temperatures. In particular, the conclusions of that work state that “if sludge containing ascaris ova is heated to 55OC and maintained at this temperature for 2 hours the treated material will be free of viable ascaris ova.”

• Jones and Martin25(2003) reference D-values (the amount of time to cause a 1 log10 reduction for a given organism) of 1.3 minutes at 60OC for ascaris and 32 minutes at 55OC and 19 minutes at 60OC for poliovirus. They also cite 7 minutes at 55OC as the time required for destruction of Ascaris.

As the recent investigations by Ferran and Aitken suggest that operating regimes defined by the Alternative 1 “Time and Temperature” equation may have a major level of conservatism for ascaris ova and enterovirus, the PEC is interested in identifying other mechanisms that may be responsible for the reported results. Others argue that the time and temperature equations, which were appropriately conservative at the time of their development and publication based on the data available at the time, are not closely reflective of the minimums required to achieve Class-A performance. At the same time, additional research is advancing on the minimum requirements to achieve Class-A treatment levels with respect to fecal coliform and on the effectiveness of a variety of possible stressors (in addition to heat) that occur normally within digesters. SUMMARY The combination of recent research shows that unionized ammonia and volatile acids can aid in the disinfection of biosolids under certain operating regimes and have little or no effect under other regimes. These results support the foundation of the 2PAD argument that the volatile acids contribute to the disinfection observed in their pilot testing of low-pH, acid-phased digestion. The same results also support the CBFT3 research claims that, at neutral pH, ammonia and volatile acids have little or no effect and that the observed pathogen destruction is due to thermal stressors alone. The CBFT3 results are also supported by D-value research from the 1980s. Within the research work for thermophilic anaerobic digestion, there appears to be a large amount of conservatism in the Class A time/temperature equation with respect to certain pathogens (ascaris ova and viruses, for example). Also, more general pathogen research work indicates that more rapid dieoff of certain pathogens is very supportable (i.e., more rapid dieoff than would be predicted by the EPA Class A time/temperature equation). For fecal coliform,

#1114

there appears to be less conservatism in the Class A time/temperature equation, and the amount of conservatism can be argued based on a wide variety of data. Ongoing work with respect to fecal coliform reactivation and regrowth and other aspects of indicator organism inactivation may provide helpful information on this issue. REFERENCES 1 “40 CFR Part 503: Standards for the Use or Disposal of Sewage Sludge,” Federal Register,

Volume 58, No. 32 (1993), pp.9387-9404. 2 Garber, W. F., Ohara, G., Colbaugh, J., and Raksit, S., Thermophilic digestion at the Hyperion

Treatment Plant. Journal WPCF Vol 47, No. 5, May 1975 3 Popova and Bolotina (1963) The present state of purification of town sewage and the trend in

research work in the City of Moscow. International Journal of Air and Water Pollution Vol 7, 145.

4 Golueke, C. G. (1958) Temperature effects on anaerobic digestion of raw sewage sludge. Sewage and Industrial Wastes, Vol 30, 1225.

5 Malina, J. F. Jr. (1961) The effect of temperature on high rate digestion of activated sludge. Proceedings of 16th Industrial Waste Conference, Purdue University, p. 232

6 Rimkus, R., Ryan, J., and Cook, E. (1982) Full-scale thermophilic digestion at the West-Southwest Sewage Treatment Works, Chicago, Illinois. Journal WPCF, Volume 54, number 11.

7 Krugel, S., Nemeth, L., and Peddie, C., Extended thermophilic anaerobic digestion for producing Class A biosolids at the Greater Vancouver Regional District’s Annacis Island Wastewater Treatment Plant. Presented at IAWQ conference in Vancouver, Canada, June 1998

8 Han, Yue, and Dague, Richard, Laboratory studies on the temperature-phased anaerobic digestion of mixtures of primary and waste activated sludge. Presented at WEFTEC Conference, October 1996.

9 Schafer, P., Farrell, J., Newman, G., and Vandenburgh, S. Advanced anaerobic digestion performance comparisons. Presented at WEFTEC conference, 2002.

10 Schafer, Perry (1999) Site visit and data evaluation at Wilhelmshaven, Germany’s Main Wastewater Treatment Plant.

11 Farrell, J. 2003. Discussion with Dr. Joseph Farrell, November 2003. 12 Feachem, Bradley, Garelick and Mara, Sanitation and Disease: Health Aspects of Excreta and

Wastewater Management, John Wiley and Sons – 1983. 13 Willis and Gottschalk, Operational Improvements from Start-up of OWASA’s Class-A,

Thermophilic Anaerobic Digestion System, WEFTEC 2001, Atlanta, GA – Oct. 2001. 14 Iranpour, et. al, “Changing Mesophilic Wastewater Sludge Digestion into Thermophilic

Operation at Terminal Island Treatment Plant”, Water Environment Research, (Volume 74; Number 5) pp. 494-507 – Oct./Sept. 2002.

15 Iranpour, R. 2005. Discussion with Dr. Reza Iranpour, June 2005. 16 Schimp, et. al, “Anaerobic Digestion: Retooling an old process to meet a ‘Class A’ objective”,

Water Environment and Technology, May 2003, pp. 45-49.

#1114

17 Letter from Lauren V. Fondahl (EPA Region 9) to Douglas Drury (Inland Empire Utilities

Agency), December 21, 2001 18 Iranpour R, et. al, Full-scale Class A biosolids production by two-stage continuous-batch

thermophilic anaerobic digestion at the Hyperion Treatment Plant, Water Environment Research (in press) – 2005.

19 Iranpour R, et. al, Thermophilic anaerobic digestion to produce Class A biosolids; initial full-scale studies at Hyperion Treatment Plant, Water Environment Research (in press) – 2005.

20 Iranpour, et. al, Production of EQ Biosolids at the Hyperion Treatment Plant: Problems and Solutions for Reactivation/Growth of Fecal Coliforms, WEFTEC 2003, Los Angeles, CA – Oct. 2003.

21 Ferran, et. al, Two-Phase Anaerobic Digestion of Municipal Sewage Sludge Optimization of the Pathogen Destruction, WEFTEC 2002, Chicago, IL – Oct. 2002.

22 Willis, et. al, The State of the Practice of Class-A Anaerobic Digestion: Update for 2005, WEFTEC 2005, Washington, DC – Oct.-Nov. 2005.

23 Barnard, et. al, “Ascaris lumricoides suum: Thermal Death Time of Unembryonated Eggs”, Experimental Parasitology (Vol. 64), 1987.

24 Carrington, “Pasteurization; Effects upon Ascaris Eggs”, Inactivation of Microorganisms in Sewage Sludge by Stabilisation Processes, 1985

25 P. Jones and M. Martin, A Review of the Literature on the Occurrence and Survival of Pathogens of Animals and Humans in Green Compost, The Waste and Resources Action Programme, Nov. 2003.

#1114