Abstract:
A method of treating a waste stream comprising feeding a waste stream into a reactor constructed and arranged for a fast start and maintained in a thermophilic temperature regime of between about 50° C. and 62° C. for a HRT of less than or equal to 48 hours, wherein the reactor contents are maintained at a pH less than or equal to 6.5; drawing a portion of the contents of the reactor and feeding the drawn contents into a second reactor which is maintained in a mesophilic temperature regime of between about 28° C. to 38° C. for a HRT of at least thirteen days; and replacing the contents drawn from the first reactor with more waste from the waste stream. Alternately, the waste stream may be initially treated in a mesophilic reactor, followed by treatment in a thermophilic reactor, or a single stage thermophilic reactor may be used.

Description:
SPONSORSHIP 
     This work was supported by a grant from the Water Environment Research Foundation (Project No. 97-REM-2). The inventors acknowledge the assistance provided by Bio Vir Laboratories and the East Bay Municipal Utility District (EBMUD) laboratory staff. 
    
    
     FIELD OF THE INVENTION 
     This invention relates to the processing of wastewater, and particularly to a method for providing treatment to achieve Class A biosolids. 
     BACKGROUND OF THE INVENTION 
     Land application of treated wastewater sludge (biosolids) is controversial because the biosolids therein potentially contain human pathogens, i.e., some counties in California have banned the application of biosolids which meet the Environmental Protection Agency&#39;s (EPA) Class B pathogen reduction standards, as set forth in 40 CFR 503, which require a fecal coliform level of less than two million most-probable-number (MPN) per gram of total solids, but not Class A pathogens standards per 40 CFR 503. Wastewater treatment agency managers are concerned about the future of biosolids land applications in their states. EPA&#39;s Class A pathogen standards require the virtual elimination of pathogens in biosolids. In accordance with 40 CFR 503.32 (a)(5), Class A—alternate 3 and 40 CFR 503.32(a)(6)—alternate 4, EPA Class A pathogen requirements are met in biosolids when fecal coliform densities are less than 1,000 MPN per gram total solids; or when Salmonella densities are less than 3 MPN per four grams total solids. Additionally, enteric virus must be less than 1 plaque-forming unit per four grams of total solids, and helminth ova is less than one viable helminth ova per four grams of total solids. Meeting Class A standards will significantly increase the opportunity for biosolids recycling, however, known processes which achieve Class A pathogen levels in biosolids are generally cost prohibitive. The provision of a low-cost method of biosolids treatment, which will meet Class A standards will offer additional biosolids management options Known thermophilic anaerobic digestion processes are not classified as a Process to Further Reduce Pathogens (PFRP) under 40 CFR Part 503. 
     Anaerobic digestion has been one of the most widely used processes for the stabilization of primary and secondary sludges produced at municipal wastewater treatment facilities. The majority of applications of anaerobic digestion to wastewater sludges have been in the mesophilic temperature range, from 35° C. to 40° C. (95° F. to 104° F.). Anaerobic sludge digestion in the thermophilic temperature range from 45° C. to 65 ° C. (113° F. to 149° F.) has been practiced to only a limited extent. 
     The limited use of anaerobic digestion at temperatures above the mesophilic range is due to higher energy requirements to obtain the higher thermophilic temperature, poor process stability, increased odor, and lower quality supernatant (filtrate/centrate). The advantages of thermophilic anaerobic digestion over mesophilic anaerobic digestion have accrued from increased stabilization and methane production rates, and from improvements in sludge dewatering properties. Since the advent of the 40 CFR Part 503 Regulations, more studies have focused on the destruction of pathogenic organisms. 
     Thermophilic anaerobic digestion has an advantage of improving pathogen destruction, and has the potential to meet the pathogen quality requirements of EPA&#39;s Class A biosolids. While the economic disadvantages of thermophilic anaerobic digestion has outweighed the advantages of the process, the implementation of 40 CFR Part 503 and the use of a two-stage digestion system, having a thermophilic or mesophilic first-stage and a mesophilic or thermophilic second-stage, may negate the economic disadvantage. 
     Recent studies have introduced some improvement to the stability and performance of the thermophilic anaerobic digestion process. These improvements include: 
     Fast start-up method. The fast start-up method produces a more robust thermophilic culture that is relatively tolerant of temperature fluctuations and increases process stability. 
     Draw/fill operation. The draw/fill cycle improves pathogen removal by reducing the potential for short-circuiting. 
     Multiple-stage digestion. Multiple-stage reactors reduce the potential for short-circuiting, wherein flow exits the reactor before it should, provide a low pH environment in the first stage if methanogenesis is minimized or eliminated in this stage, which apparently enhances pathogen reduction; and, if the second-stage reactor is mesophilic, produce a higher quality sludge in terms of odor and supernatant. 
     Research has shown that pathogen destruction in municipal sludge digestion follows a typical time/temperature relationship, wherein higher temperatures require shorter exposure times for pathogen destruction. Data has been collected demonstrating survival rates of various pathogens in municipal sludge digestion. These data suggest that thermophilic digestion achieves pathogenic bacteria reduction rates about two orders of magnitude higher than mesophilic digestion, and may meet the pathogenic bacteria levels required for Class A sludge. EPA studies and full-scale research results indicate enhanced pathogen destruction in acid-phase digestion. 
     U.S. Pat. No. 5,525,228 to Dague et al., for Temperature-phased anaerobic waste treatment process, granted Jun. 11, 1996, describes a process having a long term hydraulic retention time (HRT) and which operates in a temperature range sufficient to Pasteurize the waste. 
     U.S. Pat. No. 5,624,565 to Lefevre et al., for Method of regulating aeration in a basin for biological treatment of Waste water, granted Apr. 29, 1997, describes a method which requires elimination of carbon in aerobiosis, nitrification and denitrification. 
     U.S. Pat. No. 5,746,919 to Dague et al., for Temperature-phased anaerobic waste treatment process, granted May 5, 1998, is a continuation of the &#39;228 patent. 
     U.S. Pat. No. 5,900,150 to Fayoux et al., for Method of purifying waste water biologically, granted May 4, 1999, describes a treatment method which is conducted at temperatures between 25° C. and 40° C. 
     SUMMARY OF THE INVENTION 
     A method of treating a waste stream comprising feeding a waste stream into a reactor constructed and arranged for a fast start and maintained in a thermophilic temperature regime of between about 50° C. and 62° C., for a HRT of less than or equal to 48 hours, wherein the reactor contents are maintained at a pH of 6.5 or less; drawing a portion of the reactor contents and feeding into a second reactor which is maintained in a mesophilic temperature regime of between about 28° C. to 38° C. for a HRT of at least thirteen days; and replacing the volume of sludge drawn from the thermophilic reactor with more waste from the waste stream. Alternately, the waste stream may be initially treated in a mesophilic reactor, followed by treatment in a thermophilic reactor, or a single stage thermophilic reactor may be used. 
     An object of the invention is to meet Class A requirements with a thermophilic anaerobic digestion processes. 
     Another object of the invention is to provide a digestion process for fecal coliform, Salmonella, enteric virus and helminth ova reduction. 
     A further object of the invention is to provide a treatment method which will produce less methane than do prior art methods. 
     Another object of the invention is to provide a treatment method which will produce biosolids which are substantially pathogen-free and are suitable for application with respect to pathogen densities where ever commercial fertilizers or soil conditioners are used. 
     This summary and objectives of the invention are provided to enable quick comprehension of the nature of the invention. A more thorough understanding of the invention may be obtained by reference to the following detailed description of the preferred embodiment of the invention in connection with the drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a flow chart depicting the various processes used to demonstrate the method of the invention. 
     FIG. 2 is a schematic representation of the structure used to demonstrate the method of the invention. 
     FIG. 3 depicts the distribution of fatty acids in the second embodiment of the invention at Thermo 55. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     This invention includes three embodiments of various thermophilic anaerobic digestion processes for disinfecting sludge, in some instances, to meet the EPA&#39;s Class A pathogen requirements, as set forth in 40 CFR Part 503, the requirement of which have been previously identified. Three embodiments of the invention, schematically represented in FIG. 1, are assessed for their ability to consistently meet fecal coliform requirements, and include: (1) a two-stage, thermophilic acid phase/mesophilic methane phase; (2) a single-stage thermophilic; and (3) a two-stage, mesophilic acid phase/thermophilic methane phase. Each process is evaluated at three different thermophilic temperatures, i.e., 45° C., 50° C., and 55° C., against a mesophilic control at 35° C. As used herein, a mesophilic temperature range includes temperatures between 35° C. and 40° C., while a thermophilic temperature range includes temperatures between 40° C. and 70° C. Temperatures above 70° C. are in a range which will pasteurize an organic material in thirty minutes per 40 CFR 503, Appendix B.7. Pasteurization is a physical, rather than a biological process. What is significant about the three embodiments of the invention is that the initial HRT is of short duration, and introduction of a waste stream into a thermophilic reactor is accompanied by a rapid start up, i.e., the contents of the reactor chamber are quickly brought up to the thermophilic temperature regime. 
     Experimental data using the methods of the invention was gathered in three phases. In Phase 1, the three embodiments of the invention were screened at bench-scale. Because the acid stage of the first embodiment was expected to provide significantly greater disinfection power than the mesophilic methane stage, only the acid stage was operated for evaluation purposes in Phase 1. Each embodiment was examined at or about three different thermophilic temperatures: 45° C., 50° C., and 55° C., referred to herein as Thermo 45, Thermo 50 and Thermo 55, respectively, against a mesophilic control at 35° C., referred to herein as Meso 35. In Phase 2, the better performing embodiments of Phase 1 were examined in more detail at bench-scale over a longer time period for assessment of process stability and actual pathogen destruction using feed sludges seeded with pathogens of concern. Phase 3 is a further continuation of the Phase 2 assessment at full-scale. 
     The method of the invention was tested, as shown in FIG. 2, generally at  10 , using laboratory-scale Plexiglas reactors  12  (digesters) with an operating capacity of approximately 30 liters. Peristaltic pumps  16 ,  18  control reactor influent and effluent flows, respectively. 
     The reactors were brought to thermophilic temperatures rapidly from a mesophilic seed sludge and operated in a draw/fill mode. As used herein, “rapidly” means that the seed sludge, at an initial temperature of about 35° C., was brought up to the thermophilic temperature regime in less than one day. At corresponding temperatures, single stage thermophilic processes (2) demonstrated greater fecal coliform removals than the first, thermophilic acid phase, stage of the two-stage process (1). Temperatures greater than 50° C. for embodiment 1, having a 2-day residence time, or 45° C. for embodiment 2, having a 15-day residence time, produced average effluent fecal coliform concentrations less than 1,000 MPN/g.TS (most probably number of pathogen/gram of total solids). 
     The reactor contents are mixed by an externally-mounted variable-speed DC motor  20  that turns a stainless steel shaft  22  and two 4-inch diameter propellers  24 ,  26 . The mixing system is sealed and anaerobic conditions are maintained by a small tube  28  that covers the stainless steel shaft. Each reactor was heated by a 520-watt heat tape  30 , such as Cole Parmer, E-36110-08. To avoid direct contact between the Plexiglas reactor walls and heat tape the exterior vertical area of the cylinders was covered with aluminum foil (not shown), which also distributes heat more evenly. Digester temperature is controlled by a proportional-response temperature controller  32 , such as Dynasense Model 02149-20, which receives a signal from an 18-inch immersible RTD probe  34 , such as Cole Parmer, E-08 117-74. As the temperature controller does not include a temperature display, temperatures are obtained from an external bimetal thermometer (not shown), and the heat tape output adjusted accordingly. Gas production was measured using a wet-tip gas meter  36 , as supplied by Wet Tip Gas Meter Co. Reactor effluent was stored in a container  38  with a gas seal  40  that allowed gas to escape but prevented air from entering. 
     The various hydraulic residence times (HRT), two days for embodiment 1 and fifteen days for embodiment 2, were achieved by adjusting flow rates and pumping intervals. The 2-day HRT of the reactors was achieved for part of the test period by cycling one-fourth of the reactor volume twice a day. This feeding regime approximated batch behavior to maximize fecal coliform removal, however, a semi-continuous flow regime, having hourly feed cycles, was adopted for another part of the test period to better control reactor levels. Feed cycles for embodiment 2 were six hours apart. Feeding regimes include the many variations from batch-feed, intermittent fee and continuous-feed. 
     Reactor feed, the waste stream, was a mixture of the a main wastewater treatment plant&#39;s primary sludge (PS) and thickened waste activated sludge (TWAS). The PS was filtered through a 5×5 wire mesh, having a 63.2% open area, and mixed in a 1:1 volume ratio with TWAS. The feed was kept in a refrigerator at approximately 4° C. and was mixed using a submerged pump and recirculation loop prior to feeding. For embodiment 1, the feed lines were immersed in a pre-heating water bath at 35° C. to avoid large reactor temperature fluctuations. Pre-heating was not required for embodiment 2 because of the smaller feed volumes and longer feed cycle periods. 
     All tests were performed in accordance with procedures detailed in the Standard Methods for the Examination of Water and Wastewater. Reactor feed and effluent sludges were analyzed for a number of parameters to monitor digester performance. The average and standard deviation for each parameter were calculated for embodiment 1, and are shown in Table 1: “General Characteristics of Reactor Behavior” and for embodiment 2 in Table 2 “General Characteristics of Reactor Behavior.” Digester performance, particularly for fecal coliform removal, is assessed as a function of temperature and hydraulic residence time. 
     
       
         
               
             
               
               
               
               
             
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 General characteristics of Reactor Behavior for Embodiment 1 
               
             
          
           
               
                   
                 #of 
                   
                 Reactor Operating Temperature 
               
             
          
           
               
                 Parameter 
                 Samples 
                   
                 35 
                 45 
                 50 
                 55 
                 Feed 
               
               
                   
               
             
          
           
               
                 Residence time (days)* 
                 56 
                 average 
                 3.3 
                 2.4 
                 2.8 
                 2.7 
                   
               
               
                   
                   
                 st. dev. 
                 46 
                 09 
                 22 
                 21 
               
               
                 Temperature (° C.)* 
                 112 
                 average 
                 35 
                 45 
                 52 
                 55 
               
               
                   
                   
                 st. dev. 
                 2 
                 2 
                 3 
                 3 
               
               
                 Gas production (l/hr)** 
                 57 
                 average 
                 0.5 
                 0.0 
                 0.8 
                 0.2 
               
               
                   
                   
                 st. dev. 
                 0.3 
                 0.1 
                 0.6 
                 0.5 
               
               
                 Methane in gas produced (%)*** 
                 5 
                 average 
                 23.6 
                 24.2 
                 32.6 
                 33.8 
               
               
                   
                   
                 st. dev. 
                 29 
                 38 
                 51 
                 70 
               
               
                 pH*** 
                 58 
                 average 
                 584 
                 607 
                 658 
                 665 
                 5.88 
               
               
                   
                   
                 st. dev. 
                 0.20 
                 0.12 
                 0.17 
                 0.16 
                 0.13 
               
               
                 Total Alkalinity (mg CaCO 3 /l)* 
                 26 
                 average 
                 3415 
                 4162 
                 4942 
                 5085 
               
               
                   
                   
                 st. dev. 
                 502 
                 461 
                 544 
                 520 
               
               
                 Volatile Acids (mg acetic acid/l)* 
                 26 
                 average 
                 6681 
                 7477 
                 7265 
                 7462 
               
               
                   
                   
                 st. dev. 
                 747 
                 893 
                 691 
                 863 
               
               
                 VFA/TALK ratio 
                 26 
                 average 
                 1.98 
                 1.81 
                 1.48 
                 1.47 
               
               
                 (mg acetic acid/mg CaCO 3 )* 
                   
                 st. dev. 
                 0.23 
                 0.21 
                 0.15 
                 0.13 
               
               
                 Acetic Acid (mg/l)**** 
                 7 
                 average 
                 2800 
                 3471 
                 3243 
                 3571 
                 1729 
               
               
                   
                   
                 st. dev. 
                 351 
                 315 
                 447 
                 571 
                 180 
               
               
                 Propionic Acid (mg/l)**** 
                 7 
                 average 
                 1757 
                 1429 
                 1514 
                 1714 
                 871 
               
               
                   
                   
                 st. dev. 
                 336 
                 214 
                 279 
                 248 
                 176 
               
               
                 Butyric Acid (mg/l)**** 
                 7 
                 average 
                 1250 
                 1543 
                 1486 
                 1343 
                 730 
               
               
                   
                   
                 st. dev. 
                 210 
                 140 
                 212 
                 244 
                 172 
               
               
                 Isobutyric Acid (mg/l)**** 
                 7 
                 average 
                 1189 
                 2246 
                 1430 
                 729 
                 380 
               
               
                   
                   
                 st. dev. 
                 1770 
                 2675 
                 1928 
                 87 
                 23 
               
               
                 Valeric Acid (mg/l)**** 
                 7 
                 average 
                 473 
                 426 
                 194 
                 187 
                 187 
               
               
                   
                   
                 st. dev. 
                 72 
                 40 
                 20 
                 18 
                 33 
               
               
                 Isovaleric Acid (mg/l)**** 
                 7 
                 average 
                 533 
                 703 
                 801 
                 849 
                 229 
               
               
                   
                   
                 st. dev. 
                 67 
                 72 
                 98 
                 126 
                 45 
               
               
                 Total Solids (%)* 
                 26 
                 average 
                 3.5 
                 3.6 
                 3.2 
                 3.3 
                 4.3 
               
               
                   
                   
                 st. dev. 
                 0.4 
                 1.2 
                 0.3 
                 0.4 
                 0.8 
               
               
                 Volatile Solids (%)* 
                 26 
                 average 
                 2.8 
                 2.6 
                 2.5 
                 2.5 
                 3.5 
               
               
                   
                   
                 st. dev. 
                 0.4 
                 0.3 
                 0.3 
                 0.3 
                 0.7 
               
               
                 Vol. Sol. Removal efficiency (%)* 
                 26 
                 average 
                 169% 
                 22.8% 
                 20.9% 
                 21.5% 
               
               
                   
                   
                 st. dev. 
                 11.1% 
                 21.1% 
                 12.2% 
                 10.9% 
               
               
                 Fecal Coliform (MPN/g TS)* 
                 26 
                 average 
                 15196 
                 4445 
                 905 
                 449 
                 309846 
               
               
                   
                   
                 st. dev. 
                 19661 
                 5008 
                 1538 
                 1133 
                 600687 
               
               
                 Fecal Log reduction* 
                 26 
                 average 
                 1.24 
                 1.74 
                 3.07 
                 3.60 
               
               
                   
                   
                 st. dev. 
                 0.65 
                 0.61 
                 1.11 
                 1.05 
               
               
                   
               
               
                 *58 Days  
               
               
                 **57 Days  
               
               
                 ***31 Days  
               
               
                 ****39 Days  
               
             
          
         
       
     
     
       
         
               
             
               
               
               
               
             
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
             
           
               
                 TABLE 2 
               
             
             
               
                   
               
               
                 General characteristics of Reactor Behavior for Embodiment 2 
               
             
          
           
               
                   
                 #of 
                   
                 Reactor Operating Temperature 
               
             
          
           
               
                 Parameter 
                 Samples 
                   
                 35 
                 45 
                 50 
                 55 
                 Feed 
               
               
                   
               
             
          
           
               
                 Residence time (days)* 
                 112 
                 average 
                 16.3 
                 15.7 
                 14.2 
                 15.1 
                   
               
               
                   
                   
                 st. dev. 
                 4 
                 4 
                 4 
                 4 
               
               
                 Temperature (° C.)* 
                 222 
                 average 
                 34 
                 45 
                 51 
                 56 
               
               
                   
                   
                 st. dev. 
                 2 
                 2 
                 3 
                 12 
               
               
                 Gas production (l/hr)** 
                 113 
                 average 
                 1.5 
                 2.0 
                 1.5 
                 1.5 
               
               
                   
                   
                 st. dev. 
                 0.6 
                 0.7 
                 0.6 
                 0.7 
               
               
                 Methane in gas produced (%)*** 
                 20 
                 average 
                 61.4 
                 58.1 
                 54.1 
                 55.0 
               
               
                   
                   
                 st. dev. 
                 8.3 
                 11.5 
                 12.1 
                 9.4 
               
               
                 pH*** 
                 113 
                 average 
                 7.95 
                 8.05 
                 8.08 
                 8.07 
                 5.92 
               
               
                   
                   
                 st. dev. 
                 0.18 
                 0.18 
                 0.15 
                 0.25 
                 0.22 
               
               
                 Total Alkalinity (mg CaCO 3 /l)* 
                 67 
                 average 
                 6769 
                 6803 
                 6661 
                 6518 
               
               
                   
                   
                 st. dev. 
                 437 
                 475 
                 386 
                 534 
               
               
                 Voltile Acids (mg acetic acid/l)* 
                 67 
                 average 
                 270 
                 488 
                 1525 
                 2199 
               
               
                   
                   
                 st. dev. 
                 260 
                 465 
                 733 
                 1129 
               
               
                 VFA/TALK ratio 
                 67 
                 average 
                 0.04 
                 0.07 
                 0.23 
                 0.34 
               
               
                 (mg acetic acid/mg CaCO 3 )* 
                   
                 st. dev. 
                 0.04 
                 0.08 
                 0.11 
                 0.21 
               
               
                 Acetic Acid (mg/l)**** 
                 58 
                 average 
                 280 
                 460 
                 578 
                 982 
                 1611 
               
               
                   
                   
                 st. dev. 
                 167 
                 330 
                 283 
                 834 
                 692 
               
               
                 Propionic Acid (mg/)**** 
                 58 
                 average 
                 61 
                 95 
                 796 
                 948 
                 786 
               
               
                   
                   
                 st. dev. 
                 95 
                 118 
                 457 
                 447 
                 438 
               
               
                 Butyric Acid (mg/l)**** 
                 58 
                 average 
                 28 
                 27 
                 40 
                 89 
                 602 
               
               
                   
                   
                 st. dev. 
                 14 
                 8 
                 37 
                 147 
                 406 
               
               
                 Isobutyric Acid (mg/l)**** 
                 58 
                 average 
                 67 
                 93 
                 156 
                 252 
                 298 
               
               
                   
                   
                 st. dev. 
                 25 
                 31 
                 64 
                 148 
                 118 
               
               
                 Valteric Acid (mg/l)*** 
                 58 
                 average 
                 26 
                 25 
                 40 
                 45 
                 98 
               
               
                   
                   
                 st. dev. 
                 4 
                 1 
                 25 
                 19 
                 65 
               
               
                 Isovaleric Acid (mg/l)**** 
                 58 
                 average 
                 28 
                 31 
                 76 
                 158 
                 188 
               
               
                   
                   
                 st. dev. 
                 16 
                 19 
                 74 
                 174 
                 134 
               
               
                 Total Solids (%)* 
                 67 
                 average 
                 2.2 
                 2.2 
                 2.2 
                 2.2 
                 4.2 
               
               
                   
                   
                 st. dev. 
                 0.3 
                 0.3 
                 0.3 
                 0.4 
                 1.0 
               
               
                 Volatile Solids (%)* 
                 67 
                 average 
                 1.4 
                 1.4 
                 1.5 
                 1.5 
                 3.2 
               
               
                   
                   
                 st. dev. 
                 0.2 
                 0.2 
                 0.2 
                 0.2 
                 0.8 
               
               
                 Vol. Sol. Removal efficiency (%)* 
                 67 
                 average 
                 42.8% 
                 44.1% 
                 40.2% 
                 38.5% 
               
               
                   
                   
                 st. dev. 
                 17.0% 
                 15.9% 
                 15.8% 
                 17.8% 
               
               
                 Fecal Coliforms (MPN/g TS)* 
                 58 
                 average 
                 1100 
                 240 
                 110 
                 43 
                 120000 
               
               
                   
                   
                 st. dev. 
                 1200 
                 770 
                 320 
                 170 
                 150000 
               
               
                 Fecal Log reduction 
                 58 
                 average 
                 1.94 
                 3.07 
                 3.69 
                 4.16 
               
               
                   
                   
                 st. dev. 
                 0.67 
                 0.86 
                 0.89 
                 0.92 
               
               
                   
               
               
                 *58 Days  
               
               
                 **57 Days  
               
               
                 ***31 Days  
               
               
                 ****39 Days  
               
             
          
         
       
     
     Embodiment 1 
     Two-Stage Thermophilic Acid Phase/Mesophilic Methane Phase Digestion 
     Embodiment 1 was evaluated using only the first stage (thermophilic acid phase) of the two-stage process because it was expected that this is where the greatest pathogen destruction would occur. Reactor temperatures appeared to have a proportional relationship with total alkalinity (TALK), pH, methane production, fecal coliform removal, and an inversely proportional relationship with volatile fatty acids (VFA)/TALK ratio, as shown in Table  1. TALK and pH levels were interrelated with the proportion of methane in gas production —alkalinity is a by-product of methanogenesis in that it results in the consumption volatile of fatty acids. As expected, all of the reactors in this process had high VFA concentrations because of the short HRT, but it appeared there was no direct relationship between total VFA concentration and temperature. Because of higher alkalinities of the higher-temperature reactors, VFA/TALK ratios exhibited an inverse relationship with temperature. The preferred temperatures in the thermophilic regime are in a range of about  50° C. to 62° C. 
     Because of the perceived impact of feeding regime, i.e., semi-batch vs. semi-continuous, on fecal coliform removal, a statistical analysis was performed to determine significance. Semi-batch feeding showed slightly higher removals at 35° C. and 45° C., and semi-continuous feeding showed slightly higher removals at 50° C. and 55° C. An Analysis of Variance (ANOVA) test and a Tukey comparison showed that these differences were not statistically significant. However, the embodiment 1 data presented here includes only the semi-continuous feeding results for consistency and comparison to embodiment 2. 
     Fecal coliform log removal rates increased noticeably with temperature. At 50° C. and higher, average fecal coliform concentrations in the digested sludge were less than 1,000 MPN/g. TS, the fecal coliform requirement for Class A sludge. The 50° C. and 55° C. thermophilic reactors achieved removals greater than three logs. As previously noted, the fecal coliform removal was only for the first stage of a two-stage process. Inclusion of the second stage, mesophilic methane phase, of the first embodiment would likely result in removals which are higher in all temperatures regimes. Preferred mesophilic temperature regime is in a temperature range of about 28° C. to 38° C. 
     Embodiment 2 
     Single-Stage Thermophilic Digestion 
     As shown in Tables 1 and 2, pH and TALK values for each reactor were substantially higher for embodiment 2 than embodiment 1. VFA/TALK ratios were also proportionately greater at higher temperatures. Propionic acid appeared to accumulate in the higher-temperature reactors, 50° C. and 55° C., further increasing VFA/TALK values. VFA/TALKs at 50° C. and 55° C. were much greater than 0.1 mg VFA as acetic acid/mg TALK as CaCO 2 , a standard benchmark for conventional mesophilic digesters. 
     During the initial period of embodiment 2 testing, much lower fecal coliform concentrations were measured in the reactor feed than were measured during the embodiment 1 evaluation. To determine if this decrease was significantly different from the expected values, a statistical Mann-Whitney test was performed on the feed sludge fecal concentrations. ANOVA was also performed but the data did not have a normal distribution. The results showed that fecal coliform concentrations in this initial period, log Fecals=4.3606 xxx 0.7922, were significantly less than those that followed, log Fecals=5.1840 xxx 0.3977. The reduced concentrations in the beginning of the process were related to decreased frequency of fresh sludge feed. Subsequently, the frequency was increased, to daily, and the feed sludge fecal coliform concentrations recovered to their former levels. However, during this period the reduced concentration of feed sludge fecal Coliforms also decreased the removal rates, especially for the lower-temperature reactors. The feed sludge fecal coliform data at reduced levels is included in the embodiment 2 data in Table 2. 
     Toward the end of embodiment 2 testing, the VFA/TALK ratio for the 55° C. reactor increased to almost 1.0 mg acetic acid/mg CaCO3. As expected, this condition was accompanied by a drop in gas production and a lower methane concentration, although the pH remained relatively constant and the TALK concentration high. It is possible that propionic acid accumulation, as depicted in FIG. 3, Distribution of Fatty Acids in Thermo 55, embodiment 2″, was not easily metabolized by the methanogenic bacterial populations under thermophilic conditions. FIG. 3 depicts levels of fatty acids over approximately twenty weeks of testing, corresponding to the data in Table 2. Trace  50  represents acetic acid, trace  52  represents butyric acid, trace  54  represents valeric acid, trace  56  represents propionic acid, trace  58  represents isobutyric acid, and trace  60  represents isovaleric acid. The actual operating temperature of Thermo 55 was 62° C. At this temperature, it appears that methanogenic activity is significantly impaired. 
     Fecal coliform log removal rates increased noticeably with temperature. At 45° C. and higher, embodiment 2 achieved 3-4 log removals of fecal coliform and average fecal coliform concentrations in the digested sludge were less than 1,000 MPN/g. TS, satisfying Class A sludge requirements. For embodiment 2, the preferred temperatures are in the range of 45° C. to 62° C. for the thermophilic reactor. 
     A summary of the conditions and results for the three embodiments are set forth in Table 3: 
     
       
         
               
               
               
               
             
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
             
           
               
                   
                 TABLE 3 
               
             
             
               
                   
                   
               
               
                   
                   
                   
                 % of time in 
               
               
                   
                 Temperature 
                   
                 compliance 
               
             
          
           
               
                 Embodiment 1 
                 Nominal 
                 Actual 
                 Mean 
                 Std Dev 
                 n 
                 n &lt; 1000 
                 with Class A 
               
               
                   
               
             
          
           
               
                   
                 45° C. 
                 48° C. 
                 3.378 
                 4.515 
                 37 
                 12 
                 32 
               
               
                   
                 50° C. 
                 52° C. 
                 934.57 
                 1516.94 
                 37 
                 27 
                 73 
               
               
                   
                 55° C. 
                 62° C. 
                 339.43 
                 971.74 
                 37 
                 32 
                 86 
               
               
                 Embodiment 2 
                 45° C. 
                   
                 243.6 
                 767.78 
                 43 
                 42 
                 98 
               
               
                   
                 50° C. 
                   
                 107.3 
                 319.68 
                 44 
                 42 
                 95 
               
               
                   
                 55° C. 
                   
                 42.7 
                 172.3 
                 44 
                 43 
                 98 
               
               
                 Embodiment 3 
                 35° C. 
                   
                 706.9 
                 1614.4 
                 38 
                 33 
                 87 
               
               
                   
                 45° C. 
                   
                 128 
                 367 
                 38 
                 35 
                 92 
               
               
                   
                 50° C. 
                   
                 3.8 
                 6 
                 38 
                 38 
                 100 
               
               
                   
                 55° C. 
                   
                 2.8 
                 4.6 
                 38 
                 38 
                 100 
               
               
                   
               
             
          
         
       
     
     As previously noted, only the first stage thermophilic reactor was tested for embodiment 1 using a two-day HRT. Embodiment 2 at Thermo 45 and 55 and embodiment 3 at Meso 35 followed by Thermo 50, 55 produced significant results, meeting EPA Class A requirements 98% and 100% of the time, respectively. Sludge treated following the method of the invention as set forth in these embodiments is suitable, with respect to pathogen densities, for most applications where commercial fertilizers and soil conditioners are used. 
     A summary of the performance of the three embodiments is set forth in Table 4: 
     
       
         
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
               
               
               
               
               
             
           
               
                   
                 TABLE 4 
               
             
             
               
                   
                   
               
               
                   
                 Embodiment 1 
                   
                 Embodiment 2 
                   
                 Embodiment 3 
               
             
          
           
               
                 Temperature (° C.) 
                 48 
                 52 
                 62 
                   
                 45 
                 50 
                 55 
                   
                 35 
                 45 
                 50 
                 55 
               
               
                   
               
             
          
           
               
                 Pop. Mean &lt; 1000 
                   
                 Fail 
                 0.26 
                 3.76 
                   
                 6.46 
                 18.52 
                 36.85 
                   
                 1.07 
                 14.65 
                 1023.5 
                 1336 
               
               
                 t calculated (All 
               
               
                 numbers are −ve) 
               
               
                 degree of freedom 
                   
                   
                 35 
                 35 
                   
                 40 
                 40 
                 40 
                   
                 40 
                 40 
                 40 
                 40 
               
               
                 4 @ 0.005 level 
                   
                   
                 2.724 
                 2.724 
                   
                 2.705 
                 2.705 
                 2.705 
                   
                 2.705 
                 2.705 
                 2.705 
                 2.705 
               
               
                 (All numbers are 
               
               
                 −ve) 
               
               
                 Fails/Succeeds 
                   
                 F 
                 F 
                 S 
                   
                 S 
                 S 
                 S 
                   
                 F 
                 S 
                 S 
                 S 
               
               
                 (F/S) 
               
               
                 Interval 
                 LCL 
                 1356* 
                  255** 
                 0 
                   
                 0 
                 0 
                 0 
                   
                 0 
                 0 
                 1.2 
                 0.8 
               
               
                   
                 UCL 
                 5400* 
                 1614** 
                 834 
                   
                 561 
                 237 
                 113 
                   
                 1449 
                 289 
                 64 
                 4.8 
               
               
                 Portion of 
               
               
                 Population 
               
               
                 Meeting Class A 
               
               
                 90% of the Time 
               
               
                 z calculated 
                   
                 N/A 
                 N/A 
                 N/A 
                   
                 1.73 
                 1.1 
                 1.73 
                   
                 N/A 
                 0.44 
                 2.05 
                 2.05 
               
               
                 p-value 
                   
                 N/A 
                 N/A 
                 N/A 
                   
                 .0418 
                 .1357 
                 .0418 
                   
                 N/A 
                 0.33 
                 .0202 
                 .0202 
               
               
                 Confidence Level 
                   
                 N/A 
                 N/A 
                 N/A 
                   
                 95.82 
                 86.43 
                 95.82 
                   
                 N/A 
                 67 
                 97.98 
                 97.98 
               
               
                 (%) 
               
               
                   
               
               
                 *The 99% confidence interval for embodiment 1 population mean for fecal coliform densities @ 48° C. ranges from 1356 to 5400 MPN, which is well about the Class A limit of 1000 MPN/100 ml.  
               
               
                 **At 52° C., this embodiment is a near miss, but should provide consistent compliance with EPA Class A when the mesophilic digester is added as a second stage.  
               
             
          
         
       
     
     Comparison of Process 1 and 2 
     Results from embodiment 1 and embodiment 2 testing were compared, and the results presented in Table 5. 
     
       
         
               
             
               
               
               
             
           
               
                 TABLE 5 
               
             
             
               
                   
               
               
                 Statistical Similarities Among Embodiments 1 and 2 
               
               
                 Average Fecal Coliform Log Concentrations 
               
             
          
           
               
                 Reactor 
                 Embodiment 1 
                 Embodiment 2 
               
               
                   
               
               
                 Meso 35 
                 3.6871 ± 0.6407 
                 2.9448 + 0.5927 
               
               
                 Thermo 45 
                 3.1341 ± 0.6765 
                 1.8011 ± 0.9091 
               
               
                 Thermo 50 
                 2.0751 ± 1.1216 
                 1.1986 ± 0.8145 
               
               
                 Thermo 55 
                 1.6061 ± 1.0182 
                 0.7232 ± 0.6693 
               
               
                   
               
             
          
         
       
     
     A number of conclusions may be made regarding fecal coliform removals: 
     1. Longer residence times showed greater fecal coliform reductions. 
     2. A reactor operating at a 2-day HRT and 50° C. had a statistically similar ability to disinfect as a reactor operating at a 15-day HRT and 45° C. 
     3. A reactor operating at a 2-day HRT and 55° C. had a statistically similar ability to disinfect as a reactor operating at a 15-day HRT and 45° C. or 50° C. 
     4. Embodiment 2 showed significantly greater removals at thermophilic temperatures, although it must be noted that embodiment 2 had lower feed sludge concentrations and that embodiment 1 used only one stage of a two-stage process. Operation of the systems according to identical conditions for embodiments 1 and 2 would likely produce more similar removal rates. 
     From the results of embodiments 1 and 2, the following conclusions may be drawn: 
     1. Average effluent fecal coliform concentrations were less than 1,000 MPN/g. TS, meeting EPA 503 fecal coliform requirements for Class A treatment for embodiment 1 operating at 50° C. and 55° C.; and for embodiment 2 operating at 45° C., 50° C. or 55° C. 
     2. Embodiment 2 showed greater fecal coliform removals than embodiment 1 removals at the same temperature. However, adding a methanogenic second stage to embodiment 1 and more consistent feed sludge fecal coliform levels may reduce this difference. 
     Thus, three embodiments of a method for pathogen destruction has been disclosed. The second and third embodiments produce fecal coliform reductions in biosolids to a level sufficient to apply these treated biosolids in a manner similar to that of commercial fertilizers or soil conditioners. It will be appreciated that further variations and modifications thereof may be made within the scope of the invention as defined in the appended claims.