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Pretreatment of Sludge With Microwaves for Pathogen Destruction and Improved Anaerobic Digestion Performance

Posted on: Wednesday, 15 February 2006, 06:00 CST

By Hong, Seung M; Park, Jae K; Teeradej, N; Lee, Y O; Et al

ABSTRACT:

A new way of generating Class A sludge using microwaves was evaluated through a series of laboratory-scale experiments. Microwaves provide rapid and uniform heating throughout the material. Other benefits of microwave treatment include instant and accurate control and selective and concentrated heating on materials, such as sludge, that have a high dielectric loss factor. Sludge was irradiated with 2450-MHz microwaves, and fecal coliforms were counted. Fecal coliforms were not detected at 65C for primary sludge and anaerobic digester sludge and at 85C for waste activated sludge when sludge was irradiated with 2450-MHz microwaves. During the bench-scale anaerobic digester operation, the highest average log reduction of fecal coliforms was achieved by the anaerobic digester fed with microwave-pretreated sludge (≥2.66 log removal). The anaerobic digester fed with microwave-irradiated sludge was more efficient in inactivation of fecal coliforms than the other two digesters fed with raw sludge and externally heated sludge, respectively. It took more than three hydraulic retention times for a bench-scale mesophilic anaerobic digester to meet Class A sludge requirements after feeding microwave-irradiated sludge. Class A sludge can be produced consistently with a continuously fed mesophilic anaerobic digester if sludge is pretreated with microwaves to reach 65C. Water Environ. Res., 78, 76 (2006).

KEYWORDS: anaerobic digestion, Class A wastewater sludge, microwave irradiation, pathogen reduction.

doi: 10.2175/106143005X84549

Introduction

Approximately 8 to 9 million metric dry tons of biosolids are produced each year by municipal wastewater treatment facilities in the United States. Sludge treatment and disposal may account for up to 40 to 60% of the total wastewater treatment cost (Barrett, 1996). Beneficial use of biosolids is expected to increase from 63% in 2000 to 70% in 2010. In 1992, the U.S. Environmental Protection Agency (U.S. EPA) promulgated a regulation (40 CFR part 503; Standards for the Use or Disposal of Sewage Sludge, 2005) to protect public health and the environment from reasonably anticipated adverse effects of certain pollutants in wastewater sludges (58 FR 9248, 1993). For Class A sludge, fecal coliform densities must be less than 1000 most probable number (MPN)/g-total dried solids (TS), or Salmonella sp. bacteria must be less than 3 MPN/4 g-TS (U.S; EPA, 1999b). Many wastewater treatment plants in the United States are currently evaluating ways of increasing pathogen destruction in biosolids to make Class A sludge. The processes approved by U.S. EPA (1999a) are rather expensive and may not be applicable for many wastewater treatment plants.

Microwave irradiation can be an appropriate method for destruction of pathogens in sludge. Pathogen-free sludge may be recycled through land application as a soil conditioner or fertilizer. Destruction of an indicator organism, fecal coliform, by microwave irradiation may be achieved at temperatures lower than by external heating methods (e.g., boiler).

Park et al. (2002) found from biochemical methane potential tests that when only primary sludge (PS) and waste activated sludge (WAS) were irradiated with microwaves, gas production increased by up to 57% compared with the conventional mesophilic operation. Park et al. also claimed that no fecal coliform was detected in sludge samples when sludge was irradiated with microwaves for one minute (with a corresponding temperature of 65C), while the temperature had to be raised to 99C when external heating was used. The objectives of this study were to determine microwave penetration depths for PS and WAS, assess the effect of microwave irradiation on methanogenic bacteria, and evaluate whether a benchscale anaerobic digester receiving microwave-irradiated sludge can consistently and continuously produce Class A sludge under various solids retention times (SRTs).

Microwave Penetration Depth Theory

The electrical field strength and the frequency (f) represent the energy source. It is noted that the increasing electric field strength has a dramatic effect on the power density. The energy transfer is influenced by the electrical properties of the material.

According to eqs 6 and 7, if frequency (f) is constant, and the variations of material density (p) and specific heat (C^sub p^) are relatively small, the temperature increase by microwave irradiation will be a function of ε".

Materials and Methods

Sludge Sampling and Experiment Preparations. Sludges tested were obtained from the Nine Springs Wastewater Treatment Plant (WWTP) in Madison, Wisconsin. Primary sludge was taken from a gravity thickener underflow pipe. The thickened WAS was taken from the dissolved air flotation (DAF) thickened sludge pipe. The anaerobic digester sludge (ADS) was obtained from the recirculation line for heating. All samples were stored separately in a 4C refrigerator until they were tested.

Total Coliforms and E. coli Analysis. The membrane filter test (MF, Standard Method 9222B; APHA et al., 1995) was adopted for finding the total coliform count for sludges or pure cultured samples. An appropriate volume of sludge sample or its dilution (10^sup 2^, 10^sup 3^ or 10^sup 6^) was passed through a MF that retained the bacteria present in the sample. The filter containing microorganisms was placed on an absorbent pad saturated with m- ColiBlue 24 (U.S. EPA approved method 10029) (Cole Farmer, ATCC 25922, PN# 14020-01; Cole Farmer, Vernon Hills, Illinois) in a petri dish. The dish was incubated at 35 0.5C for 24 2 hours. After incubation, the typical red (coliforms) and blue (E. coli) colonies were counted under low magnification, and the number of total coliforms and total number of blue and red colonies were recorded either per 100 mL of the original sample or per gram total dried solids (U.S. EPA, 1987).

Microwave Penetration Depth Measurement. To obtain the penetration depth, an acryl-air baffled vessel was constructed and placed in a 2450-MHz microwave unit, as shown in Figure 1. The vessel had five reservoirs, with volumes of 50, 50, 110, 110, and 130 mL, in which each reservoir was separated by two 1-mm thick acryl walls and 5 mm of air space to prevent heat transfer from one reservoir to adjacent reservoirs. The right side of the vessel wall was tightly attached at the hole of microwave exit to reduce the wave loss caused by leakage before testing. Temperature was measured using a thermo-coupled thermometer (Cole Farmer p-92900-20, T-type) within 30 seconds, as soon as the microwave power stopped. Deionized (DI) water, tap water, and 10% sodium chloride (NaCl) water were tested at the initial temperature of 20 0.2C and sludge samples at 10 0.2C. The microwave irradiation times were 30, 60, and 90 seconds.

Determination of the Survival Fraction of Fecal Coliforms. The purpose of the determination of the survival fraction of fecal coliforms was to investigate the difference between microwave irradiation and external preheating with different types of sludge and to determine the best microwave irradiation point. Fecal coliform reduction was assayed for optimal irradiation time and temperature conditions. Sludge samples were irradiated with microwaves at 0.70, 1.40, 2.80, 4.19, and 5.59 watts-h/g TS for ADS; 0.34, 0.67, 1.35, 2.02, and 2.69 watts-h/g TS for PS; and 0.53, 1.06, 2.11, 3.17, and 4.23 watts-h/g TS for WAS. This corresponded to 15, 30, 60, 90, and 120 seconds of irradiation time per 200 mL sludge sample, and the temperature increased accordingly to 25, 45, 65, 85, and 100 2C. Six samples each containing of 200 mL of PS were prepared in 500 mL beakers. The samples were irradiated with microwaves for 0 (control), 15, 30, 60, 90, and 120 seconds, and the temperature was measured as soon as microwave irradiation stopped. This procedure was repeated with WAS and ADS. Similar samples of PS, WAS, and ADS were heated in a water bath to temperatures of 10 (control), 25, 45, 65, 85, and 100C.

The membrane filter (MF, Standard Method 9222D; APHA et al., 1995) test was used to obtain the survival fraction of fecal coliforms in microwave-irradiated and externally heated sludge samples. An appropriate volume of each sample or its dilution (10^sup 2^, 10^sup 4^, or 10^sup 6^) was passed through a sterile, gridded, 0.450-m membrane filter that retained all bacteria present in the sample. This filter was placed on an absorbent pad saturated with m-FC (Cole Farmer, ATCC 25922, PN# 14020-01) in a petri dish. The dish was incubated at 44.5 0.5C for 24 2 hours. After incubation, the typical blue colonies were counted under low magnification, and the number of fecal coliforms was recorded either per 100 mL of original sample or per gram total dried solids.

Bench-Scale Anaerobic Digester Tests. Bench-scale anaerobic digester tests were performed to evaluate fecal coliform destruction during continuous operation of digesters at various SRTs. Because anaerobic digesters were operated in a semibatch mode, SRT was virtually the same as hydraulic retention time (HRT). Approximately 4 L of anaerobic digester sludge was inoculated as seed sludge in three 6-L reac\tors. The first three months of the bench-scale experiment were spent in setup, sludge seeding, and biomass acclimation, with the seed sludge obtained from the Nine Springs WWTP.

Each anaerobic digester was equipped with a mechanical mixer that operated at a speed of 20 rpm. The first reactor was a control experiment, simulating a conventional mesophilic anaerobic digester receiving primary sludge and WAS. The second reactor received the feed irradiated with microwaves. A third reactor received the feed heated in a water bath to simulate typical heating with a heat exchanger. The feed was heated to a temperature achieved by a corresponding amount of microwave irradiation. The feed contained equal volumes of PS and WAS (1:1). A 1-kW microwave oven designed for household use was used for the study. The feed was manually added once or twice a day to the anaerobic digesters based on the SRT. The SRT was controlled by removing the same volume of sludge as was added each day. After the first three months, each anaerobic digester was operated at an SRT of 20 days for 60 days, at 10 days for 20 days, at 5 days for 25 days, at 15 days for 15 days, at 10 days for 15 days, and at 7.5 days for 20 days. The typical 3 SRT rule (where the experiment is carried out through three retention times) was not observed in this experiment, because the main objectives were to evaluate coliform destruction under worst-case dynamic loading situations and to assess regrowth problems in anaerobic digesters after coliforms in the feed sludge were destructed almost completely by microwave irradiation. The temperature of the anaerobic digesters was controlled at 35 0.5C by insulating the outer wall of each digester with a 6.4-mm (0.25-in.) Tygon tube (Cole-Parmer, Vernon Hills, Illinois) and circulating water from a heated water bath through the tube (Haake Bath and Circulator, P-12203-00, Cole-Parmer). A 4.2-L gas-sampling bag (standard Teflon bags with on/off valves) was used to collect gas, and the daily gas volume was determined using a wet gas meter. The temperature and gas measurement devices were calibrated at least once every two weeks.

Figure 1-Schematic of devices for the measure of penetration depth.

A statistical analysis was performed to determine if there is a difference in the two average results. Three alternatives, such as raw sludge, microwave-irradiated sludge, and externally heated sludge, were compared to each other to evaluate the level of difference in each pair. The standard procedure for comparing two methods is to construct a null hypothesis, which is tested statistically using a paired t-test (Berthouex and Brown, 1994).

Results and Discussion

Figure 2-Temperature variations over distance with error bars after microwave irradiation (average 60-second irradiation).

Table 1-Determination of penetration depth and attenuation factor.

The left term of eq 8 indicates the absorbed microwave heating energy (w) from the surface of the material to penetration depth. The right term represents one-half of the total absorbed microwave energy. Because the equation includes the penetration depth term (D^sub p^) (implicitly in M^sub j^) and P = 0.5P^sub 0^, the penetration depth of the sample by 2450-MHz microwaves can be calculated.

As shown in Figure 2, the temperature decreased as the distance from the microwave source increased. Because penetration depth was expected to be O to 2.4 cm (first or second reservoir), the thickness of first and second reservoirs was 1.2 cm. Of three different water samples (DI water, tap water, and 10% NaCl), the 10% NaCl solution showed the highest power dissipation (225.47 watts) at the first reservoir.

Although the initial temperature of PS and WAS was 10 0.2C, PS and WAS absorbed 254.14 and 241.74 watts at the first reservoir, respectively, indicating that PS absorbed the highest microwave energy among the five samples. It appears that microwave energy absorption depends on the characteristics of materials, such as water content, ionic strength, percentage of protein and fat, and viscosity (Metaxas and Meredith, 1983). It should be noted that, as the power absorption increases, the penetration depth decreases.

The average penetration depths of various media are summarized in Table 1. The penetration depths of PS and WAS were 1.7 and 1.1 cm, respectively. These values were relatively small compared to the water samples. The NaCl solution had the highest penetration depth (2.6 cm), followed by DI water (2.2 cm). As discussed above, penetration depth of materials is determined by many complicated factors. One of the factors is solid contents (water contents). Total solids concentrations of WAS and PS used in the test was 29 800 and 41 300 mg/L, respectively. Another possible reason might be viscosity. The WAS taken from the thickener (DAF) showed a high viscosity, although viscosity was not tested. Because the heating mechanism of the microwave is regarded as the interaction of dielectric materials (dipole moments of polar materials like water molecules), heating is thought to be affected by viscosity (Metaxas and Meredith, 1983).

The penetration depth is also associated with the attenuation factor (α'), as shown in Table 1. The attenuation factor was calculated using α' = 0.347/penetration depth (Lambert's expression). All of these factors are important in influencing the penetration depth for sludge, which is essential for design of a 2450-MHz microwave unit for generating Class A sludge.

Release of Soluble Chemical Oxygen Demand from Sludge by Microwave Irradiation. It is expected that cell membranes, proteins, or some colloidal nonsoluble materials in sludge can be solubilized when sludge is irradiated with microwaves. A series of experiments was performed with 20 mL of sludge samples in a 50-mL container at microwave irradiation times of O, 3, 6, 9, 12, 15, and 18 seconds to evaluate the degree of solubilization of organics. The sludge samples tested were ADS, PS, and WAS. The changes in soluble chemical oxygen demand (COD) at various temperatures caused by microwave irradiation are shown in Figure 3.

The PS had the greatest initial soluble COD (SCOD) concentration (6220 mg/L), followed by WAS (3510 mg/L), and ADS (2340 mg/L). When the temperature reached 60.5C, the soluble COD value of WAS surpassed that of PS. In the case of ADS, the soluble COD value reached a maximum at 51.5C and then decreased slightly. Soluble COD values of PS and WAS increased when the sludge sample temperature rose to over 30C by microwave irradiation. The temperatures of initial samples ranged from 10 to 15C (the first data point of each curve in Figure 3). The WAS had a greater SCOD increase than ADS and PS. The soluble COD values of PS, WAS, and ADS increased from 6220 to 7200 mg/L, 3510 to 7890 mg/L, and 2340 to 3400 mg/L (equivalent to a soluble COD increase of 16, 125, and 45%, respectively), when the temperature reached 72.5C.

The variations of the SCOD/total COD (TCOD) ratio are shown in Figure 4. With the increase in temperature, the SCOD/TCOD ratio increased in the case of WAS. However, PS and ADS had a slight increase in the SCOD/TCOD ratio for all temperatures. The SCOD/TCOD ratios increased by 2, 9, and 5% for PS, WAS, and ADS, respectively. These increases in SCOD are thought to increase the gas production rate and the efficiency of anaerobic digestion.

Figure 3-Changes in soluble COD at various temperatures after microwave irradiation.

Figure 4-Ratios of SCOD/TCOD of ADS, PS and WAS after microwave irradiation.

Pathogen Destruction Efficiency. To compare the effectiveness of microwaves with external heating, the same volume of sludge was heated in a water bath to the corresponding temperature achieved by microwave irradiation, and total coliform, fecal coliform, and E. coli were measured. The fecal coliform counts for various treatments of primary sludge are shown in Figure 5. The x-axis is the temperature increase caused by microwave irradiation and external heating in a water bath. Initially, microwave-irradiated samples had much lower coliform counts than externally heated ones. When temperature reached 65C for microwave irradiation, the fecal coliform count was not detected, while external heating had to reach 85C to achieve the same efficiency, indicating that fecal coliforms were lower than the detection limits.

Total coliform counts were always greatest followed by fecal coliform and E. coli, as was expected. At 1.35 watts-h/g TS (65C), total coliform, fecal coliform, and E. coli were not detected. In the case of external heating, all three were not detected at 85C. For external heating (water bath), typically it took 0.9, 1.9, 2.9, 3.8, and 4.8 minutes to raise the temperatures to 25, 45, 65, 85, and 100C, respectively. In general, pathogen destruction was insignificant up to 45C. The irradiation of PS with microwaves will result in prepasteurization (or destruction of pathogens) and breakdown of paniculate organics into more readily biodegradable organics. This step will significantly destruct pathogens, sufficient for meeting Class A sludge requirements. Furthermore, the detention time in sludge stabilization processes will be shortened significantly. To reduce the volume for microwave irradiation, it is better to thicken the primary sludge with a thickener. The microwave irradiation will increase the influent sludge temperature to approximately 65C or more. Therefore, when anaerobic digestion is used as a stabilization process, microwave-irradiated sludge may have to be cooled in a heat exchanger filled with raw sludge before microwave pretreatment. This will lower the energy cost for microwave pretreatment.

Typically, sludge treatments for the pathogen reduction are classified with (1) pretreatment, (2) post treatment, and (3) submerged treatment using ADS recycling. The second trial was to test the feasibility of irradi\ating a recycle stream from a sludge stabilization process with microwaves. This irradiation will lead to pathogen destruction and heating. This will ensure the complete exposure of the sludges in a stabilization process to microwaves for generation of Class A sludge. For heating only or partial pathogen destruction, the exposure volume may be reduced. The test results for ADS are shown in Figure 6. The results were very similar to those for primary sludge. No fecal coliforms were detected when ADS was pretreated with microwaves higher than 65C. External heating had to reach 85C to have no detection of fecal coliform count. At 2.8 watts-h/g TS (65C) by microwaves, total coliform, fecal coliform, and E. coli were all not detected. In the case of external heating, all three were not detected above 85C.

The third test was the microwave irradiation of thickened WAS taken from a DAF system in the Nine Springs WWTP. Microwave irradiation of WAS is thought to destruct pathogens and hydrolyze biomass. This is a pre-pasteurization step before disposal or further treatment in sludge stabilization processes. Because of hydrolysis of biomass or breakdown of biomass into smaller molecular weight organics, the detention time in the sludge stabilization process is anticipated to be shortened. The test results for WAS are shown in Figure 7.

Figure 5-Fecal coliform reductions of PS with microwaves and thermal heating.

Figure 6-Fecal coliform reductions of ADS by microwaves and thermal heating.

Figure 8-Fecal coliform variations during bench scale anaerobic tests (MW: microwave; EH: external heating). Influent sludge = [white circle]; R1 (control) = *; R2 (MW) = [white square]; R3 (EH) = [white triangle up]; and dotted lines at the bottom of the figure = detection limits.

External heating showed almost the same trends as microwave irradiation up to 45C. At temperatures higher than 65C, fecal coliform destruction was almost the same for both microwave irradiation and external heating. Fecal coliforms were detected until the temperature reached 85C for WAS. The higher temperature needed to destruct pathogens in WAS was thought to be a result of shorter microwave penetration depth of WAS. Sample thickness in a 500-mL beaker was much greater than the microwave penetration depth of 1.1 cm for WAS. Therefore, only a fraction of sludge was irradiated with microwaves, while the rest was heated by convection. This might have lead to poorer pathogen destruction than expected. Further studies are currently being performed to determine important factors controlling the pathogen destruction in sludges such as mixing, solids content (water content), and viscosity.

Changes in Fecal Coliform during the Bench-Scale Anaerobic Digester Tests. The changes in fecal coliforms during the operation periods are shown in Figure 8 with detection limits of fecal coliform counts (~30 CFU/g TS) and SRTs (dotted line). When fecal coliforms were not detected, the detection limit value was used instead of zero in Figure 8. The initial fecal coliform values of raw sludges (WAS + PS = 1:1) ranged from 10^sup 4^ to 10^sup 5^ CFU/ g TS. During the 20-day SRT, the fecal coliform count in Rl (control) fluctuated from 10^sup 3^ to 10^sup 4^ CFU/g TS. Anaerobic digestion is known to have ability to reduce the pathogen because of microbial competition (U.S. EPA, 1999a). The fecal coliform count decreased gradually over time for R2 (MW) and R3 (EH) as a result of washout of fecal coliforms in the reactors. Conventional digester sludge still contained over 10^sup 4^ CFU/g.TS and preheated sludge had 10^sup 2^ to 10^sup 4^ CFU/g.TS, while fecal coliform counts were <10^sup 3^ CFU/g TS or not detected in microwave-pretreated sludge (R2) after 50 days of operation. It appears that three times the HRT is required to meet Class A sludge limits after irradiated with microwaves to ~65C. Note that HRT in the semi-batch fed reactors adopted in this study is the same as SRT.

Figure 7-Fecal coliform reductions of WAS by microwaves and external heating.

Raw sludge containing PS and WAS was Class B, based on the U.S. EPA regulation limit in terms of fecal coliform counts. Digesters receiving raw sludge and preheated sludge could not generate Class A sludge consistently. However, the pretreatment with microwaves provided Class A sludge consistently. The average values (log unit) of fecal coliform detection during the operation were approximately 3.5 for R1 (control: raw sludge), 2.1 for R2 (microwave-irradiated sludge), and 2.7 for R3 (externally heated sludge). The average log reductions of fecal coliforms were approximately 1.27, 2.66, and 2.08 for reactors R1, R2, and R3, respectively, as shown in Table 2. The analyzed paired t-test results from the sludge obtained by the control (R1), microwave (R2), and external heating (R3) reactors are summarized in Table 2. The results of fecal coliform counts for pairs of digesters were significantly different, and it is concluded that the second digester receiving microwave irradiated sludge (R2) is more efficient in inactivation of fecal coliforms than the other two digesters (Rl and R3).

The pH of the three digesters was maintained at neutral conditions (pH 7.0 0.2) throughout the experimental period, except the initial 10 days of SRT five-day operation. The pH of Rl, R2, and R3 dropped to 6.3, 6.5, and 6.5, respectively. The performance of anaerobic digesters was not stable at the five-day SRT for all three digesters. The digesters appeared to have organic overloading. The pH value increased to nearly neutral and fluctuated slightly in the narrow range until the SRT was reduced from 15 to 7.5 days. At the SRT of ≥7.5 days, the pH seemed to be relatively stable at near neutral. Alkalinity dropped rapidly at the initial period after changing the SRT. In addition, alkalinity fluctuated significantly when the digester operated at an SRT of 5 and 7.5 days. The alkalinity values of the effluent sludge from Rl, R2, and R3 were in the range 4000 to 7000, 4000 to 6800, and 4000 to 6500 mg calcium carbonate (CaCO^sub 3^)TL.

Table 2-Results of paired t-test of fecal conforms for each pretreatment (unit: log CFU/gTS).

Figure 9-Cumulative gas production from SRTs of 5, 7.5, 10, and 15 days.

Effluent total COD values for both R2 and R3 were slightly greater than that for Rl. The effluent soluble COD values of all three digesters were very similar. When the SRT was 5 days, the TS increased from approximately 22 000 to 25 000 mg/L, to 35 000 to 38 000 mg/L, and then decreased slightly. Volatile solids (VS) had a trend similar to TS. At SRTs of 15, 10, and 7.5 days, TS and VS values remained almost constant. More detailed information on the operation can be found in Teeradej (2002). Cumulative gas productions from the three reactors at different SRTs are shown in Figure 9. At the five-day SRT, R2 and R3 produced 68 and 55% more gas than the control reactors after 11 days. In general, R2 produced the most gas at all SRTs. Surprisingly, R3 produced the least gas at SRTs of 7.5 and 15 days. In terms of gas composition, methane accounted for 55 to 65%, carbon dioxide for 35 to 45%, and nitrogen gas for less than 2%. With the increase in SRT, the percent of methane gas composition increased, while the carbon dioxide composition decreased. This short-term, bench-scale, anaerobic digester test was performed to simply determine coliform destruction at various SRTs. Long-term controlled experiments have been performed to accurately evaluate VS reduction, gas production, and pathogen destruction among three anaerobic digesters (Pino Jelcic et al., 2004).

Conclusions

Several technologies have been developed for generating environmentally safe Class A sludge. Microwave irradiation could potentially be an attractive method because of its synergistic effect on pathogen destruction and thermal heating for anaerobic digestion at 35C. Because sludge from WWTPs can be easily heated with microwave power, and microwave energy has a strong ability to penetrate dielectric materials to produce thermal and nonthermal effects on microbes, it has been postulated that microwave irradiation technology could be applied to biosolids treatment. To accomplish the specific objectives, a series of laboratory-scale tests was performed, and the following conclusions were drawn.

The penetration depths of a 2450-MHz microwave unit were 1.7 and 1.1 cm for PS and WAS, respectively. These values can be used as a design parameter for microwave units to apply to wastewater sludge. When PS, WAS, and ADS were irradiated with microwaves to reach a temperature of approximately 70C, the soluble COD values increased from 6220 to 7200 mg/L, 3510 to 7890 mg/L, and 2340 to 3400 mg/L, which are equivalent to a soluble COD increase of 16, 125, and 45%, respectively.

During the bench-scale anaerobic digester operation, the highest log reduction of fecal coliforms was 2.66 for the digester fed with microwave-pretreated sludge. Based on the paired t-test results at the 95% confidence interval, fecal coliform counts for pairs of digesters were statistically significantly different. The digester fed with microwave-irradiated sludge was more efficient in inactivation of fecal coliforms than the other two digesters receiving raw and preheated sludge. The anaerobic digester receiving microwaveirradiated sludge was the only digester able to consistently meet Class A regulations. It should be noted that it took three times the HRT for the anaerobic digester fed with microwave irradiated sludge to meet Class A regulation. Therefore, the best application for microwave technology would be pretreatment of sludge followed by a sludge stabilization process.

References

American Public Health Association; American Water Works Association; Water Environment Federation (1995) Standard Methods for the Examination of Water and Wastewater, 19th ed.; Washington, D.C.

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Pino Jelcic, S. A.; Hong, S. M.; Park, J. K. (2004) Enhanced Anaerobic Biodegradability and Inactivation of Fecal Coliforms and Salmonella spp. in Biosolids by Using Microwaves. Submitted for publication in Water Environment Research.

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Acknowledgments

Authors. Seung M. Hong is a principal senior researcher with the Environmental Technology Research Team, Daewoo Engineering and Construction Co. Ltd., Korea. Jae K. Park is a professor, N. Teeradej was a M.S. student, and Y.K. Cho is a Ph.D. student with the Department of Civil and Environmental Engineering, University of Wisconsin-Madison. Y. O. Lee is a professor in the Division of Life Science, Daegu University, Kyung-Buk Province, Korea. C. H. Park is a professor in the Department of Environmental Engineering, University of Seoul, Korea. Correspondence should be addressed to Seung M. Hong, Environmental Technology Research Team, Daewoo Engineering and Construction Co. Ltd. 60, SongjukDong, Jangan-Ku, Suwon-City, Kyungki-Do 440-210, Korea.

Submitted for publication March 4, 2003; revised manuscript submitted May 25, 2004; accepted for publication August 9, 2004.

The deadline to submit Discussions of this paper is April 15, 2006.

Copyright Water Environment Federation Jan 2006

Source: Water Environment Research

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