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 65°C for primary sludge and anaerobic digester sludge and at 85°C 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 65°C.
During continuous operation of three mesophilic‐anaerobic digesters, the effect of microwave irradiation, as sludge thermal pretreatment (60 to 65°C), was studied. The fecal coliforms log inactivation for microwaved/digested sludge was 4.2 ± 0.4, whereas for conventionally heated/digested sludge and control were 2.9 ± 0.5 and 1.5 ± 0.5, respectively. In the case of Salmonella spp., no colonies were detected in 85% of the microwaved/digested samples. Considering the detection limit, the log inactivation of these samples was greater than 2.0 ± 0.3. The conventionally heated/digested sludge and control showed log inactivations of 1.9 ± 0.2 and 1.1 ± 0.3, respectively. At the 95% confidence level, microwaved/digested sludge increased the biogas production by 16.4 ± 5.6% and 6.3 ± 2.4%, as compared to control and conventionally heated/digested sludge, respectively. When thermally treated sludge was analyzed for soluble chemical oxygen demand, microwaved waste‐activated sludge showed considerable solubilization between 37 and 60°C. Microwaved/digested sludge showed a reduction of capillary suction times by 11.1 ± 5.9% and 10.7 ± 5.6%, as compared to control and conventionally heated/digested sludge, respectively.
The objectives of this study were to evaluate the amount of phosphorus and metals in sludge that can be released into solution by microwave irradiation when applied to sludge before anaerobic digestion and determine the effectiveness of subsequent lime precipitation. The fraction of phosphorus in the soluble form increased to 23 to 28% for thickened sludge and to 31 to 38% for unthickened sludge, after raising temperatures by microwave heating to 50 to 708C. Microwave irradiation also caused the release of arsenic, molybdenum, nickel, and selenium into solution to 33, 15, 13, and 28% for thickened sludge and 63, 61, 37, and 27% for unthickened sludge, respectively. Microwave irradiation has been found to destruct pathogens in sludge to meet Class A biosolids requirements. Therefore, the reduction of phosphorus and metals in biosolids using microwave heating is economically attractive when considered as a secondary benefit to the use of microwave heating to generate Class A biosolids. Water Environ. Res., 80, 784 (2008).
The objective of this study was to evaluate how much phosphorus and metals in sludge can be released into solution by microwave irradiation when applied to sludge prior to anaerobic digestion, and to determine the effectiveness of subsequent lime precipitation. For thickened sludge, the fraction of phosphorus in the soluble form increased to 23%~28% while for unthickened sludge, to 31%~38%, after raising temperatures by microwave heating to 50~70°C. Microwave irradiation also caused the release of arsenic, molybdenum, nickel, and selenium into solution to 33%, 15%, 13%, and 28% for thickened sludge and 63%, 61%, 37%, and 27% for unthickened sludge, respectively. Microwave irradiation has been found to destruct pathogens in sludge to meet Class A biosolids requirements. Therefore, the reduction of phosphorus and metals in biosolids using microwave heating is economically attractive when considered as a secondary benefit to the use of microwave heating to generate Class A biosolids.
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