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Reduction of Pathogens in Biosolids in Response to Stress Units using Solar Drying BEDS in an Arid/Semi-Arid Climate, Real-Time Data Acquisition

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Achieving high microbial quality of biosolids in an economical manner is a crucial endeavor in ensuring public and environmental safety and facilitating public acceptance of land application of treated sewer sludge. Field experiments were conducted to investigate patterns in microbial reduction when drying Class B biosolids to Class A using open solar drying beds in arid/semi-arid climates, i.e. La Paz County and Pima County, Arizona. In Pima County, the inactivation rates of fecal coliform and salmonella in anaerobically digested versus aerobically activated waste sludge was investigated using two different types of drainage systems. In La Paz County, anaerobically digested biosolids were treated during the drying process by tilling at different frequencies.

Class A criteria levels (fecal coliforms) were reached in 35 days for all experimental beds located at the Pima County site. The results of these baseline studies indicate that the average fecal coliform inactivation rates for aerobically and anaerobically digested biosolids were 0.179/day-1 and 0.137/day-1, respectively during the months of May and June of 2004. A studentized t-test indicated that these averages are not significantly different at an alpha level of 0.05; p=0.569.

The studies at La Paz County demonstrated that Class A criteria levels (fecal coliforms) were achieved during a maximum of 15 days for all three beds. During the month of June 2004, the inactivation rates for fecal coliform were 0.502/day-1, 0.883/day-1, and 1.188/day-1 for the control bed, the bed tilled 2x/week, and the bed tilled 6x/week, respectively. Microbial inactivation rates increased as a function of tilling intensity. The difference between inactivation rates in the control bed (no-tilling treatment) and the beds in Pima County (no tilling treatment) appeared to be a function of bed temperature.

Salmonella levels reached minimum detection values of <3 MPN/4 g of biosolids after 14 days of exposure in the baseline studies, while minimum detection levels were achieved after only 7 days in the experimental beds at La Paz County. Sequential microbial assays of soil after experimental land application of the Class A biosolids from the Pima County and La Paz County sites, demonstrated no regrowth of salmonella. Regrowth of fecal coliforms from the Class A biosolids dried in Pima County did occur, but were dependent on soil moisture content. No regrowth of fecal coliforms occurred when the biosolids dried in La Paz County were land applied.

Document Type: Research Article


Publication date: January 1, 2005

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