The Effects of Physical Stressors on Bacterial Inactivation Rates in Biosolids

Sanitation is fundamental to reducing disease and sustaining a high standard of living. The evolution of sewer systems and the modern engineering of wastewater treatment plants work to decrease health risk and manage environmental concerns associated with the reuse and disposal of treated effluent and solid wastes generated as byproducts. The recycling of treated solid wastes (biosolids) continues to be an environmental challenge due to the shear volume produced, and its potentially hazardous composition. Solar drying of biosolids was studied in semi-arid regions as a sustainable method for reducing pathogens. The initial studies were performed with no intervening treatments. Average fecal coliform inactivation rates for digested biosolids during summer experiments were determined to be 0.17 ± 0.03/day⁻¹ and 0.17 ± 0.04/day⁻¹, respectively. Salmonella inactivation rates in aerobically digested biosolids were 0.11 ± 0.08 day⁻¹ and 2.0 ± 2.0 day⁻¹ for aerobically and anaerobically digested biosolids, respectively for the summer seasons. Solar drying during warm dry seasons was effective in reducing pathogens. Microbial testing to verify the quality of biosolids can be expensive. Utilizing a mathematical model to predict pathogen density levels during the solar drying process can minimize such testing. The first order mathematical model, N(t) = N(o) * 10⁻ᵏᵈᵗ where the inactivation constant, k(d), is further defined as a function of moisture (Θ) and temperature (T), i.e. k(d) = f(Θ,T): k(d) = (k₁/( k₁ + Θ) * (T/(k₂-T)) * k₃, k₁ = 0.112, k₂ = -41.88, and k₃ = -0.5357; for all T greater than or equal to 38ºC, T=38°C provided a good estimate of the inactivation rate of fecal coliforms in biosolids. During subsequent field studies, treatments were employed to manage the drying cycle of biosolids - tilling increased the rate of drying, a covered solar drying bed increased the inactivation rate of fecal coliforms by 300%, and an automated rain shield was engineered to limit enteric bacterial regrowth due to rainfall. Finally, since biosolids are to be considered a source of nitrogen when land-applied, temporal samples of biosolids from various solar drying experiments were analyzed to ascertain the levels of NH⁺₄-N and NO⁻₃-N throughout the drying process. Chemical analyses revealed that as much as 34-92% of nitrogen was lost via volatilization during the drying process