Characterization of bottom ash from a medical waste incinerator and its potential environmental impacts: A case study of Kaloleni Hospital, Arusha, Tanzania.

Abstract

Although incineration is a prevalent method for medical waste treatment in developing countries like Tanzania, the environmental safety of the resulting ash is often overlooked. This study aimed to present a detailed characterization of bottom ash from a medical waste incinerator at Kaloleni Hospital, Arusha, to assess its chemical composition, leaching potential, and environmental risk. Herein, source-segregated medical waste from three streams, including infectious waste (red bags), laboratory/pharmaceutical waste (yellow bags), and general waste (black bags), was analyzed. The findings revealed the distinct particle-size distributions, with the infectious waste (red bag) ash exhibited the highest fineness modulus (9.29) and the largest average particle size (2.35 mm), indicating greater potential for soil penetration and dust generation. Concentrations of heavy metals in the ash exceeded USEPA permissible limits for soil, including titanium (Ti) (3600-9884 mg/kg), iron (Fe) (3683-7789 mg/kg), zinc (Zn) (4020-7449 mg/kg), copper (Cu) (304-616 mg/kg), and mercury (Hg) (0.93-1.23 mg/kg). Notably, mercury predominated in infectious waste (red bag) ash, which was associated with thermometers. Leachate concentrations of critical metals such as lead (Pb) (1093 mg/L) and chromium (Cr) (601.1 mg/L) exceeded U.S. EPA regulatory limits by more than 200 and 120 times, respectively. Calcium sulfate (CaSO₄), silicon dioxide (SiO₂), and sodium chloride (NaCl) were the main crystalline phases. These findings demonstrate that the bottom ash poses a substantial risk of heavy metal leaching and environmental contamination. This underscores the urgent need for regulated disposal and pre-treatment of such ash at this hospital. This study recommends extending this investigation to other hospitals to fully assess and mitigate the regional risk to public and environmental health.Implications: This study demonstrates that bottom ash from a typical medical waste incinerator in a low-resource setting poses a severe environmental threat. High concentrations of heavy metals (Pb, Cr, Hg, Zn) exceed regulatory limits, with leaching tests confirming alarming mobility, particularly for Pb and Cr. Particle size analysis indicates risks of atmospheric dispersion of fine particles and soil infiltration of coarser fragments. The established link between color-coded waste segregation and ash hazards provides critical insights. These findings necessitate integrated strategies, including optimized incinerator operation, mandatory pre-treatment (e.g., stabilization) of bottom ash, and context-specific regulatory frameworks to mitigate risks to air quality, soil, groundwater, and public health.