Water Sanitation Health

Findings on an assessment of small-scale incinerators for health-care waste

Summary

This report provides an analysis of low cost small-scale incinerators used to dispose of health-care waste in developing countries, specifically sharps waste (used and possibly infected syringes and needles). The report includes a situation analysis, a “best practices” guide to small-scale incineration, a screening level health risk assessment for ingestion and inhalation exposure to dioxin-like compounds, and other information related to the operation and evaluation of the incineration option for health-care waste.

The situation analysis documents the need for adequate and safe disposal. Quantities of sharps waste generated monthly range from a few kg at remote clinics, to hundreds or possibly thousands of kg at central hospitals, to approximately 1 000 tons world-wide during vaccination campaigns. With improper disposal, syringes and needles may be scavenged and reused, leading to large numbers of people becoming infected with hepatitis, AIDS and other diseases. To avoid these serious health problems, international agencies have promoted the use of low-cost small-scale incinerators. The more recent versions of simple brick incinerators, e.g., the De Montfort Mark8A, utilize primary and secondary combustion chambers, some basic operator safeguards, and a short chimney. These units are built on-site for several hundred to US$ 2 000, depending on the availability of materials and metal working facilities. The use, maintenance and management of these incinerators has been evaluated in four countries using survey-based rapid assessment techniques, and combustion parameters (temperatures, flows, etc.) and emissions (carbon monoxide, dioxins, etc.) have been measured in several field tests. The surveys show widespread deficiencies in the construction, siting, operation and management of these units. These deficiencies can result in poor performance of the incinerator, e.g., low temperatures, incomplete waste destruction, inappropriate ash disposal, high smoke emissions, fugitive emissions, etc. Still, user acceptance of small-scale incinerators appears generally high and the use of incinerators is preferable to the disposal of waste in unsecured pits or landfills, or (uncontrolled) burning in drums or pits. However, the combustion of health-care waste can form particulate matter, dioxins, furans and other toxic air pollutants.

Emission standards for modern incinerators require the use of various air pollution control devices as well as monitoring, inspection and permitting programs. Such standards cannot be met by small-scale incinerators that do not incorporate any air pollution control devices or monitoring devices. Moreover, as typically operated, small-scale incinerators do not achieve the lowest possible emissions. Installation of process monitors, emission controls, and other equipment necessary to meet modern emission standards would increase costs by at least an order of magnitude. When incinerators are used, however, “best practices” should be promoted to minimize occupational and public health risks.

“Best practices” for small-scale incineration has goals of suitably treating and disposing of waste, minimizing emissions, and reducing occupational exposures and other hazards. Best practices includes the following elements: (1) Effective waste reduction and waste segregation, ensuring that only the smallest quantity of appropriate waste types is incinerated. (2) An engineered design, ensuring that combustion conditions are appropriate, e.g., sufficient residence time and temperatures to minimize products of incomplete combustion. (3) Siting incinerators away from populated areas or where food is grown, thus minimizing exposures and risks. (4) Construction following detailed dimensional plans, thus avoiding flaws that can lead to incomplete destruction of waste, higher emissions, and premature failures of the incinerator. (5) Proper operation, critical to achieving the desired combustion conditions and emissions, e.g., appropriate start-up and cool-down procedures; achievement and maintenance of a minimum temperature before waste is burned, use of appropriate loading/charging rates (both fuel and waste) to maintain appropriate temperatures, properly disposal of ash, and various actions and equipment to safeguard workers. (6) Periodic maintenance to replace or repair defective components, e.g., including inspection, spare parts inventory, record keeping, etc. (7) Enhanced training and management, possibly promoted by certification and inspection programs for operators, the availability of an operating and maintenance manual, management oversight, and maintenance programs.

Public health risks from incinerator emissions are driven largely by dioxin and furan emissions. For these toxic, persistent and bioaccumulative chemicals, analyses must consider inhalation and ingestion exposures, the latter due to the consumption of locally-produced foods that become contaminated. Information related to hazard assessment, dose response, exposure assessment, and risk characterization for dioxin-like compounds is reviewed. Several parameters necessary to quantify exposures and risks have considerable uncertainty and high site-to-site variability, thus risk modeling utilizes a range of scenarios and sensitivity analyses. Information related to the emissions of dioxin-like compounds from incinerators without emission controls is reviewed, and three classes are considered: (1) Best practice for a properly operated and maintained unit utilizing sufficient temperatures, afterburners and other features that limit chimney (stack) concentrations to 10 ng TEQ/Nm3; (2) Expected practice for an improperly designed, constructed, operated or maintained units, giving a 500 ng TEQ/Nm3 limit; (3) Worst-case using an incinerator without an afterburner, giving a 4000 ng TEQ/Nm3 concentration. To reflect a range of settings and conditions, four incinerator usage rates were considered: (1) Low usage, equivalent to 1 hr of incineration or 12 kg waste per month; (2) Medium usage, 2 hr or 24 kg waste per week; (3) High usage, 2 hr or 24 kg per day; and (4) Universal usage – burning 12 000 to 20 000 tons per year, equivalent to sharps waste from vaccinations throughout the developing world. Uptake rates for adults and children due to food consumption were estimated by coupling emission and usage rates to estimates of individual intake fractions based on recent studies in the US and Europe. Worst-case inhalation exposures for adults and children were estimated using a plume dispersion model, a range of meteorological conditions, and the same emission conditions discussed above. This modeling also showed the effect of stack (chimney) height on exposures. The resulting uptake rates were compared to WHO’s provisional tolerable intake rate, US EPA’s cancer risk levels, and other indicators. Because of significant uncertainties, e.g., current exposures are unknown, it is prudent to keep exposures from small-scale incinerators to a small fraction of the provisional value.

Dioxin/furan emissions from a single small-scale incinerator that is operated infrequently under best practices is not likely to produce excessive ingestion exposures and risks, however, the feasibility of achieving and sustaining best practices seems doubtful. Under the expected practices emissions, only the low usage scenario keeps the intake to a small fraction of the WHO provisional intake. Ingestion intake rates and risks are unacceptable for the worst-case emission rate at any usage rate. Widespread use of incinerators gives similar exposures and risks similar to that obtained for a single unit, e.g., exposures are below 1% of the provisional WHO value, and aggregate emissions are 1 to 2 g TEQ/year. Under actual practices to worst-case emissions, adult exposures range from 1 to 12% of the WHO provisional value, exposures to children are 17 times higher, and between 60 and 800 g TEQ/year are emitted, a significant fraction of continental and global emissions. Only with best practices are exposures and risks small. The plume modeling shows that maximum airborne concentrations occur at downwind distances from 0 to 800 m, with distances increasing under stable conditions. During the day (neutral and unstable conditions), maximum concentrations occur very close to the incinerator (within 100 m). Increasing stack height from 3 to 6 m significantly lowers concentrations by 5 to 13 times during daytime, and the major effect is observed close to the source (especially relevant for operator exposure). Dilution ratios of at least 1000 are desirable. Ratios well below 1000 can occur at distances below 100 m for daytime conditions, regardless of stack height. At night, ratios below 1000 occur only with the shortest stack height (3 m) and for distances from 200 to 500 m. Low and medium usages under best practices emissions give exposures below 1% of the provisional WHO limit. Like the ingestion estimates, the inhalation assessment has many uncertainties, data gaps are large, and consequently the calculated exposures and risks reflect a wide range. Moreover, results are applicable to open sites where dispersion is not inhibited, and not to incinerators sited in forests or mountainous terrain.

In conclusion, small-scale incineration is viewed as a transitional means of disposal for health-care waste. The analysis in this report shows significant problems regarding the siting, operation, maintenance and management of incinerators. While uncertainties are high, emissions of toxic and persistent compounds from incinerators may result in human exposure at levels associated with adverse health risks. Because chronic exposures to dioxins/furans are judged to pose the major public health risk, transitioning to safer options over a period of several years would not be expected to result in significant adverse consequences, especially if most elements of best practices are followed.

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© World Health Organization 2004
WHO/SDE/WSH/04.07
To order a copy, please contact: phedoc@who.int

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