Assessment of Incineration Process Performance in a District Hospital Using Statistical Analysis ()
1. Introduction
Hospitals are experiencing problems of high fuel consumption and longer incineration cycle times, using old incinerator designs. Moreover, incinerator emissions and ash handling problems are evident especially where incinerators are poorly performing. This situation can cause diseases such as cancer, pulmonary infections, heart diseases, etc., to the health workers and societies around hospitals. The main source of hazards in the incineration process is presence of toxic and hazardous chemical compounds from poorly operated incinerators. Other problems facing the hospitals include excessive fuel consumption, extended incineration cycle times, air pollution (from toxic gases and particulate matter), soil and ground-water pollution due to poor ash disposal methods. Thus, it is important to ensure that the design, operation, testing, and maintenance of incineration process provide maximum safety and minimum risk to the environment. Therefore, the need for conducting incinerator assessment is of vital importance. The parameters studied in this work included the amounts of sharps waste and other waste incinerated in relation to amount of diesel oil used in the process and time taken to complete the entire incineration cycle. This leads to increased expenses in the operations of any district hospital.
This paper gives more insight on the incineration process, and its major challenges. The study was conducted at Temeke district hospital for 22 months consecutively covering 653 days of daily data collection. Medical wastes incinerated were segregated in two categories: sharps waste containing needles, syringe and surgical blades [1]. Other waste category included all waste such as pathological waste (human tissues, organs and body fluid), pharmaceutical waste (drugs, vaccines spoiled or expired), and chemical waste (detergents, dressing solutions), grouped together [2,3].
2. Literature Review
2.1. Medical Waste Incineration
Incineration is the waste treatment technology that involves thermal destruction of medical waste at higher temperature, usually 850˚C - 1000˚C. Incineration of waste materials converts the waste into bottom ash, flue gases, particulates and heat which can in turn be used to generate electric power. The flue gases are usually cleaned of pollutants before they are dispersed in the atmosphere [4-6]. Modern incineration systems use high temperatures, controlled air supply, and excellent mixing to change the chemical, physical, or biological character or composition of waste materials. The new systems are equipped with state-of-the-art air pollution control devices to capture particulate and acidic gaseous emission like wet scrubbers (Manyele, 2008).
Preliminary studies have shown that incineration is still the suitable method of decontaminating the medical waste in Tanzanian hospitals [7]. It is also the most often used method in developed countries as well (Switzerland, German, Sweden, USA, and Canada). The major advantages of incineration include volume reduction (by more than 90%), assured destruction (99% and above), high sterilization efficiency, high weight reduction, and the ability to manage most types of wastes with little processing before treatment [2,7-9]. However, incineration is facing political challenges when emissions are not controlled and also because it is related to mostly publically owned institutions, such that the public feels to have a say on medical waste incineration even when they lack full knowledge.
2.2. Medical Waste Incineration Process
The medical waste treatment facilities require several units combined together facilitating different tasks and the overall performance depends on the efficiency of each of these units. The major units include: waste storage unit, waste preparation and feeding, waste combustion unit, flue gas treatment and ash handling. To achieve a high degree of combustion in the combustion chamber, for instance, it is necessary to ensure an adequate combustion temperature, sufficient excess supply of air, an ample mixing of the air with the thermal decomposition gas, and an adequate reaction time. In a successful incinerator design, careful consideration is given to the provision of adequate excess air by use of blowers and the appropriate location of the air supplies. Moreover, the combustion temperature is carefully controlled; the gas flow velocities through the combustion chamber are adequately low; and the gas and solid waste residence times in the combustion chamber are of sufficient duration [10].
Some incinerator designs attempt to accomplish combustion of both the gaseous and solid phases in one combustion chamber. It appears much more preferable, however, to employ two combustion chambers connected in series. In such designs, the waste feed is loaded into the primary chamber where it undergoes thermal decomposition and where the carbon residue burnout is also (later) completed. This is followed by a secondary combustion chamber (afterburner) for oxidizing the remaining gases and volatiles, including the particulate matter carried over from the primary chamber, under conditions where there is excess air. A support burner and additional air injection equipment are usually required in the secondary chamber to maintain a sufficiently high and consistent temperature [11]. The excess air incinerators allow an excess of oxygen during the primary combustion process so that both the gaseous and solid fractions can burn directly in one combustion chamber. The controlled air incinerators limit the air supply in the primary combustion step to near or below the stoichiometric ratio, and a secondary combustion step is needed for completion of the combustion of gaseous fraction in an oxygen rich atmosphere [12].
The pyrolyzing incinerators employ secondary chambers where the ash and the gaseous products of pyrolysis are fully oxidized in an oxygen rich atmosphere. The incinerators employed to produce the slag use relatively high process temperatures to burn the carbonaceous residue by receiving heat, typically from burning fuel, thus releasing an amount of heat energy sufficient to convert all non-combustibles contained in the waste feed to molten slag. The combustion of both the solid and gaseous fractions of the waste is accomplished in one chamber and the fly ash resulting from the process leaves the chamber with the off-gas. The composition and character of the ash vary with the combustion technique used and with the composition of the waste feed. The excess air and the controlled air incineration leave a similar end product but contain less carbon [13].
2.3. Average Waste Incineration Rate
The average waste incineration rate per year for an incinerator studied is about 35 tons which is very low compared to waste generation capacity for a district hospital located in urban areas. It is estimated that the total amount of hospital waste incinerated, including the waste incinerated off-site, to be about 80 percent of the total hospital waste in the United States [12]. In 2001 the United States had 190 operating incinerators with a design capacity of 114,339 tons/day and an annual capacity of 35.5 million tons. Germany, which has the highest concentration of incinerators in Europe, has 530 units with an annual capacity of 107 million tons [14].
Incineration can be adapted to the destruction of a wide variety of wastes. This includes municipal solid wastes, industrial wastes, medical wastes, sewage, surface polluted soils and liquids, and the hazardous wastes (liquids, tars, sludge’s, solids, and vent fumes) generated by industry [15]. Unlike many other methods of waste treatment, incineration is a permanent solution. The major benefit of incineration is that the process actually destroys most of the waste rather than just disposing of or storing it. Many health facilities which were built after World War II in Europe, selected incineration as the method of medical waste disposal over landfills. There was a lack of consideration of exhaust emissions from these units in the original designs, such that, tall stacks were used for dispersion rather than proper air pollution controls. The combustion furnaces were operated at high excess air levels resulting in lower temperatures, incomeplete combustion and high levels of carbon monoxide and unburned hydrocarbons. Typical conditions surrounding these facilities were high soot and odor levels as well as corrosion from acid gas deposition. Poorly designed and ill operated incinerators led to an unhealthy and unsafe environment for the health facilities and neighbors [16].
2.4. Incinerator Siting, Planning and Quality Control
Variations in medical waste incineration processes and other innovative technologies continue to appear. At present, controlled air incinerators are popular due to their relatively low (capital, operating and maintenance) cost and their ability to meet existing air standards without air pollution controls [17]. The location of an incinerator can significantly affect dispersion of the plume from the chimney, which in turn affects ambient pollutants concentrations, deposition and exposures of workers and the community to the gaseous emissions. Best practices of siting incinerators have the goal of finding a location that minimizes potential risks to the public health and the environment [2]. Adequate plans, drawings, and quality control are necessary to construct incinerators. Dimensional drawings, tolerances, material lists, etc., are necessary. A lack of adequate quality control in the construction phase results in incorrectly-built facilities, whereby shelters, protective enclosures, and pits have not been constructed in most sites [18].
2.5. Combustion Temperature and Cycle Time
Proper design and operation of incinerators should achieve desired temperatures, residence times, and other conditions necessary to destroy pathogens, minimize emissions, avoid clinker formation and slugging of the ash (in the primary chamber), avoid refractory damage and minimize fuel consumption. Good combustion practice (GCP) should be followed to control dioxin and furan emissions [5]. A minimum residence time of one second in the combustion zone at the minimum combustion temperature specified in the design is generally considered adequate to provide high-efficiency incineration. The residence time is calculated from the point where most of the combustion has been completed and the incineration temperature has been fully developed. In multichamber incinerators, the residence time is calculated from the secondary burner(s) flame front. If air is introduced downstream of the burner flame front, residence time should also be calculated from the final air injection point [18].
2.6. Assessment of Incinerator Performance in Tanzania
The assessment of operational problems in the existing incinerator designs has been conducted at Muhimbili National Hospital (MNH), Medical Stores Department (MSD), Mwananyamala Hospital (all in Dar es Salaam), Mnazi Mmoja Hospital (MMH) in Zanzibar, and Bugando Medical Center (BMC) in Mwanza in 2004. Problems identified include low incinerator capacities (kg waste/h), high fuel consumption rate (L/h), poor combustion efficiencies, poor chamber designs, and air pollution problems [8]. However, none of the assessment studies concentrated on one unit to establish its performance based on long term behavior. This study concentrated on one unit for 22 months which is operated in a district hospital (Temeke).
3. Materials and Methods
3.1. Design Features of the Assessed Incinerator
The old Temeke hospital incinerator was a simple facility comprising of a cylindrical combustion chamber with an opening for waste feeding and a chimney for smoke outlet, as shown in Figure 1. The secondary burner is connected to the chimney, which acts as a secondary chamber. Such units are not suitable in terms of combustion efficiency and environmental acceptance. The first chamber performs pyrolytic destruction of the waste and final combustion of gases takes place in the secondary chamber. This incinerator was designed mainly for destroying placentas from labor wards, but due to scarcity of incineration facilities, it was used for destroying all medical waste generated in a district hospital.
The fuel consumption was alarmingly high, about 20 - 40 L/h, which necessitates investigation. The maximum temperature was about 700˚C only in the secondary chamber, but the primary chamber temperature was only about 400˚C - 500˚C. The incinerator had the capacity to burn only about 10 placentas per day, but was usually loaded from 60 to 120 kg of mixed medical waste due to lack of air pollution control device made the whole equipment less useful. Other waste types (like chemicals,
Figure 1. Design features of the assessed incinerator.
pharmaceuticals) could not be destroyed due to lack of air pollution control device (APCD). The incinerator was located in densely populated area, which made the smoke problem to be a continuous nuisance to the nearby community. Another incinerator problem in relation to its location was fluid leakage (blood) from the drying placentas in the primary chamber onto the floor which caused aesthetic view and odorous environment.
3.2. Experimental Model Formulation
Figure 1 shows the process flow sheet for the double chamber incinerator used in this study. In this model, the input parameters studied include: Sharps waste loaded in primary chamber, Sw(t); Other waste loaded in primary chamber, Ow(t); Total diesel oil consumed per cycle by the two burners, Do(t), and the incineration cycle time, Tc(t), where t denotes time in days. The quantity of air through the burners (stoichiometric air) and blower (excess air) were not determined. The time series recorded for N = 653 days, can be expressed as per Equation (1), in which, each parameter was recorded separately: