Organic Liquids Storage Tanks Volatile Organic Compounds (VOCS) Emissions Dispersion and Risk Assessment in Developing Countries: The Case of Dar-Es-Salaam City, Tanzania |
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Authors: | Msafiri M Jackson |
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Institution: | (1) Department of Environmental Engineering, University College of Lands and Architectural Studies, P.O. Box 35176, Dar-es-Salaam, Tanzania |
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Abstract: | The emission estimation of nine volatile organic compounds (VOCs) from eight organic liquids storage tanks companies in Dar-es-Salaam
City Tanzania has been done by using US EPA standard regulatory storage tanks emission model (TANKS 4.9b). Total VOCs atmospheric
emission has been established to be 853.20 metric tones/yr. It has been established further that petrol storage tanks contribute
about 87% of total VOCs emitted, while tanks for other refined products and crude oil were emitting 10% and 3% of VOCs respectively.
Of the eight sources (companies), the highest emission value from a single source was 233,222.94 kg/yr and the lowest single
source emission value was 6881.87 kg/yr. The total VOCs emissions estimated for each of the eight sources were found to be
higher than the standard level of 40,000 kg/yr per source for minor source according to US EPA except for two sources, which
were emitting VOCs below the standard level. The annual emissions per single source for each of the VOCs were found to be
below the US EPA emissions standard which is 2,000~kg/yr in all companies except the emission of hexane from company F1 which
was slightly higher than the standard. The type of tanks used seems to significantly influence the emission rate. Vertical
fixed roof tanks (VFRT) emit a lot more than externally floating roof tanks (EFRT) and internally floating roof tanks (IFRT).
The use of IFRT and EFRT should be encouraged especially for storage of petrol which had highest atmospheric emission contribution.
Model predicted atmospheric emissions are less than annual losses measured by companies in all the eight sources. It is possible
that there are other routes for losses beside atmospheric emissions. It is therefore important that waste reduction efforts
in these companies are directed not only to reducing atmospheric emissions, but also prevention of the spillage and leakage
of stored liquid and curbing of the frequently reported illegal siphoning of stored products. Emission rates for benzene,
toluene, and xylene were used as input to CALPUFF air dispersion model for the calculation of spatial downwind concentrations
from area sources. By using global positioning system (GPS) and geographical information system (GIS) the spatial benzene
concentration contributed by organic liquid storage tanks has been mapped for Dar-es-Salaam City. Highest concentrations for
all the three toxic pollutants were observed at Kigamboni area, possibly because the area is located at the wind prevailing
direction from the locations of the storage tanks. The model predicted concentrations downwind from the sources were below
tolerable concentrations by WHO and US-OSHA. The highest 24 hrs averaging time benzene concentration was used for risk assessment
in order to determine maximum carcinogenic risk amongst the population exposed at downwind. Established risk for adult and
children at 2.9×10-3 and 1.9×10-3 respectively, are higher than the acceptable US-EPA risk of 1×10-6. It is very likely that the actual VOCs concentrations in some urban areas in Tanzania including Dar-es-Salaam City are much
higher than the levels reported in this study when other sources such as petrol stations and motor vehicles on the roads are
considered. Tanzania Government therefore need to put in place: an air quality policy and legislation, establish air quality
guidelines and acquire facilities which will enable the implementation of air quality monitoring and management programmes. |
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Keywords: | volatile organic compounds organic liquids benzene toluene xylene atmospheric emissions waste minimization dispersion risk |
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