Treatment of chlorinated volatile organic compounds using wetland systems

Bench-scale continuous vertical flow column and microcosm studies were conducted to investigate the attenuation potential of chlorinated volatile organic compounds (CVOCs) in constructed wetland soil mixtures prepared from peat, compost and sand and in a pristine natural freshwater wetland soil. The study also determined geotechnical properties of potential synthetic peat mixtures for construction of treatment wetlands for CVOCs. Trichloroethene (TCE), cis-1,2-dichloroethene (cis-1,2-DCE) and 1,2-dichloroethane (1,2-DCA) were the main test chemicals used during the studies. Based on geotechnical and sorption characteristics, two mixtures (one comprised of sand and peat and the other comprised of sand, peat and compost product) were selected out of ten potential soil mixtures for column and sorption studies as the most promising materials for construction of the wetland. Faster removal kinetics and higher sorption potential were observed in peat/compost/sand mixture than in peat/sand mixture for all pollutants tested. Degradation kinetics of cis-1,2-DCE and 1,2-DCA were observed to be fastest under methanogenesis followed by sulfate-and sulfite-reducing conditions. Detection of the 16S rDNA gene of Dehalococcoides sp. via DNA extraction, PCR amplification, cloning and sequencing was directly correlated with complete dechlorination of TCE and cis-1,2-DCE to ethene. Two steady-state solute transport models were used to model TCE attenuation in natural and constructed wetland mesocosms, one equation had a dispersion term whereas another equation ignored the dispersion effects. Both models were able to capture the spatial concentration trends of TCE in the soil columns equally well. However, the model without the dispersion term underestimated the TCE removal rate constant by a factor of at least two compared with the other model. TCE attenuation rate constants were observed to be significantly faster in the constructed wetland columns than in the natural wetland columns. Model simulations indicated that attenuation of TCE in the constructed wetland columns was controlled by biodegradation whereas both sorption and biodegradation were equally important removal mechanisms in the natural wetland columns. The results of this study may be useful in establishing design information for treatment wetlands for CVOCs

Mapping the Gap of Water and Erosion Control Measures in the Rapidly Urbanizing Mbezi River Catchment of Dar es Salaam

In rapidly urbanizing catchments, increase in stormwater runoff may cause serious erosion and frequent floods if stormwater management systems are improper and dysfunctional. Through GIS-based modelling, field investigations, resident’s questionnaire survey, and interviews with officials, the study set out to assesses the coverage and efficiency of drainage infrastructure in Mbezi River catchment basin in Dar es Salaam, Tanzania. Between 2003 and 2016, the catchment imperviousness increased by 41%, causing flood incidents, massive erosion, and numerous pollution sources. Residents strive to address stormwater hazards using terraces, hedges, and physical barriers; however, the problems persist, indicating lack of coordination and poor causality understanding between land-use changes and catchment impacts. Small-scale stormwater harvesting was exercised by 75% of the households, pointing to water supply challenges. Municipal stormwater management efforts was limited to roadside drains covering 17% of road lengths in the catchment, and 65% of those did not meet their design standards. Interviews with officials revealed a need for improved co-understanding and collaborative initiatives to bolster integrated water management. The study suggests a need to adopt a new urban stormwater management paradigm, appropriate for both residents and authorities. Without this new discourse, the urbanization led stormwater increase might jeopardize the liveability of the entire catchment