Research Repository :: Browsing by Author "Njau, Karoli N."

Browsing by Author "Njau, Karoli N."

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Analysis of Different Techniques for Improving Performance of Flat Plate Solar Collectors for Fruits Drying
(2014-06) Bakari, Ramadhani; Minja, Rwaichi J. A.; Njau, Karoli N.
Effect of Diffusional Mass Transfer on the Performance of Horizontal Subsurface Flow Constructed Wetlands in Tropical Climate Conditions
(2011) Njau, Karoli N.; Gastory, L; Eshton, Bilha; Katima, Jamidu H. Y.; Minja, Rwaichi J. A.; Kimwaga, Richard; Shaaban, M.
The effect of mass transfer on the removal rate constants of BOD₅, NH₃, NO₃ and TKN has been investigated in a Horizontal Subsurface Flow Constructed Wetland (HSSFCW) planted with Phragmites mauritianus. The plug flow model was assumed and the inlet and outlet concentrations were used to determine the observed removal rate constants. Mass transfer effects were studied by assessing the influence of interstitial velocity on pollutant removal rates in CW cells of different widths. The flow velocities varied between 3-46 m/d. Results indicate that the observed removal rate constants are highly influenced by the flow velocity. Correlation of dimensionless groups namely Reynolds Number (Re), Sherwood Number (Sh) and Schmidt Number (Sc) were applied and log-log plots of rate constants against velocity yielded straight lines with values β = 0.87 for BOD₅, 1.88 for NH₃, 1.20 for NO₃ and 0.94 for TKN. The correlation matched the expected for packed beds although the constant β was higher than expected for low Reynolds numbers. These results indicate that the design values of rate constants used to size wetlands are influenced by flow velocity. This paper suggests the incorporation of mass transfer into CW design procedures in order to improve the performance of CW systems and reduce land requirements.
Effect of Glass Thickness on Performance of Flat Plate Solar Collectors for Fruits Drying
(Hindawi Publishing Corporation, 2014) Bakari, Ramadhani; Minja, Rwaichi J. A.; Njau, Karoli N.
This study aimed at investigating the effect of thickness of glazing material on the performance of flat plate solar collectors. Performance of solar collector is affected by glaze transmittance, absorptance, and reflectance which results into major heat losses in the system. Four solar collector models with different glass thicknesses were designed, constructed, and experimentally tested for their performances. Collectors were both oriented to northsouth direction and tilted to an angle of 10° with the ground toward north direction. The area of each collector model was 0.72 m2 with a depth of 0.15 m. Low iron (extra clear) glass of thicknesses 3 mm, 4 mm, 5 mm, and 6 mm was used as glazing materials. As a control, all collector performances were analysed and compared using a glass of 5 mm thickness and then with glass of different thickness. The results showed that change in glass thickness results into variation in collector efficiency. Collector with 4 mm glass thick gave the best efficiency of 35.4% compared to 27.8% for 6 mm glass thick. However, the use of glass of 4 mm thick needs precautions in handling and during placement to the collector to avoid extra costs due to breakage.
Energy Recovery routes from Municipal Solid Waste, A case study of Arusha-Tanzania
(Journal of Energy Technologies and Policy, 2014-01) Omari, Arthur; Said, Mahir M.; Njau, Karoli N.; John, Geoffrey R.; Mtui, Peter
A study of energy recovery from municipal solid waste was undertaken. The energy content of the solid waste is 12MJ/kg. The elemental composition shows that the municipal solid waste contains 50% and 5% of carbon and hydrogen respectively. The energy flow (exothermic and endothermic) and thermal degradation analysis were carried out using differential scanning calorimetry and thermo-gravimetric analyser respectively. Experiments were performed at heating rate of 10 K/min, 20 K/min, 30 K/min and 40 K/min in the nitrogen atmosphere at temperature between room temperature and 1273 K. The thermal degradation kinetic parameters values of activation energy (E a) ranged from 205.9 to 260.6kJ/mol. It has been observed that municipal solid waste is less reactive to combustion as compared to coal and biomass, but its reactivity can be improved through pre-treating process so as to reduce noncombustible materials such as oxygen and ash content. Also pyrolysis and gasification can be used to convert MSW to liquid or gaseous fuel.
Experimental Analysis of Air Flow Patterns in Perfomance of Flat Plate Solar Collectors
(Academic Journals, 2015) Bakari, Ramadhani; Minja, Rwaichi J. A.; Njau, Karoli N.
Solar drying is one of the promising methods of reducing post-harvest losses in rural areas. Studies have shown that, heat transfer mechanisms in a solar collector influences the performance of solar dryers. This study aims at improving heat transfer in flat plate solar collectors by designing different air flow patterns inside the collector. Three flat plate solar collectors were constructed by using Pterocarpus timber (Mninga) and tested for their effect in heat transfer at various flow patterns. Three different flow patterns namely: single duct front pass, double duct parallel flow and double duct counter flow were designed and tested. Experimental results show that collector efficiency of single duct front pass, double duct front pass and double duct counter flow were 30.6, 36.1 and 38.2% respectively. It was found that, double duct flow gives improved performance compared to single duct flow due to the increased heat transfer area. In additional, double duct counter flow showed superior performance compared to double duct parallel flow due to extended heat transfer area and the advantage of air preheating at the inlet which reduces heat losses through glazing. Through this study, it was concluded that, solar collector designs with double duct counter flow can improve collector performance for up to 8.3% compared to single duct front pass.
Influence of Flow Velocity on The Removal f Faecal Coliforms in Horizontal Subsurface Flow Constructed Wetland
(2012-09) Lohay, W. S.; Lyimo, Thomas J.; Njau, Karoli N.
In order to determine the influence of flow velocity on the removal of faecal coliforms (FC) in constructed wetlands (CWs), removal rate constants of FC (k(FC)) were studied at various flow velocities (u). Membrane filtration technique was used during analysis. Values of k(FC) were determined using Reed's equation of pathogen removal; the results were compared with the plug flow equation. According to Reed's equation, k(FC) values ranged from 1.6 day(-1) at a velocity of 4 m/day to 34.5 day(-1) at a velocity of 42.9 m/day. The removal rates correlated positively with flow velocity (r = 0.84, p < 0.05). On assuming a plug flow equation, removal rates constants ranged from 0.77 to 11.69 day(-1); a more positive correlation (r = 0.93, p < 0.05) was observed. Optimum removal rate constants were observed for the velocity ranging 36 to 43 m/day. Generally, the increase of flow velocity improved FC removal rate constants: implying that pathogen removals are influenced by diffusion of the microorganisms into the biofilms on CW media. The velocity dependent approach together with the plug flow equation is therefore proposed for incorporation in the design of CW in a tropical climate where temperature variations are minor.
Mass And Energy Balance For Fixed Bed Incinerators A case of a locally designed incinerator in Tanzania
(JMEST, 2015-10) Omari, Arthur; Njau, Karoli N.; John, Geoffrey R.; Kihedu, Joseph H.; Mtui, Peter L.
An estimation of mass and energy balance of an incinerator is an important consideration toward the design and operation of the incineration process. This paper is aimed to study the mass and energy balance of a locally made fixed bed incinerator. The results shows that the total mass rate of 49 kg/h of municipal solid waste and 9.75 kg/h of diesel consumed 458.9 kg/h of air. The incineration process generates 379,287.14 kJ/h with ash and flue gases emissions at a total mass rate of 528.51 kg/h.
Pumice Soil: A Potential Wetland Substrate for Treatment of Domestic Wastewater
(IWA Publishing, 2003) Njau, Karoli N.; Minja, Rwaichi J. A.; Katima, Jamidu H. Y.
Laboratory and fieldwork studies were carried out to evaluate the potential of pumice soil for use as a wetland substrate in wastewater treatment. The composition of pumice soil was analysed by x-ray fluorescence (XRF) and x-ray diffraction (XRD) techniques. Adsorption kinetic studies were carried out in a semi-batch recycle system. Fieldwork tests were carried out on Subsurface Flow Constructed Wetland (SSFCW) cells planted with Phragmites mauritianus and Vetiveria zizanioides. The results have shown that pumice soil composition contains among other elements Al, Ca, Fe and Mg, which are positive indicators for phosphorus adsorption. The main minerals observed by XRD were augite, hematite, and sodium titanium silicate. Phosphorus adsorption kinetics have shown that phosphorus is adsorbed on pumice soil following first order kinetics and the adsorption was highly influenced by mass transfer. Approximately 3% of the phosphorus was removed by plant uptake.
Use of Vetiver Grass Constructed Wetland for Treatment of Leachate
(2011) Bwire, Masinde K.; Njau, Karoli N.; Minja, Rwaichi J. A.
Performance of Constructed Wetland planted with vetiver grasses for the treatment of leachate was investigated in controlled experiments involving horizontal subsurface flow constructed wetland (HSSFCW). The HSSFCW experimental unit had two cells, one planted with vetiver grasses and another bare. Both units were packed with limestone gravel as substrate and were operated with equal hydraulic loading and hydraulic retention time. Collected samples of influents and effluents were analysed for COD, Cr, Pb, Fe and pH. The results showed that vetiver grasses tolerated leachate with high loading of COD up to 14,000 mg L−1. The planted cell outperformed the unplanted cell in terms of COD, Cr, Pb and Fe removal. The systems showed optimum points for COD and Pb removal as a function of feed concentrations. The optimum COD removal values of 210 mgm−2 day−1 at feed COD concentration of 11,200 mg COD L−1 and 89 mgm−2 day−1 at feed concentration of 7,200 mg COD L−1 were obtained for planted and unplanted cells respectively. Similarly Pb removal values of 0.0132 mgm−2 day−1 at 1.0 mg Pb L−1 and 0.0052 mgm−2 day−1 at 1.04 mgPb L−1 were obtained for planted and unplanted units respectively. Removal of Fe as a function of feed Fe concentration showed a parabolic behaviour but Cr removal showed linear behaviour with feed Cr concentrations in both units. The system showed very good removal efficiencies with Cr and Fe but poor efficiencies were recorded for Pb.

Browsing by Author "Njau, Karoli N."

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