Faculty of Engineering & Technology

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Browsing Faculty of Engineering & Technology by Author "Aoyi, Ochieng"

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    Anaerobic co-digestion of abattoir and textile industry wastewater in a UASB reactor
    (2015-04) Ondari, James Maati; Ndege, Maurice S.; Aoyi, Ochieng
    Textile industry effluents are carcinogenic and highly recalcitrant hence difficult to degrade especially through biological methods. Abattoir effluents are classified under high-strength wastewaters because of their characteristic high organic load hence highly biodegradable. Anaerobic co-digestion is the concept of degrading two effluent streams with complementary characteristics in order to improve the substrate removal rate. The feasibility of co-digesting abattoir and textile wastewater in a UASB reactor was evaluated at mesophilic and ambient temperature conditions. Preliminary experiments were conducted in 500 ml batch reactors to evaluate the optimum abattoir to textile synthetic wastewater ratio. The effect of COD, TVFA, alkalinity and pH on biogas yield was examined at both ambient and mesophilic temperatures. Anaerobic co-digestion of abattoir to textile wastewater in the ratio determined in the batch process was carried out in a 3 L UASB reactor by a continuous process. The continuous biodegradation process was executed at three different HRTs (22, 18 and 14 hrs) over a 60 day operation period. UASB reactor efficiency was achieved at organic loads ranging from 3.0 – 10.8 gCOD L-1 day-1. Continuous mode experiments were carried out at influent flow rates which corresponded to HRTs ranging between 1 to 8 days in order to evaluate the steady state operating parameters for the co-digestion process. The abattoir to textile effluent ratio was found to be 60:40 respectively. The COD, TVFA, alkalinity and pH and biogas yield followed a similar pattern over time at both mesophilic and ambient temperature conditions. Experimental data adequately fit the Grau first order kinetic model and average COD removal efficiencies of 85% and BOD5 of around 96% were achieved. The average biogas yield remained essentially constant, around 0.19 L/g CODremoved. The co-digested mixture was found to be biodegradable judging from the BOD:COD ratio of 0.53. TCOD removal efficiency decreased from 93% to 16% as HRT decreased from 8 days to 1 day. The kinetics of a UASB reactor co-digesting the mixture of synthetic abattoir and textile wastewater was evaluated in this study using Grau second order multicomponent substrate removal kinetic model. The Grau second order kinetic model, whose kinetic coefficient (ks) was 0.389, was found to be suitable for predicting the performance of a lab-scale UASB reactor.
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    Photocatalytic treatment of industrial wastewater containing citric acid and toxic heavy metals
    (2014-12) Baloyi, Siwela Jeffrey; Aoyi, Ochieng
    The co-existence of organic acids and toxic heavy metals in natural water creates harmful effects on human, plants and animals. Therefore, it is necessary to treat organic acids and toxic heavy metal contaminated wastewater prior to its discharge to the environment. The aim of this study was to apply co-treatment of industrial wastewater containing citric acid and toxic heavy metals in single and binary systems using photocatalysis process. The hydrothermal method was used to synthesise dandelion-like TiO2 structures. Modifications of the dandelion-like TiO2 by deposition of gold nanoparticles and immobilisation on calcium alginate were done using deposition precipitation and one-step encapsulation methods, respectively. Dandelion-like TiO2 and dandelion-like TiO2 immobilised on calcium alginate (Alg/TiO2) were used as photocatalysts for Cr(VI), Hg(II) and citric acid removal from water. The results showed that the production of dandelion-like TiO2 structures strongly depends on the reaction time and synthesis temperature as key process parameters. The characterisation of the dandelion-like TiO2 by X-ray diffraction (XRD), transmission electron microscope (TEM), scanning electron microscopy (SEM), energy-dispersive X-ray (EDX) and Brunauer-Emmett-Teller (BET) revealed the crystal structure, morphology, chemical composition and surface area. It was found that the efficiency of photocatalytic process depends on the type of pollutants, initial pH of the solution, photocatalyst dosage, contact time, substrate initial concentration, UV wavelength and light intensity. The reduction efficiency of Cr(VI) ion and citric acid increased with decreasing the initial pH values and initial concentration. On the other hand, Hg(II) reduction efficiency increased with increasing the initial pH values and initial concentration. In a binary system, the reduction of Cr(VI) and Hg(II) was found to be faster than in the single and ternary systems. The relationship of the chemical reaction rate of Cr(VI), Hg(II) and citric acid were expressed by the pseudo-first-order kinetic equation. Addition of ferric ions to Cr(VI)-citric acid complex and Hg(II)-citric acid complex enhanced the reduction of Cr(VI) and Hg(II), a complete reduction was accomplished within 30 and 60 minutes (min) of irradiation time, respectively. The reduction efficiency of both Cr(VI) and Hg(II) in the presence of citric acid in a solution was still significant after four times of Alg/TiO2 reuse. These results indicated that the UV/TiO2 photocatalysis process can be considered as a suitable method to reach a complete reduction of Cr(VI) and Hg(II) in the presence of citric acid in a solution.
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    Treatment of acid mine drainage using constructed wetland and UV/TiO₂ photocatalysis
    (2014-05) Seadira, Tumelo Wordsworth Poloko; Aoyi, Ochieng
    Acid mine drainage (AMD) is a serious problem associated with mining activities, and it has the potential to contaminate surface and ground water. The aim of this study was to evaluate the performance of constructed wetland and photocatalysis in treating AMD. Three identical unvegetated upflow constructed wetlands packed with natural zeolite (clinoptilolite) and coarse silica sand were made of a cylindrical plastic pipe, and the slurry photocatalyst was prepared using quartz material. A hydro-alcohol thermal method was used to prepare an anatase core-void-shell TiO2 photocatalyst. The results showed that the three unvegetated upflow constructed wetlands (CW) had relatively similar percentage removal of heavy metals despite their varying concentrations within the AMD. The removals were: Fe (86.54 - 90.4%); Cr (56.2 - 64.5%); Mg (56.2 - 67.88%); Ca (77.1 - 100%); and 100% removal was achieved for Be, Zn, Co, Ni, and Mn. The removal of sulphate was also 30%. Heavy metals concentration in CW packing material was significantly higher in the outlet of the constructed wetlands than in the inlet. The adsorption isotherms revealed that the experimental data fitted the Langmuir Isotherms better, which suggested a monolayer coverage of heavy metals on the surface of the adsorbents; thermodynamic studies showed that the nature of adsorption taking place was physical; the kinetics models showed that the adsorption was first order reaction. A higher photocatalytic reduction (62%) of Cr(VI) was obtained at pH 2, 30 mg/l Cr(VI) initial concentration, and three hours of irradiation time. It was also found that the presence of Fe(III) enhanced the reduction of Cr(VI). The core-void-shell TiO2 photocatalyst showed a better activity than the commercial P25 Degussa for the reduction of Cr(VI) to Cr(III). The kinetic studies showed that the reduction of Cr(VI) was first order reaction. Photocatalytic reduction of Cr(VI) in real AMD sample was achieved only for the Douglas North Discharge (DND) sample (68%), and the Fe(III) reduction was found to be 83%. Therefore it was concluded that the combination of constructed wetland and UV/ TiO2 photocatalysis employing anatase core-void-shell TiO2 as a photocatalyst has a potential to reduce the toxicity of Cr(VI)-laden acid mine drainage.