Aoyi Ochieng

Brewery wastewater treatment in a fluidised bed bioreactor

A hydrodynamic characteristic performance of a three phase fluidised bed bioreactor has been studied with brewery wastewater. The influence of operating parameters, such as phase hold up, phase mixing, aspect ratio and superficial gas velocity, on an aerobic biodegradation in a bioreactor of 0.16 m i.d. and 2.7m in height, was analysed. A low-density (960 kg/m(3)) support particle with an internal interstice was employed. The particle and liquid loading were varied in order to determine the effect of phase hold up on bed homogeneity. The ranges in which particle loading and bed height affect fluidisation, and consequently chemical oxygen demand (COD) reduction, were determined. The distributor used in this work was designed such that fluid flow pattern similar to that of a draft tube was induced in the reactor. The low-density particles enabled cost effective operation at a relatively low gas superficial velocity (2.5 cm/s). Aspect ratio significantly influenced the overall bed homogeneity, and the optimum aspect ratio was 10, with volume of the support particles being 21% of the reactor volume.

An integrated anaerobic digestion and UV photocatalytic treatment of distillery wastewater.

Anaerobic up-flow fixed bed reactor and annular photocatalytic reactor were used to study the efficiency of integrated anaerobic digestion (AD) and ultraviolet (UV) photodegradation of real distillery effluent and raw molasses wastewater (MWW). It was found that UV photodegradation as a stand-alone technique achieved colour removal of 54% and 69% for the distillery and MWW, respectively, with a COD reduction of <20% and a negligible BOD reduction. On the other hand, AD as a single treatment technique was found to be effective in COD and BOD reduction with efficiencies of above 75% and 85%, respectively, for both wastewater samples. However, the AD achieved low colour removal efficiency, with an increase in colour intensity of 13% recorded when treating MWW while a colour removal of 51% was achieved for the distillery effluent. The application of UV photodegradation as a pre-treatment method to the AD process reduced the COD removal and biogas production efficiency. However, an integration in which UV photodegradation was employed as a post-treatment to the AD process achieved high COD removal of above 85% for both wastewater samples, and colour removal of 88% for the distillery effluent. Thus, photodegradation can be employed as a post-treatment technique to an AD system treating distillery effluent for complete removal of the biorecalcitrant and colour imparting compounds.

Photodegradation of Molasses Wastewater Using TiO2–ZnO Nanohybrid Photocatalyst Supported on Activated Carbon

Molasses wastewater (MWW) is characterized by high organic loading and a recalcitrant dark brown color. A study was carried out to determine the photocatalytic efficiency of TiO2–ZnO nanohybrid supported onto activated carbon (AC) for the remediation of MWW. Immobilization of ZnO onto TiO2 formed TiO2–ZnO, which was then supported onto AC forming TiO2–ZnO–AC composite. X-ray diffraction (XRD), scanning electron (SEM), energy-dispersive X-ray (EDX) and Fourier transform infrared (FTIR) spectroscopies, and transmission electron (TEM) microscopy revealed the successful hybridization of TiO2 and ZnO, and the subsequent support of the hybrid onto AC. Photoluminescence (PL) spectroscopy further revealed a restrained electron-hole pair recombination resulting from both the successful formation of a heterojunction, and the introduction of electron accepting AC support. Photodegradation process was monitored by color and total organic carbon (TOC) reductions. It was observed that TiO2–ZnO (ratio 3:1) supported onto AC (30% loading) had a higher adsorptive capacity and catalytic activity than bare TiO2–ZnO resulting in a 14% increment in overall color reduction. UV-photodegradation was found to be very effective for color as compared to TOC reduction, with 96% and 9% reductions, respectively, after 1 hour of irradiation. A pH of 5 was found to be optimum for photodegradation.

Cobalt removal from wastewater using pine sawdust

Agricultural wastes can cause environmental problems if not well managed, but there is a lot of potential to use these wastes as raw material in other processes. In this investigation, pine sawdust was evaluated as an adsorbent in the treatment of wastewater containing cobalt ions. A two-level three-factor full-factorial experimental design with centre points was used to study the interactive effect of the operating parameters in order to achieve the best conditions for the batch adsorption of cobalt ions. A response surface analysis was also conducted to further understand the interactions amongst the factors such as adsorbent dose, solution pH and initial concentration. In addition, adsorption isotherms, namely the Freundlich and Langmuir, were used to characterize the removal of cobalt from the wastewater. It was observed that the combined effect of low adsorbent dose, high pH and high initial concentration of wastewater resulted in the highest adsorption capacity. The Freundlich isotherm provided a better fit to the experimental data than the Langmuir isotherm. Moreover, pine sawdust showed adsorption capabilities for cobalt, and hence it could be an option in the quest to use waste to treat wastewater. Key words: Adsorption, cobalt, isotherm, sawdust, wastewater.

CFD simulation of the hydrodynamics and mixing time in a stirred tank

Hydrodynamics and mixing efficiency in stirred tanks influence power draw and are therefore important for the design of many industrial processes. In the present study, both experimental and simulation methods were employed to determine the flow fields in different mixing tank configurations in a single phase system. Laser Doppler velocimetry (LDV) and computational fluid dynamics (CFD) techniques were used to determine the flow fields in systems with and without a draft tube. There was reasonable agreement between the simulation and experimental results. It was shown that the use of a draft tube with a Rushton turbine and hydrofoil impeller resulted in a reduction in the homogenization energy by 19.2 and 17.7%, respectively. This indicates that a reduction in the operating cost can be achieved with the use of a draft tube in a stirred tank and there would be a greater cost reduction in a system stirred by the Rushton turbine compared to that stirred by a propeller.

Evaluation of synergy and bacterial regrowth in photocatalytic ozonation disinfection of municipal wastewater

The use of solar and ultraviolet titanium dioxide photocatalytic ozonation processes to inactivate waterborne pathogens (Escherichia coli, Salmonella species, Shigella species and Vibrio cholerae) in synthetic water and secondary municipal wastewater effluent is presented. The performance indicators were bacterial inactivation efficiency, post-disinfection regrowth and synergy effects (collaboration) between ozonation and photocatalysis (photocatalytic ozonation). Photocatalytic ozonation effectively inactivated the target bacteria and positive synergistic interactions were observed, leading to synergy indices (SI) of up to 1.86 indicating a performance much higher than that of ozonation and photocatalysis individually (SI≤1, no synergy; SI>1 shows synergy between the two processes). Furthermore, there was a substantial reduction in contact time required for complete bacterial inactivation by 50-75% compared to the individual unit processes of ozonation and photocatalysis. Moreover, no post-treatment bacterial regrowth after 24 and 48h in the dark was observed. Therefore, the combined processes overcame the limitations of the individual unit processes in terms of the suppression of bacterial reactivation and regrowth owing to the fact that bacterial cells were irreparably damaged. The treated wastewater satisfied the bacteriological requirements in treated wastewater for South Africa.

Treating high nitrate groundwater using surfactant modified zeolite in fixed bed column

High levels of nitrate in South African groundwater used for drinking purposes are a cause of concern due to the possible human health risks associated with consuming nitrate contaminated water. In this study, nitrate removal using surfactant modified zeolite (SMZ) in fixed bed column is explored. The performance of SMZ is studied as a function of bed height, initial concentration, flow rate, and bed diameter. The number of bed volumes processed and capacity of the bed at breakthrough point are used as performance indicators. The bed performance improves with a decrease in bed height while column diameter has no influence on bed performance. Within the studied flow rate range, the highest number of bed volumes processed and bed capacity are observed at a flow rate of 5 mL/min. In an adsorption-desorption process, the performance of SMZ is found to be poor in the subsequent cycle suggesting that the media is suited for single-use only.

UV and solar light photocatalytic removal of organic contaminants in municipal wastewater

ABSTRACT This study investigated UV and solar photocatalysis of organic pollutants in municipal wastewater using metal-ion (Ag, Cu and Fe) doped TiO2 photocatalysts. The effects of pH and catalyst dosage on photocatalytic performance were determined. The best performance was obtained using 0.5 g/L catalyst concentration and pH 6, and the treated water met specific requirements of the drinking water quality standards (phenol ≤10 μg/L) and wastewater effluents for discharge chemical oxygen demand (COD) levels (<30 mg/L). The photocatalysts were effective under both UV and visible light, thereby overcoming the limitation of TiO2 which is only effective under UV light.

Impact of ozonation in removing organic micro-pollutants in primary and secondary municipal wastewater: effect of process parameters

The study investigates the influence of process parameters on the effectiveness of ozonation in the removal of organic micro-pollutants from wastewater. Primary and secondary municipal wastewater containing phenol was treated. The effect of operating parameters such as initial pH, ozone dosage, and initial contaminant concentration was studied. An increase in contaminant decomposition with pH (3-11) was observed. The contaminant removal efficiencies increased with an increase in ozone dose rate (5.5-36.17 mg L(-1) min(-1)). Furthermore, the ultraviolet absorbance (UV 254 nm) of the wastewater decreased during ozonation indicating the breakdown of complex organic compounds into low molecular weight organics. Along the reaction, the pH of wastewater decreased from 11 to around 8.5 due to the formation of intermediate acidic species. Moreover, the biodegradability of wastewaters, measured as biological and chemical oxygen demand (BOD5/COD), increased from 0.22 to 0.53. High ozone utilization efficiencies of up to 95% were attained thereby increasing the process efficiency; and they were dependent on the ozone dosage and pH of solution. Ozonation of secondary wastewater attained the South African water standards in terms of COD required for wastewater discharge and dissolved organic carbon in drinking water and increased significantly the biodegradability of primary wastewater.

Modelling energy efficiency of an integrated anaerobic digestion and photodegradation of distillery effluent using response surface methodology

ABSTRACT Anaerobic digestion (AD) is efficient in organic load removal and bioenergy recovery when applied in treating distillery effluent; however, it is ineffective in colour reduction. In contrast, ultraviolet (UV) photodegradation post-treatment for the AD-treated distillery effluent is effective in colour reduction but has high energy requirement. The effects of operating parameters on bioenergy production and energy demand of photodegradation were modelled using response surface methodology (RSM) with a view of developing a sustainable process in which the biological step could supply energy to the energy-intensive photodegradation step. The organic loading rate (OLRAD) and hydraulic retention time (HRTAD) of the initial biological step were the variables investigated. It was found that the initial biological step removed about 90% of COD and only about 50% colour while photodegradation post-treatment removed 98% of the remaining colour. Maximum bioenergy production of 180.5 kWh/m3 was achieved. Energy demand of the UV lamp was lowest at low OLRAD irrespective of HRTAD, with values ranging between 87 and 496 kWh/m3. The bioenergy produced formed 93% of the UV lamp energy demand when the system was operated at OLRAD of 3 kg COD/m3 d and HRT of 20 days. The presumed carbon dioxide emission reduction when electricity from bioenergy was used to power the UV lamp was 28.8 kg CO2 e/m3, which could reduce carbon emission by 31% compared to when electricity from the grid was used, leading to environmental conservation.