Selected operating process variables for a bioflocculant supported column flotation system

Abstract

The poultry industry generates significant volumes of slaughterhouse wastewater, which is laden with numerous pollutants, thus requiring treatment prior to discharge. However, the current typical water and wastewater treatment technologies have reached their limits due to the concentration of the pollutants therein and the large volumes of the wastewater to be treated, which influence the resultant water quality. Hence, there is a need for new technologies, re-engineering of the existing wastewater treatment equipment, and incorporating new unit designs to improve the treatment processes or system performance. Flotation is a well-known separation technology with the potential to be applied in wastewater pretreatment. Flotation is highly dependent on bubble size generated by air diffusers. However, inherent drawbacks underscore the significance of air diffusers design, making their design and study important. Currently, chemical flocculants, although widely used, are discouraged in flotation systems as some are considered harmful to humans and the environment. Meanwhile, the use of bioflocculants is considered eco-friendly, albeit their application requires further studies at an industrial scale. For the current study, the main aim was to evaluate the effect of selected operating process variables, i.e., diffuser type, bioflocculant form, and feed flow rate, on the performance of a bioflocculant-supported column flotation system for poultry slaughterhouse wastewater (PSW) pretreatment. Firstly, the design and production of laboratory-scale 3D-printed spargers using the Laser-Powder Bed Fusion, a part of additive manufacturing, was explored to determine their applicability in a flotation system for wastewater pretreatment. Furthermore, they were compared to conventionally sintered/molded diffusers through microstructural analysis employing optical microscopy, tested for Vickers hardness, and analyzed for surface topography, including composition, using a scanning electron microscope and energy-dispersive spectroscopy. The application of 3D-printed spargers was proven feasible, revealing their porous nature, albeit with fewer pores than molded diffusers. Notably, the latter’s, dense pores and better microstructure was thought to significantly enhance their suitability for optimizing the column flotation process. To overcome limitations related to pore properties, there is a need to explore new-generation alloys and optimize the 3D-printing process to make the final product more competitive and efficient than molded diffusers. Secondly, the study went on to focus on the isolation of bioflocculant-producing microorganisms from PSW. Characteristics of the produced bioflocculant were determined, including the optimum storage conditions and the flocculation mechanism. Twenty microorganisms were isolated, and the D2 isolate had maximum flocculation activity. It was identified using 16S rDNA to be a Bacillus species and using RpoD to be a Bacillus megaterium. The bioflocculant was composed of mainly polysaccharides and proteins and was better stored in a crude form under frozen conditions. Thirdly, the flocculation mechanism was assessed by Response Surface Methodology (RSM) at pH 4 (min) to 9 (max); bioflocculant dosage of 1% (min) to 3% (max) v/v with an assessment of changes in zeta potential as a measure of the changes in the electrostatic potential of the bulk solution. Zeta potential results confirmed that the bioflocculant was ionic, albeit charge neutralization was not the primary mechanism. These results were inconclusive in determining optimum conditions for flocculation activity; hence, flocs were viewed under a microscope, showing the optimum conditions for flocculation activity at pH 6.5 with a bioflocculant dosage of 2% (v/v). A bonding type test was carried out, and hydrogen bonding was identified as predominant, suggesting a bridging mechanism. This assertion was supported by the type of functional groups present in the structure of the bioflocculant produced by the D2 isolate. Fourthly, three variables, i.e., diffuser design/type, bioflocculant form, and influent flow rate, were evaluated to determine their effect on the performance of a bioflocculant-supported column flotation system. It was found that diffuser type and feed flow rate were influenced by bioflocculant efficacy, thus affecting the overall column flotation system performance. In addition, it was determined that 3D-printed air diffusers and cell-free bioflocculants were a superior type and form, respectively, compared to their counterparts, i.e., molded diffusers and cell-bound bioflocculants. Combining 3D-printed air diffusers and cell-free bioflocculants at a feed flow rate of 1 ml/min resulted in relatively high pollutant removal (COD, TSS, protein, and turbidity reduction). The study laid a foundation for exploring 3D-printed air diffusers, a relatively new technology in conjunction with bioflocculants usage that are regarded as eco-friendly, for application in wastewater pretreatment to enhance the performance of column flotation systems.