Bayer Residue Research Articles

Performance of novel media in stratified filters to remove organic carbon from lake water

Disinfection by-products (DBPs) are an ever-increasing occurrence in water networks, particularly those which abstract water from peatland areas. Although much research has been carried out to discover novel methods to remove specific DBPs, the removal of natural organic matter (NOM) from source water may provide a more sustainable solution in many areas. This study focuses on the removal of NOM by novel filters, which could be retrospectively fitted to any conventional water treatment facility. The filters comprised stratified layers of a variety of media, including sand, Bayer residue, granular activated carbon (GAC), and pyritic fill. The filters were operated under two loading regimes, continuous and intermittent, at loading rates similar to recognised design standards. The most successful filter design comprised stratified layers of sand, GAC, and pyritic fill. Over the duration of a 240 day study, these filters obtained average dissolved organic carbon removal rates of 40%, and achieved average specific ultra-violet absorbance reductions from 2.9 to 2.4 L mg−1 m−1. The study demonstrates that these novel filters may be used to reduce NOM levels, thus reducing the potential for DBP formation. Such designs can incorporate the use of waste media, making the overall design more sustainable and robust.

Performance and surface clogging in intermittently loaded and slow sand filters containing novel media

Slow sand filers are commonly used in water purification processes. However, with the emergence of new contaminants and concern over removing precursors to disinfection by-products, as well as traditional contaminants, there has recently been a focus on technology improvements to result in more effective and targeted filtration systems. The use of new media has attracted attention in terms of contaminant removal, but there have been limited investigations on the key issue of clogging. The filters constructed for this study contained stratified layers comprising combinations of Bayer residue, zeolite, fly ash, granular activated carbon, or sand, dosed with a variety of contaminants (total organic carbon (TOC), aluminium (Al), ammonium (NH4+-N), nitrate (NO3−-N) and turbidity). Their performance and clogging mechanisms were compared to sand filters, which were also operated under two different loading regimes (continuous and intermittently loaded). The study showed that the novel filter configurations achieved up to 97% Al removal, 71% TOC removal, and 88% NH4+-N removal in the best-performing configuration, although they were not as effective as sand in terms of permeability. Deconstruction of the filters revealed that the main clogging mechanism was organic matter build-up at the uppermost layer of the filters. The clogging layer formed more quickly on the surface of the novel media when compared to the sand filters, but extended further into the sand filters, the extent dependent on the loading regime. The study shows the potential for an alternative filtration configuration, harnessing the adsorption potential of industrial waste products and natural media.

Use of industrial by-products and natural media to adsorb nutrients, metals and organic carbon from drinking water

Filtration technology is well established in the water sector but is limited by inability to remove targeted contaminants, found in surface and groundwater, which can be damaging to human health. This study optimises the design of filters by examining the efficacy of seven media (fly ash, bottom ash, Bayer residue, granular blast furnace slag (GBS), pyritic fill, granular activated carbon (GAC) and zeolite), to adsorb nitrate, ammonium, total organic carbon (TOC), aluminium, copper (Cu) and phosphorus. Each medium and contaminant was modelled to a Langmuir, Freundlich or Temkin adsorption isotherm, and the impact of pH and temperature (ranging from 10°C to 29°C) on their performance was quantified. As retention time within water filters is important in contaminant removal, kinetic studies were carried out to observe the adsorption behaviour over a 24h period. Fly ash and Bayer residue had good TOC, nutrient and Cu adsorption capacity. Granular blast furnace slag and pyritic fill, previously un-investigated in water treatment, showed adsorption potential for all contaminants. In general, pH or temperature adjustment was not necessary to achieve effective adsorption. Kinetic studies showed that at least 60% of adsorption had occurred after 8h for all media. These media show potential for use in a multifunctional water treatment unit for the targeted treatment of specific contaminants.

Minimization of Bauxite Residue Neutralization Products Using Nanofiltered Seawater

Currently, open circuit Bayer refineries pump seawater directly into their operations to neutralize the caustic fraction of the Bayer residue. The resulting supernatant has a reduced pH and is pumped back to the marine environment. This investigation has assessed modified seawater sources generated from nanofiltration processes to compare their relative capacities to neutralize bauxite residues. An assessment of the chemical stability of the neutralization products, neutralization efficiency, discharge water quality, bauxite residue composition, and associated economic benefits has been considered to determine the most preferable seawater filtration process based on implementation costs, savings to operations and environmental benefits. The mechanism of neutralization for each technology was determined to be predominately due to the formation of Bayer hydrotalcite and calcium carbonate; however, variations in neutralization capacity and efficiencies have been observed. The neutralization efficiency of each feed source has been found to be dependent on the concentration of magnesium, aluminum, calcium, and carbonate. Nanofiltered seawater, with approximately double the amount of magnesium and calcium required half the volume of seawater to achieve the same degree of neutralization. These studies have revealed that multiple neutralization steps occur throughout the process using characterization techniques such as X-ray diffraction (XRD), infrared (IR) spectroscopy, and inductively coupled plasma optical emission spectroscopy (ICP-OES).

Effect of High Concentrations of Calcium Hydroxide in Neutralized Synthetic Supernatant Liquor − Implications for Alumina Refinery Residues

The presence of calcium hydroxide (Ca(OH)2) in Bayer residue slurry inhibits the effectiveness of the seawater neutralization process to reduce the pH and aluminum concentration in the residue. An increase in the slurry pH (reversion), after seawater neutralization, is caused by the dissolution of calcium hydroxide and hydrocalumite (solid components found in bauxite refinery residue). Reversion was not observed when the final solution pH was greater than 10.5, due to hydrocalumite being in a state of equilibrium at high pH. Hydrocalumite has been found to form during the neutralization process when high concentrations of calcium hydroxide are present in the residue liquor. The dissolution of hydrocalumite releases hydroxyl (OH−) and aluminum ions back into solution after the seawater neutralization (SWN) process, which causes pH and aluminum reversion to occur. This investigation looks at the effect of Ca(OH)2 and subsequently hydrocalumite on the pH and aluminum concentration in bauxite refinery residue liquors after the SWN process.

The effect of high concentrations of calcium hydroxide in neutralised synthetic supernatant liquor—Implications for alumina refinery residues

The presence of calcium hydroxide in Bayer residue slurry inhibits the effectiveness of seawater neutralisation. An increase in the slurry pH, after neutralisation, is caused by the dissolution of calcium hydroxide and hydrocalumite (both components are found in the solid residue). At increased calcium hydroxide concentrations, additional hydrocalumite forms. Hydrocalumite has been found to be unstable when the solution pH is less than 10.5, releasing OH − ions and aluminium ions back into solution (pH and aluminium reversion). Through this mechanism, the concentration of calcium hydroxide in alumina refinery residues has an impact on seawater neutralisation process efficiency.