|dc.description.abstract||Harmful algal blooms are considered threats to drinking water quality. During drinking water treatment, suspended and dissolved materials must be removed and treated, and in the case of cyanobacteria, these are intact cells and extracellular cyanotoxins. Cyanotoxins produced by different species of cyanobacteria are a major issue in drinking water treatment (Westrick et al., 2010). Microcystin-LR is among the most commonly detected and studied cyanotoxins. Microcystin-LR (MC-LR) is an hepatotoxin, which can cause irreversible damage to liver cells and cause chronic liver and kidney diseases and Health Canada’s guidelines proposes a maximum acceptable concentration of 1.5 µg/L in drinking water. Powdered activated carbon can remove extracellular cyanotoxins from drinking water and is also often implemented for control of seasonal taste and odor issues. Newly developed super-fine powdered activated carbons (SPAC) have particles sizes around 1 µm, and they have been shown to have faster adsorption kinetics and similar or even higher capacities than conventional powdered activated carbon (PAC) for organics. Most of the SPAC studies to date focus on SPAC as a pre-treatment for microfiltration membranes. Only a few studies evaluated the use of SPAC in association with conventional treatment processes like coagulation, flocculation, sedimentation (CFS) and rapid media filtration. These more recent studies introduced the idea that it may be possible to implement SPAC in conventional drinking water treatment plants (DWTPs). The main focus of this study was to investigate the applicability of SPAC for the removal of microcystin-LR as an alternative to PAC in conventional DWTPs.
Three SPACs were prepared by pulverization of three commercially available PACs (wood-based BG-HHM (Calgon Carbon), coconut-based WPC (Calgon Carbon), and coal-based COL-PL60-800 (Activated Carbon Corp.)). These PACs were chosen since an earlier study has evaluated the adsorption of MC-LR onto these PACs (Liu, 2017). A method based on scanning electron microscope (SEM) imaging and static image analysis (SIA) was then developed and validated before it was used to determine the particle size distribution of the prepared SPACs. The median sizes of prepared SPACs ranged from 1.2 to 1.3 µm which classifies them as SPAC. The adsorption of MC-LR via the prepared SPACs was initially evaluated in buffered ultrapure water (pH 7.2) to establish baseline adsorption rates and capacities. The SPACs were dosed as dry powder in experiments in the ultrapure water. The WPC (coconut-based) SPAC had the highest adsorption rate for MC-LR, followed by the BG-HHM (wood-based) and COL-PL60-800 (coal-based) SPACs. The adsorption capacities of SPACs for MC-LR were evaluated at both equilibrium and non-equilibrium (i.e. short contact times relevant to practice in DWTPs). At the short contact times in range of 5 to 120 min, the coconut-based SPAC had the highest capacity due it’s faster adsorption rate, followed by the wood-based and coal-based SPACs. However, the dry powdered dosing method caused problems with dispersion of SPACs, so a prewetted slurry dosing method was used in experiments at equilibrium and all the later experiments in this study including those done in Lake Erie water. At equilibrium the SPACs were dosed as prewetted slurry and under those conditions, the BG-HHM (wood-based) SPAC had the highest capacity, while the WPC (coconut-based) SPAC had the least capacity in ultrapure water. The performance of SPACs at both equilibrium and short contact times for adsorption of MC-LR was compared with the performance of parent PACs reported by Liu (2017). All three prepared wood-based, coal-based, and coconut-based SPACs outperformed their parent PACs in terms of adsorption rate and capacities in both equilibrium and non-equilibrium conditions.
The adsorption of MC-LR via the prepared SPACs was also evaluated in Lake Erie water, which was sampled after pH adjustment to the target pH of 7.2. The SPACs were dosed as prewetted slurry in all of the experiments in Lake Erie water. In terms of adsorption rate, The BG-HHM (wood-based) SPAC adsorbed the MC-LR the fastest in Lake Erie water whereas the WPC (coconut-based) SPAC was the slowest. At short contact times (non-equilibrium), the BG-HHM (wood-based) SPAC exhibited the highest capacity for MC-LR followed by the COL-PL60-800 (coal-based) and WPC (coconut-based) SPACs. At equilibrium, the order of capacities was the same as one for the short contact times. The simplified equivalent compound model (SEBCM) was applied to the adsorption data of SPACs in Lake Erie water to describe the competitive adsorption of dissolved natural organic matter (NOM) and MC-LR at the short contact times in range of 5 to 30 minutes representing the relevant contact times in practice. The SEBCM model was proved successful in prediction of the required SPAC doses for reduction of a range of MC-LR influent concentrations to a target effluent concentration. Based on the SEBCM results all three SPACs can be reasonably dosed in range of 1 to 25 mg/L to reduce MC-LR concentration as high as 100 µg/L to below the strictest of the regulated drinking water concentrations (i.e. 0.4 µg/L set by USEPA for bottle-fed infants) in Canada, however, the 1.5 µg/L MAC is in effect. The predicted doses of SPACs were also compared with those of the parent PACs (Liu 2017), and the required doses for SPACs were several degrees of magnitude lower compared to the parent PACs.
The removal of SPACs from the Lake Erie water was evaluated with a series of jar tests, simulating typical CFS processes in DWTPs. The addition of SPACs to the low turbidity (2 NTU) Lake Erie water increased its turbidity substantially. However, the results from the CFS experiments showed that the SPACs can be removed via conventional CFS using aluminum sulfate (alum) as a coagulant. Alum doses as low as 10 mg/L were sufficient for reduction of turbidity in Lake Erie water without addition of SPAC and also the water dosed with the BG-HHM (wood-based) SPAC to below 1 NTU. The water dosed with COL-PL60-800 (coal-based) SPAC required 15 mg/L of alum to achieve the same level of turbidity removal. The WPC (coconut-based) SPAC increased the turbidity of the Lake Erie by the most (>100 NTU) and required at least 30 mg/L of alum to achieve a residual turbidity below 1 NTU.
Overall, SPAC as a treatment method for removal of MC-LR from drinking water is a promising alternative to PAC. The benefits of SPAC, including the faster adsorption rate and higher short contact time capacities are essential to conventional DWTPs in terms of managing seasonal MC-LR presence in source waters. Further studies are required to evaluate the adsorption of other cyanotoxins via SPAC and to assess the removal of spent SPAC residual particles during rapid media filtration. With experimental commercial SPAC products slowly being developed it is necessary to assess the performance of SPAC for removal of variety of cyanotoxins in surface water under various conditions.||en