Toxic cyanobacteria and drinking water: Impacts, detection, and treatment

Detection of contaminants in water supply: A review on state-of-the-art monitoring technologies and their applications

Water monitoring technologies are widely used for contaminants detection in wide variety of water ecology applications such as water treatment plant and water distribution system. A tremendous amount of research has been conducted over the past decades to develop robust and efficient techniques of contaminants detection with minimum operating cost and energy. Recent developments in spectroscopic techniques and biosensor approach have improved the detection sensitivities, quantitatively and qualitatively. The availability of in-situ measurements and multiple detection analyses has expanded the water monitoring applications in various advanced techniques including successful establishment in hand-held sensing devices which improves portability in real-time basis for the detection of contaminant, such as microorganisms, pesticides, heavy metal ions, inorganic and organic components. This paper intends to review the developments in water quality monitoring technologies for the detection of biological and chemical contaminants in accordance with instrumental limitations. Particularly, this review focuses on the most recently developed techniques for water contaminant detection applications. Several recommendations and prospective views on the developments in water quality assessments will also be included.

Mesoporous carbon for efficient removal of microcystin-LR in drinking water sources, Nak-Dong River, South Korea: Application to a field-scale drinking water treatment plant

Microcystin-LR (MC-LR) is a growing issue as it is toxic and difficult to remove in drinking water treatment plants (DWTPs). Mesoporous carbon (MC) is evaluated as an alternative adsorbent for MC-LR removal and compared with three widely-used powdered activated carbons (PACs). MC was more favorable for MC-LR removal than PACs. MC-LR adsorption on MC was a rapid process (k₂ = 1.02  ×  10^-4 g/μg/min) that completed within 15 min, while adsorption on PACs took 60 min. The maximum adsorption capacity of MC-LR was 18,008 μg/g (MC), which was higher than that of the PACs. Two mechanisms were associated with adsorption: the small hydro-dynamic diameter of MC in an aqueous solution increased the instantaneous attraction of MC-LR to its surface, and the numerous mesopores enhanced pore diffusion. The MC could remove MC-LR to meet the drinking water guidance level (1 μg/L) from an the MC-LR concentration range of 5-20 μg/L in drinking water sources, and 10 min of treatment was sufficient to meet this level (MC dose = 20 mg/L). The field-scale DWTP was operated by adding 1 or 5 mg/L MC to the mixing basin, and 49.49% and 74.50% of MC-LR was removed, respectively. Geosmin and 2-methylisoborneol were slightly reduced when 5 mg/L of MC was applied.

Simultaneous uptake of NOM and Microcystin-LR by anion exchange resins: Effect of inorganic ions and resin regeneration

This study investigated the efficiency of a strongly basic macroporous anion exchange resin for the co-removal of Microcystin-LR (MCLR) and natural organic matter (NOM) in waters affected by toxic algal blooms. Environmental factors influencing the uptake behavior included MCLR and resin concentrations, NOM and anionic species, specifically nitrate, sulphate and bicarbonate. A860 resin exhibited an excellent adsorption capacity of 3800 μg/g; more than 60% of the MCLR removal was achieved within 10 min with a resin dosage of 200 mg/L (∼1 mL/L). Further, kinetic studies revealed that the overall removal of MCLR is influenced by both external diffusion and intra-particle diffusion. Increasing NOM concentration resulted in a significant reduction of MCLR uptake, especially at lower resin dosages, where a competitive uptake between the charged NOM fractions and MCLR was observed due to limited active sites. In addition, MCLR uptake was significantly reduced in the presence of sulphate and nitrate in the water matrix. Moreover, performance of the resin proved to be stable from one regeneration cycle to another. Approximately 80% of MCLR and 50% of dissolved organic carbon (DOC) were recovered in the regenerated brine. Evidences of resin saturation and site reduction were also observed after 2000 bed volumes (BV) of operation. For all the investigated water matrices, a resin dosage of 1000 mg/L (∼4.5 mL/L) was sufficient to lower MCLR concentration from 100 μg/L to below the World Health Organization guideline of 1 μg/L, while simultaneously providing more than 80% NOM removal.

Combining hydrogen peroxide addition with sunlight regulation to control algal blooms

The concentration, light conditions during treatment, and the number of hydrogen peroxide (H₂O₂) additions as well as the H₂O₂ treatment combined with subsequent shading to control algal blooms were studied in the field (Lake Dianchi, China). The cyanobacterial stress and injury due to H₂O₂ were dose dependent, and the control effectiveness and degradation of H₂O₂ were better and faster under full light than under shading. However, H₂O₂ was only able to control a bloom for a short time, so it may have promoted the recovery of algae and allowed the biomass to rebound due to the growth of eukaryotic algae. A second addition of H₂O₂ at the same dose had no obvious effect on algal control in the short term, suggesting that a higher concentration or a delayed addition should be considered, but these alternative strategies are not recommended so that the integrity of the aquatic ecosystem is maintained and algal growth is not promoted. Moreover, shading (85%) after H₂O₂ addition significantly reduced the algal biomass during the enclosure test, no restoration was observed for nearly a month, and the proportion of eukaryotic algae declined. It can be inferred that algal blooms can be controlled by applying a high degree of shading after treatment with H₂O₂.

Temporal Dynamics of the Microbial Community Composition with a Focus on Toxic Cyanobacteria and Toxin Presence during Harmful Algal Blooms in Two South German Lakes

Bacterioplankton plays an essential role in aquatic ecosystems, and cyanobacteria are an influential part of the microbiome in many water bodies. In freshwaters used for recreational activities or drinking water, toxic cyanobacteria cause concerns due to the risk of intoxication with cyanotoxins, such as microcystins. In this study, we aimed to unmask relationships between toxicity, cyanobacterial community composition, and environmental factors. At the same time, we assessed the correlation of a genetic marker with microcystin concentration and aimed to identify the main microcystin producer. We used Illumina MiSeq sequencing to study the bacterioplankton in two recreational lakes in South Germany. We quantified a microcystin biosynthesis gene (mcyB) using qPCR and linked this information with microcystin concentration to assess toxicity. Microcystin biosynthesis gene (mcyE)-clone libraries were used to determine the origin of microcystin biosynthesis genes. Bloom toxicity did not alter the bacterial community composition, which was highly dynamic at the lowest taxonomic level for some phyla such as Cyanobacteria. At the OTU level, we found distinctly different degrees of temporal variation between major bacteria phyla. Cyanobacteria and Bacteroidetes showed drastic temporal changes in their community compositions, while the composition of Actinobacteria remained rather stable in both lakes. The bacterial community composition of Alpha- and Beta-proteobacteria remained stable over time in Lake Klostersee, but it showed temporal variations in Lake Bergknappweiher. The presence of potential microcystin degraders and potential algicidal bacteria amongst prevalent Bacteroidetes and Alphaproteobacteria implied a role of those co-occurring heterotrophic bacteria in cyanobacterial bloom dynamics. Comparison of both lakes studied revealed a large shared microbiome, which was shaped toward the lake specific community composition by environmental factors. Microcystin variants detected were microcystin-LR, -RR, and -YR. The maximum microcystin concentrations measured was 6.7 μg/L, a value still acceptable for recreational waters but not drinking water. Microcystin concentration correlated positively with total phosphorus and mcyB copy number. We identified low abundant Microcystis sp. as the only microcystin producer in both lakes. Therefore, risk assessment efforts need to take into account the fact that non-dominant species may cause toxicity of the blooms observed.

Varied influence of microcystin structural difference on ELISA cross-reactivity and chlorination efficiency of congener mixtures

Enzyme-linked immunosorbent assay (ELISA) is an antibody-based analytical method that has been widely applied in water treatment utilities for the screening of toxic cyanobacteria metabolites such as microcystins (MCs). However, it is unknown how the minor structural difference of MCs may impact their chlorination kinetics and measurement via ELISA method. It was found in this study that, regardless of the experimental conditions (n = 21), there was no MC-YR or MC-LY residual, while different removal rates of other MCs were observed (MC-RR > MC-LR > MC-LA ∼ MC-LF) as measured by liquid chromatography tandem mass spectrometry (LC-MS/MS), which was consistent with the relative reactivity of the amino acid variables with free chlorine. The removal of total MCs was generally lower as measured by ELISA than by LC-MS/MS. By incorporating both analytical results, existence of ADDA-containing byproducts or byproducts that had a higher sensitivity toward the ELISA kit was demonstrated, after excluding the contribution of the cross-reactivity of the parent MCs. It should be noted, however, that the cross-reactivities of MCs could be influenced not only by MC congeners, but also by other conditions such as mixtures and the applied ELISA kit.

Addressing harmful algal blooms (HABs) impacts with ferrate(VI): Simultaneous removal of algal cells and toxins for drinking water treatment

Although ferrate(VI) has long been recognized as a multi-purpose treatment agent, previous investigations regarding ferrate(VI) for addressing harmful algal blooms (HABs) impacts in drinking water treatment only focused on a single HAB pollutant (e.g. algal cells or algal toxins). Moreover, the performance of ferrate(VI)-driven coagulation was poorly investigated in comparison with ferrate(VI) oxidation, though it has been widely acknowledged as a major ferrate(VI) treatment mechanism. We herein reported ferrate(VI) as an emerging agent for simultaneous and effective removal of algal cells and toxins in a simulated HAB-impacted water. Ferrate(VI)-driven oxidation enabled algal cell inactivation and toxin decomposition. Subsequently, Fe(III) from ferrate(VI) reduction initiated an in-situ coagulation for cell aggregation. Cell viability (initial 4.26 × 10⁴ cells/mL at pH 5.5 and 5.16 × 10⁴ cells/mL at pH 7.5) decreased to 0.0% at ≥ 7 mg/L Fe(VI) at pH 5.5 and 7.5, respectively. Cell density and turbidity were dramatically decreased at pH 5.5 once ferrate(VI) doses were beyond their respective threshold levels, which are defined as minimum effective iron doses (MEIDs). However, the particulate removal at pH 7.5 was poor, likely because the coagulation was principally driven by charge neutralization and a higher pH could not sufficiently lower the particle surface charge. Meanwhile, algal toxins (i.e., microcystins) of 3.98 μg/L could be substantially decomposed at either pH. And the greater degradation achieved at pH 5.5 was due to the higher reactivity of ferrate(VI) at the lower pH. This study represents the first step toward the ferrate(VI) application as a promising approach for addressing multiple HABs impacts for water treatment.

Response of bacterial communities to cyanobacterial harmful algal blooms in Lake Taihu, China

Cyanobacterial harmful algal blooms are prevalent around the world, influencing aquatic organisms and altering the physico-chemical properties in freshwater systems. However, the response of bacterial communities to toxic cyanobacterial blooms and associated microcystins (MC) remain poorly understood even though global concentrations of MC have increased dramatically in the past few decades. To address this issue, the dynamics of bacterial community composition (BCC) in the water column and how BCC is influenced by both harmful cyanobacterial blooms and environmental factors were investigated on a monthly basis from August 2013 to July 2014 in Lake Taihu, China. Non-metric multidimensional scaling (NMDS) revealed that seasonal variation in BCC was significant, and that the succession of BCC greatly depends on changes in environmental conditions. Redundancy analysis (RDA) results showed that the overall variation of BCC was explained mainly by dissolved oxygen (DO), nitrate nitrogen (NO₃^--N), and Microcystis. The alpha biodiversity of the bacterial community was different among months with the highest diversity in February and the lowest diversity in October. Furthermore, significant negative relationships were found between alpha biodiversity indices and Microcystis abundance as well as with intracellular MC concentrations, indicating that Microcystis and associated MC may influence the bacterial community structure by reducing its biodiversity. This study shows that potential associations exist between toxic cyanobacterial blooms and bacterial communities but more investigations are needed to obtain a mechanistic understanding of their complex relationships.

Spatial and temporal variability in the nitrogen cyclers of hypereutrophic Lake Taihu

Harmful cyanobacterial blooms (cyanoHABs) are a major threat to freshwater ecosystems worldwide. Evidence suggests that both nitrogen and phosphorus are important nutrients in the development and proliferation of blooms, yet much less is known about nitrogen cycling dynamics in these systems. To assess the potential nitrogen cycling function of the cyanoHAB community, surface water samples were collected in Lake Tai (Taihu), China over a 5-month bloom event in 2014. The expression of six nitrogen cycling genes (nifH, hzsA, nxrB, nrfA, amoA, nosZ) was surveyed using a targeted microarray with probes designed to provide phylogenetic information. N-Cycling gene expression varied spatially across Taihu, most notably near the mouth of the Dapu River. Expression of nifH was observed across the lake and attributable to both Proteobacteria and Cyanobacteria: Proteobacteria were major contributors to nifH signal near shore. Other N transformations such as anaerobic ammonia oxidation and denitrification were evident in the surface waters as well. Observations in this study highlight the potential importance of heterotrophic bacteria in N-cycling associated with cyanoHABs.

Critical assessment of chitosan as coagulant to remove cyanobacteria

Removal of cyanobacteria from the water column using a coagulant and a ballast compound is a promising technique to mitigate nuisance. As coagulant the organic, biodegradable polymer chitosan has been promoted. Results in this study show that elevated pH, as may be common during cyanobacterial blooms, as well as high alkalinity may hamper the coagulation of chitosan and thus impair its ability to effectively remove positively buoyant cyanobacteria from the water column. The underlying mechanism is likely a shielding of the protonated groups by anions. Inasmuch as there are many chitosan formulations, thorough testing of each chitosan prior to its application is essential. Results obtained in glass tubes were similar to those from standard jar tests demonstrating that glass tube tests can be used for testing effects of coagulants and ballasts in cyanobacteria removal whilst allowing far more replicates. There was no relation between zeta potential and precipitated cyanobacteria. Given the well-known antibacterial activity of chitosan and recent findings of anti-cyanobacterial effects, pre-application tests are needed to decipher if chitosan may cause cell leakage of cyanotoxins. Efficiency- and side-effect testing are crucial for water managers to determine if the selected approach can be used in tailor-made interventions to control cyanobacterial blooms and to mitigate eutrophication.

Adsorption of microcystin-LR on mesoporous carbons and its potential use in drinking water source

Microcystin-LR (MC-LR) is a common toxin derived from cyanobacterial blooms an effective, rapid and non-toxic method needs to be developed for its removal from drinking water treatment plants (DWTP). For an adsorption-based method, mesoporous carbon can be a promising supplemental adsorbent. The effect of mesoporous carbon (MC1, MC2, and MC3) properties and water quality parameters on the adsorption of MC-LR were investigated and the results were analyzed by kinetic, isotherm, thermodynamic, Derjaguin-Landau-Verwey-Overbeek (DLVO), and intraparticle diffusion models. MC1 was the most appropriate type for the removal of MC-LR with a maximum adsorption capacity of 35,670.49 μg/g. Adsorption of MC-LR is a spontaneous reaction dominated by van der Waals interactions. Pore sizes of 8.5-14 nm enhance the pore diffusion of MC-LR from the surface to the mesopores of MC1. The adsorption capacity was not sensitive to changes in the pH (3.2-8.0) and the existence of organic matter (2-5 mg/L). Furthermore, the final concentration of MC-LR was below the WHO guideline level after a 10-min reaction with 20 mg/L of MC1 in the Nak-Dong River, a drinking water source. The MC-LR adsorption mainly competed with humic substances (500-1000 g/mole); however, they did not have a great effect on adsorption.

Estimating microcystin levels at recreational sites in western Lake Erie and Ohio

Cyanobacterial harmful algal blooms (cyanoHABs) and associated toxins, such as microcystin, are a major global water-quality issue. Water-resource managers need tools to quickly predict when and where toxin-producing cyanoHABs will occur. This could be done by using site-specific models that estimate the potential for elevated toxin concentrations that cause public health concerns. With this study, samples were collected at three Ohio lakes to identify environmental and water-quality factors to develop linear-regression models to estimate microcystin levels. Measures of the algal community (phycocyanin, cyanobacterial biovolume, and cyanobacterial gene concentrations) and pH were most strongly correlated with microcystin concentrations. Cyanobacterial genes were quantified for general cyanobacteria, general Microcystis and Dolichospermum, and for microcystin synthetase (mcyE) for Microcystis, Dolichospermum, and Planktothrix. For phycocyanin, the relations were different between sites and were different between hand-held measurements on-site and nearby continuous monitor measurements for the same site. Continuous measurements of parameters such as phycocyanin, pH, and temperature over multiple days showed the highest correlations to microcystin concentrations. The development of models with high R² values (0.81-0.90), sensitivities (92%), and specificities (100%) for estimating microcystin concentrations above or below the Ohio Recreational Public Health Advisory level of 6μgL^-1 was demonstrated for one site; these statistics may change as more data are collected in subsequent years. This study showed that models could be developed for estimates of exceeding a microcystin threshold concentration at a recreational freshwater lake site, with potential to expand their use to provide relevant public health information to water resource managers and the public for both recreational and drinking waters.