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Cyanotoxins pose a health threat when present in the drinking water supply since conventional water treatment processes are not effective in removing extracellular metabolites hence, advanced treatment techniques are usually applied. Powdered activated carbon (PAC) is an effective adsorbent for removing toxins. However, since a high volume is necessary, alternative adsorbents have been investigated. Biochar, especially from renewable sources, is a potential adsorbent material that could replace PAC for removing toxins. This paper aimed to investigate which PAC properties play key roles in cyanotoxin adsorption by a systematic review addressing the adsorption of toxins such as microcystins-LR (MC-LR), cylindrospermopsin (CYL), and saxitoxins (STXs). As a result, the review showed that some commonly adopted indices (i.e. total surface area) are not relevant to cyanotoxin adsorption, especially if appraised alone. Along with a multi-barrier approach, PAC has to be applied taking into account the complexity of the water system, which includes a better understanding of the characteristics of the adsorbent, the target toxin, and the aqueous medium. The biochar systematic review showed that no studies have yet been designed specifically for the removal of toxins. Since biochar has not yet been applied to water treatment processes, the knowledge gap is even greater than for PAC.

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RESUMO A escassez hídrica e o aumento da demanda de água para usos múltiplos incentivaram a construção de diversos reservatórios e sistemas de transferências hídricas entre bacias hidrográficas em diversas partes do mundo, principalmente em regiões áridas e semiáridas. Apesar de essas medidas mitigarem os aspectos quantitativos do abastecimento, a diminuição da qualidade da água disponível pode se tornar uma limitação importante. Este estudo teve por objetivo avaliar a qualidade da água no sistema de transferência que abastece a Região Metropolitana de Fortaleza, Ceará. Foram analisados 20 parâmetros de qualidade de água ao longo do sistema. Aplicou-se em seguida a análise de agrupamento utilizando a distância euclidiana associada ao método de ligação de Ward para agrupar os 13 pontos de coleta que foram estabelecidos conforme suas similaridades. A piora da qualidade da água ficou evidente ao longo do sistema, com o aumento da concentração de cianobactérias, pH, nitrogênio total, oxigênio dissolvido, turbidez e clorofila-a. Com relação às cianobactérias, duas espécies mostraram-se dominantes — Oscillatoria sp. e Microcystis sp. — e sete abundantes — Merismopedia sp., Cylindrospermopsis sp., Pseudanabaena sp., Anabaena sp., Aphanizomenon sp., Aphanocapsa sp. e Dolichospermum sp. A presença de cianobactérias potencialmente tóxicas e a piora da qualidade da água tornam a realização de monitoramentos frequentes e o desenvolvimento de sistemas de alerta essenciais para a adequada gestão dos recursos hídricos, tanto do ponto de vista ambiental quanto do de saúde pública.

ABSTRACT Water scarcity and increasing water demand for multiple uses encourage the construction of reservoirs and water transfer systems between watersheds in various parts of the world, especially in arid and semi-arid regions. Although these measures may mitigate the quantitative aspects of water supply, the quality decrease of the available water may become an important limitation. This study aimed to evaluate the water quality in the water transfer system that supplies the metropolitan region of Fortaleza, Ceará. Twenty water quality parameters were analyzed throughout the system followed by the application of the Euclidean distance and Ward's method to group the 13 sampling points according to their similarities. The worsening of the water quality was evident with increasing cyanobacterial concentration, pH, total nitrogen, dissolved oxygen, turbidity, and chlorophyll-a. Regarding cyanobacteria, two species were dominant: Oscillatoria sp. and Microcystis sp. while seven were abundant: Merismopedia sp., Cylindrospermopsis sp., Pseudanabaena sp., Anabaena sp., Aphanizomenon sp., Aphanocapsa sp. e Dolichospermum sp. The presence of potentially toxic cyanobacteria and the worsening of water quality make frequent monitoring and the development of alert systems essential for the management of water resources, both from an environmental and public health point of view.

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RESUMO As florações de cianobactérias vêm aumentando em intensidade e frequência em todo o mundo, ameaçando a sustentabilidade dos recursos aquáticos e o abastecimento humano. A construção de reservatórios, o uso de fertilizantes artificiais e o descarte de esgotos não tratados são os principais motores dessa expansão. As alterações climáticas, como o aquecimento global, as chuvas e as secas extremas, têm contribuído para a acelerar esse processo. Qualquer medida de mitigação dessas florações deve considerar os impactos sobre os outros organismos, os custos, além dos resíduos gerados. Nessa perspectiva, a aplicação do peróxido de hidrogênio em reservatórios artificiais traz uma série de vantagens para o controle a curto prazo, sendo uma das principais a destruição seletiva de cianobactérias. Apesar de vários estudos indicarem que as cianobactérias são o alvo preferencial do peróxido de hidrogênio, uma explicação bioquímica desse fenômeno ainda não foi encontrada. Este trabalho, portanto, procura congregar assuntos relacionados a esse tema e traz também uma coletânea de experiências recentes sobre o uso em escala real do peróxido de hidrogênio, apontando as principais lacunas de conhecimento que deverão ser preenchidas caso sua utilização em reservatórios seja para abastecimento. Uma das lacunas a ser preenchida está relacionada às vantagens do peróxido de hidrogênio sobre outros métodos de controle, pois apresentam maior eficiência no controle de cianobactérias posto que são preferencialmente destruídos pelo peróxido de hidrogênio, em relação aos fitoplânctons, sendo, dessa forma, importante o estudo de técnicas de aplicação do peróxido de hidrogênio em reservatórios de grande porte.

ABSTRACT Cyanobacterial blooms are increasing in intensity and frequency throughout the world, threatening the sustainability of aquatic resources and human supplies. The construction of reservoirs, the use of artificial fertilizers and the disposal of untreated sewage are the main engines of this expansion. Climate change, such as global warming, rainfall and extreme droughts, has contributed to accelerate this process. Any mitigation measure of the blooms should consider the impacts on the other organisms, the costs beyond the waste generated. In this perspective, the application of hydrogen peroxide in the artificial reservoirs has a number of advantages for short-term control, one of the main ones being the selective destruction of cyanobacteria. Although several studies indicate that cyanobacteria are the preferred target of hydrogen peroxide, a biochemical explanation for this phenomenon has not yet been found. This work, therefore, seeks to gather subjects related to this topic and brings a collection of recent experiences on the real scale use of hydrogen peroxide, pointing out the main knowledge gaps that should be filled if their use in reservoirs is for supply. One of the gaps to be filled is related to the advantages of hydrogen peroxide over other control methods, as they present greater efficiency in the control of cyanobacteria, as they are preferentially destroyed by hydrogen peroxide in relation to phytoplankton, therefore, it is important to study the application techniques of hydrogen peroxide in large reservoirs.

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Although some studies have investigated the impact caused by chemicals used on water treatment (coagulants and oxidants) on cyanobacteria integrity, the isolated effect of shear stress during coagulation is still not fully understood. This study evaluated the impact of different velocity gradients, mixing times, and the addition of powdered activated carbon (PAC) on the integrity of Microcystis aeruginosa, Raphidiopsis raciborskii, and Dolichospermum circinale, known producers of toxin and taste and odor (T&O) compounds. No association was found between R. raciborskii cell lysis and velocity gradient, with or without PAC, demonstrating the high resilience of this taxon to shear stress. In contrast, an association was found for M. aeruginosa at the highest velocity gradient evaluated (1000 s-1) and for D. circinale above the lowest velocity gradient studied (600 s-1). After PAC addition, there was a reduction in the chances of finding M. aeruginosa intact cells above velocity gradient 800 s-1 at 45 s, while D. circinale show cell lysis in all the scenarios expect at 600 s-1 and 10 s of agitation. The additional impact of PAC on cell lysis may lead to more release of metabolites and shows the need to adjust the hydraulic conditions in the rapid mixing stage, especially when more "fragile" cyanobacteria are present. Neither cyanobacterial cell size nor morphology was shown to be relevant to shear stress sensitivity, indicating that cell wall composition might have been an important factor in controlling cell lysis.

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Taste and odor compounds affect drinking water safety perception and may drive consumers to less secure water sources. Adsorption, using powered activated carbon, is the most common method to remove these compounds but greatly increases the amount of sludge generated. Another way of removing taste and odor compounds is to use filters with granular activated carbon (GAC) but little is still known on how to design them. In this work, the homogeneous surface diffusion model (HSDM) was used to model bench-scale kinetic and isotherm experiments and to simulate the removal of geosmin in a full-scale GAC filter. Geosmin adsorption isotherm was best described by the Freundlich model in all used carbons and film resistance (Kf) was more relevant to adsorption kinetics than pore diffusion (Ds). The simulation showed that in a filter with an empty bed contact time of 5 minutes and raw water with geosmin concentrations of 50, 75, and 100 ng.L-1, the effluent would exceed the trash-hold concentration (10 ng.L-1) in 98, 77, and 66 days, respectively, without considering biological removal.

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Cyanobacteria are known to produce a wide variety of bioactive, toxic secondary metabolites generally described as hepatotoxins, neurotoxins, cytotoxins, or dermatoxins. In Brazil, the regular monitoring of cyanobacterial toxins has intensified after the death of 65 patients in a hemodialysis clinic in Caruaru in the state of Pernambuco due to microcystin exposure. The primary objective of this study was to use multivariate statistics that incorporated environmental parameters (both biotic and abiotic) to forecast blooms of cyanobacteria and their toxic secondary metabolites in 20 drinking water reservoirs managed by the Water Treatment Company of Ceará (CAGECE) in the semi-arid region of Ceará, Brazil. Across four years (January 2013 to January 2017), 114 different phytoplankton taxa were identified, including 24 cyanobacterial taxa. In general, Ceará reservoirs were dominated by cyanobacteria due to eutrophication but also because of the dry and warm climate found throughout the region. Interestingly, specific cyanobacterial taxa were influenced by different biotic and abiotic factors. For example, nitrogen-to-phosphorus (N:P) and evaporation were positively related to saxitoxin-producing taxa, especially Raphidiopsis raciborskii, while temperature, electrical conductivity, total phosphorus, and transparency (measured as Secchi depth) were positively associated with microcystin-producing taxa, such as Microcystis aeruginosa. Climate forecasts predict higher evaporation and temperatures in the semi-arid Ceará region, which will likely magnify droughts and water scarcity as well as promote toxic cyanobacterial blooms in reservoirs in the future. Therefore, understanding the factors associated with algal blooms dominated by specific taxa is paramount for water resource management.

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RESUMO O problema causado pelas florações de cianobactérias em mananciais para abastecimento humano vem se tornando cada dia mais preocupante não só em regiões tropicais mas em todo o mundo. Os gestores dos sistemas de tratamento de água, muitas vezes, têm que recorrer ao uso da pré-oxidação como forma de auxiliar no tratamento apesar do incremento do potencial de liberação de metabólitos e da formação de subprodutos tóxicos. Por isso, a pré-oxidação tem sido vista como vilã, tornando-se um paradigma para os profissionais de tratamento de água. Este trabalho apresenta o estado da arte do conhecimento e evidências de que a pré e a pós-oxidação podem ser usadas com sucesso como primeira e última barreira à ameaça imposta pelas cianobactérias, desde que as características da água bruta sejam conhecidas. A partir daí, o oxidante, a dosagem e o tempo de contato podem ser selecionados e definidos com mais segurança, auxiliando na produção de água potável.

ABSTRACT The problem caused by cyanobacteria blooms in water sources for human consumption is becoming of more concern each day not only in tropical countries but worldwide. Water treatment systems often resort to the use of pre-oxidation as a form of auxiliary treatment although increasing the potential for release of metabolites and formation of toxic byproducts. Therefore, the pre-oxidation has often been seen as becoming a paradigm for the water treatment professionals. This paper presents the state of art and provides evidences that the pre and post-oxidation can be used successfully as first and last barrier to the problems caused by cyanobacteria provided that the raw water characteristics are known. From there, the oxidant, dosage and contact time can be selected and defined with greater confidence in order to produce drinking water.

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A implantação de uma unidade de pré-tratamento a montante de um filtro ascendente (FA) existente objetiva condicionar a água bruta eutrofizada à capacidade de tratamento do FA. Essa solução esbarra na necessidade de se instalar essa unidade de pré-tratamento com cota mais elevada que o filtro a jusante. Nesta pesquisa, a instalação de um filtro de pressão de fluxo descendente possibilitou a eliminação do problema do desnível geométrico e a adequação da água bruta com elevada presença de fitoplâncton às limitações da filtração direta ascendente (FDA). O pré-filtro de pressão com melhor eficiência, o PFP1, apresentou uma remoção de 50% de cor aparente e 63% de turbidez. A eficiência global do sistema PFP/FA foi satisfatória, apresentando turbidez final inferior a 0,5 uT, cor aparente em torno de 5 uH, além de reduzir as perdas da água tratada de 11,7 para 4,5%.

The implementation of a pretreatment unit before existing upward filter, to condition the eutrophic raw waters to its capacity, is an option which runs into a problem of technical nature, the need of a pretreatment unit with enough hydraulic load to properly feed the existing filter. Typically, the available studies on double filtration present upward filters followed by descendant ones. In this study, the installation of a pressure filter downflow adapted the raw water with high algae concentration to the upward direct filtration system limitations, improving the quality of water produced and reducing losses by washing. The pressure filter with better efficiency was the PFP1 filter showed a removal of 50% of apparent color and 63% of turbidity. The overall efficiency of the PFP/FA system was satisfactory, presenting final turbidity of less than 0.5 uT, apparent color around 5 uH and reducing losses from 11.7 to 4.5% of the total treated water.

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A determinação da quantidade de percolado gerado em aterros sanitários continua sendo uma tarefa difícil, principalmente no semi-árido nordestino onde predominam caracteríticas meteorológicas e geológicas bem peculiares. O programa HELP (avaliação do comportamento hidrológico em aterros sanitários), desenvolvido por Schroder et al (1984), foi utilizado na simulação de várias configurações de drenagem de fundo. Aplicou-se ao modelo, dados meteorológicos e de solos da região onde está o aterro sanitário de Caucaia (ASMOC), a qual é representativa do semi-árido nordestino. Três características da drenagem de fundo foram variadas (configuração da drenagem, inclinação de fundo e adição de uma camada drenante com alta permeabilidade) com o objetivo de avaliar a altura da coluna de percolado, a quantidade de percolado infiltrado no solo e o volume coletado para tratamento. Nas trincheiras sem camada drenante, o aumento da inclinação de fundo e a alteração da configuração dos drenos tiveram pouca influência sobre as variáveis estudadas. A adição da camada drenante proporcionou uma redução significativa da altura da coluna e na infiltração de percolado no solo, mostrando que é um componente fundamental para o bom funcionamento do sistema de drenagem horizontal em aterros sanitários.

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