In early August 2014, a harmful algal bloom (HAB) in the western basin of Lake Erie produced toxins that, due to their inability to be treated by local water treatment facilities, rendered drinking water unsafe for nearly 500,000 residents of Toledo, Ohio over the course of several days. The drinking water ban that was imposed raises questions about the nature of HABs and strategies that can be used to maintain the availability of clean water for cities and industries when they occur. What causes HABs and what can be done to prevent water bans in the future?
HABs are linked to microorganism populations, particularly algae, phytoplankton and certain varieties of cyanobacteria, which form the base of the food chain in aquatic ecosystems. These microorganisms depend on light and nutrients, particularly “limiting nutrients” such as iron, phosphate and nitrogen, to grow and multiply. In ambient environmental concentrations, limiting nutrients sustain microorganism life while maintaining stable population levels.
Anthropogenic and natural pressures cause limiting nutrient concentrations to increase in aquatic ecosystems. For example, runoff from farmland containing fertilizers rich in nitrogen and phosphorous can cause elevated levels of each chemical to enter lakes, and wind patterns can transport and deposit iron-rich sands into oceans. In shallow layers of water bodies where light is plentiful, an influx of limiting nutrients prompts eutrophication, whereby microorganisms quickly multiply, or bloom, to atypical population levels. In the western basin of Lake Erie, where light can easily penetrate shallow water and nutrient-rich fertilizers drain from vast farmlands by way of the Maumee River, eutrophication risk is clearly pronounced in the region.
Microorganism blooms reach HAB status when they begin to threaten human health, natural resources, and man-made resources such as water treatment facilities. For example, cyanobacteria blooms characteristically produce a toxin known as microcystin. Microcystin persists anywhere from weeks to months in the environment, causes liver damage, irritation of the skin, eyes and throat, and other negative health effects in humans. The toxin can be fatal to fish and other smaller animals as well. Based on these health risks and impacts, it has been found that it is not uncommon for HABs to cause upwards of millions, if not billions, of dollars of damage on the commercial and recreational fishing and fishing trades in coastal regions.
Conventional water treatment facilities that are not designed to handle increased solids loading, toxicity or other detrimental, HAB-driven water quality factors are susceptible to shut down events to protect public health and resources. Older facilities which have lost their abilities to provide robust water treatment as a function of age are vulnerable to shut down events as well. Such was the case in Toledo, Ohio this month: the Collins Park Water Treatment Plant, a 73-year old facility known to be in need of upgrades was shut down for several days to protect residents from potential impacts of the cyanobacteria bloom on the potable water supply.
Despite the breadth of knowledge of the factors that drive HAB development in the United States and abroad, HABs are still understood to be widely unpredictable. Studies also show that HAB frequency and distribution is on the rise. For example, a report by the Woods Hole Oceanographic Institute indicated that regions worldwide impacted by paralytic shellfish poisoning (PSP), an indicator of HAB activity, increased nearly six-fold between 1972 and 2006. Therefore, if HABs are essentially moving targets whose incidence is increasing, what can be done to mitigate their negative impacts?
Modern water treatment technologies and strategies offer opportunities for cities and industries to maintain clean, safe water supplies for consumption throughout the duration of HAB events. Several promising water treatment strategies and technologies can be categorized as detection and monitoring systems, intake designs, and treatment technologies.
Incidence of PSP between 1970 (top) and 2006 (bottom). Photo: WHOI
Detection and Monitoring – Analyzers and remote sensing technologies can be used to identify HAB organisms and indicators of HAB activity. For example, nitrogen analyzers can be used to gauge potential risk of eutrophication activity based on levels of the limiting nutrient available in a water supply. Chlorophyll-A analyzers, which detect light-capturing pigments available in photosynthetic organisms, can track concentrations of organisms comprising an HAB. Remote sensing technologies, such as NASA’s MODIS satellite imaging system, can also be used to detect HAB presence and movement through the environment. Facility operators can use these tools to their advantage to track HAB activity and optimize water treatment processes in response, in order to maximize plant uptime during a bloom.
Intake Systems – Design and location selection of structures used to draw water from a water body for treatment are important considerations for HAB risk mitigation. For example, open intakes which extend farther and deeper into water bodies can avoid regions where biomass accumulate, as well as the photic zone where HAB organisms tend to grow. Submerged pipelines which extend along lake or sea beds can be used for similar purposes. Well systems, such as beach wells employed in desalination facilities, can leverage natural filtration through surrounding geology to alleviate HAB impacts. With proper intake selection and placement, HAB material can be avoided, and plant availability may improve.
Treatment Technologies – Technologies such as screens, dissolved air flotation (DAF) systems, and membrane filtration systems are powerful tools that can be used to enable production of clean, continuous water supplies during HAB events. Screens, typical first-tier treatment technologies for many water treatment facilities, can be designed to limit HAB impingement and allow adequate flow capacity during a bloom event. Advanced DAF systems have proven highly effective in separating algae and other low density solids from raw water. For example, in late 2008, a severe and persistent red tide prompted shutdowns of numerous seawater desalination facilities along the coast of the UAE, except for a pilot facility outfitted with an advanced DAF technology at the Fujairah 2 plant. Membrane filtration systems, including microfiltration and ultrafiltration, have proven effective in removing elevated solids loads while resisting biological fouling. Further, when combined with well-designed detection, monitoring and intake systems, the benefits of these treatment technologies can be amplified. Use of these technologies, individually or in conjunction with one another, can thus be used to provide continuous water treatment during an HAB event.
Despite the promising aspects of modern water treatment technologies and strategies, numerous tradeoffs must be considered when evaluating their use in existing facilities or in new builds. For example, use of certain analyzers and remote sensing technologies can be cumbersome, time-consuming, and expensive. Intake systems may not prevent pollution such as dissolved toxins from entering water treatment facilities, and can add cost for both new facilities and others which are examining retrofit options. Screens, DAF systems, and membrane filtration processes can be expensive to construct and operate, and space availability may be limited at facilities which are considering upgrades.
Nonetheless, drinking water bans, such as that which impacted Toledo, Ohio this month, illustrates the remarkable impact that HAB events can impose on cities and industries in the United States. As water treatment plants age across the United States, new facilities undergo design and construction, and HAB risks continue to rise, it will become increasingly important to carefully consider leveraging modern water treatment technologies and strategies to ensure the resilience of water treatment facilities. In this way, unforeseen HAB risks can be mitigated, and safe water supplies can be assured for cities and industries in years to come.
Photos courtesy of Haraz N. Ghanbari, Associated Press; Woods Hole Oceanographic Institution (WHOI).