Suppression of Harmful Algae Blooms in Reservoirs: Lessons Learned from Projects
Annual Conference
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Suppression of harmful algae blooms (HAB) in drinking water reservoirs is a common and growing concern for utilities worldwide. Effects of HAB include risk of cyanotoxin exposure, formation of taste and odour compounds, and operational impacts to water treatment plants.
The evidence from projects is that suppression of HAB formation in reservoirs is feasible and practical. Evolution of engineering and science over decades has matured into a suite of effective technologies. Successful reservoir management technologies operate through nutrient denial, and (2) buoyancy disruption of cyanobacteria.
Summer is the central season of thermal stratification in reservoirs. Warm surface waters (epilimnion) float on colder, denser bottom waters (hypolimnion). Sediments consume dissolved oxygen, creating anoxia in the hypolimnion. This shift in reservoir geochemistry solubilizes iron, manganese, ammonium, and phosphate from sediments into the water column. It often causes formation of hydrogen sulphide. Increase in phosphate concentrations is the central driver of HAB, but ammonium, iron, and hydrogen sulphide are common co-drivers.
Phytoplankton need abundant light and dissolved nutrients to form HAB. Whatever nutrients are available in the photic (light penetration) zone are quickly scavenged by fast-growing diatoms or green algae, which then sink into the deep. Cyanobacteria control their buoyancy, acquiring nutrients from deep waters and light from the surface on a daily cycle built-in to their physiology.
Destroying anoxia denies cyanobacteria their deep water “pantry” by keeping nutrients locked in sediments. Access to deep nutrients is critically important to cyanobacteria blooms. Diatom blooms often occur in the fall or winter when cooling causes nutrient-rich bottom waters to rise to the surface. Summer anoxia is a major diver of these blooms.
Pure oxygen injection is far more effective at destroying anoxia than is aeration or mixing. By practical application of the law of partial pressures a pure oxygen bubble has about five times greater mass transfer capacity of oxygen to water than does a bubble of air. Comparison of reservoirs that have gone through aeration and oxygen injection phases clearly demonstrates the superiority of pure oxygen.
It is possible to disrupt cyanobacteria buoyancy by destratification. If the reservoir mixes from to top bottom frequently enough, then cyanobacteria cannot sink or float fast enough to maintain their life-strategy advantage in the reservoir ecosystem. Demonstrably effective, theory sets limits to application of this method.
Attention to reservoir geochemistry is critical. Providing abundant dissolved oxygen will not sequester phosphate in sediments if there is not enough ferric iron to bind it. Thus, iron amendments are essential in iron-deficient reservoirs. In shallow reservoirs, increasing the dissolved aluminium concentration may be necessary to bind phosphate into sediments. Ecologically safe criteria for doing so were established by the US EPA in 2018 after many years of research.
Together, these technologies, all of which are in the public domain, now form core means to suppress HAB formation in drinking water reservoirs and point toward future improvements.