Frequent outbreaks of algal blooms are degrading water in drinking water reservoirs worldwide, making the water unusable.
Algal bloom causes
Algal blooms can sometimes be so massive that they are visible from outer space. Rising temperatures, prolonged water stagnation, extreme weather and increased runoff of nutrients from urban and agricultural lands are all compounding the algae problem. Especially in the summer months, when water temperatures increase, algal concentrations can grow exponentially and form dense surface scums.
The water then turns green, sand or gravel filters can become clogged, and some algae and bacteria present can produce volatile metabolites such as geosmin and methyl-Isoborneol (MIB), giving the water an earthy smell and flavour that cannot readily be removed by conventional water treatments. Cyanobacteria (blue-green algae) are major producers of geosmin and MIB, causing taste and odour problems in surface waters. Some species of cyanobacteria can release harmful toxins (HABs) which are poisonous to humans, aquatic fauna and livestock.
In addition, due to regular and prolonged algal bloom events, dissolved oxygen concentrations decrease dramatically with depth and reach anoxic levels due to oxygen-demanding organic matter. Bacteria, which decompose this organic matter, leach oxygen from the water and produce acidic by-products. This causes delicate organisms, which cannot survive under these chemical conditions, to die.
Causes of algal blooms
- Stagnant water
- High temperatures
- Fertilizer runoff
- Excess nutrients
- Chemical discharge and waste
The effects of algae in drinking water
Removing algae or algal by-products from water treatment plants where water is taken from a drinking water reservoir is costly and time consuming (e.g. the removal and cleaning of the sand filters) and therefore impractical.
Often, massive algal growth occurs within the water treatment plant itself, causing various problems within the process, including clogging of intake screens, fouling of weirs, increased chemical demand, bad tastes/ odours, and release of toxins. Chemical interventions in water treatment plants that aim to reduce algae and fouling problems involve the use of various additives such as chlorine and algaecides (e.g. copper sulphate). Their use, in turn, creates problems with hazardous chemical by-products such as THM (trihalomethane) or lead as well as an excessive release of harmful toxins in the treated water due to the rapid decay of algae. Such compounds make the water unsuitable for human consumption or any other use.
It is therefore important to tackle the problem at the source of algal blooms, which is the drinking water reservoir, in order to prevent impacts on the entire treatment system.
Impact of cyanotoxins on drinking water
- Public health risk
- Decrease or restriction of water use
- Taste and odour problems
- Increased chemical consumption
- Limited water supply
Action must be taken to make the water safe for drinking purposes. Algae need to be controlled in order to reduce the frequency, duration and magnitude of harmful algal blooms. Algal blooms are dynamic, as they can move up and down in the water column and are physically distributed throughout the reservoir. These blooms may appear at dusk or dawn and disappear during the day. Real-time water monitoring and control of the algae before it turns into a major bloom is essential.
Monitoring of water quality parameters such as chlorophyll-a, phycocyanin, temperature, turbidity, dissolved oxygen, pH and redox can provide direct and indirect information about algal growth. Based on this water quality information, it is possible to forecast the occurrence of potential harmful algal blooms. With this knowledge, plant operators can better handle the negative side effects of bloom outbreaks.
Treating algae in drinking water
Classic treatment options include chemicals, ultraviolet (UV) lighting, and aeration. Chemical algal bloom intervention involves treating the water with different compounds, usually copper sulphide, alum or lanthanum, or any other products that precipitate or sequester ionized orthophosphates. Use of chemicals for treatment and controlling algae in smaller ponds may be efficient, but since this method is expensive and the ecological consequences are uncertain, these products are not suitable for treatment of larger water bodies.
Furthermore, frequent dosing renders the use of chemicals troublesome and expensive. UV lighting is one of the most popular current in-lake restoration technologies for management of blue-green algae. This method destroys waterborne algae by running the water through ultraviolet light, which sterilizes the algae and prevents it from reproducing. The aeration method is a process that adds oxygen to water. This can be done by bubbling air into the water, or letting water fall through the air.
Algal bloom solutions
- Ultraviolet light
An effective solution for algae water supplies is ultrasonic sound waves. Ultrasound is made of sound waves with frequencies higher than the upper audible limit of human hearing (22 kHz). At specific frequencies, these sound waves can be used to control algae growth. Controlling algae with ultrasound is a well-established technology that has existed for years. This is an environmentally-friendly technology that is harmless to non-target aquatic organisms such as zooplankton, fish and plants.
Due to the direct effect of the ultrasound on the vertical distribution of algae in the water column, the ultrasound influences the ability of algae to form a dense bloom near the surface. It is, however, important for the efficiency of the technology that specific frequency programs be used based on the type of algae that requires controlling. Moreover, due to the adaptability of algae during seasonal changes within a waterbody, the ability to change these ultrasonic frequencies is of importance for long-lasting algal control.
How do sound waves control algae?
Algae can move vertically in the water due to their gas vesicles. When they rise to the water surface, where sunlight is abundant, for photosynthesis, they can form high densities and create visible algal blooms. During treatment, the ultrasonic sound waves create ultrasonic pressure in the top layer of the water that prevents algae from moving vertically through the water. In this way, algae are pushed towards deeper water and isolated in water layers where light conditions are limited.
Therefore, the algae are not capable of absorbing light for photosynthesis and perform other critical functions for their growth. In more detail, the generated ultrasound frequencies target specific cell structures of the algae, such as the gas vesicles and contractile vacuoles, therefore disrupting their vital functions such as photosynthesis, nutrient uptake and internal pressure control. Without these functions, these single-celled algae will eventually die. This limits the growth of new algae during treatment. During treatment, the cell wall of the algae remains intact, preventing the release of potentially harmful toxins from the algae to the water.
Interactive ultrasonic algae control can target common types of algae (e.g. green algae, cyanobacteria, diatoms, and filamentous algae).
How it works
- Monitor water quality
- Use specific sound waves to target specific algal species
- Restore ecosystem
- Prevent future algal blooms
Low power ultrasound to control algae growth does not control algae through the high pressures and temperatures associated with cavitation. Cavitation is a phenomenon in which high-power ultrasound causes the formation of microbubbles that implode, causing intense heat, local high pressure, and the generation of hydrogen radicals. This process can destroy cells and results in the release of algal toxins into the reservoir. Furthermore, the required power consumption for this technique, even treating a small volume is high-cost, making this technology unsuitable for lake or even pond treatment. A more effective solution is the use of low-power ultrasound to control algae growth. This also prevents the release of algal toxins into the water.
The MPC-Buoy makes use of this type of ultrasound. It is a floating solar-powered system that combines real-time water quality monitoring with interactive ultrasound technology. The collected water data is delivered to an online server, where algal blooms can be predicted, and ultrasonic programs can be automatically updated. In this way, algal overgrowth is detected and treated daily.
Sound waves solving algal blooms
Ultrasound technology combined with real-time water quality monitoring provides a cost-effective solution to control algal blooms in drinking water reservoirs. By handling the problem at the source, the intake of algal cells and their potential by-products (toxins, geosmins, MIB) into water treatment plants is reduced, resulting in fewer operational problems within the plant itself.
Ultrasonic Treatment of Algae in a New Jersey Reservoir (USA)
In 2014, America’s largest publicly traded water and wastewater utility company, American Water, installed four MPC-Buoy algae control systems at the Canoe Brook water treatment plant in Short Hills, New Jersey. Episodic taste and odour events were common here due to high algal concentrations. The primary objective was to reduce algae and, therefore, reduce taste and odour-causing compounds (geosmin and MIB), while a secondary objective was to decrease chemical doses and increase filter run times.
Interactive ultrasonic treatment was used to control algae as an alternative to copper-based algaecides. Real-time water monitoring also continuously measured the water quality and progress of the ultrasonic treatment. Based on chlorophyll-a, phycocyanin and turbidity data, the buoys controlled algae growth very well. When new algal species (Aphanizomenon sp.) began to bloom in the reservoir due significant water input from an adjusted reservoir, the ultrasonic program was adjusted to target this species and resulted in the active control of their growth (See Figure 5).
The algae eventually sank to the bottom of the reservoir. Because the algal cells are not lysed with this method, metabolites (including T&O compounds, pigments and toxins) were contained within the cells and not released into the water. Extensive analytical measurements conducted by American Water showed that the interactive ultrasonic system improved the water quality. The values for algal counts and pigments returned to baseline levels while undesirable taste and odour-causing compounds were well controlled in the treated water delivered to customers.
Additionally, chemical consumption was reduced by more than 20% compared to the previous year, while longer filter runs (127% better) and higher unit filter run volumes (83% higher) were estimated than the year before the treatment. Based on the economic comparisons, a payback time of 1.8 years was calculated.
Watercare Case study: Ultrasonic Treatment of Lower Nihotupu Dam (New Zealand)
The Lower Nihotupu Dam is one of five reservoirs in the Waitakere Ranges that supply water to Auckland. It covers an area of 52.9 hectares and is managed by Water Care Services Limited, a council-owned company. Five MPC-buoy systems were installed in 2017 with full coverage of the reservoir in order to control algae at the lower levels and prevent bloom incidents. Based on the online monitoring data and developed algorithm, the ultrasonic programs were modified remotely to the specific water conditions.
The real-time water monitoring data shows relatively stable and low algal concentrations during spring. During warm periods, the ultrasonic programs were adjusted to prevent growth of algae and cyanobacteria and revert it to normal levels. Summer algae levels during the treatment year were 90% reduced compared to the year before. Moreover, an optimal pH range was maintained during treatment in accordance to the thresholds set by the user.
Empresas Públicas de Medellín Case study: Ultrasonic Treatment of La Fe Water Reservoir (Colombia)
The La Fe reservoir is a freshwater reservoir in Medellín, Colombia. The lake measures 1.1 square kilometres and is used to supply drinking water to about 55% of the people in Medellín. Due to algal bloom problems, both the drinking water supply and recreation were hampered. Empresas Públicas de Medellín (EPM) is the company responsible for the reservoir’s water quality. Previously, the water treatment plants operators removed algae using chemicals such as copper sulphate and other algaecides. Such methods are considered economical and fast working but have to applied continuously and involve high toxicity rick due to direct lysis of the algae and negative effects on non-target organisms.
In order to control algae effectively and in an environmentally friendly way, EPM installed eight MPC-Buoy systems in 2015 at La Fe Reservoir. During the first months of the treatment in La Fe reservoir, the photic area in the lake had already increased due to decreasing algal concentrations. EPM’s own water quality tests also indicated a drastic reduction of diatoms and cyanobacteria throughout the water column in the La Fe reservoir.
In addition, since the system installation, algal blooms have been controlled effectively, even during the extreme environmental conditions of ‘El Nino’ from 2015–2016. The interactive algae control systems allowed the treatment to be adjusted for a highly dynamic and fast-growing algal population. This significantly reduced the chemical treatment costs as well.
About the author
Valentini Maliaka is the R&D Manager of LG Sonic B.V in the Netherlands. She graduated from Wageningen University with an MSc Degree in Water Quality Management and Aquatic Ecology and she is a PhD(c) of Radboud University in Nijmegen. She has done applied research on the biogeochemical processes of eutrophic lake systems and developed restoration methods which aim to improve their water quality.