Algal blooms in drinking water reservoirs
Drinking water reservoirs worldwide often suffer from frequent outbreaks of algal blooms. Their water quality degrades and becomes undrinkable. Algal blooms can grow so massive that they are even visible from outer space.
Rising temperatures, extreme weather, stagnant water, and excess of nutrients accelerate algae growth. On hot summers, algae can grow very fast forming dense surface scums. The water turns green, clogging sand or gravel filters.
Causes of algal blooms:
- Stagnant water
- High temperatures
- Fertilizer runoff
- Excessive nutrients
- Chemical discharge and waste
Cyanobacteria (blue-green algae) can produce volatile metabolites like geosmin and methyl-Isoborneol (MIB). They give water an odd smell and taste that is difficult to remove by conventional water treatment.Release of toxins
Some species of cyanobacteria can release harmful toxins (HABs). These are poisonous to humans, aquatic fauna, and livestock. Regular and prolonged algal blooms decrease the dissolved oxygen concentrations with depth. Bacteria decomposing the organic matter, leach oxygen from the water. This process creates acidic by-products. Delicate organisms cannot survive such chemical conditions and die.
Impacts of cyanotoxins in drinking water:
- Public health risks Decrease or restriction of water use
- Taste and odor problems
- Increased chemical consumption
- Limited water supply
Water taken from a drinking water reservoir can contaminate the water treatment plants too. Removing algae and algal by-products from the water treatment system is impractical. Measures like replacing and cleaning the sand filters are too costly and time-consuming. So, water treatment plants also often suffer from massive algal growth. This creates significant problems within the water treatment processes. Typically, clogging the intake screens, fouling the weirs, and increased chemical demand. Odd tastes and odors and the release of toxins are another big concern.Algal bloom solutions
To make the water safe for drinking, algae must be carefully controlled. Reducing the frequency, duration, and magnitude of harmful algal blooms must always be a priority. Algal blooms are dynamic, they can move up and down in the water column. They are also distributed throughout the reservoir. Blooms may appear at dusk or dawn and disappear during the day. Real-time water monitoring and controlling the algae before it turns into a major bloom is crucial.
Solutions for algal bloom control in drinking water systems include:
- Ultraviolet light
Algae and fouling problems in water reservoirs and treatment plants can be addressed with chemical intervention. It involves treating the water with different additives like chlorine and algaecides. Copper sulfide, alum, or lanthanum precipitate or sequester the ionized orthophosphates. These compounds can be efficient for the treatment and control of algae in smaller ponds. However, they also create problems with hazardous chemical byproducts. For example, THM (trihalomethane) or lead. Rapid decay of algae triggers an excessive release of harmful concentrations of toxins in the treated water.
In this case, the water becomes unsuitable for human consumption or any other use. This can compromise the entire water treatment system. This method is risky and expensive. It requires frequent dosing with significant ecological impacts. It is inadequate for larger waters. The best solution is to resolve the problem at the source of algal bloom, the dam.Aeration
The Aeration method relies on adding oxygen to the water. Either by bubbling air into water or letting the water fall through the air. It can be used for large ponds if the complete water surface is treated. But, this method is expensive and does not have a direct effect on algae. So its efficiency is variable.Ultraviolet light
UV Lighting is a popular in-lake restoration technology for managing blue-green algae. This method destroys waterborne algae by running the water through ultraviolet light. UV sterilizes the algae and prevents it from reproducing. It is effective for bacteria and parasites. But, it is only useful for low volumes of water. Its efficiency in turbid waters is low. It also needs frequent maintenance to keep the UV sleeve clean.Ultrasound
Ultrasonic sound waves offer an effective solution for keeping drinking water supplies safe. At specific frequencies, they control algae growth well. Ultrasound has a direct effect on the vertical distribution of algae in the water column. It influences the ability of algae to form dense blooms near the surface. This method is environmentally-friendly and harmless to other aquatic organisms.
Various ultrasonic solutions are available for treating smaller water areas (up to 200 meters/650ft). Typical devices comprise floating systems with transmitters emitting ultrasound waves underwater. This helps control common types of algae. Products for larger water surfaces like drinking water reservoirs use solar-powered buoy constructions.
The physicochemical parameters, water flow, and microorganisms can differ within the water body. Types of algae can vary and even change throughout the season. Monitoring the key water quality parameters provides valuable information about algal growth. The obtained data allows timely forecasting of potentially harmful algal blooms. This empowers the plant operators to mitigate the negative side effects of the bloom outbreaks effectively.
For this purpose, LG Sonic has developed MPC-Buoy, a solar-powered platform conducting continuous water quality monitoring to control algae with ultrasound. It uses scientifically accurate ultrasonic frequencies to control different types of algae. The devices were proven safe for fish, zooplankton, and plants.
The right frequency is determined based on the size of the algae cell, morphology, and water quality. Monitoring water quality is a crucial step. Chlorophyll α, Phycocyanin, Turbidity, DO, pH, Redox, and temperature must be monitored carefully. This helps predict an algal bloom and identify the exact type of algae. Effective ultrasonic treatment is achieved using these parameters and predictions.