Prevention and control
Prevention and control
There are a range of measures that can be used in the prevention management and control of blue–green algal blooms.
Catchment management is a long–term solution to the minimisation of blue–green algal blooms. Protecting soils from erosion and maintaining vegetation cover within a catchment will ultimately lead to better water quality as less sediments and nutrients will be able to enter waterways. Nutrients encourage the growth of blue–green algae, so reducing the nutrient inputs will reduce the frequency of algal blooms. Measures to improve catchment management will generally not show immediate results, but they will have long term benefits to the environment.
The main ways of reducing the nutrient load of a water body are:
- Avoiding the excessive use of fertilisers and manures on agricultural land within the catchment
- Protecting soil from erosion
- Treating sewage to remove the nutrients nitrogen and phosphorus.
Sediments and nutrients can be prevented from entering a waterway by protecting the strip of land adjacent to the water body. This strip of land is known as the riparian zone and vegetation within the riparian zone, is known as riparian vegetation. Riparian vegetation is important for maintaining and protecting water quality and performs the following tasks:
- Acts as a buffer zone
- Filters runoff and prevents pollutants from entering the water body
- Prevents river bank erosion which can increase turbidity and sedimentation of the water body
- Shades the water, which reduces the available light and keeps the water temperature lower so algal growth is not encouraged.
Managing blue-green algae blooms in water storages
Algal blooms in water storages such as lakes or dams can be dealt with by using a number of management strategies:
A water body becomes thermally stratified when two distinct temperature layers form. During spring the sun will warm the surface layers of water. They become less dense, but will be mixed with cooler 'bottom' water by wave action. As heating continues, the wave action will become less able to drive the mixing. When mixing ceases, the warmer surface water will lie over cooler, dense bottom waters. During autumn this process is reversed, and the water body will 'turn over'. During summer, algal blooms often occur in the warm stable conditions of the upper layer. The bottom layer often has very low concentrations of dissolved oxygen that creates favourable conditions for the release of nutrients from the sediments.
Artificial destratification involves increasing the movement of water that circulates between the shallower and deeper layers of the reservoir. This can be achieved by introducing a plume of bubbles near the bottom of the reservoir or installing a propeller or impeller in or near the dam wall. A circulation pattern is set up that reduces the differences in temperature, oxygen and nutrients between the top and the bottom waters.
Artificial destratification can reduce algal growth by:
- Reducing the sediment phosphorus load available to the water column and so starving the algae of nutrients
- Mixing algae deeper into the water column and starving them of light.
Once nutrients enter a water storage they are very hard to remove. Therefore the most effective strategy is to prevent nutrients from entering the storage in the first place. There are a number of measures which can be used to reduce the input of nutrients in water storages:
- Artificial wetlands and/or pre–reservoirs upstream of the water storage act as a nutrient sink and prevent the inflow of nutrients into the storage. These systems often require a lot of maintenance and their effectiveness in improving water quality has been varied. Examples of artificial wetlands used for these purposes are Carcoar Wetland and Lake Pillans
- Planting trees and shrubs around a water body will reduce the input of sediments and therefore nutrients into the water body. The plants also provide shade to the water body which reduces the temperature and solar radiation reaching the water body. This will help in the prevention of blue–green algal blooms
- Managing the whole catchment above the storage by reducing the use of fertilisers and fencing off waterways to prevent stock access will also reduce the soil erosion and amount of nutrients entering a waterway.
Biomanipulation or biological control is a method of altering the ecosystem to reduce the growth of algae. Biomanipulation is not yet a viable control mechanism in most waterbodies for algal blooms and is still being actively researched to see whether it will work or not.
Algae can be removed from water through a number of treatment methods. These include filtration, coagulation using aluminium and ferric iron salts or organic polymers and the use of algicides.
Short–term control techniques for drinking water supplies include changing the position or depth of offtakes so they are away from where algal cells scums accumulate, and the use of barriers to restrict scum movement.
The most reliable method of algal toxin removal is using activated carbon filtration. This approach uses either powdered activated carbon, which can be added intermittently whenever the need arises, or granular activated carbon absorbers, which are used continuously. Accordingly, granular activated carbon may be more expensive than powdered activated carbon when only used intermittently, but it is also generally more effective and more reliable for consistent removal of soluble organic compounds.