Water, Sewage & Effluent March-April 2018 | Page 23

Controlling aquatic vegetation in canal systems Historically, aquatic vegetation control measures applied in South Africa have Aquatic vegetation in the Marico-Bosveld canals. Filamentous algae (Cladophora glomerata). Water Sewage & Effluent March/April 2018 21 algaecides and herbicides with heavy metals as the active ingredient were also tested over the years. But the problems associated with these products were many, as their usage resulted in the build-up of toxic residues in irrigation soils, were corrosive to canal infrastructure and irrigation equipment, and necessitated downtime for the canals, as well as withholding periods from crops. An alternative herbicide, with the trade name of MAGNACIDE™ H Herbicide (L8655), has recently been introduced to the South African market from the United States. This water-soluble aldehyde, which comes in liquid form and has acrolein as its active ingredient, can be used in operational water conveyance systems without interrupting irrigation water deliveries, and is characterised by superior effectiveness and quick dissipation without any residue. What makes acrolein a good prospective is that the product does not accumulate in crops, soil, or groundwater, and treated water can be applied directly to crops. Furthermore, acrolein (an organic molecule) dissipates to harmless carbon dioxide (CO 2 ) and water (H 2 O) as end products, which makes it more environmentally friendly. It is also effective on both algae and submersed macrophytes and is used widely in the United States, Australia, Canada, and other countries with similar problems. Scientific trials with MAGNACIDE™ H Herbicide (L8655) have been successfully completed in Roodeplaat and Hartbeespoort canal systems. In the case of Hartbeespoort, water been symptomatically orientated. The problem was treated only once it visibly influences the operation of the system. Different control options have been investigated over the years and implemented with varying levels of success. As the majority of South African canals are in operation 365 days per year, one of the greatest challenges was finding a control option that takes the canal out of operation for as short a time as possible. In the past, mechanical removal of the aquatic plant biomass from the canals had been a popular method; however, it was expensive, labour intensive, and time consuming. It has also been found that in peak summer months, the biomass cannot be removed efficiently and fast enough. Long dry periods to do mechanical cleaning and maintenance were found to be unproductive. To date, attempts to control the biomass mechanically in systems that continuously supply water seem to be relatively unsuccessful and uneconomic. In terms of chemical control methods, the so-called ‘lower pH method’ was favoured in the 1980s and 1990s, using copper sulphate after lowering the pH of the water with sulphuric acid. In those years, local herbicides/algaecides options were limited to the treatment of aquatic vegetation with mainly copper and diquat as the active ingredient. Unfortunately, both substances have their limitations under local conditions. The copper products are more effective on algae than on submersed macrophytes, while the diquat- based product offers greater efficacy on macrophytes than algae. Other conveyance structures, such as reducing hydraulic capacity and flow-speed in affected canals — some to such a degree that the supplied water cannot reach the terminal point of the canal system. Aquatic vegetation can replace significant volumes of the capacity of a canal. This results in spillage and water loss out of the system, as well as crop losses owing to under-supply of water to irrigators. Another operational problem is that of overestimation of the volume of water supplied to the user. As aquatic vegetation biomass replaces water in a canal, a higher volume of water is measured at measuring devices, compared to what is in fact passing at that specific location. The overestimation of flow makes the optimal operation of water supply a difficult task, as less water is passing at a specific point than what is measured. To compensate for these artificially high water levels, more water is often released into the system, giving rise to spilling with subsequent water loss and damage to canal infrastructure. Other problems with the presence of aquatic vegetation in canal systems include impediment of sluice gates working at dividing structures, water logging of long-weirs, structure failure of concrete-lined irrigation canals due to flooding, and blocking of irrigation systems and filters at water purification works by aquatic vegetation fragments.