Reducing Development of Cyanobacteria in Water Reservoirs

The ASIO Company has recently dealt intensely with the issue of pollution of surface waters and reducing of cyanobacteria development in water reservoirs.

Over the past ten years, the issue of excessive growth of cyanobacteria has become an ever-increasing problem. There are several methods of resolving it, starting with preventive methods of reducing hypotrophication and finishing with actions that resolve consequences of growth of cyanobacteria [1].

The methods of reducing of development of algal bloom include e.g.:

  • Chemical methods (coagulant, flocculant dosing),
  • Biological methods (bioseparation of phosphorus, bioaugmentation, allelopathy),
  • Mechanical (removal of cyanobacteria biomass, excavation of sediment),
  • Physical (use of ultrasound, microwave radiation)[1].

The ASIO Company has recently dealt intensely with the issue of pollution of surface waters and reducing of cyanobacteria development in water reservoirs.

Of the wide range of methods and ways of reducing of cyanobacteria bloom, we have developed two ways:
1, a chemical method of coagulant dosing,
2, a mechanical method of removal of cyanobacteria off the water surface.

Dosing of coagulant to a lake water column

The first method – dosing of coagulant under the water surface seems to be a simple way at first sight but a detail investigation finds out that there is a number of facts that should be considered more thoroughly. Based on the study of the issue of cyanobacteria growth reduction and based on discussions with specialists in the field, we have developed a vessel which is able to administer exact doses of coagulant or another liquid substance in a sophisticated way under the water surface. Dosing is performed using an application frame through which the coagulant enters the water. The application frame can be placed to the depth of 10 to 30 cm under the water surface. The effective range of area around the application frame is 10 m. The coagulant-dosing vessel is equipped with GPS navigation and sonar which enable systematic application across the water surface. What does that mean? It means that the helmsman has immediate information about position of the vessel in GPS coordinates, speed of vessel movement and depth of water profile above which the vessel is currently situated. The information is supplemented with the trajectory of the route covered so far, so as to avoid double or multiple dosing of the same place. All this information is needed for the vessel operator who sets the amount of the coagulant to be dosed, while dosage of the applied substance depends on a coagulation test, the vessel speed and depth of water in the applied profile. Because the operator calculates this immediately according the aforementioned requirements, the immediately optimal concentration for each profile means that the coagulant is administered into water in an ECOLOGICAL as well as ECONOMICAL way – we always administer just the calculated (required) dose – no more, no less – which is ecological in terms of the reservoir eco-system and economical from  the investor’s point of view. Since all the above-mentioned devices are equipped with data-loggers, the records enable to determine what amount of the coagulant was administered, when, where and in what profile, which is very important information for analysing and reporting of the application.

The boat used for application of substances under water surface has a supply tank on board which can hold approx. 6 tons of coagulant. The coagulant is stored in two reserve tanks on the lakeside from where it is pumped to the vessel. The reserve tanks are filled from a tanker truck.    

The boat, like all vessels, is subject to regulations and approval by the State Navigation Administration and must be equipped with navigation permit for a particular navigation zone. The helmsman must also have a licence for navigating the vessel.

Last year we completed several applications using this vessel. The task was treating of a reservoir with coagulant polyaluminium chloride. The water management board decision determined the conditions of application that had to be respected. The reservoir had an area of approx 68 ha; the application lasted 3 days as specified by the water management board decision. During that time the whole volume of the reservoir was treated. Transparency of water measured by Secchi disk was 0.2 m before application and 3.0 m after application.

 Skimming floating biomass in reservoir (skimming of cyanobacteria off the surface)

The function of another vessel we have developed is based on mechanical removal of floating biomass off the water surface. The method is a non-chemical method of treating of water surface so no chemical is administered into reservoir. Water bloom of cyanobacteria is separated via filtration. The separated biomass which has the appearance and consistence of liquidised spinach is transferred to a container of the boat and when this container is full, it is replaced by another one and the cyanobacteria is taken away for ecological disposal. The filtered water is either drained back to lake or treated by another, e.g. physical or biochemical method.   

 As mentioned above, there are many methods for limiting the growth of cyanobacteria or removal of cyanobacteria, and development of them has not been finished yet. But definitely, we can say that attempts for removal of cyanobacteria or limiting its growth should be preventive in the first place and systematic above all in order to prevent an increase in concentration of phosphorus in reservoirs and sediments in them. As we also perform research and technological applications in this field, we are aware of the fact that resolving the issue of cyanobacteria requires systematic steps and that each method which enables control and ecological removal of cyanobacteria or limiting their growth is a significant asset.    

 Precipitation of phosphorus at a dam influent

Preventive methods also include precipitation of phosphorus influent to a water reservoir.

Why did we choose this solution?

Because the basic sources of phosphorus in the basin are as follows:

  • Phosphorus flowing from small municipal water treatment plants that have no legislative duty to precipitate phosphorus at the outlet of the plant.
  • Phosphorus washed off agricultural soil. Private agriculturists farming in the Czech Republic are not bound by any regulations regarding application of phosphate fertilizers and growing crops on steep land in river basins.

Owing to the two sources of influx of phosphorus, excessive concentrations of it occur in recipients, including water reservoirs. Thus, the level of nutrients – hypotrophication of water increases. Due to feeding of high concentrations of phosphorus from the basin lying above a reservoir, massive growth of cyanobacteria occurs. By precipitating phosphorus with, for instance, iron (III) sulphate, we achieve phosphorus degradation efficiency as high as 93 %. Thus only a minimum concentration of phosphorus enters the water reservoir, a fact which contributes to suppression of massive development of cyanobacteria in the water reservoir. We have experience with phosphorus precipitation in the Brno valley dam where we have been operating the system with remote transfer of data for three years. With 93% efficiency of phosphorus precipitation along with combination of aeration of the main lake, we have achieved suppression of excessive development of cyanobacteria Microcistis aeruginosa down to minimal levels.

Aeration of dam reservoirs using aeration towers.

Mobile aeration towers

We have several years of experience in building and operating aeration towers. Aeration towers were used for the first time at the Brno dam in 2009. In that year, the Brno dam was emptied from 12 mil. m3 to 2 mil. m3. Mobile aeration equipment was used to ensure sufficient oxygen concentration close to the bottom (to prevent fish death). The equipment was installed on two vessels. Thus the aeration equipment was mobile and flexible in the scope of depths of 0-6 m below the surface. The maximum depth of the emptied dam was 6 m.

Aeration of re-filled Brno Dam

Basic information about the dam:
Area of aerated reservoir: 114 ha
Volume of aerated reservoir: 12 mil. m3
Average depth: 12 m
Maximum depth: 17 m

 Since 2010,the re-filled dam has been aerated using 20 aeration towers, in order to remove the anoxic layer in  the  hypolimnion layer. In 2008 (before installation of mobile and stationary aeration towers), the anoxic layer in the reservoir ranged from 5 to 17 m under the surface.

At present, oxygen concentration at the bottom of more than 3 mg/l is ensured thanks to 20 aeration towers (the patent of the ASIO Company). The aeration towers along with phosphorus precipitation at the influent to the Brno Dam have ensured ideal swimming conditions and a dam without the feared cyanobacteria for 2 seasons now.

Aeration using the aeration towers is currently a unique project. It is one of the complex of measures projected for the Brno Dam to suppress growth of cyanobacteria.

 The project of aeration towers designed by the ASIO, spol. s r.o. was awarded with the scientific accolade  “Česká hlava” (the “Czech Brain”)

 The project of phosphorus precipitation at the influent to reservoir, designed by the ASIO, spol. s r.o. was awarded with a “Certificate of Merit” in the global competition IWA.  

Jiří Palčík, Ing. Ph.D.,  ASIO, spol. s r.o., Tuřanka 1, 627 00 Brno, palcik@asio.cz

References:
[1] Maršálek B., Maršálková E., Vinklárková D.- Nechemické metody omezení rozvoje sinic, Vodárenská biologie 2009, pp. 84-93.