The presence of cyanobacteria in water reservoir in Slovakia and options for their removal

Health concerns regarding the presence of cyanobacteria cells and related metabolites (cyanotoxins, taste and odorous substances) in water resources have raised in the last decade. The number of cyanobacteria in water resources is higher than 10000 cells/ml in some of the cases. This values of cyanobacteria also increase the demands on water treatment technology. At the same time, exceeding the value of 20000 cells/ml in water resource is a reason for monitoring the quality of drinking water and the value of Microcystin LR in treated drinking water. Diseases caused by cyanobacteria and cyanotoxins include disorders of the digestive tract, allergic reactions, respiratory problems, contact dermatitis, liver and kidney diseases, weakening of the immune system, skin and liver cancer, nervous system diseases. The intensive physical and chemical treatment is needed, e.g. coagulation, flocculation, sedimentation, filtration, adsorption (activated carbon), disinfection (ozone, chlorination), eventually the combination of mechanical, physio-chemical and biological treatment methods with the disinfection. Removal cyanobacteria require the further options to be sought to improve the treatment technology efficiency of drinking water. Ultrafiltration (without coagulation) was investigated within the pilot-plant tests at the WTP Turček during the treatment of surface water originating from water reservoir Turček. Water treatment was focused on cyanobacteria, turbidity, colour and humic substances (expressed by the COD - chemical oxygen demand parameter) removal from water. Fully automated ultrafiltration equipment with the membrane module UA-640 (Microdyn-Nadir) was used. The ultrafiltration unit treated the water for 3-4 hours, filtration took place for 30 minutes, then washing with treated water followed for 15 seconds and the entire filtration cycle was repeated several times. On the base of filtration cycles, the effectiveness of membraned technology was evaluated. By the application of membrane technology used, the required quality of treated water has been achieved. The results show 100% cyanobacteria removal efficiency, 38.8% COD_Mn reduction efficiency, 78.6% turbidity removal efficiency, water colour was reduced by 80.8%. The turbidity value in the treated water ranged from 0.32 to 0.42 NTU. Ultrafiltration membrane processes generally remove viruses and bacteria with an efficiency of 99.99%, There are no published data on the removal of filamentous cyanobacteria in the literature. Based on the results achieved, it clearly follows that this method of treatment is suitable for this type of water and achieving the stricter limits specified in the new Directive of the European Parliament and the Council (EU) 2020/2184 for drinking water.