Artificial mixing for destratification and control of cyanobacterial growth in reservoirs

Abstract

EXECUTIVE SUMMARY: Problem Cyanobacteria and their excessive growth in reservoirs pose a major unresolved water quality problem for the water industry. The risk to water quality arises firstly from taste and odour compounds, which are problematic at low cell concentrations, and secondly from toxins which become problematic at a higher biomass. Copper - based algicides can be effective in controlling cyanobacteria but there are undesirable side effects. Disruption of the cells releases the taste and odours compounds and toxins which makes removal during treatment more difficult and the broad ecological impact may reduce the opportunity for natural controls. There is an urgent need for environmentally acceptable control methods. The primary aim of CRC for Water Quality and Treatment project 2.5.1 was to investigate control of cyanobacteria by creating mixing regimes that are unfavourable to cyanobacteria and which limit the release of nutrients from the sediments. The project evaluated a hybrid artificial mixing system, which included raft-mounted mechanical mixers and a bubble plume aerator as a means of controlling cyanobacteria and low dissolved oxygen levels in Myponga Reservoir, South Australia. A major component of the evaluation involved monitoring and understanding the behaviour of the reservoirs in terms of physical, chemical and biological processes and to demonstrate how monitoring data could be used to predict the onset of water quality hazards. The initial part of the report highlights the importance of incorporating reservoirs in a risk-based framework to manage water quality in the supply system and also the need for a high level of knowledge and understanding of the system. This is demonstrated by a conceptual model that identifies the processes leading to problems associated with cyanobacteria, pathogens and soluble metals (Fe & Mn). In particular, the significance of hydrodynamic processes in governing water quality processes is highlighted. A basic overview of cyanobacterial ecology and role of artificial mixing as an in-reservoir management strategy is also described. Approach The strategy proposed in this study involved using the bubble plume aerator to break down the primary thermocline while the surface mixers will be directed at entraining the warm surface layer including cyanobacteria into the main circulation pattern of the reservoir. Two approaches were used to evaluate artificial mixing. The first approach involved a 3-year monitoring program involving online and field measurements of physical (temperature), chemical (dissolved oxygen, Fe, Mn, nutrients) and biological parameters (chlorophyll, algal composition) in Myponga Reservoir. The second approach involved field investigations into the flow field around a surface mixer and numerical modelling of artificial mixing and reservoir hydrodynamics. This approach allowed for rapid assessment of different management strategies such as mixer entrainment volumes or mixer versus aerator configurations.