Using artificial circulation for in-reservoir management of cyanobacteria and taste and odour metabolite production

: (Alternative Format Thesis)

Abstract

The quality of raw water in reservoirs used for drinking water supply is projected to decline with the changing climate through prolonged thermal stratification and its associated water quality implications such as cyanobacterial blooms. To mitigate against water quality deterioration, in-reservoir artificial mixing techniques are often used to prevent the development of thermal stratification. Surface mixers are a relatively novel artificial mixing technique and are being increasingly installed in reservoirs to improve raw water quality, despite very limited observations of effectiveness. In this thesis, the hydrodynamics of surface mixer operation in a shallow, hypereutrophic reservoir was investigated using historical and in-reservoir measurements to assess their effects on cyanobacteria, taste and odour metabolite production, and soluble manganese concentrations. To understand the effects of surface mixers on taste and odour metabolite production in cyanobacteria, a greater understanding of the key trigger(s) of production was required. Here, water samples from the intakes of different reservoirs with varying trophic states were assessed using self-organising maps to determine the key trigger(s) in taste and odour metabolite production. The analysis revealed that high concentrations of ammonium relative to nitrate are important in the production of taste and odour metabolites in cyanobacteria. In-reservoir measurements of temperature, dissolved oxygen, and water velocities demonstrated that the range of influence of surface mixers were localised, but within the range of influence, temperature and dissolved oxygen gradients were reduced. Finally, despite the localised effects, surface mixer operation was found to decrease soluble manganese concentrations at the intake and increase cyanobacterial biomass. Although light penetration was low in the hypereutrophic reservoir, surface mixer operation did not effectively light-limit cyanobacteria to suppress photosynthesis as the water column depth was too shallow and mixing between light and nutrients created a beneficial growing environment for Planktothrix. Historical and in-reservoir observations showed that taste and odour metabolite concentrations were not affected by mixer operation but were susceptible to variable environmental conditions, such as heavy rainfall that can introduce a pulse of nutrients into the reservoir. Overall, surface mixer operation in a shallow, hypereutrophic reservoir poses a water quality management dilemma as the benefits of reduced soluble manganese concentrations at the intake need to be weighed against the cost of increased cyanobacterial biomass and greater risk of taste and odour event occurrence.

Date of Award	20 Jan 2021
Original language	English
Awarding Institution	University of Bath
Sponsors	Natural Environment Research Council & Wessex Water Services Ltd
Supervisor	Chris Blenkinsopp (Supervisor) & Antony Darby (Supervisor)