Control of Algae in Fuel Storage Ponds

Abstract

The First Generation Magnox Storage pond is one of the most hazardous and high risk facilities on the Sellafield site, due to the inventory of Magnox fuel, sludge and other wastes, and the age and condition of the pond structure. Retrieval of the waste and its transfer to safer modern storage is one of the highest priority programmes on the site. This already complicated task can be further hindered by the appearance of microbial blooms during the warmer months of the year. The blooms can cause significant loss of visibility, severely hampering the majority of waste retrieval operations such as those implemented through the operation of remotely operated vehicles. The resultant delays to waste retrieval risks extending the FGMSP programme for several years. One of the primary colonisers of the pond has been identified as a cyanobacterium Pseudanabaena catenata. The aim of this research project was to better understand growth of P. catenata in laboratory cultures that mimic pond conditions, to determine if changes can be made to the pond environment that could reduce the impact of the bloom events or prevent blooms from occurring, and to investigate whether biocides could be used for microbial growth control. Pseudanabaena catenata was grown in batch cultures in pond simulant medium to determine how cultures behave in low nutrient and highly alkaline environments. Further experiments included those using cultures grown in batch conditions in progressively lower nutrient concentrations, to discover the optimum nutrient ratio for growth and to better understand the adaptation processes supporting growth in the pond environment. Batch experiments were concluded with Mexel® 432 and Spectrus® NX 1422 biocide trials for bloom removal. Continuous culture experiments were used to assess the ability of P. catenata to adapt to flushing, nutrient renewal in high and low nutrient concentrations, and increases in pH due to sodium hydroxide dosing. Turbidity, cell and pigment concentrations, and culture pH were used to evaluate culture growth and activity. Information about nutrient requirements was obtained using ion chromatography. The culture used for these experiments was not axenic, and therefore changes in microbial community composition caused by biocide dosing, flushing, or sodium hydroxide dosing were determined by 16S rRNA gene 18 sequencing. The results presented provide insight into how biomass concentrations may be controlled by reduced nutrient levels, and the optimum nitrogen and phosphorus ratios supporting growth. The biocide dosing experiments helped to conclude that the lowest effective dose for bloom reduction was the same for both biocides trialled at 50 ppm. The continuous culture experiments helped to identify the highest doubling rates for the cultures grown in low and high nutrient concentrations, and effects of flushing, and the highest pH (11.5) that cultures can adapt to without reductions in cell concentrations. Results from the continuous culture experiments also suggest that flushing combined with increasing the pH values of the ponds could be used for bloom control. However, this may be only effective when the change is sudden, making it crucial to time this control strategy to the start of the bloom.

Date of Award	1 Aug 2019
Original language	English
Awarding Institution	The University of Manchester
Supervisor	David Sigee (Supervisor), Jonathan Lloyd (Supervisor) & Jon Pittman (Supervisor)