The microbial biochemistry of slow sand filters.

Invercannie Water Treatment Works in the Grampian Region of Scotland produces potable water by using slow sand filtration. This method of potable water production involves raw water percolating through a porous sand bed allowing the removal of particles by a combination of physical and biological mechanisms. The unique feature of slow sand filters is the ‘schmutzdecke’ that forms at the sand - water interface, which acts as an area for biofilm development. During filtration, schmutzdecke development ultimately results in the filter clogging, which causes a reduction in the amount of water filtered, known as headloss. At Invercannie Water Treatment Works, water from the River Dee is used as the raw intake water as it is of a high quality. However, like all waters from upland sources, it can become discoloured during heavy rainfall due to the presence of aquatic humic substances which are large recalcitrant molecules. Therefore, the colour of the water containing these non-biodegradable humic substances is scarcely affected by slow sand filtration, making the colour of the filtered water to the customer inconsistent. To meet the European Community Directive for Water Quality and to produce aesthetically pleasing potable water, the discoloration of the river water is removed prior to filtration using ozone. Ozone readily reacts with the humic material present in the raw water by oxidising these large recalcitrant molecules causing a reduction in water colour and the formation of a variety of biodegradable, low molecular weight compounds known as ozonation by-products. The increase in biodegradable organic carbon, caused by ozonation, results in changes in the microbial biomass of the schmutzdecke. It was found at Invercannie Water Treatment Works that slow sand filters receiving ozonated water reached terminal headloss more rapidly than those receiving non-ozonated water, especially in the later autumn / early winter. It is thought that the microflora associated with the clogging of slow sand filters consists predominantly of bacteria. Throughout the year, heterotrophic bacteria were isolated from the schmutzdecke of slow sand filters at Invercannie Water Treatment Works, as the by-products of ozonation will provide these bacteria with a fairly constant supply of nutrients throughout the year. All of the heterotrophic bacteria isolated produced highly pigmented, mucoid colonies when grown on solid media, which indicates a production of exopolymeric substances (EPS). One of the main mechanisms of headloss development is thought to be the accumulation of EPS. As headloss development occurs more frequently in the late autumn / early winter the affect of temperature upon the EPS production of these isolated heterotrophic bacteria was investigated. The isolated heterotrophic bacteria produced the most biomass and EPS at the higher temperatures investigated. During the late autumn / early winter months the production of biomass and EPS by these bacteria, the reduction in competition, an increase in the viscosity of the feed water and the rise in particulate matter on the sand surface are likely to contribute towards rapid headloss development during these months. The effect of selected ozonolysis by-products upon the growth of the isolated heterotrophic bacteria was also investigated. It was found that not all of these carbon sources supported the growth and development of the selected heterotrophic bacteria. Those ozonolysis by-products which were biodegraded by the isolated heterotrophic bacteria are likely to contribute towards biofilm development. However those ozonolysis by-products which could not be metabolised are likely to be removed from the ozonated water by adsorption. More nutrients will become available through increased rainfall during the autumn / winter months which will lead to an increase in the amount of nutrients available for the heterotrophic bacteria within the slow sand filters. This increase in nutrient availability will stimulate bacterial growth, leading to an increase in the amount of biomass present. Scanning electron micrographs of schmutzdecke samples revealed that the biofilm which forms part of the schmutzdecke is heterogeneous, therefore many intra- and interspecies relationships must be considered when investigating schmutzdecke formation and development.

Grantor dc:publisher.institution

Robert Gordon University

Year dc:date.issued

2003

Author dc:creator

• Law, Samantha P.

Advisor dc:contributor.advisor

• M.A.L. Melvin and A.J. Lamb

• Microbial biochemistry
• Slow sand filters
• Schmutzdecke
• Biofilm development
• Recalcitrant molecules
• Ozone
• Headloss
• Exopolymeric substances (EPS)
• Heterotrophic bacteria

Language dc:language

en

Identifier

oai:rgu-repository.worktribe.com:2807441
https://doi.org/10.48526/rgu-wt-2807441

OAI identifier oai:identifier

oai:rgu-repository.worktribe.com:2807441