Loch Ascog in Argyll and Bute, is a eutrophic surface water supply that experiences algal blooms from mainly filamentous species. These algae cause blankets to form across the slow sand filters, causing major throughput problems and continuous filter blockages.

Filter Clear was brought into to remove the algae and maintain plant throughput during the peak algal period. Loch Ascog Water Treatment Works (WTW) comprises two raw water intake wells and five slow sand filters. The project was concerned with the upgrading of the works to satisfy drinking water (DW) directives covering Coliforms, Manganese, Pesticides/Taste and Odour, Turbidity, and Aesthetic Quality (Disinfection Control).

Our Spruce Filter, operating on a seasonal basis, was installed between the existing raw water intake wells and the slow sand filters for removal of algal that consisted mainly of filamentous algae. Some algae are pathogenic to humans producing endotoxins that can cause gastroenteritis, while others interfere with treatment plant operations, specifically with filter operations. The existing slow sand filters were retained as they contained an activated carbon sandwich layer.

Deep bed filter

The Spruce Filter is a non-chemical deep bed filter, capable of exceptional particle filtration, combining mechanical and adsorptive filtration.

The filter bed comprises of four layers of inert particulate material. From the top layer to the bottom layer the coarseness of the particulates decrease, while the density of the particulate material increases.

The final adsorptive element of the filter bed was significant in the success of the project from several aspects:

  • Filtration performance down to 0.2micron at flowrates of up to 60m3/m2hr and maximum inlet solids loadings in excess of 500mg/l
  • It has a naturally high positive charge whereas micro-organisms such as Cryptosporidium & Legionella are negatively charged. This ‘Zeta Potential’ enhances the mechanical filtration with surface adsorption to remove up to 99.954% of solids down to 0.2micron, without chemical addition.
  • It has catalytic properties enabling significant reduction in both iron and manganese. Iron and Manganese are transitional elements that when exposed to a catalyst such as the Spruce Filter media change from soluble into insoluble through catalytic oxidation
  • As the filter is a deep bed filter, it has a large surface area for the build up of biofilm. This is enhanced by the natural Zeta Potential within the filter media. The build up of biofilm causes selected micro-organisms to oxidise Iron and manganese into dense precipitates; enabling three to five times the metal retention when compared to chemically formed precipitates
  • The Spruce media’s properties allowed the required site flowrate (45l/s) to be obtained in a 2m diameter vessel, with the added benefit of a guaranteed media lifespan of 15 years

For the WTW, the installation of the Spruce Filter has been of exceptional benefit. Initially installed as a pre-treatment to sand, the remit of the filter was to assist in reducing filamentous algae, especially during the lake turnover.

However, the system produced results that helped with all five of the key drinking water directives:

  • Coliforms – 80% average reduction
  • Manganese – 94% average reduction
  • Pesticides/Taste and Odour – both significantly improved due to removal of algae
  • Turbidity – 82% average reduction
  • Aesthetic Quality (Disinfection Control) – improved visible quality

The success of the project brought the site into compliance without any additional treatment. The loading placed on the downstream iron and manganese system was therefore significantly reduced, allowing it to run at maximum performance.

The higher filtration rates and longer filter runs that can be obtained with the Spruce Filter has provided a substantial saving with regards to capital expenditure against competing technologies.


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