Surface Water Treatment by Roughing Filters - A Design, Construction and Operation Manual (SKAT, 1996): Part 2: Design, construction and operation of roughing filters: 13. Construction of roughing filters: 13.5 Gravel and sand washing facilities

Surface Water Treatment by Roughing Filters - A Design, Construction and Operation Manual (SKAT, 1996)

Part 2: Design, construction and operation of roughing filters

13. Construction of roughing filters

		(introduction...)
		13.1 Filter box
		13.2 Filter material
		13.3 Inlet and outlet structures
		13.4 Drainage system
		13.5 Gravel and sand washing facilities

13.5 Gravel and sand washing facilities

Filter material needs to be carefully washed before it is placed into the roughing and slow sand filters. The gravel and sand must be free from organic material, silt and clay particles, as these impurities cause serious operational problems. The organic matter could decompose and affect taste and odour, and the particles slowly washed out of the filters could thus increase turbidity of the treated water. It must be noted that provision and cleaning of filter material by the community require considerable effort and time.

Filter material washing is best achieved by mechanical stirring of the aggregates in a washwater basin, as mechanical friction rubs the impurities off the aggregates' surface. The washing site should be located in the centre of the treatment plant, preferably next to the slow sand filters which are cleaned manually and more frequently to reduce the transport distance of the filter material. The site should also not be too small and allow 4 - 6 men to wash the filter medium simultaneously. Washwater can be saved and gross impurities eliminated if small filter material loads are stirred and removed with a shovel to a first tank before they are transferred to a second tank for final washing. Such a washing installation is illustrated in Fig. 49. However, centralised cleaning involves transport of the filter material. Use of the open drainage channel located along roughing filters is an alternative to the washing site as it requires less gravel movement.

Raw water of moderate turbidity can be used as washwater, and prefiltered water for final sand cleaning. A flexible hose may also be used to wash the filter material, and should thus also be available if raw water is supplied by gravity to the treatment plant. The washing site may also be equipped with a shower for the treatment plant operator if the washwater is adequately disposed.

Fig. 49 Layout of Gravel and Sand Washing Installation

A Precious Plastic Sheet

This was already my fourth visit to La Javeriana treatment plant which forms part of the water supply system of a private education centre in a rather large and well-known city in Latin America. I have always looked forward to meeting Alcibiades, the dedicated caretaker of the treatment plant. I enjoyed our interesting discussions and Alcibiades would quite often relate some of his new practical experience he gained since my last visit, or we discussed some pending operational problems for which we found appropriate solutions. We learned a lot from each other, and our working relationship grew into a personal friendship.

Since my visit last year, new houses had been constructed in the catchment area. This further enhanced soil erosion in the rainy season, and uncontrolled wastewater discharges into the rivulet deteriorated the raw water quality especially in the dry season. Alcibiades had to install a small grit chamber in front of the intake filter to reduce the silt load on the filters. He also tried to reduce the increasing bacteriological pollution by the installation of a sand layer in the roughing filter. The first alteration brought the expected amelioration, however, he faced considerable problems with the second modification. The sand layer, which developed a significant filter resistance in the roughing filter, resulted in filter clogging. Since hydraulic cleaning was no longer possible, the roughing filter had to be cleaned manually.

La Javeriana's treatment plant is located on a slightly sloping meadow in a picturesque environment - reason enough to make some photos for my project documentation. I had intended to document the recent changes at the intake filter with some snapshots. What I discovered was a carelessly disposed of plastic sheet near the intake filter, and I was surprised by this litter that did not at all correspond to Alcibiades' working manner. He normally kept the treatment plant and its surrounding in very clean condition. I thought of several reasons for his negligence; i.e., his salary may no longer meet the current standard of living, or he may have personal problems with the administration vat home.

I was about to take away the unpleasant plastic sheet which would had spoiled my photo when Alcibiades arrived on the scene. I told him my problem and asked him to remove the plastic. However, Alcibiades was strongly opposed to my suggestion since this piece of plastic was used to clean the entire filter box during the biannual removal of the gravel from the intake filter. The plastic sheet was used as temporary slab in order not to soil the cleaned gravel. Ashamed of my ignorance I took a picture - along with the plastic sheet right in the middle of the photo!