|Surface Water Treatment by Roughing Filters - A Design, Construction and Operation Manual (SANDEC - SKAT, 1996, 180 p.)|
|Part 2: Design, construction and operation of roughing filters|
|10. Detailed filter design|
Roughing filters can be considered a major pretreatment process for turbid surface water since they efficiently separate fine solid particles over prolonged periods. They are therefore placed at the treatment plant site and operated in combination with other pretreatment units such as dynamic filters or sedimentation tanks. Roughing filters precede final treatment processes, such as slow sand filtration and chlorination.
Vertical-flow roughing filters usually consist of three filter units arranged in series as shown in Fig. 38. The water to be treated flows in sequence through the three filter compartments filled with coarse, medium and fine filter material. The size of the three distinct filter material fractions is generally between 20 and 4 mm, and graded, for example, into fractions of 12-18 mm, 8-12 mm and 4-8 mm.
Vertical-flow roughing filters operate either as downflow or upflow filters. They are hence either supplied by inflowing water at the filter top or at the filter bottom. The filter material of vertical-flow roughing filters is completely submerged. A water volume of about 10 cm depth usually covers the gravel. The top should be covered by a layer of coarse stones to shade the supernatant water and thus prevent algal growth often experienced in pretreated water exposed to the sun. Drainage facilities, consisting in perforated pipes or a false filter bottom system, are installed on the floor of the filter boxes. Finally, pipes or special inlet and outlet compartments are required to convey the water through the subsequent three filter units.
Fig. 38 Layout and Design of Vertical-flow Roughing Filters
Vertical-flow roughing filters are usually operated at 0.3 to 1.0 m/h filtration rates. Vertical-flow roughing filters may be sensitive to hydraulic fluctuations, especially if loaded with large amounts of solids. Settled matter might be resuspended at increased filtration rates, causing solids to break through the filter. Filter operation at constant flow rates is, therefore, recommended. Raw water containing colloidal matter and a high suspension stability should be treated at low filtration rates and preferably with fine filter material. Filter resistance is usually less than 20 cm per filter unit and, hence, not a decisive operational criteria for properly designed and operated roughing filters.
Due to structural constraints, vertical-flow roughing filters have a relatively small filter depth of about 1 m. Total filter depth of the three filter units used in series is thus 3 m. This total available filter dentin limits vertical-flow roughig filter application. It can generally and efficiently handle moderate raw water turbidities of 50 to 150 NTU. Raw water pretreatment by intake filters, reduction of filtration rate or provision of additional filter boxes would be required to treat raw water of higher turbidities.
In vertical roughing filters in layers, where all three gravel fractions are installed in one filter box with a total filter distance of about 1 m, low turbidity raw water can be pretreated. However, due to cleaning aspects this filter design can only be used for upflow operated roughing filters. In such filters, the coarse filter material is placed at the bottom and the finest material at the top of the filter. The separated solids, which accumulate mainly in the coarse filter fraction next to the filter bottom, can be easily flushed out with the water stored in the filter. Therefore, the use of upflow roughing filters in layers is recommended. Down flow roughing filters in layers face considerable problems with hydraulic filter cleaning. The bulk of solids accumulated in the coarse filter material on top of such filters would have to be flushed through the finer rather clean filter material and would thus soil the entire filter bed.
Adequate and efficient washwater collection is important for reliable roughing filter operation. Perforated pipes or a false filter bottom can be installed in vertical-flow roughing filters. Perforated pipes, which should be laid in a coarse gravel pack to support an even washwater abstraction, would require the installation of additional filter material in vertical-flow roughing filters. Preference is given to false filter bottoms as they allow an even washwater abstraction and do not require additional gravel layers. Although special perforated concrete slabs will be necessary, they may be readily produced locally.
A comparison of downflow with upflow roughing filters reveals the following:
· Direction of flow and sedimentation are obviously the first differences which might interfere or support solids settling on the filter material. Solid removal efficiency should consequently vary in the two filter types. Theoretically, downflow filters should have a better performance than upflow filters as the solid particles are more likely to settle on top of the gravel surface in the direction of flow than under countercurrent conditions. However, practical field experience has shown a similar efficiency for both filters. In dead filter zones, where the water flow is reduced to a minimum, solids settle regardless whether the roughing filter is operated in upflow or downflow direction. Hence, filter efficiency is similar in both filter types.
· The accumulation pattern of retained solids is another difference between downflow and upflow filters. The bulk of the solids is deposited at the inlet of the filter; i.e., for downflow filters in the upper part of the filter, and for upflow filters in the filter medium located next to the filter bottom. This, however, has a tremendous impact on hydraulic filter cleaning. In downflow roughing filters, the buIk of accumulated solids has to be flushed with a relatively small washwater volume from the soiled filter top through the lower and cleaner filter part to the filter bottom. The opposite is true for upflow roughing filters. The bulk of retained solids is accumulated next to the drainage system and a relatively large washwater volume, accommodated in the upper filter part, is available to flush the solids out of the filter. Owing to the important filter cleaning aspect, use of upflow roughing filters rather than downflow filters is recommended.
The Bathtub by the Riverside
Xiaojiang township, located by the Qiantang river, the largest stream in the province, faced tremendous problems with its water supply scheme. The raw water was pumped over a distance of about 1 km from the river to the treatment plant situated to the water tower in the centre of the town. In 1986, a public water supply was constructed using the "traditional" water treatment technology, e.g. coagulants and bleaching powder were added to the raw water, which was then pumped into a sedimentation tank and thereafter filtered through rapid sand filters However, the river is known to carry large silt loads especially during the rainy season. These solids, accumulated in the raw water main, increased the headloss in the pipe and seriously reduced its hydraulic capacity. Moreover, the water consumers drinking a lot of tea were always complaining about the unpleasant taste of the distributed water.
Rehabilitation of the water treatment became inevitable after six years of operation. Embarrassed by the situation, Mr Xu and his team transformed the sedimentation tank into two upflow roughing filter units, and constructed two new slow sand filters. All these installations were built behind the surrounding wall of the existing treatment plant and did not attract much public attention. However, these modifications did not solve the clogging problem in the raw water pipe, and the small raw water pumping house could not accommodate any installation larger than the common chemical dosing equipment. The project team admitted their design errors and decided to expose themselves to public criticism by erecting a gaudy and flashy structure.
To avoid a second pumping step, two additional upflow roughing filters were constructed on an elevated positron next to the dam of the river. The walls were covered with nice white tiles as commonly used in bathrooms. However, Mr Xu and his team convinced by the efficiency of roughing and slow sand filters had, on this occasion, made the right decision. Raw water turbidity of 500 NTU is now reduced to 60 -180 NTU by the upflow roughing filters, to 12 - 30 NTU by the second prefilters, and the filtrate of the slow sand filters is now always less than 5 NTU. Even more important, the complaints of the consumers have ceased since chemicals are no longer used to treat the water, and the public has no reason to criticise the bathtub by the riverside.