|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|
Unlimited filter length and simple layout are the main advantages of horizontal-flow roughing filters. Generally, the shallow structure does not create structural problems, and the filter length is not limited to a few metres. Furthermore, its simple layout does not require additional hydraulic structures and installations as in vertical-flow roughing filters. The raw water runs in horizontal direction from the inlet compartment, through a series of differently graded filter material separated by perforated walls, to the filter outlet as illustrated in Fig. 39. Filter material also ranges between 20 and 4 mm in size, and is usually distributed as coarse, medium and fine fraction in three subsequent filter compartments. To prevent algal growth in the filter, the water level is kept below the surface of the filter material by a weir or an effluent pipe placed at the filter outlet.
Fig. 39 Layout and Design of a Horizontal-flow Roughing
Filtration rate in horizontal-flow roughing filters ranges between 0.3 and 1.5 m/h. It has been defined here as hydraulic load (m³/h) per unit of vertical cross section area (m²) of the filter. Filter length is dependent on raw water turbidity and usually lies within 5 to 7 m. Due to the comparatively long filter length, horizontal-flow roughing filters can handle short turbidity peaks of 500 to 1,000 NTU.
Drainage facilities, such as perforated pipes, troughs or culverts, allow hydraulic filter bed cleaning. These drainage systems are placed at the filter bottom perpendicular to the direction of flow. Drainage facilities in flow direction must be avoided as they could create short-circuits during normal filter operation. Hence, false filter bottom systems cannot be installed in horizontal-flow roughing filters. Since most of the solids accumulate at the inlet of each filter medium, drainage facilities should be placed at the inlet of each filter compartment to enhance hydraulic cleaning efficiency. Installation of troughs complicates construction of the filter box floor. Furthermore, since the horizontal distance Ld between the troughs is usually large, an even abstraction of the sludge is correspondingly difficult. Therefore, use of perforated pipes is the best drainage system for horizontal-flow roughing filters, as it allows easy installation of a dispersed system. Although prefabricated culverts may allow a more even solids removal, connection to the washwater effluent pipes is more complicated.
Horizontal-flow roughing filters have a large silt storage capacity. Solids settle on top of the filter medium surface and grow to small heaps of loose aggregates with progressive filtration time. Part of the small heaps will drift towards the filter bottom as soon as they become unstable. This drift regenerates filter efficiency at the top, and slowly silts the filter from bottom to top. Horizontal-flow roughing filters also react less sensitively to filtration rate changes, as clusters of resuspended solids will drift towards the filter bottom or be retained by the subsequent filter layers. Horizontal-flow roughing filters are thus less susceptible than vertical-flow filters to solid breakthroughs caused by flow rate changes. However, they may react more sensitively to short circuits induced by a variable raw wafer temperature.
Periodic cleaning is also essential for horizontal-flow roughing filters. Hydraulic cleaning is carried out by fast drainage of the water stored in the filter. During filter drainage, the small unstable heaps of accumulated solids collapse and are flushed towards the filter bottom. The solid matter stored in the filter material is washed out of the filter box through the drainage system. Drainage velocities of 60 to 90 m/h are necessary to achieve a good hydraulic cleaning efficiency. Drainage pipes of adequate size are required to achieve the recommended velocity which drains the filter within 1 to 2 minutes. Depending on the solids concentration in the raw water, regular hydraulic filter cleaning, at intervals of every few weeks, is required to avoid deterioration of filter efficiency and development of excessive filter resistance. However, filter resistance will not exceed 20 cm if normal filter operation and regular cleaning are observed. Frequent and efficient filter drainages will also defer the need for manual filter cleaning, which nonetheless becomes unavoidable after some years of filter operation.
Photo 6 Inside View of a Roughing Filter Bed during Hydraulic Cleaning
Learning from Developing Countries
Water demand of Basle's agglomeration has been increasing gradually due to industrialisation and migration into the prospering area. Newly constructed buildings and highways have contributed to surface sealing and have reduced natural groundwater recharge. Increased water demand and reduced recharging rate have led to an alarming drop in the groundwater table. The groundwater wells almost ran dry. In order to reverse this situation, an artificial groundwater recharge plant was constructed in Aesch in the early 1970s.
Raw water was pumped from the river Birs into a lagoon for coarse matter separation from where it was conveyed to horizontal-flow roughing filters for fine solids separation. It was then aerated by cascades before flowing into the large lagoon which acts as slow sand filter. The treated water was finally led to recharge wells through the impervious top layer into the aquifer. Although the original capacity of the plant was designed for 400 l/s, operation had to be reduced to 200 l/s due to operational problems. Inadequate solids removal efficiency and gradual silting of the recharge wells were the reasons for the reduced treatment plant operation and for the repeated drop in the groundwater table.
The horizontal-flow roughing filters were designed according to the layout illustrated in Fig. 22. The 15-m long roughing filters, filled with one gravel fraction amounting to 50 - 80 mm in size, were operated at 5-10 m/h filtration rates. This inappropriate design and operation resulted in poor solids removal efficiencies. The slow sand filter was rapidly clogged so that the partly treated water ran through the coarse material of the embankment directly into the recharge well. Furthermore, since the roughing filters could not be cleaned hydraulically, the gravel had to be replaced every six years - a costly undertaking which caused headaches to the management of the water authority.
In the last decade, the roughing filter technology has, however, been developed to a viable treatment alternative in the developing countries. The responsible water authority had access to the necessary information and field-tested the new roughing filters. The comparative test results revealed important improvements; i.e., the small upflow roughing filters with a total filter length of 1 m had up to a six times higher solids removal efficiencies than the old filters. Since they could also be cleaned hydraulically operating costs were reduced The high-tech society was amazed by the efficiency of this low-cost technology.