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Removal of Suspended Substances in Domestic Wastewater by Coagulation Using Slow Sand Filtration and Roughing Filtration
Water to be supplied for public use must be potable i.e., satisfactory for drinking purposes from the standpoint of its chemical, physical and biological characteristics. Drinking water should, preferably, be obtained from a source free from pollution. The raw water normally available from surface water sources is, however, not directly suitable for drinking purposes. The objective of water treatment is to produce safe and potable drinking water. Some of the common treatment processes used for water purification includes Plain sedimentation, Slow Sand filtration, Rapid Sand filtration with Coagulation-flocculation units as essential pretreatment units. Pressure filters and diatomaceous filters have been used though very rarely. Roughing filters are used, under certain circumstances, as pretreatment units for the conventional filters. This paper specifically deals with Removal of suspended substances in domestic wastewater by coagulation using slow sand filtration and roughing filtration.
The application of micro filtration (MF) and ultra filtration (UF) via slow sand and roughing filtration for drinking water purification became a standard during the past two decades. This micro filtration process which uses slow sand and roughing filtration will hence forth be referred to as MFP in this paper. The first full scale applications in this field were reported in 1988 (Amy, 2006). In the meantime MF and UF applications have developed into widely established methods of primary treatment, with a steady increase in installed production capacity in the recent decades (Furukawa, 2002) both in the European Union and the United States.
By replacing conventional treatment steps (coagulation, sedimentation, and rapid filtration) with microfiltration a more reliable, robust, effective, and cheaper treatment method is introduced (Mallevialle et al., 1996). Compared to traditional treatment methods further advantages are (a) stable process under varying feed water quality, (b) smaller footprint, and (c) highly automatic operation. Most full-scale treatment plants are designed with polymeric MF/UF membranes. On the technical lifetime basis for the purchase of membranes for an entire drinking water treatment plant, the employment of ceramic membranes compared to polymeric ones was not a competitive alternative due to higher cost. Just recently studies in pilot scale (Loi-Brugger et al., 2006; 2006a), (Lerch et al., 2005) (Lerch et al., 2006) suggested that the process of purifying coagulated surface water with monolithic ceramic microfiltration membranes in constant flux and dead end mode can be optimised to such an extend, that their employment is competitive to the application of polymeric hollow fibre membranes. Higher flux and less frequent cleaning of ceramic membranes, but also considering the longer membrane lifetime is the basis for this recent leap in productivity.
Slow Sand Filtration
Due to their size, slow sand filtration systems are typically designed specifically for each site and application. Package slow sand treatment units are available but are not commonly used.
The primary design parameter for slow sand filtration is the filtration rate. Design filtration rates typically range from 0.05 gpm/ft2 to 0.1 gpm/ft2 although rates as high as 0.15 gpm/ft2 may be tolerated for short periods during filter scraping or ripening. Filtration rates can have a significant impact on filter run lengths. Lower filtration rates may provide longer filter runs. The appropriate filtration rate should be determined by pilot study on the raw water to be treated. Using the design filtration rate, the required filter area can be determined for the design flow rate.
Number of Filter Basins
Since slow sand filtration requires that a filter be off line for up to two weeks for scraping and filter ripening, more than one filter basin is typically necessary. State regulations require multiple filter units that provide redundant capacity when filters are out of service for backwash or maintenance. This requirement may be waived for non-community water systems providing engineering justification acceptable to the WSDOH. Each filter basin that can be operated independently is considered an individual filter unit.
The number of filter basins provided will depend on the difference between average and peak flows, the anticipated filter run time, and available storage within the water system. The most conservative system design criteria would be to use the maximum day demand as the design filtration rate with one filter basin out of service.
Basin Materials of Construction
Filter basins can be constructed using concrete or earthen berm construction. For very small systems (<25 gpm), basins can be constructed from alternative materials such as polyethylene or fiberglass tanks. Except for these very small systems, surface area requirements for slow sand filters are such that these types of tanks are impractical.
Regardless of the construction material, the tank should be made as watertight as practical because filtered water is collected in the bottom of the tank. For concrete tanks, water-stop material should be used at all construction joints. Hydrostatic relief valves should not be used. For earthen berm construction, continuous geomembrane liners should be used. Integrity testing should be performed on all geomembrane liner seams to verify no leak paths are present. Care should be taken when installing underdrains and gravel materials on geomembrane liners so as not to damage the liner material. It is WSDOH's position that common wall construction not be used between basins containing filtered water and unfiltered water due to the potential for contamination.
Geomembrane lined earthen berms are typically less expensive to construct than concrete basins but they have a shorter design life. The design life of a geomembrane is typically not greater than 20 years. The design life for a concrete basin is typically 40-50 years. Geomembrane liners are also not as durable as concrete basins as they can be damaged by activities such as sand scraping and resanding. Additionally, geomembrane liners must meet the requirements of WAC 246-290-220 pertaining to materials used in public water systems.
In order to understand fully the operating principles of a slow sand filter, it is necessary to have a basic knowledge of the way in which the filter acts, both biologically and physically. The treatment process is totally natural and is simply dependent on the maintenance of the correct environment for the growth of certain 'good' micro-organisms on or near the surface of the sand filter. Soon after the start of the treatment process, a film of these biologically active microorganisms develops in the filter fabric and the top of the sand. This film breaks down the incoming disease-carrying organisms, converting them into water, carbon dioxide, and other harmless chemicals. At the same time a large amount of suspended matter (which causes the cloudy or 'dirty' look of the water) is retained in the fabric and sand by simple straining (Amy, 2006).
The continuous straining process will gradually block the pores in both fabric and sand, which allow the water to pass through. This is shown by a lowering of the water level in the head-loss indicator tube on the outside of the filter tank, while the water level above the sand remains the same. In order to maintain the same flow through the filter, it is necessary to open the outlet valve further. After a certain time (generally 3-12 weeks, depending on raw-water quality), the valve will be fully open and the filter will be so blocked that it is no longer possible to get enough clean water out of it. It is then necessary to clean the filter (Amy, 2006).
The roughing filters systems are generally constructed in T11 tanks and developed to ensure that the raw water moves up-wards, which significantly enhances their cleansing effectiveness by utilizing gravitational forces to backwash amassed suspended solids accrued within the filter. Productivity is additionally enhanced by putting media on the elevated floor having a void beneath it (Pacini et al., 2005).
The amount of roughing filters needed is determined by raw water standard and needed production capacity which ought to be assessed/calculated as well as suitable models drafted before manufacturing begins. For maximum overall performance roughing filters ought to be operated in a maximum surface-filling rate of 0.6 m3 / m2 / hour. (This particular throughput may be seen as being the standard velocity in which water moves from the filtration system.) This implies that each and every T11 tank ought to be operated to generate 3.2m3/hour. They could be run at reduced throughputs however there's small distinction in percentage elimination of suspended solids in between 0.3 and 0.6 m3 / m2 / hr. Nevertheless, the effectiveness drops off over 0.6 m 3/m2 / hour. Circulation meters haven't been supplied in this system as they're vulnerable to congestion with higher suspended solids filling. Circulation rates will have to be established physically (Pacini et al., 2005).
Roughing filters are frequently recommended to be constructed with an in-sequence range where every tank becomes a phase, utilizing steadily finer media in every tank. Unprocessed water standard will decide the number of phases, i.e. how many roughing filter tanks will probably be needed: the more phases utilized (generally not more than 3) the higher the cleansing impact on the…[continue]
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