Removal of Suspended Substances in Domestic Wastewater Peer Reviewed Journal

Excerpt from Peer Reviewed Journal :

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.

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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.

Filtration Rate

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 (

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