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f , Multiple-pass recirculating sand/gravel filters typically achieve the following average effluent levels: I cBOD5: 10 mg/L TSS: 10 mg/L NO3-n: 30 mg/L Multiple-pass recirculating sand/gravel filters (RSFs or RGFs)have been most popular in applications with medium to large wastewater flows. They are ideal wastewater treatment systems for parks, restaurants, schools;office complexes, and large developments, and they are especially suited for communities with STEP and/or STEG effluent sewer collection systems. Typical multiple-pass recirculating sand/gravel filter design criteria are: Type of Design Recire Filter Effective Uniformity Dose Operation Loading Rate Ratio Depth Size Coefficient Limit gpolf? R:R inches Dn`m C, gallorificeldose C Recirculating 5 5:1 24 1.5 to 2.5 2 0.5 to 1.5 While sand/gravel media PBFs are, and will continue to be, an excellent choice for wastewater pretreatment, certain limitations have prevented them from being considered at all sites: • Land area—Some sites lack the land area required for a sand filter. Single-pass sand filters for single-family homes typically require between 300 and 400 square feet, depending on jurisdictional design or flow criteria. Media quality and accessibility—Good quality sand media is occasionally not locally available, resulting in either high transportation costs or the use of inferior local media. In addition, getting sand to some sites—such as islands, mountainous regions, or other isolated areas—can be difficult. a Installation quality—Sand filters are typically built onsite with locally available materials, and the quality of installation is partially contingent on the consistency of these materials, and the knowledge and ability of the installing contractor. • Serviceability—The ease of maintaining a buried onsite single-pass sand filter has been a long-term design concern that resulted in robust designs with low loading rates. The low loading rates are intended to ensure 10 to 20 years of continuous, usage with little to no intrusive filter maintenance because replacing the sand media can be difficult and costly. Textile-Based Packed Bed Filters The efforts to improve loading capacities and serviceability have led to extensive research into a wide variety of media(e.g., foam, glass, styrene, plastic products, expanded clays, zeolite, limestone, furnace slag,peat, etc.). Over the past decade, this research has led to the development of an advanced technology for packed bed filters that uses an engineered textile medium assembled in a variety of 4 configurations. Textile provides all the benefits inherent in the packed bed filter design but overcomes the limitations listed above. NTP-FLT•TR3-1 Rev.1.1,11/02 Page 3 of 72