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754 CHAPTER 11: Intermittent and Recirculating Packed-Bed Filters TABLE 11-17 Typical design criteria for layered textile filter with capillary break between every 4 in of textile medium used to treat typical residential wastewater' Hydraulic loading rate,gailftZ d f=ilter Dose Type of depth, limit,' operation Designtt Peak' in galfft2 Single pass 15 30 24 0.5 Recirculation 30 45 12 0.5 'Bounds(1996). +Based on typical average daily flows(e.g.,50 gal/capita-d). I Average residential septic tank effluent from a septic tank with an effluent filter:BOD = 130 mg/L,TSS = 40 mg/L,and O and G = 20 mg/L. 'Peak residential septic tank effluent from a septic tank with an effluent filter:BOD= 200 mg/1.,TSS = 60 mg/L,and O and G = 25 mg/L. THydraulic application rate,in both cases,should be limited to 0.8 in/dose(20 mm/dose). at a hydraulic loading rate of 45 gal/ft2-d (1800 L/M2-d) and a 5 to 1 recirculation ratio, three 4-in (100-mm) layers have proven to be very effective (see Table 11-16) (Auger, 1997; Chenier, 1997; and Roy, 1997). To reduce space requirements and construction costs,the final design of the lay- ered filter is still being optimized with respect to layer thickness, number of layers, nature of the textile fibers, and compression rate. Current (1997) recommended de- sign criteria for layered intermittent and recirculating textile filters used for treat- ing typical residential septic tank effluent are presented in Table 11-17. The textile medium must be placed in compacted layers not less than 4 in (100 mm) deep. Peat Filters The use of peat filters as an alternative method of biological and physical treat- ment of septic tank effluent has been reported since 1984 (Brooks et al., 1984). These filters were used in Maine and Ontario (see Fig. 11-30). The peat is a per- meable, absorbent medium that serves: (1)to filter the wastewater, (2) as a substrate for biological treatment, and (3) to reduce phosphorus concentration. Loading rates are usually the same as for intermittent sand filters, 1 gal/ft2•d (4 cm/d). The re- sults of monitoring several onsite peat filters are reported in Table 11-18. Commer- cial peat filters are available for individual home use. One manufacturer markets a 150 ft2 (13.9 m2) filter for the treatment of 500 gal/d (1892 L/d) of septic tank ef- fluent. The manufacturer emphasizes the ability of the filter to reduce BOD, TSS, ammonia, and bacteria. The importance of the source and characteristics of the peat have been demon- strated,based on the monitoring results obtained in Maine and in Ontario. The spec- ifications of the State of Maine require a von Post decomposition of H-4, a pH of 3.5 to 4.5, and a moisture content of 50 to 60 percent. It was observed that peat from different sources provided various levels of treatment,leading to the additional! requirement for a nitrate analysis of the peat leachate extract.