500 gal 20 min = 25 gal/min (The process of pump selection is described in more detail in the section on dosing chambers in Chapter 4.) Next, divide the effluent volume per cycle by the flow rate. In the example, 100.7 gal / 25 gal/min = 4 min A recirculation ratio of 3:1 to 5:1 is desirable. To achieve a ratio of 5:1, five times the daily input of effluent must be sent to the filter. In our example, a total of 1500 gpd (300 gpd X 5) must be cycled onto the filter bed. To find the total number of cycles per day necessary to provide this flow rate, divide the total daily volume to be cycled by the volume of effluent per cycle. Continuing with our example, 1500 gpd 100.7 gal/cycle = 15 cycles To find the duration of one cycle, divide the number of hours in a day by the number of cycles: 24 hr 15 cycles = 1.6 hr/cycle Therefore, in our example, the pump should be set to run for 4 min every hr and 36 min. One cycle should occur at least every 3 hr. As with the intermittent filter, a high water alarm should be placed in the filter enclosure to warn of pump failure or a clogged filter. Also, an additional float switch can be placed in the recircula- tion tank to start the pump and prevent backup of effluent into the septic tank and your house during periods of abnormally high water usage. CONVENTIONAL SYSTEM MODIFICATIONS Elevated sand filters should be protected from heavy rainfall, windblown trash, animals, and weeds. A wooden cover is sufficient under normal conditions, and where extreme low temperatures occur, a bit of fiberglass or styrofoam insulation should be installed on the bottom of the cover to prevent freezing of the filter surface. Materials A. Subsurface Uncollected Sand Filter 1. dosing chamber (recommended) 2. 4-in. unperforated PVC supply pipe 3. 4-in. perforated PVC distribution pipe 4. 0.025 yd3 of 0.75-in. to 2.5-in. gravel/ft2 of filter area 5. 0.074 yd3 of washed coarse sand/ft2 of filter area (2-ft-deep layer) 6. untreated building paper, hay, marsh grass, or filter fabric 7. 18-mil plastic side