THE RESPONSE OF EFFLUENT ABSORPTION RATES TO RESTING TRENCHES OF A SERIALLY LOADED SEPTIC FILTER FIELD

Since the biomat can become the limiting factor in determining the effluent loading rate, a studywas conducted to 1) determine the hydraulic performance of a serially loaded filter fieldfollowing annual summer resting for fall and spring effluent absorption rates, and 2) test thehypothesis that summer resting of upper trenches of serially loaded filter fields will improve thesubsequent fall and spring effluent absorption rates. Earlier studies have assumed that thebiomat at the soil-trench interface has been degraded by aerobic microbial activity and therebyincrease effluent absorption rates. This study was initiated on a 10-year-old serially loaded septicsystem. The filter field consists of eight 9-to 17-m long, 60-cm wide, and 46-cm deep trencheson 3-to 4-m centers in a Fragiudult. Effluent absorption rates were calculated by dividing theeffluent load by the trench bottom area used. The filter field was rested during the summers ofyears 10 through 14. During years 7 through 10, the mean EAR had been near stable at about 2.8cm d-1. This mean rate increased to average 10.6 cm d-1 in the fall of years 11 through 14. SpringEAR increased from around 2.8 cm d-1 to an average of 5.0 cm d-1 in years 11 through 14. Thesedata indicate that resting increased EAR in all cases, approximately 280 percent in the fall and 70percent in the spring.