Rationale

Septic tank configurations vary widely across the United States with most of them performing satisfactory to the requirements of their AHJ. However, many of those tanks cannot comply to this ASTM standard due to its restrictive nature. This standards primary focus should be how to build a quality vessel and its secondary focus on configuration. Section 7.3.1 updates make it easier for single compartment tanks to meet the standard. Many parts of the country use single compartment tanks and there is sufficient test data that shows a single compartment tank performs as well or better than a two compartment tank. Section 7.3.4- Industry practices across the country vary widely when addressing the baffle that creates a two compartment tank. Currently, section 7.3.4 requires a hole or baffle port through the baffle to be a minimum of 50 sq-in and section 7.3.5 requires this to be located in the middle 25% of the liquid column. This requirement is most likely inherited from the top-half / bottom-half tank designs, leaving the baffle short to form the baffle port. Currently there is wide spread use of tanks built with a monolithic base and a top slab cover. The baffle is poured full height and uses a pipe fitting to transfer fluid through the baffle. The pipe fitting could be a coupler, 90 elbow or a sanitary tee. These fittings do not meet the 50 sq-in but yet are widely used and specified by local regulators across the county. IAPMOs material and property standard for septic tanks, Z1000 allows them and has since the first standard PS-1 in 1997. A 1996 septic study funded by the NPCA tested 6 different configurations, one being a baffled tank with a pipe fitting and one being a tank with a baffle slot. The conclusion reads, The best performance (based on TSS) was obtained with a 1,000 gallon, two compartment tank with a four inch diameter PVC elbow connection. Based on performance and wide spread use of a pipe fittings, requiring 50 sq-in is too restrictive. Section 7.3.5 addresses where to locate the baffle port opening. A white paper written by T.R. Bounds, P.E.* titled Septic Tank Septage Pumping Intervals develops a formula for predicting pump out interval based on a U.S. Public Health Services study published in 1955 and an eight year study of 450 septic tanks in Glide Oregon. The study also demonstrates that the sludge accumulates almost twice (1.82) as fast as the scum. Placing the baffle port at mid-point of the liquid column does not optimize sludge accumulation, leading to increased pump out interval and reduced performance. Ideally, it would be located at 70% of the liquid depth to optimize pump out interval. Changing section 7.3.5 to allow for a range of 50% to 75% allows manufactures to continue using current practices while allowing manufactures wishing to optimize their tank designs to be supported by ASTM C1227. Section 7.5.3 addresses the depth of the inlet Tee or Baffle. Ideally, the bottom of the Tee/Baffle should extend just below the scum layer into the clear zone. Changing the minimum to 25% times the Liquid Level ensures the inlet tee/baffle discharges below the scum level and keeping the maximum depth at 30% times the liquid level keeps the tee/baffle at least 6 above the sludge layer so as not to re-suspend it. 25% to 30% may seem like a narrow band but is required to accommodate a liquid level range of 36 to 72. Section 7.6.3 addresses the 4 inspection hole over the baffle wall. I question the effectiveness, due to size, placement accuracy, potential infiltration problems and what purpose it serves. It is one more requirement to discourage adoption of the standard. IAPMO only requires inspection access on tanks longer than 12. This makes sense and is very practicable.