Leach Field Data Collection and Analysis Results, Revision 1

The study was conducted under WO-LT-0006 Amendment #6 of the Afghanistan Engineering Services Program (AESP) for the USAID Office of Infrastructure, Energy, and Engineering (OIEE). After assessing the design and maintenance requirements associated with the installation of the package WWTP at SGHS, USAID requested that Tetra Tech assess the feasibility of using an on-site leach field wastewater treatment system instead of the package WWTP. The purpose of this data collection and analysis effort was to gain a better understanding of the site characteristics and wastewater quantities in order to determine the feasibility of installing an on-site septic tank and leach field system. Percolation testing was conducted at six locations within the proposed leach field area from July 10 to 12, 2011. The testing was performed per the United States Army Corps of Engineers (USACE) Afghanistan Engineering Design Requirements: Sanitary Sewer and Septic System, dated June 2010. The results of the percolation testing showed that the soils have slightly higher than average infiltration rates, with an average percolation rate of 25.22 minutes per 25 millimeter. According to the AED Design Requirements, the limits of percolation rates for a leach field system are 0.1 min/25 mm (too fast) and 60 min/25mm (too slow). Each test pit percolation rate and the average percolation rate for the area tested at SGHS have soils with percolation rates that are within the percolation rate limits for leach field soils. Tetra Tech collected wastewater information to develop a better estimate of the average daily demand and peak flow demand for sanitary flows at SGHS. Flow estimates were developed according to the design guidance in USACE AED Design Requirements: Sanitary Sewer and Septic Systems. The total daily design occupancy of SGHS is estimated to be 5,619 persons, and the wastewater system will be designed to manage this occupancy. The average daily flow (ADF) represents the total waste volume generated over a 24-hour period and is calculated using the following equation: ADF = 0.8 * P * ADD * CF, where P is the total population, ADD is the average daily water demand per capita, and CF is the capacity factor. The resulting ADF is 17,447 gallons/day. This average daily flow is the wastewater flow rate that the leach field will be designed to manage. The peak diurnal flow (PDF) represents the maximum potential flow in the sanitary sewer system and is computed by the following equation: PDF = 167 * Q * C, where Q is the ADF in gallons/day and C is a constant, 38.2. The resulting PDF is 65,221 gallons/day or 45 gallons/minute. This value can be appropriately managed by properly sized sewers and is within the normal range of sanitary peak flow rates for a leach field system. The results of the percolation testing show that the soils at SGHS in the area of the proposed leach fields have an average percolation rate of 25.22 minutes per 25 millimeter and the site soils are suitable for the implementation of a leach field. The results of the ADF and PDF analysis show that the sanitary flows at SGHS are within a manageable range for a septic tank and leach field system given the available area on site and soil infiltration rates. Based on the data gathered during this exercise, Tetra Tech recommends that USAID move forward to the conceptual design phase of the septic tank and leach field system at SGHS.